RADIATION-DOSE TO PATIENTS FROM EMI BRAIN AND BODY SCANNERS

Absorbed doses throughout the head of a Rando standard man phantom have been measured during simulated neurological examinations with the EMI Mk I brain scanner and the second generation CT 1010 brain scanner. Similar measurements have been made with the CT 5005 whole-body scanner which have also included scans through thoracic and abdominal sections of the phantom. Lithium borate thermoluminescent dosimeters placed at regular intervals throughout, and on either side of the scanned volume, have enabled estimates to be made of the total energy imparted to the phantom by each scanner as well as the absorbed dose at various locations including organs of interest in health physics such as the lens of the eye, thyroid and gonads. Isodose curves have been plotted for the central transverse section of each simulated examination. Results are reported for both the standard and high accuracy scan speed options on the 2 scanners where this choice is available. It would appear that all 3 machines when operated at their standard scan speeds impart no more energy to the phantom than that associated with a few conventional radiographs of the same part of the body. Maximum skin doses vary from 30 to 56 mGy (3.0 to 5.6 rad) being highest for the CT 1010 scanner. Use of the high accuracy scan option increases the energy imparted and the maximum skin doses by a factor of about 5, while providing much enhanced tissue density discrimination. In the case of the body scanner this higher value of energy imparted becomes comparable to that associated with lengthy fluoroscopic examinations like barium enemas. Recent work by EMI involving further refinements to the collimation system has reduced the dose levels by typically 50%. Some preliminary results with this new system are included to indicate the likely degree of improvement, but a detailed account will be the subject of a separate paper.