computed tomography equipment techniques 6.pdf

Middle Technical University (MTU) College of Health and Medical Techniques -Baghdad Radiological Techniques Department Computed Tomography EquipmentsTechniques Second stage/ 2nd coarse Title: CT system design: (SSCT & MSCT) X-Ray imaging system (gantry) X-Ray Tube , X-Ray tubes in MSCT (Straton x-ra...

Middle Technical University (MTU) College of Health and Medical Techniques -Baghdad Radiological Techniques Department Computed Tomography EquipmentsTechniques Second stage/ 2nd coarse Title: CT system design: (SSCT & MSCT) X-Ray imaging system (gantry) X-Ray Tube , X-Ray tubes in MSCT (Straton x-ray tube) Name of the instructor: Lec. Dr. Lamyaa Fadhil Abdul Hussein Target population: Students of second class 41 Scientific Content: CT system design The basic equipment configuration for CT represented by three major systems: the imaging system, the computer system, the image display, recording, storage, and communication system. The three major systems are housed in separate rooms, as follows: 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 storag Today, CT scanners are typically housed in similar physical spaces that contain the three system components identified in Figure 1. Fig.1 Components of a CT imaging system. 1, Gantry; 2, patient couch; 3, integrated console; 4, optical disk system including cassette storage; 5, high-speed processor system; 6, x-ray high-voltage generator; 7, couch control unit; 8, system transformer I; 9, system transformer II; 10, patient observation system. 42 Fig 2: Basic equipment configuration for CT, showing the major technical components. The imaging system The purpose of the imaging system is to produce x rays, shape and filter the x-ray beam to pass through only a defined cross section of the patient, Detect and measure the radiation passing through the cross section, Convert the transmitted photons into digital information. The major component of the imaging system is the gantry, that comprises several components housed in it, represented by; the x-ray tube and generator, collimators, filter, detectors, and detector electronics. The x-ray tube and generator are responsible for x-ray production. The radiation beam that emanates from the tube is filtered through a specially designed filter that protects the patient from low-energy rays and ensures beam uniformity at the detectors. The collimators help define the slice thickness and restrict the x-ray beam to the cross section of interest. The detectors capture the x-ray photons and convert them into electrical signals (analog information); the detector electronics, or data acquisition system (DAS), converts this information into digital data. 43 Gantry The gantry assembly is the largest of these systems. It is a rotating mounted scan frame that surrounds the patient in a vertical plane. Two important features of the gantry are the gantry aperture and the gantry tilting range. The gantry aperture is the opening in which the patient is positioned during the scanning procedure. 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.3) 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. Fig.3: the gantry tilting The gantry also includes a set of laser beams to aid patient positioning. There are 3 types of lasers used in the CT Scan which are : Internal laser Wall-mounted laser Overhead laser. When lasers are positioned at zero setting, their intersection point is coincident with the center of the scan plane. 44 A) Internal laser All scanners contain an internal laser to identify the scan plane. This internal laser is mounted at the scanner bore of the CT scan machine. This type of laser used to mark the patient during the scan & treatment process. B) Wall-mounted Laser These types of laser usually situated at the right and left of the room to align the patient and the couch. C) Overhead Laser These lasers also have the same function as wall-mounted laser which is used to mark the patient. It may represent isocenter, field corners or markers. It is projecting at the same fixed distance as lateral laser bot orthogonal to the scan plane. These lasers are always capable of lateral movement because the scanner couch may not. Fig.4: Types of lasers used in the CT Scan. 45 The essential parts of the gantry include: high-voltage generator x-ray tube, x-ray beam filters collimators detector array, patient support couch, slip rings DAS Mechanical support for each. x-ray system These subsystems receive electronic commands from the operating console and transmit data to the computer for image production and postprocessing tasks. high-voltage Generator Previously, CT scanners use three-phase power for the efficient production of x rays, but now use high-frequency generators, which are small, compact, and more efficient than conventional generators. These generators are located inside the CT gantry. In some scanners, the high-fre quency generator is mounted on the rotating frame with the x-ray tube; in others it is located in a corner of the gantry and does not rotate with the tube. In a high-frequency generator (Fig.5), the circuit is usually referred to as a highfrequency inverter circuit. The low-voltage, low-frequency current (60Hz) from the main power supply is converted to high- voltage, high-frequency current (500 to 25,000 Hz) as it passes through the components, as shown in Figure 5. Each component changes the lowvoltage, low- frequency AC waveform to supply the x-ray tube with high-voltage, highfrequency direct current of almost constant potential. After high-voltage rectification and smoothing, the voltage ripple from a high-frequency generator is less than 1%. This makes the high- frequency generator more efficient at x-ray production than predecessor. 46 its Fig. 5: The basic components of a high-frequency generator used in modern CT scanners. The x-ray exposure technique obtained from these generators depends on the generator power output. The power ratings of CT generators vary and depend on the CT vendor; however, typical ratings can range from 20 to 100 kilowatts. More recently CT manufacturers have generators capable of 120 kW. An output capacity of, say, 60 kW will provide a range of kilovolt and milliampere settings, where 80 and 120 to 140 kV and 20 to 500 milliamperes (mA) are typical. X-Ray Tubes The radiation source requirement in CT depends on two factors: (1) radiation attenuation, which is a function of radiation beam energy, the atomic number and density of the absorber, and the thickness of the object. (2) The quantity of radiation required for transmission. X-ray tubes satisfy this requirement. Rotating anode x-ray tubes have become common in CT because of the demand for increased output. These rotating anode tubes, an example of which is shown in Fig. 6, have large-diameter anode disk to facilitate the spatial resolution requirements of the scanner. The disk is usually made of a rhenium, tungsten, and molybdenum (RTM) alloy and other materials with a small target angle (usually 12 degrees) and a rotation 47 speed of 3600 revolutions per minute (rpm) to 10,000 rpm (high-speed rotation). Fig.6, B, shows an upgraded tube based on the technology used in the tube shown in Fig.6, A. Fig. 6: A modern rotating anode x-ray tube used in CT scanners. The tube shown in (B) is an upgraded tube based on the technology used in the tube shown in (A). (A, Courtesy Philips Medical Systems, Shelton, Conn; B, Courtesy Philips Healthcare.). The introduction of spiral/helical CT with continuous rotation scanners has placed new demands on x-ray tubes. Because the tube rotates continuously for a longer period, compared with conventional scanners, the tube must be able to sustain higher power levels. Several technical advances in component design have been made to achieve these power levels and deal with the problems of heat generation, heat storage, and heat dissipation. For example, the tube envelope, cathode assembly, anode assembly including anode rotation, and target design have been redesigned. The glass envelope ensures a vacuum, provides structural support of anode and cathode structures, and cathode. Although the provides high-voltage insulation borosilicate glass provides good between the anode and thermal and electrical insulation, electrical arcing results from tungsten deposits on the glass caused by vaporization. Tubes with metal envelopes, which are now common, problem. 48 solve this Metal envelope tubes have larger anode disks; for example, the tube shown in Figure (6) has a disk with a 200-mm diameter compared with the 120- to 160-mm diameter typical of conventional tubes. This feature allows the technologist to use higher tube currents. Heat-storage capacity is also increased with an improvement in heat dissipation rates. The cathode assembly consists of one or more tungsten filaments positioned in a focusing cup. The anode assembly consists of the disk, rotor , hub, and bearing assembly. The large anode disk is thicker than conventional disks; the three basic designs are (Fig.7): The conventional all-metal disk, The brazed graphite disk, The chemical vapor deposition (CVD) graphite disk. In conventional tubes, the all-metal disk (Fig.7, A) consists of a base body made of titanium, zirconium, and molybdenum (TZM) with a focal track layer of 10% rhenium and 90% tungsten. It can transfer heat from the focal track very quickly. Unfortunately, tubes with this all-metal design cannot meet the needs of spiral/helical CT imaging because of their weight. The brazed graphite anode disk (Fig. 7, B) consists of a tungsten-rhenium focal track brazed to a graphite base body. Graphite increases the heat- storage capacity because of its high thermal capacity, which is about 10 times that of tungsten. The material used in the brazing process influences the operating temperature of the tube, and the higher temperatures result in higher heat-storage capacities and faster cooling of the anode. Tubes for spiral/helical CT scanning are based mostly on this type of design. Is a metal-joining process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, with the filler metal having a lower melting point than the adjoining metal. 49 Fig. 7: Three types of disk designs for modern x-ray tubes used in CT scanners: (A) conventional all-metal disk; (B) brazed graphite anode disk; and (C) CVD graphite anode disk. 50 The final type of anode design (Fig. 7, C) is also intended for use in spiral/helical CT x-ray tubes. The disk consists of a graphite base body with a tungsten-rhenium layer deposited on the focal track by a chemical vapor process. This design can accommodate large, lightweight disks with large heat-storage capacity and fast cooling rates. The purpose of the bearing assembly is to provide and ensure smooth rotation of the anode disk. In modern CT, smooth rotation of the disk is improved by using a liquidbearing method (Fig. 8). The stationary shaft of the anode assembly consists of grooves that contain gallium-based liquid metal alloy. During anode rotation, the liquid is forced into the grooves and results in a hydroplaning effect between the anode sleeve and liquid. The purpose of this bearing technology is to conduct heat away from the x-ray tube more efficiently than conventional ball bearings with improved tube cooling. Additionally, the liquid-bearing technology is free of vibrations and noise. The working life of the tubes can range from about 10,000 to 40,000 hours, compared with 1000 hours, which is typical of conventional tubes. Fig. 8: The anode assembly of a modern x-ray tube used in CT imaging. The main parts of the assembly are the disk, rotor, and bearing assembly that contains liquid metal lubricant. 51 Straton X-Ray Tube: A New X-Ray Tube for MSCT Scanning As it is known, the fundamental problem with conventional x-ray tubes is that of heat dissipation and slow cooling rates. Efforts have been made to deal with these problems by introducing various designs, such as large anode disks and the introduction of the compound anode design (RTM disk), which has higher heatstorage capacities and cooling rates. Additionally, as gantry rotation times increase, higher milliampere values are needed to provide the same milliamperes per rotation. As the electrical load (milliamperes and kilovolts) increases, faster anode cooling rates are needed. Despite these efforts, the problems of heat transfer and slow cooling rates still persist with MSCT scanners. To overcome these problems, a new type of x-ray tube called Straton x-ray tube has been introduced for use with MSCT scanners. This unique and revolutionary tube was designed by Siemens Medical Solutions (Siemens AG Medical Solutions, Erlangen, Germany). Fig.9: The Straton x-ray tube, a new x-ray tube for MSCT scans. A photograph of the Straton x-ray tube is shown in Fig.9. As can be seen,. The tube is compact in design to much smaller than conventional x-ray tubes described earlier. This size ensures a fast gantry rotation of 0.37 seconds. 52 In the Straton x-ray tube, it is encased in a protective housing that contains oil for cooling, and the motor provides a rotation of the entire tube (which is immersed in oil). It is important to note that the anode is in direct contact with the oil (directly cooled tube). Because the anode is in direct contact with the circulating oil, very high cooling rates will result in, as illustrated in Fig.10. Fig. 10: The Straton x-ray tube. This diagram shows the anode and cathode structures, deflection coils, an electron beam, and a motor. Another important feature of the Straton x-ray tube relates to the electron beam from the cathode. This beam is deflected to strike the anode at two precisely located focal spots (Fig.11) that vary in size. 1.1 mm, and 0.7 mm. The electron beam alternates at about 4640 times per second to create two separate x-ray beams that pass through the patient and fall on the detectors. 53 Fig. 11: An important feature of the Straton x- ray tube is that the electron beam from the cathode is deflected to strike the anode at two precisely located focal spots that vary in size. The Advantages of Straton x-ray tubes Better Heat Dissipation Various size multiple focal spot Longer tube life high-speed volume scanning is possible Can be used in high KV and high mA technique for prolonged Duration. Ie; (High mAs & long exposure times for increasing lengths of anatomic coverage). 54