Imablog Perspectives of a Canadian in the Old/Deep/New/Geographic South: This is where I ramble on about nothing in particular and post a few nice pictures.

Posts from Sciency things

Identify this spider

Ok, there's this big nasty looking spider hanging out by the sliding door to our room at my wife's parents' house. Somebody tell me what it is.

Spider top view

SpiderBottomSmall.jpg

IoP Book Sale

If you're in the market for some physics related books, the Institute of Physics is having a huge book sale. Books are discounted up to 75%, so if they have a text you've been eyeing for a while, but have been putting off because it's too expensive, see if it's one the books on sale. The list of books sorted by title (which I'm browsing through now) is 18 pages long, so there are plenty of books to choose from. Lots of proceedings, lots of hardcore physics books, and a bunch of general interest books. The sale ends Dec 31, so go check it out!

Farewell Voyager

APOD's got a pretty cool representation of what Voyager 1 is about to encounter at the edge of the solar system.

90 AU out, and moving at over 3 AU/year. That's over 448793612 km/year (278866864 miles/year) or 51197 km/h (31819 miles/h) or 14 km/s (8.8 miles/s). Now that's fast.

The Face on Mars

APOD has a cool new picture of what that Face on Mars really looks like.

APOD Face on Mars

It still has a face-like appearance, although it does look like more of a natural, not artificially created structure.

The big high resolution version of the photo makes a really cool desktop background.

Stunning example of retrograde motion

More pretty pictures to look at from APOD.

This is a stunning image of retrograde motion of not one planet, but two planets!. Both Mars and Uranus are shown going through their motions.

2003 Chart of the Nuclides

One of the coolest things (at least I thought so) I've ever had decorating my wall was a 3'x5' Chart of the Nuclides poster from 1976 or so. I had it posted on the wall above my desk, so that every time I looked up I could check out a different element. It was big, and packed a ton of information about each element into each box. It also doubled as a place for me to post Sticky notes. Had to leave it at home when I moved to the US, and I've been missing it ever since.

Just heard that the 2003 16th edition of the chart is now available for a really decent price. Only $25 for the wall chart or the book (the book is only $15 if you're a student).

Think I'll have to go place an order...

Back to the moon

Hmm, so President Bush wants to go back to the moon. While I am cautiously optimistic about renewed NASA funding, especially in light of the significant budget cuts NASA has experienced over the past decade, I'm still a little skeptical about Bush's vision, especially considering this is an election year. Along with his immigrant worker proposal, it just smacks of political pandering to the masses to me. But in any case, more funding for NASA is always a good thing.

I know we've been to the moon before, but considering how we can hardly keep a space station in Earth orbit, I can't see a moon base coming in my lifetime. I'll be very happy to be proven wrong though.

Personally, I would like to see ISS construction and expansion continue with more support and participation from other countries, with the goal of using it as a launching pad for missions to the moon and Mars. Make ISS and future lunar and planetary missions a truly global effort.

No reprieve for Hubble Space Telescope

Well, this is disappointing. The BBC article (via Slashdot) reports NASA has decided Hubble is doomed to fall back to earth. No servicing mission to fix Hubble's gyroscopes, no possibility of carting it back to display in a museum. NASA's deemed a Hubble mission as too unsafe.

From the article,

Readdy, a former shuttle astronaut, said Nasa had already analysed the question of whether to send astronauts to fix Hubble, and determined that it was unsafe.

He added that Hubble offers no "safe haven" for astronauts seeking refuge from a damaged shuttle, while the ISS does.

Well, at least there's still Herschel and the James Webb Telescope to look forward to..

Solar system family photos

Today's APOD photo is a very cool shot of Saturn taken from the rapidly approaching Cassinni probe. With arrival at Saturn scheduled for July 1, I'm looking forward to some exciting pictures and science in the coming months.

More Hubble goodness at APOD

Coming today to APOD, the newest deep space photo from Hubble, the Hubble Ultra-Deep Field image. The Hubble Deep Field showed hundreds of galaxies in an area previously thought to be dark and empty. Now, with the HUDF containng data from over 400 orbits looking at the same region of space over a 3 month period, I'm sure there will be even more to see, looking even farther back into time.

Lots of cool juicy info in the STSI newsletter

Venus Transiting

Today's APOD is an amazing picture of Venus at the edge of it's transit, taken by the Swedish Solar Telescope.

You can also see some cool movies taken by the Dutch Open Telescope.

Countdown to Cassini Saturn Orbit Insertion

In just two short days, Cassini will fire it's engines to slow down for Saturn orbit. 15 years in the making and after an almost 7 year trip, it's finally arriving. Looking forward to some pretty sweet data from Cassini and later from Huygens when it drops down to Titan.

Saturn's rings up close

Tons of pictures from Cassini today. Most of them are closeups of the rings during the orbital insertion. Very cool if you're into that kind of thing. For the rest, it's just a bunch of boring pictures of grey stripes.

Images at JPL and NASA
Images from CICLOPS

Modelling a radionuclide generator in Excel

We've been teaching our first year (second year now) residents about nuclear medicine physics, and one of the topics we've covered is producing radionuclides for medical use. So I'm trying to whip up a spreadsheet that will model the activity of a typical Mo-Tc radionuclide generator to show off transient equilibrium and what happens when the generator is eluted.

Modelling the Mo/Tc activity in the generator is easy. What I'm finding a little more difficult is including the effects of generator elution where some of the Tc activity is removed from the generator. I'm thinking if I can reformulate the transient equilibrium equation as a recursive equation that looks something like A(t+dt) = f(A(t)) then I can get it to work. Spreadsheets are good at dealing with recursive equations. Should be simple.

So let's start with the Bateman equation (need to learn some MathML). For a 2 radionuclide (parent/daughter) setup, it looks like

Ad(t) = Ap(0)(λd/(λdp))(exp(-λpt)-exp(-λdt)) + Ad(0)exp(-λdt)

Now, it's a fairly simple exercise to show that when transient equilibrium is established, the recursive equation has the form

Ad(t+1) = Ad(t)exp(-λp)

But, when the generator is eluted, transient equilibrium no longer exists, and we need to go back to the Bateman equation to determine the daughter activity.

So, supposing that at time t=0, we have no initial daughter activity. Our equation looks like

Ad(0) = Ap(0)(λd/(λdp))

And at time t=1, we have

Ad(1) = Ap(0)(λd/(λdp))(exp(-λp)-exp(-λd))

At time t=2,

Ad(1) = Ap(0)(λd/(λdp))(exp(-2λp)-exp(-2λd))

Already we can see that the term containing the difference of exponentials

exp(-λp)-exp(-λd)
is going to cause a lot of grief. A recursive Bateman equation may not be possible. I may have to come up with another way to do my spreadsheet.

Help find gravity waves!

There's another interesting new distributed computing project on the horizon coming soon. Like SETI@Home or Distributed.net, Einstein@Home is a project aiming to recruit masses of personal computers to process data from the LIGO (Laser Interferometer Gravitational-wave Observator) project. The purpose of LIGO is to look for gravitational waves by measuring teeny tiny changes in the length of the two arms of the observatory that might be caused by gravitational waves.

From the Einstein@Home homepage:

Einstein@Home is a project developed to search data from the Laser Interferometer Gravitational wave Observatory (LIGO) for signals coming from rapidly rotating neutron stars, known as pulsars. Scientists believe that some pulsars may not be perfectly spherical, and if so, they should emit characteristic gravitational waves, which LIGO will begin to detect in coming months.

Bruce Allen of the University of Wisconsin-Milwaukee's (UWM) LIGO Scientific Collaboration (LSC) group is leading the development of the Einstein@Home project.

Einstein@Home is one, small part of the LIGO scientific program. It is being set up as a distributed computing project, which means that it relies on computer time donated by private computer users like you to search for pulsars.

The project is scheduled to start sometime in 2005 as one of the events for the World Year of Physics 2005, so sign up now to get the word when it starts!

Found at UIUC's Physics Blog.

We are very messy people

Well, this is interesting.

THE MASSIVE NORTHEAST BLACKOUT of a year ago not only shut off electricity for 50 million people in the US and Canada, but also shut off the pollution coming from fossil-fired turbogenerators in the Ohio Valley. In effect, the power outage was an inadvertent experiment for gauging atmospheric repose with the grid gone for the better part of the day. And the results were impressive. On 15 August 2003, only 24 hours after the blackout, air was cleaner by this amount: SO2 was down 90%, O3 down 50%, and light-scattering particles down 70% over "normal" conditions in the same area. The haze reductions were made by University of Maryland scientists scooping air samples with a light aircraft. The observed pollutant reductions exceeded expectations, causing the authors to suggest that the spectacular overnight improvements in air quality "may result from underestimation of emission from power plants, inaccurate representation of power plant effluent in emission models or unaccounted-for atomospheric chemical reactions." (Marufu et al., Geophysical Research Letters, vol 31, L13106, 2004.)

Perhaps it will start people thinking a little more about what gets spewed out of industrial and power generating plants. One can only hope.

From Physics News Update 696

Splat!

Oooo, sadly NASA's Genesis capsule went splat into the desert after the parachutes failed to deploy. There was video of the splattage at NASA TV (along with video coverage of other unrelated things). There is a press conference scheduled for 2:30 EDT (also on NASA TV) where presumably we'll be told if there was anything salvagable from the sun-dust Genesis collected and what might have gone wrong with the parachutes.

Found via Slashdot

In the light of a blood red moon...

overcash1_med.jpgGet ready for it! Another lunar eclipse to ooo and ahh over. This one is happening relatively early too, starting October 27 just after 9PM EDT and ending just before 1 AM with totality somewhere in the middle (around 10:23 PM EDT).

I've always found lunar eclipses much more interesting than solar eclipses. For one, you don't need dark welder's glass to see them. They happen at night when it's quieter, which isn't a problem as long as you've got a decent supply of coffee or whatever your stimulant of choice is. And I like the colours.

So in two weeks, head out with your comfy lawn chair, some blankets and a thermos of coffee or other caffeinated beverage and find a nice dark field to plop yourself down into. Chill out, watch the moon slowly dissolve into an orange disk and enjoy.

The photo is a lunar eclipse on May 15, 2003, photographed by Loyd Overcash of Houston, Texas nabbed from the Lunar Eclipse Gallery.

Talk about your big bathtubs

Cleaning the Super Kamiokande PMTssk_01.jpgThere's a neat photo in the January issue of Physics Today that shows just how big the Super Kamiokande neutrino detector is. I'd heard that it was big, and even when I saw the pictures from the shattered PMTs incident, I never realized just how big it was.

There are more pictures that also show just how big SK is. Those are some guys in a raft at the far right of the this image. They're cleaning the huge PMTs that make up the light detecting system of SK.

All about CANDU

The latest issue of Physics in Canada is a cool theme issue with 4 really interesting articles about the features and capabilities of CANDU's newest ACR-700™ reactor. The first article, The CANDU™ Reactor — Past, Present, and Future talks about the evolution of CANDU reactors and design considerations that went into the ACR-700.

Another article (that I haven't gotten around to reading yet), ACR-700™ Reactor Physics and Fuel goes into some of the physics behind the various fuel core designs of the CANDU™ reactors.

It's a very interesting and instructive series of articles that shows what makes the CANDU™ reactor design simple to build (relatively), easy to operate and cheap to run.

Einstein@Home is officially active!

The Einstein@Home project has officially gone public! Go help find gravity waves!

I've had a couple of computers crunching part time for the past couple of weeks during the beta testing (2 computers 12 h/day) and like most other distributed computing projects, it's pretty unobtrusive. Download and install the BOINC client and follow the instructions. With E@H, you can tell it the start and end times you want the client to crunch away. When the client detects any user activity it stops, giving you back the CPU until you're finished. Then it goes back to work.

And if gravity waves aren't your thing, well then check out the other BOINC projects

A twist on the double-slit experiment

This is a sweet twist on the classic double-slit experiment. Instead of two physical slits that you shine light through, send a really short laser pulse through a gas and and let the electric field of the light pulse act the double slit.

Paulus and co-workers focused a train of pulses from a Ti:sapphire laser into a chamber containing a gas of argon atoms. The pulses were so short - just 5 femtoseconds - that each one contained just a few cycles of the electric field.

The team was able to control the output of the laser so that all the pulses were identical. The researchers could, for example, ensure that each pulse contained two maxima of the electric field (thatis, two peaks with large positive values) and one minimum (a peak with a large negative value). There was a small probability that an atom would be ionized by one or other of the maxima, which therefore played the role of the slits, with the resulting electron being accelerated towards a detector. If the atom was ionized by the minimum, the electron travelled in the opposite direction towards a second detector.

The team registered the arrival times of the electrons at both detectors and then plotted the number of electrons as a function of energy. The researchers observed interference fringes at the first detector because it was impossible to know if an electron counted by the detector was produced during the first or second maximum.

Clever, very clever indeed

Found at Slashdot.

Women should be on this list

My wife will probably smack me for the title of this post :)

This New Scientist article about the 13 things that do not make sense is an interesting read, and presents some of the more perplexing problems that still defy explanation. There are some pretty interesting ones in the list.

Found at Slashdot.

Immediate/Delay parathyroid ratios

Every summer, one or two med students show up at my door looking for help with a research project. They get sent to me by the radiologist they happen to be doing the project for, because I'm the Guy That Knows Stuff about image and data analysis (usually more than they know at any rate).

A few years ago (2001-ish or so), I helped out a med student on an interesting project that eventually became a poster that was accepted at an SNM meeting. A paper was written up and was submitted to JNM but ended up getting rejected for whatever reason.

So, for the sake of posterity and in the hopes that someone else might notice it, or have a similar idea, here is the paper that was submitted.

Note, this version has not undergone any form of peer review, other than being reviewed and edited by the authors. This is the latest version of the paper that I have, and there may have been revisions of some kind made in response to reviewer/editor comments when the paper was originally submitted. Those changes may or may not have made it into this version you see here. Questions and comments about this paper are welcome.

Dual Phase Tc-99m Sestamibi Imaging: Its Utility in Parathyroid Hyperplasia and Use of Immediate/ Delayed Image Ratios to Improve Diagnosis of Hyperparathyroidism

Abstract

Objective: Dual-phase Tc-99m sestamibi (MIBI) imaging is the technique of choice for hyperparathyroidism (HPT), especially for localizing parathyroid adenomas. Prior studies show its utility for detecting hyperplasia is equivocal. Quantitation to differentiate benign cases from hyperplasia and adenoma is introduced as a ratio between immediate and delayed images of counts/pixel (I/D ratio). This ratio should be significantly higher in benign parathyroid vs. hyperplasia. Method: Anterior pinhole and upper thorax images with a LEHR collimator at 20 minutes and again at 2 hours after sestamibi injection were obtained in 53 subjects. Retrospective interpretation of the scans as hyperplastic, adenomatous, or benign by a reader blinded to all data was based on the persistence of diffuse activity in two or more foci, a solitary focus, or no activity on the delayed images. These were compared to pathology when available. Regions of interest over the thyroid and background were drawn on immediate and delayed anterior pinhole images, and background subtracted counts/pixel were calculated. Immediate/delay ratios (I/D ratio) were computed for all scans and average ratios were calculated for each type of pathology (benign, hyperplasia, and adenoma). The resulting ratios were analyzed with a t-test to determine significant differences between ratios. Results: Sensitivity and specificity were for parathyroid hyperplasia. Mean I/D ratios were 2.26±0.68, 2.80±0.95, and 3.10±0.77 for subjects with hyperplasia, adenoma, and benign parathyroid respectively (hyperplasia vs. benign P=0.020, adenoma vs. benign P=0.381, hyperplasia vs. adenoma P=0.033. Thyroid ratio data was found to be somewhat independent of the time delay between immediate and delay image acquisition. Conclusion: Dual phase Tc-99 sestamibi imaging is more sensitive and specific for parathyroid hyperplasia than previously reported, supporting its use to localize hyperplastic glands preoperatively and help guide resection. A thyroid ratio between immediate and delayed images will aid in distinguishing hyperplasia from benign parathyroid in uncertain cases.

Keywords: hyperparathyroidism; 99mTc sestamibi; dual-phase parathyroid imaging

Introduction

Primary hyperparathyroidism has an incidence of 100-200/100,000 in the general population. The etiology is unknown, but, data from parathyroid ademomas and hyperplastic glands support a genetic cause linked to chromosome 11, which is also implicated as the cause of MEN I. Ninety-five percent of cases of primary hyperparathyroidism are caused by an adenoma (80-85%) or hyperplasia (10-15%)1. Adenomas are nearly always solitary. Hyperplasia usually involves all four glands, but can involve two or three and occur with varying degrees of asymmetric glandular involvement. Secondary hyperparathyroidism is another cause of parathyroid hyperplasia, often resulting from chronic hypocalcemia in the setting of renal failure and resolving when the hypocalcemia is corrected as with renal transplant. However, transplant failure can result in recurrent hyperparathyroidism that may require parathyroid removal2.

Parathyroid imaging is important for preoperative localization of hyperfunctioning parathyroid tissue. Originally advocated for patients who underwent previous neck exploration and had persistent or recurrent hyperparathyroidism, preoperative parathyroid imaging has proven to be beneficial for identification hyperfunctioning glands because it reduces operative time, costs, and failure rates. Past imaging techniques have involved Tl-201 and Tc-99m pertechnetate, and Tc-99m sestamibi with I-123. Currently, a single radionuclide, dual-phase Tc99m sestamibi imaging protocol is accepted as the standard for localizing hyperfunctioning parathyroid tissue given its combined sensitivity and cost effectiveness3,4,5. Tc-99m sestamibi (Cardiolite; DuPont Pharma, Billerica, MA) has a high affinity for thyroid and parathyroid tissue and a clear differential washout between thyroid and parathyorid tissue. Studies using this technique for initial preoperative detection of parathyroid adenomas have shown sensitivites and positive predictive values ranging from 82-100% and 89-100% respectively. However, results from studies using dual phase Tc-99m sestamibi for preoperative diagnosis of parathyroid hyperplasia have been poor with sensitivities ranging from 37-80%6,7,8. One group reported a sensitivity of 84% recently for detecting adenomas using a criterion whereby two or more foci of prolonged retention of radiotracer was interpreted as hyperplasia9.

We believe dual-phase Tc-99m sestamibi to be more sensitive than previously reported for parathyroid hyperplasia. Thus, we designed a study with two purposes. First, we aim to show dual phase Tc-99m sestamibi imaging is more sensitive than previously reported using a larger patient population than has been used in the past and using more updated interpretive criteria. Secondly, a quantitative ratio is introduced using a thyroid region of interest on the immediate and delayed images to differentiate between parathyroid hyperplasia, parathyroid adenoma, and benign parathyroid when the diagnosis is unclear from image interpretation alone. The immediate/delay image ratio (I/D ratio) is expected to be different for benign, hyperplastic, and adenomatous parathyroid. At 10-15 minutes after injection, there is uptake of Tc-99m sestamibi in both the thyroid and parathyroid glands. Since sestamibi washes out of the thyroid much faster than the parathyroid, the I/D ratio for a benign parathyroid scan should be fairly high and greater than one. In cases of parathyroid hyperplasia, residual activity in the parathyroid glands would result in slightly higher activity during the delayed imaging, resulting in a slightly lower I/D ratio. For parathyroid adenomas, small intense regions of focal uptake would result in higher residual activity, and therefore lower I/D ratio compared to the benign case but higher than the hyperplasia case. The expected values for the I/D ratio are summarized in Table 1.

Table 1: Summary of expected Immediate/Delay (I/D) and right/left (R/L) thyroid ratios
DiseaseI/D Ratio
Benign>> 1
Hyperplasia> 1
< benign
Adenoma> 1
> hyperplasia
< benign

Materials and Methods

A retrospective study of 54 patients (34 female, 20 male) who underwent dual-phase Tc-99m sestamibi imaging in our nuclear medicine department between February 1997 to March 2001 was performed. The average age of patients was 53.3±15.6 years (range 16-86 years). There were 8 (14.8%) benign cases (normal scan or negative pathology) including, 21 (38.9%) parathyroid hyperplasia (positive pathology or positive scan) and 25 (46.3%) parathyroid adenoma (positive pathology).

Dual Phase Parathyroid Imaging Protocol

Parathyroid images were acquired using a dual phase imaging protocol with patients receiving 740 MBq (20 mCi) Tc-99m sestamibi. Images were acquired at 20 minutes (immediate phase) and 2 hours (delay phase) post injection. Immediate phase imaging consisted of a 5 minute anterior pinhole image followed by a 5 minute image of the upper thorax using a low-energy, high-resolution parallel hole collimator. Imaging for the delay phase consisted of the same acquisitions as for the immediate phase plus additional 5 minute right and left anterior oblique pinhole images. All images were acquired using a 256x256 matrix on a dual head gamma camera (Picker PRISM 2000, Marconi Medical Systems, Cleveland, OH).

Images of all patients were retrospectively interpreted by a board certified nuclear medicine physician blinded to the clinical diagnosis and histopathology results when applicable. Scans were interpreted for hyperfunctioning parathyroid tissue as follows: prolonged retention of radiotracer on the delayed images relative to thyroid activity appearing as a solitary focus was interpreted as an adenoma; two or more foci of persistent radiotracer activity on delayed images as hyperplasia; no radiotracer retention on delayed images relative to thyroid activity was considered a normal scan/benign parathyroid.

Bilateral neck exploration and parathyroidectomy was performed on 46/54 patients. Resected parathyroid tissue specimens were submitted for pathological examination to obtain a definitive diagnosis. A scan was considered true positive if it showed a solitary focus of activity on delayed imaging corresponding to the location of the adenomatous tissue resected and found to be parathyroid adenoma on histopathology, or if it showed multiple foci of activity on delayed imaging corresponding to the locations of hyperplastic tissue resected and found to be hypercellular parathyroid on histopathology. Scans were also true positive if no activity focus was seen and parathyroidectomy was performed anyway, and specimens were found normal by histopathology.

Thyroid ROI Analysis

Images were analyzed using the Picker Odyssey (Marconi Medical Systems, Cleveland, OH) workstation platform. Only the immediate and delay anterior pinhole images were analyzed for this study. Regions of interest (ROI) were drawn over the thyroid on the immediate and delayed images (Figure 4). The counts per pixel were calculated and background subtraction was performed for each region. An I/D ratio was calculated by dividing the background subtracted immediate thyroid region counts/pixel by the background subtracted delay thyroid region counts/pixel. For each pathology (benign, hyperplasia and adenoma), average I/D ratios were calculated for the thyroid region. The results were analyzed using a two-tailed Student’s t-test to determine the significance of the differences in the ratios between benign and hyperplasia, benign and adenoma, and hyperplasia and adenoma.

Figure 4: Regions of interest used for I/D ratio calculation.

Results

The sensitivity and specificity of dual phase Tc-99 sestamibi parathyroid scans was found to be 82%/96% and 91%/88% for parathyroid adenoma and parathyroid hyperplasia respectively in 46/54 patients who underwent parathyroid resection. Figures 1, 2 and 3 show examples of true positive scans for parathyroid adenoma, hyperplasia, and benign from the study patient population.

Figure 1: Dual phase images from a patient diagnosed with bilateral hyperplasia confirmed by histopathology after subtotal parathyroidectomy
Figure 2: Dual phase images from patient with NM diagnosis of left inferior adenoma that was confirmed by pathology after left inferior excision
Figure 3: Typical benign dual phase Tc-99m sestamibi parathyroid images

The mean I/D thyroid ratio for benign patients was 3.10±0.77 (range 2.40-4.71). As predicted, the mean I/D thyroid ratio for the parathyroid hyperplasia cases was lower (2.26±0.68, range 0.78-3.73) than the benign ratio. For adenoma cases, the mean I/D thyroid ratio was 2.80±0.95 (range 1.07-4.72), also as predicted. Figure 5 shows a graph of the I/D ratio data and illustrates the range in the calculated ratios. Figure 6 shows a bar graph of the immediate/delay ratio for the thyroid region. Error bars in Figure 6 represent ±1 standard deviation. The average ratios from the thyroid region are summarized in Table 2 for the benign, hyperplasia and adenoma groups. A two-tailed Student’s t-test was performed to evaluate whether the differences in the I/D ratios were statistically significant (P < 0.05 considered to be statistically significant). The difference between the I/D ratio was found to be significant for benign parathyroid vs. parathyroid hyperplasia (P = 0.020) and for parathyroid hyperplasia vs. parathyroid adenoma (P = 0.033). The difference in the I/D ratio for benign parathyroid vs. parathyroid adenoma cases was not significant (P = 0.381). Results of the Student t-test are given in Table 3.

Figure 5: I/D thyroid ratios for benign, hyperplasia and adenoma
Figure 6: Average immediate/delay ratios of the thyroid region for benign, hyperplasia and adenoma.

Discussion

The sensitivity and specificity we obtained for parathyroid hyperplasia, equal or exceed other studies (6,7,8,9) to date examining the diagnostic utility of dual phase sestamibi parathyroid imaging for parathyroid hyperplasia prior to parathyroidectomy in which imaging findings were correlated with pathology findings. This study is retrospective, but includes the largest number of patients with proven parathyroid hyperplasia who underwent preoperative dual-phase Tc-99m sestamibi imaging. Given the results for parathyroid hyperplasia using the scan interpretation criteria used, and that dual-phase Tc-99m sestamibi imaging is already established as the best imaging tool with regard to cost effectiveness and diagnostic utility for parathyroid adenoma prior to surgery, we believe dual-phase Tc-99m sestamibi imaging as the diagnostic imaging test for localizing hyperfunctioning parathyroid tissue of any type prior to parathyroidectomy.

Our purpose was to establish a quantitative method to corroborate scan interpretation and distinguish adenomas from hyperplasia when scan interpretation was/is uncertain. Although the differences between the I/D ratios were not as large as initially expected, we believe the differences in mean ratios support using this quantitation method to substantiate scan interpretation in uncertain cases. There is considerable variation in the I/D ratios and the overlap in the range made it difficult to determine a precise I/D ratio for each type of pathology. Part of the variation is a result of normal physiologic differences between patients. More importantly, a larger number of patients with benign parathyroid, adenomas, and hyperplasia would decrease the error for the mean ratios and allow calculation of the I/D ratios to be more a robust diagnostic tool for supporting scan interpretation. Patient and camera positioning differences between the immediate and delay acquisitions also cause variation. In some cases, the delayed images were acquired with the camera at a different distance from the patient, or with the patient turned or tilted relative positioning used for the immediate scan. Such differences affect the size and shape of the thyroid region between the two acquisitions.

Another potential source of variation was the time interval between immediate and delay image acquisitions. Although the imaging protocol calls for the delayed images to be acquired two hours post-injection, this was not always possible and some delay images were acquired as long as 3 hours post-injection. The average time between the immediate and delay acquisitions was 1.87±0.36 hours (range 1.20-2.97 hours). Because of the rapid thyroid clearance time reported for Tc-99m sestamibi (27±13 minutes)10, it might normally be expected that increasing or decreasing the interval between immediate and delay acquisitions would affect the I/D ratio. However, a plot of the I/D ratio vs. I/D acquisition interval time (Figure 7) did not appear to be very well correlated. A linear regression analysis performed on the data yielded y = 0.3824x + 1.916 for the line of best fit (R2 = 0.0247), where y was the I/D thyroid ratio and x was the I/D imaging time interval (Figure 6). Thus, the I/D ratio appears to be relatively insensitive to fluctuations in the time interval between immediate and delay image acquisitions. There does appear to be a slight trend towards an increasing ratio as the delay time interval is increased however.

Figure 7: Plot of the I/D imaging time interval vs. the calculated I/D thyroid ratio. The line represents the line of best fit having the equation y = 0.3824x + 1.916, R2 = 0.0247.

Conclusion

Using more appropriate criteria for scan interpretation, dual-phase Tc-99m sestamibi parathyroid imaging is more sensitive in diagnosing parathyroid hyperplasia than previously reported. In addition, it is appropriate preoperatively in guiding parathyroidectomy for all types of hyperparathyroidism. The differences in the I/D ratio seen between cases of benign parathyroid and parathyroid hyperplasia and between parathyroid hyperplasia and parathyroid adenoma support the use of calculating an I/D ratio to help clarify diagnosis when interpretation is uncertain. However, more precise values for the I/D ratios are needed using an increased number of patients and better control of immediate and delay acquisition parameters. There did not appear to be the expected correlation between the image acquisition delay time and I/D ratio. However, any correlation may have been masked by the wide variations in I/D ratio. A prospective study is currently being conducted to determine a more precise I/D ratio for benign, hyperplasia and adenoma.

References

  1. Heath H, III, Hobson SF, Kennedy MA: Primary hyperparathyroidism: incidence, mortality and potential economic impact in a community. N Engl J Med 302:189, 1980.
  2. Heath DA: Localization of parathyroid tumours. Clin Endocrinol 43:523, 1995.
  3. Majors JD, Burke GJ, Mansberger AR, Wei JP: Technetium-99m sestamibi scan for localizing abnormal parathyroid glands after previous neck operations: preliminary experience in reoperative cases. South Med J 88:327, 1995.
  4. Taillefer R, Boucher Y, Potvin C, Lambert R: Detection and localization of parathyroid adenomas in patients with hyperparathyroidism using a single radionuclide imaging procedure with technetium-99m sestamibi. J Nuc Med 33:1801-7.
  5. Casas AT, Burke GJ, Mansberger AR, Wei JP: Impact of technetium-99m MIBI localization on operative time and success of operations for primary hyperparathyroidism. Am Surg 60:12, 1994.
  6. Light VL, McHenry CR, Jarjoura D, Sodee DB, Miron SD,: Prospective comparison of dual-phase technetium-99m scintigraphy in the evaluation of abnormal parathyroid glands. Am Surg 62:562, 1996.
  7. O'Doherty MJ, Kettle AG, Wells P, Collins R, Coakley AJ: Parathyroid imaging with technetium-99m sestamibi: preoperative localization and tissue uptake studies. J Nuc Med 33:313, 1992.
  8. McHenry CR, Lee K, Saadey J, Neumann DR, Esselstyn CB: Parathyroid localization with technetium-99m sestamibi: a prospective evaluation. J Am Coll Surg 183:25, 1996.
  9. Klieger P, O'Mara R: The diagnostic utility of dual phase Tc-99m sestamibi parathyroid imaging. Clin Nuc Med 23:4, 1997.
  10. Foldes I, Levay A, Stotz G. Comparative scanning of thyroid nodules with technetium-99m pertechnetate and technetium-99m methoxyisobutylisonitrile. Eur J Nucl Med 1993;20:330-333

Hybrid MR/X-ray fluoroscopy in Physics Today

Physics Today also had a much easier to read article describing the hybrid MR/X-ray system being developed at Stanford.

Since my PT happened to arrive in my mailbox before the MedPhys, I read about the new system there, and then discovered the paper in MedPhys. The MedPhys paper goes into a lot of gory detail about the system, so for those of you who don't have the stomach for it, the PT article is probably the more interesting one for you.

Thanks to Charles Day for the reminder about the article.

LNT prevails

Supporters of the linear-no threshold model of radiation risk got a big boost from the National Academy of Science in a report (BEIR VII Phase 2) released yesterday.

Personally, I'm not a fan of LNT. It's an overly conservative model and considering that every living organism on the planet evolved in a sea of terrestrial and cosmic background radiation, it just doesn't make much intuitive sense. The problem is that at low doses, the statistics just aren't there, and what is there is incredibly noisy. So trying to extract anything meaningful in terms of risk/adverse effect is stretching things pretty thin.

However, considering that regulatory agencies already develop recommendations and regulations based on LNT, I don't expect there to be much of an impact on the way us radiation people work.

From the NAS:

Low Levels of Ionizing Radiation May Cause Harm
June 29 -- A preponderance of scientific evidence shows that even low doses of ionizing radiation, such as gamma rays and X-rays, are likely to pose some risk of adverse health effects, says a new report from the National Research Council. In living organisms, such radiation can cause DNA damage that could eventually lead to cancers. The report provides a comprehensive assessment of these risks based on a review of the scientific literature from the past 15 years. It is the seventh in a series of assessments from the Research Council called the Biological Effects of Ionizing Radiation.

From the Report-in-Brief:

the current scientific evidence is consistent with the hypothesis that, at the low doses of interest in this report, there is a linear dose-response relationship between exposure to ionizing radiation and the development of solid cancers in humans. It is unlikely that there is a threshold below which cancers are not induced, but at low doses the number of radiation-induced cancers will be small. Other health effects (such as heart disease and stroke) occur at higher radiation doses, but additional data must be gathered before an assessment of any possible dose response can be made between low doses of radiation and non-cancer health effects. The report also concludes that with low dose or chronic exposures to low-LET irradiation, the risk of adverse heritable health effects to children conceived after their parents have been exposed is very small compared to baseline frequencies of genetic diseases in the population.

Relativity circa 1921

I just happened to be browsing around Nature's website and noticed that they were going to be launching Nature Physics in a few months. But, more interesting than that (I thought anyway) was that they've made available (for free) PDF copies of the articles from a 1921 issue celebrating GR.

Many of the articles make for interesting reading and provide a neat look back into history.

In 1921, Nature published a special issue celebrating Einstein's general theory of relativity, with contributions from Eddington, Weyl, Lorentz and Einstein himself, among many others. Nature Physics is making that special issue available online for the first time.

Einstein had published his general theory of relativity in 1915, a decade after the special theory. In 1919, observations of a solar eclipse - from expeditions led by Eddington and Dyson - offered some of the first evidence in support of the theory.

Einstein was awarded the 1921 Nobel Prize for physics - not, of course, for his theory of relativity, but for his work on the photoelectric effect. Publication of that work in 1905, with papers on special relativity and brownian motion, marked the annus mirabilis of the Swiss Patent Office clerk. The centenary is now celebrated in World Year of Physics 2005.

Hurricane history from tree rings

From the Earth System Processes 2 meeting comes an interesting method of tracking past hurricanes using tree growth rings.

Centuries of hurricane records have been discovered in the rings of southeastern US pine trees. This arboreal archive may contain critical information about how the Atlantic hurricane factory responds over the long term to natural and human-induced climate changes, say researchers at the University of Tennessee, Knoxville.

Apparently a record of past tropical storms and hurricanse can be gleaned by looking at the O-18 concentration in tree growth rings.

What makes drops in oxygen-18 so telling is that it matches up with a little known talent of all hurricanes: they are very good at depleting the air of oxygen-18, Mora says. Consequently, there are unusually low concentrations of oxygen-18 in the water that rains out of hurricanes. So when shallow roots of Southeastern trees like the longleaf pine and slash pine suck up that low-O-18 hurricane rain water, the same unusual isotopic signal is preserved in the woody tree cells that start growing as soon as the sun breaks through the storm clouds.

Neat stuff. Who knew trees could store all this interesting information.

Found via ScienceDaily.

The 2006 PACS

Just an entry to remind myself where to find the newest PACS® (Physics and Astronomy Classification Scheme®).

2006 PACS®

Biological and medical physics is category 87. The medical imaging categories go from 87.57 to 87.63.

Literature searching

Some people look at doing literature searches as a chore. Me, I think it's fun.

My typical project protocol normally goes like this:

  1. Come up with an idea
  2. Do a quick literature search to see what else there is on the topic
  3. Draft up a protocol for the experiment
  4. Do the experiment
  5. Analyze the results
  6. Do a more extensive literature search in preparation for writing it up
  7. Write, revise, rewrite
  8. Submit for publication somewhere

Step 6 is the fun literature search, because it means trips to the library. I usually find everything I need using PubMed, although sometimes it takes a while to find the right search terms to use. Get a collection of abstracts, highlight the ones I want to get and it's off to the library to dig out dusty old journal volumes for the paper I'm looking for. When I'm done, it's always fun to browse through the journals to see what interesting articles were being published at the time. That's my favourite part of the literature search. Now and then I'll even stumble onto a bonus article that I need to copy. You'd be surprised at how much interesting reading you can find in a 10 or 20 year old journal volume. Usually it's mostly from a historical perspective, but sometimes there's that long forgotten article that just happens to have exactly what you're looking for and didn't show up in the literature search.

Reducing Radiation Exposure to Pediatric Cardiac Catheterization Patients Using Copper Filtration

A long, long time ago during my undergrad days, one of the requirements for my 4th year physics lab course (way back in 1990 or so) was to do some kind of research project. The project my lab partner and I settled on was to investigate the use of copper filters in the x-ray beam to reduce skin entrance exposure to pediatric cardiac cath patients. For the past few years it's something that's been pretty standard in fluoroscopic equipment now (although not something I can claim any credit for, I don't think). I thought for posterity, I'd post our original write-up here.

Disclaimer: This work is from a project I did for one of my undergraduate labs a few years ago. The only review process this project has undergone was being scrutinized by my supervisor for this project and by the TA who marked it. Check my bibliography for other published papers on the subject.

If you'd like to comment on or criticize this, email me at eugenem@ix.netcom.com (or just leave a comment)

Abstract

Pediatric patients undergoing cardiac catheterization procedures can be subjected to large skin entrance exposures from both fluoroscopy and cine modes. To reduce the radiation exposure to the patient while minimizing loss in image quality, a small amount of copper filtration is added. In conjunction with standard radiation protection procedures (shielding and minimizing procedure time) the reduction in patient exposure can be reduced quite significantly. Preliminary phantom investigations demonstrate a decrease in exposure of up to 70% with the addition of 0.2 cm of copper filtration and little image quality degradation.

Introduction

Cardiac catheterization procedures involving pediatric patients can subject both the patient and operators to high radiation exposures. Because of the small size of the patients involved, children receive higher gonadal and thyroid exposures from scattered radiation. Children are also more sensitive to the effects of radiation than adults.

Several methods of reducing patient dose have been suggested such as minimizing procedure time, reducing the x-ray field size or placing lead shielding underneath the patient. Newer pulsed fluoroscopy systems where the x-ray beam is pulsed at the same rate as the video refresh rate of the monitor (30-60 pulses/second) has also shown to result in a significant saving in patient dose as well as improving image quality [7, 8, 13].

In keeping with the principle of ALARA, a method of reducing patient exposure by introducing additional beam filtration is proposed and investigated in this work. The additional filtration reduces radiation exposure by hardening the beam and removing the low energy x-rays which do not contribute to the formation of the x-ray image. Since the average kVp for pediatric patients is between 50-60 kVp, this should result in a large reduction in patient exposure without significant degradation in image quality. Aluminum filtration was also examined to compare the reduction in exposure between copper and aluminum. A similar method employed by den Boer et al [8] demonstrated a 55% reduction in patient exposure and 69% reduction in operator exposure with the combination of a high-output pulsed fluoroscopy and 0.4 mm of copper filtration.

Materials and Methods

Skin entrance exposures in typical cardiac catheterization procedures were examined for a Siemens cardiac catheterization laboratory equipped with a Siemens Polydoros 100 high frequency generator, Bicor biplane C-arms with variable FOV image intensifiers, Cinematic AEC 35 mm cine camera and Hicor digital imaging system.

Figure 1: Phantom picture A simple phantom (Figure 1) consisting of a number of Lucite sheets was assembled to mimic a small patient approximately 6 cm thick. A small length of catheter was placed in the center of the phantom to serve as a reference marker and a rough indication of the image quality as the filters were added.
Figure 1: Phantom

Skin entrance exposures for both fluoroscopy and cine were measured with a Keithly electrometer and a small ion chamber placed underneath the phantom. For fluoroscopy and cine, the skin entrance exposures for various amounts of aluminum and copper filtration were measured and graphed. Aluminum thicknesses ranging from 0-4.5 cm and copper thicknesses from 0-2.0 mm were used.

Figure 2: Low contrast phantom Fluoroscopic and cine image quality was tested using a low contrast 2% phantom (Figure 2) and high contrast resolution fluoroscopy phantom (Figure 3). The low contrast phantom was made up of a 0.4 cm thick plate of aluminum with 8 holes of 4 different sizes, 1.6, 3.2, 4.8 and 6.4 mm in diameter. This plate was sandwiched between two 0.8 cm blocks of aluminum so that the holes in the aluminum sheet were 2% of the total thickness (2.0 cm).
Figure 2: Low contrast phantom
Figure 3: High contrast phantom The high contrast resolution phantom is a 0.5 cm thick block of plastic with 8 metal grids of varying line pair densities embedded in it. Line pair densities range from 20 lpi (lines/inch) to 60 lpi.
Figure 3: High contrast phantom

Both phantoms were placed on the table top 30 cm away from the image intensifier and 40 cm above the x-ray tube and examined using fluoroscopy and cine. Image quality was evaluated using each phantom for a variety of filter thicknesses using the fluoroscopy display monitors and cine film.

Results

Image Quality

For the low contrast phantom and up to 0.2 mm of copper filtration added, all 4 holes were still easily visible. With 0.4 mm of copper filtration, the smallest hole (1.6 mm) became barely visible and was no longer visible with 0.5 mm of copper filtration.

High contrast resolution (HCR) was measured at about 28 lpi (lines/inch) with up to 0.2 mm of copper filtration. At 0.4 mm and 0.5 mm of copper filtration, HCR dropped to 24 lpi.

Exposure Rates

With the phantom placed on the table and the ion chamber underneath the phantom, fluoroscopic skin entrance exposure rates in mR/min were measured first with no aluminum filtration, and then with increasing amounts of aluminum added to the x-ray tube. X-ray techniques (kVp and mAs) were also recorded. Table 1 shows the results with the results illustrated graphically in Figure 4.

Table 1: Fluoroscopic skin entrance exposure rates for Al filter
Amt of Al (cm) kVp mAs Exposure rate (mR/min)
0.00461.6225.0
0.60502.789.0
0.90523.169.2
1.20553.657.0
1.50564.051.3
2.25594.643.0
2.70625.238.6
3.00645.636.2
3.75686.332.8
4.20716.331.5
4.50746.330.8
Figure 4: Fluoro skin entrance w/ Al graph
Figure 4: Fluoro skin entrance w/ Al

Figure 4 illustrates the dramatic decrease in skin entrance exposure with the addition of just 0.6 cm of additional aluminum filtration. The additional filtration results in a 60% reduction in patient exposure and only a slight increase in x-ray technique (from 46 kVp to 50 kVp and 1.6 mAs to 2.7 mAs).

The same measurements were performed using copper filtration, with the results tabulated in Table 2 and illustrated in Figure 5.

Table 2: Fluoroscopic skin entrance exposure rates for Cu filter
Amt of Cu (mm)kVpmAsExposure rate (mR/min)
0.0461.8267.0
0.1482.3141.0
0.2523.088.0
0.3543.470.0
0.4563.957.0
0.5584.353.0
1.5586.338.4
1.7716.435.3
2.0756.434.7
Figure 5: Fluoro skin entrance w/ Cu graph
Figure 5: Fluoro skin entrance w/ Cu

The reduction in patient exposure is nearly 50% with 0.1 mm of copper and 67% with 0.2 mm with very little change in x-ray technique (Figure 5). For larger filter thicknesses, the reduction in patient exposure is small relative to the increase in x-ray technique and the expected degradation in image quality resulting from the higher techniques.

Table 3 shows the cine skin entrance exposure with copper filtration, illustrated in Figure 6.

Table 3: Cine skin entrance exposure rates for Cu filter
Amt of Cu (mm)kVpmAsExposure rate (R/min)
0.050.5283.32
0.254.0290.64
0.460.5310.40
Figure 6: Cine skin entrance w/ Cu graph
Figure 6: Cine skin entrance w/ Cu

For cine procedures where exposure rates can be in the R/min range, 0.2 mm of copper filtration resulted in an 80% decrease in patient exposure with little change in technique. Unfortunately, due to time limitations and the arrival of a patient, fluoroscopy exposure rates were not obtained for copper filter thicknesses between 0.5 mm and 1.5 mm and only 3 measurements could be made in cine mode.

Conclusion

Inserting a small amount of additional filtration is a simple and effective way of obtaining significant exposure savings for pediatric patients undergoing cardiac catheterization procedures where they may be subjected to high radiation doses. From a practical viewpoint, copper would appear to be the most ideal filter, since copper provides much greater reduction in patient exposure and is much thinner than the equivalent amount of aluminum. An additional 0.2 mm of copper filtration appears to provide an optimum amount of filtration which maximizes the radiation exposure reduction and minimizes the degradation in image quality.

Bibliography

  1. Rueter FG, "Physician and patient exposure during cardiac catheterization", Circulation, 58:134-139 1978
  2. Martin EC, Olson A, "Radiation exposure to the paediatric patient from cardiac catheterization and angiocardiography", Br J Radiol, 53:100-106 1979
  3. Wu JR, Huang TY, Wu DK, Hsu PC, Weng PS, "Radiation exposure of pediatric patients and physicians during cardiac catheterization and balloon pulmonary valvuloplasty", Am J Cardiol, 68:221-225 1991
  4. Leibovic SJ, Fellows KE, "Patient radiation exposure during pediatric cardiac catheterization", Cardiovasc Intervent Radiol, 6:150-153 1983
  5. Martin EC, Olson AP, Seeg CN, Casarella WJ, "Radiation exposure to the pediatric patient during cardiac catheterization and angiocardiography", Circulation, 64:153-157 1981
  6. Waldman JD, Rummerfield PS, Gilpin EA, Kirkpatrick SE, "Radiation exposure to the child during cardiac catheterization", Circulation, 64:158-163 1981
  7. Schueler BA, Julsrud PR, Gray JE, Stears JG, Wu KY, "Radiation exposure and efficacy of exposure reduction techniques during cardiac catheterization in children", Am J Roentgenol, 162:173-177 1994
  8. den Boer A, de Feyter PJ, Hummel WA, Keane D, Roelandt JRTC, "Reduction of radiation exposure while maintaiing high quality fluoroscopic images during interventional cardiology using novel x-ray tube technology with extra beam filtering", Circulation, 89:2710-2714 1994
  9. Li LB, Kai M, Takano K, Ikeda S, Matsuura M, Kusama T, "Occupational exposure in pediatric cardiac catheterization", Health Phys, 69:261-264 1995
  10. Johnson LW, Moore RJ, Balter S, "Review of radiation safety in the cardiac catheterization laboratory", Cathet Cardiovasc Diagn, 25:186-194 1992
  11. Miller SW, Castronovo Jr FP, "Radiation exposure and protection in cardiac catheterization laboratories", Am J Cardiol, 55:171-176 1985
  12. Faulkner K, Love HG, Sweeney JK, Bardsley RA, "Radiation doses and somatic risk to patients during cardiac radiological procedures", Br J Radiol, 59:359-363 1986
  13. Holmes DR, Wondrow MA, Gray JE, Vetter RJ, Fellows JL, Julsrud PR, "Effect of pulsed progressive fluoroscopy on reduction of radiation dose in the cardiac catheterization laboratory", J Am Coll Cardiol, 15:159-162 1990

In purple, it is stunning!

Well, probably not.

From Hubble and the ESA comes a stunning view of the Crab nebula.

Pretty, huh.

Crab Nebula

Energizer bunny, meet the Mars rovers

Heading into their third year on Mars, Spirit and Opportunity are still motoring along taking cool pictures, finding neat stuff and checking out more terrain.

Not bad for a couple of robots that were only supposed to last 3 months. Sounds like they're beginning to suffer from aging though.

While showing signs of wear, Spirit and Opportunity are still being used to their maximum remaining capabilities. On Spirit, the teeth of the rover's rock abrasion tool are too worn to grind the surface off any more rocks, but its wire-bristle brush can still remove loose coatings. The tool was designed to uncover three rocks, but it exposed interiors of 15 rocks.
On Opportunity, the steering motor for the front right wheel stopped working eight months ago. A motor at the shoulder joint of the rover's robotic arm shows symptoms of a broken wire in the motor winding. Opportunity can still maneuver with its three other steerable wheels. Its shoulder motor still works when given extra current, and the arm is still useable without that motor.

Cool.

Don't eat me!

Doesn't today's APOD make you think of The Doomsday Machine episode from Star Trek:TOS?

Chandra Images of the Year

The votes are in and the best images of the year taken by the Chandra X-Ray Observatory over the past 7 years are posted. Some of them are pretty spectacular too.

My favourite of the winners is the 2002 image of the Crab Nebula which shows x-ray emissions from the cloud of gas surrounding the pulsar at the nebula's center.

Chandra X-Ray Observatory image of the Crab Nebula

There's another really cool composite image of the Crab Nebula made up of the Chandra image, an optical image from Hubble and an infrared image from the Spitzer Space Telescope.

Composite image of the Crab Nebula

Pretty, huh?

New life for Hubble!

Huzzah!! NASA has given the go-ahead for one final Hubble servicing mission for the spring of 2008.

From the ScienceDaily release:

Shuttle astronauts will make one final house call to NASA's Hubble Space Telescope as part of a mission to extend and improve the observatory's capabilities through 2013.

The flight is tentatively targeted for launch during the spring to fall of 2008. Mission planners are working to determine the best location and vehicle in the manifest to support the needs of Hubble while minimizing impact to International Space Station assembly. The planners are investigating the best way to support a launch on need mission for the Hubble flight. The present option will keep Launch Pad 39-B at the Kennedy Space Center, Fla., available for such a rescue flight should it be necessary.

There will be a lot for the astronauts to do on this servicing mission. Two new cameras are to be installed, gyroscope and battery replacement and an attempt to repair the Space Telescope Imaging Spectrograph are among many tasks to do.

This last mission should allow Hubble to operate at least until 2013 and fill in the gap until the James Webb telescope goes online.

Evaluation of a Triple Energy Window Scatter Correction Method for SPECT Imaging

Back when I was doing my medical physics residency (oh about 6 or so years ago now), I was asked to evaluate a triple-energy window technique for scatter reduction in nuclear medicine imaging. I stashed a copy of what I wrote up over on my other much neglected website and just thought I'd move it over here for storage and posterity. It's pretty rough and not anywhere near any kind of publishable state, but it was fun to work on and provided a good learning experience.

Objective

To test and evaluate the usefulness of the Triple Energy Window (TEW) method for scatter correction proposed by Ogawa et al.

Background

In a scatter correction method proposed by Ogawa et al1, the number of scattered photons in each pixel is estimated using two energy windows adjacent to the main photopeak window. The number of scattered photons in the photopeak window is determined by calculating the area of the trapezoid underneath the line joining the two scatter windows in the scatter spectrum.

For each pixel in the photopeak projections, the number of scatter counts is determined using the ollowing equation

Cscat ~ (Clower/Ws + Cupper/Ws)*Wm/2

where

Clower = counts in left window
Cupper = counts in right window
Ws = width of left and right scatter windows (keV)
Wm = width of photopeak window (keV)
Cscat = number of scatter counts

Phantom studies and computer simulations performed by Ogawa et al and Ichihara et al2 showed the method could estimate the number of scattered photons fairly accurately. However, neither paper addresses the potential for increased noise when the scatter counts are subtracted.

A Monte Carlo investigation of the method by Ljungberg et al3 questioned the use of the upper (right) scatter window noting that using the right scatter window might make the TEW method more susceptible to noise. When only the lower energy window is used, Clower = 0 and the estimated number of scatter counts becomes

Cmax = (Clower*Wm)/(2*Ws)

Monte Carlo simulations performed by Buvat et al4 demonstrated an 18% overestimate of the scatter counts when both scatter windows were used, and a 14% underestimate of the scatter counts when only the lower scatter window was used. Good relative activity quantification was also demonstrated by the simulations when only the lower window was used.

Equipment

  • Jaszczak phantom with cold rods and spheres
  • ACNP kidney phantom
  • Picker PRISM 3000 triple head SPECT camera

Method

A standard Jaszczak phantom containing the solid rod and spheres inserts was filled with water and 1110 MBq (30 mCi) of Tc-99m. A SPECT acquisition was acquired using three energy windows

  1. Photopeak - 15% window centered at 140 keV.
  2. Scatter1 - 3 keV window centered at 126 keV.
  3. Scatter2 - 3 keV window centered at 153 keV.

with these acquisition parameters: 120 projections, 30 s/projections, 3 degrees/projections and a 128x128 projection matrix. The images from each energy window was stored in a separate file.

The ACNP (American College of Nuclear Physicians) kidney phantom consists of two fillable kidney objects with a cold spot defect located in the middle of the right kidney and superior portion of the left kidney. The objects were filled with approximately 370 MBq (10 mCi) of Tc-99m and placed within a water bath. A SPECT acquisition was acquired using the same energy windows used for the Jaszczak phantom, 120 projections, 30 s/projection, 3 degrees/projection and a 128x128 projection matrix.

Processing

Once the acquisition is complete, there are three files, the photopeak projections, and two scatter projections. The TEW method was performed two different ways; with both scatter windows (TEW2) and with just the lower scatter window (TEW1) as suggested by Ljungberg et al. All image processing was done through the Odyssey software.

TEW2

  1. Divide the counts in each pixel of the scatter projections by the width in keV of the scatter window.
  2. Add the two scatter window projections together.
  3. Multiply the resultant projections by half of the photopeak window width in keV.
  4. Subtract the result from the original projections.

TEW1

  1. Divide the counts in each pixel of the scatter projections from the lower window by the width in keV of the scatter window.
  2. Multiply the resultant projections by half the photopeak window width in keV.
  3. Subtract the result from the original projections.

With the window settings used in this experiment (3 keV scatter windows, 15% (21 keV) photopeak window), the scatter correction using both lower and upper scatter windows was performed by adding the scatter projections together and multiplying the result by a factor of 3.5. The scatter projections were then subtracted from the main photopeak projections. For the TEW1 method, the pixels of the lower scatter window projections were scaled by a factor of 3.5 and subtracted from the photopeak window projections

Reconstruction

A 3 pixel thick slice was reconstructed with a ramp filter through the center of the spheres from each of the corrected and uncorrected projections. To investigate uniformity and noise, a 9 pixel thick slice was reconstructed with a ramp filter through the uniform water section of the phantom. Attenuation correction was applied using the system attenuation correction software and an attenuation coefficient of 0.11 cm-1. No additional postfilters were applied to the reconstructed images.

The ACNP kidney phantom was reconstructed using a ramp filter and single pixel thick slices, and then filtered using a Wiener 3-D postfilter. The filtered images were reformatted to 3 pixel thick coronal and transverse slices. No attenuation correction was applied to the phantom images.

Results and Discussion

The total counts per projection in the ACNP kidney phantom images ranged from 15-20 kcounts in the photopeak window, 1.5-2.6 kcounts in the lower scatter window and 300-600 counts in the upper scatter window. The count loss when both scatter windows were used to estimate the scatter counts in the photopeak window was almost 50% (6-10 kcounts/projection subtracted from the photopeak projections) and around 35% (5-9 kcounts/projection subtracted) when only the lower scatter window was used.

Circular ROIs was drawn through the each sphere of the Jaszczak phantom to obtain the mean counts/pixel within the sphere for the corrected and uncorrected images. The same ROIs were used to obtain the mean counts/pixel from the center of the phantom. The percent contrast for each sphere was calculated using the equation

% Contrast = (bkg counts - ROI counts)/bkg counts

Table 1: Contrast levels for solid spheres
% Contrast
  Uncorrected 2 Window 1 Window
36 mm 80 90 92
31 mm 66 75 83
25 mm 52 68 69
19 mm 49 60 65
15 mm 23 41 34

The most noticeable problem with the scatter corrected images is a significant decrease in counts and increase in noise. The inherent noisiness of the ramp filter also contributes to the noise in the reconstructed images. An interesting item to note is the improved contrast when only the single window is used compared to when both windows are used. This suggests that using only the lower scatter window may produce better results as suggested by Ljungberg et al3. Neglecting the higher scatter window for the higher energy photopeak is also recommended by Ogawa to avoid increasing statistical noise.

The additional noise introduced by the TEW scatter correction may be compensated for somewhat by applying a different filter to the projections or postfiltering the reconstructed images.

The slices from the uniform section of the phantom was used to determine the uniformity and noise of the corrected and uncorrected slices. The mean, standard deviation, maximum and minimum counts per pixel from a 15x15 pixel ROI were used to calculate the integral uniformity and the RMS noise level for the uniform section. The relatively high uniformity values are a result of the ramp filter which is inherently noisy.

Table 2: Integral uniformity using 15x15 pixel ROIs with no scatter correction applied. The last row gives the average over all ROIs used.
Mean cts/pix Max cts/pix Min cts/pix Int Unif
19619 24975 14598 26.2%
19750 23922 16182 19.3%
19134 24246 14715 24.5%
19371 24336 14940 23.9%
18785 23391 14598 23.1%
19331.8 24174 15006.6 23.4%

When both the upper and lower energy windows are used the integral uniformity increases significantly. This a result of the subtraction of the scatter counts from the projections.

Table 3: Integral uniformity (15x15 pixel ROIs) using upper and lower energy windows for scatter correction. The last row gives the average over all ROIs used.
Mean cts/pix Max cts/pix Min cts/pix Int Unif
11789 20709 5661 57.1%
12451 18261 7326 42.7%
12139 179555661 52.1%
12391 18513 6084 50.5%
11921 20781 6039 55.0%
12138.2 19243.8 6154.2 51.5%

Using a just the lower energy window to estimate the scatter improves the integral uniformity slightly, although the values remain relatively high compared to the uncorrected images.

Table 4: Integral uniformity (15x15 pixel ROIs) using only the lower energy window for scatter correction. The last row gives the average over all ROIs used.
Mean cts/pix Max cts/pix Min cts/pix Int Unif
13551 21951 67.32 53.1%
13896 19827 8163 41.7%
13227 21870 6732 52.9%
13999 19611 7785 43.2%
13512 20601 7614 46.0%
13637 20772 7405.2 47.4%

The noise level in the reconstructed images can be largely alleviated by filtering the images as is commonly done with clinical studies. However, the uniformity of the scatter corrected filtered images will still be greater than the uncorrected images simply because of the loss of counts incurred when the estimated scatter counts are subtracted.

A Wiener 3-D postfilter was applied to the same images resulting in a significant improvement in image noise. However, the scatter corrected filtered images still demonstrated greater non-uniformity and appeared noisier than the uncorrected filtered images.

Evaluation of the ACNP kidney phantom was performed qualitatively on both the filtered and unfiltered images. In all images, both defects were clearly visualized, although the scatter corrected images showed a lower count density and noise was increased significantly. Applying a Wiener 3-D postfilter improved the appearance of the images considerably.

Conclusion

The triple energy window scatter correction method evaluated using the Jaszczak and ACNP kidney phantoms showed decreased count images and increased noise, although contrast was improved. The phantom studies suggest that the triple energy window method may not be well suited for studies involving large distributions of radioactivity such as brain or liver studies because of the poor noise and uniformity properties. This scatter correction method may prove to be more applicable to studies involving smaller discrete radioactivity distributions such as cardiac or renal studies. Planar studies may also benefit from this correction method, although it was not investigated here. The ramp filter used to reconstruct the corrected and uncorrected images is inherently noisy, so the use of more optimal filters in addition to or instead of the ramp filter when reconstructing images should be considered. Further research using more clinically relevant phantoms is needed to further evaluate the noise and uniformity properties of the TEW method.

Bibliography

  1. Ogawa K, Harata Y, Ichihara T, Kubo A, Hashimoto S, A practical method for position dependent Compton scatter correction in single photon emission CT, IEEE Trans Nucl Med, 10:408-412 1991
  2. Ichihara T, Ogawa K, Motomura N, Kubo A, Hashimoto S, Compton scatter compensation using the triple-energy window method for single and dual isotope SPECT, J Nucl Med 34:2216-2221 1993
  3. Ljungberg M, King MA, Hademenos GJ, Strand SE, Comparison of four scatter correction methods using Monte Carlo simulated source distributions, J Nucl Med 35:143-151 1994
  4. Buvat I, Rodrigues-Villafuerte M, Todd-Pokropek A, Benali H, Di Paola R, Comparative assessment of nine scatter correction methods based on spectra analysis using Monte Carlo simulations, J Nucl Med 36:1476-1488 1995

111 years of x-rays

111 years ago today, a physicist by the name of Wilhelm Röntgen caught a glimmer of light coming from a fluorescent screen located across the room from his cathode ray tube setup. The strange phenomenon caught his interest and he spent several weeks investigating the matter.

Röntgen had just "discovered" x-rays. A month later he created the first radiograph (röntgenograms as they were called back then), an x-ray image of his wife's hand. The new rays were a huge hit among physicists, enabling the discovery of all kinds of wonderful things about the properties of matter. Once word reached the medical community of this new discovery, it wasn't long before Röntgen's new rays were used to image and treat patients.
The first x-rays were used to treat a breast cancer patient a mere two months after Röntgen's discovery. Shortly after came Edison's fluoroscope machines for visualizing x-rays in real-time. From there, the rest was history.

References:
Juan A del Regato, Radiological Physicists, American Association of Physicists in Medicine, 1985
Seliger HH, "Wilhelm Conrad Röntgen and the Glimmer of Light", Physics Today, 1995(11), 25-31

Mercury Transit

Later on this afternoon with a properly configured telescope configuration, you'll be able to catch Mercury's transit across the sun. If you don't have access to a properly configured telescope, you can catch the action live from SOHO's point of view. The transit is supposed to start at 1912UTC, or 1412EST (2:12 PM). Mercury's only going to do this 14 times this century, and this is the second one so far.

Merctransit2006a.jpg

Mars claims another one

Looks like Mars may have may have claimed yet another probe. After 9 years in operation in Mars orbit, the Mars Global Surveyor has gone silent.

From the NewScientest blurb:

An unexpected break in communications has NASA struggling to restore contact with its Mars Global Surveyor (MGS) spacecraft. If communication cannot be restored soon, NASA may try to diagnose the problem by having another spacecraft, the Mars Reconnaissance Orbiter, take pictures of MGS.

If no signal is heard on either day, NASA may call on the Mars Reconnaissance Orbiter (MRO) to take pictures of MGS early next week. The two spacecraft pass within about 100 kilometres of each other several times each week.

MRO's camera is easily powerful enough to determine the orientation of the spacecraft and its solar arrays. This would reveal whether the spacecraft has gone into safe mode or not.

Considering MGS is in it's 4th mission extension, it's lasted remarkably well. Hope the NASA eggheads can manage to figure out the problem and recover the orbiter.

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