CT Aortography
CT aortography is an electrocardiogram gated or non-gated CT angiogram of the thoracic and abdominal aorta and its branches, from the neck base (or the aortic root) to the common femoral arteries. It uses a bolus of iodinated contrast (80 to 120 mL of high concentration non-ionic medium at 4 to 5 mL each second, followed by saline).
Bolus tracking in the descending aorta triggers the arterial phase; a non-contrast phase is added when intramural haematoma or calcification must be shown, and a delayed phase (60 to 90 seconds) for endoleak or leak. Thin slices (0.5 to 1.0 mm) are reconstructed and reviewed with multiplanar, curved planar, maximum intensity and volume rendered images.
Electrocardiogram gating (prospective or retrospective) removes cardiac motion from the aortic root and ascending aorta. Scanning takes 5 to 10 seconds, with effective dose of about 5 to 15 mSv depending on range and phases.
It is the first line investigation for suspected acute aortic syndromes (dissection, intramural haematoma, penetrating ulcer, rupture), aneurysm, trauma and pre- and post-procedural planning.
Acute aortic dissection affects 3 to 6 in 100 000 people each year; mortality without treatment is 1 to 2% each hour in the first 24 to 48 hours, and about half of patients with type A dissection die within a week if untreated.
CT has a sensitivity and specificity of 98 to 100% for aortic dissection and over 95% for rupture. Thoracic aortic aneurysm affects 6 to 10 in 100 000 people each year, and abdominal aortic aneurysm affects 4 to 8% of men over 65.
Indications. Suspected acute aortic syndrome (sudden severe chest or back pain, pulse deficit, syncope, new aortic regurgitation, neurological deficit, mediastinal widening). Known or suspected aneurysm: diagnosis, size, growth, symptomatic or rupture. Pre-operative and pre-endovascular planning (neck length, angulation, access vessels, branch vessels).
Surveillance after open repair or endovascular repair (endoleak, migration, graft infection, pseudoaneurysm, sac enlargement). Blunt chest trauma with suspected aortic injury (part of whole body trauma CT).
Aortitis (Takayasu arteritis, giant cell arteritis, infection), coarctation, congenital aortic anomalies, connective tissue disease (Marfan syndrome, Loeys-Dietz syndrome, Ehlers-Danlos type IV) follow up, mycotic aneurysm, aortic graft infection, aorto enteric fistula, and planning for transcatheter aortic valve implantation (dedicated technique).
Acute limb ischaemia and mesenteric ischaemia (aortic and branch assessment). Not for routine surveillance of small aneurysms when ultrasound is adequate.
Contraindications and precautions.
- Previous severe contrast reaction (premedication or MR angiography).
- Renal impairment (estimated glomerular filtration rate under 30 mL each minute): in suspected rupture or dissection do not delay the scan; otherwise use low contrast techniques or MR angiography without gadolinium.
- Pregnancy (avoid unless life threatening; MR is an alternative).
- Metformin, and thyroid disease.
Large cannula (18 G) in the right antecubital vein. Beta blockers are not given in suspected dissection unless instructed by the treating team, and the patient is monitored; hypotensive patients are scanned with resuscitation equipment available.
Preparation and technique. No preparation in emergencies. The patient lies supine with arms raised over the head. A non-contrast acquisition of the chest and abdomen (3 to 5 mm) is optional for intramural haematoma (crescentic hyperattenuating wall thickening over 60 HU) and to recognise calcification displaced from the intima.
A gated arterial phase from the thoracic inlet to the groins (or whole body from neck to feet for vascular lesions) is acquired in a single breath hold, in a craniocaudal direction, with bolus tracking in the descending thoracic aorta. A delayed phase of 60 to 90 seconds is added for endoleak (type II), slow flow, or active bleeding.
Dual energy and low kilovoltage techniques reduce contrast load. Reconstructions include centreline measurements perpendicular to the vessel axis, with sizing of diameters at set levels (annulus, sinuses of Valsalva, sinotubular junction, ascending aorta, arch, descending aorta at the diaphragm, infrarenal aorta, iliac arteries).
