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Review |
1 From the Department of Diagnostic Imaging, M.D. Anderson Cancer Center, 1515 Holcombe Blvd, Box 57, Houston, TX 77030 (E.P.T., P.M.S.); and Department of Diagnostic Imaging, Evergreen Hospital Medical Center, Kirkland, Wash (W.P.S.). Received October 23, 2001; revision requested January 14, 2002; revision received July 23; accepted August 8. Address correspondence to E.P.T. (e-mail: etamm@di.mdacc.tmc.edu).
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ABSTRACT |
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© RSNA, 2003
Index terms: Genitourinary system, calculi, 80.811 • Genitourinary system, CT, 80.12111 • Genitourinary system, MR, 80.12141, 80.12143 • Genitourinary system, US, 80.12981 • Urography, 80.1221 • Review
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INTRODUCTION |
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In the past several years, thin-section nonenhanced computed tomography (CT) has evolved, especially with the development of single–detector row helical CT and, later, multi–detector row CT, as a means of rapid examination of patients suspected of having urolithiasis but without the limitations of radiography, IVU, US, or nuclear medicine. In many institutions, nonenhanced CT has largely supplanted IVU as the primary modality for evaluation of these patients. Nonenhanced CT can help identify calculi and their location, determine their size, and guide management. However, questions remain regarding the optimal use of this technique because of concerns about radiation exposure and cost.
In this article, we will review the state of the art in diagnosing flank pain due to suspected urolithiasis. We will summarize the current state of the art on the use of nonenhanced CT and provide information on techniques and relevant findings. The role of the different imaging modalities in various patient subgroups will also be examined. The issues of radiation exposure and cost will be briefly examined.
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CLINICAL FINDINGS |
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Acute ureteral obstruction usually manifests as renal colic, a severe pain that is often spasmodic, increases to a peak level of intensity, then decreases before increasing again. However, the pain can also manifest as steady and continuous. The pain generally begins abruptly in the flank and increases rapidly to a level of discomfort that often requires narcotics for adequate control. Over time, pain may begin to radiate to the lower abdomen and into the scrotum or labia as the stone moves into the more distal portion of the ureter (3,4).
Urinalysis is often the initial laboratory examination. Unfortunately, the most common laboratory finding, hematuria, is absent in up to 15% of patients, usually due to a completely obstructing stone (1). Clinical symptoms (ie, fever), leukocytosis, and urine gram staining can help identify a superimposed urinary tract infection. Subsequent urine cultures can be used to determine the spectrum of sensitivity to antibiotics. Close clinical assessment is important to help identify such factors as severe obstruction, uncontrollable pain, substantial bleeding, and infection, which are usually indicators for stone removal, and to determine if a patient is a potential surgical candidate if surgery is deemed necessary (3). Percutaneous nephrostomy has been shown to reduce mortality from 40% to 8% when there is obstruction associated with gram-negative septicemia (5). Imaging is used to confirm urolithiasis as the cause of the patient’s pain, to identify the location and degree of obstruction, and to identify potential complications.
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TREATMENT |
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Currently, the primary method for treatment of symptomatic urolithiasis is extracorporeal shock-wave lithotripsy (ESWL). This technique has eliminated the need for percutaneous treatment in many patients. Authors of a recent article (6) showed that both the size and the location of a calculus within a ureter were important predictors of the success of ESWL. In that report, the overall success rate was 87.4% regardless of stone size or position, but the success rate was only 60% (success defined as free of stones) for renal pelvis staghorn calculi, and the rate declined even more sharply for calculi larger that 24 mm in size (6). One of the complications of ESWL for large calculi is the accumulation of multiple stone fragments in the distal ureter, causing an appearance called "stein-strasse" on radiographs (1).
Despite the advancements in ESWL, there remains a need for interventional techniques for extraction of impacted calculi that cannot be reduced with ESWL or for residual calculi that remain after ESWL. Percutaneous nephrostomy is used for drainage of an obstructed infected urinary tract. A percutaneous nephrostomy tract can also be subsequently used for placement of a variety of equipment, including endoscopes, catheters, stone baskets, and lithotriptors, for removal of calculi. Advancements have also occurred with ureteroscopy, including the use of stents, lithotriptors, lasers, and other devices for successful extraction of calculi (7–13). As a consequence, open stone surgery is being used less frequently and often is used only after other less-invasive approaches have failed (14,15).
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RADIOGRAPHIC EXAMINATION |
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Rounded calcifications in other structures can be difficult to distinguish from renal calculi, although specific features on conventional radiographs can help in making the right diagnosis, which preserves the utility of radiographs. Arterial calcifications are usually linear and can often be readily distinguished from ureteral calculi. While at times it may be impossible to distinguish phleboliths or the uncommon entity of calcified mesenteric lymph nodes from ureteral stones by using radiographs alone (1), the radiographic findings (central lucency, anatomic position) can be a useful adjunct to a nonenhanced CT scan.
Overall, the sensitivity of radiographs remains limited. A recent study with 178 patients showed a sensitivity of 45% but a specificity of 77% (16), which are similar to findings in other studies (17,18). In contrast, the former study (16) showed nonenhanced CT to have a sensitivity of approximately 90%.
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IVU EXAMINATION |
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IVU has the advantage of providing physiologic information with reference to time, unlike CT and US. As with conventional radiographs, however, it can be difficult to identify partially obstructing calculi at IVU when extensive feces or bowel gas are present or when the calculus is overlying osseous structures. Secondary signs, particularly the formation of a column of contrast material proximal to a point of obstruction, can be useful in such cases. The rare instances of nonobstructing calculi may be difficult to detect because of the lack of secondary signs (19). Another limiting factor is that the technique cannot be used when patients have poor renal function or are allergic to iodinated contrast material.
In studies comparing nonenhanced CT with IVU (20–22), CT was shown to have a sensitivity of 94%–100% and a specificity of 92%–100%, while IVU was shown to have a sensitivity of 64%–97% and a specificity of 92%–94%.
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US EXAMINATION |
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When urinary tract calculi cause obstruction, US is very effective in demonstrating the secondary sign of hydronephrosis. Although the results of one study (24) in which IVU was compared with US in 123 patients showed that US had a sensitivity of only 37% for ureteral calculi (direct visualization), when hydronephrosis was included as a positive sign for ureteral calculi the sensitivity increased to 74%. However, renal sinus cysts can mimic the appearance of hydronephrosis. Additionally, early in the course of ureteral obstruction, hydronephrosis reportedly may be absent or only of minimal severity in as many as 11%–35% of cases (25–28).
Nonobstructive pelvocaliectasis, such as that due to reflux or prior episodes of obstruction, can be mistaken for collecting system dilatation secondary to obstruction (29). Recently, it has been suggested that duplex renal Doppler US, primarily when used for determining the resistive index, be used to help discriminate between obstructive and nonobstructive causes of renal collecting system dilatation. The authors of one study suggested that resistive index values greater than 0.70 are indicative of obstruction (25), but others have called this into question (29,30). Partial obstruction may not result in an elevated resistive index, and the use of nonsteroidal antiinflammatory drugs for pain control or an antecedent IVU may alter the resistive index (29,30).
The greatest challenge with regard to US is the identification of ureteral calculi, primarily due to obscuration by overlying bowel gas and the deep central location of ureters within the retroperitoneum. Endovaginal or transperineal scanning have been successfully used to identify calculi in the distal ureters (31–33). A ureteral calculus will be identified as an echogenic focus within the distal portion of a dilated ureter. The ability to identify ureteral jets, which can be evaluated at transabdominal examination, is helpful for assessing the presence of obstruction (34). The ability to detect jets is particularly improved when color Doppler is used and the patient is well hydrated. In the absence of obstruction, ureteral jets can be seen intermittently and bilaterally. Obstruction manifests as either complete absence of the jet on the affected side or continuous low-level flow on the affected side (34). In a study with 17 healthy subjects and 26 patients with ureteral calculi, Burge et al (34) showed that abnormalities were seen in ureteral jets in 11 of 12 patients with high-grade obstruction; however, only three of 11 patients with low-grade obstruction showed ureteral jet abnormalities.
Currently, US is recommended in patients in whom radiation exposure is a concern, such as pregnant or pediatric patients.
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NUCLEAR MEDICINE EXAMINATION |
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The nuclear medicine examination has the advantage of being able to demonstrate renal function quantitatively. The examination is applicable in patients allergic to iodinated contrast material or those with impaired renal function. It is also of value as a follow-up to CT when functional information is required regarding the physiologic severity of ureteral obstruction secondary to obstructing calculi (36,37). In our anecdotal experience, however, this technique has largely been supplanted in day-to-day use by nonenhanced helical CT.
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NONENHANCED HELICAL CT EXAMINATION |
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Techniques
The majority of studies have been evaluations of the use of single–detector row helical CT for depicting ureteral calculi, and only limited information is available regarding the use of newer multi–detector row CT technology. In the majority of studies, images are obtained from the top of the kidneys through the bladder base by using a conventional helical CT scanner, with the patient in a supine position and without orally or intravenously administered contrast material. Images are obtained at a section thickness of 3–5 mm, pitch of 1.0–1.8, and one or more patient breath holds (18,38,41,47–57). However, limiting the number of breath holds helps eliminate respiratory misregistration. Images are reconstructed either contiguously or with an overlap of up to 50%, particularly if multiplanar or three-dimensional reconstructions are contemplated.
At the time this review was written, only limited information was available in the literature on the use of multi–detector row CT for the demonstration of ureteral calculi. Authors of one article (58) reported on the use of multi–detector row CT with intravenously administered furosemide and 2.5-mm collimation, achievable because of the faster acquisition possible with multi–detector row CT.
Special techniques may be used in particular circumstances to aid in making a correct diagnosis. In cases where a calculus is identified in or immediately adjacent to the ureterovesicular junction, prone imaging (Fig 1) is added to determine if a calculus is free within the bladder lumen or fixed within the ureterovesicular junction (59). Postprocessed reformatted images, such as curved reformatted images and coronal oblique images, aid in communicating findings to clinicians (55). Profile analyses of calcifications (graphs demonstrating attenuation of each pixel of a line drawn through each calcification), magnification views (or reduced–field-of-view images), and supplemental images reviewed with bone window settings help to differentiate phleboliths from ureteral calculi (60).
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Approach to Interpretation
While one of the primary advantages of this technique is the ability to visualize calculi directly, it is often useful to begin by evaluating for the presence of hydronephrosis. Hydronephrosis is often associated with obliteration of renal sinus fat. It is important not to use the presence of a prominent renal pelvis alone as a sign of hydronephrosis, since an extrarenal pelvis or parapelvic cysts may be mistaken for hydronephrosis; inspection for clubbing of calyces in the upper and lower poles can help differentiate these diagnostic possibilities (Fig 2). In cases where direct visualization of hydronephrosis is thought to be equivocal, ancillary signs of hydroureter, asymmetric perinephric stranding, enlargement of the kidney in question, and lower attenuation of the kidney (63) help increase diagnostic certainty.
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Once the presence of hydronephrosis has been established, it is then important to evaluate the ipsilateral ureter. Beginning at its junction with the renal pelvis, one should follow the ureter throughout its course. Evaluation should be made for obstructing calculi, obstructing masses, changes in caliber (and whether the change in caliber is gradual or sudden), and extrinsic causes that could also account for obstruction. When the ureter cannot be followed in the superior-to-inferior direction, it may be possible to follow the ureter in question from the level of the ureterovesicular junction upward. It is important to follow the ureter throughout its course to avoid confusion with adjacent vascular structures. Identification of the ureter on all sections is often challenging. Viewing on a workstation in interactive cine mode is often helpful, in our anecdotal experience, especially in problematic cases.
It is not sufficient to identify only the presence of an obstructing ureteral calculus. Various other characteristics, such as the size of a ureteral calculus (57,65), the presence of perinephric edema (48,57), and the presence of perinephric fluid collections (57), can be identified at CT and can be useful in treatment planning. In a study with 69 patients (57), the mean diameter of ureteral calculi that underwent spontaneous passage was approximately 2.9 mm ± 2.0, whereas the mean diameter of ureteral calculi for which conservative therapy failed was 7.9 mm ± 3.3; this difference was statistically significant. It has also been reported that the success of ESWL declines sharply for calculi larger than 24 mm in diameter. The degree of perinephric edema identified on CT images has been shown to correlate with the degree of functional obstruction on IVU images. Extensive perinephric edema was associated with high-grade obstruction on IVU images, while lack of perinephric edema or mild perinephric edema was associated with no functional obstruction or low-grade obstruction on IVU images (48). Boridy et al (48) compared CT with IVU in 47 patients with acute ureterolithiasis and found that by performing a qualitative assessment of the degree of perinephric edema seen on CT images, they were able to accurately predict the degree of ureteral obstruction seen on IVU images in 44 (94%) patients. The presence of substantial perinephric edema and of perinephric fluid collections have also been shown to be associated with a higher likelihood of spontaneous passage of ureteral calculi (57).
Pitfalls
There are several pitfalls that must be avoided in the course of attempts to identify the cause of flank pain with the use of this CT technique. The most common dilemma is the differentiation of phleboliths from ureteral calculi. In rare cases, vas deferens calcifications can also mimic the appearance of ureteral calculi (66). Certain secondary signs can be used to differentiate extraurinary calcifications, such as phleboliths, from obstructing ureteral calculi.
A "rim" of soft tissue may be identified surrounding a calcification; this finding can be seen in 50%–77% of ureteral calculi (Fig 3) but is seen in only 0%–8% of extraurinary calcifications (ie, phleboliths) (51,67). Phleboliths may be seen with an associated "tail" of soft-tissue attenuation (representing the associated vein) that extends to the calcification (Fig 4). Boridy et al (68) showed that 65% of phleboliths were associated with this sign, while ureteral calculi lacked this finding.
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Other less common entities can mimic a ureteral calcification. Orally administered contrast material from a recent examination that is trapped in the appendix (Fig 6) or in a diverticulum can mimic the appearance of a stone surrounded by the soft tissue of the ureteral wall. Pelvic pathologic conditions containing calcifications, such as a dermoid cyst, can also mimic a ureteral stone (Fig 7). Radiation seeds implanted for treatment of prostate cancer can mimic a bladder calcification or a calcification near the ureterovesicular junction (Fig 8).
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A wide variety of other diseases can result in the clinical symptom of acute flank pain. Many of these can be identified on nonenhanced CT images. In a study by Chen and Zagoria (40) in 1999, 100 consecutive patients with urinary colic were studied with helical CT and the following breakdown was obtained: ureteral calculi (n = 49), renal calculi (n = 17), extraurinary lesions (n = 16), and normal findings (n = 18). In a follow-up study, Chen et al (41) showed that clinicians had expanded the role of nonenhanced CT beyond that for evaluation of the symptoms of renal colic, with ureteral calculi identified in only 28% of patients and extraurinary lesions identified in 45%.
Entities that need to be considered include renal arterial or venous thrombosis, appendicitis, diverticulitis, bowel obstruction or herniation, intraabdominal fluid collections (eg, abscess, hematoma), gynecologic conditions (eg, tubo-ovarian abscess), aortic aneurysm, pancreatitis, and neoplasm (Fig 7). Evaluation for renovascular disease is limited with nonenhanced CT; contrast material–enhanced CT greatly improves sensitivity for detection of these lesions. The kidneys of patients with acute renal infarct secondary to renal arterial occlusion may appear unremarkable during the very early stages on nonenhanced CT images; however, when infarction involves large regions of an involved kidney, the kidney may become enlarged, with preservation of its reniform shape (61,84). Renal venous thrombosis may manifest on nonenhanced CT images as ipsilateral renal enlargement with edema in the perinephric space (61). A ureteral transitional cell carcinoma, while uncommon, should also be considered, especially if there are signs of chronic obstruction; careful inspection of the ureters may be necessary (Fig 9). Many of these entities may require follow-up imaging with intravenous contrast material to establish a definite diagnosis. Renal tumors can often be missed at nonenhanced CT performed for the evaluation of ureteral calculi (85). It is, therefore, important to evaluate such studies comprehensively.
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There are reports in the literature of attempts to reduce the radiation dose. Recently, Liu et al (87) used nonenhanced helical CT with 7-mm collimation and a pitch of 2 to reduce radiation dose. Their measured effective dose equivalent was 2.8 mSv, compared with a dose of 1.33 mSv for IVU at their institution, which they measured as approximately 75% and 50%, respectively, of the dose reported for earlier CT protocols by Smith et al (70) and Sommer et al (55). Diel et al (88) examined the use of 5-mm collimation with pitch of 1.5–3.0 for a helical CT scanner. Entrance doses were 461, 553, and 913 mR (0.119, 0.143, and 0.236 mC/kg) at pitches of 3.0, 2.5, and 1.5, respectively. Diel et al also noted no change in diagnostic accuracy for detection of ureteral calculi when using a pitch of 2.5 or 3.0; ratings of image quality decreased at a pitch of 3.0 as compared with those at a pitch of 2.5. They did not assess the accuracy of making diagnoses for extraurinary pathologic conditions.
Few authors have reported use of CT in adolescents (89,90). Given the concerns about radiation exposure, however, it is probably advisable that techniques that do not involve ionizing radiation, such as US, be used first. In cases where further imaging is necessary, a limited IVU would probably be the most prudent approach. For pediatric patients, such a limited IVU might include a scout abdominal image, a coned anteroposterior image of the kidneys 1–2 minutes after contrast material injection, an abdominal image approximately 7–10 minutes after injection, and a delayed image 25 minutes after injection; however, studies need to be monitored closely so that the technique can be tailored to the particular clinical question (91,92).
The diagnosis of ureteral calculi is particularly problematic in pregnant patients (93). A study with 80 pregnant patients in whom renal colic had been diagnosed showed that in 99% of cases, the onset of symptoms was in the second or third trimester, and in 84% of cases stones passed spontaneously (94). In the case of pregnant patients, transabdominal US would again probably be the study of first choice. However, the typical hydronephrosis due to pregnancy can make diagnosis of a superimposed obstructing ureteral calculus difficult. The additional use of endovaginal US, when appropriate, to evaluate for ureteral jets and possible obstructing ureterovesicular junction calculi may provide assistance. Butler et al (95), in a study with 57 pregnant patients, showed that 40% of calculi were not detected by using US; however, their report did not include details as to the techniques used. They recommended "single-shot" intravenous pyelography (single abdominal radiograph obtained 30 minutes after contrast material injection) in patients with negative sonograms (95). However, a review by Boridy et al (96) of the literature regarding possible urolithiasis in pregnant patients showed considerable controversy in the recommendations as to how such a limited urographic examination should be performed. Boridy et al advocated acquisition of a scout radiograph, a radiograph collimated to include only the kidneys at 1 minute (to assess nephrograms), an image at 15 minutes to assess pyelograms, and delayed images as needed (45–180 minutes and beyond, if necessary) on the basis of the appearance of the nephrograms (96).
There has been promising research on the use of heavily T2-weighted magnetic resonance (MR) images for evaluation of the renal collecting systems and ureters. Roy et al (97,98) reported effective use of rapid acquisition with relaxation enhancement MR urography in 17 pregnant patients for identifying both the level of obstruction and whether the cause of the obstruction was intrinsic or extrinsic. These techniques will be discussed in more detail in a subsequent section.
Cost
Another consideration is the cost of CT versus that of IVU. Smith et al reported in 1995 and in 1996 (44,70) that, at their institution, the charge for a nonenhanced CT "flank pain protocol" study was equal to that of IVU. Sommer et al (55) noted in their 1995 study that the charge for such a CT examination was similar to that for US plus conventional radiography but that the charge for IVU was 60% higher, though they noted that at their institution the charge for IVU was "relatively high." Chen and Zagoria (40), in a 1999 study, noted that at their institution the charge for a nonenhanced CT study was $600, versus $400 for IVU.
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FUTURE DIRECTIONS: MR IMAGING |
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MR imaging has the inherent advantage over CT in that MR does not use ionizing radiation. Several articles have been published on MR urography techniques; our discussion will focus on those techniques as they are applied to evaluation for ureteral calculi. The different published techniques can be summarized into two broad groups: (a) techniques that use heavily T2-weighted imaging to demonstrate static fluid and (b) techniques that use T1-weighted imaging to demonstrate gadolinium-based contrast material excreted into the urinary tract.
In studies in which the utility of T2-weighted imaging has been evaluated, a variety of techniques have been used. Some investigators (97) have acquired thick coronal slabs from the level of the kidneys through to the bladder to enable visualization of the entire urinary tract. Others (99,100) have acquired multiple thin slabs in the coronal plane; these images have then been used to create three-dimensional views of the entire urinary tract by using maximum intensity projection postprocessing algorithms. These postprocessed images and the source images were then both reviewed. Both breath-hold and rapid acquisition sequences have been described, including such techniques as half-Fourier single-shot turbo spin-echo and rapid acquisition with relaxation enhancement sequences (97,99–102). Some researchers (99,101) have also administered furosemide to enhance distention of the urinary tract or have had patients ingest material to suppress background high signal intensity from the gastrointestinal tract. In a study with 115 patients (99) in whom 35 urinary tract calculi were ultimately identified with a variety of imaging techniques, 24 (69%) calculi were identified by using a breath-hold half-Fourier single-shot turbo spin-echo technique. Authors of a study who compared T2-weighted with T1-weighted techniques reported sensitivities between 54% and 58% and specificities of 100% for identification of ureteral calculi for the T2-weighted half-Fourier acquisition single-shot turbo spin-echo technique (100). Other authors have noted that small urinary tract mucosal lesions, clotted blood, and debris can all mimic calculi, and the high intensity signal from urine can mask a very small calculus (103).
Other researchers have attempted to use the excretion of gadolinium-based agents into the urinary tract to enable visualization of calculi and other lesions on T1-weighted images (100,104–107). In these studies, images have been typically acquired over the course of 25 minutes (though delayed images may be obtained several hours later), often following the administration of furosemide.
Nolte-Ernsting et al (104) compared respiratory-gated and breath-hold gradient-echo techniques, both two- and three-dimensional, with gradient-echo echo-planar imaging. They found that while echo-planar imaging required shorter breath holds (14–20 seconds) than did conventional gradient-echo imaging (breath holds of approximately 20–30 seconds) and that echo-planar imaging markedly reduced ghost artifacts from ureteral peristalsis in comparison with conventional gradient-echo imaging, the greater image detail provided by conventional gradient-echo imaging rendered it superior to echo-planar imaging for depiction of the urinary tract. Calculi also had diameter measurements that were 0.8%–21.7% greater on echo-planar images than on conventional gradient-echo images, owing to increased susceptibility artifacts on echo-planar images.
In a study by Jung et al (107) with 82 patients and in two studies by Sudah et al (100,106), breath-hold conventional gradient-echo techniques were used. Jung et al detected 90% (65 of 72) of ureteral calculi. Sudah et al (100) compared gadolinium-enhanced T1-weighted gradient-echo techniques with T2-weighted half-Fourier acquisition single-shot turbo spin-echo techniques and found sensitivities of 96%–100% for ureteral calculi and specificities of 100%, as compared with sensitivities of 54%–58% and specificities of 100% described earlier for T2-weighted half-Fourier single-shot turbo spin-echo imaging. As with the T2-weighted techniques, small urinary tract lesions, blood clots, and debris can mimic calculi, and calculi can be mistaken for other pathologic conditions. In the study by Jung et al (107), five of the missed calculi were misconstrued as peristalsis and two were misidentified as small tumors.
In summary, the two techniques offer different advantages and have different limitations. The T2-weighted techniques are rapid and do not require the administration of gadolinium chelates. This technique has been utilized in studies in which MR urography was used in patients who were pregnant (102,108). However, the T2-weighted techniques currently yield lower sensitivities for detection of urinary tract calculi. Gadolinium-enhanced techniques appear to offer greater sensitivity; however, imaging generally occurs over a longer time period, has typically been performed with furosemide, and requires the administration of gadolinium chelates. This limits its utility, because the intravenous use of gadolinium chelates in pregnant patients has not been approved by the U.S. Food and Drug Administration.
Recently, Sudah et al (106) compared MR urography performed with T2-weighted and contrast-enhanced T1-weighted sequences with nonenhanced helical CT (106). In that study, the sensitivities of nonenhanced CT for two observers were 90.6% and 90.6% and the specificities were 94.1% and 100%. In comparison, the sensitivities for MR urography were 93.8% and 100% and the specificities were 100% and 100%. Sudah et al noted that interpretation of MR images, in contrast to that of CT images, was not confounded by the presence of phleboliths. However, they also noted that CT offered an advantage in that it could reveal the extent of calculus burden, including that of nephrolithiasis. Also, in at least one case, one of the observers of MR images could not rule out the possibility of a tumor as the underlying abnormality when the actual cause was a calculus.
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SUMMARY |
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However, the radiation dose for the most commonly used CT techniques is higher than that for IVU, particularly limited IVU. In children, and pregnant patients, US should be used as a first-line modality, with limited IVU used when US is insufficient in providing information necessary for diagnosis and treatment. Developments in MR imaging are promising in that they potentially provide an alternative modality when use of ionizing radiation is a concern. The recent introduction of multi–detector row CT promises a continued evolution of this technique.
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FOOTNOTES |
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REFERENCES |
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ABSTRACT
INTRODUCTION
