In multi-slice cone beam CT imaging, there are artifacts known as windmill artifacts. These artifacts are due
to not satisfying the Nyquist criteria in the patient longitudinal direction. This paper quantifies and compares
these artifacts as a function of the number of rows, pitch, collimation, and image thickness of the CT scanner.
Scanners with rows of 16, 64 and 128 are measured and compared with simulated data, using both Helical and
Axial scanning modes. In addition three focal spot switching modes are compared: the traditional within image
plane mode; diagonal mode; and quad mode. All images are compared via four criteria: artifacts, MTF, SSP
Results show that the frequency of the artifact, or number of blades on the windmill and magnitude of each
blade, is dependent on the rate at which the rows are crossed for an image. For example, for a given pitch,
doubling the rows doubles the frequency of the artifact, with each artifact approximately the magnitude. A
similar result can be obtained by keeping the number of rows constant and varying the pitch. The artifact
disappears as the Nyquist criteria is satisfied by either increasing the slice thickness or incorporating one of the
focal spot switching modes that switch in the patient longitudinal direction. For a given MTF and SSP, the
diagonal focal spot switching mode has slightly more noise while the other two are approximately equal. The
artifact varies with the quad mode being the best and traditional mode being the worse.
A complete 32 slice CT detector system has been constructed which uses back illuminated photodiodes (BIPs). Individual detector modules in the system incorporate the BIPs along with highly integrated A/D conversion electronics on the same substrate. A symmetrical mechanical structure allows the system to be compact and lightweight for use at high rotational speeds. The unique design also has the advantage of having no internal cables. The current BIP exhibits a higher level of crosstalk between photosensitive elements when compared to a conventional photodiode. Differences in the crosstalk level at detector module boundaries can cause artifacts unless the crosstalk can either be reduced or a software correction made. In order to show that the BIP is a viable technology for use in multislice CT, a performance evaluation of the complete BIP system along with its associated mechanical, electrical and software components is required. The 32 slice detector system has been mounted to a rotating CT scanner for image performance evaluations. Measurements of low contrast sensitivity, MTF, limiting resolution and other parameters have been done. A crosstalk correction algorithm has also been developed and evaluated under different conditions. Low contrast sensitivity, MTF and limiting resolution of the system match those of a current conventional CT scanner of similar geometry. The crosstalk correction effectively eliminates artifacts caused by non-uniform crosstalk at module boundaries. MTF and noise properties before and after crosstalk correction match theoretical values.