summarizes state-of-the-art multidetector-row CT (MDCT) hardware for the main CT scanner manufacturers. Most scanners are described as single source and employ a single x-ray tube mounted on a gantry opposite a detector array. One scanner type, the dual-source CT scanner, has 2 x-ray tube/detector systems mounted on the same gantry, 95° apart.
Summary of CT hardware and parameters for state-of-the-art CT scanners from the major CT manufacturers
Most scanners use ceramic (solid state) scintillation detectors coupled to photodiodes, which have improved spatial resolution and decreased noise compared to older xenon gas detector systems.1,2
The detectors are arranged in rows and columns; the number of active detector rows and the z-axis width of detectors in an array define the detector configuration. Detector configurations available on state-of-the-art scanners for cardiovascular imaging are illustrated in .
Schematic of the detector row configurations for current state-of-the-art multidetector row CT scanners from the major CT manufacturers.
The total nominal beam width (i.e., the total x-ray beam width per gantry rotation defined at the scanner isocenter) is determined by the detector configuration selected in a CT protocol. For example, a detector configuration of 64 × 0.5 mm corresponds to a total nominal beam width of 32 mm, and a detector configuration of 320 × 0.5 mm corresponds to a total nominal beam width of 160 mm. The total nominal beam width for state-of-the-art scanners is listed in .
The detector row width defines the minimum thickness of the reconstructed CT image and is influenced by the z-axis dimension of the individual detector element.3
For example, a detector row width of 0.6 mm allows a slice thickness of 0.6 mm or greater. Through-plane or z-resolution is dictated by the z-axis width of individual detector elements. To achieve improved through-plane spatial resolution, some systems use an x-ray focal spot that alternates between 2 z-positions (ie, z-flying focal spot) to acquire 2 overlapping slices for each detector row.4
This feature has been reported to improve z-plane spatial resolution to 0.4 mm.5
In-plane spatial resolution is determined primarily by the number of x-ray projections available for reconstruction, the scan field of view, and the image matrix. The highest in-plane spatial resolution of current MDCT scanners has been reported to be in the range of 0.23 mm to 0.4 mm.5,6
During helical scanning (described in detail below), some overscanning in the longitudinal direction (z-overscan) is required to ensure that sufficient data are available for reconstruction. Overscanning exposes organs adjacent to the desired scan range that are not of clinical interest. Dynamic or adaptive collimation is a hardware-based solution for collimating the x-ray beam such that extraneous radiation exposure is blocked by retractable collimator blades.7,8, 9
Dynamic collimation has the greatest effect in reducing dose for shorter scan lengths and higher pitch values.
Z-overscanning can also occur during axial scanning. In this case, the amount of z-overscan depends on the planned scan length and the total beam collimation. If the planned scan length is not an integer multiple of the total beam collimation, the number of axial scans required to cover the anatomy will result in unnecessary exposure of organs adjacent to the range of clinical interest, as occurs with helical scanning. Adaptive collimation is a hardware solution offered on certain wide-detector array MDCT scanners; with this technique the detector collimation is automatically selected from a set of beam collimations in increments of 10 mm based on the planned scan length so as to minimize extraneous exposure.9
Adaptive collimation for axial scanning has the greatest effect in reducing dose with wide-detector array MDCT scanners because the portion of the total x-ray exposure in the axial scan mode attributed to z-overscanning increases with z-axis detector coverage.10
While early CT image reconstruction algorithms needed 360° of projection data to generate images, newer algorithms for cardiac imaging require only approximately 180° of data. For a single-source scanner, the time required to collect all data needed for reconstruction of cardiac images (ie, the acquisition time) is approximately one-half the gantry rotation time. The fastest single-source scanner currently available spins at 270 ms per rotation, for a nominal acquisition time and temporal resolution of approximately 135 ms. For a dual-source scanner, the acquisition time is approximately one-fourth the gantry rotation time, because the 2 x-ray source/detector systems collect data in half the time needed for a single x-ray source/detector array. The fastest dual-source scanner currently available spins at 280 ms per rotation, for a nominal acquisition time of approximately 70 ms.
In some instances, there is an opportunity to effectively improve temporal resolution during image reconstruction beyond the limits imposed by the gantry rotation time through the use of multicycle reconstruction. During helical scanning, attenuated photons passing through any given slice level of the patient’s body strike each detector row in succession as the patient table moves through the gantry. Each consecutive detector row collects data from that particular slice of the patient’s body, each at a slightly different time point within the cardiac cycle. However, if the heart rate is sufficiently high, the table speed sufficiently slow, and the detector array sufficiently wide, the detector rows may collect data from the same location 2 or 3 times. Rather than reconstructing images from a single 180° arc of attenuation data obtained during 1 heartbeat, images can be reconstructed from 2 adjacent or overlapping arcs totaling 180° and obtained during 2 consecutive heartbeats.11
For example, the first 90° of data might be obtained from the first heartbeat, and the second 90° might be obtained from the next heartbeat. Because the acquisition of data within each heartbeat is now occurring over a smaller scan angle, the time required for acquisition is shorter, effectively improving the temporal resolution. For cases in which exactly 90° of data is obtained from each cardiac cycle, for a single-source system, temporal resolution would effectively be improved 2-fold to a value equal to one-fourth of the gantry rotation time (eg, 67 ms for a gantry rotation time of 270 ms). Alternatively, 3 adjacent or overlapping datasets could be used from 3 consecutive cardiac cycles, again given a high enough heart rate and slow enough table speed, to effectively improve temporal resolution 3-fold up to a value equal to one-eighth of the gantry rotation time for a single-source system (eg, 45 ms for a gantry rotation time of 270 ms).
There are some caveats associated with multicycle reconstruction. Radiation exposure tends to be higher because of the requirement for overlapping x-ray exposure. In addition, the best temporal resolution can be achieved only at certain heart rates for which it is possible to acquire equal and spatially adjacent datasets from consecutive cardiac cycles (eg, scan angles equal to 0° to 90° from the first cardiac cycle and 90° to 180° from the second cardiac cycle). At most heart rates, datasets from consecutive cardiac cycles overlap, with 1 dataset spanning more than 90° and requiring a longer acquisition time. Finally, multicycle reconstruction requires a regular cardiac rhythm: the heart must come to rest in the same position, with the same cardiac cycle length, for every beat of the scan. Any variation, especially in cardiac cycle length, during the scan may result in motion artifacts. Still, the advantages of multicycle reconstruction are believed to outweigh the disadvantages for single-source systems and are automatically implemented for retrospective ECG-gated helical scanning on all CT systems.
Multicycle reconstruction is not limited to helical scanning. A scanner with a wide enough detector array to cover the entire heart in 1 rotation, such as a 320-row scanner, can image the entire heart in diastasis multiple times (up to 5 times) using prospective ECG-triggered axial techniques and can combine data to improve the effective temporal resolution. Adjacent data arcs can always be obtained, ensuring, for example, a 2-fold improvement in effective temporal resolution with 2-cycle reconstruction at all heart rates. However, a major disadvantage of using multicycle reconstruction with a wide detector array is the increase in radiation dose with repeated acquisitions; 2-cycle, 3-cycle, 4-cycle, or 5-cycle reconstruction results in a 2-, 3-, 4-, or 5-fold increase in radiation dose compared to single cycle reconstruction.