For more than three decades, CT has been an important diagnostic imaging tool.1,2 In particular, technologic advances in the last 5 years have influenced the patterns of use of CT. Simply stated, we are using CT more frequently.3 The principal technical advancement responsible for this increase has been multidetector CT (MDCT), offering both faster scanning and the potential for higher image quality.2 There have been several important consequences of this technology. First, there are more options (and therefore protocols) for CT evaluation. These options involve manipulating a variety of CT parameters that control the amount of radiation delivered, a direct determinant of image quality.4,5 While there are many options, some of these are inappropriate in that the amount of radiation a patient receives is in excess of what is necessary for obtaining a diagnostic examination.6,7 Since the amount of radiation that CT delivers overlaps with the amount of radiation that has been reported to cause cancer,8 this is a cost of CT. It is this recognition that there is a potential (and many argue a tangible) risk of radiation that has driven many of the recent technical developments with CT. That is, there is an increasing call to balance image quality against the risk. For these reasons, the following material will review recent and projected patterns of use, discuss the technology and technologic advancements of CT and their effect on clinical applications, and summarize some of the recent clinical applications as well as what we know (and do not know) about radiation risk with CT.
Patterns of CT Use
Since its introduction in the early 1970s, CT has become an invaluable tool in diagnostic imaging.1 More important, the use of CT is increasing. It is not absolutely known how many CT examinations are performed either in the United States or worldwide per year. Estimates include as many as 65 million CT examinations performed annually in the United States.9 If we assume that the United States accounts for about 25% of the world total,3 this means that there are potentially 260 million CT examinations performed worldwide. If one just considers the number of CT examinations performed in the United States, given the US population of 290,000,000,10 according to the 2002 census, then CT examinations are performed at the rate of one for every four or five persons. For children, estimates have ranged from 600,000 to 1.3 million examinations in the United States per year.3 However, recent data by Mettler et al have suggested that this may be an underestimate as approximately 11% of all CT examinations may be obtained in the pediatric age group.11 Applying this percentage to 65 million annual examinations in the United States, the number of pediatric CT examinations may be more than tenfold greater than assumed.
The number of CT examinations has also risen dramatically over the past 20 years. Several sources help to underscore this point.3,9,12 For example, in a 14-year period ending in 1995, there was a sevenfold increase in the number of CT examinations. Another estimate covering an 18-year period was that the number of examinations rose from 3.6 million to 33 million, more than an 800% increase. Other sources suggest that CT use is expected to increase at a rate of about 10% to 15% per year.13 Moreover, these numbers do not reflect the trajectory of current use. That is, the newest multidetector technology is continuing to drive as well as accelerate use through improved value in both traditional applications as well as new applications. The traditional applications include improved evaluation of trauma, and cancer detection and surveillance. New applications, over the past 5 years, include CT angiography (CTA) of cardiac and vascular structures, evaluation of pulmonary embolism, urologic assessment (eg, renal stones), assessment for appendicitis, small bowel obstruction, and screening CT for coronary artery disease and cancer. Notably, these new applications are frequently encountered medical conditions, translating to frequent CT evaluation. It is this burgeoning application of new MDCT technology for common medical indications, especially screening CT, that will likely accelerate the frequency of examinations with important socioeconomic health care ramifications.14,15 Parallel to this increasing use, and partly due to the increased attention to radiation risks, there has been a call for regulation and practice standards.3,16 A review of these standards is beyond the scope of this article, but the reader is referred to a source to be available early in 2004.3
During a CT examination, the individual lies on a bed, also known as a table. This table passes through a gantry that contains the x-ray source opposite (180?) the x-ray detectors. This gantry continuously rotates around the patient while the table moves through the gantry. The images are formed based on the character (energy and amount) of the x-ray beam that strikes the detectors after it passes through the individual. The character is affected by the various organs and structures through which it passes. As with a conventional 35 mm camera, a variety of settings or parameters (these are selected by the CT technologist on the scanner console) controls the amount and energy of x-rays. Examples of these settings include tube current (milliamperage or mA), peak kilovoltage (kVp), and the speed of rotation of the gantry equipment or speed of movement of the table through the gantry. These settings contribute to image formation and image quality.
In the early 1990s, there was a great leap forward in CT: slip ring technology. It freed the gantry to rotate continuously, unimpeded by wires and cables that previously meant that one to two clockwise rotations had to be followed by one to two counterclockwise rotations to keep the winding apparatus from binding. This leap was called helical (or spiral) CT; the term basically represents a tracing of the spiral path of the x-ray beam along the patient due to the gantry rotating continuously while the table (and the patient) moved through the gantry. In addition, the x-ray detectors have continued to evolve through several iterations usually resulting in a more effective and efficient trapping and conversion of x-rays. In 1998, the detector technology again moved forward so that multiple rows of detectors could simultaneously capture and convert x-rays. This advancement is called multislice or multidetector CT. Over the past 5 years, the number of detector rows has increased from a single row (the first helical CT) so that now manufacturers offer 16-row (or 16-slice) MDCT. Basically, the increased number of detectors allows conversion of a wider x-ray beam for each rotation. One benefit of this wider beam is that the patient can now travel through the x-ray scanner quicker. For example, MDCT scans in the chest or abdomen in young children can be completed routinely in 2 to 5 seconds. Improved image quality has also resulted from this evolving technology.
Faster scanning has several benefits. First of all, in children, sedation is less often needed. This is a substantial benefit compared with the long image acquisition times for MRI (each MRI sequence can take several minutes to acquire, and the total number of sequences means an examination duration typically of 30 to 60 minutes). Fast imaging also reduces the use of resources required for pediatric sedation, a great savings in cost.17 This is one reason that, in children, MDCT is being performed more frequently than MR for similar applications. Faster imaging also decreases motion artifact, particularly in patients that have limited breath-hold ability during scanning, such as young children. Faster scanning has also been applied to “freeze” periodic motion such as that with the heart for cardiac and coronary artery evaluation. While throughput is potentially improved by faster scanning, much of the patient’s scan time is spent setting up the scan, preparing the patient, and cleaning the room. Still, there is some improvement in throughput with faster MDCT.
|Figure 1. Ten-year-old boy with pain and swelling of his left lower extremity for several months. (a) Lateral radiograph shows thickened, sclerotic cortex of the left tibia. (b) Axial image from a CT examination through the mid tibia shows a small area of lucency with a more central area of dense sclerosis ? the nidus (large arrow). Note the surrounding thickening and sclerosis of the tibia (small arrows). This lucent area represents the center of a benign bone tumor, an osteoid osteoma. (c) Reconstructing the axial data set into a sagittal plane nicely demonstrates the nidus and scelorosis with no artifacts.
Another benefit of MDCT technology has been that thinner slices can be obtained. The advantage of thinner slices is improved detail, in particular image sharpness (or spatial resolution). The most recent MDCT using thin (submillimeter) slice thicknesses offers the opportunity for multiplanar (for example, coronal and sagittal) and three-dimensional depictions of structures (Figure 1), which are essentially free from artifacts that plagued older CT technology .18 Images can now be reconstructed rapidly and efficiently in multiple planes with detail that would have been achieved if the scan was actually obtained in the plane. This obviates additional planes, for example for CT scans of skeletal abnormalities, thus reducing time, cost, and radiation exposure. Another technical advancement includes more efficient detectors and new technology that improves quality and speed of the reconstructed pictures.
Other technologic advancements include CT fluoroscopy, and the combination of positron emission tomography and CT (PET-CT). With CT fluoroscopy, interventional procedures such as aspirations, biopsies, and abscess drainages can be facilitated by using cross-sectional information for guidance.19,20 PET-CT represents a “blending” where functional images from PET (areas of increased metabolic activity of cancer, for example) are combined with CT (for improved anatomic localization).21, 22 PET-CT, in particular, has been a powerful and rapidly expanding tool in many practices.
MDCT has, with each increase in the number of detectors, often been met with some skepticism about the touted benefits, but each time has been quickly embraced as valuable for faster examinations, more flexible scan options, and improved image quality, equating improved diagnostic opportunity and clinical application.
Some of the recently reported applications for MDCT include chest and abdomen evaluation. In the chest, these investigations include evaluation of nodules, pulmonary embolism, cardiovascular structures including the coronary arteries, the airway, and the chest wall.23-28 Abdomen and pelvis indications include urinary tract virtual endoscopy; evaluation of urinary tract cancer, stones, and congenital disorders; vascular disorders; appendicitis; and bowel obstruction.29-35 Screening CT includes detection of lung cancer, colon cancer, coronary artery disease, and whole body screening.14 Together, these reviews and investigations demonstrate the broad and expanding role new CT technology has in medicine.
CT Costs include Radiation Risk
With these benefits, however, has come a recognition of one potential costthat of radiation. More than 2 years ago, this issue was emphasized through a series of articles in the American Journal of Roentgenology dealing with children and cancer risk from CT radiation, an excess in radiation exposure, and techniques to reduce this exposure.36-38 Since that time, the focus of manufacturers and the practice of CT examinations, in both adults and children, have been slowly changing, acknowledging the potential cost of radiation. This was relatively neglected for many years.
There is some debate as to what the risk of radiation is. Basically, investigations can be found that support the contention that the amount of radiation (low-level exposure) in CT is not associated with increased risk of developing fatal cancer,39,40 and investigations can be found that show that the amount of radiation in CT is a risk factor for cancer.38 At this point, the more prevalent posture is the latter: the doses of radiation delivered by CT scanning overlap those that have been shown to have a significant increased risk of cancer. Proponents of this viewpoint have pointed out that even a single CT scan in a child can increase the risk of lifetime cancer mortality.38 What is not debated are the facts that children are more susceptible to radiation than adults, have a longer lifetime to manifest radiation-induced cancer (which can take decades to develop), and have been routinely exposed to an excess amount of radiation from CT. CT doses overlap and can even exceed low-level exposure.3 Moreover, CT is the single largest source of radiation after background (including radon) exposure.11,41 Irrespective of one’s posture, it is prudent to minimize unnecessary radiation exposure. As stated in the 2000 report from the United Nations Scientific Committee on Effects of Atomic Radiation Report (UNSCEAR), “It should be noted, however, that the inability to detect increased [cancer] risks at very low doses does not mean that those increases do not exist.”41 Our posture should be to minimize the amount of radiation individuals are exposed to during CT. Clearly, the recent CT innovations, and market positions taken by manufacturers, have been to emphasize steps taken toward managing radiation.
Many of the technical advancements, particularly in the last 2 years, are aimed at managing radiation dose. These include automatic tube current modulation (ATCM) and recommendations for size-based scanning in the pediatric population.
ATCM is a new method in which one of the scan settings, the x-ray tube current, is automatically adjusted during scanning to account for patient thickness, shape, or part of the body being scanned.42-44 The principle underlying ATCM is that there may be different requirements for tube current during the scan. Higher tube current (which generates more x-ray particles) is required to pass through denser tissues such as the liver versus the air-filled lungs; for thicker cross-sectional areas, such as the body side-to-side versus front-to-back, during the 360? rotation of the x-ray beam around the patient; or in small children or thinner adults versus thicker adults. Up to this point, a single tube current (usually relatively high to penetrate the densest tissues) was used for the entire CT scan. Most manufacturers now have incorporated some sort of ATCM that will adjust the tube current to the necessary level (thus lowering radiation) in those regions of the body or portions of the scan where less tube current is required.
One downside of modern MDCT technology is that scanning has been more complex with many more options. It can be quite difficult to determine what type of setting should be used for various medical conditions. Industry has recently been providing pediatric CT guidelines and protocols that incorporate settings that are either age- or size-based45 since small children do not require or do not need the same types of settings (such as tube current) as those in adults.36 Three years ago, this type of adjustment was rare, with most practices using a “one-size-fits-all” philosophy.
Another downside is that CT technology is relatively expensive: a new MDCT scanner costs $1 to $1.5 million. This is particularly a problem with the rapid advancements over the last 5 years. By the time a new scanner was installed, newer technology was often available or soon to be. The justification for spending this kind of money is beyond the intent of this article. Be that as it may, the penetration of the newest 16-slice scanners is increasing rapidly in the United States. Whether this is market-driven (having the newest technology), due to the recognized benefits, or (probably the case) a combination3 of both is irrelevant. This conversion is happening.
Ultimately, what needs to be determined is the cost-benefit ratio. This will depend on a multitude of factors shaped by individual experience, practice guidelines, and standards, all helped by scientific investigation. While much has been done to define the diagnostic quality of CT in medical imaging in, for example, diagnosis of appendicitis, urologic disorders, and pulmonary embolism, balancing these against risk (ie, radiation) is less clear, and assessment of actual change in patient outcome (particularly with screening CT) is still in its infancy. There is a long way to go to define the cost-benefit ratio for modern CT. What is clear is that the empirical experience, particularly evident through the purchase of the newest technology scanners, is driving use. Implied is that radiologists have accepted that CT is an increasingly useful tool.
In conclusion, CT is a central imaging modality. Recent technology has been responsible for increasing use, through both novel applications as well as applications for common disorders. The benefits of this technology, particularly faster scanning and the ability to obtain thin, high-quality slices, now must be balanced against costs. One notable cost is radiation exposure. Advances have provided new opportunities to scan but also important opportunities to manage radiation dose. The role of CT will need to be clarified with a combination of research, education (including practice standards), and manufacturer innovations.
Donald P. Frush, MD, is chief of pediatric radiology, division of pediatric radiology, associate professor of radiology, Department of Radiology, Duke University Medical Center, Durham, NC.
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- Society for Pediatric Radiology and National Cancer Institute. Radiation and pediatric computed tomography: a guide for health care providers. 2002. Available at: www.cancer.gov/cancerinfo/causes/ radiation-risks-pediatric-CT. Accessed July 2, 2003.
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- UNSCEAR 2000 Medical radiation exposures, annex D. United Nations Scientific Committee on the Effects of Atomic Radiation Report to the General Assembly. New York.
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