What is an AVM?
In the circulatory system, arteries are normally connected to veins by tiny capillary vessels:
An arteriovenous malformation (AVM) is a congenital vascular defect where an artery is connected directly to a vein without the intervening capillary network:
An AVM provides less resistance to blood flow than the capillary network, so a lot of blood flows through the AVM, diverting blood from the surrounding tissue. To compensate, the blood vessels in the surrounding tissue dilate, increasing the blood flow through the AVM, and depriving blood from tissue further afield. As AVMs occur almost exclusively within the brain, this can cause vision loss, headaches, and loss of conciousness.
The increased volume of blood moving through the AVM increases the pressure on the vessels, which can cause hemorrhage and the formation of aneurysms -- balloon-like formations -- in the draining veins. An aneurysm has a 4% chance per year of bursting, killing the patient.
About 1 in 1000 people is born with an AVM. There are no known predisposing genetic or lifestyle conditions; a parent with an AVM has the same chances of having a child with an AVM as a parent without an AVM. AVMs cannot be acquired after birth; most AVMs take 20 to 60 years to grow large enough to cause noticeable symptoms, though, so they tend to affect older people more than younger ones.
There are three known treatments for AVMs: surgical removal, embolization, and radiation treatment. Surgical removal is currently the preferred treatment, although some AVMs are too deep in the brain or too close to a critical region of the brain to be operated upon.
In embolization, the physician attempts to seal off the AVM's feeder vessels from within using an epoxy applied by an angiographic catheter. Sealing off all the feeder vessels is difficult, though, so embolization is usually only performed in preparation for surgery, to reduce the AVM's effect on blood flow.
In radiation therapy, the physician irradiates a region of the brain containing the AVM. Over a number of treatments, the AVM reduces in size, until it eventually disappears. Unfortunately, the irradiated tissue temporarily swells; there is a maximum volume above which the amount of associated swelling can cause serious damage. Thus, radiation therapy can only treat AVMs up to a certain size.
In all AVM treatments, the AVM must be completely removed. If any portion remains after treatment, the AVM will eventually grow back, since the remaining portion provides low resistance to blood flow, just as the original shunt did.
Our goal is to establish more accurate bounding regions for AVMs. With more accurately established boundaries, a wider range of AVMs can be safely treated using radiation therapy. We believe that we can attain higher accuracy by using MRI velocity data and subvoxel classification methods.