Introduction

The aorta can be evaluated by a variety of techniques (Figure 6.1). X-ray contrast angiography was the gold standard method for many years but it is invasive and uses ionizing radiation and nephrotoxic contrast agents. Noninvasive options are TOE, CT and CMR (Figure 6.2). TOE is portable but relatively invasive and offers incomplete coverage of the aorta with restricted visualization at the aortic arch. CT is fast and widely available but uses ionizing radiation and nephrotoxic contrast agents. CMR avoids these limitations and offers multiplanar imaging of the aorta with a wide field of view and concurrent cardiac functional assessment. However, it is less available and evaluation of critically ill patients can be hampered by MRI-incompatible life support and monitoring equipment.

Figure 6.1 Diagrammatic representation of the normal aorta.

Figure 6.2 Normal CMR study of the aorta from a 60-year-old man referred for further investigation of hypertension with a family history of an ascending aortic aneurysm and marfanoid features.

These images show the oblique sagittal plane (known as the ‘hockey-stick’ or ‘candy cane’ view) from which several measurements can often be made using the (a) GE or (b) SE sequence. This aortic long-axis view is a plane obtained from the initial transaxial FSE images by placement of three markers—at the ascending aorta, at the aortic arch and at the descending thoracic aorta. Quoted aortic dimensions are usually intraluminal, at end-diastole, and should state clearly the sequence used in order to facilitate serial evaluation.

CMR reports of scans pertaining to the aorta follow the general pattern of:

Thoracic aortic aneurysm

An aortic aneurysm is a focal dilatation of the aorta. According to the shape the aneurysm is described as fusiform, with symmetrical dilatation of the circumference of the aorta, or saccular where only a part of the aortic wall is dilated. Aortic aneurysms can also be classified into true and false aneurysms. A true aneurysm consists of dilatation of all the layers of the aortic wall and characteristically has a wide neck. By contrast, in a false aneurysm perforation of the intima and media is contained by the surrounding adventitia and peri-aortic tissue, and the neck is usually narrow.

Aortic aneurysms are usually the consequence of atherosclerotic disease and most commonly occur at the descending aorta (Figure 6.3). Other causes are trauma, connective tissue disorders such as Marfan and Ehlers–Danlos syndromes, congenital abnormalities, and infections such as syphilis. Post-stenotic aneurysms are found distal to AV stenosis and aortic coarctation or recoarctation.

Figure 6.3 Hockey-stick (a) SSFP cine and (b) T2W SE views from a 74-year-old man with a 6.2 cm fusiform descending thoracic aortic aneurysm (large white arrow) containing a large amount of mural thrombus (small white arrows).

CMR can readily detect aortic aneurysms and scan protocols evaluate the following:

Figure 6.4 LVOT SSFP GE cines from a 20-year-old man with Marfan syndrome showing an 11.5 cm ascending aortic aneurysm (Ao) associated with severe AR (solid white arrows), dilated LV, and a small global pericardial effusion (dashed white arrows). There was no evidence of dissection.

SE images acquired in the transverse and long-axis planes are useful for measuring the diameter of the aorta at different levels and relationship of the aneurysm to major vessels. Coronal and oblique sagittal views clearly depict the aortic root and tortuous segments. Slow or turbulent blood flow can produce images which mimic mural thrombus on SE imaging. SSFP cines and velocity flow mapping help to differentiate slow flow from intraluminal thrombus. Where intraluminal thrombus is present, its thickness and extent should be determined. CMR allows characterization of intraluminal thrombus based on signal changes caused by the paramagnetic properties of deoxyhaemoglobin and methaemoglobin. Methaemoglobin forms from red blood cell lysis and shortens T1 but increases T2, causing hyperintensity on both T1W and T2W SE sequences. Thrombus with homogeneous low signal intensity on both T1W and T2W images corresponds to macroscopic organized thrombus. Some organized thrombi may have an internal rim of hyperintensity which represents recent clot apposition on the luminal surface of the thrombus. Thrombi with homogeneous high signal intensity on T1W and T2W imaging represent unorganized thrombi, composed mainly of fresh clot. Some thrombi may appear partially organized, with areas of high and low signal intensity (Figure 6.5). Inflammatory aortic aneurysms may show an area of peri-aortic inflammation that enhances following gadolinium contrast. This can be better visualized using a T1W CMR sequence with an added fat saturation prepulse. MRA is useful for assessing aortic flow and involvement and patency of aortic branch vessels. Images are acquired in the aortic long-axis plane and can then be reformatted into the transaxial plane. Post-processing techniques, such as maximum intensity projections (MIPs) and shaded surface displays, are used for representation (Figure 6.6). CMR is useful for follow-up evaluation of aneurysm size and serial measurements must be made at the same level.

Figure 6.5 Modified (a) coronal, (b) sagittal and (c) transaxial T2W SE planes from a 70-year-old woman demonstrate a 10 ×10 ×6cm³ aortic arch pseudoaneurysm on the outer curvature of the distal aortic arch containing partially organized mural thrombus (white arrows).

Figure 6.6 MIP reconstruction of the CE-MRA data obtained from the case illustrated in Figure 6.5.

Of note, the large outer aortic arch pseudoaneurysm (white arrow) is clearly seen to be in communication with the aortic lumen via a 2.5 cm entry site (solid black arrow). Additionally, a second, smaller pseudoaneurysm (3.5 ×2.3 ×6.0cm³) is demonstrated on the inner aortic arch (dashed black arrow).

Percutaneous stent-graft placement has emerged in the recent years as an effective way of treating aortic aneurysms and dissection. CMR is useful in planning these interventions allowing customization of stent design according to aortic anatomy, and detection of post-stent leaks. Accurate quantification of aortic calcification prior to stenting requires CT rather than CMR.