Distinguishing stent from stone: use of bone windows

Computed tomography window affects kidney stones measurements

OBJECTIVES

Measurements of stone features may vary according to the non-contrast computed tomography (NCCT) technique. Using magnified bone window is the most accurate method to measure urinary stones. Possible differences between stone measurements in different NCCT windows have not been evaluated in stones located in the kidney. The aim of this study is to compare measurements of kidney stone features between NCCT bone and soft tissue windows in patients submitted to retrograde intrarenal surgery (RIRS).

MATERIALS AND METHODS

Preoperative and 90th postoperative day NCCT were performed in 92 consecutive symptomatic adult patients (115 renal units) with kidney stones between 5 mm to 20 mm (< 15 mm in the lower calyx) treated by RIRS. NCCT were evaluated in the magnified bone window and soft tissue window in three axes in a different time by a single radiologist blinded for the measurements of the NCCT other method.

RESULTS

Stone largest size (7.92±3.81 vs. 9.13±4.08; mm), volume (435.5±472.7 vs. 683.1±665.0; mm3) and density (989.4±330.2 vs. 893.0±324.6; HU) differed between bone and soft-tissue windows, respectively (p<0.0001) 5.2% of the renal units (6/115) were reclassified from residual fragments > 2 mm on soft tissue window to 0-2 mm on bone window.

CONCLUSION

Kidney stone measurements vary according to NCCT window. Measurements in soft tissue window NCCT of stone diameter and volume are larger and stone density is lesser than in bone window. These differences may have impact on clinical decisions.

Comparison of ureteric stone size, on bone window versus standard soft-tissue window settings, on multi-detector non-contrast computed tomography

Objective

To compare the difference in mean stone size, as measured on bone window vs standard soft-tissue window setting using multi-detector computed tomography (MDCT) in patients with a solitary ureteric stone.

Patients and methods

In all, 60 patients presenting to the emergency and outpatient departments of a University Hospital from May 2015 to October 2015 and fulfilling the inclusion criteria were included in the study. A 64-slice MDCT was used to assess the locations and size of the ureteric stones. A consultant radiologist independently analysed the MDCT scans of all the patients. The mean difference in stone size was calculated between both window settings in axial and coronal planes.

Results

The mean (SD) age of the patients was 37.13 (11.9) years. Males constituted ∼68% of the cohort and 32% were female. In all, 85% of the patients had left ureteric stones and 15% had right ureteric stones. The mean (SD) stone size, as measured on the soft-tissue window setting was 6.68 (2.01) mm, and on the bone window setting was 4.8 (1.9) mm. The mean (SD) difference in stone size between the two window settings was +1.85 (0.55) mm. The two means were compared using Student’s t-test, and the difference was found to be statistically significant (P < 0.05).

Conclusion

The stone size measured using the soft-tissue window setting on a MDCT is significantly different from the measurement on the bone window setting.

Is computed tomography mandatory for the detection of residual stone fragments after percutaneous nephrolithotomy?

The introduction of minimally invasive endourologic procedures for upper urinary stone disintegration has closed the curtain on the era of open surgery for upper urinary tract stones in which complete stone eradication was the rule. This shift to minimally invasive procedures has led to the introduction of new terminology, such as stone-free rates and residual stone fragments, the presence of which after treatment was considered an acceptable therapeutic end point. Percutaneous nephrolithotomy (PCNL) is currently considered the procedure of choice for large renal stones. Its use has been greatly facilitated by the favorable profile of multidetector CT with regard to its sensitivity in detecting small stones. Despite the fact that CT is considered essential for the diagnosis and exact localization of stones and has been used for the creation of percutaneous tracts in PCNL, however, its routine use for the post-PCNL detection of residual stones has not been established. There is evidence that routine application of post-PCNL CT provides additional advantages compared with other imaging modalities-namely, the identification of the presence and location of even small residual fragments, which has been shown to cause significant trouble and necessitate secondary procedures in a significant cohort of patients after PCNL. On the other hand, the issues of cost, availability of CT scanners, and radiation exposure along with the acceptable sensitivity, cost, and availability of other imaging studies has raised doubts as to whether CT should be the routine imaging study after PCNL. The present review will discuss the concept of clinically significant residual fragments and comment on the advantages and drawbacks of different imaging studies used for the detection of residual stones after PCNL. This review also aims to clarify the indications in which CT should routinely be performed or could be omitted in the follow-up after PCNL.

Computed tomography versus ureteroscopy in identification of renal tract stone with ureteral stent in situ

INTRODUCTION

The aim of this study was to validate the use of non-contrast computed tomography (CT) with a ureteral stent in situ instead of ureteroscopy for identification of renal tract stones.

METHODS

All patients who had stents inserted for renal tract stones and underwent non-contrast CT with the stent in situ followed by ureteroscopy between May 2008 and October 2009 at The Canberra hospital, Australia, were analysed retrospectively. Statistical analysis was performed to compare any differences between CT and ureteroscopy in the identification of stones.

RESULTS

Overall, 57 patients were included in the study. The difference between CT and ureteroscopy findings was statistically significant. CT identification of stones with a stent in situ had a sensitivity of 86%, a specificity of 46%, a positive predictive value of 63%, a negative predictive value of 76% and an accuracy of 67%.

CONCLUSIONS

Our study suggests that non-contrast CT is inferior to the 'gold standard' of ureteroscopy. It lacks sensitivity, specificity, positive predictive value, negative predictive value and accuracy. Therefore, we cannot recommend using non-contrast CT to replace ureteroscopy.

Diagnosis and management of postpercutaneous nephrolithotomy residual stone fragments

Residual stone fragments can occur in up to 8% of patients who are treated with percutaneous nephrolithotomy (PCNL). When left untreated, approximately half of these patients will experience a stone-related event, for which more than half will need a secondary surgical intervention. Predictors of adverse events are a residual fragment larger than 2 mm that is located in the pelvis or ureter. Preventive measures for the creation of residual fragments include a carefully selected access giving exposure to the bulk of the stone, the creation of multiple tracts, the use of single pulse pneumatic lithotripsy, the prevention of stone migration with ureteral balloons or stone cones, and careful flushing of the stones from the collecting system. Plain radiography of the kidneys, ureters, and bladder, linear tomography, and ultrasonography have all been used to judge the result of PCNL and to detect the presence of residual fragments. Thin-slice, unenhanced helical CT, however, is more sensitive and should be performed at 1 month after surgery. While medical therapy and shockwave lithotripsy possess a minor role, second-look flexible nephroscopy and/or flexible ureterorenoscopy seem to be the treatments of choice for residual stone fragments after PCNL.

Imaging modalities for urolithiasis: impact on management

PURPOSE OF REVIEW

Urolithiasis is a common urological problem, often requiring efficient workup, accurate diagnosis, and treatment. The purpose of this review is to summarize the imaging modalities employed for the diagnosis of calculi and the caveats of different clinical situations.

RECENT FINDINGS

Noncontrast computed tomography has become the most universally used imaging tool for diagnosing urolithiasis, although ultrasound and magnetic resonance imaging maintain specific roles. Noncontrast computed tomography may provide prognostic information regarding the success of specific management strategies for urolithiasis. Additionally, noncontrast computed tomography is being tested in lower-radiation dose protocols with promising results.

SUMMARY

Considering the well supported accuracy and relative ease of use of noncontrast computed tomography, it has become a logical choice for the urologist to use the technique as a diagnostic tool for stone disease. The future of imaging for intervention and surveillance of stone disease lies in the continued progress of noncontrast computed tomography in terms of patient safety. This will need to be done by developing low-dose radiation computed tomography that can replicate the efficacy of current noncontrast computed tomography.

Confident intraoperative decision making during percutaneous nephrolithotomy: does this patient need a second look?

OBJECTIVES

To evaluate intraoperative decision making regarding the necessity of second-look nephroscopy after percutaneous nephrolithotomy.

METHODS

Percutaneous nephrolithotomy was performed cooperatively by a urologist and radiologist equipped with a high-resolution C-arm. Ultrasonic lithotripsy was performed followed by thorough flexible nephroscopy and fluoroscopy. At conclusion patients were prospectively classified as radiologically and/or endoscopically stone free. If confident that complete stone clearance had been atraumatically achieved, a ureteral stent was placed. If there was suspicion of trauma or residual calculi, a nephrostomy tube was inserted. Postoperative CT was performed and allowed fragment classification as stone free, less than 2 mm, 2 to 4 mm, or 4 mm or greater.

RESULTS

Average stone dimensions were 640.2 +/- 412.5 mm2 in 39 consecutive renal units. Computed tomographic imaging demonstrated that 26 renal units (66.7%) were stone free after primary procedure, with 5 (12.8%), 5 (12.8%), and 3 (7.7%) having fragments less than 2 mm, 2 to 4 mm, and greater than 4 mm, respectively. Of the 34 renal units considered endoscopically and fluoroscopically stone free, postoperative CT demonstrated 9 with residual fragments, all less than 4 mm. Of the 5 renal units not considered radiologically and endoscopically stone free, 4 had fragments on CT. Nephrostomy tubes were avoided in 33 patients. This intraoperative decision was supported by negative predictive values of 100%, 88%, and 73% at postoperative CT fragment detection thresholds of 4 mm, 2 mm, and 0 mm, respectively.

CONCLUSIONS

Rigorous fragment detection allows confident intraoperative decision making regarding the necessity of second-look nephroscopy.

Discrimination between ureteral stent and stone by non-contrast helical computed tomography in the clinical setting

OBJECTIVE

In urolithiasis with indwelling ureteral stent, an adequate visualization of retained stone is often obscured by the stent. In functional solitary kidney, it is essential to detect retained fragments before stent removal. We examined the capability of non-contrast helical computed tomography (NCHCT) in discriminating stone from stent.

PATIENTS AND METHODS

We assessed the CT of 21 radiopaque stones of 3-5 mm and four stents, and compared their attenuation values. In addition, two other cases with stent, in which only NCHCT by manipulating the setting could detect retained fragments.

RESULTS

No overlaps of attenuation value were found between the stones and stents. Although they had the same appearance in NCHCT with abdominal-window setting, in bone-window setting the contrasts of stones were reduced, while stents were still bright white.

CONCLUSIONS

We would discriminate the residual stone from the stent by manipulating window and level setting, and bone-window setting is easy to use.