computed tomography equipment techniques 2.pdf

Middle Technical University (MTU) College of Health and Medical Techniques -Baghdad Radiological Techniques Department Computed Tomography EquipmentsTechniques Second stage/ 2nd coarse Title: Basic principles of CT Scanners : Generations of CT First-generation Second-generation Third-generation Four...

Middle Technical University (MTU) College of Health and Medical Techniques -Baghdad Radiological Techniques Department Computed Tomography EquipmentsTechniques Second stage/ 2nd coarse Title: Basic principles of CT Scanners : Generations of CT First-generation Second-generation Third-generation Fourth-generation Fifth-generation CT , electron beam (EBCT) Name of the instructor: Lec. Dr. Lamyaa Fadhil Abdul Hussein Target population: Students of second class 9 Introduction: Generations of CT scanners CT scanners were first introduced in 1971 with a single detector for brain study under the leadership of Godfrey Hounsfield, an electrical engineer at EMI (Electric and Musical Industries Ltd). Thereafter, it has undergone several changes with an increase in the number of detectors and decrease in the scan time. The changes were majorly on the X-ray tube and detector arrangements. Scientific Content: A Major adjustment in the technology of CT scanners was manifested in: Tube orientation and shape of beam (from pencil beam through narrow beam to fan beam) Number of detectors (from single detectors to multiple detectors). Detector arrangement. (Parallel-Beam Geometry) 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 directing across the 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 10 advantages of this design are simplicity, good view-to-view detector matching, ameters (such as resolution and contrast), and the highly collimated beam provides excellent rejection of radiation scattered in the patient. This scanner was limited because; 1. Only head scans could be performed. 2. Generates a lot of heat, therefore, require an elaborate cooling system. 3. Scan time was very slow. About 1 minute per slice therefore the duration of scan (average): 25-30 mins. Figure (1) : First Generation: Parallel-Beam Geometry. 11 Second Generation: (Fan Beam, Multiple Detectors) Second-generation CT systems use the same translate/r 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 wide range sizes can be easily scanned with the second-generation scanners. The reconstruction algorithms are slightly more complicated than those for first-generation algorithms because they must handle fan-beam projection data. Figure (2) : Second Generation: Fan Beam, Multiple Detectors. Third Generation: (Fan Beam, Rotating Detectors) 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) 12 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 bigger amount of sensors in the detector array. Figure (3) : Third Generation: Fan Beam, Rotating Detectors Fourth Generation: (Fan Beam, Fixed Detectors) 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. 13 Figure (4): Fourth Generation: Fan Beam, Fixed Detectors. Figure (5): The Four Generations of CT scan 14 Fifth Generation: (Electron beam scanning EBSCT) 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 beams 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. Projections data can be acquired in approximately (