Radiation doses to patients during computed tomographic (C.T.) examinations of the head

Abstract

Since the discovery of x-rays in 1895, and more recently the use of computerized Tomography (CT) there has been a growing interest in the assessment of the radiation energy absorbed by the patients during CT and other diagnostic x-ray procedures. Consequently, there has been a development of radiation detectors for patient dose measurements. Among such devices is the (LiF) Lithium Fluoride, Thermoluminescent Dose Meter (TLD), which is placed on the skin surface of the patient being monitored. Some of the desirable properties of TLDs are high sensitivity and near energy -independence.

Tests have been conducted to assess the suitability of TLDs during -CT procedures, with positive results. Between September, 1993 and February 1994, a total of 75 patients who underwent CT-examinations of the head, at Kenyatta National Hospital were studied. While they were scanned their radiation doses were determined. The anatomical landmarks were chosen to correspond to the organs under dose assessment. Thermoluminescent. dosimeters (LiF) were placed on the site of interest before commencement of the CT procedure. After the procedure was completed, the TLDs were taken for readout using the TOLEDO 654-TLD Reader. The organs monitored were the frontal bone, eyes (lens), parotid, submandibular and thyroid glands. other peripherally located organs were also surveyed, but only for a few patients, in order to establish how much stray radiation reached them. These organs were sternum, ovaries and testes. Of the 75 patients, (25.3%) were referred for trauma while (74.7%) were non-trauma patients. Adults represented 82.7% of the patients while only 17.3% were children. Among patients attended, 64.0% were male and 36.0% were female. The number of slices per CT-examination of the head ranged from 13 to 52. The mean doses to the thyroid among adult patients recorded to be O.28mGy while the testes and ovaries among adults were O.06mGy and 0.07mGy respectively. The difference of the radiation doses among undergoing CT-examination of the head were significant at P<0.05. adult patients statistically The current study shows that by scanning the head through the orbit, the mean absorbed dose to the eyes is 1.48mGy and 1.19mGy for scanning to avoid the orbits; four times less than those reported in the studies done elsewhere. Infact scanning to avoid the orbits was 5.58mGy. Scanning through the orbits in the same study was reported to be 43.44mGy higher than figures obtained in this study. This study also reveals that post contrast mean doses recorded are higher than pre-contrast doses. It agrees with the figures reported in studies conducted under reference above. An attempt to address the issue of gantry tilt versus absorbed doses has been made, as angling the gantry cranially can contribute to reduction in radiation dose to the lens of the orbit. Among radiation detector devices, TLDs are highly recommended during cT-scanning of the head because of their overall sensitivity. They do not produce artifacts on radiographic image. This study reveals that the absorbed doses to the eyes, frontal bone and parotid glands are higher than the rest organs around the Head. During CT-scan of the Head, this series shows that organs around the head receive low mean absorbed dose when the gantry is angled cranially to avoid the eyes. Thus setting on angle the gantry cranially to avoid the orbits during Head scanning can contribute to the reduction of the radiation dose. Further studies of patient radiation dose during CT-examination not covered in this work are suggested. A comparative study of patient radiation doses between different CT-models available in Kenya ought to be undertaken in future