Data Acquisition

The first step, scanning the patient, is the radiographic portion of the study. The method used to scan is dependent upon the equipment.

CT equipment has undergone many changes since it was first used for diagnostic imaging. The geometry has evolved through four basic generational designs. A fifth-generation scanner, the electron beam CT scanner, allows physicians to produce high-resolution images of moving organs, free from motion artifacts.

The four basic generations of CT scanners incorporated the following geometric designs: First-generation CT scanners used a pencil x-ray beam, a single detector, and a rotate-translate parallel beam 180-degree geometric design. Second-generation scanners incorporated a fan x-ray beam with multiple detectors and a rotate-translate 180-degree geometric design. The first- and second-generation CT scanners required very long scan times to complete a study. The scan time was improved in the third- and fourth-generation designs to the range of 2 to 10 s. Third-generation scanners used the fan beam with 360-degree rotation of the x-ray tube and multiple detector array, considered a rotate-rotate design. Fourth-generation equipment incorporated a rotating fan beam with a 360-degree stationary ring of detectors, or a rotate-stationary geometric design. These scanners used two methods: rotation of the x-ray beam within the detector array or rotation outside a “tilting” (nutating) detector array. One disadvantage of this geometric design is that the system must be reset after only two complete revolutions of the x-ray tube. This is due primarily to the arrangement of the cables within the gantry. It can be seen that there is a time delay between periods when the x-ray tube is activated. This is referred to as the interscan delay (ISD).

All of the older generation methods use slice by slice scanning. In this process, the scan is made, followed by a delay while the scanner is reset, and then the patient is repositioned for the next slice. The sequence is repeated until the area to be scanned has been covered. Among the limitations of the slice by slice scanning method are the following:

A change in the fourth-generation beam geometry led to a scanning technique known as volume data acquisition scanning, commonly referred to as spiral, or helical, CT scanning, depending upon the manufacturer of the equipment. In these systems, a continuous x-ray beam is used to produce the scan while the patient is continuously moved (transported) through the gantry. The continuous movement of the tube around a moving patient yields a spiral geometric path that allows for the collection of a large volume of data in a short period (Fig. 9-2). In fact, the scan times have been reduced to less than a second in this type of system. This type of scanning is made possible through the use of the slip ring geometric design in which there is a set of stationary conductive rings that allow the components of the CT system to continuously rotate by means of conductive brushes that serve as sliding contact points for the transmission of electrical current. Figure 9-3 illustrates the components of the spiral CT scanner within the gantry. This design eliminates the need for the electrical cables that restricted the movement of the x-ray tube in the previous generation of CT scanners.

Computed tomography has undergone a major change in the technology used to accomplish image acquisition. Initial helical scanners had the capability to acquire 4 slices per rotation. Manufacturers have been able to increase the number of slices acquired per rotation from this level through 8, 16, 32, and 64 slices per acquisition. This has also been accompanied by an increase in the resolution produced by the systems. This growth has been mainly attributed to improved detector materials and design.

The system is designed so that the x-ray tube is located opposite the detector system in a circular configuration. The detector is composed of several parts: a scintillator, photodiode, electric channels, and an analog to digital converter.

In earlier computed tomographic units xenon gas detectors were used. They operated by means of ionization of the xenon gas in response to the x-radiation. The ions collected on parallel plates connected to amplifiers to produce the signal. Today scintillators are primarily used to convert the x-rays to light. A photodiode is incorporated into the system to convert the light produced by the scintillator into an electrical signal that is collected and used to produce the image. Figure 9-4 illustrates the process by which the remnant radiation exiting the patient is converted into a digital signal that is used to produce the image. The detectors used vary with the manufacturer. Siemens has developed what they refer to as an ultrafast ceramic (UFC). This material has a very short afterglow, which improves the resolution of the finished image (Fig. 9-5). Note the chessboard pattern of the plate. The illustration shows 16 rows or lines. Older systems had only one detector line producing only one slice per acquisition. The multislice systems discussed here utilize detectors with multiple lines allowing a wider section of the patient to be imaged in the same acquisition period. Among the advantages of this technology is the reduction in examination time. Higher end systems are capable of imaging the entire body in less than 10 seconds. The advantage to this is that the patients are required to hold their breath for shorter periods. There is also a corresponding decrease in motion artifacts.

The multislice units, especially the 64 slice systems, allow for thinner tomographic sections. As we know from basic tomographic principles, the thinner slices will allow the physician to study extremely small anatomic details and increase the accuracy of the diagnoses.

Image Reconstruction

The scanning process produces the image by the attenuation of the x-ray beam; the patient absorbs the radiation in varying amounts depending upon its interactions with the various tissue types. The exit radiation is collected by the detector array and transmitted to the computer for processing. This process is termed image reconstruction.

The information (measurements) acquired from the scan is recorded in digital form by the computer. From this information, the computer reconstructs the image. The computer software that runs the image reconstruction procedure processes the data. The computer programs are generally referred to as algorithms, or more specifically, reconstruction algorithms. The processing procedure can affect both the quality and the appearance of the image—selection of the matrix size can affect the resolution. In general, the larger the matrix, the greater will be the resolution (and quality) of the image.

The algorithm is a part of the computer program and cannot be altered. Many different algorithms are used for processing the data; however, the algorithms are specific for the type of equipment and software options used.

CT is the production of reconstructed images from information acquired from the remnant radiation through one of the image reconstruction algorithms. These transmitted x-ray photons represent some amount of attenuation within the patient. They are compared with the intensity of the radiation from the x-ray tube, which is measured by a special “reference” detector, to give relative transmission values after digitization. The attenuation values of various tissues are related to the attenuation value of water and may be arranged as a scale (Fig. 9-6). These scale values are called EMI, or Hounsfield, numbers and represent the various CT digital numbers used to reconstruct the image. When the image is produced, the scale (CT digital) numbers represent a certain brightness level. Figure 9-7 illustrates how the scale numbers and brightness level are related. The brightness level (gray scale) can be manipulated to demonstrate different structures in the image. This manipulation, or variation in the relation between the scale numbers and level of brightness, is often called windowing, or setting a window.

The window is controlled by the operator of the CT unit and usually set as a window width and window level. The window width represents the range of scale numbers used for the gray scale. Adjusting the window width is equal to adjusting the contrast of the image. The window level represents the midpoint of the gray scale and can be considered a density adjustment. When viewing an image, these values can be adjusted, usually by the radiographer, to enhance certain anatomic structures. The effect of varying window width and window level is illustrated in Figure 9-8.

Image Display and Storage

CT images are digitally captured and manipulated. The reconstructed image can be displayed on a cathode ray tube (CRT) monitor for viewing. The image can also be recorded and stored for future viewing. One common method for viewing and storage of the study is by producing hard copy images on medical x-ray film using laser cameras. The images can also be stored on discs, or optical storage media. The images can usually be manipulated through the use of various software packages.

Most institutions have added a digital centralized storage system that provides easy access to images which can be transmitted to any workstation on its network. The system is referred to as PACS (picture archiving communications system). One advantage of the PAC system is that the physician has access to not only the patient’s film but also the patient information data. PACS eliminates the need for processing facilities and allows the images to be transmitted and manipulated at any one of the network workstations without affecting the stored image. The system greatly improves communications and productivity. A full discussion of PACS is beyond the scope of this text; however, some resources have been included in the Suggested Readings at the end of the chapter.