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CT Basics: Computed Tomography Fundamentals Module 1 1. Title Screen Welcome to Module 1 Computed Tomography Fundamentals. This module was written by Nikkole Weber, R.T. (R)(CT)(M), Patty Hiddinga, B.S. R.T.(R)(CT) and Myke Kudlas, M.Ed., R.T.(R)(QM). 2. License Agreement 3. Objectives After you completing this module, you will be able to: Describe the events leading to the discovery of computed tomography. Explain the design of CT scanner generations. Identify the location and function of major CT components. List the most common uses of computed tomography in medical imaging. Discuss the basic processes related to digital image processing. 4. CT Defined CT stands for computed tomography, but this term has evolved since the CT scanner was first invented. Other terms such as computerized transaxial tomography and computerized axial tomography also were used to identify the CT scanner. To understand the term computed tomography more clearly, let s first look at the origin of tomography, as well as explore how other terms were first used to describe this type of image acquisition. 5. Tomographic Principle Tomography is a method of imaging various planes in the body. Motion is used to blur certain portions of the patient s anatomy in order to see a particular location better. Tomography has been around for more than 80 years, and although its use has declined over the past two decades as other forms of imaging have emerged, many institutions still perform tomography for excretory urography and other specialized examinations. Tomographic equipment works in the following way. Before image acquisition, the technologist locates the anatomical area of interest and adjusts the equipment accordingly. The radiographic tube, which is generally located above the patient, moves in one direction as the image receptor moves in the opposite direction. The anatomy in the patient s body is blurred during this process with the exception of one thin section of anatomy that contains the area of interest. This pivotal point, where no motion occurs, is called the fulcrum. The entire process is termed the tomographic principle. Click on the button to see the tomographic principle in action. As the equipment moves, you can see that the area of interest remains relatively still in relation to the x-ray tube and the image receptor. The objects located above and below the area of interest are blurred during the exposure. If a plain radiograph were made of these objects, it would be difficult to see all of them, let alone determine their exact location. By using the tomographic principle, we can identify one object by blurring the images of the other two objects. 6. Planes
The human body can be shown in three planes. The sagittal plan divides the body into a right side and a left side; the transverse plane divides the body into a superior and an inferior portion; and the coronal plane divides the body into an anterior and posterior portion. The type of tomography described on the previous page is known as axial tomography because the x-ray tube and the image receptor move along the long axis of the patient. If the patient lies on his or her back during the exam, a coronal section of anatomy is demonstrated; if the patient lies on his or her side, a sagittal section of anatomy is seen. The only way to produce a transverse sectional image would be to have the patient stand on top of the radiographic table. This is not a practical solution for most exams. 7. Planes A CT scanner images a transverse section of anatomy as it scans a patient s body. In effect, the patient s anatomical images are presented like individual slices from a loaf of bread. The observer can look at each section individually to see the anatomy of interest. The first CT scanners were only capable of reconstructing images in the transverse plane and were called computerized transaxial tomography scanners or computed axial tomography scanners. This is the origin of the colorful term CAT scan. Although still a popular term, CAT scan no longer adequately describes present day CT units. Most of the scanners used today obtain a volume of data during the acquisition process that can be reconstructed in many different ways. The image data can be displayed in a sagittal, transverse or coronal plane, depending on user preference. Click on the buttons above to see how the algorithms used in computed tomography can display the same anatomy in three different planes. 8. Computed Tomography Computed tomography is an exciting medical imaging modality that continues to expand rapidly. The use of CT imaging is increasing not only with respect to the types of imaging procedures possible, but also in terms of the range of anatomy and body systems that can be imaged with CT. Today we are able to examine a wide range of body parts that may have been obscured only a few years ago. CT is pushing the limits of technology and is poised for future evolution. To whet your appetite for this and subsequent CT learning modules, let s look a few images from routine CT examinations. More specialty examination images will be presented in later modules. 9. Routine CT Chest Examination Several different sets of images can be reconstructed from one axial CT data set. These images were reconstructed from a chest, abdomen and pelvis data set. Note that the images did not come from the same patient. Images 1 and 2 show sections of an axial chest at a 5-mm slice thickness using standard algorithms. Image 1 is presented with a soft tissue window width/window level, and image 2 is displayed with a lung window width/window level. Image 3 is also of the chest, but a bone algorithm has been applied to the data, and the slice thickness is 1.25 mm. The last reconstruction of the chest is an axial image using a maximum intensity projection, or MIP. This image is reconstructed with a bone algorithm to a 20-mm slice thickness and 10- mm interval. 10. Routine CT Abdomen/Pelvis Examination Images 5, 6 and 7 show common reconstructions of the abdomen and pelvis. Image 5 is an axial image through the liver at 5-mm slice thickness. When we reformat the data in the coronal plane, we get an image like number 6. Image 7 is a coronal reformat of the entire chest, abdomen and pelvis.
11. 2-D and 3-D CT Imaging Images of the musculoskeletal system often are used to create a surgical care plan for the patient. These axial, coronal and sagittal 2-D images of the ankle show a fracture of the distal tibia. 12. 2D and 3D CT Imaging The axial shoulder image on the left shows bony erosions. The 3-D image of this shoulder, on the right, complements the 2-D image and aids in surgical planning for shoulder replacement. 13. Soft Tissue and Bony Anatomy Imaging can be performed to show soft tissue and bony anatomy. The images of the head on this page show the soft tissue of the orbits on the left and, on the right, the bony anatomy of the sinuses in the axial and coronal planes. 14. CT Angiography CT is commonly used as an alternative to or in conjunction with diagnostic catheter angiography to demonstrate vascular anatomy. The axial image on the left shows a chest during the arterial phase. The ascending and descending aorta is easily visualized using this type of protocol because of the presence of contrast in the vessels. The image on the right is a volume-rendered (3-D) image of the chest, abdomen and pelvis. This image shows the entire aorta through the aortic bifurcation. 15. Postsurgical CT Scan Physicians use postsurgical CT imaging of abdominal aortic aneurysm (AAA) repair to demonstrate stent placement, monitor the size of the aneurysm and to look for any stent leakage. The postsurgical repair is clearly evident on this volume-rendered (3-D) image of the abdomen and pelvis. 16. CT History Now that you ve had a brief look at the types of images CT is capable of producing, let s look at how CT has evolved since it was first invented and what major components make up a modern CT scanner. 17. CT Terminology When the first CT scanner was introduced, the process was called computerized transverse axial scanning. As time went on and newer types of equipment were invented, other names such as reconstructive tomography, computer-assisted tomography or computerized transaxial transmission reconstructive tomography were used. To better describe the capacity of the CT scanner to display anatomy, we ve removed many of the limiting terms such as axial and transaxial. Computed tomography is now universally accepted as the name for CT scanning. It is a much more descriptive term for today s scanners because CT equipment is able to display the anatomy of interest in any plane desired. As you have already seen, the algorithms used in computed tomography are capable of much more than simply reconstructing anatomy in various planes. Modern CT scanners can display only a desired organ or body part, measure the amount of calcium in the coronary arteries and even provide a virtual anatomical model that physicians can use in place of invasive exploratory surgery. We will look at these applications in later modules. 18. CT Roots
We can trace the roots of the modern CT scanner back to the early part of the twentieth century. In 1917 Johann Radon, a mathematician living in the Austro-Hungarian empire, developed an algorithm that reconstructed a two- or three-dimensional object based on multiple measures from several different locations. His algorithm, the Radon transform, was developed for use in integral geometry. Radon probably never imagined the application of his work to computed tomography, but the Radon transform provides the foundation for many of the algorithms used in CT. Several decades later, at Tufts University in Medford, Massachusetts, Allan MacLeod Cormack, a professor of physics originally from South Africa, was also laying the groundwork for the algorithms needed in CT. He wrote two articles for the Journal of Applied Physics that answer many of the mathematical problems inherent to computed tomography image reconstruction, even though the CT scanner had yet to be invented. 19. Godfrey Newbold Hounsfield Although many people contributed to the development of the CT scanner, Sir Godfrey Newbold Hounsfield brought together the pieces to create the first workable unit. In 1967 Hounsfield, an Englishman, was working with early computers at Electric and Musical Industries Limited, or EMI. You might recognize EMI as the record label of the Beatles. Hounsfield s work centered on pattern recognition and image reconstruction. Although his background was not in medicine or medical imaging, he theorized that an image could be reconstructed from data taken at multiple angles. The first primitive CT scanner used the element americium as the radiation source, and it took 9 days to scan an object. In addition, reconstructing an image required more than 2 hours. Hounsfield then substituted an x-ray tube for the americium source to increase the power to the machine and reduce the time needed to scan an object. 20. The First Scans Before this new device could be used to image humans, it first needed to be tested. For this step, Hounsfield turned to James Ambrose, a radiologist at Atkinson-Morley s Hospital in West Wimbledon, England. Together Hounsfield and Ambrose experimented with imaging various tissues, most notably brain tissue with a known mass and the brains of bulls. The first CT scanner used to image a living human was installed at Atkinson-Morley s in 1971, and the first clinical images were acquired in 1972. This first scan, and all scans performed with first generation scanners, was of a patient s head. In this case, the patient was a woman with a suspected brain tumor. For his part in the development of the CT scanner, Hounsfield was awarded the Nobel Prize for medicine and physiology in 1979. Allan MacLeod Cormack also shared in the 1979 prize for his work in solving the mathematical problems of CT image reconstruction. 21. CT Implemented The first commercially available CT scanners began to be installed in hospitals and clinics around the world in 1973. Remember that these first generation scanners were only capable of imaging the head. The following year, Robert Ledley, a professor at Georgetown University in Washington, D.C., developed the first CT scanner capable of imaging the entire body. This invention was just the beginning of a series of modifications to Hounsfield s original design that continues to this day. 22. Practice Question 23. Practice Question 24. CT Generations
The design and use of computed tomography scanners changed considerably after their initial introduction in the early 1970s. Today s CT scanners bear little resemblance to the first unit designed by Hounsfield. Each time the configuration of the scanner changes, we say that a new generation of computed tomography has occurred. There have been 7 major generations of CT equipment to date, with each generation building on previous design configurations. There is some disagreement in the medical imaging community regarding the number of generations and the type of equipment found in a particular generation. This is likely because of the rapid development of medical imaging technology in the past decade. The dates of development of a particular type of equipment and the type of equipment that fall within a specific generation are probably less important than the overall evolution of the CT scanner and the design configurations. A review of the different generations of CT scanners provides not only an appreciation for the technological advancements within the field, but also a glimpse at the future of computed tomography imaging. 25. First Generation The first generation of CT scanners was designed specifically for imaging the head. The scanner was smaller than those used today because the opening only needed to be large enough for the patient s skull. The computed tomography scanner displayed on this page is the first CT scanner in North America, which was installed by the Mayo Clinic in Rochester, Minnesota, in 1973. The scanner was acquired for the Mayo Clinic department of radiology for $350,000 by Hillier Baker, Jr., who had learned about the new technology at the annual Radiological Society of North America (RSNA) scientific assembly. Baker went to England with the authorization to purchase the equipment if he thought the technology was a viable investment. The first scan was taken on June 19, 1973, by Darrel Holtz, R.T.(R), a special procedures technologist. It took approximately 4 minutes to acquire one image and 7 minutes for the computer to analyze the data. A Polaroid camera was used to capture the image from the monitor. 26. First Generation A bag of water was placed inside the CT scanner to improve image quality. This rubberized bag of water, called a "head cap," enclosed the patient's head to reduce the dynamic range of the radiation reaching the detectors. No other generation of CT scanners has routinely used water bags in the design. The design of the first generation CT scanner was almost primitive by today s standards. An x-ray tube was mounted on one side with 2 detectors located on the opposite side. The x-rays passed through the patient and the water bag before being absorbed by the detectors on the other side. 27. First Generation First generation scanners used what has been described as rectilinear pencil beam scanning. The x-rays emitted by the tube were very tightly collimated so that no divergence of the x-ray beam was evident. The size of the x-ray beam was very small, approximately 3 mm x 26 mm, with the x-rays nearly parallel to each other. Click on the button to begin the demonstration of the first generation scanner. The x-ray tube and the detectors moved from one side of the patient to the other, emitting x- rays and collecting data as they went. This motion from side to side during the scan is known as translation. Once an entire pass was completed, the x-ray beam stopped, the entire assembly rotated 1 degree and then the scanner performed the same action again. This rotational motion is called indexing. The process of translation and indexing was repeated 180 times until the examination was completed, a process that took approximately 5 minutes. Because 2 detectors were used, 2 sections were scanned at a time. An entire examination of a patient s head consisted of approximately 20 sections, so the CT scanner completed around 10 scans to
collect the data. Therefore, for the entire examination, the patient had to remain still for about 30 minutes. In addition, it took approximately 5 minutes for each scan to be processed by the computer and reconstructed into a viewable image. 28. CT Comparison Even with lengthy scan times, the best quality images produced by first generation CT scanners would be considered unacceptable today. This was mainly because of the small amount of image data collected, which resulted in a matrix size of only 80 voxels x 80 voxels. By comparison, modern scanners have matrix sizes of 512 voxels x 512 voxels or 1024 voxels x 1024 voxels. The images on this page are examples for comparative purposes and are not actual first generation scans. 29. Second Generation Two major differences separated the first generation CT scanners from second generation units. One major difference was that second generation CT scanners used a much larger x-ray beam and a larger number of detectors. The term fan beam is used to describe the x-ray beam in a second generation scanner because of the divergent nature of the x-ray beam. The other major difference was that a second generation unit completed a scan with far less indexing motion than a first generation scanner. Click on the button to begin the demonstration of a second generation scanner. You can see that the scanner completes a translation motion similar to a first generation scanner, but more data is collected during the translation. During indexing, a larger motion is completed. Instead of the 1 indexing used by a first generation scanner, the second generation scanner moves 30 between exposures. This allowed a scan to be completed in approximately 6 exposures rather than the 180 exposures required by a first generation scanner. The reduction in number of exposures also decreased the overall scan time per section to less than a minute. The overall scan time was determined by the number of detectors built into the CT scanner. If a unit contained a large number of detectors, the scan could be completed in as little as 10 seconds; CT scanners that contained fewer detectors took up to three and a half minutes to gather data for a single section. 30. Third Generation Third generation CT scanners also capture image data using a fan beam of x-rays and an array of detectors. The fan beam is much larger in third generation scanners, however. Another difference between the third generation scanner and previous generations is the curved array of detectors that gather the image data. In first and second generation CT scanners, the detectors were arranged in a linear fashion. Click on the button to see how a third generation scanner works. Unlike the first and second generation units, the third generation scanner operates continuously, without stopping to translate. Both the x-ray tube and the row of detectors rotate so that that an entire 360 arc of image data can be collected. In some cases, only a portion of the entire arc is used. Because scanning is continuous, the acquisition time per section is reduced to just a few seconds. 31. Fourth Generation Fourth generation scanners use a stationary array of detectors that do not rotate. During the scanning process, the x-ray tube completes an entire 360 rotation, sending image data to hundreds of detectors surrounding the patient. Click on the button to see a fourth generation scanner in operation. In some cases, the tube may rotate in an arc greater than 360 during scanning. This process is known as overscanning. Once a section of anatomy is scanned, the tube moves in the opposite direction to acquire the data from the next section. This procedure continues until all anatomy of interest is scanned. The total scan time for a fourth generation unit ranges from.5 second to 10 seconds.
32. Nutation Another adaptation of the fourth generation scanner placed the x-ray tube outside the detector array, and the ring of detectors were tilted slightly to one side to allow the x-rays to pass. This process of tilting the detector ring was known as nutating. The goal of the design was to improve the beam geometry and the signal acquired by the detectors. Nutating computed tomography scanners are generally not used today. 33. Fifth Generation Fifth generation CT scanners look quite different from previous generations. The fifth generation scanner is designed specifically for cardiac imaging. Rather than using an x-ray tube to create an x-ray beam that passes through the patient and conveys image data to the detectors, the fifth generation CT scanner uses an electron beam gun to fire electrons at a tungsten target. Click on the button to see how a fifth generation CT scanner works. After the patient is in position, the electron gun fires a stream of electrons at the tungsten target. The semicircular target lies beneath and to the sides of the patient. The electron beam strikes the target in a sweeping side-to-side motion, producing a fan beam of x-rays that passes through the patient and delivers image data to the detectors. The detectors also are arranged in a semicircular pattern and are located above the patient. The tungsten target is actually made of 4 distinct semicircular layers or rings. After the electron beam completes a pass along the entire length of 1 layer, it quickly shifts to the second layer. Once the second layer has been exposed to the beam of electrons, the third and forth rings are also exposed. The result is 4 x-ray beams passing through the patient with an 8-millisecond delay between each beam. Because of the detector configuration, 8 sections can be imaged. This entire procedure is completed in a very short period of time, approximately 224 milliseconds. 34. Fifth Generation The fast scan time of a fifth generation scanner allows dynamic imaging of certain structures, especially the heart. For this reason, fifth generation CT scanners often are referred to as high-speed CT scanners. These scanners can make simple CT movies of organs such as the heart; thus, the term cine CT also is used to describe a fifth generation scanner. A final name given to fifth generation units is electron beam computed tomography, or EBCT, which refers to the way the x-ray beam is created in this type of equipment. 35. Sixth Generation In many ways, sixth generation scanners are a return to the technology used in third generation scanners. As you might recall, in third generation scanners both the x-ray tube and the detector array rotate around the patient during data acquisition. The major change for sixth generation scanners is the ability of the x-ray tube and detectors to continuously rotate around the patient while the patient and table move through the gantry. A third generation scanner acquires image data section by section so that the scanner pauses between each scan to allow the table to move into a new position. Sixth generation scanners acquire data continuously during the scan, so an entire volume of tissue can be imaged during one breath hold. This means that the entire chest or abdomen can be scanned in approximately 30 seconds. Sixth generation scanners first appeared in 1989, a little more than 20 years after Hounsfield began work on the original CT scanner.
Sixth generation CT scanners often are referred to as spiral or helical scanners because of the geometry of the x-ray tube motion in relation to the anatomy of interest. Different manufacturers use the terms spiral or helical to describe the same motion. 36. Sixth Generation The unique construction of the sixth generation CT scanner makes the spiral-helical motion possible. Click on the button to see the sixth generation CT scanner in operation. During the scan, the x- ray tube and the detectors rotate around the patient. Slip rings help maintain a constant connection between the x-ray tube and its electrical source, which helps improve image quality. 37. Dual Source CT A more recent development in spiral-helical scanners is the addition of a second set of imaging equipment into the CT scanner. This unit is called a dual-source CT scanner, or DSCT. Click on the button to see the DSCT in operation. A second x-ray tube and detector array are placed in the CT scanner at a 90-degree angle to the first set. This arrangement increases resolution for specific scans and helps to counterbalance the forces within the CT scanner as the moving parts rapidly rotate around the patient. 38. Seventh Generation Spiral-helical scanners can acquire multiple sections during the scanning process. This type of scanner is known as a seventh generation CT scanner. As you might remember, the first through fifth generation scanners were able to acquire only 1 to 4 sections per rotation. Depending on the configuration of the detectors, spiral-helical scanners can acquire data from between 16 and 320 sections per rotation. The scanning process collects a great deal of data, but the total scan time for a volume of tissue is reduced to approximately 15 seconds. In addition, seventh generation scanners can reduce patient exposure because the photons from a single x-ray beam can convey image data to multiple rows of detectors simultaneously. Click on the button to see the scanner in operation. Notice the multiple rows of detectors that acquire image data during the scanning process. Because of the seventh generation scanner s ability to image more than 1 section per 360 rotation, this type of scanner is referred to as a multisection, multidetector or multiple detector array scanner. 39. Practice Question 40. Practice Question 41. How CT Scanners Work A modern CT scanner contains several electrical and mechanical components, including an x-ray tube and rows of x-ray detectors opposite the tube. During a CT scan, the x-ray tube and detector system rotate around the patient. X-rays are transmitted through the patient, recorded by the detectors and converted into electrical signals. The signals then are transformed into digital data and transferred to a computer. The computer completes the image reconstruction process, producing digital images that can be displayed, shared, printed and digitally stored. Modern scanners allow the entire data acquisition and reconstruction process to be completed in typically less than 20 seconds. These scanners are known as multidetector CT, generally abbreviated MDCT, or multislice CT, referred to as MSCT. 42. Beam Geometry
Beam geometry is the way in which the x-ray beam passes through the patient and is acquired by the CT detectors. There are three major types of beam geometry: pencil beam, fan beam and cone beam. The type of beam used by a particular CT scanner is important because it affects the speed, quality and image reconstruction algorithms during the acquisition process. 43. Pencil Beam Pencil, or parallel, beam geometry refers to a highly collimated, thin pencil-shaped x-ray beam. This type of beam geometry was used by Hounsfield in the first generation CT scanner. That system used a translate-rotate method to collect data. The x-ray tube, which was located on one side of the patient, and one or two detectors located on the opposite side of the patient moved across the patient in a straight line collecting several transmission measurements, called projections. The tube then rotated one degree and repeated the data collection process until 180 of transmission data were acquired, which was sufficient data for one reconstructed CT image. Each image took about 5 minutes to acquire, and then the patient had to be moved manually to acquire the next slice. One reason for the long scan times in the first generation scanners was the tightly collimated x-ray beam. The pencil beam was so tiny that it only collected data on a very small part of the body during each data acquisition. 44. Fan Beam Fan beam geometry was introduced in second generation scanners. Unlike the pencil beam of first generation equipment, the x-rays diverged slightly from the x-ray source, forming a narrow fan-like shape of about 10. Instead of one or two detectors, second generation scanners had a row of about 30 detectors. The movement of the x-ray tube and detector system was similar to first generation systems in that the x-ray tube translated across the patient and then rotated. The improved design using fan beam technology shortened the scan time per image to around 20 seconds, making second generation CT scanners 10 times faster than the previous generation. 45. Cone Beam The second generation configuration using fan beam technology and a single row of detectors was quickly replaced by an improved design. Third generation scanners, which can have 800 or more detectors, use a much wider fan beam that encompasses the patient. This type of beam is commonly used today and has virtually replaced single-slice scanners. In a single-slice CT scanner, the x-ray tube was coupled to a curved, single-row detector array with an arc of 30 to 40 or greater from the apex of the fan. As the tube and detectors rotated, data was obtained for every fixed point of the tube and detector. This configuration permitted greatly reduced scan times; however, the generator cables, which supply power to the x-ray tube, and the data cables limited the acquisition process. The scanner frequently had to stop and reverse direction to unwind the cables. This problem was overcome by the development of the slip ring, which will be discussed later in greater detail. With slip ring technology, CT scanner gantries can rotate continuously, a development that led to spiral, or helical, scanning. Unlike axial, or sequential, image acquisition in which the table is shifted after each gantry rotation, with spiral acquisition the table moves continuously as the tube rotates. Continuous acquisition dramatically reduces scan times. 46. Multidetector CT
Multidetector CT, which is generally considered third generation technology, has become the new standard in most computed tomography departments. Manufacturers have designed scanners with up to 256 data channels, making single-slice scanners essentially obsolete. The benefits of MDCT technology include faster scan times, increased anatomical coverage and thinner slice capabilities for higher resolution images. Multidetector CT uses a wide beam geometry, with the detector array covering typically more than 20 mm in the longitudinal, or z-axis, direction. The very wide x-ray beam no longer has a flat fan shape but spreads out in three dimensions in a shape similar to a narrow pyramid. This type of geometry is commonly called cone beam, a term that should not be confused with some specialty scanners that use an even wider beam and are marketed as conebeam scanners. These special-purpose scanners are not addressed in this module. The wide cone-beam geometry made the scanning process more complicated because of the difference in the x-ray path angles from the central plane to the planes on the edges. Special reconstruction algorithms have been developed to correct for the increased beam divergence. The improvement of cone-beam technology and multidetector CT now enables scans to be performed faster than 3 rotations per second with greater patient coverage. The faster technology permits an entire abdomen to be scanned in typically less than 15 seconds. 47. Digital Imaging For a CT image to be created, the image data must go through a specific set of steps. Subsequent modules will explain the details of this process; what is important to understand in this introductory module is that the quality of the finished image largely depends on the scanning parameters used to acquire the data. Algorithms used by the computed tomography processing computer can manipulate the collected data to achieve a desired appearance, but their capability is limited if the input data is substandard. 48. Sequential Scanning Data acquisition refers to the systematic collection of information from the patient to produce the CT image. The two methods of data acquisition are sequential, or slice-by-slice, acquisition, and spiral, or helical, volume acquisition. Click on the button to see the scanner in operation. In sequential data acquisition, more than one slice can be obtained per rotation; however, the scanner gathers image data in distinct sections, one after another. The x-ray tube rotates around the patient and collects data for that single rotation. The tube then stops emitting x-rays, and the table moves the patient to the next scan position. This process continues until all positions have been scanned individually. 49. Spiral or Helical Scanning In spiral or helical scanning, an entire volume of data is acquired without the x-ray tube stopping for the patient to move to the next position. The x-ray tube rotates around the patient and emits x-rays while the table transports the patient through the gantry opening. Click on the button to see the scanner in operation. From the perspective of the patient, the tube traces a spiral/helical path to scan an entire volume of data. The table speed can be adjusted so that helical bands of data are adjacent to each other, overlapped or have gaps. The spacing of the bands of data is referred to as pitch. A pitch of 1 results in adjacent bands, a pitch of less than 1 implies the bands overlap and a pitch greater than 1 produces gaps between the bands. Spiral acquisitions can be much faster than sequential acquisitions, especially for pitches greater than 1; however, pitches greater than 1 may have an inherent increase in actual slice width and possibly a decrease in image quality, depending on other scanning parameters used.
50. Visual Differentiation of Tissues After the image data has been acquired by the CT scanner and reconstructed by the computer, the image can be displayed. The CT image is very much like a radiograph in appearance. Areas of high density appear white, and areas of very low density appear black on the finished image. Look at the radiograph of a foot on the left side of this page. You can see four main densities that differentiate different areas of the body. The air surrounding the foot appears black on the image, the bone looks white, the muscle appears light gray and the fat displays as a darker shade of gray. Compare this image to the CT section on the right. The same differentiation of tissues is evident on this image. The scapula and the humerus appear white, the air in the lungs appears black and the remaining tissues display as different shades of gray. One of the significant aspects of a CT image, or any digital image for that matter, is that the image data not only can display a visual difference between tissue types, but there also is an actual mathematical difference between the individual image components. When image data is acquired by a computed tomography scanner, it goes through a process known as quantization. During quantization, each voxel, or individual image element, is assigned a specific number to represent its brightness. Voxels that make up bone have a very specific range of numbers, as do air, muscle and fat. Quantization helps the computer to display the image data appropriately, depending on what tissue type the individual viewing the image is examining. 51. CT Components and Operations The CT scanner consists of several major components, each critical to the proper functioning of the imaging chain. In this section of the module, we will look at the components of a generic CT scanner. The scanner that you are using may differ slightly from what is presented here based on the manufacturer of your equipment. 52. Scanning Console Today s scanning console looks more like a personal computer than a specialized piece of diagnostic equipment. It consists of a high-quality, flat-panel display monitor, keyboard and mouse, and central processing unit. The console is the hub of communication between the CT technologist and the imaging system. The technologist enters pertinent patient information here, defines default scan parameters, selects the appropriate scan protocols, starts the scanning process, manipulates the images during postprocessing, and archives the images or uses a network that provides access to patient information from other imaging modalities and databases. 53. Gantry The gantry is a mounted framework that surrounds the patient in the vertical plane. Several components are housed within the gantry, and these components work together to acquire the image data. The x-ray generator, x-ray tube, detectors and other components are mounted on a rotating scan frame within the gantry. The gantry typically also houses imaging components such as the slip rings, a high-tension generator, collimators, detectors, the digital acquisition system known as DAS, as well as the x-ray tube. A few gantries now contain two x-ray tubes. In certain scanner models, some of the components, such as the generator, may be located outside the gantry in some cases, in an adjacent room. Scanners can weigh more than 6,800 pounds, and often the floor beneath the equipment requires reinforcement. Two major features of the gantry are the gantry aperture and gantry tilting range. The gantry aperture is the opening in which the patient is positioned during the scanning procedure; the opening typically ranges from 70 to 78 cm in diameter. The degree of tilt varies between systems, but plus or
minus 12 to plus or minus 30 is standard. The gantry also includes a set of laser beams that indicate the center of the scan plane to aid in patient positioning. 54. Table The patient lies on a couch, or patient table, during the examination. The couch rests on a pedestal housing the electrical and mechanical components that allow vertical and horizontal movement into the gantry aperture. The table usually is made of carbon fiber composites because those materials have low absorption and excellent vibration dampening capabilities. In addition, tables made of carbon fiber can support up to 450 pounds and still provide accurate slice reproducibility. Some bariatric scanners can accommodate loads of more than 600 pounds and have a gantry diameter of 85 cm. The mechanics that move the table must be very precise because the entire reconstruction process assumes that the correct positioning of the patient. In fact, weight limits are usually defined by the maximum load at which the table can be moved accurately, not the weight at which the table will break. In any case, the maximum weight load of the table should never be exceeded. 55. Radiographic Tube The x-ray tube used in a CT scanner is very similar in design to the conventional rotating anode x-ray tube, although at least one manufacturer has developed a tube that deviates significantly from the standard design. The x-ray tube is located inside the CT gantry and rotates around the patient during the data acquisition process. 56. Radiographic Tube Let s digress for a moment and review some details about radiographic tubes. The standard x- ray tube contains two electrodes, the cathode and the anode. CT uses a higher voltage than radiography, typically more than 100,000 volts. When high voltage, referred to as tube voltage, kv or kvp, is applied across the cathode at a specified tube current, electrons flow from the cathode and collide with the target material on the anode. This collision causes the electrons to decelerate rapidly, and x-rays are produced. Remember that x-rays created in this manner are called bremsstrahlung, or braking, radiation. The cup-shaped cathode contains two filament wires, one which is slightly longer than the other. The size of the focal spot is directly proportional to the length of the filament that is used. Filament lengths of 0.6 mm and 1.2 mm or 0.75 mm and 1.5 mm are common. Small filaments are preferred for CT scans with thin slices and high resolution. For scans that cover a larger volume, such as an abdominal CT, a larger focal spot with higher power ratings is used. When heated, the cathode boils off electrons, which are then accelerated toward the anode because of the voltage difference between the anode and cathode. The temperature of the filament, which is controlled by the tube current, determines the amount of electrons that flow to the anode. Therefore, the hotter the filament, the greater the amount of electrons, and ultimately the greater number of x-rays. Tube currents are usually in the range of tens to hundreds of milliamperes, or ma. In addition to controlling the number of x-rays generated by changing the ma, it is also necessary to control the energies of the x-ray photons that are emitted from the anode target material. The photon energies are controlled by adjusting the tube voltage. CT tube voltages range from 80 to 140 kv. Remember that 1 kilovolt equals 1,000 volts. The tube voltage determines how fast the electrons are accelerated from the cathode to the anode higher voltages result in higher-energy electrons, which in turn result in higher-energy x-rays.
Note that the x-ray beam contains a broad distribution of energies, ranging from essentially zero to any specific kvp setting. Often the highest kvp settings in CT reach the 120 kvp to 140 kvp range. Increasing the kvp also increases the number of x-rays that are produced. Higher tube voltages are typically used for very attenuating body parts, such as the skull or shoulders, or for large patients, because higher-energy photons are more penetrating than lower-energy photons. The kvp also affects image contrast and patient dose. Higher kvp levels reduce radiographic contrast, but significantly reduce patient dose. The CT tube housing is nearly opaque to x-rays except for the tube window, which is the only part of the tube that allows x-rays to pass through it. Therefore, although x-rays are emitted from the focal spot in all directions, they can only escape the tube housing through the tube window. The majority of the x-rays produced never leave the tube. The impact of the high-energy electrons on the anode produces an enormous amount of heat; it is not uncommon for the anode to glow white during long exposures. Therefore, the tube s heat storage capacity and ability to dissipate heat are very important. Typical storage capacities range from 2 to 5 million heat units (HU), and the typical heat dissipation rate is 400,000 HU per minute. Some tubes are cooled by using onboard oil-to-air heat exchangers, some are water cooled and others use ambient air inside the gantry. Tube heat characteristics can affect the minimum amount of time allowed from the end of one scan to the beginning of the next. Shorter interscan delays are typically preferred, especially for dualphase scans when timing is critical. 57. Generator Scanners require high instantaneous power. A high-voltage generator produces the large voltages required for adequate penetration of body tissue. The generator may be located in the gantry near the x-ray tube or outside of the gantry. When inside the gantry, the generator is usually a small, solid-state, high-frequency device mounted on the rotating frame. The power ratings of generators for CT scanners range from 30 kilowatts (kw) to 80 kw or more, depending on the equipment. The CT technologist needs to understand the effect of tube current and tube voltage because these two parameters are critical in producing high-quality CT images. 58. Slip-rings Slip-ring technology revolutionized the design of the modern CT scanner. Slip rings eliminate the need for cables to transfer power to the gantry components and relay image data back to the computer. The slip ring uses stationary metal brushes as electrical contacts to a metal ring along the perimeter of the gantry. As the components within the gantry rotate, the brushes remain in contact with the slip rings and permit large amounts of digital data and electrical power to be transferred. All power is conveyed to the gantry components via the slip rings. 59. Filters Two types of filtration are used in CT. The computer in the CT system uses mathematical filters such as bone or soft tissue algorithms to change the appearance of an image. The digital process of filtration is integral to CT image reconstruction as part of the back projection process. Subsequent modules in this series describe back projection. The other type of filter used in CT is a beam-shaping filter. Beam-shaping filters generally are made of aluminum or Teflon and are located just beyond the tube window within the gantry. These filters are used to change the spatial distribution of the x-rays.
For example, a common type of beam-shaping filter is called a bow-tie filter, which is thin at the center and thicker at the edges. The thicker edges remove more photons, and x-rays that pass through the thick edges eventually pass through the lateral sides of the patient, where the x-ray path length is the shortest. The thin part of the bow-tie filter is centrally located over the patient, where the patient is thickest. Thus, the bow-tie filter equalizes the distribution of the x-rays when they reach the detectors. Beam-shaping filters also remove lower-energy x-rays from the beam. These lower-energy x-rays most likely would stop and be absorbed in the patient, thereby contributing to dose and not to image quality. 60. Collimators As CT scanners have become faster and acquire greater volumes of data, patients have become more concerned about how much radiation they receive and the risks associated with it. The CT technologist s job is to reassure patients that every safeguard is taken to ensure the lowest possible radiation dose is used without compromising the diagnostic quality of the exam. Collimation is a very effective way to restrict the path of photons emitted from the x-ray tube and keep radiation dose to a minimum. Two types of collimators are used in MDCT scanning to restrict the x-ray beam as it diverges from the tube: prepatient and postpatient. Prepatient source collimators are located just beyond the x- ray tube and determine the scan volume for each tube rotation for a particular CT scanning procedure. When the CT technologist selects a detector configuration, he or she determines tube collimation by narrowing or widening the beam in the longitudinal direction. A second set of collimators, called postpatient collimators, located directly below the tube collimators ensures that the beam width is constant as it travels toward the patient. 61. Detectors This page shows the position of the CT detection system within the gantry. CT detectors pick up and measure the radiation passing through a cross-section of the patient, convert the transmitted photons into an electrical signal, and then send the signal to the computer system for processing. Modern detectors are scintillation devices that use a crystal to produce light when it is struck by an x-ray photon. A lens focuses the visible light onto a semiconductor photodiode and transforms it into an electrical signal. The strength of the detector signal is proportional to the number of photons that strike the crystal. Scintillation detectors are about 99% efficient. Earlier designs used xenon gas detectors that converted x-ray energy directly to electrical energy. One of the problems associated with these elements was that at times they would fluoresce, or glow, more than necessary. The afterglow altered the strength of the detector signal causing inaccuracies during computer reconstruction. Gas-filled detectors were approximately 35% to 40% efficient. 62. Detector arrays Single-slice scanners have a single row, or array, of detectors, and the collimation of the x-ray beam determines the slice thickness of the image. One major problem with single-slice CT is related to the length of time needed to acquire the data, especially for thinner slices. This is because the scan coverage per rotation is equal to the slice thickness. A two-dimensional detector array enables CT scanners to acquire multiple slices simultaneously and greatly increases the speed at which the CT data can be acquired. Multislice detector systems typically have more coverage per rotation because a volume of data is acquired that can then be divided into thinner slices after the acquisition.
Multislice CT detector systems contain several rows of detector elements. The number of rows of elements varies for different scanner models and does not necessarily correspond to the maximum number of slices. For example, the detector configuration displayed on this page represents a 16-slice system that has 24 detector rows. There is a discrepancy because the detector elements are not the same size the innermost 16 detectors are 0.625 mm wide and the four detectors on each side are 1.25 mm wide. If just the inner 16 detectors are used for the detector configuration, a maximum of 16 slices, each 0.625 mm wide, is possible. When the configuration is set so that all 24 elements are used, a maximum of 16 slices, each 1.25 mm thick, is possible. Two of the inner 0.625-mm elements are paired together to yield one 1.25-mm slice. Detector systems in which the elements are not all the same size are referred to as adaptive arrays. Systems with equal-size detector elements are called symmetric arrays. An additional significant advantage of multislice systems is the option to reconstruct different slice thicknesses without rescanning the patient. 63. Data Acquisition Systems The data acquisition system (DAS) is responsible for amplifying and digitizing the signals from the detectors before they are sent to the computer for processing. The system has three functions. 1) First, it measures the radiation beam that is transmitted through the patient. 2) Second, it encodes these measurements into binary data. 3) And finally, it transmits the binary data to the array processor computer. 64. Array Processors After the signal from the detectors has undergone analog-to-digital conversion, it is read as raw, or unprocessed, data into the computer, where it is reconstructed into a cross-sectional image. The computer system generally includes input-output devices, a central processing unit, an array processor, interface devices, back-projector processors, storage devices and communication hardware. The system also includes software that allows each hardware component to perform specific tasks. In addition, the computer system performs image manipulation and various image processing operations such as windowing, image enhancement, image enlargement and measurements and quantitative measurements. Advanced visualization tools require powerful computer workstations and considerable memory to handle the vast amount of data used in various processing techniques. Some high-resolution CT applications that use 3-D postprocessing include CT angiography, CT colonography with virtual 4-D flythrough technology and other advanced cardiac imaging applications that will be covered in Module 10 of this series. Other postprocessing applications include retrospective reconstruction, volume rendering, multiplanar reconstruction, maximum intensity projection and 3-D surface shading. 65. Monitors and Archival Devices Because of the trend toward filmless radiology departments and the need to transfer data and images to and from the CT scanner and workstations, networking an important feature of any CT system. Workstations must comply with the Digital Imaging and Communications in Medicine (DICOM) standard and be able to connect with picture archiving and communication systems, radiology information systems and hospital information systems. If only a small archival system is needed, data and image storage devices include optical disks, 8- mm magnetic tape, and writable CD or DVDs. The storage capacity of these media varies depending on the system. Printing images directly onto x-ray film is also an option. 66. Practice Question