Date Published: August 16, 2012
Publisher: Hindawi Publishing Corporation
Author(s): P. D. Mc Laughlin, L. Crush, M. M. Maher, O. J. O’Connor.
Objective. To critically evaluate the current literature in an effort to establish the current role of radiologic imaging, advances in computed tomography (CT) and standard film radiography in the diagnosis, and characterization of urinary tract calculi. Conclusion. CT has a valuable role when utilized prudently during surveillance of patients following endourological therapy. In this paper, we outline the basic principles relating to the effects of exposure to ionizing radiation as a result of CT scanning. We discuss the current developments in low-dose CT technology, which have resulted in significant reductions in CT radiation doses (to approximately one-third of what they were a decade ago) while preserving image quality. Finally, we will discuss an important recent development now commercially available on the latest generation of CT scanners, namely, dual energy imaging, which is showing promise in urinary tract imaging as a means of characterizing the composition of urinary tract calculi.
Since its first description by Smith et al. in 1995, noncontrast computed tomography (CT) of the urinary tract has become the imaging investigation of choice in patients with acute renal colic [1–3] due to its significantly higher sensitivity and specificity for detection of urinary stones when compared with plain radiography and intravenous urography and also due to superior capability for accurate characterization of the size and location of obstructing urinary calculi, thus allowing clinicians to predict the likelihood of spontaneous passage [4, 5].
One of the most feared adverse events associated with exposure to ionizing radiation is carcinogenesis, which is a stochastic effect, that is to say, it is random. Cancer induction does not exhibit an upper or lower threshold of occurrence, and the probability of cancer induction is variable . The overall risk of cancer is currently believed to be small, but a cause for concern with regard to radiation exposures in the diagnostic imaging range is that there is no radiation dose below which cancer induction does not occur. In addition, it is important to highlight that carcinogenesis may transpire many years following exposure, and it is accepted that exposure to ionizing radiation in early life magnifies the risk of tumor induction .
Many studies have examined the diagnostic efficacy of low-dose CT in the setting of renal colic, and a variety of protocols have described that result in effective radiation dose reductions of up to 95% from greater than 10 mSv to as low as 0.5–3.5 mSv [14–24]. Low-dose CT is uniformly associated with an increase in image noise, but successes in dose reduction in the setting of renal colic have been aided by the inherent high contrast of renal calculi against the relatively low-density soft tissues surrounding the urinary tract.
Studies which initially compared noncontrast CT of the urinary tract with plain radiography, ultrasound, and intravenous urography found significantly increased detection of urinary calculi with noncontrast CT leading to its immediate adoption as the imaging investigation of choice in the setting of suspected urinary tract calculi [1, 37, 38]. A recognized limitation in studies, which report the diagnostic performance of noncontrast CT of the urinary tract, is the choice of gold standard investigation upon which sensitivity and specificity calculations are based. In many studies, the identification of false negative cases is dependent on additional calculi being subsequently identified with clinical followup, urography, and/or endoscopy in some cases. This may potentially lead to an inappropriately low detection of false negative cases and consequently a spurious increase in the reported sensitivity of noncontrast CT .
The attenuation value of different subtypes of renal calculi overlaps greatly on conventional single-energy CT datasets , but their attenuation values differ significantly when imaged with high- and low-energy CT. Uric acid stones, which are predominantly composed of low-molecular-weight elements (oxygen, carbon, and nitrogen), have different X-ray attenuation properties at high- and low-energy CT compared with other types of renal calculi such as calcium oxalate, hydroxyapatite, or cysteine stones. These stones are composed of high-molecular-weight elements (phosphorus, calcium, and sulfur) and therefore, as a consequence, will have a higher Hounsfield unit value at lower-energy CT.
As a result of CT dose reduction measures that have been outlined in this paper, a cancer risk that was small to begin with is being systematically reduced . Average radiation exposures associated with CT scanning of the urinary tract for urinary tract calculi are likely to reduce further and may eventually reach doses similar to those currently encountered in plain radiography with the help of iterative image reconstruction and other techniques. The statistical risks associated with performing a clinically indicated CT will therefore be reduced, but individual justification for performing CT will still be required.