Physics Corner was a series of articles I wrote for an incarnation of the department newsletter between 2000 - 2001. They're collected here for posterity and maybe to provoke me into starting it up again as an online column.
A common attitude in diagnostic radiology is that most procedures are relatively safe and carry very little risk of causing further harm to patients. For the most part, this is true. However, some may be surprised to learn that in recent history, there have been several documented cases of radiation induced skin injury to patients resulting from prolonged fluoroscopic procedures. In fact, an advisory was issued by the FDA in 1994 warning of the potential for radiation injury resulting from prolonged fluoroscopic procedures such as PCTA, stent placements, ERCP, cath lab procedures and RF ablations. The text of the advisory is available online at http://www.fda.gov/cdrh/fluor.html for those interested in reading it.
The first effect of excessive radiation exposure to the skin is erythema (skin reddening). Erythema, which has a threshold of 200-600 rad (2-6 Gy), looks just like a sunburn and often has the same shape as the x-ray beam (circular or square). For an average sized patient, the skin entrance exposure rate to the patient can vary between 4-7 R/min (35-60 mGy/min), depending on the projection angle, size of the patient and type of tissue in the field. At these typical exposure rates, the threshold for skin erythema can be reached with less than an hour of fluoro time. For cine runs, patient exposure is even higher, around 50-100 R/min (440-875 mGy/min), and the skin erythema threshold can be reached with a few minutes of cine.
In the majority of fluoroscopic studies performed, the radiation exposure to the patient is well below the threshold for inducing skin erythema. However, even with todayÃ¢â‚¬â„¢s technology, the potential exists for radiation induced skin damage to a patient, and it is this potential that every fluoroscopist should be aware of when performing a procedure on a patient.
The table below, taken from http://hna.ffh.vic.gov.au/phb/hprot/rsu/pubs/fluoro2.html, summarizes a few recent incidents in which patients received radiation injuries from their procedures.
|Patient Sex and|
|Procedure||Nature of Injury||Fluoroscopic Exposure Time
and Skin Dose
|Female, 53, New Zealand (1996)||coronary angiography followed by coronary angioplasty||skin lesion||101 minutes, 78 cinefluorography runs, 18 Gy|
|Female, 25, USA||RF cardiac catheter ablation||skin breakdown 3 weeks post procedure||unknown, procedure time of 325 minutes|
|Female, 62, USA||balloon dilation of bile duct anastomosis||burn-like back injury on back requiring skin graft||unknown|
|Female, 61, USA||renal angioplasty||skin necrosis requiring skin graft||unknown, procedure time of 165 minutes|
A very interesting case study can be found at http://www.fda.gov/cdrh/rsnaii.html discussing one particular incident of radiation injury from a coronary angioplasty procedure.
There are several methods that operators can use to minimize the radiation exposure to themselves and the patient:
- Keep the total fluoro time for the procedure as low as practical. Most fluoro systems in this institution come with a last image hold. Learn to us it.
- Keep the II as close to the patient as possible and the x-ray tube as far away as practical. As the II is moved away from the patient, the system increases the fluoro technique to maintain image quality. Keeping the x-ray tube far away from the patient reduces the patientÃ¢â‚¬â„¢s exposure via the inverse square law.
- Collimate, collimate, collimate.