Kidney: Renal Colic: The Role of MDCT (Multidetector) and Spiral CT

In the United States urinary tract stone disease affects up to 5% of women and 12% of men sometime during their lifetime. It occurs approximately three times as frequently in males as females with over 1,000,000 Americans requiring treatment each year. The classic presentation is flank pain or pain in the costovertebral region which is often acute and debilitating with nausea and vomiting or can be more chronic with the pain dull and persistent. Although the presentation is often classic with flank pain and hematuria in other cases it is atypical. Hematuria is common and may be either gross or microscopic. Yet up to 15% of patients with stone disease do not have hematuria. Urinary calculi are of variable composition but are most commonly composed of calcium oxalate, calcium phosphate, uric acid or cystine. Urinary stones can nearly all be seen on CT with the exception of stones due to protease inhibitors such as indinavir (Merck Co.) (table 1).

The classic radiologic work-up for patients with suspected urinary tract stone disease was a plain abdominal radiograph followed by an intravenous urogram (IVU). The plain film was obtained in an attempt to see the stone while the IVU was obtained to look for evidence of obstruction manifested by delayed renal function, as well as dilatation of the renal pelvis and ureter. The IVU was used as a guide for subsequent patient management decision by determining the degree of obstruction and in helping determine whether or not the stone would pass on its own or intervention would be needed. The obvious limitations of this combination study was that the patient was typically not prepped for the exam (i.e.stool in colon) as well as the need to use iodinated contrast material. In cases where the study was positive it could take hours or more to complete. In addition the study focused on a single etiology for the patients clinical symptoms while they might be due to other causes ranging from appendicitis to pancreatitis to torsion of an ovary to a dissecting aneurysm.

During the 1980's and 1990's computed tomography continued to develop technologically and soon became one of the mainstays of renal imaging. Due to its superior cross sectional imaging capabilities CT replaced the IVU as the initial study for most renal problems relegating the IVU to a secondary study. Whether the application is to evaluate hematuria, to stage a renal cancer or exclude infection CT became the study of choice. With the advent in the early 1990's of spiral CT, a critical rethinking of many of the uses of CT was begun. The ability to image a volume of data became a focus and this led to applications of CT that might have previously been considered the purview of other imaging studies.

In 1995 Smith et al. reported on the use of non-contrast CT for evaluation of acute flank pain and compared it directly with IV Urography in 20 patients. In that series 12 patients had CT and IVU findings consistent with obstruction while 8 had normal studies. Of the 12 patients with obstruction, 5 have a ureteric stones seen on both studies but 6 had a stone seen only on CT. This led the authors to conclude that "Non-contrast enhanced CT is more effective than IVU in precisely identifying ureteric stones and is equally effective as IVU in the determination of the presence or absence of ureteric obstruction." Although CT had been used for this application for many years this was the first article to quantify the value of CT. In addition, the authors also used spiral CT for the last 6 patients and noted that it provided advantage over non-spiral studies by decreasing volume averaging with thinner slice collimation (5 mm vs. 10mm), as well as the advantage of narrow interscan spacing which increase the detection of small stones.

Smith et al. also noted the other advantages of CT which included the speed of exam completion, the lack of need for IV contrast injection, as well as the ability to detect non stone disease as the causes for the patients symptoms. In addition to the detection of a ureteric stone there are other findings seen with obstruction. Katz et al. reviewed 54 patients with CT positive scans of ureteric obstructed and noted that in addition to the stone common findings included hydronephrosis, hydroureter, perinephric soft tissue changes and periureteral edema. Only 2 of the 54 patients had no evidence of at least one of these findings.

Levine et al. helped further define the weakness of the plain radiograph/IVU paradigm by looking at the ability to detect the presence of a ureteral calculus when one was present on CT. Even when knowing that CT showed a stone and knowing where it was an unblinded retrospective reading still had a sensitivity of 59% leading the authors to conclude that "Plain radiography is of limited value for aiding the diagnosis of ureteral stones. All patients with acute flank pain for whom radiologic imaging is recommended can directly undergo unenhanced helical CT; plain radiographs need not be obtained first." Sourtziz et al. came to similar conclusions and found that "compared with excretory urography, unenhanced helical CT is better for identifying ureteral stones in patients with acute ureterolithiasis. Secondary CT signs of obstruction, including renal sinus fat blurring, were frequently present even when the stone was eliminated before imaging."

Another article reviewed 106 adult patients suspected of having ureterolithiasis who had both a non-contrast CT scan as well as a urogram. Of the 75 patients proven to have stone disease CT was correct in 72 of the 75 cases. Urograms were correct in 65 of the 75 patients. Based on these results Miller et al. concluded that "NNCT accurately diagnose ureterolithiasis in patients presenting with acute flank pain." NCCT is significantly better than IVU in determining the presence of urolithiasis."

Fielding et al. looked at the features that could have a predictive value in the diagnosis of renal colic. The findings with the strongest positive predictive values (>0.90) were a ureteral stone, hydronephrosis, hydroureter and periureteal stranding. In fact the results were strong enough for the authors to suggest that if there is no hydronephrosis or hydroureter than an alternative diagnosis out to be explored.

Chu et al. looked at whether the digital CT scout radiograph is good enough for detecting stones and might provide the urologist with a replacement for plain films. In a review of 215 consecutive cases chosen only 49% of stones were visible on the topogram. However, in these cases the topogram can be used to follow up stone passage on subsequent plain films if conservative management is chosen.

With more than two dozen articles published on the subject the past 5 years careful attention has been placed on specific signs of diagnostic value in the detection of ureterolithiasis on a CT scan. Kawashima et al. analyzed the tissue rim sign, "a rim or halo of soft tissue attenuation seen around the circumference of an intraureteral calculus on unenhanced CT" and found that" a positive tissue rim sign is specific for the diagnosis of ureterolithiasis. However, a negative tissue rim sign does not preclude such a diagnosis." In this series of 59 patients with suspected ureterolithiasis 32 had evidence of a stone, with 16 having a positive tissue rim sign. Heneghan et al. reached a similar conclusion and found that "Helical CT can be used in place of excretory urography to plan treatment of patients with flank pain caused by obstructing ureteral stones." Stones that are larger than 5 mm, located in the proximal two thirds of the ureter, and seen on two or more consecutive CT images are more likely to require endoscopic removal, lithotripsy or both." Not suprisingly perinephric edema correlated well with the degree of obstruction. Extensive edema was seen in patients with high grade obstruction at urography while limited perinephric edema was seen in patients with low grade obstruction. Boridy et al concluded that "the extent of perinephric edema on nonehanced helical CT images can be used to predict the degree of ureteral obstruction in acute ureterolithiasis."

The analysis of a renal or ureteral stone has also been an area of interest. The attenuation of a calcification and a phlebolith are often different. Bell et al concluded that "Differences were significant between the mean attenuation of calculi and phleboliths. The mean attenuation of phleboliths was 160 HU (range, 70-278HU) and of calculi was 305 HU (range, 221-530 HU). The probability that a calcification represented a phlebolith was 0.03% when the mean attenuation was 311 HU or more."This may be a helpful finding in difficult diagnostic dilemmas when it is hard to distinguish a stone from a phlebolith.

CT of Renal Colic: Study Protocols

The specific study protocol will vary on the scanner available. In our practice we use either a single detector or a multidetector CT scanner. All studies are done without oral or IV contrast material. We scan from the level of the mid-liver to just beneath the level of the mid symphysis pubis. We have the patient hold their breath and in a single breathhold scan the entire volume. Typical scan collimation is 3mm with an interscan spacing of 3mm. In select cases reconstruction is done at 1 mm intervals if necessary. All images are reviewed at soft tissue windows and the study can be reviewed on either film or a workstation. In select cases multiplanar reconstruction and/or 3D imaging can be used to better define the location of a stone or to determine whether or not a stone is indeed in the ureter. MDCT has several advantages over single detector CT including faster study performance which is important in the patient with severe enal colic who may have trouble remaining still. The ability to obtain thin collimation helps provide better datasets for 3D rendering.

If the CT scan is negative for ureterolithiasis then the study is typically considered completed unless select findings are detected which result in the need for IV contrast. These conditions can include suspected aortic dissection, acute pancreatitis, acute pyelonephritis or ischemic bowel. This however is unusual and will be necessary in a significant minority of patients.

The Role of CT in Patient Management

One of the advantages of an IVU was that the function of the kidney could be defined. However the use of a non-contrast CT can not define function which might suggest a limitation in patient management. Boulay et al addressed this problem and found that "stone size alone was found to correlate with patient treatment. Stone location and the presence and severity of secondary signs of obstruction did not affect patient treatment." He therefore concludes, "Because the degree of obstruction and relative renal function are not relevant to the initial treatment of patients with ureteral stone obstruction, CT is adequate for both diagnosis and treatment." In this series patients with stones around 4 mm in size were managed conservatively while patients with stones around 8 mm in size usually underwent a urologic procedure. Takahashi et al. also found that stone size was a critical criteria for stone passage with stones that passed spontaneously having a mean of 2.9 mm and those which did not pass being 7.8 mm Takahashi et al also found that " In addition to stone size, the degree of perinephric fat stranding and the presence of perinephric fluid collections are useful ancillary signs for help in predicting the likelihood of stone passage."

Patients with increased perinephric fluid or stranding were more likely to pass the stone. Finally Fielding et al found that based on a review of 100 patients with ureteral stones and a clear outcome that " Helical CT can be used in place of excretory urography to plan treatment of patients with flank pain caused by obstructing ureteral stones. Stones that are larger than 5 mm, located in the proximal two thirds of the ureter, and seen on two or more consecutive CT images are more likely to require endoscopic removal, lithotripsy or both."

Pitfalls in CT Diagnosis

The single biggest pitfall as well as the most difficult diagnostic dilemma is differentiating a stone from a phlebolith or other vascular calcifications. Several successful solutions to this problem have included imaging signs including the 'rim sign" which represents ureteral edema around a stone and is not seen with a phlebolith. Similarly the "tail sign" has been described as a finding seen in phleboliths. This sign is that the soft tissue of the venous structure which contains the phlebolith is seen as a 'tail" continuous with the phlebolith. This sign is only useful when present however. The lack of secondary signs of hydroureter or hydronephrosis also makes the presence of a stone unlikely. Fielding et al in fact noted that the "Absence of hydroureter and hydronephrosis on spiral CT images should prompt a search for a diagnosis other than an obstructing ureteral stone."

The use of multiplanar reconstruction and/or 3D imaging may be helpful in defining the orientation and location of a stone or phlebolith in difficult cases Volume displays may soon prove to be the ideal technique for display of these volume datasets. If despite the use of all of the signs discussed and the use of multiplanar and 3D imaging one is still uncertain as to whether or not a calcification is in the ureter the easiest solution is to inject 50 ml of nonionic contrast intravenously, wait 3 -5 minutes and get scans through the area in question. This will definitively tell you where the stone is. It is rare however for this to be needed.

Noncalculus Causes of "Renal Colic"

One of the major benefits of CT in the evaluation of renal colic is the global view provided by the CT examination. The ability to detect the presence of other causes of flank pain ranging from pyelonephritis, to diverticulitis to appendicitis can all be recognized. Chen et al. also noted the phenomenon of increasing use of non-contrast enhanced studies by clinicians, as they became more familiar with it. "As clinicians developed familiarity with this technique, the indications for performance of unenhanced helical CT were expanded with a consequent reduction in the rate of detection of stone disease and identification of an increased number of extraurinary lesions, which suggest a demand for emergency abdominal CT scans."Our personal experience is similar with CT scans being obtained in many ER patients with even mild flank pain and the pre-test probability of stone probably low. Yet, the ability of CT to detect both suspected as well as unsuspected pathology has made it into a frontline study, at times even seeming to replace a physical examimation.

Conclusion

Table 1 CT attenuation of renal stone disease -calcium-based stone>1,000 HU -uric acid stones300-500 HU -indinavir-related stone<100 HU (non-opaque) (note: Excreted indinavir stones are a result of treatment with Cruxivan which is a protease inhibitor. They are non-opaque on CT scans.)

Quotes from the Literature on Renal Stone Disease

"Non-contrast enhanced CT is more effective than IVU in precisely identifying ureteric stones and is equally effective as IVU in the determination of the presence or absence of ureteric obstruction." Acute Flank Pain: Comparison of Non-Contrast-enhanced CT and Intravenous Urography
Smith RC et al.
Radiology 1995; 194:789-794

"In patients with ureteral calculi imaged with nonenhanced CT for acute renal colic, associated findings included hydronephrosis, hydroureter, perinephric soft tissue changes, and periureteral edema. These common findings provided supportive evidence that an acute obstructive process was present." Unenhanced Helical CT of Ureteral Stones: Incidence of Associated Urinary Tract Findings
Katz DS et al.
AJR 1996; 166:1319-1322

"Noncontrast helical CT was a rapid and accurate method for determining the presence of ureteral calculi causing renal colic. The reformatted views produced images similar in appearance to excretory urograms, aiding greatly in communicating with clinicians." Detection of Ureteral Calculi in Patients with Suspected Renal Colic: Value of Reformatted Noncontrast Helical CT
Sommer FG et al.
AJR 1995;165:509-513

"Absence of hydroureter and hydronephrosis on spiral CT images should prompt a search for a diagnosis other than an obstructing ureteral stone." Spiral CT in the Evaluation of Flank Pain; Overall Accuracy and Feature Analysis
Fielding JR et al.
J Comput Assist Tomogr 1997; 21(4):635-638

"The sensitivity of non-contrast CT in assessing ureteral calculi was 97% and the specificity was 96% and the negative predictive value was 97%. The accuracy was 97%." Diagnosis of acute flank pain: Value of unenhanced Helical CT
Smith RC, et al.
AJR 1996; 166:97-101

"Plain radiography is of limited value for aiding the diagnosis of ureteral stones. All patients with acute flank pain for whom radiologic imaging is recommended can directly undergo unenhanced helical CT; plain radiographs need not be obtained first." Ureteral calculi in Patients with Flank Pain: Correlation of Plain Radiography with Unenhanced Helical CT
Levine JA et al.
Radiology 1997; 204:27-31

"A positive tissue rim sign is specific for the diagnosis of ureterolithiasis. However, a negative tissue rim sign does not preclude such a diagnosis." Unenhanced Helical CT of Ureterolithiasis: Value of the Tissue Rim Sign
Kawashima A et al.
AJR 1997;168:997-1000

"In patients with flank pain, the presence of a rim sign is a strong indicator that a calcification along the course of the ureter is a stone. Absence of the rim sign indicates that a calcification remains indeterminate." Soft Tissue "Rim" Sign in the Diagnosis of Ureteral Calculi with use of Unenhanced Helical CT Heneghan JP et al. Radiology 1997; 202:709-711

"In addition to stone size, the degree of perinephric fat stranding and the presence of perinephric fluid collections are useful ancillary signs for help in predicting the likelihood of stone passage." Ureterolithiasis: Can Clinical Outcome Be Predicted with Unenhanced Helical CT Takahashi N et al. Radiology 1998; 208:97-102

"NNCT accurately diagnoses ureterolithiasis in patients presenting with acute flak pain. NCCT is significantly better than IVU in determining the presence of urolithiasis." Miller OF et al. Urology 1998; 52:982-987

"Helical CT can be used in place of excretory urography to plan treatment of patients with flank pain caused by obstructing ureteral stones. Stones that are larger than 5 mm, located in the proximal two thirds of the ureter, and seen on two or more consecutive CT images are more likely to require endoscopic removal, lithotripsy or both." Unenhanced Helical CT of Ureteral Stones: A Replacement for Excretory Urography in Planning Treatment Fielding JR et al. AJR 1998; 171:1051-1053

"Differences were significant between the mean attenuation of calculi and phleboliths. The mean attenuation of phleboliths was 160 HU (range, 70-278HU) and of calculi was 305 HU (rang, 221-530 HU). The probability that a calcification represented a phlebolith was 0.03% when the mean attenuation was 311 HU or more." Unenhanced Helical CT Criteria to Differentiate Distal Ureteral Calculi from Pelvic Phleboliths Bell TV et al. Radiology 1998; 207:363-367

"The preferred technique is to utilize a helical data acquisition with 5mm thick sections and a pitch of 1. Refocused images are obtained in 1-2mm increments through the levels of suspicious calcifications to determine if they are intraureteral." Noncontrast helical CT in the evaluation of acute flank pain Smith RC et al. Abdom Imaging 23:10-16 (1998)

"Compared with excretory urography, unenhanced helical CT is better for identifying ureteral stones in patients with acute ureterolithiasis. Secondary CT signs of obstruction, including renal sinus fat blurring, were frequently present even when the stone was eliminated before imaging." Radiologic Evaluation of Renal Colic: Unenhanced Helical CT Compared with Excretory Urography Sourtzis S et al. AJR 1999; 172:1491-1494

"The extent of perinephric edema on nonehanced helical CT images can be used to predict the degree of ureteral obstruction in acute ureterolithiasis." Acute Ureterolithiasis: Nonenhanced Helical CT Findings of Perinephric Edema for Prediction of Degree of Ureteral Obstruction." Boridy IC et al Radiology 1999; 213:663-667

"Stone size alone was found to correlate with patient treatment. Stone location and the presence and severity of secondary signs of obstruction did not affect patient treatment. Because the degree of obstruction and relative renal function are not relevant to the initial treatment of patients with ureteral stone obstruction, CT is adequate for both diagnosis and treatment." Ureteral Calculi: Diagnostic Efficacy of Helical CT and Implications for Treatment of Patients Boulay I et al. AJR 1999;172:1485-1490

"The tail sign is an important indicator that a suspicious calcification represents a phlebolith. Absence of the tail sign indicates that the calcification remains indeterminate." Ureterolithiasis: Value of the Tail Sign in Differentiating Phleboliths from Ureteral Calculi at Nonenhanced Helical CT Boridy IC et al. Radiology 1999; 211:619-621

"As clinicians developed familiarity with this technique, the indications for performance of unenhanced helical CT were expanded with a consequent reduction in the rate of detection of stone disease and identification of an increased number of extraurinary lesions, which suggest a demand for emergency abdominal CT scans." Trends in the Use of Unenhanced Helical CT for Acute Urinary Colic Cen MYM et al. AJR 1999;173; 1447-1450

Low Dose Nonenhanced Helical CT of Renal Colic: Assessment of Ureteric Stone Detection and Measurement of Effective Dose Equivalent Liu W et al. Radiology 2000; 215:51-54

"When secondary signs of obstruction are present but no stone is present, differential diagnostic considerations include a recently passed stone, pyelonephritis, urinary tract obstruction unrelated to stone disease, and protease inhibitor deposition disease." Pearls and Pitfalls in the Diagnosis of Ureterolithiasis with Unenhanced Helical CT Dalrymple NC et al. RadioGraphics 2000; 20:439-447

References

• 1.Bell TV, Fenlon HM d, Davison BD, Ahari HK, Hussain S. Unenhanced Helical CT criteria to differentiate distal ureteral calculi from pelvic pheleboliths. Radiology 1998; 207:363-367.
• 2.Boridy IC, Kawashima A, Goldman SM, Sandler CM. Acute ureterolithiasis: nonenhanced helical CT findings of perinephric edema for prediction of degree of ureteral obstruction. Radiology 1999; 213:663-667.
• 3.Boridy IC, Nikolaidis P, Kawashima A, Goldman SM, Sandler CM. Ureterolithiasis: value of the tail sign in differentiating phleboliths from ureteral calculi at nonenhanced helical CT. Radiology 1999; 211:619-621.
• 4.Boulay I, Holtz P, Foley WD, White B, Begun FP. Ureteral calculi: diagnostic efficacy of helical CT and implications for treatment of patients. AJR 1998; 172:1485-1490.
• 5.Chen MYM, Zagoria RJ. Can noncontrast helical computed tomography replace intravenous urography for evaluation of patients with acute urinary tract colic? J Emergency Med 1998; 17(2):299-303.
• 6.Chen MYM, Zagoria RJ, Saunders HS, Dyer RB. Trends in the use of unenhanced helical CT for acute urinary colic. AJR 1999; 173:1447-1450.
• 7.Chen MYM, Scharling ES, Zagoria RJ, Bechtold RE, Dixon RL, Dyer RB. CT diagnosis of acute flank pain from urolithiasis. Sem in Ultrasound, CT, and MRI 2000; 21(1):2-19.
• 8.Chu G, Rosenfield AT, Anderson K, Scout L, Smith RC. Sensitivity and value of digital CT scout radiography for detecting ureteral stones in patients with ureterolithiasis diagnosed on unenhanced CT. AJR 1999; 173:417-423.
• 9.Dalrymple NC, Verga M, Anderson KR, Bove P, Covey AM, Rosenfield AT, Smith RC. The value of unenhanced helical computerized tomography in the management of acute flank pain. Journal of Urology 1998; 159:735-740.
• 10.Dalrymple NC, Casford B, Raiken DP, Elsass KD, Pagan RA. Pearls and pitfalls in the diagnosis of ureterolithiasis with unenhanced helical CT. RadioGraphics 2000; 20:439-447.
• 11.Fielding JR, Fox LA, Heller H, Seltzer SE, Tempany CM, Silverman SG, Steele G. Spiral CT in the evaluation of flank pain: overall accuracy and feature analysis. JCAT 1997; 21(4):635-638.
• 12.Fielding JR, Silverman SG, Samuel S, Zou KH, Loughlin KR. Unenhanced helical CT of ureteral stones: a replacement for excretory urography in planning treatment. AJR 1998; 171:1051-1053.
• 13.Freed KS, Paulson EK, Frederick MG, Preminger GM, Shusterman DJ, Keogan MT, Vieweg J, Smith RHA, Nelson RC, Delong DM, Leder RA. Interobserver variability in the interpretation of unenhanced helical CT for the diagnosis of ureteral stone disease. JCAT 1998; 22(5):732-737.
• 14.Heneghan JP, Dalrymple NC, Verga M, Rosenfield AT, Smith RC. Soft-tissue Rim sign in the diagnosis of ureteral calculi with use of unenhanced helical CT. Radiology 1997; 202:709-711.
• 15.Katz DS, Lane MJ, Sommer FG. Unenhanced helical CT of ureteral stones: incidence of associated urinary tract findings. AJR 1996; 166:1319-1322.
• 16.Katz DS, Hines J, Rausch DR, Perlmutter S, Sommer FG, Lumerman JH, Friedman RM, Lane MJ. Unenhanced helical CT for suspected renal colic. AJR 1999; 173:425-430.
• 17.Kawashima A, Sandler CM, Boridy IC, Takahashi N, Benson GS, Goldman SM. Unenhanced helical CT of ureterolithiasis: value of the tissue rim sign. AJR 1997; 168:997-1000.
• 18.Levine JA, Neitlich J, Verga M, Dalrymple N, Smith RC. Ureteral calculi in patients with flank pain: correlation of plain radiography with unenhanced helical CT. Radiology 1997; 204:27-31.
• 19.Levine J, Neitlich J, Smith RC. The value of prone scanning to distinguish ureterovesical junction stones from ureteral stones that have passed into the bladder: leave no stone unturned. AJR 1999; 172:977-981.
• 20.Liu W, Ealer SJ, Kenny BJ, Goh RH, Rainbow AJ, W. SG. Low-dose nonenhanced helical CT of renal colic: assessment of ureteric stone detection and measurement of effective dose equivalent. Radiology 2000; 215:51-54.
• 21.Miller OF, Rineer SK, Reichard SR, Buckley RG, Donovan MS, Graham IR, Goff WB, Kane CJ. Prospective comparison of unenhanced spiral computed tomography and intravenous urogram in the evaluation of acute flank pain. Urology 1998; 52(6):982-987.
• 22.Olcott EW, Sommer FG, Napel S. Accuracy of detection and measurement of renal calculi: in vitro comparison of three-dimensional spiral CT, radiography, and nephrotomography. Radiology 1997; 204:19-25.
• 23.Preminger GM, Vieweg J, Leder RA, Nelson RC. Urolithiasis: detection and management with unenhanced spiral CT - a urologic perspective. Radiology 1998; 207:308-309.
• 24.Smith RC, Rosenfield AT, Choe KA, Essenmacher KR, Verga M, Glickman MG, Lange RC. Acute flank pain: comparison of non-contrast-enhanced CT and intravenous urography. Radiology 1995; 194:789-794.
• 25.Smith RC, Dalrymple NC, Neitlich J. Noncontrast helical CT in the evaluation of acute flank pain. Abdom Imaging 1998; 23:10-16.
• 26.Smith RC, Varanelli M. Diagnosis and management of acute ureterolithiasis: CT is truth. AJR 2000; 175:3-6.
• 27.Sommer FG, Jeffrey RB, Rubin GD, Napel S, Rimmer SA, Benford J, Harter PM. Detection of ureteral calculi in patients with suspected renal colic: value of reformatted noncontrast helical CT. AJR 1995; 165:509-513.
• 28.Sourtzis S, Thibeau JF, Damry N, Raslan A, Vandendris M, Bellemans M. Radiologic investigation of renal colic: unenhanced helical CT compared with excretory urography. AJR 1999; 172:1491-1494.
• 29.Takahashi N, Kawashima A, Ernst RD, Boridy IC, Goldman SM, Benson GS, Sandler CM. Ureterolithiasis: can clinical outcome be predicted with unenhanced helical CT? Radiology 1998; 208:97-102.
• 30.Traubici J, Neitlich JD, Smith RC. Distinguishing pelvic phleboliths from distal ureteral stones on routine unenhanced helical CT: is there a radiolucent center? AJR 1999; 172:13-17.