Summary
An image superposing several images obtained by scanning in thin
slice thickness can reduce some artifacts such as Hounce field dark space
in the posterior cranial fossa. The method, however, has some disadvantages
such as a reduction in time resolution due to increased scanning time,
inefficient throughput, and unneccesary exposure in the patient.
We examined the table movement speed and reconstruction pitch. In order to measure low contrast resolution and SSPz (slice sensitivity profiles in the z direction), suitable scanning conditions were determined by approximate equations so that the single noise ratio would be equal between the image in 5mm slice thickness using conventional CT (conventional image) and the image made from five images in 1mm slice thickness by helical CT (additional image).
Low contrast resolution in the additional image was the same as that in the conventional image. SSPz showed that a 2:1 helical pitch equalized the time resolution and thr throughput in both images. We confirmed that the additional image was usuful for the posterior fossa, the superiorclavicular region, and the lumbar vertebrae.
Key words:Helical CT, additional image, Hounce field dark space,
low contrast resolution, SSPz
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