MR Angiography

Technique

MR angiography (MRA) can be performed without the use of contrast or with contrast (gadolinium). Often both methods are utilized on the same exam. Most commonly, the aortic arch, great vessels, neck vessels, and intracranial vessels are evaluated. On non contrast MRA bright signal is generated by forward blood flow within a vessel but not the vessel itself. In fact, if a vessel is patent but there is very slow flow or flow reversal, no flow will be evident on non contrast MRA. Contrast enhanced MRA uses contrast to create the bright signal in the vessel lumen. Sequential images are obtained over the same area after IV contrast administration. This allows us to not only gauge vessel caliber (is it too small, is it too large, aneurysmal) but also to assess temporal dynamics. This means we can see what fills fast, what fills slow, and what the drainage is. This method was actually developed here at UW and is called TRICKS (Time Resolved Imaging of Contrast Kinetics). The second set of images illustrate early and late phase TRICKS angiography.

Indications

  • Carotid stenosis, vascular occlusion
  • Screening for aneurysms
  • Arteriovenous malformation, fistula

Advantages

  • No radiation
  • Don't have to give contrast
  • Can get temporal information for bloodflow

Disadvantages

  • Long exam, claustrophobia
  • Expense
  • Can over-estimate stenosis
  • Contraindications to MRI

Time Of Flight MRA

MRA of the head, obtained without the use of contrast. The image on the left is an an axial reconstruction showing an abrupt cutoff (occlusion) of the left middle cerebral artery. The image on the right is snapshot of a rotating 3D reconstruction of this patient's circle of Willis also showing the abrupt occlusion of the left middle cerebral artery.

Time Resolved MRA

MRA with contrast using the TRICKS (Time Resolved Imaging of Contrast Kinetics) sequence. On the surface, the earlier image (left) illustrates contrast filling the aortic arch, great vessels, neck arteries, and intracranial arteries while the later image (right) illustrates subsequent contrast filling of the venous drainage (dashed arrows pointing at the jugular veins). This sequence of images however also illustrates a phenomenon called subclavian steal. To learn more about subclavian steal and how TRICKS can be helpful in diagnosing it, click HERE.

MR Perfusion

As with CTA, MRA can be combined with contrast to perform perfusion MRI. The transit time of blood to a location, the flow of blood to a location and the blood volume are depicted in perfusion MRI. These are the same parameters as with CT perfusion. This information can be critical in evaluating the significance of a stenosis or describe hemodynamics to an area in the context of stroke.

The above images illustrate MR perfusion parameters for a patient with a left middle cerebral artery (MCA) occlusion. The image on the left is the FMT (first moment transit time) map which reflects prolonged transit time for blood in the left MCA territory. The middle image is the flow map showing that the flow of blood (volume per unit time) to the left MCA territory is low compared to the right side. Lastly, the cerebral blood volume (CBV) map is shown on the right image. In this patient the cerebral blood volume is decreased in the left MCA territory. A decreased CBV suggests that the normal autoregulation of the blood flow to the brain in the left MCA territory has failed and this area is now decompensated.

MR Venography

Occasionally the larger cerebral veins, including the dural venous sinuses, need to be evaluated. In these cases we are primarily looking for venous thrombosis which can lead to a wide range of symptoms such as headache, seizure, unresponsiveness, and stroke. MR venongraphy (MRV) is the most common mode of evaluation for venous occlusion.

3D reconstruction of MRV with thrombosis and occlusion of the left transverse sinus (dashed). The right transverse sinus is patent with normal flow (solid).