CT Basics: Equipment and Instrumentation Module 2

Transcription

1 Module 2 Transcript 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.

2 CT Basics: Equipment and Instrumentation Module 2 1. Title Screen Welcome to CT Basics Module 2 Equipment and Instrumentation. This module was written by Rebecca R. Blankley, R.T.(R)(M)(CT)(MR) 2. License Agreement 3. Objectives After completing this module, you will be able to: Explain the major components of the computed tomography computer system. Trace the sequence of events in CT scanning from the application of the electrical current to the radiographic tube to image display. Explain how adjusting operator console parameters affect CT image data. 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: 1. An imaging system that consists of a gantry and patient table. 2. A computer capable of processing the CT image data. 3. 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.

3 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 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 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

4 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. 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. The crystal absorbs the x-ray photons and then emits flashes of light that are directly proportional to the energy of the collected photons. The photomultiplier amplifies the light and converts the light into a digital signal for computer processing. Scintillation detectors are associated with lower patient dose and reduced image noise. 12. Collimators The goal of the collimators is to provide a consistent beam width, which is defined by slice thickness. The beam width is measured in the z-axis at the center of the rotation for a single-row detector array. Collimation limits the amount of x-ray exposure to the patient by reducing scatter radiation and improves image contrast. CT scanners contain both prepatient and postpatient collimators.

5 Prepatient collimators are located just outside the x-ray tube where the beam leaves the tube. These collimators, which are made of thick metal plates, define beam width and restrict the shape of the x-ray beam before it ever reaches the patient. In single-slice CT scanning, the collimators define the thickness of the cross-sectional slice. Prepatient collimators also define the thickness of the x-ray beam in multislice CT scanning, which spreads the beam over the entire detector array, or multiple rows of detectors. In multidetector CT scanning, however, image reconstruction rather than prepatient collimation determines the slice thickness. Postpatient collimators are positioned just above the detector array. These collimators improve image quality and axial resolution. Postpatient collimation also works in conjunction with prepatient collimation to help define slice thickness. If postpatient collimation is reduced, the slice thickness decreases. Thin collimation results in better resolution, but it takes longer to scan a particular area of anatomy. Wider collimation results in lower resolution, but it provides better volume coverage speed. 13. Practice Question 14. Practice Question 15. Scanner Configurations From the development of CT in the 1970s until the present time, all CT scanners have used a fairly similar construction. A gantry containing an x-ray tube and set of detectors rotates around the patient and collects image data that are processed by a computer. Significant developments in modern CT scanner design have led to increased image quality, faster scans times and decreased patient radiation exposure. Helical scanning is a major leap forward in CT scanner construction. Let s briefly look at its development and the unique properties that make helical scanning possible. 16. Historical Development of the Helical Scanner The development of computed tomography required advances in digital computing and special mathematics. The first generation CT scanner developed by Sir Godfrey Hounsfield used an x-ray source and detector to collect data for a single slice. The scanning motion of the first generation equipment was called translate and rotate because the scanner moved across the patient, rotated slightly and then made the next translation. The second generation scanner was a big improvement in design because it had a larger fan beam, but the equipment still used the translate and rotate motion. Third generation CT scanners used an even larger x-ray beam that was capable of covering the entire patient cross-section, and the tube and assembly both rotated around the patient. Current CT scanners have a similar configuration in which the tube and the detectors rotate; however, modern scanners contain multiple detector rows. For each gantry rotation, the scanner acquires multiple sections. Similar to single-slice configurations, the scan can be taken in either a stepand-shoot or helical mode. The biggest advantage to this type of configuration is that large areas of anatomy can be covered more quickly. Modern scanners also have better contrast resolution and less patient motion artifacts than their predecessors. 17. Conventional Axial Scanning Axial scanning is characterized by the start-and-stop rotation of the x-ray tube around the patient. After the data are acquired during a single revolution, the table is indexed to the next position

6 and the process is repeated. This process continues for the duration of the scan until the prescribed anatomy is covered. Click on the button to see a CT scanner in a conventional axial scan mode. Often referred to as step-and-shoot, axial scanning is time consuming, but the method is still used today for brain imaging because it provides better image quality than helical scanning. Most neurologists request axial scanning of the brain because they believe it s superior to other types of CT scanning. 18. Multidetector Computed Tomography (MDCT) Mulitdetector-row CT, or MDCT, scanners are similar to single detector-row scanners in that they obtain images in one rotation of the x-ray tube. The difference is that MDCT scanners cover more anatomy in one rotation because the detector array acquires multiple, parallel slices. For example, for a 16-slice detector, the CT scanner obtains 16 slices per rotation, and then the table is indexed to the next position to obtain the next 16 slices. MDCT is much faster than single-row CT, but not as fast as helical scanning. The disadvantages of MDCT include the need for more contrast to complete a scan, the possibility of misalignment between scans that can affect postprocessing and the chance that anatomy can be missed if patient breathing is not consistent. 19. Volumetric Data Acquisition Volumetric data acquisition involves continuous movement of the x-ray tube as the table moves through the gantry. The beam traces a helical, or spiral, path around the patient, allowing much faster scan times compared to axial scanning. Click on the button to see a CT scanner in a helical scan mode. You can see the x-ray tube rotate continuously around the patient in a helical path as the table moves through the gantry. Spiral and helical are interchangeable terms when describing this type of scanning. Interscan delay refers to the time period between the end of one scan and the beginning of the next scan. The purpose of the delay is to allow time for tube cooling. Interscan delays are less noticeable with multislice CT, and delays don t occur in helical scanning because the scan is continuous. 20. Volumetric Data Acquisition One of the major advantages of helical scanning is that it allows much faster imaging because large volumes of data can be collected during each scan. Helical scan times are now seconds rather than minutes or hours, and scans usually can be completed in a single breath hold. For example, a helical scan from the chest through the pelvis can be obtained in a single breath hold, whereas an axial scan would require many breath holds for that amount of anatomy. A single breath hold reduces the likelihood of patient motion during the scan, as well as problems that arise when patients must hold their breath multiple times. If the patient has to take several breath holds throughout the scan, each breath hold might be different, leading to misregistration artifacts that occurs when the images are reconstructed and the patient s anatomy does not line up properly. Faster scans often require less contrast material. For example, the contrast injection rate during a CT scan for suspected pulmonary embolism is 4 to 5 cc per second. If the area of anatomy is covered quickly, the scan can be stopped before the entire amount of contrast is used. Another advantage of helical scanning is that the images can be reconstructed at a smaller slice width than the slices of the original scan. For instance, if the protocol calls for 3-mm cuts through the abdomen and pelvis, the scan often can be reconstructed using 2-mm slices. The reconstruction has a smoother appearance that no longer shows the stair step artifact found in some reconstructed images. During a helical scan, the x-ray tube must be able to rotate around the patient without stopping.

7 The ability of the tube to rotate continuously was made possible by the development of slip-ring technology. 21. Slip Rings Before the introduction of slip-ring technology, cables connected the gantry components. The gantry rotated and acquired the image, and then the cables had to unwind before the next acquisition to avoid tangling the tube and detectors. Now, slip rings allow the gantry components to be coupled without cables. The x-ray tube rotates continuously around the gantry without hanging up the electronic mechanisms. The technology eliminates the time-consuming, start-and-stop process of earlier CT scanners and permits data acquisition to begin very quickly. Faster scan times led to the development of continuous acquisition exams such as computed tomography angiography. Slip-ring technology made helical CT scanning a reality. Slip rings are composed of electrical conductive rings and brushes. The slip ring transmits an electrical current across the rotating surface. Slip rings supply the electrical power to the x-ray tube and help transfer the signals from the detectors to the computer for image reconstruction. High-voltage slip rings provide greater voltage capacity, typically more than 600 volts. The slip-ring power supply uses either a disk or cylinder design.the disk design consists of conductive rings located within the plane of the rotation of the disk. The brushes transfer electrical power by sliding across and coming in contact with the grooves on the slip ring. Two types of brush designs are common: wire brushes and composite brushes. Wire brushes use conductive wire such as copper to make contact with the ring. The composite brush is a conductive block, often a silver graphite alloy, that acts as a sliding contact. Several different configurations are available to maintain contact between the brushes and the ring. One example is an arrangement of compressions and springs that push the brushes forward onto the stationary slip ring to provide contact. 22. Slip Rings CT scanners use both high-voltage and low-voltage slip rings; the main difference between the two is how each component is positioned within the gantry. Low-voltage slip-ring design uses alternating current (AC). The x-ray signals are transmitted to the slip rings by passing a low-voltage current to the conductive brushes.these brushes are in contact with the stationary ring, or back plate. The slip ring provides power to the high-voltage transformer that powers the x-ray tube. The components are arranged in the following sequence: AC main power > Slip ring > High-voltage generator > X-ray tube. High-voltage slip-ring design is similar to the low-voltage design except the high-voltage generator does not rotate with the x-ray tube. The components are arranged in the following order: AC main power > High-voltage generator > Slip ring > X-ray tube. 23. Practice Question 24. Practice Question 25. Operator Console and Data Acquisition The CT technologist enters instructions into the system console. The system is responsible for controlling the high-voltage generator, the gantry operations and the patient table. The console consists of a liquid crystal display, or LCD, monitor and keyboard. The technologist interacts with the CT scanner by typing on a keyboard, using a mouse or using touch screen sensors. The patient s information, sequence selection and sequence parameters are entered into the console before the scan starts. The console also is used for postprocessing tasks and data transfer to a

8 picture archiving and communication system, or PACS. All this information is processed through the host computer and passed on to the system controller. The operator console contains all of the controls necessary for completing a CT scan. Decisions made on the operator s console have a direct impact on image quality that, in many cases, cannot be reversed. Let s briefly look at some of these controls and see how they affect the CT image. 26. Pitch The pitch is the distance the table travels during one revolution of the x-ray tube. Pitch is expressed as the ratio of distance the table travels per rotation of the total collimated x-ray beam width. If that distance equals the slice thickness (in other words, the thickness of the collimated beam) the pitch is said to be 1:1. A pitch of 1 provides the best image quality because data are collected on all anatomy. Click on the different pitch setting buttons to begin the animation and see the difference between a pitch of 1 and a pitch setting greater than 1. As the pitch increases, the patient moves through the gantry at a faster rate. This is because the helical path is stretched, and so more anatomy can be covered in a shorter period of time. Therefore, when the pitch increases, patient dose decreases because of shorter exposure to the x-ray beam. If the pitch is increased by a factor of 2, the patient dose decreases by one-half. This relationship is only valid if all other parameters remain the same. 27. Scan Field of View (SFOV) The scan field of view, or SFOV, is the actual area of interest selected by the CT technologist before the scan begins. The transmission measurements of this region are recorded during the duration of the scan. The scan field of view determines the number of detectors needed to collect data for a particular scan. The scan field of view, or SFOV, must be larger than the area of interest because any anatomy that falls outside the scan box won t be recorded by the detectors and therefore can t be used in image reconstruction. The result is out-of-field artifacts. The manufacturer often preselects SFOVs with the idea that the technologist will align the scan box to each specified body part. 28. Display Field of View (DFOV) The display field of view, ordfov, is also called the reconstruction field of view. The DFOV is the reconstructed area seen on the image monitor, and it can be equal to or less than the scan field of view. The DFOV is taken from the original scan and can be manipulated with reconstruction algorithms, or particular areas can be zoomed or panned. Using a narrow display field of view shows a smaller area than the scan field of view and can be useful in certain diagnoses. The display field of view affects the resolution and noise of the image. Using a wider DFOV increases the number of photons from which the original data were collected, so noise is reduced but spatial resolution also decreases. As the field of view decreases, spatial resolution improves but image noise increases. To compensate for increased noise, the mas can be increased, but this also increases the radiation dose to the patient. Anatomy also appears larger in a smaller field of view because the area of interest is enlarged to fill the displayed image. 29. Annotation Annotation is the process of marking an image with relevant information. Annotations are used to include additional information regarding the study that might be beneficial to the radiologist. CT technologists can annotate a scan by selecting a predetermined note or using the comments section to make multiple notations. For example, information such as precontrast, postcontrast or 5-minute

9 delay can be included. Annotations also can be made after the image has been sent to a PACS. The technologist can note important information by using different font sizes and colors. 30. Scout A scout film is essential for every CT scan. Other terms for the scout image include scanogram, pilot, topogram and survey. It is basically a plain-film radiograph used to set the scan box, or area of anatomy to be scanned. The film is taken using a stationary tube and translating the table through the gantry. Both AP and lateral scouts can be taken, with the main difference being the position of the x-ray tube. If an AP scout image is required, the x-ray tube is positioned above the patient. The lateral scout is taken with the x-ray tube positioned to the left or right of the patient, or 90 from the AP scout. Scouts of different lengths are programmed into each protocol, but the CT technologist can choose the scout length. Common scout lengths are 128, 256, 512, 768, 1,024 and 1,536 mm, depending on the examination and anatomy to be scanned. A typical scout length for a CT abdomen/pelvis is 512 mm. The technologist also can stop the scout after it has adequately covered the area of anatomy, so that the patient is not irradiated unnecessarily. Click on the buttons on the animation to view an AP and lateral scout scan. 31. Region of Interest Region of interest, or ROI, is a tool used in CT scanning to circle an area on the image and obtain a CT number. The tool helps the radiologist identify a specific tissue and obtain certain statistical information. ROIs can be placed and dragged to select a larger area of coverage. 32. CT/Hounsfield Numbers Before we discuss the next two controls on the operator console, window width and window level, let s look at the concept of CT/Hounsfield numbers. Individual anatomy on the CT image is represented by different shades of gray. The mathematical unit that describes the shade of gray is called the Hounsfield unit (HU) or the CT number. The CT number represents the gray scale value seen on the final image and is related to the linear coefficients of the tissue within a CT image section. The information found in each digital image unit is assigned a CT number. Water is always assigned zero and serves as the basis from which other CT numbers are calculated. All other values represent various shades of gray. Atoms that are dense are assigned the highest CT numbers. Bone, with a CT number near +1000, appears white on a CT image. Air is associated with the lowest CT number at and is displayed as black on the CT image. 33. Window Width The window width, abbreviated WW, is the range of CT numbers displayed in the CT image and is represented by a gray scale. Window width determines the maximum number of shades of gray that can be displayed on the CT monitor. It also determines how much contrast appears in the image. Increasing the window width provides a wider range of tissue information in the image. Consequently, a wide window width displays less variation between tissues with similar densities and often is used for CT studies that have a great deal of subject contrast, such as a CT scan of the lung. A narrow window width is used for anatomy that has minimal inherent contrast between structures, such as the brain. The white matter and gray matter of the brain only differ by 5 to 10 Hounsfield units, so CT scans of the brain benefit from a narrow window width because it increases the contrast differences between the tissues. A narrow window width enhances image contrast and ensures that the transition from black to white takes place over a relatively few CT numbers.

10 Although a CT scanner is capable of producing approximately 2,000 shades of gray, the LCD monitor can only display 256 shades and the human eye can only distinguish Window Level The window level, abbreviated WL, designates the center or midpoint of the range of CT numbers. It can be positioned anywhere within the window width and determines how bright the image appears. A low WL makes the image appear brighter, and a high WL makes the image appear darker. The window level should be set to the CT number of the anatomy being imaged. For instance, for a CT scan of the brain, the technologist should set the window level at approximately 40 Houndsfield units. 35. WW/WL Example Let s look at an example of window width and window level. Suppose that you set the WW to 600 and the WL to 200 for the CT scan you want to perform. These settings would represent a gray scale of +300 and -300, with a WL setting of 200. Therefore, would equal an upper setting of 500 and would equal a lower setting of So the range is 500 to Tissues with CT numbers falling below -100 would appear black on the scan and anatomy with CT numbers above 500 would appear white on the image. This page shows some typical window widths and window levels. Remember that the range of CT numbers encompasses the CT number of the tissue of interest. 36. Practice Question 37. Practice Question 38. Computer The image data acquired by the scintillation detectors and converted into electrical signals are eventually sent to the computer for processing. The analog signals must be amplified and then digitized by the analog-to-digital converter (ADC). The raw data are stored until converted into the final image by the array processor. The image then is transferred to the host computer where it is displayed on the LCD monitor. After the image is amplified, it s sent to the sample/hold unit, which is located between the amplifier and the ADC. The sample/hold unit determines the relative attenuation of the x-ray beam by the patient s tissues and assigns various shades of gray to the image. The array processor is responsible for applying algorithms to the attenuation data to produce the final CT image. Its computing capacity can perform several mathematical calculations at lightning speed on an enormous volume of projection data, so that the CT technologist is able to view an image after only a fraction of a second delay. The array processor also performs retrospective reconstructions. In addition to image data, the main computer stores patient information so that it can be retrieved for postprocessing. The host computer is linked to other hospital systems and is DICOM compatible. DICOM stands for digital imaging and communications in medicine and is the industry standard for distributing and viewing any kind of medical image data regardless of how the image was created and stored. 39. Digital Imaging The final, processed CT image data are a type of digital image, just like cassette-based and cassetteless digital images currently replacing film in many radiography departments. In fact, CT scans were some of the first digital images found in radiology. Digital imaging is a complex process that includes data acquisition, image processing, image display, image communication and storage. At each

11 step, from collecting the initial data to viewing the final digital image, computers are an essential part of manipulating and managing the image data. Data acquisition involves collecting the image information from the patient. Collection devices include the x-ray tube and the detectors that record the information. Data acquired from the patient are in the form of linear attenuation coefficients that are based on the type of tissue scanned. The information is an electrical signal that must be converted to a digital signal by the analog-todigital converter. Image processing consists of the steps needed to create a CT image. The raw data set produced by the ADC is converted back into an analog signal for image display. Various image processing techniques such as filters and algorithms are applied to improve image quality, reduce noise, increase image sharpness or enhance detail. Image processing also allows the CT technologist to view an image at the same time the scan is taking place. The final image is the result of multiple computer operations and decisions made by the technologist. The process takes an amazing amount of computing power to process enormous amounts of information. The digital-to-analog converter (DAC), makes image display possible by converting the digital data to an analog signal for the LCD monitor. After the image is ready for display, it is stored electronically by the PACS. PACS are capable of archiving vast quantities of radiology images without taking up a large amount of physical storage space. Another advantage of a PACS is accessibility. Physicians at multiple facilities or remote sites are able to retrieve diagnostic images easily and quickly. 40. Pixel Digital image data are acquired as extremely small, individual pieces of information, just a fraction the size of a millimeter. These individual units are known as pixels, a term that is short for picture elements. A pixel is a two-dimensional element that is assigned a gray value in the form of a CT number. The CT number is displayed on an image matrix according to the mean attenuation value of the corresponding anatomy. The pixel information directly corresponds to the attenuation coefficient of the object. So, the pixel number is related to the atomic number and the mass of the scanned tissue. The field of view and the matrix determine the size of an individual pixel. To calculate pixel size, we use the equation: pixel = field of view/matrix. So, for example, let s suppose the CT technologist selects a 24-cm field of view with an image matrix of 1024 x We first convert the field of view from centimeters to millimeters and then divide by the matrix size. In this case, the field of view is 240 mm; 240 divided by 1024 is Therefore, the size of an individual pixel would be mm. Use the slider bar on this page to see how the pixels in the resulting image change as the field of view changes. 41. Matrix The image matrix is a two-dimensional array of numbers made up of multiple pixels arranged in columns and rows. Typical image matrices in CT are 256 x 256, 512 x 512 or 1024 x As the matrix number gets higher, the image resolution increases. For instance, a 1024 x 1024 matrix has a better resolution than a 512 x 512 matrix. To calculate how many pixels are in an image matrix, simply multiply the columns by the rows. So for a 512 x 512 matrix, there are 262,144 pixels. The CT technologist selects the matrix when the field of view is selected. 42. Voxel

12 A voxel is a volumetric, or three-dimensional, picture element that represents a volume of tissue in the reconstructed CT image. Voxel size depends on three factors: slice thickness, matrix size and the field of view. The voxel size is determined by multiplying the pixel size by the slice thickness. Voxel reconstruction is classified as isotropic and anisotropic. When the x, y and z plane (that is, the length, width and height) are equal, the volume data set is isotropic. For CT, isotropic display means that the size of each voxel in a given volume data set is equal. In other words, an isotropic voxel represents a perfect cube. This relationship is achieved by making the slice thickness equal to the pixel size. Isotropic imaging increases spatial resolution in all imaging planes. The more a multislice scanner is capable of increasing the number of slices obtained per rotation of the x-ray tube, the better the scanner is at scanning isotropically. Isotropic scanning is important because it enhances 3-D and multiplanar reconstructions. Detector configuration also affects isotropic imaging. As the scanner increases the number of detector rows, isotropic scanning becomes more effective. The benefits of isotropic imaging in CT include improved image quality in all three dimensions, reduction of the stair step artifact and the ability to achieve a resolution of less than 0.4 mm. Anisotropic imaging, on the other hand, occurs when the slice thickness is not equal to the pixel size. In this case, an anisotropic voxel displays as a rectangular shape rather than a cube. 43. Digital Image Characteristics in CT Decisions made at the operator s console or during patient positioning can affect the display characteristics of the digital image. Some display characteristics are inherent in the image data set based on patient size, anatomical structure or pathology. 44. Sampling and Aliasing The very first step in CT data collection is sampling. Sampling occurs at the CT detectors where the incoming x-ray beam is recorded (or sampled) to ensure that enough signal is present to produce an image. The detector will assign a grayscale quantity based on the amount of radiation absorbed by the detector; this grayscale quantity is the CT number or Hounsfield unit. The sampling process must follow a specific set of rules to avoid potential artifacts such as streaking. To avoid sampling artifacts thinner slices may be acquired or more compressed detectors can be used. Some vendors have redesigned the x-ray tube geometry to reduce the occurrence of sampling errors. Sampling data are different for each of the different beam geometries, such as parallel beam, fan beam and cone beam. Aliasing refers to a sampling problem that arises when structures and spaces cannot be distinguished. These artifacts show up as streaks and are caused by an insufficient number of samples available for image reconstruction. Aliasing often is referred to as a sampling error. 45. Spatial Resolution Spatial resolution describes the degree of blurring in an image and is a measure of how small an object can be imaged. It is the ability to discriminate objects of varying density a small distance apart and against a uniform background. CT is a superior imaging modality with respect to spatial resolution because it s able to distinguish tissue with density differences of less than 0.5%. In comparison, conventional radiography can only differentiate densities as low as to 10%. A high-contrast area such as a bone-soft tissue interface is more difficult to image than a lowcontrast area such as the liver-spleen boundary. Spatial resolution in CT can be improved by using thinner slice thicknesses, adjusting the image matrix to ensure a smaller pixel size, using a reconstruction filter such as a high-frequency convolution filter or using a small detector size. 46. Contrast Resolution

13 Contrast resolution describes the caability to image adjacent tissues that have similar mass density and effective atomic density. In other words, contrast resolution is the ability to distinguish between similar tissues. Examples of areas that have similar tissue densities are the liver/spleen interface and the gray and white matter of the brain. CT has superior contrast resolution because narrow x-ray beam collimation reduces scatter radiation. 47. Temporal Resolution Another type of scanning should be briefly mentioned here. Half-scanning techniques are, in some ways, a return to the early days of CT when only a portion of the 360 arc was used to create the data set. Although the scanner components rotate around the patient in a complete 360 circle, only a portion of the image data is used to create the image. Special algorithms are able to create a diagnostic quality image from a smaller data set. Halfscanning techniques are used to reduce the appearance of motion in a CT scan from involuntary movement such as a beating heart. The ability to resolve or view an object in motion is termed temporal resolution. The half-scan mode is beneficial simply because it shortens the scan time considerably and improves temporal resolution. 48. Noise Noise, which is also referred to as quantum mottle, is the primary factor that affects lowcontrast resolution. Noise appears as a grainy texture throughout the image and presents a nonuniform image. Factors that contribute to noise include patient size and density, detector size, patient dose, slice thickness, image matrix and the display field of view. Increasing mas reduces the noise in a CT image because more x-ray photons hit the detectors. The matrix size also affects the noise seen on an image. A smaller matrix results in larger pixels and a decrease in noise, but at the expense of spatial resolution. 49. Partial Volume Averaging Partial volume averaging occurs when the CT numbers are misrepresented because the anatomy extends into the adjacent slice thickness. CT numbers are based on the linear attenuation coefficients for a volume of tissue. If the anatomy contains only one tissue type, then the voxel will accurately represent the CT number. However, if the anatomy is divided between two slices, an average CT number is assigned to the tissue. For example, in the human brain, white matter with a CT number of 46 and gray matter with a CT number of 43 can be represented on the same voxel. Therefore, the voxel can be assigned a single CT number that is the average for the tissues, in this case a CT number of 44 or 45. Partial volume averaging can lead to misdiagnosis because the assigned average CT number doesn t accurately represent the tissue. Partial volume averaging can be improved by selecting thinner slices, which allows the anatomy to be evenly distributed over each pixel and assigned a correct CT number. 50. Practice Question 51. Practice Question 52. Conclusion This concludes CT Basics Module 2 Equipment and Instrumentation. You should now be able: Explain the major components of the computed tomography computer system.

14 Trace the sequence of events in CT scanning from the application of the electrical current to the radiographic tube to image display. Explain how adjusting operator console parameters affect CT image data. Discuss the elements of a digital image. 53. Bibliography Papp J. Quality Management in the Imaging Sciences. 3rd ed. St. Louis, MO: Mosby; Seeram E. Computed Tomography: Physical Principles, Clinical Applications, and Quality Control. 3rd ed. St. Louis, MO: Saunders; Development Team 55. Final Slide