In most cases, acute and short-term obstruction of urinary outflow tract is caused by stones. Dilatation of the renal pelvis and of the ureters is only mild in these cases. The complete picture of a hydronephrosis develops only with chronic renal outflow obstruction (tumor, apparent crossing vessel at the renal pelvis, ureteropelvic junction stenosis).
In chronic hydronephrosis, T1-weighted images demonstrate the low SI dilated renal calyceal system and a loss of the corticomedullary differentiation. In addition, the renal parenchyma may be thinned out as a sign of chronic parenchymal damage. On T2-weighted images, the renal calyceal system is hyperintense.
Coronal images demonstrate the dilated ureter. At this, particularly heavily T2-weighted images such as RARE (rapid acquisition with relaxation enhancement) and HASTE sequences are suitable to acquire fast images with high T2-weighted SI. A different approach for MR-imaging of hydronephrosis or suspected urinary outflow obstruction is to use Gadolinium chelates and postcontrast T1-weighted images. Typically, the excretion of the contrast agents starts immediately after the contrast injection.
After as little as 1–2 min, initial enhancement can be seen in the renal pelvis and ureters. Heavily T1- weighted sequences, such as 3D-GRE sequences that are mainly used for MRA, can also be applied to demonstrate the ureters. Due to the high concentration of gadolinium contrast agent in the bladder T2* effects may arise, frequently rendering some parts of the bladder non-diagnostic. Sequential acquisition of heavily T1-weighted post-contrast images allows the generation of a dynamic urography series. For T1-weighted techniques, the administration of furosemide is also advantageous.
MRI also allows identification of the cause of hydronephrosis, such as retroperitoneal tumors compressing or invading the ureter. Carcinoma of the ureter cannot regularly be depicted. Stones of the renal pelvis and the ureter generally have a low SI on T1-weighted and T2- weighted images. Therefore, only larger stones may be well marked off on T2-weighted images where they appear as black holes in the hyperintense urine.
Ureteral obstruction can also be caused by ectopic endometrial tissue in patients with endometriosis. In these typically younger patients, MR demonstrates hyperintense lesions on fat-saturated T1-weighted sequences pre-contrast caused by hemorrhage into the endometrial lesions.
Retroperitoneal fibrosis is another rare cause of hydronephrosis and is due to encasing and compressing the ureters. This disease entity is discussed in detail in the retroperitoneum section.