MR angiogram and venogram
MR Angiography
MR angiography (MRA) is used to assess the lumen of the arteries for example in:
- aneurysm diagnosis and follow up
- arterial dissection
- arterial stenosis
- vasculitis
It can be performed with or without contrast.
Time of Flight MRA
No contrast is injected in these studies. Instead, the flow of blood in a particular direction is imaged creating an arterial map. At 3T these produce very good images of the vessels. They are also useful for assessing retrograde flow e.g. arterialisation of veins in arteriovenous fistulas.
However, they are susceptible to metal artefact and slow flow will result in no signal being acquired, mimicking occlusion.
Contrast Enhanced (CE) MRA
This is performed after an injection of gadolinium which fills the vessels highlighting the lumen. It also means that a post-contrast image of the brain can be acquired.
The most common use for this is for follow up of aneurysm treatment as the signal dropout caused by coil and clip metal artifact can make it look like the vessels are completely occluded; the gadolinium fills the vessels and overcomes some of this effect. It also overcomes the slow flow effect caused by TOF MRA.
Notice how the contrast-enhanced MRA reveals much smaller arteries at the periphery of the study.
MR Venography
MR venography (MRV) is used to assess the size and patency of the intracranial veins. Usually, this is only accurate for the dural venous sinuses and the larger cortical veins. MRV is not sensitive enough for accurate assessment of the smaller veins and venous plexuses. Clinical situations in which this is useful includes:
- dural venous stenosis
- dural venous enlargement
- dural venous and cortical vein thrombosis
- infiltration of the dural venous sinuses by tumor e.g. meningioma
MRVs can be performed without or with contrast.
Phase-contrast / TOF MRV
These are performed without contrast. They use physics principles to form an image based on the direction of flow. Both methods are susceptible to metal artifact. Venous blood flows in multiple directions intracranially and if you don’t sample in a particular direction you may underestimate the venous flow. Because of this there will also be some arterial contamination.
Contrast-enhanced MRV
Contrast is injected and the images acquired at a slight delay when the contrast fills the venous system. This technique overcomes the artifacts created by metal and hemorrhage and the unexpected direction of flow.
Tips
- When there is the potential for artefact (e.g. prior surgery, hemorrhage, metal implants/coils) or when you need to see the smaller veins or arteries then contrast enhanced vascular studies are more sensitive.
- If the direction of flow is important (e.g. in arteriovenous fistulas) then phase-contrast and TOF studies are more useful.
- Also, there’s nothing to stop you from doing both but just keep in mind time constraints!