Spiral Versus Dynamic Incremental CT
The benefits of spiral volumetric computed tomography (SVCT) over dynamic incremental computed tomopraphy (DICT) have been well described [1–4]. They include the absolute contiguity of the reconstructed image. This overcomes the reliance on the patient reproducing an exactly identical respiratory excursion to produce sequential images. This should theoretically make it impossible to skip over small lesions. The lack of interslice delay means that all total data acquisition times are much shorter. This should lead to improved overall vascular and organ enhancement as more images will be obtained during peak levels of enhancement. This feature will be most noticeable when using a simple small bolus injection of contrast medium given by hand, as at our institution. These benefits are predicted to be available with little effect on image quality. Work on the CT investigation of pulmonary nodules is confirming the usefulness of slice contiguity [2, 3]. However, as regards image quality, there are certain inherent theoretical disadvantages attached to SVCT. In order to remain within present tube heating specifications, there are limitations on the available current, and these become more marked as the acquisition time increases. For example, when examining the liver in 1-cm thick slices on a standard algorithm using our version of the spiral Somatom Plus, the maximum allowable current for volumes up to 8 cm in length is 210 mA at 1 cm/s; for 8–12 cm this falls to 165 mA. For volumes between 12 and 24 cm in length a ‘soft tissue’ algorithm has to be used, with subsequent reconstruction using a standard algorithm.
KeywordsImage Quality Pulmonary Nodule Slice Contiguity Subsequent Reconstruction Spiral Technique
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