Volume 9 Issue 10 - October 2011
                       
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Non-contrast MR Angiography
     
  NC-MRA has numerous applications for visualizing the arterial and venous systems in most regions of the body

  NC-MRA is best suited to patients who have contraindications for the use of intravenous contrast materials (gadolinium) such as renal failure or allergic reaction

  NC-MRA is an alternative to CT Angiography in some circumstances, such as contraindications to iodinated contrast materials, or when radiation exposure is undesirable

  NC-MRA sequences are generally more time-consuming than contrast-enhanced MRA sequences, but overall exam times are similar in most applications

     


  Figure 1. NC-MRA (inflow inversion recovery) shows normal arterial vasculature of the lower extremities. PA, popliteal artery; AT, anterior tibial arteries; PT, posterior tibial arteries; and PER, peroneal arteries.
  Figure 1. NC-MRA (inflow inversion recovery) shows normal arterial vasculature of the lower extremities. PA, popliteal artery; AT, anterior tibial arteries; PT, posterior tibial arteries; and PER, peroneal arteries.

Techniques for performing non-contrast MR angiography (NC-MRA) have been available since the introduction of MR imaging. These techniques were largely displaced by contrast-enhanced MR angiography (CE-MRA) owing to shorter acquisition times, greater anatomical coverage, and decreased pulsatility and flow artifacts. However, recent concerns about the safety of gadolinium-based contrast materials in high-risk patients, specifically in patients with renal failure, have revived interest in NC-MRA. At the same time, the initial disadvantages of NC-MRA have largely been overcome by advances in scanner hardware, the development of new imaging sequences, and the advent of cardiac and respiratory gating. NC-MRA also offers intrinsic advantages beyond, or in addition to, CE-MRA. For example, NC-MRA does not require precise timing of a power-injected bolus, while CE-MRA relies on perfect timing of the first pass of contrast through the arterial or venous circulation. The timing of this acquisition can be a challenge in the presence of cardiac disease, vascular disease, or difficult intravenous access. NC-MRA also allows for multiple acquisitions, whereas CE-MRA is a "single-shot" technique.

NC MRA techniques utilize the natural differences in the signal characteristics of flowing protons in the bloodstream versus stationary tissues. Several techniques have been developed to exploit this phenomenon. In "black blood" techniques, the pulse sequences are optimized to null the signal from flowing blood (i.e. "flow voids"). These techniques, which are based upon traditional spin-echo sequences, are a staple of cardiac imaging and allow robust characterization of the surrounding soft tissue anatomy. "White blood" imaging techniques are based upon rapid gradient-recalled echo techniques and result in images that show bright blood signals in flowing vessels against a dark background of the surrounding tissue.

Although NC-MRA is more commonly used for arterial applications, imaging techniques are available that can be used to visualize the venous system.

Table 1. Common Applications of Non-Contrast MRA
Arterial Applications Venous Applications
Steno-occlusive and aneurysmal diseases of:
> Carotid and cerebrovascular system
> Abdominal and thoracic aorta
> Pulmonary
> Renal
> Mesenteric
> Celiac
> Iliac/lower extremity
> Transplant planning and follow up

> Pelvic congestion
> May Thurner syndrome (Iliac vein)
> Pelvic DVT
> Mesenteric/portal thrombosis


Indications for NC-MRA
  Figure 2. NC-MRA, using a modern time-of-flight-based balanced steady-state free precession sequence, demonstrates normal native renal arteries.
  Figure 2. NC-MRA, using a modern time-of-flight-based balanced steady-state free precession sequence, demonstrates normal native renal arteries.

While CTA offers robust, rapid imaging in most vascular beds and is the only way to achieve reliable noninvasive imaging of the coronary arteries, NC-MRA often can have a role in vascular imaging. NC-MRA is best suited to those patients with high radiation exposure risks (such as children and women of childbearing potential) or who have contraindications to CT or MR contrast materials (i.e. pregnancy, renal insufficiency, or gadolinium contrast material allergy). Although NC-MRA sequences can be more time-consuming than CE-MRA, NC-MRA has almost the same sensitivity for detecting vascular abnormalities and has a very high negative predictive value. NC-MRA (like CE-MRA) can be superior and complementary to ultrasound when evaluating most parts of the body because it is not limited by acoustic windows, particularly in the thoraco-abdominal vasculature.

Peripheral Vasculature
NC-MRA imaging of peripheral vessels can play an important role in the management of patients with limb ischemia, which is often a chronic disease requiring intense longitudinal follow-up. In this case, the technique known as 2D time-of-flight (TOF) is often used because blood flow is generally orthogonal to the imaging plane. Inflow inversion recovery techniques are optimized to capture images of straight-path arteries, such as the femoral and tibial arteries, with better vessel enhancement, very robust fat and background suppression and minimized pulsatile artifacts (Figure 1). These efficient inflow sequences have been further accelerated with parallel imaging, resulting in 50% reduction in scan time compared to the conventional TOF exam. Inflow techniques can be applied with a three-dimensional volume readout to allow high-resolution images of more tortuous arteries. Pulse sequences can be optimized for imaging of the arterial or venous systems. In the periphery, plethysmography is used to separate pulsatile arterial and venous flow in inflow techniques. An image of the entire leg with 1.0 mm x 1.0 mm in-plane resolution can be acquired in about 5 minutes; a fraction of the scan time needed for a conventional TOF scan.

Figure 3. (A) Axial NC-MRA with inflow 3D volume acquisition sequence in a patient with a transplanted kidney shows high grade stenosis at the junction of the left external iliac artery and transplanted renal artery. (B) Image postprocessing (MIP) confirms high-grade stenosis of the left pelvic renal transplant in the same patient.
Figure 3. (A) Axial NC-MRA with inflow 3D volume acquisition sequence in a patient with a transplanted kidney shows high grade stenosis at the junction of the left external iliac artery and transplanted renal artery. (B) Image postprocessing (MIP) confirms high-grade stenosis of the left pelvic renal transplant in the same patient.


Neurovascular MRA
In the brain, the tortuous nature of the intracranial vessels and the need for high-resolution images offer a challenge. NC-MRA, using the technique known as time of flight (TOF) with 3D sampling is commonly used to assess both arterial and venous systems. Post-processing of the imaging data displays the arterial or venous vessels in a single image that resembles a digital subtraction angiogram. The ability to use MRA to rapidly and accurately visualize the cerebrovascular system is a valuable diagnostic tool in stroke patients because it can detect intracranial stenoses and vascular occlusions. In addition, 2D TOF MR venography is used to detect venous sinus thrombosis, although this technique is less sensitive than CT and contrast enhanced MR venography and is only used in patients who cannot receive contrast materials.

TOF MRA has been found to have comparable accuracy to digital subtraction angiography (DSA) or CT angiography (CTA) for the assessment of neurovascular aneurysms, reliably detecting aneurysms greater than 3 mm. TOF MRA can also detect residual flow within an aneurysm after treatment. TOF MRA is routinely used to follow patients with Moya Moya disease after synangiosis as well as for following arteriovenous malformations. MRI combined with TOF MRA has comparable diagnostic quality to CTA for the detection of intracranial carotid and vertebral dissections. In the neck, 2D TOF MRA is less sensitive than CE MRA but can be used to detect critical stenoses at the carotid bifurcation, larger dissections, and altered flow in patients who cannot receive contrast materials.


  Figure 4. NC-MRA in a patient with renal and pancreatic transplants. MIP reconstruction demonstrates normal right pancreas transplant artery and normal left renal transplant artery.
  Figure 4. NC-MRA in a patient with renal and pancreatic transplants. MIP reconstruction demonstrates normal right pancreas transplant artery and normal left renal transplant artery.

Thoracic MRA
Although the availability and simplicity of CTA often relegates MRA to a second tier diagnostic examination, MRA plays an increasing role in the workup, diagnosis, and management of aortic disease. Balanced steady-state free precession (SSFP) techniques are commonly used for thoracic applications because they yield bright blood images with little dependence on blood inflow.

Cardiac and respiratory motions pose unique challenges to thoracic imaging. However, gating techniques have now been developed to synchronize image acquisition with the patientís electrocardiographic leads and/or the peripheral plethysmograph. Certain applications, such as accurate imaging of the aortic root, require cardiac gating. However, in some applications, rapid "real-time" sequences can be used to mitigate cardiac motion without gating. Alternatively, "navigator-gated" sequences can be used in which images are acquired over numerous cardiac and respiratory cycles and data acquired during unfavorable respiratory excursions are rejected. Both these techniques greatly increase imaging times.

NC-MRA is particularly valuable in patients with genetic disorders such as Marfan's syndrome or Loey-Dietz syndrome, who need repeated imaging over time, because the technique is robust and can be accomplished without radiation exposure or administration of contrast material. Coronary MRA has in limited studies demonstrated similar performance to coronary CTA for the exclusion of significant stenosis. However, the inability to depict calcified lesions and the relatively long examination times makes MRA a second or third choice test for most patients.


  Figure 5. Maximum intensity projection coronal reformatted time-of-flight-based, plethysmography-gated image optimized for venous-only flow demonstrates left common iliac vein compression (arrow) in a patient with May-Thurner syndrome (chronic left leg deep vein thrombosis).
  Figure 5. Maximum intensity projection coronal reformatted time-of-flight-based, plethysmography-gated image optimized for venous-only flow demonstrates left common iliac vein compression (arrow) in a patient with May-Thurner syndrome (chronic left leg deep vein thrombosis).

Abdominopelvic MRA
There are several important NC-MRA applications in the abdomen and pelvis (Figures 2-4). NC-MRA is a valuable and robust technique for imaging large vessels to examine abdominal aortic aneurysms and dissections. Although imaging of small and distal vessels can be limited by several artifacts, NC-MRA is useful in young patients and those who are able to breath-hold. For example, NC-MRA, in conjunction with MRI anatomic imaging is an acceptable method for preoperative planning prior to uterine artery embolization in patients with contraindications to gadolinium.

In patients with suspected renovascular hypertension, NC-MRA is particularly useful in the presence of poor renal function. In a small study of 67 patients, renal artery stenosis (Figure 3) was detected with a sensitivity of 82-94% and a specificity of 76-96% compared to CE MRI.

NC-MRA is also an important tool for visualizing compression of the left common iliac vein and associated findings that are characteristic of May-Thurner syndrome (Figure 5) as well as for examinations for pelvic congestion, pelvic deep vein thrombosis, and mesenteric or portal thrombosis.


Scheduling
Appointments can be scheduled by calling 617-724-9729 or through the Radiology Order Entry system, http://mghroe/. MRA is available at the main campus, the Mass General / North Shore Center for Outpatient Care, and at the Mass General Imaging Centers in Chelsea and Waltham.


Further Information
For more information about non contrast MRA imaging, please contact George R. Oliveira, MD or Brian Ghoshhajra, MD, Vascular Imaging and Intervention, Mass General Hospital, at 617-643-6315.

We would like to thank Dr. George R. Oliveira, Sanjeeva Kalva, MD, Vascular Imaging and Intervention, and Brian Ghoshhajra, MD, MBA, Cardiac Imaging, Massachusetts General Hospital, for their assistance and advice for this issue.




References

Glockner JF, Takahashi N, Kawashima A, Woodrum DA, et al. (2010). Non-contrast renal artery MRA using an inflow inversion recovery steady state free precession technique (Inhance): comparison with 3D contrast-enhanced MRA. J Magn Reson Imaging 31: 1411-1418.

Hartung MP, Grist TM and Francois CJ (2011). Magnetic resonance angiography: current status and future directions. J Cardiovasc Magn Reson 13: 19.

Khoo MM, Deeab D, Gedroyc WM, Duncan N, et al. (2011). Renal artery stenosis: comparative assessment by unenhanced renal artery MRA versus contrast-enhanced MRA. Eur Radiol 21: 1470-1476.

Venkatesh V, Verdini D and Ghoshhajra B (2011). Normal magnetic resonance imaging of the thorax. Magn Reson Imaging Clin N Am 19: 489-506, viii.




©2011 MGH Department of Radiology

Janet Cochrane Miller, D. Phil., Author
Raul N. Uppot, Editor