CT Peripheral Angiography

CT peripheral angiography images the arteries of the lower limbs (and, less often, the upper limbs) from the infrarenal aorta to the feet after an intravenous bolus of iodinated contrast (80 to 120 mL of high concentration non-ionic medium at 3 to 5 mL each second, followed by a saline flush), with bolus tracking in the abdominal aorta and a table speed matched to the contrast front.

A multidetector scanner of 64 rows or more acquires the whole run of 100 to 120 cm in 10 to 25 seconds, with slices of 0.6 to 1.0 mm. Data are reconstructed with multiplanar, curved planar, maximum intensity projection and volume rendered images, and bone and calcium subtraction software improves the tibial vessels.

Effective dose is about 5 to 10 mSv for a run off study.

Peripheral arterial disease affects about 5 to 10% of people over 55 years and 15 to 20% of those over 70, and critical limb ischaemia develops in 1 to 3% of patients with intermittent claudication each year.

CT angiography has a sensitivity of about 95% and a specificity of 96% for stenosis over 50% in the lower limb arteries compared with catheter angiography, and is now the first line imaging test for treatment planning in most centres.

It also shows the arterial wall, calcification, aneurysms and soft tissue disease, and takes under 15 minutes.

Indications. Intermittent claudication not responding to supervised exercise, rest pain and tissue loss (critical limb ischaemia), to plan revascularisation. Acute limb ischaemia (embolism, thrombosis). Suspected popliteal artery entrapment and cystic adventitial disease.

Aneurysm of the iliac, femoral and popliteal arteries. Trauma with suspected arterial injury (penetrating limb injury, knee dislocation, long bone fracture with absent pulses). Surveillance of bypass grafts, stents and endografts.

Planning of free tissue transfer (perforator mapping) and of dialysis access. Vasculitis (Takayasu arteritis, Buerger disease, polyarteritis). Vascular tumours and malformations.

Diabetic foot with suspected ischaemia. Aortoiliac disease planning (Leriche syndrome). Not for screening of asymptomatic people, and not for mild claudication before conservative treatment.

Contraindications and precautions. Previous severe reaction to iodinated contrast (premedication or MR angiography). Renal impairment (estimated glomerular filtration rate under 30 mL each minute: weigh benefit against risk; many patients with peripheral disease have diabetes and chronic kidney disease, so hydrate and use the lowest contrast dose; in acute limb ischaemia do not delay the scan).

Metformin (follow local renal function advice). Pregnancy: avoid unless the limb or life is at risk. Children: rare, use low dose settings.

Heart failure and large contrast loads need care. A cannula of 18 G in an antecubital vein is used.

Preparation and technique. No special preparation; fasting for 4 hours is common where sedation or an unstable patient is possible. The patient lies supine, feet first, with both legs together, feet internally rotated and secured to avoid motion, and arms raised over the head.

Scanning runs from the level of the renal arteries (or the diaphragm for aortoiliac disease) to the toes.

Bolus tracking in the infrarenal aorta triggers the scan, with a speed that does not outrun the contrast. A dual phase or delayed run of the calf is added for slow flow, and a non-contrast run for calcified grafts and haematoma. A dual energy or low kilovoltage technique reduces contrast load.

Results are reported for each segment (aortoiliac, femoropopliteal, infrapopliteal, pedal arteries) with the degree of stenosis, length, calcification, occlusion and runoff.

Normal appearances. The infrarenal aorta divides into common iliac arteries, which divide into internal and external iliac arteries; the external iliac artery becomes the common femoral artery at the inguinal ligament, dividing into the superficial and deep femoral arteries.

The superficial femoral artery passes through the adductor canal to become the popliteal artery, which divides into the anterior tibial artery and the tibioperoneal trunk (posterior tibial and peroneal arteries).

Vessel diameters are about 8 to 10 mm (common iliac), 6 to 8 mm (common femoral and superficial femoral), 5 to 6 mm (popliteal), 2 to 4 mm (tibial) and 1 to 2 mm (pedal). The lumen is smooth and uniformly opacified with a thin wall; mild calcification is common with age.

Three vessel runoff to the ankle is the normal pattern, and the foot arch is continuous.

Interpretation.

• Stenosis: graded by lumen diameter reduction on cross-sectional images (mild under 50%, moderate 50 to 69%, severe 70 to 99%, occlusion 100%); length of lesion (focal under 5 cm, long over 10 cm) and location predict the choice between angioplasty, stenting, atherectomy, endarterectomy or bypass.
• Chronic occlusion: abrupt cut off with reconstitution by collaterals, calcified cap, and long segment with a tapered stump; collateral networks (profunda to popliteal, geniculate) show the severity.
• Calcification: heavy circumferential calcification limits luminal assessment (blooming), predicts difficult angioplasty, and is common in diabetes and renal failure (medial calcification).
• Acute occlusion: filling defect or cut off with a meniscus and little calcification, often at a branch point (common femoral artery or popliteal bifurcation) in embolism; thrombosis of a pre-existing plaque shows a diseased vessel.
• Aneurysm: popliteal (over 2 cm, often bilateral, with thrombus and distal embolisation), femoral, and iliac aneurysm; false aneurysm after puncture or trauma.
• Trauma: intimal flap, occlusion, pseudoaneurysm, active extravasation, arteriovenous fistula, and spasm; vascular injury is suspected when bone fragments lie close to the artery.
• Popliteal entrapment: medial deviation of the artery, abnormal slip of gastrocnemius or fibrous band, and compression on plantar flexion.
• Cystic adventitial disease: smooth tapered stenosis ("scimitar" sign) with a cyst in the wall.
• Vasculitis: Takayasu arteritis (long smooth stenoses and occlusions of large arteries, wall thickening), Buerger disease (distal "corkscrew" collaterals, tapered occlusions in young smokers with sparing of proximal vessels), fibromuscular dysplasia (beading).
• Grafts and stents: patent lumen, graft stenosis at anastomoses, pseudoaneurysm, kinking, thrombosis, infection (gas, fluid, stranding), stent fracture, and in stent restenosis (limited by stent artefact).
• Soft tissue findings: soft tissue and bone changes of diabetic foot (gas, abscess, osteomyelitis), tumours, and muscle oedema.
• Venous findings: the late phase may show deep vein thrombosis, varicose veins and arteriovenous malformation.
• Incidental: aortic aneurysm, renal artery disease, adrenal and pelvic lesions in the field.

Critical values and findings needing immediate action.

• Acute limb ischaemia (absent flow, sensory and motor deficit): immediate vascular surgery or interventional radiology for thrombolysis, thrombectomy or bypass, with heparin; the window for limb salvage is hours.
• Arterial injury with active bleeding, expanding haematoma or pseudoaneurysm: emergency surgery or endovascular repair.
• Critical limb ischaemia with tissue loss: urgent revascularisation planning within days, with infection control.
• Ruptured or symptomatic aneurysm: emergency repair.
• Popliteal aneurysm with thrombosis or distal embolisation: urgent vascular review.
• Compartment syndrome after reperfusion: fasciotomy.
• Infected graft or mycotic aneurysm: antibiotics and surgery.

Limitations and artefacts.

• Heavy calcification causes blooming and may hide significant stenosis or exaggerate it; the tibial and pedal vessels are the hardest, and a digital subtraction angiogram at the time of intervention remains the reference.
• Poor opacification of the distal vessels results from slow flow, tight proximal disease, or the scan outrunning the bolus; delayed imaging, a lower table speed, and a second run help.
• Venous contamination in the calf (early venous return in infection, arteriovenous shunting and diabetes) obscures tibial arteries.
• Metal from hip prostheses, stents and surgical clips cause streak artefact in the pelvis and in stented segments.
• Motion of the legs and overlap of vessels reduce quality in tibial segments.
• CT shows anatomy and not haemodynamics; the degree of ischaemia requires ankle brachial index, toe pressure or transcutaneous oxygen measurement.
• Radiation dose and contrast load are not trivial; use duplex ultrasound for first assessment where it answers the question, and MR angiography for patients with severe renal disease or allergies.
• Stenosis over estimation of 10 to 20% is possible; correlate with symptoms and pressures.

Special groups.

• Diabetic patients: heavy medial calcification, tibial and pedal disease, and neuropathic foot; high risk of contrast nephropathy.
• Patients with chronic kidney disease: consider ultrasound or non-contrast MR angiography; weigh limb risk against contrast risk.
• Young smokers with distal disease: Buerger disease, and popliteal entrapment in athletes.
• Women who are pregnant: avoid except for limb threatening conditions.
• Elderly frail patients: the choice of endovascular rather than surgical revascularisation depends on anatomy found on CT.
• Patients with prosthetic grafts: surveillance by duplex first.
• Regions: peripheral arterial disease is rising with diabetes and smoking; trauma (gunshot, stab, road injury), thromboangiitis and rheumatic heart disease with emboli are frequent; late presentation with gangrene is common, access to CT angiography and revascularisation is limited, and duplex ultrasound is often the available test.

Follow up tests. Ankle brachial index and toe pressures, duplex ultrasound of the arteries and grafts, MR angiography, digital subtraction angiography with endovascular treatment, echocardiography and cardiac rhythm monitoring for embolic sources, lipid profile, glucose and haemoglobin A1c, renal function, inflammatory markers and vasculitis screen in young patients,

thrombophilia testing in young patients with thrombosis, microbiology of wounds, foot radiographs or MR for osteomyelitis, risk factor treatment (smoking cessation, statin, antiplatelet therapy, blood pressure and glucose control), supervised exercise, and referral to vascular surgery, interventional radiology or podiatry.