Interpretation

1. The kidney and surrounding structures

MR scanning, in contrast to CT, does not require ionizing radiation, utilizes non-nephrotoxic contrast, and will produce better contrast resolution in soft tissues, but is more expensive, less readily available, and therefore only plays a secondary, supportive role in the investigation of kidney diseases.

■ Renal lesions which remain ambiguous (e.g., hemorrhagic cyst) following CT and USS, can be clarified using MR

■ Cysts do not demonstrate contrast enhancement (T1 weighted), in contrast to solid lesions

■ Better detection of angiopyolipoma and renal lymphoma compared to CT

■ Accuracy of renal cancer staging by MR is between 70% and 80%

■ Evaluation of venous involvement is correctly demonstrated in 90% to 100% of cases

• Renal vasculature

■ MRA provides a useful, non-invasive technique for the assessment of renal vasculature

■ Can detect renal artery stenosis in transplant, as well as non-transplant patients

■ Avoids use of potentially nephrotoxic contrast media

■ Gadolinium-enhanced MR scanning can satisfactorily map the whole of the collecting system and ureter if scenarios where intravenous contrast is contraindicated (e.g., pregnancy, in children and patients with a history of contrast allergy)

■ A small degree of diuresis (e.g., following administration of furosemide) can enhance image quality in the detection of urothelial neoplasms and ureteric obstruction

■ MR at least as accurate as CT in the detection/characterization of adrenal masses

■ Particularly useful in distinguishing between adenomas and adrenal metastases

■ Can confirm the presence of pheochromocytoma (both adrenal and ectopic)

2. MR of the pelvis

Bladder

Manipulation of MR technique will allow reasonable demonstration of all the layers of the bladder wall, but still remains inferior to direct cystoscopy in the diagnosis of bladder tumors. Intravenous gadolinium produces intense detrussor enhancement. MR (T2-weighted images) will detect muscle-invasive or locally advanced bladder carcinoma with an accuracy of >90% and has become the standard tool for investigation of invasive bladder cancer prior to radical surgery or radiotherapy. In addition, lower ureteric tumors, tumor within diverticula, and urachal tumors are best demonstrated by MR. Ideally, MR must be deferred for at least 6 weeks following endoscopic resection or biopsy. Artifactual overstaging due to post-resection edema remains a distinct possibility, and all attempts must be made to obtain an MR prior to resection. In addition, non-specific scarring due to post-radiation or peri-vesical inflammation can be confused with malignant extension, resulting in tumor over-staging.

Prostate

In contrast to the bladder, under-staging is seen more often in patients undergoing MR for prostatic adenocarcinoma. MR, especially with an endorectal surface coil, is being extensively utilized to stage prostate cancer locally. T2-weighted images with an endorectal surface coil will provide excellent visualization of the prostatic zones, capsule, and neurovascular bundles. Pelvic lymphadenopathy is best demonstrated on Ti-weighted body coil images. Although many studies have claimed a sensitivity of >90% for the identification of extracapsular disease, the overall accuracy of MR for the prostatic adenocarcinoma lies between

51% and 82%. MR must be deferred for at least 4 weeks in patients following transrectal biopsy or transurethral prostate resection.

Size criteria for lymph nodes is the same as for CT, although MR is more likely to demonstrate smaller nodes.

3. MR of the external genitalia

Penis

MR can be employed to delineate penile anatomy and is a useful tool in the management of Peyronie's disease (for plaque demonstration), penile fractures, and erectile dysfunction (indications not clearly defined).

Testis

Although MR will readily distinguish between testicular torsion, inflammation, and tumors, the combined high diagnostic accuracy of USS and clinical examination almost negates its routine use in urological practice. MR can however, be utilized in non-emergency cases where doubt exists, as anatomical demonstration is superior with MR compared to USS.

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