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Editorials |
1 From the Department of Radiology, University of California Davis Medical Center, 4860 Y St, ACC Ellison Bldg, Suite 3100, Sacramento, CA 95817. Received January 28, 2006; final version accepted February 1. Address correspondence to the author (e-mail: jmboone{at}ucdavis.edu).
Until recently, multidetector computed tomography (CT) represented an interesting but incremental change in the way CT was performed. However, with the advent of 64–detector row scanners, CT has taken a huge, if not revolutionary, step into the future of medical imaging. Earlier generations of multidetector CT scanners (enabling the acquisition of two to 16 sections) could achieve faster scanning speeds or higher z-axis resolution at traditional helical CT speeds. Extreme multidetector CT scanners enabling the acquisition of 64 or more sections can produce isotropic spatial resolution while simultaneously delivering exceptional temporal resolution (sufficient for cardiac imaging) with excellent z-axis coverage (4–8 cm/sec).
In this editorial, I address the opportunities that extreme multidetector CT offers, the challenges that those of us in the radiology community face in using this technology, and the concerns that extreme multidetector CT raises regarding radiation exposure to patients.
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OPPORTUNITIES |
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The increased temporal capabilities of extreme multidetector CT scanners may find new applications outside of vascular imaging. While cardiac imaging with extreme multidetector CT exploits the periodic nature of the cardiac cycle to achieve very short time windows, the fast (<0.5-second) rotation combined with partial angle reconstruction techniques and multiple x-ray sources in some scanners also lead to very short (eg, 0.08–0.30-second) acquisition times for imaging nonperiodic organ motion outside of the heart. Dynamic CT examinations will likely allow orthopedic surgeons to better understand the nature of joint injuries in the elbow, knee, and shoulder. Having supine patients perform a shortened "crunch" during scanning may help neurosurgeons better understand the dynamics of vertebral disk injuries. Rheumatologists may better understand the location of painful joints. The need for pediatric sedation for CT will be reduced. In addition, a number of new (and old) pulmonary imaging techniques may help physicians take advantage of the temporal resolution of extreme multidetector CT scanners.
Multidimensional Viewing
Contemporary use of preextreme multidetector CT scanners is primarily for diagnostic evaluation of static anatomy. The drastically improved three-dimensional spatial resolution of extreme multidetector CT scanners will ultimately change and improve the way CT images are interpreted. These scanners produce CT volume data sets with isotropic spatial resolution: The 0.5–0.7-mm pixel dimensions that CT has traditionally delivered in the transverse plane (x and y dimensions) now extend to the z dimension as well. Use of extreme multidetector CT will eliminate those blurry, stair-stepped multiplanar reconstruction CT images. Thick-section (eg, 100-mm) imaging in any orientation will allow radiologists to view good-spatial-resolution radiographic projection images from the CT data set, on the fly. Quantitative volume imaging will replace the crude linear metrics typically used to determine the size of solid tumors, and monitoring tumor response during therapy will be more accurate with the smaller voxel dimensions of extreme multidetector CT. A 0.50 x 0.50 x 0.50-mm voxel contains only 125 µg of unit-density tissue, such that a 10-mm-diameter spherical lesion should be seen on 4200 pixels spanning 20 transverse CT sections. These scanners are capable of essentially digitizing the anatomy of a 70-kg human into over a half billion individual voxels.
Axial anatomic viewing, the traditional mode of CT image interpretation, was not established by choice or scientific consensus but rather by the necessity of CT acquisition geometry. Extreme multidetector CT scanners have changed this by enabling coronal, sagittal, and nonorthogonal multiplanar image viewing. Given the changes in the way the CT data stream is managed (discussed below), radiologists will be free to dynamically adjust the viewing orientation to optimize interpretation accuracy. I expect this to be the subject of scores of articles in the clinical CT literature in coming years.
Functional Imaging
CT traditionally has been considered a purely anatomic imaging modality, while combination positron emission tomography–CT and functional magnetic resonance examinations yield functional data. However, the rapid acquisition that is possible with extreme multidetector CT will likely change this perception. With existing contrast materials, real-time imaging can reveal kidney function, organ perfusion, angiogenesis, and lesion uptake kinetics. The four-phase liver examination is certainly a functional study. Pulmonary imaging for chronic obstructive pulmonary disease, with or without inhalation gas contrast material (eg, xenon), will become more feasible with shorter imaging times and better thoracic coverage. Motion is the function of the musculoskeletal system, and, as mentioned previously, joint injuries and abnormalities will likely be better diagnosed by using extreme multidetector CT systems. Thus, these extreme multidetector systems will open the door to many important functional imaging applications that were not possible or were of lesser quality with the older scanners.
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CHALLENGES |
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Infrastructure
It would be foolhardy to think that an imaging facility could simply replace an older multidetector CT scanner with an extreme system, with no other changes. For the purchase of an extreme multidetector CT scanner to be cost effective, a complete evaluation of the infrastructure needed for CT image acquisition and interpretation would be necessary. These systems are much faster; therefore, more patients can be scanned over the course of the day. In vascular facilities, more CT sections are produced per patient, further increasing the daily radiation output of the CT suite. Consequently, the amount of lead shielding for each scanner room will need to be reevaluated, and likely increased, to accommodate the increased workload with these scanners.
Extreme multidetector CT scanners produce numerous images—about two images (1 MB) per millimeter of patient anatomy. For a 30-cm-long scanning length, pre- and postcontrast CT will produce 600 MB of image data, or 1200 images. To accommodate the larger data sets produced with extreme multidetector CT, it is likely that workstation memory and disk space will need to be increased, and the local area network bandwidth may need to be improved. For radiology facilities with older PACS primary diagnostic workstations, a software upgrade or replacement may be necessary to make maximal diagnostic use of the volume data sets typical of extreme multidetector CT. Because referring physicians will want to see some of the images discussed in the radiology report, the hardware, software, and bandwidth for physician review workstations also may need to be upgraded.
Necessary but costly improvements in the infrastructure to support the installation of an extreme multidetector CT scanner are likely; thus, a team of stakeholders in CT should be assembled to assess and advise in the purchase of and infrastructure development for extreme multidetector CT systems.
Contrast Material Protocols
CT needs to be performed in appropriate synchronization with patient breathing and contrast material injection. Because of the faster scanning, CT protocols will need to be adjusted when the extreme multidetector CT scanner arrives. While application specialists will install the standard scanning protocols thought to be useful by the CT vendor, radiologists and CT technologists will need to devote substantial time revising nonenhanced, contrast material–enhanced, and breathing protocols so the scanner is optimized to the specific needs of the institution. Revision of CT protocols should involve a team of radiologists in the various subspecialty areas. All CT protocols should involve, in addition to contrast material timing, the use of automatic milliampere and milliampere modulation techniques to optimize not only the timing of the contrast material bolus but also the radiation dose levels.
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CONCERNS |
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Cardiac vascular imaging and other vascular imaging procedures require multiple scans of the same anatomic section to render four-dimensional images. Cardiac CT scans typically have a pitch of around 0.2, and dose is proportional to the inverse of the pitch. Thus, a single x-ray-source cardiac CT scan with a pitch of 0.2 is really the equivalent of five CT scans with a pitch of 1.0. This is particularly troubling for younger female patients, in whom the radiation dose to breast tissue will be substantially greater than the dose delivered at supine cardiac catheterization.
Many protocols call for CT scanning at several time intervals after contrast material injection and thus an increased radiation dose to the patient. Radiologists who make CT protocol decisions should be cognizant of the patient's age and sex and make every effort to reduce the number of examinations performed in all patients, but especially younger ones. More research is necessary in this area; we all need more knowledge, education, and guidance regarding the acceptable risk-benefit trade-offs.
Most new CT scanners have acquisition protocols that allow automatic selection of the milliampere setting. When these "auto-mA" modes are used (instead of manually setting the milliampere or milliampere-second value, as is usual in many institutions), the scanner adjusts the CT technique factors to the size of the patient. This is a very important and effective way to reduce the radiation dose to pediatric patients and small-stature adult patients. Automatic milliampere selection is essentially phototiming in CT. Caution is necessary, because using these techniques increases the technique factors and radiation dose in larger patients. Thus, the upper limits of milliampere values (part of the auto-mA setup on most scanners) should be adjusted with the dose to large patients in mind. For obese patients, the images will be noisy and the radiologist can compensate for this by viewing thicker CT sections at the workstation.
Extreme multidetector CT scanners have, in addition to automatic milliampere selection, milliampere modulation protocols that should be used for virtually all patients. With angular milliampere modulation, the x-ray tube output changes as the tube rotates around the patient's body, in response to the projected thickness of the patient (determined by the x-ray fluence measured near the center of the detector)—decreasing the milliampere value when thinner projections are being imaged and reducing the dose. This allows a reduction in dose by up to 30% for patient cross sections that are more elliptic than circular, as in the pelvis of most patients. The dose reduction essentially comes free: Image quality is not compromised, since the milliampere value for the thickest projections of the patient determines the noise levels on the reconstructed CT images. Milliampere modulation also occurs along the z-axis of the patient, where the milliampere levels decrease when body sections have a smaller total attenuation. For example, the effective milliampere-second value per CT section will decrease as the lungs are imaged and then increase as the scanner continues to interrogate the diaphragm and abdomen.
Increasing CT Use
CT is experiencing enormous growth: Approximately 62.8 million examinations were performed in 2005 (3)—an 80% increase since 2000 (when 34.9 million examinations were performed). It is a virtual certainty that the increased clinical applications (eg, pediatric, cardiac, vascular) made possible by the fast scanning capabilities of extreme multidetector CT will drive even greater CT use in the near future. CT is already the largest contributor of radiation dose to the U.S. population (4,5), and this will surely increase as the number of examinations per capita increases. This is a serious consideration that the radiology community needs to face. The large uncertainties in radiation risk estimates, long delays between exposure and cancer manifestation, and fact that carcinogenesis is proved by statistical inference rather than by direct observation tend to reduce the perceived urgency for reduced CT radiation doses.
However, we in the radiology community need to realize that the small but acceptable risk-benefit decisions made at the individual patient level are amplified by the huge number of CT procedures performed each year. The overall chance of a radiation-induced solid tumor fatality from a 10-mGy CT examination is estimated in the most recent BEIR (Biological Effects of Ionizing Radiation) VII report (6) to be about 0.00041 (average for male and female patients), a very small risk. But multiply this number by 62 000 000 CT examinations per year and the data suggest that 25 420 fatal cancers are induced by CT per year, with cancer onset occurring substantially later in the patient's life. This calculation has a number of major flaws, the most important being the fact that the risk factors were derived for generally healthy members of the population (of Japanese A-bomb survivors), whereas people who undergo CT are older and sicker than individuals in the general population. Moreover, the health benefit of CT-derived diagnostic information is immediate, whereas the risk of induced cancer is decades away. Nevertheless, this mathematic exercise was meant to underscore the importance of restraint in CT utilization.
Given these numbers, what are radiologists supposed to do? Should they refuse to perform CT in the patients referred to them? Clearly, this is not sound clinical practice. Conservative estimates of the benefit-to-risk ratios for CT are 100:1 and higher. This discussion does suggest, however, that CT should not be performed for dubious or trivial clinical indications. Appropriateness criteria need to be vigilantly applied for all patients recommended for CT. Better medical student training in radiation risk management would be helpful in reducing the number of inappropriate requests for CT. Academic radiologists should push for this better training and give such lectures in their medical schools. In training institutions, perhaps the CT examinations requested by interns and first-year residents should be approved by more senior physicians.
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SUMMARY AND CONCLUSIONS |
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The increase in clinical applications and image quality that extreme multidetector CT scanners offer can lead to a sea change in disease assessment and diagnostic medicine. To remain masters of this technology, radiologists need to (a) know when to use it and when not to, (b) be conversant and knowledgeable about radiation risk issues, and (c) develop radically new interpretation practices that will improve the diagnostic accuracy of every CT examination.
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References |
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This article has been cited by other articles:
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  K. Togashi Invited Commentary RadioGraphics, July 1, 2007; 27(4): 917 - 918. [Full Text] [PDF]
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