Editing Pathophysiology of Urinary Tract Obstruction (section)

==== MRI ====

* '''Poor detection of renal and ureteral stones in comparison to CT because stones appear as signal voids on T1- and T2-weighted images'''
* '''The MRU measurement of contrast excretion is the renal transit time''', which is defined as the time it takes for contrast to pass from the renal cortex to the proximal ureters
** Interpretation of contrast excretion time
*** '''≤4 minutes: normal'''
*** > 4 and < 8 minutes: equivocal
*** '''≥ 8 minutes: obstructed'''
* MRU has been demonstrated to have an excellent correlation with the renal isotope GFR in the adult and pediatric patients with obstructed kidneys.
* '''IV gadopentetate-DTPA allows a dynamic, functional assessment of the collecting system that correlates well with diuretic renal scintigraphy, yet provides far greater anatomic detail than nuclear studies.'''
** Differential GFR can be assessed with post-imaging processing, and contrast washout can be measured to calculate renal clearance, differentiating dilated systems from obstructed systems

'''Excretory urography depends on glomerular filtration and renal excretion of iodinated contrast medium; therefore, the utility of excretory urography is limited in patients with renal insufficiency.'''

* '''The risk for contrast nephropathy increases with worsening renal function.'''
* '''Excretory urography should not be performed in patients with a history of contrast allergy or those in whom radiation exposure is a concern (i.e., pregnancy).'''