MR of Renal Masses
Renal cysts and masses are incredibly common finding on cross sectional imaging exams. Differentiating benign from malignant lesions is critical for patient management. Most renal cell carcinomas grow very slowly, and equivocal lesions may require multiple serial follow up studies before benign etiology is confirmed.
MR of renal masses and the ureters is a powerful complement to CT and ultrasound imaging techniques and offers several advantages. As operator independent techniques, MRI and CT provide a global assessment of the renal parenchyma and allow reliable evaluation of each lesion over time. Unlike CT, MRI does not involve ionizing radiation. Serial follow up MRI studies pose no risk to the patient.
During a single MRI renal mass study, multiple post gadolinium series are generated in rapid succession. This permits characterization of cystic renal masses with a level of detail which is simply not possible with CT or ultrasound. Such information often allows an otherwise indeterminate lesion to be reliably characterized as a complex or hemorrhagic cyst. An unnecessary biopsy, surgical intervention or years of follow up exams can be avoided.
MR urography may be performed with and without gadolinium. It is useful for identifying lesions in the renal pelvis, urogenital anomalies such as duplicated collecting system or abnormal ureteral insertion, and causes of hydronephrosis.
Patient Selection and Indications
Many renal masses are silent lesions, and the most common indication for renal MRI is an indeterminate renal cyst or mass on another imaging study. Patients who cannot receive iodinated contrast for CT urogram (CTU) due to allergy or mild renal insufficiency (GFR >30) can often safely be given gadolinium.
Patients with painless hematuria are ideal candidates, particularly younger patients who want to avoid radiation exposure.
In patients with painful hematuria from a suspected renal stone, a non-contrast CT scan remains the study of choice. Although MR ureterogram may detect obstructing renal stones, it is less sensitive than CT for calcified renal lesions and stones. However, it may be more sensitive than CTU for detection of radiolucent ureteral stones.
Hydronephrosis is extremely common in pregnancy. It is often difficult to differentiate partial ureteral obstruction due to mass effect of the gravid uterus from the effect of ureteral stones. Pregnant patients with suspected renal obstruction are ideal candidates for a non contrast MR urogram. It provides more reliable imaging of the ureters than ultrasound and does not expose the fetus to ionizing radiation like CT.
Standard MRI Precautions should be employed. No other patient preparation is necessary.
Breath holds of short duration may be required and are particularly important for the contrast enhanced portion of the exam to allow detection of small areas of cyst wall thickening and nodularity.
In patients allergic to gadolinium or in those with renal failure who are at risk for NSF, a non–contrast version of the study can be performed. Pregnant patients with suspected renal obstruction should not be given gadolinium. However, gadolinium provides much additional information and is used whenever possible.
Gadolinium is given to assess renal vessels, renal perfusion, and contrast excretion into the collecting systems. It is critical for characterization of cystic renal masses. Subtraction series are performed to maximize detection of small areas of cyst wall thickening and nodularity. Presence of these features increases the probability of an underlying renal cell neoplasm. Subtraction series can also exclude enhancement in hemorrhagic or proteinaceous cysts which have intrinsic high T1 signal.
Reporting and Outcomes
The exam is used to evaluate all imaged abdominal and pelvic organs, with special emphasis on renal parenchyma and masses. Renal vascular anatomy can also be evaluated, although not with the same sensitivity as a CT or MR angiogram.
While the examination surpasses that of conventional imaging studies for assessment of the kidneys, cystoscopy still provides superior evaluation of the bladder mucosa.
During the contrast portion, the urothelial system is evaluated for masses and congenital abnormalities. All cystic renal lesions are fully characterized. Fatty renal lesions such as angimyolipoma may often be reliably differentiated from renal cell cancers.
All studies are interpreted by radiologists with subspecialty training in advanced body imaging. A comprehensive report detailing findings and any recommendations regarding patient management is provided to the referring clinician. If urgent findings are encountered, these are relayed in real time by telephone. A copy of the examination and report is furnished to the patient upon request. The exam may also be reviewed directly with the radiologist in the radiology department at a Wellstar facility.