Module 2 For educational and institutional use. This transcript is licensed for noncommercial, educational in-house or online educational course use only in educational and corporate institutions. Any broadcast, duplication, circulation, public viewing, conference viewing or Internet posting of this product is strictly prohibited. Purchase of the product constitutes an agreement to these terms. In return for the licensed use, the Licensee hereby releases, and waives any and all claims and/or liabilities that may arise against ASRT as a result of the product and its licensing. CT Basics: Equipment and Instrumentation Module 2 1. Title Screen 2. License Agreement 3. Objectives After completing this module, you will be able to: 1) Explain the major components of the computed tomography computer system. 2) Trace the sequence of events in CT scanning from the application of the electrical current to the radiographic tube to image display. 3) Explain how adjusting operator console parameters affects CT image data. 4) Discuss the elements of a digital image. 4. Major CT Scanner Components Modern CT scanners are an elegant blend of function and technology. To the untrained eye, a CT scanner appears to be the model of simplicity — a patient lies on the table, the table moves through the scanner and an image appears. On further inspection, however, the true complexity of the CT systems becomes apparent. The observer is astonished at the intricacy of design and the need for all pieces of CT equipment to work perfectly in unison to produce the highest quality diagnostic image with the least possible dose to the patient. Regardless of CT vendor, each computed tomography room contains three major pieces of equipment: an imaging system that consists of a gantry and patient table, a computer capable of processing the CT image data, and an operator’s console that controls the entire imaging process and displays the final image. These three components work together to create and manipulate the x-rays that are transformed into a digital image. This digital image then can be reconstructed in various ways to diagnose disease processes within a patient’s body. 5. Patient Table The patient table, sometimes referred to as the patient couch, is made of a material that will absorb the least amount of radiation possible while still supporting the weight of the patient. Carbon fiber is the most common material used in CT tables. The table does much more than simply transport the patient into the scanner; its movements determine which part of a patient’s anatomy is scanned and the thickness of the image sections. The table even keeps the patient safe during the examination. Because the table must move precisely and protect the patient, it is important to know your table’s weight limit. Most modern scanners can handle patients weighing up to 450 pounds, but you should check the specifications of your equipment before imaging. 6. X, Y and Z Axes For the CT scanner, direction is based on the patient and the table the patient is lying on. Three coordinates define direction: the x, y and z axes. The x-axis is referred to as the sagittal plane because it ©2010 ASRT. All rights reserved. CT Basics: Module 2 divides a patient lying on the table into a left side and a right side. The y-axis, or the coronal plane, divides the patient’s body into anterior and posterior sections. The z-axis is called the axial plane because it divides the body into superior and inferior parts. In a multislice CT scan, the gap between each slice is called the z-gap. The z-gap is determined by the pitch. If the pitch increases, the z-gap decreases and image quality improves. 7. Gantry The largest piece of CT equipment is the large, circular apparatus known as the gantry. The gantry houses most of the functional equipment parts needed to acquire image data. The x-ray tube, detectors and even most generators are found inside the gantry. The gantry also contains a cooling system that allows the x-ray tube to operate at increased speed for a longer time. All of these components rotate within the gantry as the patient moves through a large hole known as the aperture. To line up pertinent anatomy during the scan, the gantry can tilt forward or backward between 12° and 30°. 8. X-ray Tube The x-ray tube is mounted inside the gantry and rotates continuously around the patient. The introduction of multislice CT scanners increased heating and cooling demands on the x-ray tube. Some manufacturers replace the tube on an annual basis to avoid tube arcing. Modern CT tubes can last for 150,000 to 200,000 slices, which in a busy department equals approximately one year of scanning. The tube consists of two major components: a cathode and an anode. The electron beam travels from the cathode and strikes a target on the anode. The tube then generates high-energy photons from the anode. The cathode contains compact tungsten filaments that set the current of the electrons flowing to the anode. The temperature of the tungsten filaments affects the current of these electrons. The anode assembly consists of a rotor, hub and a bearing unit that permit rapid rotating speeds of 3,600 to 10,000 rotations per minute. The rotating anode usually is composed of an alloy of tungsten, molybdenum and rhenium, and the target area of the anode is made of tungsten. Tungsten is an ideal metal for use in multislice CT scanners because of its high heat tolerance and high melting point of 3,400° C. Tungsten also dissipates heat quickly so that the target area can cool rapidly and be ready for the next bombardment of electrons. The target is fixed at an angle of approximately 11° to 12°. The cathode and anode are enclosed in a metal tube. Glass tubes have been used in the past, but they form tungsten deposits that can result in quicker tube degradation. Metal envelopes are able to withstand higher tube currents. Click the button to see an x-ray tube in operation. The CT technologist can change the tube voltage, kilovoltage (kV), and tube current, milliamperes (mA), that move the electron beam from the cathode to the anode. Changing the mA changes the cathode filament temperature so that the cathode produces the desired number of electrons. The technologist can control the energy level of these electrons by adjusting the kV. Altering the kV affects the penetrating power of the electrons that pass through the patient’s body. Use the slider bars on this animation to see how changing the mAs or the kVp will affect the output from the x- ray tube. 9. Generator The generator is responsible for the high voltage needed to create x-rays. It produces voltages from 90 to 140 kV, with a typical CT scan using 120 kV. Some generators are found outside the gantry in a fixed location within the examination room, while others rotate next to the x-ray tube. Most modern generators are so compact and efficient that the unit can be located inside the gantry. ©2010 ASRT. All rights reserved. CT Basics: Module 2 Generators convert the low-voltage alternating current to a high-voltage direct current that powers the x-ray tube with constant energy. The incoming power supply of 60 hertz (Hz) is transformed into a high-voltage, high-frequency current of 500 to 25,000 Hz. The power demands on a multislice CT unit are enormous, typically 20 to 100 kilowatts (kW). A 60-kW generator produces enough voltage to provide 80 to 120 kV and 20 to 500 mA. 10. Detectors The detectors, which measure the patient’s x-ray attenuation data, are located opposite the x- ray tube. Detectors are very sensitive. They recognize the ionizing radiation that has passed through the patient, capture the signal and then transport the signal to the digitizer. X-rays produce an analog signal that must be converted into a digital signal so that the computer can read the information and produce the final CT image. The detector geometry is the relationship of the tube, the beam shape and the detectors. As you can see on this page, current CT scanners use hundreds of detectors that are arranged in a curved array and aligned with the x-ray tube. Both units rotate simultaneously around the patient. Detector efficiency determines how accurately the CT image is reproduced every time and with every patient. Different terms describe the detector’s efficiency. Capture efficiency is the measurement of how efficiently the detectors gather the photons coming from the patient. Absorption efficiency describes how efficiently the photons are captured by the detectors. Stability is the measurement of how consistently the detectors respond. The response time is how fast the detectors record the photons and how quickly they recover for the next event. Dynamic range refers to the accuracy of the detector’s response to both high-energy and low-energy radiation. Finally, reproducibility describes how consistently the detectors respond to similar transmitted radiation events. 11. Detectors Two types of detectors currently are used in multislice CT scanners: gas ionization detectors and scintillation detectors. Gas ionization detectors convert the x-rays directly into an electrical signal. Scintillation detectors first change the x-rays into light, and then the light is transformed into an electrical signal. Scintillation detectors are the industry standard because they are more sensitive and they need less frequent calibration than gas-filled detectors. Scintillation detectors use a solid-state scintillation crystal mounted next to a photomultiplier tube.