Differences Between Single Detector and Multidetector CT

On a single detector CT (SDCT) scanner, the detector consists of a single slab of ceramic material and the slice thickness is simply determined by the collimator (Fig. 2). In comparison, on a multidetector CT (MDCT) scanner the detector has a matrix array, which consists of a ceramic detector divided into small individual pieces separated by thin metallic septae (29,30). With these scanners, the slice thickness is determined not only by the collimator but also by the detector configuration (Fig. 3). In addition, multidetector CT scanners have X-ray tubes with a much higher heat-loading capacity and have shorter gantry rotation times (e.g.,

Figure 2 Diagram of a single detector helical CT scanner: It is noted that the detector consists of a single slab of ceramic material (e.g., total of 20 mm in the z-axis) and that the slice thickness is determined by the collimation width.

Figure 3 Diagram of a MDCT (multidetector computed tomography) scanner. It is noted that the detector consists of a row of small individual squares of ceramic material (e.g., four slice scanner consisting of 16 detectors each measuring 1.25 mm in the z-axis for a combined footprint of 20 mm) and that the slice thickness is determined both by the collimation width and the detector configuration.

Figure 3 Diagram of a MDCT (multidetector computed tomography) scanner. It is noted that the detector consists of a row of small individual squares of ceramic material (e.g., four slice scanner consisting of 16 detectors each measuring 1.25 mm in the z-axis for a combined footprint of 20 mm) and that the slice thickness is determined both by the collimation width and the detector configuration.

0.75-1.0 seconds for SDCT vs. 0.4-0.8 seconds for MDCT). These short gantry rotation times are in part facilitated by shorter tube-to-isocenter distances, which effectively reduce rotational inertia (31). An additional advantage of a shorter tube-to-isocenter distance is the fact that there is less X-ray flux thereby requiring lower mAs. For example, reducing the distance from 630 to 541 mm increases the X-ray flux by 36% [(630/541)2 = 1.36], thereby allowing for a 74% (1/1.36 = 0.74) reduction in the mA (32).

One of the interesting features of MDCT scanners is the phenomenon of focal spot wobble (32). This focal spot motion causes the X-ray beam to move back-and-forth on the detector so that the collimators must be opened to irradiate the specified detectors consistently within the matrix (Fig. 4). As a result, focal spot wobble can cause a substantial increase in radiation dose. Currently, all of the manufactures provide hardware and software solutions to this problem.

Figure 4 Focal spot wobble. Because of rotational factors the focal spot moves back-and-forth on the anode (white arrow). This causes the X-ray beam to move back-and-forth on the detector, as well. To prevent partial radiation of key detectors (black arrow) the collimators are opened, thus further increasing the radiation dose.

Figure 4 Focal spot wobble. Because of rotational factors the focal spot moves back-and-forth on the anode (white arrow). This causes the X-ray beam to move back-and-forth on the detector, as well. To prevent partial radiation of key detectors (black arrow) the collimators are opened, thus further increasing the radiation dose.

In general, radiation doses are significantly higher on MDCT scanners compared to SDCT scanners (32). First, the X-ray beam is collimated to a much wider width. For example, on a SDCT scanner the beam collimation to achieve a 5-mm thick slice X-ray width is 5 mm. On a MDCT scanner, the beam collimation for a 5-mm thick slice width can vary from 10 to 20 mm. To minimize this effect, the combination of a narrower collimation and a higher pitch is preferred over the combination of a wider collimation and a slower pitch. For example, comparing the 4 x 5.00 mm detector configuration at a pitch of 0.75 to that of a 4 x 2.50 mm detector configuration at a pitch of 1.5, one diminishes the radiation dose by about 25% without changing the table speed (15 mm per gantry rotation) or acquisition time (33). Second, is the effect of the penumbra. The X-ray beam has two components, (i) the umbra that is the central and most usable portion of the X-ray beam and (ii) the penumbra, which is the peripheral and unusable portion of the beam (Fig. 5). On the four-slice MDCT scanner, the penumbra represents a significant percentage of the beam; therefore, the collimation width must be increased to irradiate specified detectors with the umbra. Fortunately, on 8- and 16-slice scanners, the penumbra, represents a much smaller percentage of the total beam and therefore, has a lesser effect.

The thin septations in a matrix detector are in the order of 90 to 100 ^ in thickness and represent dead space since X-ray photons striking these metallic septae do not contribute to the image. This results in a higher radiation dose. Furthermore, scanners with 8- and 16-slice capability have more septations resulting in more dead space and a further increase in radiation dose. Fortunately, these scanners can partially counteract this effect by using higher pitches and as a result, faster table speeds.

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