|Fig 1: Chest X-Ray PA view|
|Fig 2:Axial section of Plain CT thorax|
|Fig 3:Axial section of Plain CT thorax|
|Fig 4:Axial section of Contrast CT thorax|
|Fig 5:Axial section of Contrast CT thorax|
Thoracic aortic aneurysm:
Relatively uncommon compared to abdominal aortic aneurysms. There is a wide range of causes, and the ascending aorta is most commonly affected. CTA and MRA are the modalities of choice to image this condition.
The term aneurysm is used when the axial diameter of the thoracic aorta is > 5 cm and when it measures 4-5 cm the term dilatation is used.
Most commonly occur in 50 to 60-year-old age group and incidence is estimated at ~7.5 per 100,000 patient years 8. There is a male predominance (M:F=3:1).
Thoracic aneurysms are often identified incidentally on imaging of the chest. A symptomatic presentation may be due to mass effect on airway or oesophagus. Alternatively, they may present due to a complication, including rupture.
Pseudoaneurysms of the thoracic aorta are usually the result of significant thoracic trauma, both penetrating and blunt, and carry a very high mortality, with 80-90% of patients dying before reaching hospital.
Aneurysmal dilatation can affect any part of the thoracic aorta. Relative frequencies are (with some involving more than one segment):
- aortic root/ascending aorta: 60%
- aortic arch: 10%
- descending aorta: 40%
- thoracoabdominal segment: 10%
The location and shape of thoracic aortic aneurysms are variable. An aortic aneurysm,can be described as saccular or fusiform. In the case of fusiform dilatation, the term aneurysm should be applied when the diameter is >4 cm.
The thoracic aorta can usually be seen on both frontal and lateral chest radiographs, and aneurysms are often obvious. However, it is difficult to assess size accurately (due to magnification effects and often poor visualisation on the side of the artery).
Mediastinal masses may mimic aortic aneurysms.
Mural calcification is seen both in atherosclerotic disease as well as various causes of aortitis.
Unlike abdominal aneurysms that can usually be readily assessed and monitored with ultrasound, thoracic aortic aneurysms are encased in bone and air making transthoracic ultrasound of no use.
Transoesophageal echocardiography can visualise much of the descending aorta, but due to its invasive nature is not routinely used.
CTA is the work-horse of aneurysm assessment able to rapidly image the relevant vascular territory with high resolution. It can visualise both the sac and the lumen and detect potential complications.
Typically aneurysms appear as dilatations of the lumen. The walls may be thin or thickened by the presence of a mural thrombus (circumferential or more frequently eccentric).
Calcified atherosclerotic disease is often identified not only in the wall of an aneurysm but adjacent arteries.
If rupture or leak has occurred haematoma/fluid may be seen adjacent to the aorta, in the left pleural cavity or the pericardium .
MRI has the advantage of not requiring ionising radiation or large volumes of iodinated contrast . This is particularly advantageous in young patients with connective tissue disorders. However, there are limitations in patients with pacemakers, and those with reduced renal function
Acquisitions capable of being reformatted in three dimensions are essential to allow for accurate luminal measurement 2.
Digital subtraction angiography (DSA)
Although angiography has long been considered the gold standard for vascular imaging, it has largely been superseded by CTA and MRA, which can obtain 3D volumetric data, and able to assess the extraluminal soft tissues.
Angiography is however used during endovascular repair.
Reporting tips for aortic aneurysms
- size and shape
- sac dimensions (outer surface to outer surface)
- luminal diameter if mural thrombus is present
- fusiform or saccular
- size of vessel proximal and distal to aneurysm
- characteristics of wall
- mural calcification
- presence of mural thrombus
- location and relationship to involved branches/structure
- renal arteries
- involvement of the origins of the renal arteries
- presence of accessory renal arteries and where they arise
- splanchnic arteries
- great vessels from the arch
- renal arteries
- characterisation of possible aetiology
- true or false
- possibility of mycotic aetiology
- proximity to bowel
- aortocaval fistula
- Thoracic aortic aneurysms
- the size and dominance of vertebral arteries should be included if the aneurysm is close to the left subclavian artery
- presence of carotid disease is important, as significant stenosis may predispose the patient to strokes during any period of reduced flow/hypotension