COMPUTED TOMOGRAPHY

Basic Computed Tomography Examination Protocols

Because of the numerous scanner types, parameters, tube rotation speeds, and detector types that are used in computed tomography (CT) imaging, it is impossible to list exact examination protocols. Technical factors are directly related to the detector configuration that is used: number of detector rows and fixed array versus adaptive array. Many scans are performed using auto tube current modulation as opposed to fixed mA. This chapter is an overview of basic CT scan protocols using an adaptive array, 16-row scanner. The values listed are close approximations of what can be used for the various examinations.

Fundamentals of Computed Tomography

Computed tomography (CT)^* is the process of creating a cross-sectional tomographic plane of any part of the body (Fig. 31-1). For CT, a patient is scanned by an x-ray tube rotating around the body part being examined. A detector assembly measures the radiation exiting the patient and feeds back the information, referred to as primary data, to the host computer. After the computer has compiled and calculated the data according to a preselected algorithm, it assembles the data in a matrix to form an axial image. Each image, or slice, is displayed in a cross-sectional format.

Fig. 31-1 CT scanner provides cross-sectional images by rotating around the patient.

In the early 1970s, CT scanning was used clinically only for imaging of the brain. The first CT scanners were capable of producing only axial images and were called CAT (computed axial tomography) units by the public; this term is no longer accurate because images can now be created in multiple planes. In the past few decades, dramatic technical advancements have led to the development of CT scanners that can be used to image virtually every structure within the human body. Improvements in scanner design and computer science have produced CT units with new imaging capabilities and reconstruction techniques. Three-dimensional reconstructions of images of the internal structures are used for surgical planning, CT angiography (CTA), radiation therapy planning, and virtual reality imaging.

CT-guided biopsies and fluid drainage offer an alternative to surgery for some patients. Although these procedures are considered invasive, they offer shorter recovery periods, no exposure to anesthesia, and less risk of infection. CT is also used in radiation oncology for radiation therapy planning. CT scans taken through the treatment field, with the patient in treatment position, have drastically improved the accuracy and quality of radiation therapy.

Computed Tomography and Conventional Radiography

When a conventional x-ray exposure is made, the radiation passes through the patient and produces an image of the body part. Frequently, body structures are superimposed (Fig. 31-2). Visualizing specific structures requires the use of contrast media, varied positions, and usually more than one exposure. Localization of masses or foreign bodies requires at least two exposures and a ruler calibrated for magnification.

Fig. 31-2 Conventional radiograph superimposes anatomy and yields one diagnostic image with fixed density and contrast.

During the CT examination, a tightly collimated x-ray beam is directed through the patient from many different angles, resulting in an image that represents a cross section of the area scanned. This imaging technique essentially eliminates the superimposition of body structures. The CT technologist controls the method of acquisition, the slice thickness, the reconstruction algorithm, and other factors related to image quality.

In the digital radiograph of the abdomen shown in Fig. 31-3, high-density bone and low-density gas are seen, but many soft tissue structures, such as the kidneys and intestines, are not clearly identified. Contrast media are needed to visualize these structures. A CT examination of the abdomen would show all of the structures that lie within the slice. In Fig. 31-4, A, the liver, stomach, kidneys, spleen, and aorta can be identified. In addition to eliminating superimposition, CT is capable of differentiating among tissues with similar densities. This differentiation of densities is referred to as contrast resolution. The improved contrast resolution with CT compared with conventional radiography is due to a reduction in the amount of scattered radiation.

Fig. 31-3 Digital kidney, ureter, and bladder (KUB).

Fig. 31-4 A, Axial image of abdomen showing liver (L), stomach (ST), spleen (SP), aorta (A), inferior vena cava (IVC), vertebral body of thoracic spine (VB), and kidney (K). B, Axial CT scan of lateral ventricles (LVah), septum (Sep), and third ventricle (3V). (B, From Kelley LL, Petersen CM: Sectional anatomy for imaging professionals, ed 2, St Louis, 2007, Mosby.)

Fig. 31-4, B, is an axial image of the brain that differentiates the gray matter from the white matter and shows bony structures and cerebrospinal fluid within the ventricles. Because CT can show subtle differences in various tissues, radiologists are able to diagnose pathologic conditions more accurately than if they were to rely on radiographs alone. Because the image is digitized by the computer, numerous image manipulation techniques can be used to enhance and optimize the diagnostic information available to the physician (Fig. 31-5).

Fig. 31-5 Image manipulation techniques used to enhance diagnostic information in CT image. A, Multiple imaging and windows. B, Image magnification. C, Measurement of distances. D, Superimposition of coordinates on the image. E, Highlighting. F, Histogram. (Courtesy Siemens Medical Systems, Iselin, NJ.)

Historical Development

CT was first performed successfully in 1970 in England at the Central Research Laboratory of EMI, Ltd. Hounsfield, an engineer for EMI, and Cormack, a nuclear physicist from Johannesburg, South Africa, are generally given credit for the development of CT. For their research, they were awarded the Nobel Prize in Medicine and Physiology in 1979. After CT was shown to be a useful clinical imaging modality, the first full-scale commercial unit, referred to as a brain tissue scanner, was installed in Atkinson Morley’s Hospital in 1971. An early dedicated head CT scanner is shown in Fig. 31-6. Physicians recognized its value for providing diagnostic neurologic information, and its use was accepted rapidly. The first CT scanners in the United States were installed in June 1973 at the Mayo Clinic, Rochester, Minnesota, and later that year at Massachusetts General Hospital, Boston. These early units were also dedicated head CT scanners. In 1974, Ledley at Georgetown University Medical Center, Washington, D.C., developed the first whole-body scanner, which greatly expanded the diagnostic capabilities of CT.

Fig. 31-6 First-generation EMI CT unit: dedicated head scanner. (Photograph taken at Reöntgen Museum, Lennep, Germany.)

After CT was accepted by physicians as a diagnostic modality, numerous companies in addition to EMI began manufacturing scanners. Although the units differed in design, the basic principles of operation were the same.

Computed Tomography Scanner Generation Classifications

CT scanners have been categorized by generation, which is a reference to the level of technologic advancement of the tube and detector assembly. The original “generation” classification of scanners was a clear distinction of tube movement versus detector rotational path. As scanner technology has progressed, the tube movement and detector rotation relationship has remained relatively constant, but the tube power source and the detector configurations have changed. Some authors have used slip ring or detector advancements to assign a generation number. These varied opinions and discussions have led to some confusion concerning scanner generation classifications. The following discussion of scanner generations follows the original standards of tube movement versus detector rotation.

The early units, referred to as first-generation scanners, worked by a process known as translate/rotate. The tube produced a finely collimated beam, or pencil beam. Depending on the manufacturer, one to three detectors were placed opposite the tube for radiation detection. The linear tube movement (translation) was followed by a rotation of 1 degree. Scan time was usually 3 to 5 minutes per scan, which required the patient to hold still for extended periods. Because of the slow scanning and reconstruction time, the use of CT was limited almost exclusively to neurologic examinations. A CT image from a first-generation scanner is shown in Fig. 31-7.

Fig. 31-7 Axial brain image from the first CT scanner in operation in the United States: Mayo Clinic, Rochester, Minnesota. The 80 × 80 matrix produced a noisy image. The examination was performed in July 1973.

The second-generation scanners were considered a significant improvement over first-generation scanners. The x-ray tube emitted a fan-shaped beam that was measured by approximately 30 detectors placed closely together in a detector array. Tube and detector movement was still translate/rotate; however, the gantry rotated 10 degrees between each translation. These changes improved overall image quality and decreased scan time to about 20 seconds for a single slice. The time required to complete one CT examination remained relatively long, however.

The third–generation scanners introduced a rotate/rotate movement, in which the x-ray tube and detector array rotate simultaneously around the patient. An increase in the number of detectors (>750) and their arrangement in a “curved” detector array considerably improved image quality (Fig. 31-8). Scan times were decreased to 0.35 to 10 seconds per slice, which made the CT examination much easier for patients and helped decrease motion artifact. Advancements in computer technology also decreased image reconstruction time, substantially reducing examination time. Most current scanners are third-generation configurations with one of the following technical variations:

Fig. 31-8 Rotate/rotate movement: tube and detector movement of a thirdgeneration scanner.

• Helical CT, single-slice helical CT (SSHCT). Slip-ring technology allows 360-degree continuous rotation of tube and detector. Reduces scan times to subsecond per slice.

• Multislice detectors (MSHCT or MDCT). Increase in number of detector rows allows multiple slices to be taken in one rotation. As detector rows increase, the fan beam geometry of the x-ray beam has been adapted. Began with two-slice scanners and quickly moved to four-slice and more.

• Volume CT (VCT). Multislice scanners with 64 detector rows or more. The x-ray beam geometry must be a cone-beam configuration to accommodate the increased length of the scanner.

• Flat-panel CT (FP-CT or FD-CT). A detector plate similar to plates used in digital radiography (DR) replaces the typical detector configuration. In dedicated breast units, the tube and detector travel a full 360 degrees. In other applications, the unit functions more like a C-arm fluoroscopy unit in which the tube and detector do not travel in a full 360 degrees. Scanners provide excellent spatial resolution but lower contrast resolution.

The fourth-generation scanners introduced the rotate-only movement in which the tube rotates around the patient, but the detectors were in fixed positions, forming a complete circle within the gantry (Fig. 31-9). The use of stationary detectors required greater numbers of detectors to be installed in a scanner. Fourth-generation scanners tended to yield a higher patient dose per scan than previous generations of CT scanners.

Fig. 31-9 Rotate-only movement: tube movement with stationary detectors of a fourth-generation scanner.

The fifth-generation scanners are classified as high-speed CT scanners because of millisecond acquisition times. These scanners are electron-beam scanners (EBCT) in which x-rays are produced from an electron beam in a fan beam configuration that strikes stationary tungsten target rings (Fig. 31-10). The detector rings are in a ±210-degree arc. These scanners were primarily used for cardiac studies.

Fig. 31-10 Electron beam CT scanner configuration. X-rays, produced from electron beam, strike four target rings.

The sixth-generation scanners are dual-energy source (two x-ray tubes) (DSCT, DE-CT) that have two sets of detectors that are offset by 90 degrees. These DSCT scanners provide improved temporal resolution needed for imaging moving structures such as the heart (Fig. 31-11). The original DSCT scanners had several technical challenges and were not widely used. The latest DSCT scanners have solved the technical issues, however, and offer dual-energy capabilities between the two CT tubes. This technology allows a marked decrease in patient radiation dose.

Fig. 31-11 Dual-source CT scanner (DSCT) configuration. This is considered a sixth-generation scanner.

Most scanners in use today are a third-generation variation that have 4 to 320 rows of detectors in a single array. This increase in numbers of detector rows has increased the length of the detector, which requires the x-ray beam to be cone-shaped to encompass the full detector array. This is a change from the original third-generation fan beam. The flat panel detector also requires cone-beam geometry. The increased detector size and the cone-beam geometry pose various challenges in maintaining image quality, but this is a discussion that is too involved for this chapter.

Technical Aspects

The axial images acquired by CT scanning provide information about the positional relationships and tissue characteristics of structures within the section of interest. The computer performs a series of steps to generate one axial image. With the patient and gantry perpendicular to each other, the tube rotates around the patient, irradiating the area of interest. For every position of the x-ray tube, the detectors measure the transmitted x-ray values, convert them into an electrical signal, and relay the signal to the computer. The measured x-ray transmission values are called projections (scan profiles) or raw data. When collected, the electrical signals are digitized, a process that assigns a whole number to each signal. The value of each number is directly proportional to the strength of the signal.

The digital image is an array of numbers arranged in a grid of rows and columns called a matrix. A single square, or picture element, within the matrix is called a pixel. The slice thickness gives the pixel an added dimension called the volume element, or voxel. Each pixel in the image corresponds to the volume of tissue in the body section being imaged. The voxel volume is a product of the pixel area and slice thickness (Fig. 31-12). The field of view (FOV) determines the amount of data to be displayed on the monitor.

Fig. 31-12 CT image is composed of a matrix of pixels, with each pixel representing a volume of tissue (voxel).

Each pixel within the matrix is assigned a number that is related to the linear attenuation coefficient of the tissue within each voxel. These numbers are called CT numbers or Hounsfield units. CT numbers are defined as a relative comparison of x-ray attenuation of a voxel of tissue with an equal volume of water. Water is used as reference material because it is abundant in the body and has a uniform density; water is assigned an arbitrary value of 0. Tissues that are denser than water are given positive CT numbers, and tissues with less density than water are assigned negative CT numbers. The scale of CT numbers ranges from −1000 (air/gas) to +3000 (dense bone). Average CT numbers for various tissues are listed in Table 31-1.

TABLE 31-1

Average Hounsfield units (HU) for selected substances

Substance	HU
Air	−1000
Lungs	−250 to −850
Fat	−100
Orbit	−25
Water	0
Cyst	−5 to +10
Fluid	0 to +25
Tumor	+25 to +100
Blood (fluid)	+20 to +50
Blood (clotted)	+50 to +75
Blood (old)	+10 to +15
Brain	+20 to +40
Muscle	+35 to +50
Gallbladder	+5 to +30
Liver	+40 to +70
Aorta	+35 to +50
Bone	+150 to +1000
Metal	+2000 to +4000

For displaying the digital image, each pixel within the image is assigned a level of gray. The gray level assigned to each pixel corresponds to the CT number for that pixel.

System Components

The three major components of the CT scanner are shown in Fig. 31-13. Because each component has several subsystems, only a brief description of their main functions is provided in the following sections.

Fig. 31-13 Components of a CT scanner: 1, computer and operator’s console; 2, gantry; 3, patient table. (Courtesy GE Medical Systems, Waukesha, WI.)

COMPUTER

The computer provides the link between the CT technologist and the other components of the imaging system. The computer system used in CT has four basic functions: control of data acquisition, image reconstruction, storage of image data, and image display.

Data acquisition is the method by which the patient is scanned. The technologist must select among numerous parameters, such as scanning in the conventional or helical mode, before the initiation of each scan. During implementation of the data acquisition system (DAS), the computer is involved in sequencing the generation of x-rays, turning the detectors on and off at appropriate intervals, transferring data, and monitoring the system operation.

The reconstruction of a CT image depends on the millions of mathematic operations required to digitize and reconstruct the raw data. This image reconstruction is accomplished using an array processor that acts as a specialized computer to perform mathematic calculations rapidly and efficiently, freeing the host computer for other activities. Currently, CT units can acquire scans in less than 1 second and require only a few seconds more for image reconstruction.

The host computer in CT has limited storage capacity, so image data can be stored only temporarily. Other storage mechanisms are necessary to allow for long-term data storage and retrieval. After reconstruction, the CT image data can be transferred to another storage medium such as optical disks. CT studies can be removed from the limited memory of the host computer and stored independently, a process termed archiving.

The reconstructed images are displayed on a monitor. At this point, the technologist or physician can communicate with the host computer to view specific images; post images on a scout; or implement image manipulation techniques such as zoom, control contrast and brightness, and image analysis techniques.

GANTRY AND TABLE

The gantry is a circular device that houses the x-ray tube, DAS, and detector array. Helical CT units also house the continuous slip ring and high-voltage generator in the gantry. The structures housed in the gantry collect the necessary attenuation measurements to be sent to the computer for image reconstruction.

The x-ray tube used in CT is similar in design to the tubes used in conventional radiography, but it is specially designed to handle and dissipate excessive heat units created during a CT examination. Most CT x-ray tubes use a rotating anode to increase heat dissipation. Many CT x-ray tubes can handle around 2.1 million heat units (MHU), whereas advanced CT units can tolerate 4 to 5 MHU.

The detectors in CT function as image receptors. A detector measures the amount of radiation transmitted through the body and converts the measurement into an electrical signal proportional to the radiation intensity. The two basic detector types used in CT are scintillation (solid-state) and ionization (xenon gas) detectors. Current detectors use scintillation (solid-state) detectors.

The gantry can be tilted forward or backward up to 30 degrees to compensate for body part angulation. The opening within the center of the gantry is termed the aperture. Most apertures are about 28 inches (71.1 cm) wide to accommodate a variety of patient sizes as the patient table advances through it.

For certain head studies, such as studies of facial bones, sinuses, or the sella turcica, a combination of patient positioning and gantry angulation results in a direct coronal image of the body part being scanned. Fig. 31-14 shows a typical direct coronal image of the paranasal sinuses.

Fig. 31-14 Direct coronal of paranasal sinuses.

The table is an automated device linked to the computer and gantry. It is designed to move in increments (index) according to the scan program. The table is an extremely important part of a CT scanner. Indexing must be accurate and reliable, especially when thin slices (1 or 2 mm) are taken through the area of interest. Most CT tables can be programmed to move in or out of the gantry, depending on the examination protocol and the patient.

CT tables are made of wood or low-density carbon composite, both of which support the patient without causing image artifacts. The table must be very strong and rigid to handle patient weight and at the same time maintain consistent indexing. All CT tables have a maximum patient weight limit; this limit varies by manufacturer from 300 to 600 lb (136 to 272 kg). Exceeding the weight limit can cause inaccurate indexing; damage to the table motor; and even breakage of the tabletop, which could cause serious injury to the patient.

Accessory devices can be attached to the table for various uses. A special device called a cradle is used for head CT examinations. The head cradle helps hold the head still; because the device extends beyond the tabletop, it minimizes artifacts or attenuation from the table while the brain is being scanned. It can also be used in positioning the patient for direct coronal images.

OPERATOR’S CONSOLE

The operator’s console (Fig. 31-15) is the point from which the technologist controls the scanner. A typical console is equipped with a keyboard for entering patient data and a graphic monitor for viewing the images. Other input devices, such as a touch display screen and a computer mouse, may also be used. The operator’s console allows the technologist to control and monitor numerous scan parameters. Radiographic technique factors, slice thickness, table index, and reconstruction algorithm are some of the scan parameters that are selected at the operator’s console.