Computed Tomography. The Fundamentals of... THE FUNDAMENTALS OF... Jason H. Launders, MSc. Current Technology
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1 The Fundamentals of... Computed Tomography Computed Tomography (CT) systems use x-rays to produce images of slices through a patient s anatomy. Despite having lower spatial resolution than other x-ray imaging modalities, CT images have 2 major advantages: anatomic details are not superimposed over each other and better soft tissue contrast. These advantages mean that CT has become an indispensable diagnostic tool for a wide range of conditions. All CT systems consist of a patient table that is mechanically moved through a large aperture in a gantry. Data is transmitted from a detector to a computer for processing into an image. The performance of the scanner depends on every component working perfectly. Potential problems include mechanical, x-ray tube, cooling system, electronic, and computer-related failures. Jason H. Launders earned a Master of Science degree in Medical Physics from the University of Leeds (UK) in From 1993 to 1998, he worked in the Facility for Assessment of X-ray Imaging Leeds (FAXiL), where he participated in and led research into image quality of digital x-ray imaging systems. Since 1998, he has served as a senior project officer in the Health Devices group at ECRI in Pennsylvania. His primary responsibility is to plan and implement evaluations of radiographic equipment. Studies have included digital x-ray and multislice CT evaluations. The evaluations have been published in the ECRI Health Devices journal. He has also authored a number of papers in peer-reviewed scientific journals in the U.S. and abroad. Jason H. Launders Computed Tomography (CT) systems use x-rays to produce images of slices through a patient s anatomy. Pictured is GE Lightspeed Plus multisplice technology. Current Technology Technical aspects of design and function The goal of the CT scanner is to collect sufficient data to allow 2 dimensional cross-sectional images of patients to be reconstructed. This is possible following the work of Johann Radon in Radon showed how a full set of parallel projections collected at an infinite number of angles through a subject could be reconstructed into a 2 dimensional image. (See figure 1.) A perfect reconstructed image requires an infinite number of individual projections, which is impossible. Instead, today s CT scanners approximate this with finite number of diverging, rather than parallel, projections. (See figure 2). The central component in any CT scanner is the gantry. The gantry contains the x-ray tube, collimators, filters, and detector that rotate around the patient. The x-ray tube emits x-rays that are shaped by the collimator and filters. After passing through the patient, the remaining x-rays are re-collimated before being detected using an arc-shaped array of about 1000 individual detector elements. Efficient imaging requires that: the x-ray tube, collimators, filters, and the detector array are all perfectly aligned with each other. All modern gantries use slip-ring technology that allows continuous rotation. As the rotation speed has increased over the years (now down to 0.5 second per rotation) so the mechanical tolerances become more demanding. The x-ray tube emits x-rays in all directions. Collimators and filters are used to produce a useful beam. The collimators shape the beam to a fan shape. Biomedical Instrumentation & Technology 53
2 The Fundamentals of... Computed Tomography The filters tailor the x-ray intensity and spectrum of the fan beam. A number of collimators are needed both close to the x-ray tube and the detector. Some are fixed to match the maximum fan beam slice required, while others are adjustable to match any desired slice width. Flat filters are used to remove low energy x-rays, which contribute to the x-ray dose and not the image. In addition, a bow-tie shaped filter is used. The bow-tie shape modulates the x-ray beam so that the highest intensity is incident on the thickest part of the patient. Since so much collimation and filtration is used, a very high x-ray output is necessary. X-ray production is already very inefficient (~1%), so a lot of heat is generated. In addition, the x-ray tube must withstand the rotation forces associated with the 0.5 second rotation speed. Therefore, the wear on the x-ray tube is significantly higher than in other x-ray imaging modalities. The x-ray tubes used in CT scanners are very expensive approximately $100,000 and are a significant cost in ownership. Manufacturers have developed new tube technologies to improve the life and reduce the associated downtime. The design of the detector is crucial for good imaging. The detector must be able to convert the incident x- ray signal into a digital signal as the gantry rotates. The key components are the detector material, pre-amplifiers, and analog-to-digital converter. Manufacturers have used a number of detector materials. The most common detector materials found in CT scanners made today are ceramic scintillators. Minimizing electronic noise is very important to maintain acceptable image quality. This is achieved through good design, and most importantly, cooling. Therefore, the CT gantry must be supplied with the high voltage required for x-ray production and a cooling mechanism for both the x-ray tube and the detector. An x-ray generator supplies the high voltage through the slip-rings. A conventional cooling system supplies the coolant through hoses. The patient table is also an integral part of the CT scanner. The table must be able to move precisely and smoothly, regardless of the patient s size and position. In a standard exam, lasers are initially used to align the patient with the x-ray beam. Next, a preview scan is acquired by moving the patient through the gantry while the x-ray tube is locked in a fixed position. The resulting scan, which is similar to a standard radiograph, is used to plan the actual CT scan. Therefore, mechanically reproducing the table position, based on the preview scan, is essential for good imaging. In the original CT scanners, the x-ray tube and detector rotated around the patient then the patient was moved to the next slice location. This step and shoot operation is known as axial scanning. In modern CT Figure 1. Two sets of parallel x-ray beam projections through a patient. Figure 2. A diverging x-ray beam projection (fan beam) through a patient. 54 January/February 2002
3 scanners, the x-ray tube and detector rotate at the same time as the patient table is moved through the gantry. This continuous motion is known as helical, or spiral, scanning. Helical scanning is much faster than axial scanning. However, helical scanning is more prone to artifacts so some exams are still made with a helical scanner working in axial mode. Successful helical scanning calls for smooth and accurate table movement. The final major component is the computer system. The most important duty of the computer is to reconstruct the image data. Thousands of data values from the detector corresponding to individual attenuation projections (see figure 2) are transferred to the computer. The reconstruction algorithms used are based on Radon s theories. Today, scanners come supplied with a wide range of proprietary algorithms. For example, some reproduce smooth images with little fine detail, while others reproduce fine detail at the expense of more image noise. The computer also acts as the interface between the user, gantry, and any other networked device. As a user interface, the computer is used to control all aspects of the system. The gantry generates about 20 megabytes per second of raw data. This data must be stored and analyzed at the same rate to avoid delaying future scans. Therefore, efficient scanning requires powerful processors and well designed computer architecture. Also, any data storage must be properly configured to ensure that data can be easily retrieved for reprocessing if necessary. Historical Dates of the CT Scanner 1895 W.C. Roentgen discovers x-rays 1917 J.H. Radon develops cross-sectional image reconstruction mathematics 1972 G.N. Hounsfield develops first practical CT for head scanning CT scanners installed worldwide 1975 First full body CT scanner developed ,000 CT scanners installed worldwide 1987 Slip-ring technology introduced to CT 1989 First helical CT scanner 1992 First helical CT scanner with 2 rows 1998 First multislice CT scanner with 4 rows ,000 CT scanner installed worldwide 2001 First multislice CT scanner with 8 rows Being connected to a network of other systems means that the network configuration is important for successful and efficient use. Managing the Device A CT scanner should be treated as a complex x-ray imaging system. Good equipment management, or quality assurance, is essential for reliable long-term equipment performance. Maintaining good image quality and acceptable radiation dose depends on both the mechanical and electrical integrity of the system. Therefore, it is important to follow the manufacturer s recommended preventive maintenance (PM) schedule. The PM usually takes about a day and is undertaken by a trained engineer at least twice a year. Most moving parts are checked, cleaned, and lubricated. Components subject to wear should be replaced at the recommended interval. For example, the slip-ring assembly and cooling system are particularly important. In addition to regular PM procedures, the CT technologists are responsible for most of the routine equipment management. A warm up is needed daily and a calibration usually 2 or 3 times a week. These procedures, which are usually automated, take about 15 minutes. The most expensive component needing regular replacement is the x-ray tube. In routine use the x-ray tube must withstand considerable mechanical and heat related stresses. The most obvious measure of x-ray tube life is the number of rotations it has been through, similar to the mileage on a car. However, other factors also need to be considered: x-ray tube design, variable manufacturing quality, exposure factors (kvp and mas), rotation speed, focal spot size, cooling efficiency, and patient workload. Analogies could be made with a car; a car used for city driving will age quicker than a car used for long distance road trips. In the same way, it is very difficult to predict the longevity of an x-ray tube. As the tube ages, the x-ray output gradually decreases and it becomes more susceptible to electrical arcing, eventually it will fail and need replacing. Replacing the tube takes about a day due to the need for careful re-calibration and aligning of the system. Regulations The basic requirements for a CT scanner sold within the United States are itemized in Title 21 of the Code of Federal Regulations (CFR) part 1020 sections 30 and 33. Among the requirements are definitions on how radiation dose should be measured, the information and Biomedical Instrumentation & Technology 55
4 The Fundamentals of... Computed Tomography tools that must be provided by the manufacturer, and the minimum acceptable performance, in terms of accuracy, of the system. In particular, every CT scanner must be supplied with a test phantom that allows the performance and calibration of the system to be quantitatively measured. Once installed, CT scanners are regulated under individual state codes. Normally, the same rules that govern other medical x-ray equipment are extended with additional requirements to CT scanners. Among the responsibilities of hospitals are credentialing appropriately qualified personnel to operate and test CT scanners. Owners must also ensure that the required radiation protection, warnings, etc. are in place. The principal concerns of regulators are radiation safety and image quality. Most states require annual dose measurements and regular assessments of easily quantifiable image quality parameters. In addition to these regulations, the following guidelines regarding the use and quality assurance of CT scanners are available: American College of Radiology (ACR) Standard for Diagnostic Medical Physics Performance Monitoring of Computed Tomography Equipment. The ACR also publishes specific clinical guidelines. The National Council on Radiation Protection and Measurements (NCRP) Quality Assurance for Diagnostic Imaging. NCRP report no. 99. This report includes suggested performance criteria and testing frequencies. The American Association of Physicists in Medicine (AAPM) Specification and Acceptance Testing of Computed Tomography Scanners. AAPM report no. 39. This report specifies how radiation dose and image quality measurements should be undertaken. Risk Management Issues CT exams are regarded as safe, however hazards remain. The main hazards are patient injury due to moving parts, contrast media related problems, radiation injury, and misdiagnosis. As has been described, a CT scanner contains multiple moving parts, including a rapidly rotating gantry. Despite this, the only moving part accessible to patients and staff is the patient table. Patients must be briefed to keep fingers etc. away from the side of the table to prevent them from getting caught. Also, to reduce the chances of injury, the operator must manually initiate all table movements. It is important that the operator has an Toshiba Aquilion multislice technology. The whole system (no external x-ray generator or cooling). unobstructed view of the patient. Contrast media is used for a number of CT procedures. The standard precautions needed with any other use of contrast media must be followed to minimize the risk of a severe patient reaction. However, unlike many fluoroscopic procedures that use contrast media injected intra-arterially, CT uses contrast media intravenously. Intravenous administration is intrinsically safer. The successful use of contrast media in CT relies on the timing of the scan following the injection. Manufacturers now provide options that allow for the automated synchronization between the contrast injector and the scanner. The main problem with contrast media is that you only get one chance to inject. If a problem occurs for example if the computer crashes while scanning or any other hardware failure the procedure cannot be repeated for 24 hours. This may have serious implications. While some failures are beyond a hospital s control, good PM will minimize the risk. The radiation doses from CT examinations are among the highest for any diagnostic imaging modality. Also, unlike for other x-ray imaging modalities, dose is not usually controlled during an exposure. Since most radiation damage is not immediately apparent, but may take years to develop, users have little incentive to minimize dose. Unfortunately, this means that some patients, especially pediatrics, are receiving unjustifiably high doses. Some modern CT scanners can display the dose. Unfortunately, this is of limited use to operators. A better solution is an automatic dose control mechanism. Some modern CT scanners now have such systems. 56 January/February 2002
5 Manufacturers indicate that these systems can lead to significant dose reduction without compromising image quality, and importantly, without additional work for the operator. Misdiagnosis is always going to be a problem in any diagnostic procedure. However, the number of simple mistakes, such as non-communication of results, can be minimized with good patient and image management. Assuring that the image quality and calibration are optimized is the best way to minimize genuine errors by radiologists. Effective equipment management is the key to minimizing the chances of misdiagnosis. Troubleshooting Modern CT scanners rely on system-generated error codes to aid troubleshooting. Such codes are best suited for electrical or electronic problems. A lot of problems are mechanical. For example, the patient table, which requires sub-millimeter accuracy for good imaging, can be knocked around a lot and fail. Therefore, mechanical aspects should always be considered when tracing a problem. It is always important to discover the root cause of a problem, particularly if the problem is recurring. The cooling system could be the reason some components fail. Also, the quality of the power supply should be checked. Systems with power conditioners tend to have fewer service problems. Training/Serving Each model of CT scanner has its own specific service issues. Most manufacturers will train customers to service specific models. Usually this is the same training that is given to the manufacturer s own service engineers. Some manufacturers also share service experience with customer s own engineers. Third-party training organizations also offer training on some older CT models. In addition to the training, it is necessary to obtain the appropriate service licenses from the manufacturer. It is essential for any CT engineer to be familiar with computer operating systems, networking, and Digital Image Communications in Medicine standard (DICOM) management. Other than standard service tools, the service engineer will need the phantoms supplied with the scanner and, preferably, a means to measure x-ray output on a CT scanner. The prices demanded by manufacturers for training and licenses can be very high. Also, individual engineers need to work regularly on a particular type of scanner to gain experience. For these reasons, it is recommended that a hospital or group only undertake in-house service if it owns a number of similar CT scanner models. Future Development The latest development of CT has been the introduction of multislice technology that was introduced in late Before 1998 most scanners could only acquire 1 slice with each rotation. The ability to acquire 4 slices simultaneously was enabled by advances in detector design and data transfer rates. Not only is multislice faster, but it also allows thinner slices to be acquired and increases the clinical applications for CT. For example, 3-D imaging is becoming routine. Manufacturers are now indicating that they are developing scanners that can acquire 8, 16, or even 32 slices per rotation. However, in the near future, these will simply be more, but thinner, slices. The overall speed to volume coverage ratio will not increase. This is because artifacts caused by the cone shaped x-ray beam limit the maximum width of the beam. Some manufacturers are indicating that true cone beam CT is being developed. Such a device would enable CT to make true snapshot CT images of large anatomical regions. The clinical applications can now only be imagined. For More Information... Books Computed Tomography, Fundamentals, System Technology, Image Quality, Applications. W.A. Kalender. Publicis MCD Verlag. ISBN Understanding Helical Scanning. C. Blank. Williams and Wilkins. ISBN X. X-Ray Repair. J.J. Panichello. Charles C Thomas Publisher Ltd. ISBN Web Sites of Interest General Clinical information and tutorials on multislice CT, and American College of Radiology, Manufacturer Web Sites GE Medical Systems, Philips Medical Systems, Philips Medical Systems, Shimadzu, Siemens Medical Systems, Toshiba America Medical Systems, Biomedical Instrumentation & Technology 57
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