Computed Tomography Equipment Techniques PDF

‫جامعة العين‬ ‫كلية التقنيات الصحية والطبية‬ ‫قسم تقنيات االشعة والسونار‬ C o m p u t e d Tomography E q u i p m e n t T e c h n i q u e s Basic principles of CT Scanners : Generations of CT Dr. Hussein A.Dakhild...

‫جامعة العين‬ ‫كلية التقنيات الصحية والطبية‬ ‫قسم تقنيات االشعة والسونار‬ C o m p u t e d Tomography E q u i p m e n t T e c h n i q u e s Basic principles of CT Scanners : Generations of CT Dr. Hussein A.Dakhild Ph.D. Medical Imaging Technology  Basics Principle The basic principle behind CT is that the internal structure of an object can be reconstructed from multiple projections of the object. The ray projections are formed by scanning a thin cross-section of the body with a narrow X- ray beam and measuring the transmitted radiation with a sensitive radiation detector. CT scanning is a systematic collection and representation of projection data.  Parts of CT Scan Machine The basic technology employed in the CT scanner is designed to provide a source of X-rays to be transmitted through the patient and then detected by the detectors. The CT system consists of: 1. a computer workstation for operation of the scanner, 2. image processing computers, 3. electronic cabinets, 4. the gantry 5. and the patient imaging table.  Composition Of Gantry The gantry houses the key components of the scanner. Production of the x-ray beam and detection and acquisition of the beam must be located within the rotating portion of the gantry. The fan-beam x-ray tube sits opposite the detector array within the rotating gantry. The three-phase power generator is also within the gantry module. The X-ray tube in a CT scanner is designed to produce a fan beam of X-rays approximately as wide as the body. Tissue attenuation is measured over a large region from one X-ray tube position. On the opposite side of the patient is the detector array that measures the strength of the X-ray beam at various points laterally across the body.  Generations Of Computed Tomography Computed Tomography (CT) has undergone several generations of technological advancements since its inception. Each generation represents a significant improvement in terms of image quality, speed, and capabilities. Here is an overview of the different generations of CT: 1. First-generation 2. Second-generation 3. Third-generation 4. Fourth-generation 5. Fifth-generation CT, electron beam (EBCT)  First-generation First-generation CT systems are characterized by a single X-ray source (pencil beam or parallel-beam geometry). Multiple measurements of X-ray transmission are obtained using a single highly single collimated X-ray pencil beam and detector directed across the patient isocenter. Both, the source and the detector, translate simultaneously in a scan plane, where the beam is translated in a linear motion across the patient to obtain a projection profile. This process (translate–rotate scanning motion) is repeated for a given number of angular rotations, by approximately 1 degree, and another projection profile is obtained until the source and detector have been rotated by 180 degrees. The advantages of this design are simplicity, good view-to-view detector matching, flexibility in the choice of scan parameters (such as resolution and contrast), and the highly collimated beam provides excellent rejection of radiation scattered in the patient.  Limitations of first generation 1. Only head scans could be performed. 2. Generates a lot of heat and, therefore, requires an elaborate cooling system. 3. Scan time was very slow. About 1 minute per slice therefore the duration of scan (average): 25-30 mins.  Second-Generation Scanners Second-generation CT systems use the same translate/rotate scan geometry as the first generation. The difference here is that a pencil beam is replaced by a fan beam and a single detector by multiple detectors (5-30) so that, a series of views can be acquired during each translation, which leads to correspondingly shorter scanning times, about 20 seconds per slice therefore duration of scan (average): less than 90 sec. So, objects of a wide range of sizes can be easily scanned with second-generation scanners. The reconstruction algorithms are slightly more complicated than those for first-generation algorithms because they must handle fan-beam projection data.  Third-Generation Scanners A fan beam of X-rays is rotated 360 degrees around the isocenter. No translation motion is used; however, the fan beam must be wide enough to completely contain the patient. A curved detector array consisting of several hundred independent detectors (500-1000) is mechanically coupled to the X-ray source, and both rotate together. As a result, these rotate-only motions acquire projection data for a single image in as little as 1 s. Typically, third-generation systems are faster than second-generation systems. The detectors here have incorporated a bigger amount of sensors in the detector array.  Fourth-Generation Scanners In a fourth-generation scanner, the x-ray source and fan beam rotate about the isocenter, while the detector array remains stationary. The detector array consists of 600 to 4800 (depending on the manufacturer) independent detectors in a circle that completely surrounds the patient. Scan times are less to those of third-generation scanners (~ 2sec.).The number of views is equal to the number of detectors. Two detector geometries are currently used for fourth-generation systems: (1) a rotating x-ray source inside a fixed detector array and (2) a rotating x-ray source outside a nutating detector array Both third- and fourth-generation systems are commercially available with advanced configurations.  Fifth-Generation Scanners Fifth-generation scanners are unique in that the X-ray source becomes an integral part of the system design. The detector array remains stationary, while a high–energy electron beam is electronically swept along a semicircular tungsten strip anode. X-rays are produced at the point where the electron beam hits the anode, resulting in a collimated fan beam X-rays that rotates about the patient with no moving parts. Projection data can be acquired in approximately (