MRI angiogram of the head (MRA): what it shows, how it works and what to expect

An MRI angiogram of the head is a type of magnetic resonance imaging focused on the arteries and, in some protocols, the veins of the brain and neck. Magnetic resonance imaging uses strong magnetic fields and radio waves to produce detailed images of the inside of the body, according to NHS inform, and the same equipment is used for magnetic resonance angiography.

Angiography is the general term for imaging blood vessels. The NHS describes the classic form as a type of X-ray in which a contrast dye is injected so the vessels show up. It adds that angiography may occasionally be done using scans instead of X-rays, called CT angiography or MRI angiography. In practice, MRA of the head is a non-invasive option because it avoids the catheter that is used in conventional angiography.

Doctors request vessel imaging when they need to know whether the arteries supplying the brain are narrowed, blocked, abnormally shaped or bulging. The NHS names a brain aneurysm, a bulge in a blood vessel in the brain, as one of the problems angiography is used to diagnose or investigate. It also says most brain aneurysms are small and do not cause symptoms, and are often found during a test for another condition.

Symptoms that may lead to this kind of scan include unexplained persistent or worsening headaches, vision changes, or other neurological symptoms. NHS pages on brain aneurysm list pain above or around the eye, changes in vision such as double vision, dizziness and balance problems, numbness or weakness on one side of the face, and problems with concentration or speech as possible symptoms of a larger aneurysm. These symptoms have many other causes, and an MRA is one of several tests a specialist may choose from.

A scan may also be arranged to follow up a known aneurysm. The NHS says small aneurysms that are not causing symptoms often need no treatment, and that regular follow-up scans may be used to check whether the aneurysm is getting bigger.

Preparation follows the usual MRI routine. The NHS says you may be asked not to wear anything with metal in it, such as jewellery or hearing aids, to remove coins and keys, and to change into a gown. You will fill in a safety form that asks about pregnancy, previous surgery, kidney problems, metal in the body such as surgical clips or a pacemaker, and allergies.

During the scan you lie flat on a bed that moves into a short tunnel. For head imaging a frame or coil may be placed around the head. NHS inform notes that such frames contain receivers that pick up signals from the body and help to create a better quality image. The scanner makes loud bangs and clicks, so you are given ear plugs or headphones, and you can speak to the radiographer, who controls the scanner from another room. You need to lie as still as you can, because movement blurs the pictures.

The NHS says an MRI scan usually takes between 15 minutes and 1 hour, but can take longer. NHS inform gives a range of 15 to 90 minutes depending on the area and the number of images. A head MRA is often combined with a standard brain MRI, which can lengthen the session.

Some MRA protocols need no injection, because the scanner can pick up flowing blood directly. Others use a contrast agent injected into a vein. NHS inform says contrast agents make certain tissues and blood vessels show up more clearly and in greater detail, and are only given if a radiologist decides this is right for you.

The NHS says side effects of contrast are usually mild and short-lived and can include feeling or being sick, a skin rash, a headache, a metallic taste in the mouth and dizziness. Allergic reactions are rare but can include coughing, wheezing, fainting, difficulty breathing or swallowing, or a rash. You may be asked to wait around 30 minutes afterwards. NHS inform adds that contrast agents can cause tissue and organ damage in people with severe kidney disease, so a blood test of kidney function may be arranged first. Tell staff about any history of allergic reactions or clotting problems.

MRI itself does not use ionising radiation. NHS inform describes it as painless and safe and says no evidence has been found of a risk from the magnetic fields and radio waves used. Some people feel tingling or warmth briefly. If you worry about enclosed spaces, say so before the day, because the NHS says a medicine to help you relax may be possible. If you have sedation, someone will need to take you home and you should not drive for 24 hours.

A head MRA can show the shape and calibre of the larger arteries, narrowings, occlusions, and bulges such as aneurysms, as well as unusual tangles of vessels. It cannot always show very small vessels or very small aneurysms, and results can be affected by movement or by metal near the area. The radiologist reports what is visible and may recommend a further test, such as CT angiography or catheter angiography, if an abnormal finding needs a closer look or if treatment is being planned.

A normal MRA is reassuring about the larger vessels but does not explain every headache or neurological symptom. Your doctor will interpret the result alongside your history and examination, which is why the report goes to them rather than straight to you.

The NHS says MRI results usually take between 1 and 2 weeks, since the images need to be looked at by a radiologist, who then writes to the doctor who referred you. You may need a follow-up appointment to discuss them, and the NHS advises contacting the referring doctor if you have heard nothing after a few weeks.

If an aneurysm is found, the next steps depend on its size, position and your health. The NHS says treatment is not usually needed for small aneurysms that are not causing symptoms, and that you may just need regular follow-up scans. If there is a risk of bursting, surgical clipping or endovascular treatment may be discussed. Stopping smoking and keeping blood pressure at a healthy level are advised to reduce the risk of the aneurysm getting bigger and bursting.

A burst brain aneurysm causes a bleed called a subarachnoid haemorrhage. The NHS says the main symptom is a sudden, severe headache sometimes described as a thunderclap headache. It also lists weakness in the face, weakness or numbness in one arm or one side of the body, speech problems and blurred vision or loss of sight as reasons to call 999. These are emergencies and should not wait for a scan appointment or result. The NHS adds that you should not drive yourself to A&E.

For persistent or worsening headaches, pain above or around the eye or other symptoms that do not go away, the NHS says to ask for an urgent GP appointment or get help from NHS 111.