PATIENT DOSES FROM STANDARD AND SPIRAL CT OF THE HEAD USING A FAST TWIN-BEAM SYSTEM

Investigations were carried our on a novel type of CT scanner, the Els cint CT-Twin, for comparison and optimization of the patient dose caus ed by standard and spiral CT of the head. For selected CT parameters, organ doses of the Alderson head phantom were measured with thermolumi nescent dosemeters. Organ doses were also calculated using the normali zed computed tomography dose index (CTDIn) combined with organ dose co nversion factors. Then effective doses were deduced. For standard and spiral head CT examinations brain, red bone marrow and bone surface re ceive the main contributions to effective dose. This amounts to 0.9 an d 0.8 mSv for routine standard and spiral CT, respectively, if the com bination ''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 det ermination of low and high contrast resolution. The effective dose val ues 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 s lice widths between 0.5 and 1 mm as well as packing factors greater th an 1.0 should be restricted to really necessary cases.