Contrast-enhanced magnetic resonance angiography for detection of thoracic aneurysm

Christoph U Herborn
MD

Mathias Goyen
MD
Medical Prevention Centre
Hamburg
Germany

Invasive catheter-based X-ray angiography of the aorta has largely been replaced in clinical practice by its noninvasive counterparts, CT angiography and MR angiography. MR imaging has several distinct advantages over CT, which may not always be relevant for every clinical evaluation. It offers both black blood and bright blood techniques, with and without the use of contrast material. Recent advances in fast imaging, such as steady-state free precession (SSFP) and time-resolved contrast-enhanced (CE) MR angiography, allow quick examinations with rapid initial screening evaluations of the aorta. Here we assess the role of contrast-enhanced MRA for evaluation of aneurysmal disease of the thoracic aorta.

Three-dimensional contrast-enhanced MR angiography
3D CE MR angiography is probably the most profound advance in vascular MR imaging in the last decade.(1–6) For proper planning, the imaging volume is typically placed as a sagittal or oblique sagittal acquisition that includes the long axis of the thoracic aorta. A coronal acquisition is sometimes better for evaluation of the supra-aortic vessels (carotid, vertebral or subclavian arteries). To achieve rapid data acquisition within a comfortable breath hold, parallel imaging techniques are used. These include sensitivity encoding (SENSE, Philips Medical Systems); array spatial sensitivity encoding (ASSET, General Electric Health Care); and integrated parallel acquisition (iPAT, Siemens Medical Solutions). Here, phased-array surface coils are employed which significantly reduce data acquisition times while maintaining spatial resolution, although this is at the expense of signal-to-noise ratio.(7–9) Alternatively, a faster imaging speed can be used to increase the spatial resolution of MR angiography while keeping the acquisition time constant. CE MR angiography can be performed using a contrast media dose that is weight-based (ie 0.1–0.2 mmol/kg) or volume-based (eg, 20 or 30 ml). In patients with severe renal impairment, nephrogenic systemic fibrosis has been associated with use of gadolinium (Gd)-containing MRI contrast agents.(10)

However, MRI contrast agents are generally safe and well tolerated. If the contrast agent is bound to albumin, such as gadofosveset (Vasovist, BayerSchering Pharma AG, Berlin) or gadobenate dimeglumine (MultiHance, Bracco, Italy), an interaction with other plasma protein-bound active substances (eg, ibuprofen and warfarin) is generally possible. In other words, a competition for the protein-binding site can occur. In the case of gadofosveset, for example, a series of in-vitro drug interaction studies (in 4.5% human serum albumin and human plasma), no adverse interaction occurred at clinically relevant concentrations with digitoxin, propranolol, verapamil, warfarin, phenprocoumon, ibuprofen, diazepam, ketoprofen, naproxen, diclofenac or piroxicam. In-vitro studies using human liver microsomes did not indicate any potential to inhibit the cytochrome P450 enzyme system. In a clinical study, it was shown that gadofosveset does not affect the unbound fraction of warfarin in plasma. The anticoagulant activity of warfarin was not altered, and the efficacy of the medicinal product was not influenced. Nevertheless, interaction of fibrinolysis with coagulation and platelet aggregation might be important for synergistic interactions with other antiplatelet or anticoagulant drugs.(11,12)

3D CE MR angiography is primarily interpreted on an independent computer workstation equipped with specialised postprocessing software to perform multiplanar projection (MPR) or one of various image postprocessing techniques, such as maximum intensity projection (MIP) or volume rendering (VR).

Ascending aorta and arch
Aortic dissection is one of the most serious and potentially life-threatening conditions involving the aortic root and ascending aorta.(13–16) The DeBakey and Stanford criteria divide aortic dissections into those that involve the ascending aorta or aortic arch (Stanford type A or DeBakey I and II) and those that are delimited to only the descending thoracic aorta beyond the left subclavian artery origin (Stanford type B or DeBakey III; Figure 1). Dissection may be fatal if it extends proximally into the aortic root, the aortic valve and the coronary arteries, potentially resulting, respectively, in intrapericardial hemorrhage and cardiac tamponade, acute aortic insufficiency and myocardial ischaemia.

[[HHE07_fig1_R39]]

Type B aortic dissections tend to be more stable and can be managed medically. Subacute haemorrhage (methemoglobin) appears as high intramural signal with adjacent normal signal void on unenhanced studies. SSFP can also be used successfully to demonstrate dissection with the intimal flap and its two vascular channels.(17) Intimal flaps must not be misinterpreted as artifacts within the aortic root and ascending aorta which can occur secondary to the normal pulsatility of aortic blood flow, the movement of the aortic valve leaflets and cardiac motion.

The normal aortic diameter on ECG-gated black blood images has been reported to be aortic root, 3.3 cm; mid-ascending aorta, 3.0 cm; aortic arch, 2.7 cm; and descending aorta, 2.4 cm.(18) Aneurysms larger than 5–6 cm and expanding rapidly, or that are symptomatic, will usually be repaired (Figure 2). Special care must be taken to obtain aortic diameter measurements in a plane perpendicular to the aorta. On an axial image, measurement at the level of the horizontal portion of the right pulmonary artery will ensure that the aorta is generally vertical, and that diameter measurements accurately reflect the aorta’s cross-sectional size. The use of an axial MR image will also permit comparisons with dimensions measured on other cross-sectional studies. In children, the ascending aorta is aneurysmal if the ratio of the ascending aorta to the descending aorta diameter is greater than 1.5.(19) Dilatation of the ascending aorta can also occur owing to aortic valve disease.

[[HHE07_fig2_R40]]

In almost three-quarters of individuals, the classic configuration of three aortic arch vessels is the origin of the innominate artery, the left common carotid artery, and the left subclavian artery in the proximal-to-distal direction. Variant anatomy is common, particularly a common origin of the innominate and left common carotid arteries.(20) When performing 3D CE MR angiography for evaluation of the arch vessels, it is generally preferable to use a coronal acquisition to ensure adequate coverage of the arteries mentioned above within the imaging volume.

Descending thoracic aorta
The descending thoracic aorta extends from the level of the ligamentum arteriosum to the aortic hiatus of the diaphragm. True aortic aneurysms involve all three layers (intima, media and adventitia) of the aortic wall and are typically due to atherosclerotic changes or related to connective tissue disorders (Figure 3). Penetrating atherosclerotic ulcers need to be differentiated from focal saccular aneurysm and intramural haematoma. In patients where a penetrating ulcer and aortic dissection or intramural hematoma is suspected, it is usually advisable to perform precontrast T1-weighted images to detect haemorrhage in the vessel wall.

[[HHE07_fig3_R41]]

Summary
3D CE MR angiography can provide angiographic and high-spatial-resolution images of the aorta. Diagnostic accuracy and speed can be improved by using, respectively, various postprocessing tools and parallel imaging. The careful combination of traditional black blood and bright blood techniques permits an adequate and accurate evaluation of aneurysmal disease of the thoracic aorta.

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