AUA Update SeriesLesson 37 Volume 32 2013

Imaging in the Management of Ureteral Calculi*Learning Objective: At the conclusion of this continuing medical education activity, theparticipant will be able to discuss the currently accepted recommendations for imagingureteral calculi in the settings of initial diagnosis, observation of known calculi andfollow-up after either spontaneous passage or definitive treatment, as well as understandthe evidence-based rationale for these recommendations.

Jodi A. Antonelli, MDDisclosures: Nothing to disclose

Endourology FellowUT Southwestern Medical Center

Pat F. Fulgham, MDDisclosures: Nothing to disclose

Chairman, Department of UrologyTexas Health Presbyterian

and

Margaret S. Pearle, MD, PhDDisclosures: Nothing to disclose

Professor of Urology and Internal MedicineUT Southwestern Medical Center

Dallas, Texas

*This AUA Update addresses the Core Curriculum topics of Urolithiasis and Uroradiology.

This self-study continuing medical education (CME) commercial interest. The AUA must determine if the W Introduction of a debate format with an unbiasedenduring material activity is designed to provide individual’s relationships may influence the moderator (point-counterpoint)urologists, Board candidates and/or residents educational content and resolve any conflicts of W Inclusion of moderated panel discussionaffordable and convenient access to the most recent interest prior to the commencement of the W Publication of a parallel or rebuttal article to andevelopments and techniques in urology. educational activity. The intent of this disclosure is article believed to be biased

not to prevent individuals with relevant financial W Limit equipment representatives to providingAccreditation:The American Urological Associationrelationships from participating, but rather to provide logistics and operation support only in procedural(AUA) is accredited by the Accreditation Council forlearners information with which they can make their demonstrationsContinuing Medical Education (ACCME) to provideown judgments. W Divestiture of the relationship by facultycontinuing medical education for physicians.Planners and Authors: Update Series Advisory Board Off-Label or Unapproved Use of Drugs or Devices: It isCredit Designation: The AUA designates this CME disclosures are listed in the Table of Contents. the policy of the AUA to require the disclosure of allactivity for a maximum of 1.0 AMA PRA Category 1 Authors’ credentials and disclosures are listed on the references to off-label or unapproved uses of drugs orCredit™. Physicians should claim only the credit title page of each lesson. devices before presentation of educational content.commensurate with the extent of their participation inResolution of Identified Conflict of Interest: All The audience is advised that this CME activity maythe activity.disclosures will be reviewed by the program/course contain reference(s) to off-label or unapproved uses of

AUA Disclosure Policy: All persons in a position to directors or editors for identification of conflicts of drugs or devices. Please consult the prescribingcontrol the content of an educational activity (ie interest. Peer reviewers, working with the program information for full disclosure of approved uses.activity planners, presenters, authors) and directors and/or editors, will document the Evidence-Based Content: It is the policy of the AUA toparticipating in an educational activity provided by the mechanism(s) for management and resolution of the review and certify that the content contained in thisAUA are required to disclose to the provider any conflict of interest and final approval of the activity CME activity is valid, fair, balanced, scientificallyrelevant financial relationships with any will be documented prior to implementation. Any of rigorous and free of commercial bias.the mechanisms below can/will be used to resolve

Disclaimer: The opinions and recommendationsconflict of interest: expressed by faculty, authors and other expertsW Peer review for valid, evidence-based content of all whose input is included in this program are their ownmaterials associated with an educational activity by and do not necessarily represent the viewpoint of thethe course/program director, editor and/or AUA.Education Content Review Committee or its

Release date: December 2013subgroupW Limit content to evidence with no recommendations Expiration date: December 2016

© 2013 American Urological Association, Linthicum, MD

KEY WORDS: ureteral calculi, epidemiology, imaging

OVERVIEW

Urolithiasis is a common problem worldwide, and has been associ-ated with an increase in incidence and prevalence during the last25 years.1 Patients diagnosed with upper urinary tract stones, partic-ularly ureteral calculi, often undergo repeated imaging studies dur-ing the management and treatment of the disease. The risk ofexcessive cumulative radiation exposure, coupled with cost con-cerns, has prompted efforts to standardize the use of imaging studiesin the management of ureteral calculi. We review the evidence andrecommendations for the type and frequency of imaging as it per-tains to the diagnosis, management and follow-up of ureteral calculiwith special emphasis on the AUA clinical effectiveness protocolsfor imaging in the management of ureteral calculus disease.2

EPIDEMIOLOGY

Kidney stone disease in the U.S. has demonstrated a linear increasein prevalence during the last several decades. An assessment ofthe NHANES (National Health and Nutrition Examination Survey)II and III datasets suggested that the lifetime risk of forming stonesfor U.S. adults increased significantly from 3.2% in 1976 to 1980to 5.2% in 1988 to 1994.3 The most recent NHANES data (2007 to2010) indicate that this trend has persisted, with a current estimatedprevalence of stone disease in U.S. adults of 8.8%.4 A literaturereview that included data from 5 European countries, Japan andthe United States suggested that increases in the incidence andprevalence of stone disease are global phenomena that have beenattributed in part to global warming and changes in dietary habits.1

Consequently, imaging studies obtained for the diagnosis and man-agement of urinary calculi are likewise on the rise. With a recur-rence rate of 26% to 53%,5, 6 a first time stone former is likelyto be subjected to numerous imaging studies throughout his orher lifetime.

Furthermore, there has been a trend toward increased use ofmodalities associated with higher radiation exposure for the evalua-tion of acute flank pain. Hyams et al used the National Hospitaland Ambulatory Medical Care Survey dataset from 2005 to 2009to analyze trends in emergency room evaluations for acute flankor kidney pain and found a significant increase in CT use (p<0.0001) and a decrease in conventional x-ray use (p=0.035), withstable ultrasound use during this time.7 Likewise, analysis of datafrom the Centers for Medicare and Medicaid Services revealed a31% decrease in the use of IVU and a threefold increase in the useof non-contrast CT for the diagnosis of urolithiasis between 1992and 1998.8 That trend has continued, with a 1.8-fold increase inCT use between 2002 and 2007 among Medicare beneficiaries forevaluation of urinary tract stones.9

CONSIDERTIONS WHEN CHOOSING AN IMAGINGMODALITY

Sensitivity and specificity: The sensitivity and specificity of differ-ent imaging modalities are of paramount importance when establish-

ABBREVIATIONS: BMI (body mass index), CT (computerized tomography), IVU (intravenous urography), KUB (plain film of thekidneys, ureters and bladder), MRI (magnetic resonance imaging), NCCT (non-contrast CT), SSD (skin-to-stone distance), SWL (shockwave lithotripsy), URS (ureteroscopy)

374

ing utility in the diagnosis of ureteral calculi. As shown in table 1,a ‘‘standard’’ protocol NCCT has the highest median sensitivityand specificity (98% and 97%, respectively) of all conventionalimaging modalities for detecting ureteral calculi. However, alow dose protocol CT, typically one that is associated with ≤4mSv, sacrifices little in the way of sensitivity or specificity inthe detection of ureteral stones (97% sensitivity and 95% speci-ficity). Regardless, the diagnostic success of an imaging modalitymust be weighed against its potentially adverse effects, notablyradiation exposure and cost.

Radiation exposure: ‘‘Effective radiation dose’’ (mSv), the sumof the equivalent doses of radiation to exposed organs, is used toquantify the potential adverse biological effects of radiation onpatients and health care providers. The effective dose does notequate to the absorbed dose for an individual, as that measure isdependent upon the scanning protocol for the study and the specificequipment used. Table 2 shows the estimated effective doses associ-ated with conventional imaging techniques. Of note a low doseprotocol CT has the same estimated effective dose as an IVU andless than a third the effective dose of a standard protocol CT.2

The concept of ALARA (As Low as Reasonably Achievable),originally promulgated in pediatric radiology, represents aneffort to minimize the total radiation exposure to a patientthrough modifications in machine design (exposure rate andexposure/image) and machine operation (fluoroscopy time and

Table 1. Sensitivity and specificity of conventional imaging modal-ities for the detection of ureteral calculi

Imaging Modality Median % Sensitivity Median % Specificity

Standard protocol CT 98 97

Low dose protocol CT 97 95

Conventional radiography 57 76

Intravenous urogram 70 95

Ultrasound 61 97

MRI 82 98.3

Table 2. Estimated effective radiation dose for each imagingmodality

EffectiveImaging Technique Dose (mSv)

Ultrasound of the abdomen + pelvis 0.0MRI of the abdomen + pelvis 0.0KUB 0.7KUB with tomograms 3.9Intravenous urogram 3.0NCCT of abdomen + pelvis 10.02-Phase CT of abdomen + pelvis without and with 15.0

contrast3-Phase CT of abdomen + pelvis without and with 20.0

contrastNCCT of abdomen + pelvis (low dose protocol) 3.0

number of images).10 The need for ALARA is highlighted byseveral recent retrospective reviews that quantified the radia-tion exposure associated with a single stone event.

Ferrandino et al performed a multicenter, retrospective review of108 first time stone formers to determine the radiation exposureassociated with an acute stone event through 1 year of follow-up.11

Although the median effective radiation dose per patient was 29.7mSv, notably 20% of patients received greater than 50 mSv ofradiation, the maximum recommended yearly dose for occupationalexposure set by the International Commission on Radiological Pro-tection.

In a retrospective review of 60 patients presenting with renal and/or ureteral calculi ultimately treated either conservatively or withsurgical intervention, John et al determined the radiation exposureassociated with a single stone episode from the time of diagnosisto when the patient was deemed stone-free. 12 The median effectiveradiation dose per patient was 5.3 mSv. Radiation exposure washighest for renal calculi, followed by middle and distal ureteralcalculi. The 14 patients who underwent CT imaging during thistime had a significantly higher median effective radiation dose thanthose not imaged with CT (14.46 mSv vs 4.25 mSv, respectively,p <0.0001).

Finally, Fahmy et al analyzed the charts of 104 patients followedfor 2 years from the time of an acute stone episode.13 During thefirst year of follow-up the median effective dose per patient was29.29 mSv, decreasing significantly to 8.04 mSv in the second year(p <0.01) by performing less CT imaging and more ultrasonography.Although the radiation exposure in 17.3% of patients exceeded 50mSv within the first year, this threshold was not exceeded in anypatient during year 2 of follow-up.

Cost: The diagnosis and management of urolithiasis significantlyimpact the economics of health care through direct and indirectcosts. In 2000 the total annual expenditure for urolithiasis wasestimated at $2.1 billion including $971 million for inpatient ser-vices, $607 million for physician office and hospital outpatientservices, and $490 million for emergency room services.14 Includedin this expenditure are costs associated with imaging which, ac-cording to the Medicare Payment Advisory commission, accountfor 16% of the total cost incurred during a single stone episode.15

The exact cost of each imaging technique varies greatly, and isdependent on market-driven factors and the source of the payer.According to national averages of Centers for Medicare and Medic-aid Services charge data for 2012, if a KUB has a relative valueof 1 the average allowable charge is 5 for an ultrasound or IVU,10 for CT and 30 for magnetic resonance imaging.2 While thisinformation provides only a relative measure of cost, it demonstratesthat advanced imaging techniques such as CT and MRI areassociated with substantially higher cost than basic imagingmodalities such as plain radiography or ultrasound.

PROGNOSTIC INFORMATION

In addition to the simple detection of stones, imaging can provideother useful information about stone and patient characteristicswhich can be used to guide management and treatment decisions.

Size and location: Hubner et al performed a meta-analysis of 6natural history studies of stones comprising 2704 patients whounderwent observation for ureteral calculi.16 The incidence of spon-taneous stone passage varied according to the location of the stone

375

in the ureter at initial presentation, with passage rates increasingthe more distal the stone was located in the ureter (12%, 22% and45% for stones in the proximal, middle and distal ureter, respec-tively).

Miller and Kane prospectively followed 75 patients with ureteralcalculi, most of which were <4 mm, to determine the natural historyof these stones.17 They found that 83% of ureteral stones passedwithout the need for intervention, with an average time to passagefor stones ≤2 mm, 2.1 to 3.9 mm and ≥4 mm of 8.2, 12.2 and 22.1days, respectively. Furthermore, of stones ≤6 mm that passedspontaneously 95% did so within 6 weeks of initial presentation.Eisner et al retrospectively reviewed the CT scans of patients pres-enting with obstructing ureteral calculi, and proximal ureteral stoneshad a significantly longer coronal diameter than distal ureteralstones (9.9 mm vs 8.3 mm, respectively, p=0.005) and were associ-ated with significantly more ureteral dilatation (6.1 mm vs 5.3 mm,respectively, p=0.01).18

While these studies demonstrate that size and location are impor-tant predictors of spontaneous stone passage, it is important to notethat stone size may not be accurately measured in all dimensions onCT. Several studies have compared the transverse and longitudinalmeasurements of stones on CT and plain radiography and, althoughthe transverse diameters were comparable on the 2 imaging modal-ities, CT overestimated the longitudinal (cephalocaudal) size ofstones by 0.8 to 1.4 mm.19-21 Post-processing CT parameters canbe optimized to increase the accuracy of stone measurement. Eisneret al used calipers to measure the diameter of 24 human stones andthen embedded the stones in potatoes the size of human kidneysbefore imaging them with a 64 slice multidetector CT.22 In an invivo correlate of the study, they measured the size of 41 spontane-ously passed stones in patients diagnosed with distal ureteral calculiby CT. In the in vitro and in vivo studies the smallest discrepancybetween stone size on imaging and actual stone size occurred whenthe stone was measured in a 4.0× magnified bone window. There-fore, while size and location can serve as general guides uponwhich to advise patients of the likelihood of spontaneous stonepassage, stone size is best determined in the magnified bonewindow of a CT image.

Stone composition: Knowledge of current or prior stone composi-tion is useful when advising patients on appropriate therapy, asstone composition is an indicator of stone fragility, an importantdeterminant of shock wave lithotripsy success. Additionally, stonecomposition can determine if dissolution therapy is a reasonabletherapeutic option, as for uric acid and some cystine stones.

Several investigators have attempted to correlate CT attenuationcoefficient (HU) with stone composition, although this has beenonly marginally successful because of significant overlap in therange of HUs for each stone type.23 However, several studies havedemonstrated an inverse relationship between CT attenuationcoefficient and shock wave lithotripsy success,24-26 allowing thisparameter to be used as a measure to identify patients whomight be successfully treated with SWL and those better servedwith endoscopic management. Dual energy CT, on the other hand,may hold greater promise in determining actual stone compositioncompared to single source CT.27

Skin-to-stone distance: Pareek et al introduced another importantprognostic imaging parameter, skin-to-stone distance, which is ob-tained by averaging the distance from the skin to the stone at 0°,45° and 90°.28 Wiesenthal et al conducted a retrospective review

of 204 patients with a solitary renal or ureteral calculus on CTtreated with SWL.29 Using univariate and multivariate analyses todetermine the influence of mean stone density and SSD on SWLsuccess, they found that HU >900 (OR 0.49, 95% CI 0.32 to 0.75,p >0.01) and SSD >11 cm (OR 0.49, 95% CI 0.31 to 0.78, p <0.01)were the best independent predictors of SWL failure. Of note thesesame authors developed a nomogram to predict SWL outcomesfor renal and ureteral calculi and determined that SSD was anindependent predictor of SWL success for renal calculi while meanstone density was not.30 For ureteral calculi neither of these parame-ters was a predictor of SWL success, although body mass index,which likely correlates with SSD, was a negative predictor.

Ng et al performed logistic regression analysis to identify factorspredictive of SWL success or failure.31 Using data from 94 patientswith proximal ureteral stones treated with SWL, they determinedthat stone volume and mean stone density were independent pre-dictors of SWL outcomes, while SSD showed only a minimal effect.Cut points for each parameter were determined using ROC curvesand a scoring system was constructed based on stone volume <0.2cc, mean stone density <593 HU and SSD <9.2 cm representingfavorable parameters for SWL. For patients with 0, 1, 2 and 3factors the stone-free rates were 17.9%, 48.4%, 73.3% and 100%,respectively. While these and other authors have shown that SSDis a useful prognostic indicator,32 others have not supported thisconclusion.33, 34

Urinary obstruction: For patients with ureteral stones, CT maydemonstrate secondary signs of obstruction including hydroureterin 82.7%, hydronephrosis in 80%, periureteral edema in 59% andunilateral renal enlargement in 57.2%.35 However, these findings,particularly hydronephrosis, have not been shown to be reliablepredictors of the degree of obstruction.

Bird et al reviewed the records of 77 patients seen in the emergencydepartment with renal colic who were diagnosed with ureteral stonesby CT and who then underwent a diuretic 99mtechnetium mercap-toacetyltriglycine renal scan within a mean of 6 hours.36 In anattempt to correlate CT findings, such as renal parenchymal edema,hydronephrosis, stranding and urinary extravasation, with the de-gree of obstruction seen on renal scan they found no difference inthe number of secondary CT findings among those with high grade,partial and no obstruction but they did find a greater number ofsecondary CT findings in those with any obstruction vs no obstruc-tion. Of note, no CT findings distinguished high grade from partialobstruction and only perinephric/periureteral fat stranding distin-guished any obstruction from no obstruction.

These results suggest that CT can identify the presence but notthe degree of obstruction and, therefore, it may not be the besttool with which to determine the duration of time that a ureteralstone can be safely observed. With limited availability of IVUand renography, in the acute setting or at all, hydronephrosison CT and/or ultrasound is still the most commonly used findingto determine the need for and timing of intervention. However,for the conservative treatment of ureteral calculi, a follow-upfunctional study (IVU, diuretic renogram or CT urogram) maybe advisable if prolonged observation is entertained.

IMAGING MODALITIES

Computerized tomography: NCCT is the gold standard imagingmodality for the initial evaluation of a patient with abdominal

376

pain and a suspected ureteral stone.37 CT is associated with highsensitivity and specificity for the diagnosis of ureteral stones (table1). Prior to the widespread availability of CT, IVU constituted thegold standard for the diagnosis of ureteral stones. CT and IVU areequally effective at demonstrating the presence or absence of ure-teral obstruction but CT is more accurate than IVU in preciselyidentifying a ureteral stone.38

CT provides information about the surrounding viscera as well,which can be useful when attempting to identify the source ofabdominal, pelvic or flank pain. Hoppe et al reviewed the recordsof 1500 patients who underwent CT for acute flank pain and 69%had urolithiasis but an additional diagnosis was identified in 47%.39

A total of 24% of patients had a diagnosis other than stones and14% required immediate or deferred attention for the diagnosis.The information garnered from CT, including stone size and loca-tion, stone density, SSD and presence of secondary signs of obstruc-tion, is important for prognostic indicators that can be used whenmaking recommendations for the management and particularly sur-gical treatment of stones. Furthermore, visualization of organs out-side the genitourinary tract can provide information necessary tocorrectly diagnose the etiology of flank and/or abdominal pain.

Ultrasound: The main advantage of ultrasound is that it is associ-ated with no ionizing radiation but this benefit comes at the expenseof lower sensitivity for the detection of ureteral stones comparedto other imaging modalities (table 1). Viprakasit et al comparedultrasound and CT for sensitivity, specificity and accuracy in de-tecting urinary stones.40 Sensitivity for the identification of a stoneon ultrasound decreased as stone size decreased (sensitivity 35%for stones ≤4 mm and 86% for those ≥10 mm) and was lower forureteral compared to renal stones (sensitivity 15% for ureteral and44% for renal stones). However, evaluation of ureteral stones im-proved to 67% with the use of secondary characteristics such ashydronephrosis or absence of ureteral jets.

While the sensitivity of ultrasound for detecting ureteral stonesin general is moderate at best, this modality has its greatest effective-ness in diagnosing distal ureteral stones. Moesbergen et al followed152 patients initially diagnosed with an impacted ureteral stone byCT (81% had a distal ureteral or ureterovesical stone) with concur-rent CT and ultrasound examinations.41 The sensitivity and specific-ity of ultrasound for diagnosing persistent distal ureteral calculiduring follow-up using CT as the reference was 94.3% and 99.1%,respectively. Furthermore, distal ureteral stones within 35 mm ofthe ureterovesical junction could be reliably followed on ultrasound.The authors concluded that findings on ultrasound indicative ofa stone included a hyperechoic intraureteral focus with posterioracoustic shadowing, a circumferential anechoic rim of urine (halosign) and random color encoding seen with color Doppler imagingin the area where shadowing would be expected (the twinkle arti-fact). Other investigators have reported greater sensitivity of colorDoppler (97%) in detecting stones compared to gray scale technique(66%).42

Implementation of color Doppler ultrasound imaging can alsoaid in the diagnosis of ureteral obstruction through assessmentof the resistive index43, 44 and identification of ureteral jets.45

Shokeir and Abdulmaaboud evaluated 109 patients who presentedwith unilateral flank pain and underwent CT, Doppler ultrasoundand IVU.43 Compared to IVU, the predetermined gold standard,the sensitivity and specificity for detecting ureteral obstructionwas 96% and 96%, respectively, for CT, and 90% and 100%,

respectively, for ultrasound. Others have defined a resistive indexof 0.7 and a difference in resistive index between kidneys of 10%as diagnostic of obstruction.44

Ultrasound affords the opportunity to assess other viscera, albeitin a more limited fashion than CT or MRI. The sensitivity ofultrasound in diagnosing other causes of abdominal pain, such asappendicitis, has been reported to be as high as 85%. 46 However,the lower sensitivity of ultrasound in diagnosing small stones (<4mm), overestimation of stone size and limited use in obese patientsreduce its overall usefulness in the management of acute flankpain.47

Intravenous urography: IVU has been largely supplanted byNCCT as the imaging modality of choice for the evaluation ofacute flank pain because CT has greater sensitivity for detectingureteral calculi (table 1), more rapid acquisition time and the poten-tial to diagnose non-urinary causes for pain. However, the use ofintravenous contrast with IVU provides functional information,qualitatively assesses renal obstruction and delineates the anat-omy of the collecting system, which can be valuable in preoperativeplanning.

Plain radiography: The sensitivity and specificity of plain abdom-inal radiographs for the detection of ureteral calculi are considerablylower than the other available imaging modalities (table 1). Inclu-sion of tomography with plain radiography increases the detectionrate of urolithiasis.48 However, the effective radiation dose of aKUB and 3 tomograms is nearly 4 mSv, equaling or exceedingthe dose associated with low dose CT (table 2).

The radiopacity of a stone on plain film has potential implicationsfor surgical management and follow-up. In most cases ureteralstones are initially diagnosed by CT. The CT scout image, whichmimics a plain abdominal radiograph, may demonstrate a ureteralstone but is not as sensitive as either CT or KUB. Approximately53% of stones visible on CT are not visible on CT scout.49 Further-more, a stone not visible on CT scout can be seen on KUB in 10%to 43.5% of cases, and nearly all stones visible on CT scout canbe visualized on KUB.49-51 Therefore, if a stone is visible on CTscout, it can be reliably followed by KUB imaging. However,if the stone is not visible on CT scout, a KUB should be obtainedas this will determine whether KUB is sufficient for follow-up or if repeat CT is necessary. The combination of KUB andultrasound has sensitivity and specificity for ureteral calculus detec-tion that exceeds 90%.42

Magnetic resonance imaging: MRI has limited use in the manage-ment of ureteral calculi, in part because calculi cannot be directlyvisualized. However, MRI can diagnose urinary tract obstructionand pathology outside the genitourinary tract. For patients in whomradiation exposure must be avoided and ultrasonography provesnon-diagnostic, as is often the case in pregnancy, MRI may havediagnostic benefit.

Historically the use of T2-weighted MRI was thought to reason-ably recognize ureteral stones and obstruction, as urine is readilyidentified during this phase and a filling defect and/or hydrouretero-nephrosis can indirectly diagnose an obstructing stone. Addition-ally, novel techniques applied to standard MRI, such as 3DFLASH52 and HASTE MR urography,53 demonstrate augmentedsensitivity and specificity compared to standard MRI or CT andhave been advocated for use in pregnancy.

377

AUA CLINICAL EFFECTIVENESS PROTOCOL

As the prevalence of urolithiasis, a disease known for its propensityfor recurrence, continues to increase, standardization of manage-ment strategies becomes increasingly important to improve out-comes, minimize adverse effects and reduce the economic burdenof the disease. While the European Association of Urology54 andthe AUA55 have established guidelines addressing the managementof ureteral calculi, these guidelines focus on treatment strategiesand notably exclude recommendations directed at the type andfrequency of imaging studies to use in the diagnosis, managementand follow-up of ureteral stones. This void prompted the AUA todevelop a clinical effectiveness protocol regarding the role of im-aging in the management of ureteral calculi.2

Adults, initial presentation: For adults presenting with flankand/or abdominal pain suggestive of renal colic, the goals areexpeditious, accurate diagnosis and prompt disposition. Conse-quently, NCCT of the abdominal pelvis is recommended be-cause of its high sensitivity and specificity for detecting ureteralstones, rapid image acquisition time and the widespread avail-ability of CT scanners.2

While standard NCCT of the abdomen and pelvis has the highestsensitivity and specificity for the detection of urolithiasis, low doseNCCT (≤4 mSv) can provide acceptable diagnostic accuracy witha significantly lower effective radiation dose. In their meta-analysisof published series from 1995 to 2007 Niemann et al found thatthe pooled sensitivity and specificity of low dose NCCT (<3 mSv)in the diagnosis of urolithiasis was 96.6% and 94.9%, respectively.56

Additionally, no difference between low dose and standard NCCThas been reported in the measurements of other important parame-ters gleaned from CT including stone size, HU and SSD.

However, low dose NCCT has shown limited usefulness in theevaluation of obese patients. The sensitivity and specificity of lowdose NCCT for the detection of ureteral calculi are considerablylower in patients with BMI >30 kg/m2 than in those with an index≤30.57-60 As such, AUA recommends the use of low dose NCCTof the abdomen and pelvis for the diagnosis of ureteral calculiin patients with a BMI ≤30 kg/m2 and standard protocol NCCTfor those with a BMI >30 kg/m2(fig. 1).2

It is also important at initial diagnosis to determine if theureteral stone is radiopaque on KUB as this will impact recom-mendations for follow-up imaging. If the stone is visible on CTscout and/or KUB, follow-up imaging with a KUB should suffice.Oblique films may further enhance the ability to identify a ureteralcalculus not seen on anteroposterior views. However, when thestone is not identifiable on KUB with or without oblique views,CT limited to the region of interest (eg pelvic CT for a distalureteral calculus) should be judiciously obtained for follow-up.

Adults, observation of known calculus/calculi: Imaging in thesetting of follow-up of a known ureteral calculus should conveyinformation regarding the progression of a stone along the ureterand the presence of new or persistent hydronephrosis. Accordingto the AUA guidelines for the management of ureteral calculi,medical expulsive therapy should be considered for patients witha ureteral stone <10 mm and minimal hydronephrosis, no evidenceof renal damage and well controlled pain symptoms.55 Figure 2depicts the follow-up imaging algorithm for patients undergoingobservation with or without medical expulsive therapy, beginningwith determination of whether the stone is radiopaque as this charac-

Figure 1. Algorithm for imaging of suspected ureteral calculi at initial presentation

teristic determines the type of follow-up study that will identifythe stone. For a radiopaque stone, KUB and ultrasound providethe best combination of acceptable sensitivity and specificity,and minimal radiation exposure and cost to follow the progres-sion of the stone. If a patient remains symptomatic despite negativeKUB and ultrasound, then supplemental imaging such as obliqueviews and/or limited low dose NCCT (if BMI is ≤30 kg/m2) mayidentify a persistent stone.

If a patient has a radiolucent stone, then the follow-up algo-rithm begins with low dose NCCT of the abdomen and pelvisat a reasonable interval if the stone has not obviously passed.Imaging is repeated until documented passage of the stone or needfor surgical intervention is apparent. No further imaging is neces-sary for a patient with either a radiolucent or radiopaque stonewho reports relief of symptoms and passage of the stone (stonein hand).2 However, if an asymptomatic patient does not haveevidence of stone passage, low dose NCCT of the abdomen andpelvis is recommended (fig. 2).

The interval(s) at which follow-up imaging should be performedafter the diagnosis of a ureteral calculus is not explicitly stated inthe imaging protocol because of a lack of definitive informationin the literature regarding the duration of obstruction before theonset of irreversible loss of renal function. Consequently, it is leftto the discretion of the practitioner to decide when to intervenesurgically and when to continue to observe. However, a functionalimaging study should be considered in a patient with hydronephrosiswho undergoes observation for an extended time. Furthermore, themajority of stones that will pass spontaneously do so within 6

378

weeks of diagnosis17 and, therefore, observation beyond 6 weeksis discouraged.

Adults, follow-up after definitive treatment: Follow-up imagingafter surgical management of ureteral calculi is aimed at docu-menting the absence of the stone and resolution of obstruction.In the setting of URS postoperative hydronephrosis is likely aconsequence of an obstructing retained ureteral stone/fragment,transient ureteral edema or ureteral stricture. The incidence of stric-ture after URS has been reported as 1% to 4%, but alarmingly,obstruction does not always cause symptoms.55 Although postoper-ative stricture formation after SWL is highly uncommon, postopera-tive imaging in this setting is important to confirm stone clearanceand resolution of hydronephrosis.

Despite the general consensus regarding the need for imagingstudies that identify residual stones following procedures that areassociated with stone fragmentation (SWL and URS with stonefragmentation), the need for imaging that identifies hydronephrosisafter URS and SWL is controversial. Although the incidence ofpostoperative stricture is low after URS and SWL, postoperativeobstruction is not reliably associated with symptoms. Silent ob-struction following URS has been reported with an incidenceof 0% to 4.8%,61-67 compelling some investigators to advocatethe use of routine imaging following URS.61-64 On the otherhand, some studies have found that silent obstruction after URS isexceedingly rare and that nearly all cases of obstruction can beidentified by selectively imaging patients at high risk (those withureteral injury or an impacted stone, or those experiencing postoper-ative pain).65-67

Figure 2. Algorithm for imaging during observation of known ureteral calculi

Figure 3. Algorithm for imaging of ureteral calculi after SWL and stone passage

379

Figure 4. Algorithm for imaging of ureteral calculi after ureteroscopy. A, intact stone. B, stone requiring fragmentation.

Routine imaging of all patients after URS necessitates imagingof many patients unnecessarily. However, Manger et al reviewedthe records of 289 patients who underwent URS with ultrasoundfollow-up and 4.8% had silent obstruction.62 According to theirdata, only 18 asymptomatic patients need to be imaged to diagnose1 case of silent obstruction. Sutherland et al compared the economicconsequences of routine postoperative imaging for all patientstreated with URS versus selective imaging of symptomatic patients,

380

taking into account the cost of imaging and the cost of complicationsassociated with unrecognized loss of renal function, such as theneed for dialysis or transplantation and the consequences of chronickidney disease.68 Assuming a silent obstruction rate of 2% averagedfrom the literature, they calculated that selective postoperative im-aging is associated with a $130 cost savings per patient comparedto routine postoperative imaging in all patients after URS. However,a strategy of selective imaging could lead to the loss of 2 kidneys

for every 100 patients undergoing URS. Sutherland et al concludedthat this small cost savings is not worth the potential loss of renalfunction and associated sequelae, and they advocated routine im-aging in all patients as recommended in the AUA protocol.

In an effort to balance diagnostic accuracy against radiation andcost concerns, the recommendations for imaging after URS, SWLand medical expulsive therapy are shown in figures 3 and 4. Fora patient with a radiopaque ureteral calculus who undergoesSWL, KUB and ultrasound are sufficient to assess for the pres-ence of residual fragments and hydronephrosis at follow-up. Ifthe stone is radiolucent, ultrasound alone is initially sufficientbut if hydronephrosis is evident, low dose NCCT of the abdomenand pelvis should be obtained to identify residual stone (fig.3). Persistent symptoms following presumed successful medicalexpulsive therapy or spontaneous stone passage should initially beevaluated with ultrasound but if hydronephrosis is demonstrated,CT of the abdomen and pelvis with and without contrast shouldbe obtained to assess for an additional stone or edema as the causefor persistent obstruction (fig. 3).

A distinction is made between URS with intact removal of a stone(fig. 4, A) and URS with fragmentation (fig. 4, B). Renal ultrasoundis recommended after intact removal of a stone in an asymptom-atic patient to ensure that there is no residual obstruction.If the patient is symptomatic postoperatively or has unexplainedhydronephrosis on renal ultrasound, a contrast CT of the abdomenand pelvis should be obtained. After URS with fragmentation ofradiopaque stones, KUB and ultrasound are recommended todocument absence of hydronephrosis or residual fragments.If a patient has either of these findings on imaging, or remainssymptomatic despite negative studies, continued observation/im-aging or secondary intervention is left to the discretion of thepractitioner. Ultrasound imaging is sufficient for patients withradiolucent stones if they are asymptomatic. However, if hydro-nephrosis is evident or if the patient is symptomatic, low doseNCCT of the abdomen and pelvis is advised to identify obstructingfragments or stricture, and further intervention is left to the discre-tion of the practitioner. Patients with persistent symptoms despitenegative imaging can undergo continued observation or furtherimaging as indicated.

SUMMARY

When considering imaging for the management of ureteral calculione must weigh the desire for diagnostic accuracy against thepotential risks of radiation exposure and cost. Adhering to theprinciple of ALARA and understanding the recommendations setforth in the AUA clinical effectiveness protocol should provide aframework upon which decisions about imaging are made.

REFERENCES1. Romero V, Akpinar H and Assimos DG: Kidney stones: a global

picture of prevalence, incidence, and associated risk factors. RevUrol 2010; 12: e86.

2. Fulgham PF, Assimos DG, Pearle MS et al: Clinical effectivenessprotocols for imaging in the management of ureteral calculousdisease: AUA technology assessment. J Urol 2013; 189: 1203.

3. Stamatelou KK, Francis ME, Jones CA et al: Time trends in re-ported prevalence of kidney stones in the United States: 1976-1994. Kidney Int 2003; 63: 1817.

381

4. Scales CD Jr, Smith AC, Hanley JM et al: Prevalence of kidneystones in the United States. Eur Urol 2012; 62: 160.

5. Ljunghall S and Danielson BG: A prospective study of renal stonerecurrences. Br J Urol 1984; 56: 122.

6. Trinchieri A, Ostini F, Nespoli R et al: A prospective study ofrecurrence rate and risk factors for recurrence after a first renalstone. J Urol 1999; 162: 27.

7. Hyams ES, Korley FK, Pham JC et al: Trends in imaging useduring the emergency department evaluation of flank pain. J Urol2011; 186: 2270.

8. Pearle MS, Calhoun EA and Curhan GC: Urolithiasis. In: UrologicDiseases in America. US Department of Health and Human Ser-vices, Public Health Service, National Institutes of Health, NationalInstitute of Diabetes and Digestive and Kidney Diseases. Vol 07-5512. Edited by MS Litwin and CS Saigal. Washington, D.C.: USGovernment Printing Office 2007; p 281.

9. MS Litwin and CS Saigal: Urinary tract stones. In: Urologic Dis-eases in America. US Department of Health and Human Services,Public Health Service, National Institutes of Health, National Insti-tute of Diabetes and Digestive and Kidney Diseases. Vol 12-7865.Washington, D.C.: US Government Printing Office 2012; p 345,table 9-33.

10. Strauss KJ and Kaste SC: The ALARA concept in pediatric inter-ventional and fluoroscopic imaging: striving to keep radiation dosesas low as possible during fluoroscopy of pediatric patients—awhite paper executive summary. AJR Am J Roentgenol 2006;187: 818.

11. Ferrandino MN, Bagrodia A, Pierre SA et al: Radiation exposure inthe acute and short-term management of urolithiasis at 2 academiccenters. J Urol 2009; 181: 668.

12. John BS, Patel U and Anson K: What radiation exposure can apatient expect during a single stone episode? J Endourol 2008;22: 419.

13. Fahmy NM, Elkoushy MA and Andonian S: Effective radiationexposure in evaluation and follow-up of patients with urolithiasis.Urology 2012; 79: 43.

14. Pearle MS, Calhoun EA, Curhan GC et al: Urologic Diseases inAmerica project: urolithiasis. J Urol 2005; 173: 848.

15. Medicare Payment Advisory Commission: Impact of physicianself-referral on use of imaging services within an episode. Washing-ton, D.C.: Medicare Payment Advisory Commission 2009.

16. Hubner WA, Irby P and Stoller ML: Natural history and currentconcepts for the treatment of small ureteral calculi. Eur Urol 1993;24: 172.

17. Miller OF and Kane CJ: Time to stone passage for observed ureteralcalculi: a guide for patient education. J Urol 1999; 162: 688.

18. Eisner BH, Pedro R, Namasivayam S et al: Differences in stonesize and ureteral dilation between obstructing proximal and distalureteral calculi. Urology 2008; 72: 517.

19. Parsons JK, Lancini V, Shetye K et al: Urinary stone size: compari-son of abdominal plain radiography and noncontrast CT measure-ments. J Endourol 2003; 17: 725.

20. Tisdale BE, Siemens DR, Lysack J et al: Correlation of CT scanversus plain radiography for measuring urinary stone dimensions.Can J Urol 2007; 14: 3489.

21. Narepalem N, Sundaram CP, Boridy IC et al: Comparison of helicalcomputerized tomography and plain radiography for estimatingurinary stone size. J Urol 2002; 167: 1235.

22. Eisner BH, Kambadakone A, Monga M et al: Computerized tomog-raphy magnified bone windows are superior to standard soft tissue

windows for accurate measurement of stone size: an in vitro andclinical study. J Urol 2009; 181: 1710.

23. Motley G, Dalrymple N, Keesling C et al: Hounsfield unit densityin the determination of urinary stone composition. Urology 2001;58: 170.

24. Joseph P, Mandal AK, Singh SK et al: Computerized tomographyattenuation value of renal calculus: can it predict successful frag-mentation of the calculus by extracorporeal shock wave lithotripsy?A preliminary study. J Urol 2002; 167: 1968.

25. Shah K, Kurien A, Mishra S et al: Predicting effectiveness ofextracorporeal shockwave lithotripsy by stone attenuation value.J Endourol 2010; 24: 1169.

26. Gupta NP, Ansari MS, Kesarvani P et al: Role of computed tomog-raphy with no contrast medium enhancement in predicting theoutcome of extracorporeal shock wave lithotripsy for urinary cal-culi. BJU Int 2005; 95: 1285.

27. Manglaviti G, Tresoldi S, Guerrer CS et al: In vivo evaluation ofthe chemical composition of urinary stones using dual-energy CT.AJR Am J Roentgenol 2011; 197: W76.

28. Pareek G, Hedican SP, Lee FT Jr et al: Shock wave lithotripsysuccess determined by skin-to-stone distance on computed tomog-raphy. Urology 2005; 66: 941.

29. Wiesenthal JD, Ghiculete D, D’A Honey RJ et al: Evaluating theimportance of mean stone density and skin-to-stone distance inpredicting successful shock wave lithotripsy of renal and uretericcalculi. Urol Res 2010; 38: 307.

30. Wiesenthal JD, Ghiculete D, Ray AA et al: A clinical nomogramto predict the successful shock wave lithotripsy of renal and ureteralcalculi. J Urol 2011; 186: 556.

31. Ng CF, Siu DY, Wong A et al: Development of a scoring systemfrom noncontrast computerized tomography measurements to im-prove the selection of upper ureteral stone for extracorporeal shockwave lithotripsy. J Urol 2009; 181: 1151.

32. Patel T, Kozakowski K, Hruby G et al: Skin to stone distance isan independent predictor of stone-free status following shockwavelithotripsy. J Endourol 2009; 23: 1383.

33. El-Nahas AR, El-Assmy AM, Mansour O et al: A prospectivemultivariate analysis of factors predicting stone disintegration byextracorporeal shock wave lithotripsy: the value of high-resolutionnoncontrast computed tomography. Eur Urol 2007; 51: 1688.

34. Jacobs BL, Smaldone MC, Smaldone AM et al: Effect of skin-to-stone distance on shockwave lithotripsy success. J Endourol 2008;22: 1623.

35. Ege G, Akman H, Kuzucu K et al: Acute ureterolithiasis: incidenceof secondary signs on unenhanced helical CT and influence onpatient management. Clin Radiol 2003; 58: 990.

36. Bird VG, Gomez-Marin O, Leveillee RJ et al: A comparison ofunenhanced helical computerized tomography findings and renalobstruction determined by furosemide 99m technetium mercap-toacetyltriglycine diuretic scintirenography for patients with acuterenal colic. J Urol 2002; 167: 1597.

37. Westphalen AC, Hsia RY, Maselli JH et al: Radiological imagingof patients with suspected urinary tract stones: national trends,diagnoses, and predictors. Acad Emerg Med 2011; 18: 699.

38. Smith RC, Rosenfield AT, Choe KA et al: Acute flank pain:comparison of non-contrast-enhanced CT and intravenous urogra-phy. Radiology 1995; 194: 789.

39. Hoppe H, Studer R, Kessler TM et al: Alternate or additionalfindings to stone disease on unenhanced computerized tomographyfor acute flank pain can impact management. J Urol 2006; 175:1725.

382

40. Viprakasit DP, Sawyer MD, Herrell SD et al: Limitations of ultraso-nography in the evaluation of urolithiasis: a correlation with com-puted tomography. J Endourol 2012; 26: 209.

41. Moesbergen TC, de Ryke RJ, Dunbar S et al: Distal ureteral calculi:US follow-up. Radiology 2011; 260: 575.

42. Mitterberger M, Aigner F, Pallwein L et al: Sonographic detectionof renal and ureteral stones. Value of the twinkling sign. Int BrazJ Urol 2009; 35: 532.

43. Shokeir AA and Abdulmaaboud M: Prospective comparison ofnonenhanced helical computerized tomography and Doppler ultra-sonography for the diagnosis of renal colic. J Urol 2001; 165: 1082.

44. Pepe P, Motta L, Pennisi M et al: Functional evaluation of theurinary tract by color-Doppler ultrasonography (CDU) in 100 pa-tients with renal colic. Eur J Radiol 2005; 53: 131.

45. Jandaghi AB, Falahatkar S, Alizadeh A et al: Assessment of ureter-ovesical jet dynamics in obstructed ureter by urinary stone withcolor Doppler and duplex Doppler examinations. Urolithiasis 2013;41: 159.

46. Sheafor DH, Hertzberg BS, Freed KS et al: Nonenhanced helicalCT and US in the emergency evaluation of patients with renalcolic: prospective comparison. Radiology 2000; 217: 792.

47. Ray AA, Ghiculete D, Pace KT et al: Limitations to ultrasound inthe detection and measurement of urinary tract calculi. Urology2010; 76: 295.

48. Goldwasser B, Cohan RH, Dunnick NR et al: Role of linear tomog-raphy in evaluation of patients with nephrolithiasis. Urology 1989;33: 253.

49. Johnston R, Lin A, Du J et al: Comparison of kidney-ureter-bladderabdominal radiography and computed tomography scout films foridentifying renal calculi. BJU Int 2009; 104: 670.

50. Jackman SV, Potter SR, Regan F et al: Plain abdominal x-rayversus computerized tomography screening: sensitivity for stonelocalization after nonenhanced spiral computerized tomography. JUrol 2000; 164: 308.

51. Ege G, Akman H, Kuzucu K et al: Can computed tomographyscout radiography replace plain film in the evaluation of patientswith acute urinary tract colic? Acta Radiol 2004; 45: 469.

52. Sudah M, Vanninen R, Partanen K et al: MR urography in evalua-tion of acute flank pain: T2-weighted sequences and gadolinium-enhanced three-dimensional FLASH compared with urography.Fast low-angle shot. AJR Am J Roentgenol 2001; 176: 105.

53. Semins MJ, Feng Z, Trock B et al: Evaluation of acute renal colic:a comparison of non-contrast CT versus 3-T non-contrast HASTEMR urography. Urolithiasis 2013; 41: 43.

54. Tiselius HG, Ackermann D, Alken P et al: Guidelines on urolithi-asis. Eur Urol 2001; 40: 362.

55. Preminger GM, Tiselius HG, Assimos DG et al: 2007 Guidelinefor the management of ureteral calculi. Eur Urol 2007; 52: 1610.

56. Niemann T, Kollmann T and Bongartz G: Diagnostic performanceof low-dose CT for the detection of urolithiasis: a meta-analysis.AJR Am J Roentgenol 2008; 191: 396.

57. Kim BS, Hwang IK, Choi YW et al: Low-dose and standard-doseunenhanced helical computed tomography for the assessment ofacute renal colic: prospective comparative study. Acta Radiol 2005;46: 756.

58. Liu W, Esler SJ, Kenny BJ et al: Low-dose nonenhanced helicalCT of renal colic: assessment of ureteric stone detection and mea-surement of effective dose equivalent. Radiology 2000; 215: 51.

59. Tack D, Sourtzis S, Delpierre I et al: Low-dose unenhanced multi-detector CT of patients with suspected renal colic. AJR Am JRoentgenol 2003; 180: 305.

60. Hamm M, Knopfle E, Wartenberg S et al: Low dose unenhancedhelical computerized tomography for the evaluation of acute flankpain. J Urol 2002; 167: 1687.

61. Weizer AZ, Auge BK, Silverstein AD et al: Routine postoperativeimaging is important after ureteroscopic stone manipulation. J Urol2002; 168: 46.

62. Manger JP, Mendoza PJ, Babayan RK et al: Use of renal ultrasoundto detect hydronephrosis after ureteroscopy. J Endourol 2009;23: 1399.

63. Karadag MA, Tefekli A, Altunrende F et al: Is routine radiologicalsurveillance mandatory after uncomplicated ureteroscopic stoneremoval? J Endourol 2008; 22: 261.

64. Bugg CE Jr, El-Galley R, Kenney PJ et al: Follow-up functionalradiographic studies are not mandatory for all patients after ure-teroscopy. Urology 2002; 59: 662.

383

65. Karod JW, Danella J and Mowad JJ: Routine radiologic surveil-lance for obstruction is not required in asymptomatic patients afterureteroscopy. J Endourol 1999; 13: 433.

66. Beiko DT, Beasley KA, Koka PK et al: Upper tract imaging afterureteroscopic holmium:YAG laser lithotripsy: when is it neces-sary? Can J Urol 2003; 10: 2062.

67. Adiyat KT, Meuleners R and Monga M: Selective postoperativeimaging after ureteroscopy. Urology 2009; 73: 490.

68. Sutherland JW, Parks JH and Coe FL: Recurrence after a singlerenal stone in a community practice. Miner Electrolyte Metab1985; 11: 267.

Study Questions Volume 32 Lesson 37

1. A 35-year-old man underwent uncomplicated ureteroscopyfor a 6 mm radiopaque right distal ureteral calculus and thestone was removed intact. The follow-up office visit shouldinclude

a. no imagingb. renal ultrasoundc. KUBd. KUB and renal ultrasound

2. A 42-year-old man with BMI <30 underwent SWL for a 9mm left proximal ureteral calculus. He passed a few frag-ments and is asymptomatic. At 3-week follow-up renal ultra-sound shows no hydronephrosis but KUB reveals a smallcolumn of fragments in the mid ureter. The next step is

a. medical expulsive therapy and re-image with KUB andultrasound in a few weeks

b. non-contrast low dose CT of the abdomen and pelvisc. ureteroscopy and laser lithotripsyd. placement of percutaneous nephrostomy tube

3. A patient with a BMI of 32 was treated with medical expul-sive therapy for an 8 mm left mid ureteral stone. Despitepassing the stone (which he has in hand) last week, he contin-ues to complain of left flank pain. Renal ultrasound revealsmild left hydronephrosis. The next step is

a. no further imaging is needed if the passed stone is 8 mmb. low dose protocol non-contrast CT of the abdomen

and pelvisc. KUBd. standard protocol non-contrast CT of the abdomen

and pelvis

Take this test online at http://www.auanet.org/eforms/cme/

4. A 42-year-old man is seen in the emergency department withright flank pain. Non-contrast CT of the abdomen and pelvisreveals a 6 mm right distal ureteral stone not visible on CTscout. He is afebrile and the pain is well controlled. The nextstep is

a. obtain KUBb. proceed with SWLc. discharge on medical expulsive therapy and pain medi-

cationd. discharge on potassium citrate and pain medication

5. A 25-year-old healthy woman at 26 weeks of gestation isseen in the emergency department with fever and right flankpain. Renal and abdominal ultrasound shows bilateral hydro-nephrosis (right>left) with no obvious stone seen in the kid-neys or visualized ureters. Ureteral jets were not appreciatedon either side after 10 minutes of observation. What is thenext step?

a. Limited IVPb. KUB with tomogramsc. Low dose CT of the abdomen and pelvisd. Placement of right percutaneous nephrostomy tube