Digital Subtraction Angiography

Digital subtraction angiography (DSA) is a fluoroscopic study of arteries, veins and the vascular supply of organs and lesions. A catheter is passed into a vessel (usually the common femoral artery by the Seldinger technique, or the radial or brachial artery) and iodinated contrast is injected while images are acquired at 1 to 6 frames each second.

A mask image taken before contrast arrives is subtracted from the images taken with contrast, so that bone and soft tissue disappear and only the opacified vessels remain. Selective catheterisation of the carotid, vertebral, coronary, renal, mesenteric, bronchial and other arteries gives high resolution images of small branches, and rotational acquisition gives three dimensional reconstructions.

Contrast volumes range from 3 to 10 mL for a selective injection to 20 to 40 mL for an aortic injection, and carbon dioxide or gadolinium are used when iodinated contrast must be avoided. The effective dose is about 2 to 10 mSv for diagnostic studies and much higher when combined with intervention, and the skin dose is the main concern in long procedures.

CT angiography and MR angiography have replaced diagnostic DSA for most indications, because they are non-invasive and show the vessel wall and surrounding tissue.

DSA remains the reference test for spatial resolution and for flow dynamics, and it is the basis of endovascular treatment: it is the test for small vessel and intracranial disease (aneurysm, arteriovenous malformation, dural fistula, vasculitis), for active bleeding that may be embolised, for planning embolisation, and for problems where non-invasive imaging is inconclusive or cannot be done.

Indications. Intracranial aneurysm, arteriovenous malformation, dural arteriovenous fistula and moyamoya disease when non-invasive imaging is inconclusive or treatment is planned. Cerebral vasculitis and unexplained subarachnoid haemorrhage. Carotid and vertebral stenosis, dissection and occlusion (when non-invasive imaging is unclear).

Peripheral arterial disease: level and extent of occlusion before revascularisation. Acute limb ischaemia. Acute gastrointestinal, bronchial, renal, pelvic or traumatic haemorrhage (for localisation and embolisation). Renal artery stenosis and fibromuscular dysplasia. Aortic and visceral aneurysms, and vascular malformations. Mesenteric ischaemia. Pre-operative mapping of tumour blood supply.

Assessment of dialysis access. Pulmonary angiography for arteriovenous malformation. Not as a first line test for screening, and not when CT or MR angiography answer the question.

Contraindications and precautions. Allergy to iodinated contrast (premedication or carbon dioxide, depending on the territory). Renal impairment (eGFR under 30 mL each minute for 1.73 m2): contrast nephropathy risk is raised, so minimise the volume, hydrate and consider carbon dioxide.

Coagulopathy (international normalised ratio over about 1.5 to 1.8, platelets under 50 000 in each microlitre) and anticoagulant treatment, handled as the local rule requires. Pregnancy: avoid unless essential. Severe uncorrected hypertension, heart failure, or active infection at the puncture site. Metformin: handled as the local rule requires. Thyroid disease (iodine load).

Previous groin surgery or bypass graft, and peripheral occlusion, may need another access route.

Preparation and technique. Informed consent, fasting for 4 to 6 hours (clear fluids allowed), hydration, and a check of renal function, coagulation and medicines. Under local anaesthetic and ultrasound control, the artery is punctured, a sheath is placed, and a wire and catheter are advanced under fluoroscopy to the target.

Contrast is injected by hand or pump, with images in several projections, and the patient is asked to hold still, hold the breath and avoid swallowing. Intravenous heparin is used in long procedures.

After the study the catheter is removed and haemostasis is secured by manual pressure for 10 to 20 minutes, a closure device or a radial compression band, followed by bed rest for 2 to 6 hours (femoral) or 1 to 2 hours (radial) and monitoring of pulses, puncture site, and neurological state.

Normal appearances. Vessels fill in sequence (arterial, capillary, venous phase) with smooth, tapering walls of even calibre, regular branching, and no filling defects, with prompt venous return. Aorta: tapering from about 3 cm at the arch to 2 cm at the bifurcation. Common carotid arteries: about 6 to 7 mm, with a smooth bulb at the origin of the internal carotid.

Intracranial circulation: the circle of Willis, anterior, middle and posterior cerebral arteries with symmetrical filling, and venous sinuses draining in about 4 to 6 seconds. Renal arteries: a single main artery to each kidney (in about 70%), tapering with a regular cortical blush. Mesenteric vessels: superior mesenteric artery branches in an arcade pattern with no beading.

The parenchymal phase shows a uniform blush of organs. Circulation time is brief, with contrast cleared from the arteries within a few seconds.

Interpretation.

• Stenosis: focal narrowing of the lumen, measured as percentage diameter reduction compared with a normal adjacent segment, with post-stenotic dilatation and collaterals in significant disease; for carotid disease, the percentage narrowing of the internal carotid artery measured against the normal distal vessel is used to decide on surgery or stenting.
• Occlusion: abrupt cutoff ("stump") of the vessel, with reconstitution of the distal vessel through collaterals. Acute thromboembolic occlusion shows a meniscus sign; chronic occlusion shows a tapered end and well developed collaterals.
• Atherosclerosis: irregular, ulcerated plaque (an irregular lumen outline), multiple stenoses, and tortuous, elongated vessels; ulceration is a marker of embolic risk.
• Aneurysm: a saccular or fusiform outpouching that fills with contrast, with the neck, dome, size, and relation to branches defined, a daughter sac (a point of rupture), mural thrombus (which reduces the contrast filled lumen smaller than the true size), and vasospasm after subarachnoid haemorrhage.
• Dissection: a long smooth tapered narrowing ("string sign"), an intimal flap, a double lumen, and a pseudoaneurysm.
• Vasculitis and fibromuscular dysplasia: alternating stenoses and dilatations ("string of beads", medial fibroplasia), multiple smooth stenoses, aneurysms (polyarteritis nodosa shows multiple microaneurysms in renal and mesenteric branches), and occlusions in Takayasu arteritis (long smooth narrowing of the aorta and its branches).
• Arteriovenous malformation and fistula: a nidus of tangled vessels, enlarged feeding arteries, early draining veins (venous filling during the arterial phase), and steal from adjacent territory. Dural fistulae show early filling of a venous sinus and cortical venous reflux.
• Tumour blood supply: hypervascular tumours show a dense blush, tortuous irregular vessels, arteriovenous shunting and early venous drainage (hepatocellular carcinoma, renal cell carcinoma, paraganglioma, meningioma); hypovascular tumours encase or displace vessels.
• Active bleeding: extravasation of contrast that pools and persists through the series (a blush of contrast outside the vessel), pseudoaneurysm, or arteriovenous fistula.
• Traumatic injury: vessel cutoff, intimal flap, extravasation, pseudoaneurysm and transection.
• Venous disease: thrombosis, stenosis, and sinus occlusion on the venous phase.
• Mesenteric ischaemia: occlusion or stenosis of the origin of the coeliac and mesenteric arteries, pruning of branches, and slow flow (non-occlusive ischaemia shows spasm and diffuse narrowing).
• Moyamoya disease: stenosis or occlusion of the distal internal carotid arteries, with a "puff of smoke" network of collaterals.

Critical values and findings needing immediate action.

• Ruptured intracranial aneurysm or arteriovenous malformation: neurosurgical or endovascular treatment (coiling, clipping), blood pressure control and nimodipine.
• Active arterial haemorrhage with haemodynamic instability: resuscitation and embolisation or surgery.
• Acute limb ischaemia (a sudden occlusion): thrombolysis, thrombectomy or surgical revascularisation, with heparin at once.
• Acute aortic dissection or rupture: emergency surgery or endovascular repair, with blood pressure and heart rate control.
• Stroke from large vessel occlusion: urgent thrombectomy within the time window.
• Acute mesenteric ischaemia: heparin, revascularisation, resection of dead bowel.
• Procedural complications: femoral haematoma or retroperitoneal bleed with hypotension (resuscitation, CT, intervention), pseudoaneurysm at the puncture (compression under ultrasound control, thrombin injection or surgery), arterial dissection or distal embolism (heparin, thrombectomy), stroke (during cerebral angiography, 0.1 to 1%) and anaphylaxis (stop, adrenaline, oxygen and fluids).

Limitations and artefacts.

• DSA is invasive: major complications occur in about 1 to 2% of diagnostic studies (puncture site haematoma, pseudoaneurysm, arterial occlusion or dissection, contrast nephropathy, allergic reaction, stroke, cholesterol embolism); cerebral angiography carries a risk of permanent neurological deficit of about 0.1 to 0.5%.
• Patient movement, respiration, bowel gas, peristalsis, swallowing and cardiac motion cause misregistration artefacts in the subtracted image; the study shows the lumen only and not the vessel wall, so mural thrombus, wall thickness, and plaque composition are underestimated, and a mural thrombus can make a large aneurysm look small.
• Two dimensional projections can overlap vessels and hide a lesion, so multiple projections and rotational three dimensional imaging are used.
• Laminar flow, mixing of unopacified blood and slow flow cause false filling defects; vasospasm mimics stenosis and is reduced by nitrates or a vasodilator.
• Contrast opacification may be poor in low flow states, and in the venous phase when the catheter position is wrong.
• Radiation dose is moderate to high, and operators must record the dose and avoid repeated studies without clear indication.
• DSA does not show tissue outside the vessels, so CT or MR is needed for the parenchyma and for extravascular disease.
• Reading is subjective in the percentage stenosis measurement, depending on the reference segment.

Special groups.

• Children: rarely needed (vascular malformation, Moyamoya, tumour embolisation) with general anaesthesia, a low dose technique and small contrast volumes.
• Pregnancy: avoid; MR angiography without contrast is the alternative.
• Patients with renal impairment or diabetes: minimise contrast, hydrate, use carbon dioxide in the lower body, and monitor creatinine for 48 to 72 hours.
• Elderly patients: tortuous vessels and atherosclerosis increase risk of embolism and puncture complications.
• Patients on anticoagulants or antiplatelet drugs: assess the bleeding and thrombosis risk before changing treatment.
• Regions: trauma with vascular injury, Takayasu arteritis, rheumatic and syphilitic aneurysms, peripheral arterial disease with late presentation, and Moyamoya are seen; access to DSA is limited to a few centres, and non-invasive CT angiography is used when possible.

Follow up tests. CT angiography or MR angiography for follow up and planning, duplex ultrasound of vessels, echocardiography, transcranial Doppler, blood pressure measurement, lipid profile and glucose, inflammatory markers (erythrocyte sedimentation rate, C-reactive protein), autoimmune tests and vasculitis screen (antinuclear antibody, antineutrophil cytoplasmic antibody),

renal function after contrast, haemoglobin after bleeding, and referral to vascular surgery, interventional radiology, neurosurgery or stroke services.