Lecture (4)....._Basic principles of CT.... .pdf

Basic Principles of CT Dr. Nahla Atallah  TABLE OF CONTENTS 1. Scanner Generation 2. Detector Electronics 3. Patient table 4. Collimator 5. Detectors  Scanner Generation The configuration of the x-ray tube to the detectors determines scanner g...

Basic Principles of CT Dr. Nahla Atallah  TABLE OF CONTENTS 1. Scanner Generation 2. Detector Electronics 3. Patient table 4. Collimator 5. Detectors  Scanner Generation The configuration of the x-ray tube to the detectors determines scanner generation. The first generation. A thin x-ray beam passed linearly over the patient, and a single detector followed on the opposite side of the patient. The tube and detector were then rotated slightly, and the process was repeated until a 180° arc was covered. Scan times were very long. This design is no longer in use. The second-generation design is one in which the x-ray beam also passed linearly across the patient before rotating. However, a fan- shaped x-ray beam was used, rather than the thin beam used with first-generation designs. Only part of the field of view could be covered with this fan beam. A detector array was also incorporated in the second-generation design. Although scan times were shorter than that of the original design, they were still very long. This type of design is also no longer used.  The next advance in CT technology brought the third generation design. This design consists of a detector array and an x-ray tube that produces a fan-shaped beam that covered the entire field of view and a detector array (Fig. 2-7). Reference detectors are typically located at either end of the detector array to measure the un-attenuated x-ray beam. The third-generation design made it no longer necessary to translate the beam and detector as both could move in a circle within the gantry.  The rotating detector design allows all of the readings that make up a view to be recorded instantaneously and simultaneously. This greatly reduced scan times and helped to reduce artifact resulting from patient motion. An advantage of the third-generation system is that the tube is directly focused on the detector array (Fig. 2-8). The fixed relationship between the x-ray source and the detectors allow the beam to be highly collimated, which greatly reduces scatter radiation, thereby improving image quality. A disadvantage of the third- generation design (as compared with the fourth-generation design described in the next paragraph) is the more frequent occurrence of ring artifacts.  Because the same bank of detectors is used repeatedly, even a very small misalignment of a single detector will result in visible ring artifact (more about artifacts in Chapter 7). Third-generation systems are sometimes referred to as rotate-rotate scanners. The third-generation..  The design is the most widely used configuration in the industry today. All new multi-detector CT systems sold in the United States use the third-generation design. Fourth-generation scanners use a detector array that is fixed in a 360° circle within the gantry. The tube rotates within the fixed detector array and produces a fan-shaped beam (Fig. 2-9). Although many more detector elements are included in this design, the number of detectors in use at any one time is controlled by the width of the beam. In the stationary detector design, the readings that make up a view are recorded consecutively during approximately one-fifth of the scan time.  Because the emerging beam does not strike the detectors at exactly the same time, motion artifacts are more of a problem. Fourth generation systems often use over scans to address this problem. An over scan is a tube arc greater than 360°. The use of an over scan technique will increase the radiation dose to the patient. In addition, because the tube is closer to the patient, the same milliampere-seconds (mAs) and kilovolt-peak (kVp) setting will produce a higher dose when a fourth- generation system is used (as compared with the same settings used in a third-generation system). However, because the x-ray source is closer to the patient, techniques necessary to produce an adequate image are generally somewhat lower than that used on a third generation system. Fourth-generation scanners may also be called rotate-only systems. Many variations of these basic designs have been introduced and then abandoned.  The only other design currently in use is called electron beam imaging, also referred to as EBCT or ultrafast CT. It differs from conventional CT in a number of ways. This system, which was originally produced by Imatron, uses a large electron gun as its x-ray beam source. A massive anode target is placed in a semicircular ring around the patient. Neither the x-ray beam source nor the detectors move, and the scan can be acquired in a short time (Fig. 2-10). Invented in the 1980s, its superior speed compared with traditional CT scanners of the time made it particularly suited to cardiac imaging.  However, shortfalls in spatial resolution kept EBCT from use in routine imaging, dramatically limiting the technology’s clinical versatility. Additional drawbacks were high cost and difficulties obtaining insurance reimbursement. The future of EBCT is uncertain as the newer multi-detector row technology applied to third-generation scanners has increased scanning speed so that they compare favorably with EBCT.  Detector Electronics X-ray photons that strike the detector must be measured, converted to a digital signal, and sent to the computer. This is accomplished by the data-acquisition system (DAS), which is positioned within the gantry near the detectors. Signals emitted from the detectors are analog (electric), whereas computers require digital signals. Therefore, one of the tasks of the DAS is to convert the analog signal to a digital format. This is accomplished with the aptly named analog-to-digital converter or ADC.  To measure the x-ray photons that have penetrated the patient, the detectors are sampled many times, as many as 1,000 times per second by the DAS. The number of samples taken per second from the continuous signal emitted from the detector is known as the sampling rate, sample rate, or sampling frequency. Artifacts, such as streaking, can appear on the image if the number of samples is insufficient.  PATIENT TABLE The patient lies on the table (or couch, as it is referred to by some manufacturers) and is moved within the gantry for scanning. The process of moving the table by a specified measure is most commonly called incrementation, but is also referred to as feed, step, or index. Helical CT table incrementation is quantified in millimeters per second because the table continues to move throughout the scan. The degree to which a table can move horizontally is called the scannable range, and will determine the extent a patient can be scanned without repositioning  A numeric readout of the table location relative to the gantry is displayed. When the patient is placed within the gantry, an anatomic landmark, such as the xiphoid or the iliac crest, is adjusted so that it lies at the scan point. At this level, the table is referenced, which means that the table position is manually set at zero by the technologist. Accurate table referencing helps to maintain consistency between examinations.  For example, if a lesion is seen on an image that is 50 mm inferior* to the xiphoid landmark (zero point), the patient is removed from the gantry, and a ruler is used to measure 50 mm inferior from the xiphoid. This point provides an approximation of the location of the lesion. This system is also helpful if the scan will be repeated at a later date, exclusively through the area of interest determined on the earlier scan. For this reason, the setting of landmarks must be consistent among CT staff.  The specifications of tables vary, but all have certain weight restrictions. If the patient’s weight exceeds the specified limits, scanning is often still possible. However, the table increments may not be as accurate. This problem affects small table increments more than those 5 mm or larger. On most scanners, it is possible to place the patient either head first or feet first, supine or prone. Patient position within the gantry depends on the examination being performed. Various attachments are available for specific types of scanning procedures. For example, attachments for direct coronal scanning of the head and for therapy planning are common.  SUMMARY The first step in creating a CT image is to acquire data that result from the attenuation of the x-ray beam as it passes through the patient to strike the detector. The mechanisms housed within the gantry and the patient table are the components necessary for data acquisition. Figure 2-11 diagrams the basic data-acquisition scheme in CT.  References  Radiologic Science for Technologists: Physics, Biology, and Protection, 11th edition.  Computed tomography for technologists a comprehensive text, by Lois Romans