Magnetic Resonance Angiography

Magnetic resonance angiography (MRA) is a group of techniques based on Magnetic Resonance Imaging (MRI) to image blood vessels. Magnetic resonance angiography is used to generate images of the arteries in order to evaluate them for stenosis (abnormal narrowing), occlusion or aneurysms (vessel wall dilatations). MRA is often used to evaluate the arteries of the neck and brain, the thoracic and abdominal aorta, the renal arteries, and the legs called "a run-off".

Magnetic resonance angiographic images, unlike conventional or CT angiography do not display the lumen of the vessel, rather the blood flowing through the vessel.

Normal Neck MRA

Contrast enhanced (CE-MRA):

Injection of MRI contrast agents is currently the most common method of acquiring MRA. The contrast medium is injected into a vein, and images are acquired during the first pass of the agent through the arteries. Provided that the timing is correct, this may result in images of very high quality. An alternative is to use a contrast agent that does not, as most agents, leave the vascular system within a few minutes, but remains in the circulation up to an hour (a "'blood-pool agent'"). Since longer time is available for image acquisition, higher resolution imaging is possible. A problem, however, is the fact that both arteries and veins are enhanced at the same time if higher resolution images are required.

Time-of-flight (TOF) or Inflow angiography:

It uses a short echo time and flow compensation to make flowing blood much brighter than stationary tissue. As flowing blood enters the area being imaged it has seen a limited number of excitation pulses so it is not saturated, this gives it a much higher signal than the saturated stationary tissue. As this method is dependent on flowing blood, areas with slow flow (such as large aneurysms) or flow that is in plane of the image may not be well visualized. This is most commonly used in the head and neck and gives detailed high resolution images.

Phase-contrast (PC-MRA):

It can be used to encode the velocity of moving blood in the magnetic resonance signal's phase. The most common method used to encode velocity is the application of a bipolar gradient between the excitation pulse and the readout.

Fresh blood imaging (FBI):

An imaging technique using fast or super fast spin echo sequences (FSE/SFSE). Takes advantage of the longer T2 relaxation of blood compared to surrounding tissue. The images are acquired by fast spin echo sequences that can be synchronized with heart beats.

4D Dynamic MR Angiography (4D-MRA):

The first images, before enhancement, serve as a subtraction mask to extract the vascular tree in the succeeding images. Allows to divide arterial and venous phases of a blood-groove with visualisation of its dynamics. Much less time has been spent researching this method so far in comparison with other methods of MRA.

BOLD venography or Susceptibility weighted imaging:

This method exploits the susceptibility differences between tissues and uses the phase image to detect these differences. The magnitude and phase data are combined (digitally, by an image-processing program) to produce an enhanced contrast magnitude image which is exquisitely sensitive to venous blood, hemorrhage and iron storage. The imaging of venous blood with SWI is a blood-oxygen-level dependent (BOLD) technique which is why it was (and is sometimes still) referred to as BOLD venography. Due to its sensitivity to venous blood SWI is commonly used in traumatic brain injuries (TBI) and for high resolution brain venographies.