For lateral skull radiography the minimum required radiation patient exposure to ensure adequate image quality was determined for digital luminescence radiography (DLR) in comparison with a screen-film system (speed class 200). Radiographs were produced with a grid technique on conventional X-ray equipment. A real prepared female head including a true fracture above the pars petrosa ossis temporalis was imaged. The tube current-time product (mAs), and thus the surface entrance dose, was varied systematically. Surface entrance dose was measured with TLD-100 rods. Image quality was judged by experienced radiologists according to the criteria: visual resolution, mean optical density, contrast and perceptibility of specific bone structures. Surface entrance dose was reduced from 0.46 to 0.20 mGy by application of DLR instead of speed class 200 screen-film system without loss of diagnostic information in clinical routine. This corresponds to a dose reduction potential of 57% showing a good agreement with the dose reduction potential of 52% obtained in a previous study using the Alderson head phantom.
Investigations were carried out on a novel type of CT scanner, the Elscint CT-Twin, for comparison and optimization of the patient dose caused by standard and spiral CT of the head. For selected CT parameters, organ doses of the Alderson head phantom were measured with thermoluminescent dosemeters. Organ doses were also calculated using the normalized computed tomography dose index (CTDIn) combined with organ dose conversion factors. Then effective doses were deduced. For standard and spiral head CT examinations brain, red bone marrow and bone surface receive the main contributions to effective dose. This amounts to 0.9 and 0.8 mSv for routine standard and spiral CT, respectively, if the combination "dual-slice" mode, 250 mAs per rotation, 5 mm nominal slice width and a packing factor of 1.0, is applied. In clinical practice, for spiral CT head examinations the effective dose has been reduced to 0.7 mSv while guaranteeing adequate image quality, as assessed by determination of low and high contrast resolution. The effective dose values obtained are in the lower part of the range of values published in the literature. The dose determinations showed that, from the aspect of radiation protection of the patient, CT examinations with nominal slice widths between 0.5 and 1 mm as well as packing factors greater than 1.0 should be restricted to really necessary cases.
Spiral CT angiography seems to yield enough topographical information for the accurate planning of stereotactic surgery for brain lesions. CT angiography with the helical technique is rapid and less invasive than digital subtraction angiography.
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