CT scanners are designed for imaging of the internal structures of the body. They provide detailed anatomical information by utilizing the principle that different types of tissue structures scanned are displayed in the image as different shades of grey. Intravenous or oral contrast media may be used to further enhance differentiation between tissues.
The basic components of a CT scanner are an x-ray tube and an arc banana of detectors or a flat panel, mounted on a gantry with a circular aperture. Along the patient long axis (Z) there are many rows of these arcs of detectors, giving rise to the term multi-slice CT.
Multi-detector CT is also a commonly used term. The extent of patient coverage by the detector rows currently ranges from 12mm to 160mm in length, depending on the CT scanner model.
CT scanner technology has advanced rapidly in recent years, moving to more efficient and stable detectors, more refined engineering and data acquisition systems and electronics, and faster computers.
These CT scanner developments have been largely directed towards faster scanning of further lengths of the patient, using finer slices. As a result, CT scanners have evolved from a slice-by-slice diagnostic imaging system into a truly volumetric imaging modality, where images can be reconstructed in any plane without loss of image quality. This has lead to the increased use of multi-planar and 3D display modes in diagnosis.
However, it is also important to recognize that the performance of CT scanners in practice depends on the trade-off between image quality and radiation dose. As a result, each system should also be assessed in terms of clinical performance, with close observation of the radiation dosage utilized.
Generally, multi-slice scanners cover the patient volume between 20 and 40mm in length per rotation. The latest diagnostic multi-slice CT scanners can image patient volumes of up to 160mm per rotation.
The length of the detector array of the CT scanners determines the number of rotations needed to cover the total scan length, and consequently, the overall scan time. The ability to scan a given length with fewer rotations also helps to minimize head load on the x-ray tube, thereby allowing the scanning of longer lengths.
Detector arrays of the CT scanners are broadly divided into two types: fixed and variable. Fixed arrays have detectors of equal z-axis dimension over the full extent of the array, while with variable arrays; the central portion comprises finer detectors. With variable arrays, the total scan time for a given length for the finest slice acquisition is longer, because the z-axis coverage is reduced.
All CT scanners with greater than 64-slice acquisition have fixed array. Complete coverage of an organ offers advantages for both dynamic perfusion and cardiac studies. The z-axis detector array lengths on the current 64-slice scanners, of up to 40 mm, are adequate to cover these organs in only a few rotations. A coverage length of 160mm usually allows complete organ coverage in a single rotation, so the function of the whole organ can be monitored over time.
The evolution of CT scanner designs reflect different strategies to accommodate future developments and allow for production costs. There is also some small dose saving where larger detector elements are used on the lower slice category scanners.
Spatial resolution is the ability of the CT scanners to image an object without blurring. It is often described as the sharpness of an image. It may be quoted as the smallest object size able to be distinguished, and as such, is evaluated using high-contrast test objects where signal-to-noise level is high and does not influence perception.
Modern CT scanners should be capable of achieving isotropic resolution: a z-axis resolution that is equal to or approaching the scan plane resolution, as this is essential for good-quality, multi-planar and 3D reconstructions.
It is helpful to remember that the cost of high-spatial resolution of CT scanners is either in the high image noise or in a high-patient radiation dose when the tube current is raised to reduce the image noise.
Contrast resolution of CT scanners is the ability to resolve an object from it surroundings, when the CT numbers are similar. It is sometimes referred to as low-contrast detectability. The ability to detect an object is dependent on its contrast, the level of image noise and its size. Contrast resolution is usually specified as the minimum size of object of a given contrast difference, that can be resolved for a specified set of scan.
The temporal resolution of CT scanners is defined as the time taken to acquire a segment of data for image reconstruction. In CT scanners, temporal resolution is usually considered in the context of cardiac scanning. The goal in cardiac CT is to minimize image artifacts due to the motion of the heart. This can be achieved using ECG-gating techniques, and imaging the heart during the period of least movement in the cardiac cycle, resulting in temporal resolution requirements of very short periods, as compared with the heart cycle.
There is an optimum combination of pitch, gantry rotation time, and number of segments for each given heart rate. CT scanner detectors capture the radiation beam from the patient and convert it into electrical signals, which are subsequently converted into binary coded information for onward transmission to a computer system for further processing.
CT scanners detectors must be capable of responding with extreme speed to a signal, without lag, must quickly discard the signal, and prepare for the next. They must also respond consistently and be small in size. CT scanners detectors should have high capture efficiency, high absorption efficiency and high conversion efficiency. These three parameters are called the detector dose efficiency.
The capture efficiency is how well the detectors receive photons from the patient. It is primarily controlled by detector size and the distance between detectors.
Absorption efficiency is how well the detectors convert incoming x-ray photons. It is primarily determined by the materials used, as well as the size and thickness of the detector.
Conversion efficiency is determined by how well the detector converts the absorbed photon information to a digital signal for the computer.
In recently manufactured CT scanners, the entire array of detectors consists of groupings of detectors, with each group known as a detector module, which is plugged into a motherboard unit of the detection system.
Flat-panel detectors have been developed for use in radiography and fluoroscopy, with the defined goal of replacing standard x-ray film, film-screen systems and image intensifiers by an advanced solid state sensor system. Flat-panel detector technology offers high dynamic range, dose-reduction, and fast digital conversion – yet keeping to a compact design. It appears logical to employ the same design for CT scanners, as well.
The use of flat-panel detectors for CT scanners provides a very efficient way of x-ray detection and acoustics. Flat-panel detectors provide high-spatial resolution. However, there are also some disadvantages: relatively lower dose efficiency, smaller fields or view and lower temporal resolution.