Single-detector helical CT uses an X-ray tube coupled with a single-detector array (Fig. 1). The X-ray beam is collimated prior to reaching the patient using a multistep prepatient collimator. Scan parameters for a single-detector helical CT scanner are determined by beam energy and dose (in kilovolts and milliamperes), collimation, table speed, and gantry rotation speed. The amount of collimation is determined prospectively by the operator and determines the effective image slice thickness. Due to scatter and continuous table motion during helical scanning, actual slice thickness can vary slightly depending on
Figure 1 Single-detector helical CT. An X-ray beam is collimated by a prepatient collimator prior to entering a single-detector array. Slice thickness is mainly determined by collimator setting.
table speed and pitch. Pitch is defined as table feed per 360° gantry rotation divided by image slice thickness. Higher pitch results in faster scan time, but also can result in image reconstruction artifacts. Normally, pitch is kept between 1 and 2 for most studies. Gantry rotation speed varies between 0.7 and 1 sec/360° rotation.
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