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Commentary |
1 Department of Radiology, Winthrop-University Hospital, 259 First St., Mineola,
NY 11501.
2 Department of Radiology, Stanford University Medical Center, 300 Pasteur Dr.,
Stanford, CA 94305.
3 Radiological Sciences Laboratory, Stanford University School of Medicine,
LUCAS MRS Center, Rm. P-287, Stanford, CA 94305.
Received July 30, 2002;
revised July 30, 2002;
Address correspondence to D. S. Katz.
As any reader of the American Journal of Roentgenology is aware, a spate of articles and editorials have been published recently on the significant issue of radiation dose from CT, especially in younger patients who may receive higher effective doses and are at greater risk from radiation exposure [3, 5,6,7]. The question of dose is especially problematic when considering CT studies performed on multidetector scanners. Such scanners permit shorter scanning times and significantly greater volumetric coverage, and they are being rapidly installed worldwide [8]. However, using imaging parameters on multidetector scanners that are similar to those used with single-detector helical scanners can result in greater dose exposures [3, 7,8,9,10]. Unlike conventional radiography units, CT scanners have no standardized automated exposure control. An overexposed image only improves image quality, which encourages continuation of CT protocols using such technical factors. Unfortunately, only a few prospective studies have been published on dose reduction for helical CT of the abdomen and pelvis, in either adults or children—and even fewer using multidetector CT.
The oldest investigations of radiation dose reduction on CT concentrated on the thorax, where the intrinsic high contrast is ideally suited for such studies. Although multiple reports over the past 12 years have shown that reduced-dose CT can be performed for a variety of indications without sacrificing diagnostic accuracy, we agree with the recent comments of Prasad et al. [7] that low-dose CT is not routinely accepted or practiced in adults, even for thoracic imaging. More recent investigations have focused on the abdomen, specifically CT colonography—a CT study in which the high intrinsic contrast between the air-filled colonic lumen and the higher density colonic wall might indicate that reduced dose examinations are suitable. A recently published study confirmed a prior report from the Mayo Clinic group, in finding that reduced-dose helical CT did not decrease the detection rate of clinically significant colonic polyps [9].
Unenhanced helical CT for the evaluation of patients with suspected renal colic would initially appear to be another situation in which radiation dose reduction could be instituted because of the high contrast between almost all urinary tract stones and the adjacent, much lower density, soft tissues [3, 11]. Several studies have been published in the past 3 years that examined whether lower-dose CT could be successfully implemented. Diel et al. [11] increased the pitch to either 2.5 or 3.0 when using a single-detector helical scanner by keeping the slice thickness constant while increasing the table speed. Although image quality did decrease (principally from increased volume-averaging artifacts related to the broadened slice profile), accuracy remained at a relatively high level of 93% for 59 patients. Similarly, using a pitch of 2, Liu et al. [12] reported a 97% accuracy rate in 60 patients, but achieved only a modestly lower dose than in previously reported protocols. Because of the linear inverse relationship of pitch to dose on a single-detector helical scanner, keeping slice thickness constant, the higher the pitch, the lower the corresponding dose. Caution must be taken if this approach is used on multidetector CT scanners, however, because on some CT units the tube current (i.e., mA) setting may be automatically increased to maintain the same noise level if the pitch is increased, which would cancel out any dose reduction [6].
Most recently, instead of increasing the pitch, attempts to reduce dose from CT examinations for suspected renal colic have focused on decreasing the tube current, as was done for almost all of the thoracic studies that have examined the utility of reduced-dose CT. Radiation exposure from CT is directly related to the product of X-ray tube current and time (i.e., mAs); as expected, as the mAs is reduced, the dose decreases proportionately—although image noise also increases, reducing image quality [3, 9, 11,12,13]. Spielmann et al. [3] performed a phantom study that showed that renal calculi as small as 2 mm were identifiable at CT exposures obtained at 60 mAs. On their multidetector scanner, the CT dose index decreased from 14.9 mGy at 170 mAs to 5.2 mGy at 60 mAs. Hamm et al. [13] used a protocol of 70 mAs, 5-mm slice thickness, and 2.0 pitch on a single-detector helical scanner and reported high accuracy in 109 patients.
In a study of 106 patients published in this issue of the AJR, Tack et al. [1] further pursue the goal of reduced-dose multidetector CT for suspected renal colic by routinely decreasing the dose using only 30 mAs at a peak kilovoltage of 120. Supplemental images were obtained around an equivocal ureteral stone or areas of significant noise, but this proved to be useful in only a small number of these patients when their studies were retrospectively reviewed. Accuracy was high, between 93% and 98% for the three radiologists who interpreted the studies, and there was excellent interobserver agreement. The mean effective dose was estimated at only 1.2 mSv for men and 1.9 mSv for women. The authors estimated that if one were to use more routine multidetector CT protocols, the dose could be increased by up to 10 times or more in comparison with their low-dose protocol [1].
Several aspects of the current investigation by Tack et al. [1] deserve further discussion. First, until recently, body habitus and the specific mAs that should be used for CT scanning have often been ignored in daily radiology practice. Noise on CT images increases with increasing body size (i.e., patient diameter), if other imaging factors are kept constant. It is easier to reduce the mAs in this patients and small children without sacrificing image quality as much, compared with in larger patients. Some practices have implemented CT technique charts that are based on patient diameter [10]; alternatively, patients can be triaged to the appropriate dose by calculating body mass index. However, a better solution is the application of relatively new software now available from all of the major CT equipment manufacturers, which adjusts the mAs continuously, depending on the thickness of the patient being penetrated by the beams at that point in time [8, 10]. This software could also be used in conjunction with generalized reductions in mAs levels when appropriate, by setting maximum mAs thresholds for the particular examination.
Second, the authors did not formally address the identification of associated findings, such as perinephric edema, which are almost always present to some extent if an obstructing ureteral stone is present or has recently passed. In cases in which it is less clear whether a ureteral stone as opposed to a phlebolith is present, reducing the dose could make identification of these secondary findings more difficult [3]. The images shown in the article by Tack et al. [1] suggest that this was not a major problem, however. Similarly, Hamm et al. [13] claimed no deterioration in the detection of these secondary findings of obstruction when they used their low mAs protocol.
Third, Tack et al. [1] acquired 2.5-mm-thick images and reconstructed 3-mm sections at 2-mm increments. They believe that this is the ideal slice thickness for multidetector CT and suggest that the use of this slice thickness (combined with multiplanar reconstructions) accounts for the high accuracy in their study compared with other, single-detector helical CT investigations. At both of our institutions, when imaging patients with suspected renal colic, we routinely obtain 5-mm-thick images on our multidetector scanners, and we do not agree that the use of 5-mm images significantly worsens z-axis resolution. These relatively thicker images preserve image quality at a lower level of dose exposure, because when thinner images are acquired on multidetector CT scanners, radiation dose efficiency decreases [10]. We routinely identify renal and ureteral calculi as tiny as 1 mm on our 5-mm images. Although we agree that the quality of multiplanar reconstructions worsens when thicker images are obtained, we do not believe multiplanar reconstructions are important for the accurate interpretation of most, if not all, unenhanced helical CT scans obtained to detect suspected ureteral stones.
Finally, and most important, our greatest concern regarding the protocol proposed by Tack et al. [1] is the potentially increased risk of missing alternative and additional significant diagnoses on unenhanced CT. Such identification is probably the most powerful advantage of performing CT in patients with suspected renal colic, compared with the previous examination of choice, excretory urography [2, 4, 11]. These patients have been shown to have innumerable alternative diagnoses that can be identified on unenhanced CT. Although all 13 patients in the current study with such diagnoses were reported to have been retrospectively correctly identified by all three radiologists (as they were in all patients in the study by Hamm et al. [13]), in our opinion, more research is needed before this issue can be satisfactorily resolved. In the study by Diel et al. [11], the less experienced radiologist correctly identified fewer alternate and additional diagnoses.
Especially in patients without much intraabdominal and intrapelvic fat, clinically significant alternative or additional diagnoses can be subtle and difficult to correctly identify [2]. Also, in the study by Katz et al. [2], one quarter of the 101 patients in whom significant abnormalities other than urolithiasis were revealed on unenhanced CT had both a ureteral stone and an additional diagnosis evident. As with all CT examinations that include a global look at that portion of the body, careful attention must always be paid so that such diagnoses are not overlooked. The presence or absence of a ureteral stone is therefore the primary but not the solitary issue to be considered when deciding what should be acceptable CT image quality. If CT using very low mAs, as proposed here, is to be adopted broadly, there must not be a major trade-off with diagnostic accuracy. The delays in diagnosis, potential misdiagnosis, and additional testing (i.e., repeated CT with a higher dose or with IV contrast) that might result would not justify the theoretical benefits of dose reduction [3]. Additionally, although missing tiny stones on unenhanced CT with very low mAs would not be as important as missing a significant alternative or additional diagnosis, it is still desirable to correctly label or exclude patients as "stone formers," so that appropriate urologic workup can be performed when urolithiasis is initially diagnosed, to attempt to prevent recurrent episodes.
As Tack et al. [1] correctly point out, another problem with validating the accuracy of low-dose multidetector CT scans of patients with suspected renal colic is that, paradoxically, the CT examination itself has become the gold standard [11]. Several approaches have been taken to address this problem, because repeating each entire examination at both higher and lower doses at the same time presents significant ethical problems. Investigators have gotten around this by using the imperfect comparison standard of hematuria testing, clinical follow-up, and imaging studies other than CT [1, 11, 13], by using phantoms [3] or animal models, or by simulating noise with postprocessing techniques [9]. The ideal research approach in this situation remains unclear.
In summary, the article by Tack et al. [1] is a valuable addition to the imaging literature. Their carefully done study will, we hope, prompt further detailed and larger prospective investigations. We strongly agree with the points made by Rogers [5] and several other researchers who have recently commented on radiation dose reduction in the pages of the American Journal of Roentgenology [1, 3, 6, 7]: radiologists themselves are ultimately responsible for their patients and for how CT equipment is used, and it is diagnostic accuracy and not image quality that is most important. The current research by Tack et al. and prior reports about imaging other areas of the body support the concept of substantially reducing dose by decreasing mAs, resulting in decreased image quality but not necessarily decreased diagnostic information. We believe that this trade-off is usually acceptable, especially if a ureteral stone is actually present, but we think that in the United States a setting of approximately 60 mAs would probably be a more reasonable dose to use in scanning average size patients. We remain unconvinced that 30 mAs is appropriate for scanning average size—or, in particular, obese—individuals. Scanning patients at 60 mAs would still very significantly reduce the dose compared with the multidetector CT protocols currently in use [13]. Caution should be urged before multidetector CT protocols with very low mAs are universally endorsed for imaging patients—particularly those who present with an initial episode of suspected renal colic—because of our concerns and the concerns of others [3] regarding the alternate and additional diagnoses that could be missed if the dose is lowered too much. We encourage continued research in this area, both to validate the work of Tack et al. and to further refine the balance between radiation dose reduction and prompt and accurate diagnosis of the acute abdomen using unenhanced multidetector CT.
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