Creating Waves of Awareness
ur masters told us to draw our attention towards the patient as a person, as a whole, as an individual and not to waste our energy in studying diseases itself. But, times have changed. Several short methods are currently available to assist us in making a more perfect diagnosis and ascertaining a correct prognosis leading to a true remedy with a suitable dose and reliable posology i.e. absolute similimum. With various means of investigations to reach the diagnosis and plan for treatment, I will elaborate on the Computerized Tomography or CT scan. The purpose of this article is to present a brief overview of CT systems. Various computer components and their features that are particularly relevant to CT will be described. It is assumed that the basic aspects of radiologic equipment and physics are generally known. Overall performance characteristics such as resolution and image artifacts will also be discussed.
CT systems have been classified according to-
COMPONENTS OF A CT SCANNER
The main components of a CT scanner are:
X-RAY SOURCE AND GENERATOR
To achieve good contrast sensitivity, a sufficiently large number of x-ray photons must be detected. However, only 1% of the energy incident on the x-ray tube anode is converted to x-rays; the rest merely produces heat. Furthermore, only a small fraction of the generated x-rays are used, because only a narrow fan beam exits the collimator. As a result, techniques as high as 200 mAs to 800 mAs at 120 kVp to 140 kVp are used for each scan.
The newer and faster rotate-only scanners use rotating anode tubes, which can operate at higher instantaneous power but only for reduced scan times.
"Off-focus radiation" is the term used to refer to x-rays that do not originate from the focal spot. Limiting or correcting for off-focus radiation is important, because off-focus radiation can reduce contrast and cause halo like artifacts around high-contrast image edges.
Because of the long exposure times, high voltage stability with little ripple in beam intensity is desired. X-ray systems can be operated in either a pulsed or continuous mode. Ideally, a pulsed system with a low duty cycle is preferred, because it provides an almost instantaneous measurement and reduces blurring caused by gantry motion during data acquisition. Furthermore, scan time reductions force the view acquisition rate to be increased while at the same time require higher average x-ray intensities (higher average mA) during the scan. These factors combine to make x-ray tube pulsing prohibitive. At the same time, the development of more stable detectors and data acquisition systems have reduced the need for electronic offset stabilization in the data acquisition system. As a result, many newer third-generation scanners use continuous x-ray sources.
Prepatient collimators are used to limit the thickness of the scan section, and must be accurately adjustable for several thicknesses in the 1- to 10-mm range. Thus, the focal spot should be as small as possible and the focal spot-to-collimator distance as large as possible. The slice sensitivity profile, or width of the imaging slice, can be further limited by a postpatient collimator but at the expense of wasted radiation dose.
Typically, the rays measured near the center of the fan beam experience much more attenuation because of the patient than rays toward the edges of the fan beam. Some systems use beam-shaping filters to pre-attenuate the edges of the fan beam prior to irradiating the object.
All CT detectors are operated in the "current integration" mode, in which the signals produced by the x-ray photons incident on the detector over a brief period of time are summed. The high x-ray intensities used preclude counting individual x-ray photons, because detectors able to count at rapid enough rates are not readily available.
Scintillator materials that have been used include sodium iodide (NaI), bismuth germanate (BGO), and cesium iodide (CsI). Cadmium tungstate (CdWO4) and other tungstate crystals are now frequently used. Technique changes such as kV, mA, and collimator aperture can account for another factor of 100:1. Thus, CT detectors must be able to handle signals that cover a 10,000:1 range.
To maximize the information generated from patient dose, high quantum detection efficiency (QDE) is desirable. QDE is the product of absorption and geometric efficiencies. Scintillators can be designed to achieve almost 100% absorption.
The data acquisition system (DAS) performs the function of converting signals from the x-ray detector into measurements for use by the reconstruction hardware. Signals received from each detector channel must be integrated or smoothed to obtain a measure of the number of x-ray photons received by the detector in a given period of time. These analog signals are amplified and converted into digital form by an analog-to-digital converter (ADC).
A CT DAS must be able to handle signals covering an amplitude range of 10,000:1. This can be compared, for example, to a high-quality TV camera used in digital radiography, which handles a signal range of no more than 1000:1.
GANTRY AND PATIENT TABLE
The x-ray tube, collimator, detector array, and DAS are mounted on the gantry. This contains the mechanisms for driving the source and detector precisely through the scanning movements. Because obtaining accurate positional data is required for artifact-free images, alignments between focal spot, collimator, and detector are critical. The gantry also contains a cable take-up mechanism to handle the high-voltage cables to the x-ray tube and, for all but fourth-generation systems, the signal cables to the DAS. Most scanners alternately rotate clockwise and counterclockwise to acquire data. In most CT scanners, the complete gantry can be tilted ±20° to the vertical to allow the imaging of planes other than perpendicular to the long axis of the patient.
The table on which the patient is placed is motorized to provide for accurate positioning of the required body plane in the scan plane, and for automatic incrementation between scans. Manual positioning can usually be accomplished by light beam markers; longitudinal movement can be remotely controlled to assist in performing a series of scans at set intervals.
COMPUTATIONAL AND DISPLAY SYSTEMS
The computational subsystem usually consists of a general purpose computer and associated peripheral devices, such as disks and magnetic tape drives. In addition, it often includes special computational hardware such as array processors and back projectors. During scanning the computer functions as a digital controller for the gantry and table motions, x-ray generation, and data acquisition. Patient throughput, the ability of the system to perform data acquisition, image reconstruction, and display quickly, can be improved by careful attention to data paths and by including large memories to hold raw projection data and reconstructed images. The computational requirements for fast image reconstruction with no sacrifice in image quality have necessitated using array processors for the calibration and filtering operations of image reconstruction, and using special-purpose hardware to perform the back projection operation. Depending on the requirements, array processors able to perform 5 million to 50 million floating point operations per second are used.
Display subsystems consist of a display controller, image memory, digital-to-analog convertor (DAC), and monitor (CRT). As described elsewhere, CT image data consist of numbers among - 1000 to 3000. The human eye, however, cannot distinguish between more than 256 gray levels. The digital numbers are converted to analog signals using an eight-bit DAC, and a window level control is used to control the mapping of CT numbers into gray levels. Software for calculating the mean and standard deviation within a region and for image manipulations such as filtering and magnification is often included as part of the display program.
Related terms with CT Scanning
During the discussion of components, the effect of scanning geometry was mentioned.
PARAMETERS AFFECTING PERFORMANCE
The major components of a CT system have been briefly described above from the viewpoint of a design engineer, in which the requirements and advantages or disadvantages of the components were considered. From the viewpoint of the user, it is the overall system performance that is important.
Spatial resolution is characterized by the ability to visualize individual, small, high-contrast objects that are spaced closely together. Resolution is usually measured by using phantoms consisting of sets of air-filled holes or bars in Plexiglas.
In general, there will be a resolution loss toward the edges of the field of view. Loss of radial resolution results mainly from an enlargement of the apparent focal spot size toward the edges of the fan beam.
LOW CONTRAST DETECTABILITY
Low contrast detectability is defined as the smallest object visible at a given percent contrast level. It involves both image noise and spatial information.
Scan time is the time necessary to acquire projection data. Faster scan times can improve image quality by minimizing the effect of peristaltic motion and of moving organs in abdominal studies and in trauma. Faster scan times also increase the potential number of scans that can be performed during peak enhancement periods following contrast injection.
The radiation dose at any point in the region of the object refers to the energy absorbed per unit mass. The radiation dose can vary considerably in different parts of the object. Not only does it vary in a direction perpendicular to the section, but it also varies within the scan plane.
Any deterministic structure, pattern, or CT number variation in the reconstructed image that does not correspond to a structure in the object scanned is an artifact. Artifacts always detract from image quality, and in some cases could result in a misinterpretation of a pathologic condition. Some artifacts, such as beam hardening and partial volume, are inherent in the physics of CT imaging.