EBT Information for Healthcare Providers

Radiation Dosage of EBT Procedures

Radiologists and scientists alike agree that radiation in large doses causes cancer; the current controversy centers on what dose should be considered "acceptable" before cancer risk begins to increase.

As a general rule, the electron beam CT scanner will deliver about 20% of the radiation to the patient that a conventional CT scanner would. The primary explanation for this is that the Electron Beam CT scanner is essentially a fast shuttered camera only turning on for brief periods of 50 to 100 msec as needed to acquire the images. Conventional CT Scanners have an X-ray emitter on one side of the patient and the detector on the opposite side. In this configuration, the X-ray emitter is always on during the acquisition of the image data.

When it comes to using radiation for screening or diagnostic purposes, the key is to decide whether its use is justified, then optimize it.

Most experts ascribe to the currently accepted linear "no-threshold" theory for radiation carcinogenesis risk, which holds that any radiation dose, no matter how small, can cause cancer, meaning that the risk is never zero. Recently this theory has come into question, with some experts suggesting that the risk of developing cancer after low levels of radiation exposure are overstated. These experts believe that radiation induced cancer risk only increases after the total radiation exposure passes a certain threshold level and that exposure to amounts of radiation below this threshold dosage do not significantly increase the risk of developing cancer. No risk of adverse health conditions has been established for exposures of 5,000 milli-rem (mrem) or less. The Health Physics Society recommends against quantitative estimation of health risk for individual exposures of 5,000 mrem in one year or 10,000 mrem lifetime. The threshold dose of radiation above which the risk of developing cancer begins to increase is proposed to be about 10,000 mrem.

CT scanning is a relatively high dose procedure that is becoming much more common worldwide. In the mid-1990s, CT scanning accounted for only 4% of the total X-ray procedures done but 40% of the collective dose from all diagnostic X-ray. The introduction of helical, fluoroscopic, and multi-slice CT technology has increased the usage of CT while doing nothing to diminish the radiation dosage. In US hospitals today, CT scanning accounts for about 15% of imaging procedures and 75% of radiation exposure. When multiple CT scans are performed on the same patient, the cumulative absorbed radiation doses rise to the range at which small but statistically significant increases in cancer have been found in the atomic bomb survivors.

Because CT procedures involve far higher radiation exposures than those received in conventional x-ray exams, there is growing worry that such exposure could contribute to the development of a radiation-induced cancer later in life. The FDA is currently investigating this situation. Effective radiation doses from conventional diagnostic CT procedures range from 100 to 4,000 millirem, not much less than the lowest doses of 500 to 2000 mrem received by Japanese survivors of atomic bombs, who were shown to have a small but increased relative risk for cancer mortality due to radiation exposure. A typical conventional CT scan of the abdomen delivers up to 1000 mrem of radiation, equivalent to 500 chest x-rays. Marconi (Marconi Medical Systems, Inc. 595 Miner Road, Cleveland, OH 44143) reportedly has calculated the typical patient dose received when performing their non-FDA approved cardiac calcium scoring protocol when performed on their single slice spiral CT scanner using the Win Dose program. In their cardiac calcium scoring protocol (130 kV, 200 mA, Spiral, 1.25 pitch, 77 images) the effective patient radiation dose delivered to the patient was 4,300 mrem, equivalent to over 200 chest x-rays. It would take more than three years to absorb this amount of radiation from the natural background radiation people receive from the sun and the soil.

In contrast, the Electron Beam CT (EBT) Scanner delivers only about 40-70 mrem of radiation to the patient when acquiring images for the coronary calcium scoring exam. A low risk location's background radiation exposure is about 340 mrem/year. Background radiation varies substantially from location to location depending on elevation, soil and latitude. For example, a resident of Denver, CO experiences about double the dose of background radiation due to the higher altitude than someone living at sea level.

Estimates of radiation exposure are given on the following pages in rem (radiation equivalent man) which is based on the total amount of X-ray expected to be absorbed by the patient during an average study. (100,000 millirem = 1 milliSievert)

The estimation of radiation dosage is an inexact science, so these numbers listed should be considered as best approximations.

References: Radiation Dose in Computed Tomography of the Heart: Morin RL, Gerber TC, MD & McCollough CH; Circulation 2003;107:917-922.

** All calculations based upon 3mm contiguous slice thicknesses encompassing the entire heart HCT Exposure: 135 Kv, 300mAs Measurements from CDTI phantoms scanned at University of Iowa: MultiDetector (MD) CT: Measurements used non-helical scan modes but scanning 4 detector rings/exposure. State of art scanners without modifications: MDCT radiation exposure were about 2.3x SD CT radiation exposures which were about 1.8x EBCT exposures. SDCT and MDCT were Toshiba Aquilion 500ms scanners.