CT scanners are designed to obtain images of structures inside the body. They provide detailed anatomical information using the principle that the different types of scanned tissue structures are shown in the image as different shades of gray. Intravenous or oral contrast agents can be used to further improve the differentiation between tissues.
The basic components of a CT scanner are an X-ray tube and a detector banana arc or a flat panel, mounted on a gantry with a circular aperture. Along the long axis of the patient (Z) there are many rows of these detector arches, giving rise to the term multislice CT.
Multidetector CT is also a term in common use. 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 and electronics systems, and faster computers.
These developments in CT scanners have largely been directed at faster scanning of additional lengths of the patient, using finer cuts. As a result, CT scanners have evolved from a slice-by-slice imaging system to a truly volumetric imaging modality, where images can be reconstructed in any plane without loss of image quality. This has led to increased use of multiplanar and 3D display modes in diagnostics.
However, it is also important to recognize that the performance of CT scanners in practice depends on a balance between image quality and radiation dose. As a result, each system must also be evaluated in terms of clinical performance, with close observation of the radiation dose used.
Generally, multi-slice scanners cover the patient volume between 20 and 40 mm in length per rotation. The latest diagnostic multi-slice CT scanners can image patient volumes up to 160mm per rotation.
The length of the detector array in CT scanners determines the number of rotations required to cover the full length of the scan, and consequently the total scan time. The ability to scan a specified length with fewer rotations also helps minimize head load on the X-ray tube, allowing longer lengths to be scanned.
Detector assemblies in CT scanners are divided into two types: fixed and variable. Fixed matrices have detectors of the same z-axis dimension throughout the matrix, while variable matrices; the central part contained finer detectors. With variable matrices, the total scan time for a given length for the acquisition of finer cuts is longer, because the coverage of the z-axis is reduced.
All CT scanners with acquisition of more than 64 slices have a fixed matrix.
Complete coverage of an organ offers advantages for both dynamic perfusion and cardiac studies. The z-axis detector array lengths in today’s 64-slice scanners, up to 40mm, are adequate to cover these organs in just a few rotations. A coverage length of 160mm generally allows complete coverage of the organ in a single rotation, so the function of the entire organ can be monitored over time.
The evolution of CT scanner designs reflects different strategies to accommodate future developments and take production costs into account. There is also a small dose saving when using larger detector elements in lower cut category scanners.
Spatial resolution is the ability of CT scanners to image an object without blurring. It is often described as the sharpness of an image. It can be cited as the smallest distinguishable object size and as such is evaluated using high contrast test objects where the signal-to-noise level is high and does not influence perception.
Modern CT scanners must be able to achieve isotropic resolution – a z-axis resolution that is equal to or close to the scan plane resolution, as this is essential for good quality, multiplanar, 3D reconstructions.
It is useful to remember that the cost of high spatial resolution of CT scanners is in high image noise or high radiation dose to the patient when tube current is increased to reduce image noise.
Contrast resolution of CT scanners is the ability to resolve an object in its environment, when the CT numbers are similar. It is sometimes called low contrast detectability. The ability to detect an object depends on its contrast, the image noise level, and its size. Contrast resolution is generally specified as the minimum size of the object for a given contrast difference, which can be resolved for a specific scan set.
The temporal resolution of CT scanners is defined as the time required to acquire a segment of data for image reconstruction.
In CT scans, temporal resolution is generally considered in the context of the cardiac examination. The goal of cardiac CT is to minimize image artifacts due to movement of the heart. This can be accomplished using ECG triggering techniques and images of the heart during the period of least motion in the cardiac cycle, resulting in very short time resolution requirements, compared to the cardiac cycle.
There is an optimal combination of tone, 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 then converted into binary-encoded information for transmission to a computer system for further processing.
CT scan detectors must be able to respond with extreme speed to one signal, without delay, quickly discard the signal and prepare for the next. They must also respond consistently and be small in size. CT scanner detectors must have high capture efficiency, high absorption efficiency, and high conversion efficiency. These three parameters are called the detector dose efficiency.
Capture efficiency is how well the detectors receive photons from the patient. It is mainly controlled by the size of the detector and the distance between detectors.
The absorption efficiency is how well the detectors convert the 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 information from the absorbed photon into a digital signal for the computer.
In newer CT scanners, the entire detector assembly consists of detector arrays, each group known as a detector module, which is connected to a motherboard unit of the detection system.
Flat panel detectors have been developed for use in radiography and fluoroscopy, with the definite goal of replacing standard X-ray film, film screen systems, and image intensifiers with an advanced solid-state sensor system. Flat panel detector technology offers high dynamic range, dose reduction, and fast digital conversion, while maintaining a compact design. It seems logical to use the same layout for them as well.
The use of flat panel detectors for CT scanners provides a very efficient form of acoustic and X-ray detection. 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.