CT Scanner Gantry Basic Components
The largest component of a CT scanner system is referred to as the scan or imaging system. The imaging system primarily includes the CT scanner gantry and patient table or couch. The CT scanner gantry is a moveable frame that contains the x-ray tube, including: collimators and filters, detectors, data acquisition system, rotational components including slip ring systems, and all associated electronics such as CT scanner gantry angulation motors and positioning laser lights. In older CT systems a small generator supplied power to the x-ray tube and the rotational components via cables for operation. This type of generator was mounted on the rotational component of the CT system and rotated with the x-ray tube. Some generators remain mounted inside the CT scanner gantry wall. Some newer CT scanner designs utilize a generator that is located outside the CT scanner gantry. Slip ring technology eliminated the need for cables and allows continuous rotation of the gantry components. The inclusion of slip ring technology into a CT system scanner allows for continuous scanning without the interference of cables. A CT scanner gantry can be angled up to 30 degrees toward a forward or backward position. CT scanner gantry angulation is determined by the manufacturer and varies amount CT scanner systems. CT scanner gantry angulation allows the operator to align the applicable part of the body under examination with the scanning plane.
How the CT Scanner Gantry Aperture Operates
The opening through which the patient passes is referred to as the CT scanner gantry aperture. CT scanner gantry aperture diameters generally range from 50 – 85 cm. In general, larger CT scanner gantry diameters, 70 – 85 cm, are necessary for CT scanner departments that do a large volume of biopsy procedures. The larger CT scanner gantry aperture allows for easier manipulation of biopsy equipment and reduces the risk of injury when scanning the patient and securing placement of the biopsy needle simultaneously. The diameter of the CT scanner gantry aperture is different for the diameter of the scanning circle or scan field view. If a CT scanner system has a gantry aperture of 70 cm diameter it does not mean that you can acquire patient data utilizing a 70 cm diameter. Generally, the scanning diameter in which patient or projection data is acquired is less that the size of the gantry aperture. The lasers of projection data acquired are less than the size of the gantry aperture. Lasers or high-intensity lights are included within or mounted on the gantry. The lasers or high-intensity lights serve as anatomical positioning guides that reference the center of the axial, coronal, and sagittal planes.
The Low Voltage Slip Ring and its Role
Eliminating interscan delays requires continuous CT scanner gantry rotation, a capability made possible by the low-voltage slip ring. A slip ring passes electrical power to the rotating components, such as x-ray tube and detectors, without fixed connections. The idea is similar to the technology used by bumper cars, in that power is passed to the cars through a metal brush that slides along a conductive ceiling. Similarly, a slip ring is a drum or annulus with grooves along which electrical contactor brushes slide. Data is transmitted from detectors via various high-capacity wireless technologies, and as a result, allowing continuous rotation to occur. A slip ring allows the complete elimination of interscan delays, except for the time required to move the table to the next slice position. However, the scan-move-scan sequence is still somewhat inefficient. For example, if scanning and moving the table each take 1 s, only 50% of the time is spent acquiring data. Further more, rapid table movements may introduce “tissue-jingle” motion artifacts into the images.
Revolutionary Improvements: Helical/Spiral CT
An alternate strategy is to continuously rotate the CT scanner gantry and continuously acquire data, as the patient table is smoothly moved through the CT scanner gantry. The resulting trajectory of the tube and detectors, relative to the patient, traces out a helical or spiral path. This powerful concept, referred to synonymously as helical CT or spiral CT, allows for rapid scans of entire z-axis regions of interest, in some cases within a single breath hold. So significant were improvements in body CT quality and throughput that helical scanning became the standard of care for body CT scanners.
Scan Time and Misregistration
The rotation time of the CT scanner gantry around the patient (also known as CT scanner gantry rotation time) clearly has a direct effect on total scan time. Image quality improves with faster rotation times, as there is reduced misregistration of data both in-plane and along the patient which is due to patient movement (whether from heartbeat, breathing, peristalsis or restlessness). This misregistration of data introduces artifacts into the image.
CT scanners can now achieve rotation times of less than 0.3 seconds, but the fastest rotations are generally reserved for specialist applications such as cardiac scanning, in order to minimize image artifacts due to the motion of the heart.
Rotation Times and Applications
CT scanners are currently available with two tubes, mounted at 90 degrees to each other, requiring only a half rotation of data, so is effectively even faster. This is especially important for cardiac applications. For general body scanning, 0.5 second rotations are usually more than adequate, and for head scanning, 1 second rotation times are often sufficient.
Higher tube currents will be required for these faster rotation times, and when combined with long scan lengths there will be a need for a higher anode heat capacity or higher anode cooling rate. This effect is off-set by the use of longer detector array lengths.
Current cardiac-enabled CT scanners have low enough rotation times suitable for cardiac imaging. Reconstruction techniques that use data from a partial rotation are used in cardiac scanning and so for a single source system the temporal resolution can be taken to equal half the gantry rotation time. Whiles this is higher than the theoretical required time window for 60 bps, it appears that this can still provide good image quality for suitable patients at that heart rate or below. However, for patients with high heart rates, beta blockers can be used to slow down the heart rate and stabilize it for purposes of the CT scan.