Formula for Determining the Detector Configuration

Knowledge of what detector configuration is being used for a specific application is helpful, not only for understanding the table and the acquisition speed but also for determining what slice thicknesses are available for retrospective reconstruction. To calculate the detector configuration, one must determine two parameters, (i) the pitch and (ii) the table speed. Then, by simply dividing the table speed by the pitch (preferably the beam pitch), the collimation width can be determined. For example,

Beam Pitch = 0.75

Figure 10 Beam pitch: example of a beam pitch of 0.75. It is noted that there is a 25% overlap (1 slice in a 4-slice scanner, 2 slices in an 8-slice scanner, and 4 slices in a 16-slice scanner) following two gantry rotations. This overlap improves the quality of the dataset but increased the radiation dose.

Figure 10 Beam pitch: example of a beam pitch of 0.75. It is noted that there is a 25% overlap (1 slice in a 4-slice scanner, 2 slices in an 8-slice scanner, and 4 slices in a 16-slice scanner) following two gantry rotations. This overlap improves the quality of the dataset but increased the radiation dose.

using a pitch of 1.5 and a table speed of 15 mm per gantry rotation, the collimation width is 15/1.5 = 10 mm. The detector configuration using this example would be 4 x 2.50, 8 x 1.25, and 16 x 0.625mm on 4-slice, 8-slice, and 16-slice scanners, respectively. It is possible to reconstruct slices thicker but not thinner than acquired (minimum thickness of 2.50, 1.25, and 0.625 mm, respectively, in the above example).

Beam Pitch = 1.50

Figure 11 Beam pitch: example of a beam pitch of 1.50. It is noted that there is a 50% separation rather than an overlap following two gantry rotations. This lack of overlap diminishes the quality of the dataset but decreased the radiation dose.

Figure 12 Coronal reformations of the upper abdomen. (A) There is a well-circumscribed low attenuation mass in the lower pole of the right kidney suggesting a simple cyst (arrow). It is noted that the image is quite noisy since it has been reconstructed with a thickness of 1 mm in the z-axis (antero-posterior direction). (B) When the same coronal image is reconstructed with a thickness of 5 mm in the z-axis, there is a significant decrease in noise. The trade-off, however, is an increase in partial volume average, which can make lesion characterization difficult. This is especially problematic for small lesions.

Figure 12 Coronal reformations of the upper abdomen. (A) There is a well-circumscribed low attenuation mass in the lower pole of the right kidney suggesting a simple cyst (arrow). It is noted that the image is quite noisy since it has been reconstructed with a thickness of 1 mm in the z-axis (antero-posterior direction). (B) When the same coronal image is reconstructed with a thickness of 5 mm in the z-axis, there is a significant decrease in noise. The trade-off, however, is an increase in partial volume average, which can make lesion characterization difficult. This is especially problematic for small lesions.

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