MRI is based on the spontaneous alignment of magnetic moments of the nuclei contained in tissues when an external electromagnetic field is applied. Because H+ is abundant in the human body and has a high gyromagnetic ratio, MRI is almost exclusively carried out with protons. Images are produced by applying multiple radiofrequency signals (256 -512 pulses) over several minutes. Fat satura-
tion spin echo techniques improve image resolution by reducing the respiratory artifact caused by moving subcutaneous fat.
Although CT is superior for defining calcified and fatty structures, MRI provides better tissue contrast resolution, does not require dye, directly measures multiplanar images, and has no biological side effects. Major drawbacks include availability, cost, and the relatively long time needed for imaging. MRI is most commonly used as a problem-solving technique when ultrasonography and CT provide equivocal results. The renal venous circulation is particularly well defined, and MRI is the technique of choice for detection of renal vein thrombus or tumor invasion (Table 7). Gadolinium is approved for use as a paramagnetic contrast agent. Minor side effects occur in 2.4% of patients (nausea, vomiting, urticaria). Only 5% of the usual iodi-nated dosage is delivered. Estimated incidence for anaphylaxis is 0.001%. No renal, hepatic, or cardiovascular toxicity has been reported.
Magnetic resonance angiography (MRA) is based on the phase differences between moving and stationary objects and on the bulk spin flow in the area of interest. Renal arteries are best studied with phase contrast and the renal veins and inferior vena cava with time of flight techniques (TOF). Proximal lesions are better seen than distal or intrarenal ones, particularly limiting the study of fibromuscular dysplasia.
Magnetic resonance spectroscopy (MRS) requires use of a magnet capable of delivering at least 1.5T with a field homogeneity 100 times greater than for MRI. Nuclei used for spectroscopy include 31P, 13C, 1H and 19F. The most common element used is 31P (energy transfer, ATP hydrolysis, and intracellular pH). An increase in inorganic phosphate (Pi) from ATP hydrolysis during hypoxia leads to a decreasing ATP/Pi ratio. The phospho-monoester (MP)/Pi ratio is a good predictor of renal parenchymal viability. Acute renal failure is characterized by rapid loss of ATP, increased Pi, and decreased pH. In renal allo-grafts, a decrease in MP/Pi is suggestive of rejection.
Obstructive uropathy: Anatomy is well preserved in acute and subacute cases, while in chronic obstruction the corticomedullary interface disappears. A diuretic MRI may be useful in differentiating anatomical obstruction from its functional counterpart.
Congenital anomalies: MRI provides excellent anatomical definition of congenital anomalies including agenesis, duplication, congenital obstruction, and horseshoe or pancake kidney. Ultrasonography, more readily available and less expensive, is the procedure of choice. However, MRI is warranted with inconclusive ultrasonographic studies.
Renal masses: Ultrasonography and CT are the methods of choice. However, MRI is specifically indicated for evaluation of the renal vein and inferior vena cava. With fat saturation techniques, cysts< 1 cm can be identified. This resolution is similar to that of CT. Cysts have sharp borders and thin walls and lack contrast enhancement. A complicated cyst (bleeding or infection) will have similar changes to those described with CT. MRI
offers no advantage in the study of polycystic kidney disease. Calcifications are not detected by MRI. Angiomyolipomas are best studied byCT.
Renal cell carcinoma presents with a spectrum of findings. MRI is very helpful in identifying lesions < 2 cm and is preferred over CT. Additional advantages for staging include multiplanar imaging and better vessel visualization. MRI is the method of choice for detecting thrombus and tumor extension into the renal vessels. Gadolimium enhances tumor signals but does not affect blood signals.
Renal functional assessment: Glomerular filtration can be evaluated with injection of contrast and fast breath-hold gradient echo techniques. MRI provides better spatial and multiplanar resolution than isotopic renogra-phy and allows visualization of the kidney even in the absence of function.
Pyelonephritis: The collecting system is seen well with T1 imaging because urine has a low signal. Infected urine has a higher T1 signal than normal and can thus be identified. The kidney may appear swollen and lobar areas of high density may be noted, with abscesses identified as low-density images.
Medical renal disease: Loss of the corti-comedullary junction is a very sensitive finding indicating significant renal disease; however, it is quite nonspecific. Exceptions to this rule are sickle cell disease and paroxysmal nocturnal hemoglobinuria. In both, the cortex has a very low signal intensity when compared to the medulla, because cortical iron deposition shortens T2. MRI changes are observed in 30% of patients after extracorporeal shock wave lithotripsy. Findings include parenchymal hemorrhage contusions and subcapsular or perinephric hematomas. Bleeding increases the signal, while contusion and edema produce focal or diffuse loss of corticomedullary differentiation. In ATN, the kidney appears normal, but after contrast the signal intensity in the renal medulla increases because of proximal tubule edema.
Renovascular disease: The oblique course and small size of the renal arteries limit the usefulness of MRI. However, MRA is a promising tool in the evaluation of renovascular disease, particularly in renal allografts.
Renal allograft evaluation: MRI has no advantage over ultrasonography.
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