Newsletters - February 2010

CT Scanning and Radiation Dose

By W. Dennis Foley, MD, FACR, Chief of Digital Imaging, Professor of Radiology and Medicine

It is estimated that computed tomography (CT) scanning accounts for 17 percent of hospital departmental work load, but 70 percent to 75 percent of the collective dose from medical radiation. These figures mirror the introduction of the first single detector helical CT machine in the late 1980’s and multi-detector array CT scanning in the late 1990’s. This summary will attempt to provide basic information on radiation dose and biological risk and guidelines for the clinically responsible and effective use of CT scanning.

The average background radiation dose is 3mSv.This is variable, dependent upon altitude and background radon and is obviously higher in Denver, Colo. than in Milwaukee, Wis. International airline pilots also receive a higher annual radiation dose up to 10 mSv per year.

The lifetime risk of lethal cancer in the North American population is 25 percent. It has been estimated that a dose of 10 mSv (a usual dose from abdominal CT) increases this average population risk by 0.05 percent. This determination is based on data from Japanese atomic bomb survivors (approximately 75,000 individuals) who at the low end were estimated to have received 100 mSv. Of the 5,936 total cancer deaths reported in that population, 5,592 were expected, based on the incidence of spontaneous cancer in the Japanese population. Fifty-five percent of the excess cancers occurred in the first 25 years and 45 percent in the next 11 years. Unfortunately, there are no reliable data for individuals exposed to radiation in the range of 0-100 mSv. In addition, the Japanese population received an acute dose of radiation as distinct from repeated small doses of radiation given to patients. Although estimates of radiation bio-effects assume a “linear no threshold effect” for doses between 0 and 100 mSv, this supposition remains unproven.

The effect of radiation is greater in more rapidly dividing cells, such as bone marrow, thyroid and breast. The risk is greater in pediatric and young adult patients and declines progressively as the target population ages. Given the expected latency period, a minimal to no risk is expected for patients older than age 60. Nevertheless, the principle employed in CT scanning is to utilize the ALARA principle (i.e., as low as reasonably achievable).

In the last 20 years, a number of technical improvements have lowered radiation dose in CT scanning. Chief among them is an “automatic exposure control” that modulates dose during longitudinal acquisition by varying the tube current according to the thickness and absorption of the directly irradiated region of anatomy. Studies in which there is intrinsic high contrast, such as evaluation for renal/ureteral calculi, are deliberately under-dosed as reflected in the noisy appearance of the images.

The relatively high dose of cardiac CT (10-15 mSv) as initially implemented using retrospective gating and intended wide application in the community caused controversy. Cardiac CT is now performed with prospective gating resulting in a dose reduction of 80 percent and an average patient radiation dose of 2-4 mSv. This compares to a cardiac nuclear medicine stress test with a dose approximating 7.5 mSv.

CT vendors have been designing and developing new forms of image reconstruction which tolerate significantly higher levels of noise in the projectional data, but produce identical image quality to the standard technique of filtered back projection which has been employed for the last 40 years. This new technique is called “adaptive statistical iterative reconstruction” (ASIR). It was tested on a General Electric prototype CT scanner evaluated in our institution in 2008. A clinical unit was delivered to Froedtert & The Medical College of Wisconsin in mid-December 2009. This unit produces equivalent image quality at half the radiation dose to the standard 64 MDCT scanner. Within the next several years, this initial form of iterative reconstruction will most likely be superseded by a new technique labeled MBIR (model based iterative reconstruction) which will likely reduce the radiation dose by another 50 percent.

Importantly, when considering CT imaging, dose reduction should be balanced against overutilization, particularly in the young to middle aged adult. In addition, alternative imaging techniques, such as sonography and MRI can be considered if the image information is likely to be comparable to CT.

In our patient population, imaging of suspected inflammatory bowel disease at first presentation is usually CT scanning and CT scanning is most commonly used for suspected complications, including abscess and fistula. In monitoring of patients receiving antiinflammatory agents, MR enterography may become the technique of choice, particularly if a disease flare rather than abscess is suspected.

Urolithiasis is best evaluated with a low dose CT scan, which should be the initial imaging study. However, there should be a good clinical indication for repeat studies.

In younger patients with a normal chest X-ray, V/Q scanning for suspected pulmonary embolism has a better negative predictive value than CT and a lower radiation dose.

CT scanning in pregnancy should be used in limited circumstances, including evaluation of blunt abdominal trauma and appendicitis. The radiation dose delivered to a fetus during a single abdomen pelvis CT scan is not associated with any known deleterious biological effect.

Radiation dose in CT scanning is an important issue, but one which has been relatively over-dramatized in the public media. As relayed by Douglas Evans, MD, a former working principle at MD Anderson suggested applying the rule of 10: “If a patient was going to live 10 years and may receive more than 10 CT scans, consideration would be given to MRI.” This could be understood in the context of a 10 mSv dose per CT scan resulting in 100 mSv total accumulated dose, equivalent to the lowest levels received by the Japanese atomic bomb survivors, albeit delivered in small incremental doses rather than a single acute dose. Given the improvements in scanner performance and in particular, image reconstruction techniques (ASIR), one could anticipate an average radiation dose of 5 mSv for abdomen CT scanning and 1-2 mSv for cardiac CT scanning. Within the next several years, MBIR may drop these figures by another 50 percent.

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