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Modern diagnostic imaging provides critical anatomic confirmation of a vast array of disease processes and can also elucidate important functional information for many of the body’s internal organs. In mere seconds, a multi­detector CT scan can obtain and generate images that diagnose acute appendicitis, pulmonary embolism, intracranial bleeding, cervical spine fractures, and more. Unfortunately, these advantages come at a cost.

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In addition to the financial expense of CT screening, there are health-care costs associated with the long-term side effects of ionizing radiation as well as with the surveillance of incidentally discovered findings that are unrelated to the clinical indication for the imaging examination performed, also known as “incidentalomas.” Observational studies have shown a significant increase in the utilization of CT over the past decades, but few of these have demonstrated an increase in diagnostic yield or prevention of disease complications.

Each year, approximately 10% of U.S. citizens undergo a CT scan for detection or surveillance of disease, and this population has been increasing by approximately 10% per year.1 From 2001 to 2006, the overall use of diagnostic CT scans increased by 200%, with the highest relative increase for chest CT and neck CT at 600% and 500%, respectively.1

In February 2010, the FDA instituted an initiative to reduce the unnecessary use of the three types of tests that expose patients to the highest radiation doses—CT scanning, nuclear medicine studies, and fluoroscopy.1 Of these, CT scans are by far the most frequently ordered. CT is also the most likely to uncover incidentalomas, at a rate of up to 25% to 35%.

Experts who have reviewed these data feel that at least 30% of these imaging tests were unnecessary.1 Although plain x-rays are probably ordered too often as well, they expose patients to much less radiation and were therefore not included in the FDA initiative.

Strategies to minimize unnecessary radiation from diagnostic testing include observation without imaging, clinical decision-making instruments, alternate testing (e.g., electrocardiogram or D-dimer), and imaging that avoids or minimizes radiation exposure (e.g, plain films, ultrasound, or MRI).

Risks and benefits of diagnostic imaging

The primary benefit of CT scanning is the more expeditious and accurate diagnosis of important disease processes or the ruling out of such conditions. Although no test is perfect, and the savvy clinician should always be alert for false-negative test results, CT results generally aid the clinician and can put the patient’s mind at ease.

Nevertheless, exhaustive testing is not always in the best long-term health interest of the patient. Documented risks of radiation include fetal malformation, detrimental effects on brain development of children, increased risk of future cancer, and incidental findings that often require further downstream testing and may cause significant emotional distress (i.e., incidentalomas).2

The effects of diagnostic imaging on fetal development and malformation are beyond the scope of this article but are covered by guidelines from the American College of Gynecology and Obstetrics.3 Most of the data on intellectual development come from a Swedish study that followed the academic performance of teens who had previously received radiation treatment for facial hemangiomas.4

This study showed that the higher the dose of radiation received in childhood, the lower the child’s chance of passing a standardized school-entry examination. Causation could not be proven definitively because there was no randomization, but the strong dose-response correlation is highly suggestive of a causal role of childhood radiation exposure in negatively impacting intellectual development.

The increased risk of fatal malignancy later in life in patients exposed to ionizing radiation is most strongly supported in cases in which the degree of radiation exposure is high, such as among survivors of such nuclear disasters as the one at Chernobyl and of the atomic bombings of Hiroshima and Nagasaki. Some of these survivors experienced the disasters from a distance at which the estimated radiation exposure was extremely high, but even in survivors who were at a distance at which the radiation dose would have been similar to that of modern CT scans, a higher rate of future malignancy was noted when compared with nonexposed persons.

Other populations that have been studied and noted to have an increased rate of cancer have been radium-watch painters and patients who underwent medical radiation therapy. The magnitude of the risk of fatal cancer has been extrapolated to lower exposure levels and is thought to be as high as 1 in 1,000 for most torso CT scans. Latency is often 10 to 20 years or more but can be shorter for leukemia, thyroid cancer, and breast cancer.5,6

The degree of risk will depend on the body area imaged, the radiation dose, and the age and gender of the patient. Imaging of the head, neck, and especially the torso carries a higher risk than imaging of the extremities.7 Younger patients are at much higher risk due to more rapidly dividing cells and a longer life expectancy over which they may manifest disease. Risk can be up to 10 times higher in childhood than it is in late adulthood.7 Risk related to chest CT is significantly higher in women than in men because of the risk of breast cancer.

Finally, and most important, certain types of imaging use higher doses of radiation than do others (Table 1). Ultrasound and MRI are radiation-free. Standard x-rays use much lower doses of radiation than do nuclear medicine studies, which tend to use lower doses than do CT scans. It is estimated that the average person living in the United States is exposed to 3.0 millisieverts of natural background radiation per year.

TABLE 1. Comparative radiation doses fromdiagnostic imaging

Diagnostic study/strong> Average dose in millisieverts Equivalent dose in chest x-rays
Knee, ankle, elbow, wrist x-ray 0.02 1
Posteroanterior chest x-ray 0.02 1
Lateral chest x-ray 0.04 2
Skull x-ray 0.07 3.5
Mammogram 0.6 30
Kidneys, ureter, bladder x-ray 0.6 30
Pelvis/hip x-ray 1.2 60
CT – head 2.0 100
Lumbosacral spine series 2.0 100
Ventilation/perfusion scan 2.0 100
Intravenous pyelogram 3.0 150
Yearly background exposure 3.0 100
Hepatobiliary scan 3.7 185
Bone scan 4.4 220
Positron emission tomography scan 5-14 250-700
Technetium sestamibi scan 6-12 300-600
Cardiac catheterization 5-50 250-2,500
CT-chest scan 8-16 400-800
Barium enema 8 400
CT – coronary 10 500
CT – abdomen and pelvis 15-20 750-1,000
CT – urogram 20 1,000
Thallium scan 12-24 600-1,200
Interventional radiology procedures 25 1,250
Gallium scan 40 2,000
*Actual doses may vary (by more than tenfold for CT); higher resolution requires higher dosing.

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