Renal MR Angiography

Steady-state free precession MRA of the renal arteries: Breath-hold and navigator-gated techniques vs. CE-MRA

PURPOSE

To explore the use of breath-hold and navigator-gated noncontrast Steady State Free Precession (SSFP) MR angiography (MRA) protocols for the evaluation of renal artery stenosis (RAS).

MATERIALS AND METHODS

Twenty patients referred to rule out RAS were imaged using two breath-hold and one navigator-gated SSFP MRA sequences. All patients underwent contrast-enhanced MRA (CE-MRA). Two radiologists evaluated all sequences both qualitatively (blur, artifacts, reader confidence) and quantitatively (maximum stenosis). Using CE-MRA as truth, a receiver operating characteristics (ROC) curve was generated and a statistical analysis of navigator-gated SSFP (Nav SSFP) was performed.

RESULTS

Seven patients had >50% renal artery stenosis by CE-MRA. Nav SSFP performed significantly better than either breath-hold SSFP technique in terms of blur, artifacts, and reader confidence. Using a 50% threshold for stenosis, sensitivity for detecting RAS was 100%, with a specificity of 85% and a negative predictive value of 100%. The average mean stenosis difference between Nav SSFP and CE-MRA was 9 +/- 9%.

CONCLUSION

Nav SSFP outperformed breath-hold SSFP in measures of image quality and reader confidence. Sensitivity and negative predictive value for detecting RAS with Nav SSFP was perfect, with an acceptable specificity of 85%. This suggests further study is warranted to evaluate Nav SSFP as a noncontrast screening technique for renal artery stenosis.

Parallel Imaging in MR Angiography

The recently developed techniques of parallel imaging with phased array coils are rapidly becoming accepted for magnetic resonance angiography (MRA) applications. This article reviews the various current parallel imaging techniques and their application to MRA. The increased scan efficiency provided by parallel imaging allows increased temporal or spatial resolution, and reduction of artifacts in contrast-enhanced MRA (CE-MRA). Increased temporal resolution in CE-MRA can be used to reduce the need for bolus timing and to provide hemodynamic information helpful for diagnosis. In addition, increased spatial resolution (or volume coverage) can be acquired in a breathhold (eg, in renal CE-MRA), or in otherwise limited clinically acceptable scan durations. The increased scan efficiency provided by parallel imaging has been successfully applied to CE-MRA as well as other MRA techniques such as inflow and phase contrast imaging. The large signal-to-noise ratio available in many MRA techniques lends these acquisitions to increased scan efficiency through parallel imaging.

MR angiography after renal revascularization: spectrum of expected anatomic results and postintervention complications

The use of magnetic resonance (MR) angiography in screening for renal artery stenosis has been extensively evaluated. However, the MR angiographic findings after renal artery revascularization are not as well characterized. The renal artery and parenchyma can be evaluated after revascularization with a comprehensive MR imaging protocol that includes T1- and T2-weighted spin-echo sequences, three-dimensional (3D) gadolinium-enhanced MR angiography, and 3D phase-contrast MR angiography. Because surgical techniques for revascularization vary, knowledge of the surgical procedure is necessary to ensure inclusion of the pertinent anatomy at 3D gadolinium-enhanced MR angiography and to define appropriate 3D phase-contrast MR angiography volumes. The 3D gadolinium-enhanced MR angiography volume can be manipulated to view relevant vascular anatomy at the optimal obliquity and section thickness. This protocol allows robust, noninvasive evaluation of the expected arterial anatomy after revascularization, including renal artery endarterectomy, aortorenal bypass grafts, and extraanatomic reconstructions. In cases of suspected postrevascularization complications, gadolinium-enhanced MR angiography is useful because of its lack of nephrotoxicity and radiation exposure. Immediate complications of renal revascularization include renal artery thrombosis, renal infarction, and perinephric hemorrhage. Long-term complications include aneurysms of bypass grafts and recurrent stenosis of the renal artery.