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One of the primary indications for referral to US evaluation of the kidneys is evaluation of the urinary collecting system. Enlargement of the urinary collecting system is usually related to urinary obstruction and can include the pelvis, the calyces and the ureter. [[Hydronephrosis]] is seen as an anechoic fluid-filled interconnected space with enhancement within the renal sinus, and normally, the dilated pelvis can be differentiated from the dilated calyces.<ref name="Hansen2015" /gallery> [[File:Ultrasonography of hydronephrosis due to ureteropelvic junction obstruction.jpg|thumb|Figure 13. Hydronephrosis due to ureteropelvic junction obstruction in a pediatric patient.<ref namelink="Hansen2015" />]]Several conditions can result in urinary obstruction. In both adults and children, masses, such as abscesses and tumors, can compress the ureter. In children, hydronephrosis can be caused by ureteropelvic junction obstruction, ectopic inserted ureter, primary megaureter and posterior urethral valve (Figure 13). In the latter, both kidneys will be affected. In adults, hydronephrosis can be caused by urolithiasis, obstructing the outlet of the renal pelvis or the ureter, and compression of the ureter from, e.g., pregnancy and retroperitoneal fibrosis. Urolithiasis is the most common cause Ultrasonography of hydronephrosis in the adult patient and has a prevalence of 10%–15%.<ref name="Hansen2015" /> |[[File:Ultrasonography of dilatation of the ureter due to vesicoureteric reflux.jpg|thumb|Figure 14. Bilateral dilatation of the ureters due to vesicoureteric reflux in a pediatric patient.<ref name="Hansen2015" />]]Under normal conditions, the ureter is not seen with US. However, in, e.g., urinary obstruction and vesicoureteric reflux with dilation of the ureter, the proximal part in continuation with the renal pelvis, as well as the distal part near the ostium can be evaluated (Figure 14).<ref name="Hansen2015" /> The hydronephrosis is typically graded visually and can be divided into five categories going from a slight expansion of the renal pelvis to end-stage hydronephrosis with cortical thinning (Figure 15). The evaluation of hydronephrosis can also include measures of calyces at the level of the neck in the longitudinal scan plane, of the dilated renal pelvis in the transverse scan plane and the cortical thickness, as explained previously (Figure 16 and Figure 17).<ref name="Hansen2015" /> If the fluid in the dilated collecting system has echoes, pyonephrosis should be excluded by clinical exam, blood analysis and, in special cases, puncture or drainage. Hydronephrosis can also be caused by non-obstructive conditions, such as brisk diuresis in patients treated with diuretics, in pregnant women and in children with vesicoureteral reflux.<ref name="Hansen2015" /> <gallery widths="200" heights="200">File:Ultrasonography of end-stage hydronephrosis with cortical thinning.jpg|Figure 15. End-stage hydronephrosis with cortical thinning. Measurement of pelvic dilatation on the US image is illustrated by ‘+’ and a dashed line.<ref name=Hansen2015/>File:Ultrasonography of '''hydronephrosis with dilated anechoic pelvis and calyces, along with cortical atrophy.jpg|Figure 16. Hydronephrosis with dilated anechoic pelvis and calyces, along with cortical atrophy. The width of a calyx is measured on the US image in the longitudinal scan plane, and illustrated by ‘+’ and a dashed line.<ref name=Hansen2015/>File:Ultrasonography of hydronephrosis with dilated anechoic pelvis and calyces, along with cortical atrophy - measurement.jpg|Figure 17. Same patient as in Figure 16 with measurement of the pelvis dilation in the transverse scan plane illustrated on the US image with ‘+’ and a dashed line.<ref name=Hansen2015/>''']]