Basic Instrumentation

1. The imaging system is located in the scanner room.

2. The computer system is located in the computer room.

3. The display, recording, and storage system is located in the operator’s room.

1. To display the output digital image from the computer in a form meaningful to the observer or diagnostician. CT images are now displayed on flat panel monitors or on cathode ray tube (CRT) monitors. These are highlighted subsequently.

2. To provide a hard copy of the image on a recording medium that provides for a permanent copy of the reconstructed image and accommodates the preference of the radiologist during diagnostic interpretation. Today, film images are no longer available and radiologists now have to make the primary diagnosis from a display monitor in the total digital imaging department.

3. To facilitate the storage and retrieval of digital data to address the problems of film storage and archiving and the environmental concerns of film manufacturing, consumption, and disposal. The filmless imaging department now stores images in the picture archiving and communication system (PACS), briefly described in Chapter 2.

4. To communicate images, diagnostic reports, and patient demographic data in an electronic communications network environment such as PACS and radiology information systems.

Gantry

The gantry is a mounted framework that surrounds the patient in a vertical plane. It contains a rotating scan frame onto which the x-ray generator, x-ray tube, and other components are mounted.

The gantry houses imaging components (Fig. 7-4) such as the slip rings, x-ray tube, high-voltage generator, collimators, detectors, and the data acquisition system.

The x-ray tubes of slip-ring scanners require high instantaneous power and therefore have larger anodes with a typical diameter of 5 inches or more. Some CT scanners may incorporate an on-board oil-to-air heat exchanger to assist in cooling the x-ray tube during operation.

The generator in the gantry is usually a small, solid-state, high-frequency generator mounted on the rotating scan frames. Because it is located close to the x-ray tube, only a short high-tension cable is required to couple the x-ray tube and generator. This design eliminates external x-ray control cabinets and long high-tension cables as was typical of the older CT imaging systems.

The power ratings of generators typically range from 30 to 60 kilowatts, depending on the scanner. These ratings enable a large selection of exposure techniques (values such as 80, 100, 120, 130, and 140 kilovolts and about 20 to 500 milliamperes [mA], in 1-mA increments, are not uncommon).

Gantry cooling is a prime consideration because the ambient air temperature affects several components. In the past, air conditioners were placed in the gantry. Modern cooling systems circulate ambient air from the scanner room throughout the gantry.

Two important features of the gantry are the gantry aperture and gantry tilting range. The gantry aperture is the opening in which the patient is positioned during the scanning procedure (Fig. 7-5). The technologist can approach the patient from both the front and back of the gantry. Most scanners have a 70-cm aperture that facilitates patient positioning and helps provide access to patients in emergency situations.

The CT gantry must be capable of tilting (Fig. 7-6) to accommodate all patients and clinical examinations. The degree of tilt varies between systems, but ±12 to ±30 degrees in 0.5-degree increments is somewhat standard. The gantry also includes a set of laser beams to aid patient positioning. Other gantry characteristics include scan control panels (controls gantry tilt and patient table elevation, for example), scan control box (controls emergency stop, intercom, and scan enable and pause functions, for example), slice position indicator, radiation indicator, and intercom systems with multilingual autovoice to facilitate communication with the patient in one of several languages.

Patient Table

The patient couch, or patient table, provides a platform on which the patient lies during the examination (see Fig. 7-3). The couch should be strong and rigid to support the weight of the patient. Additionally, it should provide for safety and comfort of the patient during the examination.

The couch consists of a support referred to as the table top, which floats and rests on a pedestal. The couch top is usually made of carbon fiber composites because they have low absorption and provide excellent vibration damping features and meet the strength requirements necessary to take images of heavy patients. The technologist has ample room between the gantry and table for patient access and positioning (Fig. 7-7). The pedestal houses the mechanical and electrical components that facilitate vertical and horizontal couch movements. The vertical movement should provide a range of heights to make it easy for patients to mount and dismount the table (Fig. 7-8). This feature is especially useful in the examination of geriatric, trauma, and pediatric patients. Horizontal or longitudinal couch movements should enable the patient to be scanned from head to thighs without repositioning (Fig. 7-9).

Box 7-1 lists the table characteristics for one CT system.

Processing Architectures and Hardware

The computer system in CT belongs to the class of minicomputers (see Chapter 2). The two most important characteristics of the CT computer system are a large storage capacity and fast and efficient processing of various kinds of data.

Various computer architectures for CT have been developed to accommodate fast image reconstruction and other image processing functions such as image manipulation and visualization software. For example, the evolution of computer architectures for CT scanners first used pipeline processing architectures (Fig. 7-10), followed by the use of parallel and distributed processing architectures (Fig. 7-11) as the processing and clinical application tasks became more sophisticated. The basis for these architectures depends on the way that the computer assigns various tasks (e.g., preprocessing raw data, convolution, back-projection, and visualization tasks such as 3D imaging, CT angiography, and virtual reality imaging) to the numerous processors in its electronic circuits.

An important component of computer processing architectures for CT and magnetic resonance imaging is the array processor, which is a dedicated electronic circuit capable of the high-speed calculations needed in CT. Array processors feature key elements such as speed, power, flexibility, and expandability. To accommodate these elements, the array processor architecture may consist of the following:

Scanner Control and Image Reconstruction

The operator must be able to communicate with the system to enable scanning, which may be activated through keyboard commands or a touch screen. In the case of the touch screen, the operator can select prestored protocols, modify protocol parameters, or select the sharp, smooth, or standard algorithm, depending on the CT examination. PC interfaces that use a mouse and drop-down Windows-based menus are increasingly becoming common.

Image Display and Manipulation

A wide range of image display and manipulation techniques is afforded by the CT software. Software is highlighted in a following subsection.

Operating Systems

Operating systems are programs that control the hardware components and the overall operation of the computer; they also enable the computer to run other programs. The operating system consists of a major program called the supervisor, which resides in primary memory and controls all other portions of the operating system. CT computers often use interleaved processing techniques such as multitasking, multiprocessing, and multiprogramming, which allow computers to process several programs almost simultaneously and thus increase the number of jobs the computer can handle at any given time. In addition, the system runs rapidly and efficiently. The operating system used in some CT systems is UNIX compatible and facilitates multiuser and multitasking capabilities. Microsoft Windows NT and XP operating systems have become commonplace in CT scanning.

CT Software

CT software is a special topic in itself that is beyond the scope of this book. It should be noted, however, that the software for CT has been evolving rapidly as more and more clinical applications are made possible by multislice CT scanners that can provide 16 to 64 or more slices per revolution of the tube and detectors.

CT software can be placed in one of three categories, namely, reconstruction software, preprocessing software, and image postprocessing software. Reconstruction software, which builds up the image from the raw data collected from the detectors, consists of very sophisticated algorithms with thousands of lines of coding. Preprocessing software, on the other hand, performs corrections on the data collected from the detectors before the data are sent to the reconstruction computer. Beam-hardening corrections and corrections for bad detector readings are examples of these corrections. Finally, image postprocessing software operates on reconstructed images displayed for viewing to facilitate diagnostic interpretation. For example, a list of what has been popularly referred to as visualization and analysis software is shown in Box 7-2. This software can be placed into two categories: basic and advanced image visualization and analysis tools.

Box 7-3 provides a brief description of several image visualization and analysis software tools. These include tools such as the CT viewer, multiplanar reformation (MPR), maximum or minimum intensity projection (MIP), 3D surface-shaded display reconstruction, Relate Slice, Master Cut, and custom image filters.

Finally Box 7-4 briefly describes several other tools, such as 3D small volume analysis, quantitative CT angiography (Q-CTA), combine images, and CT time lapse. Additionally, advanced clinical applications tools such as CT/MR Fusion, functional CT, brain perfusion, and virtual colonoscopy are briefly described.

Image Display

A display device for CT is generally a black-and-white or color monitor (Fig. 7-12). These can be CRT flat display or liquid crystal display flat-panel display devices. Although images are usually displayed in gray scale, nonimage data such as text fields, patient data, and option selections can be displayed in color.

The image display system includes such features as the display matrix, pixel size, bit depth, CT value scale, gray scale, image monitor and the number of lines, selectable window width and window center, single and double windows, and highlighting. CT manufacturers provide detailed specifications for each of these features.

Image Storage

Data are stored in digital form to preserve the wide dynamic range of images, including the capability for image processing and intensity transformations, and to decrease the possibility of lost records and reduce the space needed for archiving.

Digital images are stored in 2D pixel arrays; each pixel point is represented by a number of bits that determine how many gray levels can be represented by a particular pixel. CT image size varies according to the anatomy being examined. A typical CT image has a matrix size of 512 × 512 × 8 bytes (12 bits). In this case, each has a gray-level range of 512 (2⁸) to 4096 (2¹²).

A CT image of 512 × 512 × 2 bytes (16 bits) would require 0.5 megabytes (MB) of storage. If the CT examination contains about 50 images, then 25 MB of storage are needed. If 50 examinations are performed in one day, then 1.25 gigabytes (GB) of storage are needed.

Storage devices for CT include magnetic tape and disks, digital videotape, optical disks, and optical tape. It is interesting to note that the capacity of an optical disk simply is so much more than that of magnetic tapes. This is clearly illustrated in Figure 7-13 from the early days of image storage discussions. Today, compact disk (CD) writers can be used for archiving CT images as well.

The type of storage devices used, their respective storage capacities, and the typical number of images that can be stored on each device will be different depending on the manufacturer.

Laser Recording System

In the past, the requirements for hard copy recording of CT images were a major consideration during CT imaging because the images were used for diagnostic interpretation. The requirements were broad gray-scale contrast resolution to enable the perception of subtle differences in tissue contrast and high spatial resolution to detect boundaries of different tissues.

Laser image recording systems were used to meet these requirements. Although multiformat video cameras were popular in the past, laser cameras, or laser imagers replaced the video camera technology.

Two types of lasers were used for film recording in CT: solid-state laser diodes and gas lasers such as helium-neon (He-Ne), helium, cadmium, argon, carbon dioxide, and nitrogen. The He-Ne laser is the simplest and most reliable gas laser. The solid-state laser typical of the 3M laser imaging systems has a wavelength of 820 nanometers (nm), but the He-Ne laser has a wavelength of 633 nm. Both systems use infrared-sensitive films (820 nm) and He-Ne laser films sensitive to the 633-nm wavelength beam.

Today laser printing on film is obsolete and is not described further in this book.

Communications

Communications refers to electronic networking or connectivity by using a local-area or wide-area network. Connectivity ensures the transfer of data and images from multivendor and multimodality equipment according to the DICOM standard.

CT scanners must support various network speeds such as 10/100/1000 megabits per second and Ethernet switching for very fast image transfer. CT scanners now feature full implementation of the DICOM communication protocol that allows connectivity to various scanners and workstations in the digital radiology department.

• Floating keyboard: Important components include alphanumerical and special function keys, the trackball or mouse, and window controls.

• Touch panel: The touch panel allows system parameters such as scan setup and control parameters to be actuated without typed keyboard commands.

• Window controls: Window controls include the window width and window level controls, which alter picture contrast. Window width refers to the range of CT numbers. Window level is the center of the range.

• Image display: CT images are displayed on monitors for viewing and manipulation by the operator before the final image is communicated to the PACS.

• High-capacity optical disk drive and CD writer: A 9.1-GB erasable optical disk drive is not uncommon.

• Control functions: Various automated functions such as autoarchive, autowindow, and autovoice are featured on CT control consoles. These allow the technologist to devote more time to the scanning procedure and the needs of the patient.

Options

A number of options, both hardware and software, are currently available for CT scanners. The hardware options, for example, may include optical disks, optical cartridge tape, remote diagnostic stations, independent workstations, and so forth.

Software options (see Box 7-4) include packages for bone mineral analysis, dynamic scan, 3D image reconstruction, volumetric MPR, evaluation of regional cerebral blood flow (xenon CT), perfusion CT, dental CT, and networking.

Accessories

Accessories support and provide excellent immobilization of the patient to enhance the overall efficiency of the CT examination. These accessories include pediatric cradles, arm and leg supports, elevated and flat head holders, table mattresses, side rails, table extenders, knee supports, head pillows with hand rests, axial and coronal head holders, and radiation therapy table tops, to mention only a few.

Modular Design Concept

The modular design concept is intended to simplify the upgrading of scanners. The hardware modular design concept features detector modules, analog-to-digital conversion cards, tubes, generators and subassemblies, memory boards, array processors, back-projectors, display camera interfaces, and network interface boards. Software modules allow for the easy modification, updating, and revision of software packages to meet the demands of the clinical environment.

Operating Modes of the Scanner

A modern CT scanner can operate in a variety of modes to meet the requirements of various clinical examinations. Typical operating modes include routine scan mode and rapid or dynamic scan mode. Various spiral/helical scan modes such as overlap scan, skip scan, and tilt scan are available to suit the needs of the examination (Fig. 7-14).

Room Layout for CT Equipment

The room layout for CT scanners varies among institutions and depends on the particular type of scanner. A typical room layout (Fig. 7-15) includes at least three sections or rooms to house different components of the scanner, as follows:

Equipment Specifications

The acquisition of a CT scanner is an interesting experience, and the CT technologist should take advantage of the opportunity to participate in such an activity. The CT department or purchasing committee generally informs the vendor of the necessary equipment specifications. In addition, vendors will have equipment specifications available for review.

In general, several major technical specifications and features of a CT scanner to be considered are as follows: