Detector Configurations

An example of the various detector configurations on a four-slice MDCT scanner manufactured by General Electric (Milwaukee, Wisconsin, U.S.A.) is as follows (26): The matrix array on this particular scanner consists of sixteen 1.25-mm thick

Figure 5 The X-ray beam consists of the umbra, the constant and usable portion of the beam, and the penumbra, the tapering and unusable portion of the beam. As the width of the penumbra is relatively the same, it has less of an impact with wide compared to narrow collimation as well as with 8 or 16 slice compared to 4-slice MDCT scanners.

Figure 5 The X-ray beam consists of the umbra, the constant and usable portion of the beam, and the penumbra, the tapering and unusable portion of the beam. As the width of the penumbra is relatively the same, it has less of an impact with wide compared to narrow collimation as well as with 8 or 16 slice compared to 4-slice MDCT scanners.

Figure 6 Depiction of the 4 x 1.25 mm detector configuration. It is noted that the outer 12 detectors do not contribute to image reconstruction. With this configuration, 1.25, 2.50, 3.75, and 5.00-mm thick slices can be reconstructed. This configuration is associated with the slowest table speeds, the fewest artifacts, and the highest quality dataset. Owing to limited anatomic coverage, it is mainly used for single organ CT angiography.

Figure 6 Depiction of the 4 x 1.25 mm detector configuration. It is noted that the outer 12 detectors do not contribute to image reconstruction. With this configuration, 1.25, 2.50, 3.75, and 5.00-mm thick slices can be reconstructed. This configuration is associated with the slowest table speeds, the fewest artifacts, and the highest quality dataset. Owing to limited anatomic coverage, it is mainly used for single organ CT angiography.

detectors oriented in the z-axis and having a maximum footprint of 20 mm. With the 4 x 1.25 mm detector configuration, the four central detectors are irradiated and slice thicknesses of 1.25, 2.50, 3.75, or 5.00 mm can be reconstructed (Fig. 6). The maximum table speed using a pitch of 1.5 with this configuration is 7.5 mm per gantry rotation (up to 15 mm per second using a 0.5-second gantry rotation time). As a result, even though this yields the best spatial resolution in the z-axis, slower table speeds limit anatomic coverage. In the abdomen, for example, this is an acceptable tradeoff for certain organ-specific applications such as in the liver, kidneys, or pancreas.

With the 4 x 2.50 mm detector configuration, the central eight detectors are irradiated and slice thicknesses of 2.50, 3.75, and 5.00mm can be reconstructed (Fig. 7). It is noted that a slice thickness of 1.25 mm is not available with this configuration. The maximum table speed using a pitch of 1.5 is 15 mm per gantry rotation (30 mm per

Figure 7 Depiction of the 4 x 2.50 mm detector configuration. It is noted that the outer eight detectors do not contribute to image reconstruction. With this configuration, 2.50-, 3.75-, and 5.00-mm thick slices can be reconstructed. A slice thickness of 1.25 mm is not available. Due to an excellent combination of slice thickness and table speed, this configuration is widely used for CT angiography.

Figure 7 Depiction of the 4 x 2.50 mm detector configuration. It is noted that the outer eight detectors do not contribute to image reconstruction. With this configuration, 2.50-, 3.75-, and 5.00-mm thick slices can be reconstructed. A slice thickness of 1.25 mm is not available. Due to an excellent combination of slice thickness and table speed, this configuration is widely used for CT angiography.

Figure 8 Depiction of the 4 x 3.75mm detector configuration. It is noted that the outer four detectors do not contribute to image reconstruction. With this configuration, 3.75 and 5.00-mm thick slices can be reconstructed. Slice thicknesses of 1.25 and 2.50mm are not available. This configuration is widely used in pediatric patients. It is not a good choice for CT angiography.

Figure 8 Depiction of the 4 x 3.75mm detector configuration. It is noted that the outer four detectors do not contribute to image reconstruction. With this configuration, 3.75 and 5.00-mm thick slices can be reconstructed. Slice thicknesses of 1.25 and 2.50mm are not available. This configuration is widely used in pediatric patients. It is not a good choice for CT angiography.

second using a gantry rotation time of 0.5 seconds). As a result, this configuration and pitch is an excellent compromise between slice thickness in the z-axis and anatomic coverage, and for example, is widely used for routine imaging of the abdomen and pelvis (single breath-hold in 12 to 20 seconds) as well as CTA. With CTA, the images are reconstructed with a thickness of 2.50 mm at a 1.00 mm interval (60% overlap) (34,35).

With the 4 x 3.75mm detector configuration, the central 12 detectors are irradiated, yielding slice thicknesses of 3.75, 5.00, and 7.50 mm (Fig. 8). It is to be noted that at the higher pitch of 1.5 the minimum slice thickness is 5.00 mm because of the slice broadening in the z-axis and that slice thicknesses of 1.25 and 2.50 mm are not available, regardless of the pitch. This configuration is primarily used for pediatric imaging since a slice thickness of 3.75 mm is popular in these smaller individuals. Furthermore, it is used with a higher pitch (e.g., 1.5) when faster table speeds are required during the fleeting hepatic arterial phase of liver imaging (22.5 mm per rotation or up to 45 mm per second using a 0.5 second gantry rotation time). Although there tends to be more streak artifacts at the faster table speeds, they are acceptable at 22.5 mm per rotation.

With the 4 x 5.00 mm detector configuration, all 16 detectors are irradiated yielding slice thicknesses of 5.00 and 7.50 mm (Fig. 9). It is to be noted that a slice thickness of 1.25, 2.50, or 3.75 mm is not available at this configuration. This particular configuration is used infrequently since at the lower pitch radiation doses are inordinately high (compared to the 4 x 2.50 detector configuration at a pitch of 1.5) and at the higher pitch, images are significantly degraded by streak artifacts (because of aliasing).

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