Electron-beam CT technology (Imatron, South San Francisco, CA) operates by a different technology from helical and conventional CT. With those methods, a large detector array and x-ray unit rotate around the patient. The sheer mass of the x-ray apparatus limits the rate of rotation to about once every 0.Z5 to 1.0 s. With electron-beam CT, electrons are generated and deflected electromagnetically onto tungsten target rings located in the gantry below the patient. X-rays are generated by the electron bombardment of the target rings. The x-rays are then tightly collimated and directed to pass through the patient onto a double ring of detectors in the gantry above the patient. Since mechanical motion is not involved, imaging times are very rapid: images can be acquired every 58 ms (17 images per second). Similar to helical CT, image resolution is excellent: 0.1 to 0.5 mm2 At this time rate of image formation, very little blurring of cardiac structures takes place. Calcifications involving the coronary arteries are easily detected, and cine images of the myocardium are available.
Electron-beam CT and helical CT require injection of an iodinated contrast medium to enhance blood in the vessels and myocardial cavities relative to the myocardium itself. With helical CT, approximately 120 mL of iodinated contrast is delivered intravenously at a rate of 3 mL/s. Using electron-beam CT, 40 to 80 mL of contrast is given at 2 to 10 mL/s. In both cases, a power injector is used, necessitating large-bore intravenous access.
Radiation exposure is a consideration with all CT scanning methods. With conventional or helical CT, radiation levels are 2 to 4 cGy (2 to 4 rad) per CT slice. If multiple slices are taken at the same level of the heart, then the radiation increases directly in proportion to the number of slices. Electron-beam CT has less exposure to radiation for the patient: each slice has an exposure of 0.54 cGy (540 mrad). However, more imaging slices are acquired with electron-beam CT in obtaining a cine sequence of the heart, so that 10 cine frames would result in an exposure of 5.4 rad. For comparison purposes, a single x-ray of the lumbar spine is approximately equivalent to 1 cGy (1 rad), whereas radiation exposure from a chest x-ray is 0.06 cGy (60 mrad).
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