False positives ² patients who are pregnant or have

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False positives ² Patients who are pregnant or have benign prostatic hyperplasia can have mild to moderate dilatation of their collecting system because of external compression of the ureters or overdistended bladders, respect- ively. The hydronephrosis in these cases should resolve after bladder emptying. 138 Diagnostic ultrasound Renal and bladder ultrasound
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False negatives ² Patients who are severely dehydrated can have falsely negative renal scans for hydronephrosis. If there is clinical concern, a repeat renal scan should be done after some intravenous hydration. (A) (B) Figure 6.5 Bladder volume estimation: ( A ) measured width and height on transverse view; ( B ) measured depth on longitudinal view. Diagnostic ultrasound 139 Renal and bladder ultrasound
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Normal images Figures 6.6 –6.8 show examples of normal renal ultrasound scans. (A) Figure 6.6 (A) Normal longitudinal view of the right kidney. Morison’s pouch is well visualized here, too. (B) Transverse view of the kidney. 140 Diagnostic ultrasound Renal and bladder ultrasound
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Abnormal images Hydronephrosis According to Grainger and Allison’s Diagnostic Radiology [ 13 ], the following grading system is used by radiologists/sonographers: ² Grade I – slight blunting of calyceal fornices ² Grade II – blunting and enlargement of calyceal fornices but easily seen shadows of papillae ² Grade III – rounding of calices with obliteration of papillae ² Grade IV – extreme calyceal ballooning Figure 6.7 Transverse view of the bladder with no fluid seen outside the bladder wall. Figure 6.8 The bulb of the Foley catheter is visualized on ultrasound with a partially decompressed bladder. Diagnostic ultrasound 141 Renal and bladder ultrasound
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Figures 6.9 –6.11 show images of different grades of hydronephrosis. Chronic hydronephrosis can cause thinning of the renal medulla. Such distortion of the renal architecture is only seen in long-standing obstruction. MILD MODERATE SEVERE Figure 6.9 Grades of hydronephrosis ( top ). Longitudinal and transverse views of the kidney with moderate hydronephrosis ( bottom ). Figure 6.10 Dilated right renal pelvis with splaying of the renal calyces indicative of severe hydronephrosis. 142 Diagnostic ultrasound Renal and bladder ultrasound
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Bilateral evidence of hydronephrosis is less likely to be caused by two discrete ureteral events than by bladder outlet obstruction. It is expected that pregnancy and an overdistended bladder cause hydronephrosis – sometimes to a large degree. Another common finding that can be confused with acute obstruction is that of an extrarenal pelvis. This is a developmental variant in which the collecting system lies predominantly outside the kidney. Generally, in the normally hydrated patient, absence of any evidence of dilatation virtually rules out obstructive nephrolithiasis as the cause of severe pain ( Figure 6.12 ).
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