MR Urography: patient preparation, technique and pulse sequences

Magnetic resonance urography (MRU) can be used to thoroughly evaluate the renal parenchyma, the pelvicalyceal system, and the rest of the urinary tract in a single imaging study as in CT urography, and more specifically MRU is clinically useful in the evaluation of suspected urinary tract obstruction, hematuria, and congenital anomalies, as well as surgically altered anatomy. MRU can be particularly beneficial in pediatric or pregnant patients or when ionizing radiation is to be avoided. The most common MR urographic techniques for displaying the urinary tract can be divided into two categories: static-fluid MR urography and excretory MR urography. Static-fluid MR urography makes use of heavily T2- weighted sequences to image the urinary tract as a static collection of fluid, can be repeated sequentially (cine MR urography) to better demonstrate the ureters in their entirety and to confirm the presence of fixed stenoses, and is most successful in patients with dilated or obstructed collecting systems. Excretory MR urography is performed during the excretory phase of enhancement after the intravenous administration of gadolinium-based contrast material; thus, the patient must have sufficient renal function to allow the excretion and even distribution of the contrast material. Diuretic administration is an important adjunct to excretory MR urography, which can better demonstrate nondilated systems. Static-fluid and excretory MRU can be combined with conventional MR imaging for comprehensive evaluation of the urinary tract. In comparison with the CT Urography, MRU is limited by longer examination times, decreased spatial resolution, and an inability to reliably depict calcifications and calculi.

Patient Preparation

  • Routine medication should not be stopped.

  • Patients should not eat any solid food for 4-6 hours.

  • Patients should not drink any fluid for 4-6 hours, although non-sparkling water is permissible.

Technique & Patient Positioning

  • The patient should be placed in the supine position.

  • Have patients empty their bladder before imaging.

  • If no contraindications exist (eg, fluid restriction, congestive heart failure), IV hydration is needed just before the start of imaging (on the MR table).

  • IV hydration as follows: 250 ml normal saline (50 ml syringe).

  • Diuretic should be administered in the MRU.

  • Diuretic administration can improve the quality of excretory MR urography by enhancing urine flow, resulting in dilution and uniform distribution of gadolinium-based contrast material throughout the urinary tract.

  • The diuretic administration should be prior to the contrast injection.

  • A relatively low dose of furosemide (lasix) on the order of 0.1 mg/kg (ie, 5–10 mg for adults) is typically used for MR urography provided no contraindications exist.

  • For average-sized adults, a 5 mg dose of furosemide typically yields excellent image quality, while permitting the patient to finish the examination without having to empty bladder.

  • Optional: Negative contrast administration (pineapple juice or 1 cup of water + 2ml Gad) 5 minutes prior to examination. This is an attempt to reduce the high signal intensity arising from bowel content.

Sequences

  • Static-fluid MR urography highlights the urinary tract as a static column of fluid, using T2-weighted sequences (with and/or without fat suppression) that exploit the long T2 relaxation time of urine (fig. 1). Breath-hold T2-weighted MR urograms can be obtained with either thick-slab SS-FSE techniques or thin-section SS-FSE techniques.

  • 3D T2-w FSE respiratory-triggered (RTr) sequences can be used to obtain thin-section data sets that can then be postprocessed to create volume-rendered (VR) or maximum-intensity-projection (MIP) images of the entire urinary tract.

  • For cine imaging of the ureters, a thick-slab, heavily T2-weighted SS-FSE sequence can be performed. Cine MR urography is particularly helpful in confirming the existence of urinary tract stenosis.

  • Standard non-fat suppressed T1-weighted in- and opposed-phase gradient-echo sequences can be useful for detecting intracellular lipid in incidental adrenal masses and clear cell carcinoma of the kidney as well as for characterizing some angiomyolipomas.

  • For excretory MR urography, a 3D fat-suppressed T1-w gradient-echo sequence is used. Fat suppression enhances the conspicuity of the ureters and is recommended.

  • Excretory MR urography is roughly analogous to CT urography and conventional intravenous urography. Excretory MRU requires diuretic (lasix) administration and consists of the following dynamic phases (fig. 2):

  1. Corticomedullary 30-40 sec

  2. Nephrographic 90-110 sec

  3. After two postcontrast acquisitions, we immediately image the urinary bladder to ensure that we obtain images with bladder wall enhancement prior to the arrival of gadolinium-based contrast agent via the ureters (fig. 3). This procedure prevents mixing artifacts, which may obscure bladder tumors.

  4. Excretory 5-15 min

  • For renal arteries assessment: Acquire a true arterial phase instead of the corticomedullary phase.

Fig 1 Static-fluid MR urography highlights the urinary tract as a static column of fluid, using T2-weighted sequences (with and/or without fat suppression) that exploit the long T2 relaxation time of urine.

Fig. 2 Figure shows the five main dynamic phases of MR Urography.

Fig. 3 highlights the importance of immediately imaging the urinary bladder after two postcontrast acquisitions. Immediately imaging of the urinary bladder (approximately 2min post contrast injection) should be done in order to ensure that we will obtain images with bladder wall enhancement prior to the arrival of gadolinium-based contrast agent via the ureters.


Protocol


The MR Urography protocol is summarized in the table below.


References

  1. Leyendecker J. et al. MR Urography: Techniques and Clinical Applications. RadioGraphics 2008; 28:23–48.

  2. O’Connor et al. MR Urography. AJR: 195, September 2010, W201–W206

  3. Sudah M. et al. Comprehensive MR Urography Protocol: Equally Good Diagnostic Performance and Enhanced Visibility of the Upper Urinary Tract Compared to Triple-Phase CT Urography. PLoS ONE 11(7): e0158673. doi:10.1371/journal.pone.0158673

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