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Departments of 1 Imaging and 2 Urology, Johns Hopkins Bayview Medical Center, 4940 Eastern Avenue, Baltimore, Maryland 21224, USA
Correspondence: Dr F Regan, Department of Radiology, Adelaide & Meath Hospital, Dublin 24, Ireland
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Abstract |
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ge;0.75) in the finding of perirenal fluid on MRU, there was only fair interobserver agreement (Kappa<0.4) regarding perirenal stranding on CT. MRU/KUB showed ureteric calculi in 21/29 (72%) of patients with calculi seen by CT. Overall, MRU/KUB revealed 2.4 abnormalities per acutely obstructed ureter compared with 1.8 abnormalities detected by CT. MRU/KUB using HASTE sequences can diagnose the presence of acute calculus ureteric obstruction with similar accuracy to spiral CT. The technique has less observer variability and is more accurate than CT in detecting evidence of obstruction such as perirenal fluid. |
Introduction |
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TopAbstractIntroductionPatients and methodsResultsDiscussionReferences |
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Intravenous urography (IVU) has traditionally been the imaging modality of choice in evaluating patients with acute ureteric calculus disease. Unenhanced spiral CT (CT) is accurate in demonstrating kidney and ureteric calculi and signs of acute obstruction such as perirenal stranding, and hydronephrosis [2–4].
Alternatively, MR urography (MRU) has been slow to gain wide acceptance in evaluating patients with ureteric obstruction. This may be explained by the limited access to fast MRI techniques, and poor detection of kidney and ureteric calculi when compared with CT. MRU using HASTE (Half-Fourier single shot turbo spin-echo) sequences can however, demonstrate both the presence and level of ureteric obstruction [5, 6], and the technique provides good urographic type images without risk of radiation or contrast media.
The goal of this study is to compare spiral CT and MRU and to determine if HASTE MRU when combined with plain radiograph of the abdomen (KUB) can be diagnostic in patients with acute ureteric colic. A secondary objective of the study was to compare interobserver variability and ease of interpretation of CT compared with MRU.
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Patients and methods |
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TopAbstractIntroductionPatients and methodsResultsDiscussionReferences |
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This was a non-consecutive group of patients as MRI was performed only during weekday hours. Additionally only those patients imaged with both CT and MRU within 6 h of each other were included. Six claustrophobic patients refused imaging with MR and MRI was not available in a further six patients because of routine scanner maintenance. Five patients left the Emergency department before MRI could be performed and a further three patients who had received intravenous contrast agents were excluded from the study.
64 patients were included in the final study group and all these patients were imaged with both CT and MRU. Depending on availability at the time of referral, 42 (66%) patients underwent CT first and the remaining 22 patients underwent MRI prior to CT. KUBs were available in 45/64 (70%) patients. Informed consent was obtained in all patients.
Imaging technique
Unenhanced spiral CT (Siemens, Iselin, NJ) was carried out through both kidneys to the bladder base in three breath-hold clusters with the following imaging parameters: 130 kV, 83 mA, gantry rotation time 1.9 s. A slice thickness of 3 mm with 3 mm reconstruction was used. Depending on the size of the patient up to 120 images were generated per study.
Patients underwent HASTE MRI using a 1.5 Tesla Magnetom Vision scanner (Siemens, Iselin, NJ) in the axial, sagittal and coronal imaging planes to cover both kidneys to the bladder base. The following parameters were used: repetition time (TR) 11.9, echo time (TE) 95.0, acquisition time of 23 s for 19 slices. A slice thickness of either 4 mm (coronal and sagittal imaging) or 7 mm (axial imaging to cover the full abdomen) was applied with a variable field of view. 57 images (19 x 3 planes) were generated in each patient. MRU was performed using a single shot technique (TR of 2000, TE 80, single excitation and a slice thickness of 10 mm). This yielded a further 11 images per patient with a total of 68 images. A frequency selective fat suppression algorithm was used to reduce intra-abdominal fat high signal and no contrast medium was administered.
Image analysis
Images from both CT and MRU were read independently by three radiologists – a senior radiology resident (observer 1) and two attending radiologists experienced in abdominal imaging (observers 2 and 3). The readers were unaware of the final clinical outcome, results of other imaging modalities and of each other's findings. CTs and MRUs were presented in a random order to the radiologists for interpretation and read in three sessions. The KUB was read in conjunction with the patient's MRU.
The following findings were assessed on both imaging techniques: presence or absence of perirenal and periureteric stranding (CT) or fluid (MR), ureteric dilatation, and level of obstruction. A level of obstruction was determined to be either proximal, middle or distal ureter. A ureteric calculus was diagnosed as a high attenuation focus on CT or as an intraluminal signal void on MR. Calculi were documented as either kidney, proximal, middle or distal ureter in location, and their maximum diameter measured in millimetres.
The ease of interpretation was graded subjectively by each observer between 1 and 5, the most difficult interpretation being assigned a grade 1 and the easiest, grade 5. A consensus opinion was reached where there was disagreement between observers in evaluating stranding/fluid, ureteric dilation and level of obstruction.
Statistical analysis
Data from the consensus opinion of the observers was used to calculate sensitivity, specificity, positive and negative predictive values and accuracy of both techniques for detection of acute calculus obstruction. Exact confidence intervals were calculated for these results. The number of findings per acutely obstructed kidney noted by each observer and by consensus was calculated for CT and MRU/KUB techniques, and a p-value of <0.001 was considered to be significant.
Agreement between CT and MRU was measured by Kappa statistics for all three observers and for the consensus opinion. A Kappa value of ge;0.75 denotes excellent agreement, Kappa of between 0.4 and 0.75 good agreement, and less than 0.4 marginal agreement [7]. Interobserver variability regarding findings of fluid, ureteric dilation and level of obstruction and whether the right or left kidney was affected were assessed with Kappa statistics for all possible observer combinations (observers 1 and 2, 2 and 3, and 1 and 3). The size of calculi seen on CT and KUB was compared using a two sample t-test.
A McNemar's exact test compared each finding by each observer with the presence of acute ureteric obstruction. The ability of CT and MRU/KUB in predicting disease using the combined findings of fluid, ureteric dilation and calculi was compared using the McNemar's chi-squared test. Finally, the Wilcoxon Signed rank test was applied to measure the differences in ease of interpretation of CT and MRU/KUB for all three observers.
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Results |
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TopAbstractIntroductionPatients and methodsResultsDiscussionReferences |
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All CT and MR studies were considered technically adequate. No significant breathing or misregistration artefacts were noted. The sensitivity, specificity, and overall accuracy of fluid and ureteric dilatation to predict disease were 80%, 85%, and 81%, respectively, for CT (Table 1
and Figure 1), compared with 93%, 95%, and 94% for MRU. There were 61 findings of either fluid or ureteric dilatation on MRU in 44 acutely obstructed kidneys. This compared with 37 abnormalities per obstructed kidney on CT if the same parameters of fluid and dilatation were used (p<0.005). If fluid, dilatation and stone detection were assessed, the MRU/KUB combination showed 66 abnormalities in 27 acutely obstructed kidneys where all imaging modalities were available for evaluation, compared with 76 in 43 abnormalities per obstructed kidney on CT (p<0.005).
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). There was only one patient where CT demonstrated stranding not shown by MRU. The observers missed perirenal stranding on CT in four patients who had minimal intra-abdominal fat including two patients with HIV. In five further patients with acute ureteric obstruction, proximal ureteric stranding was missed by observers 2 and 3.
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).
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). There was strong agreement between all observers as to which kidney was acutely obstructed although interobserver agreement was generally greater for MRU than for CT. There was also good agreement between all observers in assessing the presence or absence of perirenal fluid on MRU but significant deviations (p<0.001) occurred between all observers in assessing perirenal stranding on CT (Table 4). Similarly there was stronger, but not statistically significant, agreement between all three interobserver combinations in the depiction of the level of obstruction on MRU.
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The location and size of ureteric calculi detected by CT and KUB/MRU are shown in Table 4
. CT detected 29 ureteric calculi ranging in size from 1 mm to 12 mm in length (mean 3.7 mm). Using CT as the gold standard KUB/MRU detected 21/29 (72%) ureteric calculi (size 2–14 mm) in patients with evidence of obstruction. MRU alone demonstrated calculi in only three patients, all of whom had calculi greater than 10 mm diameter in dilated systems and there was a significant difference (p=0.0001) in the mean size of calculi detected by CT (3.7 mm) and MRU/ KUB (5.1 mm).
There was no significant difference between CT and MRU in the probability of predicting acute calculus ureteric obstruction (McNemar's chi-squared=0, degrees of freedom (df)=1, p-value=1) when the combination of all three parameters (fluid, dilatation and calculi) was used and the combination of fluid and ureteric dilatation on both techniques was significantly more predictive of disease (p<0.001) than the individual finding of either dilatation or fluid.
Finally, the difficulty in interpreting CTs was assigned by the three radiologists an average grade of 4, 3.5, and 3.3, respectively, and 4.4, 4.4 and 4.2, respectively, for MRU. All three interpreters found the median score significantly larger (i.e. easier to interpret), for MRU than CT (p=0.001, p=0.000, p=0.000, respectively), although the least experienced radiologist (observer 1) showed less difference in difficulty between the two imaging modalities.
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Discussion |
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TopAbstractIntroductionPatients and methodsResultsDiscussionReferences |
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There have been many recent papers describing the utility of unenhanced CT in acute calculus obstruction [2–4]. Small calculi can be detected and secondary signs of obstruction such as perirenal stranding are seen without the need for administered contrast media [7, 8]. At this institution however, referring urologists were initially more comfortable at interpreting and making management decisions based on intravenous urography than CT, and interpretation difficulties may limit the value of CT in acute ureteral obstruction. Distal ureteric calculi may be difficult to distinguish from pelvic phleboliths or arterial calcifications. Although calculi have higher attenuation values than phleboliths, these values may be inaccurate if targeted views of the distal ureter are not obtained. Although analysis of calculi and reformatted images of obstructed ureters from data on the console is more accurate than hard copy images [8, 9], this is time consuming and probably inappropriate in a busy department. Signs of obstruction such as perirenal and periureteric stranding can be difficult to see on CT and this is especially true in patients with a paucity of intra-abdominal fat. Perirenal stranding is not specific to acute ureteric obstruction and occurs in diseases such as renal infection and tumour and periureteral vessels and lymphatics can be confused with stranding on CT. Numerous CT images are generated per study (up to 120 axial images compared with an average of 54 images for MRU), and these require time consuming interpretation.
Alternatively, MRU shows the effects of acute obstruction, i.e. perirenal fluid with greater sensitivity and less interobserver variability than CT shows stranding. The clarity with which this fluid is seen may be partly explained by the use of fat suppression and the fluid sensitive T2 weighting of the HASTE sequences. For the same reasons, MRU clearly shows the ureter and level of obstruction, increasing the confidence whereby ureteric calculi can be diagnosed on KUB.
Although we used a simple and subjective grading system, all three radiologists found MRU easier to interpret than CT, with more robust interobserver agreement. Although MRU fails to demonstrate most kidney and ureteric calculi, if combined with a KUB, a more confident diagnosis of ureteric calculus can be made. While MRU/KUB misses small calculi, (7 mm or less), these usually pass spontaneously [1], and do not require intervention.
There are potential design flaws in this study. Patients were imaged in a non randomized fashion, and most underwent CT first. It could therefore be argued that the delay in scanning with MR allowed further accumulation of perirenal fluid. It could also be argued however, that in this time interval, a ureteric calculus may have passed and the amount of perirenal fluid would have diminished between the two studies. Second, our CT technique used multiple acquisitions rather than a single long spiral and a stone may be missed due to misregistration, and the tube current we employed could further reduce the stone detection rate due to beam hardening artefact.
In summary, MRU using HASTE sequences is a more accurate alternative to unenhanced spiral CT in detecting signs of obstruction (perirenal fluid and ureteric dilatation) without the risk of radiation. The technique shows no statistically significant difference in predicting the presence of acute calculus obstruction, demonstrates less interobserver variability, and is easier to interpret than CT.
Received for publication February 9, 2004. Revision received November 19, 2004. Accepted for publication January 17, 2005.
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References |
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TopAbstractIntroductionPatients and methodsResultsDiscussionReferences |
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