All of these different routes of exposure to background radiation add up to 3.2 mSv/yr for the average US citizen. This has not changed, but what changed dra­matically between the NCRP reports of 1987 and 2006 was a huge increase in dose from diagnostic medical tests. These procedures do not include radiother­apy to treat cancer. On average, this amounts to 3.0 mSv/yr for the average US citizen. What are all of these procedures, and how much radiation do we get from them?

There are four general types of medical diagnostic procedures that contribute to the medical exposure: radiographs (X-rays), fluoroscopy, computed tomog­raphy (CT scans), and nuclear medicine. Other common diagnostic procedures such as ultrasound and magnetic resonance imaging (MRI) do not involve ioniz­ing radiation and do not contribute to dose. Radiography includes such things as dental X-rays, skeletal X-rays, and mammograms. Doses from these procedures are generally fairly small—a chest X-ray is only about 0.1 mSv, but a lumbar spine radiograph can be 2-3 mSv (Table 8.1).

Doses for X-rays have not always been so small. Recent experiments done with the original Crookes tubes used to generate X-rays by Heinrich Joseph Hoffmans2 in 1896 demonstrated that the doses were dramatically higher than modern tech­nology. In the modern re-creation of the experiments, the skin dose to image bones in a hand was 74 mGy—about 1,500 times that for a modern machine—and exposure time was about 90 minutes compared to 20 msec now (9). That explains why many early radiologists lost fingers to cancer and developed leukemia; there was little shielding, and radiologists often determined dose by reddening of the skin—known as skin erythema (10). The minimum dose to cause skin erythema was about 200 rads or 2 Gy (11).

Table 8.1 Doses from Common Medical Diagnostic
Procedures

Diagnostic Procedure Dose (mSv)

Chest x-ray (1 film) 0.1

Dental oral exam 1.6

Mammogram 2.5

Lumbosacral spine 3.2

PET 3.7

Bone (Tc-99m) 4.4

Cardiac (Tc-99m) 10

Cranial CT (MSAD) 50

Barium contrast GI fluoroscopy (2 min) 85

Spiral CT-full body 30-100

source: Data from DOE Ionizing Radiation Dose Ranges Chart, 2005.

Fluoroscopy is a real-time imaging medical procedure in which X-rays pass through a body and hit a fluorescent screen, where they are converted to a live image. Active processes such as a beating heart can be observed with this pro­cedure. Frequently, an X-ray absorbing material of high atomic number (Z) is injected into the bloodstream for a coronary angiograph or into the gastrointesti­nal tract to observe blockage in the GI system, for example. These procedures can result in high doses because the X-ray exposure rate is high (an average of 50 mGy/ min) and they may continue for several minutes. A two-minute barium contrast GI scan gives a dose of about 85 mSv (Table 8.1). Conventional radiography and fluoroscopy constitute about 11% of medical radiation diagnostic procedures (1).

CT scans have become the most common medical diagnostic procedure involv­ing exposure to ionizing radiation, with more than 60 million scans done annually in the United States (12), accounting for 49% of all exams (1). CT scans are X-ray procedures in which the body is exposed in slices through a full 360-degree range and the X-rays are measured by a large array of detectors. Sequences of slices provide high resolution 3-D imaging of parts of the body and are an invaluable tool for diagnostic medicine. However, in some cases they are being promoted for screening purposes where there is no indication of disease or medical prob­lems. The doses from CT scans are quite high, with typical doses of 10 to 20 mSv and as high as 80 mSv for a CT coronary angiography (12). Children are being exposed to CT scans at increasing frequencies, but the doses are not necessar­ily being adjusted to account for the smaller size and greater sensitivity of chil­dren. While CT scans are a very valuable diagnostic tool, they should not be done without a good medical reason, since the doses are high enough to entail a slight risk of getting cancer later on. This is not so much of a problem with older people, since their risk of getting cancer from a given dose of radiation is much less than it is for children (13), but it is important that the dose and number of procedures be minimized for pediatric scans and for young people to reduce the long-term cancer risk from the procedure. Increasingly, CT scans are being used for screening for colon polyps (virtual colonoscopy), early-stage lung cancer, car­diac disease, and full-body scans for a variety of diseases (12). Whether the risk from screening CT is greater than the benefit is not yet clear.

Nuclear medicine is a less well-known diagnostic (and therapeutic) procedure. It involves injecting radioisotopes to identify tumors or other physiological con­ditions. A variety of radiopharmaceuticals are used that localize in certain parts of the body. By far the most common radioisotope is technicium 99 metastable (99mTc), which has a 6-hour half-life, but many others are also used. A bone scan gives a dose of about 4.4 mSv, while a cardiac scan gives a dose of about 10 mSv (Table 8.1).

These medical diagnostic procedures have dramatically improved the practice of medicine, so the risk to benefit ratio is quite high. However, the doses are quite large in many cases, so the procedures should not be done frivolously. Together, these procedures account for nearly half of the annual exposure of the average US citizen, but of course there is a lot of variation. For many people there is no expo­sure at all, while others may have a large exposure.