The use of noncontrast helical computed tomography (NCHCT) for theevaluation of patients with acute flank pain was first suggested by Smith andcoworkers in 1995. Since that time studies have shown a sensitivity of 97%, aspecificity of 96%, and an accuracy of 97% for this technique in the diagnosisof ureteral stone disease.1-3 As a result, acceptance of NCHCT by referringdoctors for flank pain has rapidly increased.4-8 There is no one universallyutilized protocol, but most NCHCT exams for flank pain are similar. This essaywill discuss our protocol, the key points of interpretation, and the generaladvantages and disadvantages of the technique. Other imaging options also willbe discussed.

Background

Indications for CT are the same as those for the traditional IVP inpatients with suspected stone disease. Unenhanced helical CT is best used fordiagnosis and management of patients with acute flank pain and suspectedureteral obstruction. Such patients may present with hematuria on urinalysis(UA). An abdominal film may show a calcification within the course of theureter or over the kidneys. However, it should be noted that many renal stones,such as uric acid stones, are radiolucent on plain films but all stones areradiopaque on CT. In the pelvis, distinction between phleboliths and ureteralcalculi can be especially difficult with plain films. NCHCT often can clarifythe nature of these calcifications.

Protocol

The patient should be well hydrated prior to undergoing NCHCT. Experiencewith IVP has shown that a contrast or fluid load can be therapeutic, as it mayforce the stone to pass. Additionally, urine within the bladder has been foundto be helpful in interpretation; therefore, a Foley catheter should not beplaced prior to the examination. Helical acquisition is desirable, though it isnot mandatory. Axially acquired images may be used for large patients, or whena helical scanner is not available. In our institution the data is acquired in2 to 3 breath holds from the top of the kidneys, usually at T12, to thesymphysis pubis using a collimation of 5 mm with a pitch of 1.5 to 2.0. Imagesare automatically reconstructed at an interval of 2.5 mm. The larger pitch canprolong the life of the x-ray tube and decrease the number of slices per exam.However, greater collimation and pitch values increase the risk of missingsmall stones in the absence of clearly defined secondary signs ofureterolithiasis. A kVp of 120 and a minimum of 200 mAs should be used. Noenteric contrast material is administered. This study produces about 150images. We film every other image, which results in 75 images that arehard-copied onto four laser films with 20 images per sheet. Mathie andcoworkers report that the course of three-dimensional tubular structures isbest appreciated by viewing on a computer monitor in cine or stack mode.9 All150 images are then reviewed on the picture archiving and communication system(PACS) monitor or workstation in the cine or stack mode.9 Lung windows also arereviewed on the monitor. It is our routine to obtain a plain film of theabdomen as a convenient method of patient follow-up in all patients withpositive results. If the NCHCT is not definitive, contrast can be given and thestudy repeated. This may sort out phleboliths, infection, or confusingfindings. We have found this necessary in over 10% of NCHCT studies.

Interpretation

Identification of the offending stone at the ureteral vesicle junction(UVJ), ureteral pelvic junction (UPJ), or within the ureter or bladder is themost frequent finding (figure 1). The size of the stone should be measured, astreatment and follow-up is often based on stone size. A short learning curvefor accurate identification often is observed as residents and physiciansadjust to noncontrast CT. A thorough under-standing of the anatomical course ofthe ureter on axial images is essential. Secondary findings such as additionalstones within the kidney, hydronephrosis, hydroureter, perinephric fatstranding, perinephric fluid collections, thickening of renal fascia, andnephromegaly on the affected side can aid with the diagnosis of ureteralcalculi.10 These associated findings can provide supportive evidence that anacute obstructive process was present in cases of recently passed stones.Boridy has shown that the degree of perinephric edema on CT correlates with thedegree of obstruction demonstrated on urography; patients with a higher degreeof obstruction have a greater amount of perinephric edema.5 The tissue rimsign, a rim or halo of soft-tissue attenuation seen around the circumference ofan intraureteral calculus on unenhanced axial CT, has been described as usefulin differentiating ureteral calculi from extraurinary abdominal or pelviccalcifications.6,10,11 Secondary findings such as perinephric fat stranding,perinephric fluid collections, thickening of renal fascia, focal renal bulging,and nephromegaly can be seen in other processes such as infection, tumorinvasion, and renal vein thrombosis. Consideration of this differential in theabsence of renal calculi should warrant a contrast study. Hemorrhagicpyelonephritis, an uncommon variant of pyelonephritis, can demonstrate awedge-shaped or rounded area of increased density on NCHCT.12 A carefulclinical history and laboratory studies, such as a UA and WBC count, often arehelpful. If fever and chills are part of the patient's presentation, we feelthat a contrast-enhanced study is essential. Postprocessing is not alwaysnecessary, but the generated data set is suitable for reformatted multiplanarvolume reconstruction (MPVR) and 3D reconstruction. The coronal and curvedprojections are similar to IVP views and may lead to greater acceptance byreferring clinicians (figure 2). Additionally, reformatted images may behelpful in tracing the ureter to the suspect calcification. Patients maypresent with limited clinical information. Other clinical problems such aspyelonephritis, cholecystitis, appendicitis, pelvic inflammatory disease (PID),hernias, and tumors, can mimic ureteral stones. These disorders are difficultto diagnose on an IVP but are easily seen on noncontrast CT. In approximately10% of patients, differentiation of stones from phleboliths may be extremelyproblematic. A repeat study with IV contrast is indicated if the noncontraststudy is unclear and if infection or pyuria is suspected.

Advantages

There are four well defined major advantages of NCHCT when compared tourography:
1) the speed with which the examination may be completed;
2) the avoidance of the use of intravenous contrast material;
3) the ability to diagnose other intra-abdominal conditions which may mimicrenal colic clinically; and
4) the ability to directly visualize all urinary calculi, including those thatare radiolucent, on conventional plain film studies. The entire NCHCT scan maybe completed in 3 to 5 minutes of technologist time. This is in sharp contrastto the minimum time of 45 minutes to perform excretory urography and theadditional time of up to 24 hours that is required to obtain delayed films ifthe patient is actually obstructed. This marked reduction in study time willdecrease the total time spent in the emergency department and allows painmedication for symptomatic relief and definitive therapy to be administeredmuch more quickly. Additionally, CT technologists are usually available aroundthe clock, making the technology readily available in emergent situations.
With a pitch of 1.5 to 2.0, only 75 revolutions of the CT x-ray tube arenecessary, reducing radiation exposure and prolonging tube life. Also, if PACSis available, film usage is eliminated, saving further resources. Theadvantages of not using intravenous contrast material are obvious: There is norisk of adverse reaction and the patient avoids the discomfort of theinjection.
Other conditions including cholecystitis, appendicitis, diverticulitis, andcarcinoma may be confused with renal colic clinically; each of these conditionsmay be directly diagnosed on the CT images (figure 3). The ability to visualizeuric acid calculi and small stones at the UVJ allows a definitive diagnosis tobe made. In the past, with IVP, the diagnosis in many of these cases could onlybe inferred.
The size of an obstructing renal stone is associated with the clinical courseand can easily be measured on NCHCT. Takahashi et al found that mean stonediameter was significantly larger in patients in whom conservative treat-mentfailed (mean: 7.8 mm) than in patients with spontaneous stone passage (mean:2.9 mm).10

Disadvantages

Noncontrast CT does not provide physiological information about renalfunction and the degree of obstruction, though functional information generallyis not required for initial patient management. The small learning curvediscussed above may necessitate repeat performance of contrast studies untilexperience is gained with NCHCT. Helical acquisition is desirable to keepradiation dosages low while locating offending stones of only a few millimetersin size. A 10-mm axial scan may show only the secondary signs of ureteralcalculi. Also, a paucity of perinephric fat can limit the secondary findings ofureteral obstruction. Another disadvantage of NCHCT is that other procedures,such as sonography or MRI, can provide similar information without radiationexposure.

Conclusion

Noncontrast helical CT has quickly become the imaging study of choice inevaluating patients with acute flank pain. This exam can identify both stonesize and location. When renal stones are absent, careful inspection for otherCT findings, such as ipsilateral hydroureter, perinephric edema,hydronephrosis, and renal swelling can provide evi-dence of a recently passedstone. Other pathology mimicking renal colic such as cholecystitis,appendicitis, PID, pancreatitis, and diverticulitis can be identified withoutadditional imaging. Due to its speed, safety, and diagnostic accuracy, NCHCThas rapidly gained acceptance from our radiologists, technologists, clinicians,and administrators. AR