MR Angiography

MR angiography is a family of MRI techniques that show blood vessels without ionising radiation or iodinated contrast. It images arteries in the head, neck, chest, abdomen, pelvis and limbs.

There are two main approaches. Contrast enhanced MR angiography uses an intravenous gadolinium bolus (0.1 mmol each kg, often 0.05 to 0.1) and a fast 3D gradient echo sequence timed to the arterial phase, with coverage from the aortic arch to the feet using a moving table.

Non-contrast techniques use the flow of blood itself: time-of-flight (inflow of unsaturated spins, the usual choice for the intracranial arteries), phase contrast (velocity encoded), and balanced steady state free precession or quiescent interval slice selective methods for the limbs and the thorax.

Slices are 0.8 to 1.5 mm thick, with a large field of view for body imaging. The images are reformatted using maximum intensity projection, multiplanar reformats and volume rendering. The examination takes 20 to 45 minutes.

Peripheral arterial disease affects about 5 to 10% of people over 55, cerebrovascular disease causes about 11% of deaths worldwide, and aortic disease is a leading cause of sudden death in older adults.

MR angiography has a sensitivity and specificity of 90 to 98% for significant carotid, renal and peripheral artery stenosis compared with catheter angiography.

It is the preferred test for patients with allergy to iodinated contrast, renal impairment (non-contrast methods), young patients who need repeated follow up, and complex congenital vascular anomalies.

Indications.

• Intracranial: aneurysm screening and follow up, stenosis, occlusion, dissection, vasculitis, moyamoya, and vascular malformations.
• Neck: carotid and vertebral stenosis, dissection, fibromuscular dysplasia and arterial anomalies.
• Thoracic aorta: aneurysm, dissection, coarctation, aortitis (Takayasu), congenital anomalies and follow up after repair.
• Abdominal aorta and branches: aneurysm, mesenteric ischaemia, renal artery stenosis, and visceral aneurysm.
• Peripheral arteries: claudication, critical limb ischaemia, aneurysm and pre-operative bypass planning.
• Pulmonary arteries: congenital anomalies, pulmonary hypertension, sequestration and stenosis.
• Vascular malformations and tumours: mapping of arteriovenous malformations and vascular tumours.
• Pre-operative planning: transplant donors, flap surgery.
• Follow up of stents, grafts and endovascular repair when CT is not appropriate.
• Patients who need repeat angiography and should avoid radiation.

Not for emergencies where CT angiography is faster, for patients with unstable haemodynamics, or for assessment of calcification and stent lumen, where CT is better.

Contraindications and precautions.

• The usual MRI safety screen. Many aneurysm clips, stents and coils are conditional. Metal in the field causes signal loss.
• Gadolinium: avoid in severe renal impairment (estimated glomerular filtration rate under 30 mL each minute) with linear agents. Macrocyclic agents, lowest dose, or non-contrast techniques are used.
• Pregnancy: non-contrast MR angiography only.
• Claustrophobia, inability to stay still or hold the breath.
• Severe arrhythmia degrades non-contrast gated techniques.
• Previous reaction to a gadolinium agent.

Preparation and technique.

No fasting is needed for most studies. A cannula is placed in an antecubital vein for contrast enhanced studies. Gating with the electrocardiogram or pulse is used for non-contrast limb and thoracic techniques.

For time-of-flight intracranial angiography, a 3D multi-slab sequence covers the circle of Willis without contrast, in about 5 to 8 minutes. For the neck, a 2D or 3D time-of-flight or contrast enhanced sequence covers from the aortic arch to the circle of Willis.

For body contrast enhanced studies, a test bolus or bolus tracking sets the delay, and the 3D acquisition is taken in a breath hold of 10 to 20 seconds. Multi-station peripheral studies use moving table or hybrid sequences, with a fast bolus chase and sometimes calf compression to delay venous return.

Subtraction of the pre-contrast mask reduces background. Delayed phases show slow flow and endoleaks.

Normal appearances.

Arteries are bright, smooth, tapering tubes with a uniform lumen and no focal narrowing.

The circle of Willis, carotid and vertebral arteries are continuous, and common variants include a foetal type posterior communicating artery (a prominent posterior communicating artery supplying the posterior cerebral artery), hypoplastic A1 segment, and an absent communicating artery.

The aorta tapers from the root (up to 3.7 cm) to the arch (up to 3.0 cm), descending thoracic (up to 2.5 cm) and abdominal aorta (under 3.0 cm). Renal arteries arise at the level of L1 to L2.

Peripheral arteries run in the expected course with no abrupt narrowing, and the venous return is absent in the arterial phase if the timing is good.

Interpretation.

• Stenosis: focal narrowing of the lumen, graded by diameter reduction. A narrowing of over 50% is significant, and over 70% is severe. MR angiography tends to overestimate stenoses because of flow related signal loss.
• Occlusion: absent signal beyond a point, with collateral vessels reconstituting the distal artery.
• Aneurysm: focal dilatation of 1.5 times the normal diameter or more, saccular or fusiform, with thrombus (low signal in the wall on source images) and rupture signs (periaortic haematoma).
• Dissection: an intimal flap separating the true and false lumens, with different flow signal. Extension to branch vessels, false lumen thrombosis and aortic size are recorded.
• Intramural haematoma and penetrating ulcer: crescentic wall thickening with high T1 signal and no lumen enhancement.
• Aortitis and vasculitis: concentric wall thickening with enhancement and mural oedema, luminal stenosis, occlusion and aneurysm. Takayasu arteritis affects the aorta and branches in young women.
• Fibromuscular dysplasia: alternating narrowing and dilatation in the mid and distal renal, carotid and vertebral arteries.
• Carotid disease: stenosis at the bifurcation and internal carotid origin, plaque features (intraplaque haemorrhage as high T1 signal), ulceration and dissection (long smooth narrowing with a flame shaped taper and mural haematoma).
• Intracranial aneurysm: saccular outpouching, usually at branch points (anterior communicating, posterior communicating, middle cerebral artery, basilar tip). Size over 7 mm and irregular shape raise rupture risk. Sensitivity is about 90% for aneurysms over 5 mm.
• Intracranial stenosis and moyamoya: narrowing of the distal internal carotid and proximal middle and anterior cerebral arteries with a network of fine basal collaterals.
• Arteriovenous malformation: a nidus of tangled vessels, enlarged feeding arteries and early draining veins.
• Renal artery stenosis: ostial or proximal narrowing, post-stenotic dilatation, and renal asymmetry.
• Mesenteric ischaemia: stenosis or occlusion of the coeliac and superior mesenteric arteries, with collateral arcades. Acute occlusion needs CT.
• Peripheral arterial disease: segmental stenoses and occlusions in the aortoiliac, femoropopliteal and tibial arteries, with collateral reconstitution of run off vessels. Distal runoff determines suitability for bypass.
• Popliteal artery entrapment and aneurysm: medial deviation or compression of the artery, and aneurysmal dilatation with thrombus.
• Congenital: coarctation (narrowing at the isthmus with collaterals), aortic arch anomalies (double arch, right arch, aberrant subclavian artery), pulmonary artery stenosis and anomalous coronary origin.
• Stents and grafts: patency, in stent stenosis (limited by artefact), kinks, endoleak (contrast outside the graft in the aneurysm sac), and graft infection signs.
• Incidental: variants, small aneurysms, extravascular masses, nodes and organ lesions.

Critical values and findings needing immediate action.

• Aortic dissection, ruptured aneurysm or intramural haematoma: emergency vascular surgical or endovascular review and blood pressure control.
• Acute arterial occlusion with limb threat: urgent revascularisation.
• Symptomatic critical carotid stenosis or dissection with recent stroke: urgent vascular and stroke team review.
• Ruptured or large intracranial aneurysm: neurosurgical or neurointerventional treatment.
• Acute mesenteric ischaemia: laparotomy or endovascular revascularisation.
• Endoleak with sac growth: endovascular reintervention.
• Large aneurysm (aortic over 5.5 cm, rapid growth, symptoms): surgical planning.

Limitations and artefacts.

• Flow related signal loss from turbulence, slow flow or in plane flow exaggerates stenosis, especially in time-of-flight techniques at tight stenoses, bends and branch points.
• Venous contamination in the arterial phase, especially in the lower limb and renal studies, obscures arteries. Accurate bolus timing is needed.
• Metal (stents, clips, coils, joint prostheses) causes signal voids. Small stents appear occluded on MR angiography because of artefact.
• Calcified plaque is not shown, so the degree of calcification and stent integrity are better assessed by CT.
• Spatial resolution is lower than catheter angiography and CT for small vessels (distal tibial, coronary, small intracranial branches).
• Respiratory and cardiac motion degrade thoracic and abdominal studies. Gating and breath holds are needed.
• Maximum intensity projection can hide thrombus and wall disease. Source images must be reviewed.
• Contrast enhanced studies need gadolinium, with the risk of nephrogenic systemic fibrosis with linear agents in severe renal failure, and retention of gadolinium.
• Dissection can be missed in non-contrast studies with poor flow.
• Time-of-flight sequences show high T1 substances such as haematoma as bright signal that can mimic flow.

Special groups.

• Children: congenital vascular anomalies, Takayasu arteritis, moyamoya and vascular malformations. Sedation may be needed.
• Pregnant patients: non-contrast techniques only.
• Patients with renal impairment: non-contrast time-of-flight, phase contrast and balanced steady state free precession methods.
• Young adults with stroke, headache or neck pain after trauma: dissection and fibromuscular dysplasia.
• Patients with connective tissue disease (Marfan syndrome, vascular Ehlers-Danlos syndrome): serial imaging of the aorta.
• Patients with stents or grafts: check device compatibility, and use CT if artefact is severe.
• Regions: rheumatic and syphilitic aortic disease, Takayasu arteritis, tuberculous aortitis, schistosomal pulmonary hypertension, trauma, and neglected peripheral vascular disease are seen. MR angiography is scarce, and ultrasound, CT angiography and catheter angiography are the main tools.

Follow up tests.

• Duplex ultrasound for carotid, renal, aortic and peripheral arteries.
• CT angiography for emergencies, calcification and stents.
• Catheter (digital subtraction) angiography for intervention.
• Ankle brachial index, toe pressures and exercise testing for peripheral disease.
• Echocardiography for aortic root, valve and cardiac source of embolism.
• Blood tests: lipid profile, glucose, inflammatory markers, renal function, autoimmune markers for vasculitis, and genetic testing for connective tissue disease.
• Referral to vascular surgery, neurosurgery, interventional radiology, cardiology or rheumatology.