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.

Aortic dissection

Aortic dissection consists of an intimal flap separating the true and false lumen, and intimal tears are sites of communication between the true and false lumen. A dissection can spread antegradely or retrogradely and may involve the whole length of the aorta. It can lead to occlusion or obstruction of branch vessels, aortic regurgitation and pericardial effusion. Aetiology includes hypertension (the most common), atherosclerosis, congenital lesions, iatrogenic causes and trauma.

Aortic dissections are classified by the De Bakey or Stanford classification system. The De Bakey classification is based on the location of the intimal tear and the extent of the dissection:

The Stanford classification is based on prognosis and has implications for management.

Figure 6.7 CMR study from a 57-year-old man who had undergone aortic root and valve repair the previous year, followed by AV replacement with a metal prosthetic valve.

At follow-up a dilated ascending aortic aneurysm and AR were noted. (a) LVOT SSFP cine confirms an aneurysmal ascending aorta with false lumen between native aorta and aortic graft. Dehiscence of the posterior suture line of the prosthetic AV resulted in a rocking motion of the valve and severe paravalvular regurgitation (long black arrow). (b) Comparative T2W SE image highlighting the suture line dehiscence (solid white arrow). (c) Velocity mapping CMR in the same plane showing blood flowing freely between the LV and false lumen (dashed white arrow). (d) Oblique sagittal SSFP GE aortic cine showing dissection flaps in the proximal aortic arch and the descending thoracic aorta (short black arrows).

Figure 6.8 Residual type B aortic dissection from a 67-year-old man who had undergone repair of a type A dissection one year previously.

(a) Oblique sagittal T2W SE shows the false lumen of the thoracic dissection (white arrow). This extends from 4 cm beyond the left subclavian artery to 2 cm above the diaphragm and is largely thrombosed with evidence of a fresh inner layer of thrombus. (b) Oblique coronal SSFP cine demonstrates the abdominal dissection (white ellipse). (c) MIP reconstruction of the CE-MRA data showing the two separate dissections (white arrow and white ellipse). Both renal arteries receive flow from the true lumen.

Type A dissection carries a higher complication rate and mortality and is a surgical emergency, while type B dissections have a better prognosis and are usually managed medically.

CMR is the gold standard imaging technique for detecting and characterizing aortic dissections with their associated complications in the haemodynamically stable patient. Scan protocols evaluate the following after diagnosis:

SE imaging delineates the intimal flap as a linear structure separating the true and false lumens. Slow flow (usually within the false lumen) or thrombus is differentiated by SSFP cines in the same plane. Cines are useful in detecting AR, and both sequences help in the diagnosis of pericardial effusions. Velocity mapping CMR also distinguishes diminished flow from intraluminal thrombus and highlights flow directionality in the false lumen (antegrade/retrograde). AR, where present, is quantified by velocity mapping in the appropriate plane. CE–MRA shows the flap and branch vessel involvement.

Intramural haematomas and penetrating aortic ulcers

An intramural haematoma has a similar clinical presentation and prognosis to that of aortic dissection and is also associated with arterial hypertension. It is thought to be caused by spontaneous rupture of the aortic vasa vasorum with subsequent propagation of subintimal haemorrhage, and there is a high rate of progression to aortic dissection and risk of aortic wall rupture. By contrast to dissections, there is more frequent involvement of the descending aorta and less frequent AR, MI and pulse deficits since they are more localized. The location of intramural haematomas carries prognostic and management implications with those in the ascending aorta having a higher frequency of complications and requiring surgical treatment.

Intramural haematomas are characterized by the presence of intramural blood and/or increased arterial wall thickness which can be asymmetrical or circumferential (Figure 6.9). T1W and T2W SE imaging allows some determination of the age of the haemorrhage. Acutely (first 5–7 days) intramural haematoma appears as increased wall thickness of intermediate signal intensity which is isodense with the aortic wall. Subacutely (more than 8 days) there is high signal intensity similar to fat due to the presence of methaemoglobin. Transaxial planes should be acquired in preference to longitudinal views due to improved differentiation from mediastinal fat. Addition of a fat saturation prepulse sequence further improves distinction between haematoma and fat. MRA will usually not detect intramural haematomas since there is no luminal component.

Figure 6.9 (a) Oblique sagittal SSFP cine view of the aorta and (b) transaxial T2W SE image from a 79-year-old man with chronic hypertension, aneurysm of the ascending aorta (two-headed arrows) and extensive intramural haematoma of the distal aortic arch and descending thoracic aorta (single-headed arrows).

Penetrating aortic ulcers consist of an intimal erosion penetrating the aortic wall from within the aortic lumen. There is an association with intramural haematoma and a risk of progression to aortic dissection or perforation. MRA reveals a focal area of contrast extravasation communicating with the aortic lumen. Penetrating ulcers are found predominantly in the descending thoracic and abdominal aorta, with the most frequent risk factor being aortic atherosclerosis.

Aortitis

Takayasu arteritis is a granulomatous vasculitis of unknown aetiology that commonly affects the thoracic and abdominal aorta. Inflammation leads to thickening of the aortic wall and the origin of the main branches, mainly in the aortic arch, with sequelae including aortic branch thrombosis, stenosis or occlusion. The pulmonary arteries can also be affected. SE imaging demonstrates diffuse wall thickening and MRA provides good visualization of aneurysm formation and branch vessel stenosis and/or occlusion.

Marfan syndrome

Marfan syndrome is an inherited connective tissue disorder with significant cardiovascular, skeletal and ocular system complications. CMR evaluates:

Aortic dissection and rupture account for most deaths in this condition and affected individuals undergo regular CMR surveillance of the aorta to help plan elective aortic root replacement. Aortic root dimension and the rate of increase are the best predictors of those at greatest risk of aortic dissection, and prophylactic root replacement is recommended before diameters reach 55–60 mm. In patients with family history of aortic dissection, replacement is performed when the aortic root measures 50 mm. Progressive thoracoabdominal vasculopathy postoperatively necessitates continued follow-up after repair of the ascending aorta. Patients with an aortic dissection or peripheral artery aneurysms require more frequent CMR assessment.

Coarctation of the aorta

Coarctation of the aorta occurs in approximately 1 in 10 000 people and is usually diagnosed in children or adults under 40 years old. It commonly consists of a stenosis of the aorta just distal to the origin of the left subclavian artery. The area of coarctation can be a localized narrowing or a long hypoplastic segment involving the aortic arch and descending thoracic aorta. Haemodynamically significant lesions are associated with the development of collaterals from the intercostal, internal mammary and anterior spinal arteries which then can supply blood to the descending aorta (Figure 6.10). Bicuspid AV is frequently seen with aortic coarctation and this cohort can progress to AS or AR. Dilatation of the ascending aorta may precede aortic aneurysm formation and type A aortic dissection in these patients.

Figure 6.10 MIP reconstruction of CE-MRA data in the oblique sagittal view from a 25-year-old woman showing discrete coarctation of the aorta just distal to the left subclavian artery pre-catheter intervention (white circle). Associated internal mammary and intercostal artery collaterals are arrowed.

CMR is the imaging modality of choice in adult aortic coarctation. The study evaluates:

The oblique sagittal plane is imaged using SE and SSFP cines for assessment of anatomy, but several other planes may be required with increased tortuosity. Blood flow across the stenosis is recognized with cines and these are then used to position the planes required for velocity mapping CMR. Flow velocity mapping techniques across the coarctation measure blood flow velocity and a pressure gradient is then estimated with the modified Bernoulli formula. Resting peak velocity of greater than 3 m/s is significant, particularly in the presence of diastolic prolongation of forward flow—a diastolic tail. Flow velocities may normalize in significant coarctation with extensive collateralization. The percentage of collateral blood flow is calculated from measurements of flow just proximal to the coarctation site and within the descending thoracic aorta at the level of the diaphragm. Collateral blood flow is significantly increased in severe coarctation. MRA readily demonstrates tortuosity, involvement of aortic arch vessels and presence of collaterals.

Long-term outcomes following elective intervention in coarctation are optimal when performed at 2–5 years of age. Late complications include hypertension, re-stenosis, residual stenosis and aneurysm formation. CMR is performed at baseline post-intervention and then at intervals thereafter, depending on clinical and imaging findings (Figures 6.11 and 6.12). Haemoptysis in a patient with coarctation may indicate leakage of blood through a false aneurysm. Serial contiguous SE sequences should be used in this situation to identify bright para-aortic haematoma.

Figure 6.11 One-year post-catheter intervention CMR study of the patient in Figure 6.10. Oblique sagittal views show mild residual in-stent stenosis.

(a) SSFP cine reveals metallic stent artefact at the site of previous coarctation (white ellipse). Subsequent flow velocity mapping CMR performed distal to the stent demonstrated increased systolic velocity without a diastolic tail. (b) T2W SE images improve the stent artefact and allow measurement of the reduced minimum in-stent diameter (dashed white arrow). (c) MIP reconstruction of CE-MRA data shows reduced collateralization (white arrows).

Figure 6.12 Two examples of postoperative aneurysm formation shown using MIPs displayed in the oblique sagittal plane.

(a) CE-MRA from a 19-year-old man who had undergone repair of aortic coarctation with a subclavian flap and Impra patch aged 3. There is a fusiform aneurysm (solid white arrow) distal to mild recoarctation at the mid-aortic arch. (b) CE-MRA from a 51-year-old man who had undergone aortic coarctation repair with a Gelseal graft placed between the dilated left subclavian artery and descending thoracic aorta (dashed white arrow). There is a small, contained false aneurysm at the distal suture line of the graft (black arrow).

Rare congenital anomalies of the aorta

A right-sided aortic arch consists of a single aortic arch located to the right of the trachea. This congenital anomaly is associated with TOF and truncus arteriosus. A double aortic arch refers to persistence of both the right and left aortic arch leading to the formation of a vascular ring around the trachea and oesophagus. This ring may compress these structures, causing dyspnoea and dysphagia. An aberrant origin of the right subclavian artery from the descending aorta may also cause airways compromise due to direct compression as it crosses obliquely towards the right shoulder behind the trachea.

CMR with SE and cines permits rapid evaluation of the anatomy of aortic arch anomalies and their relationship to surrounding structures. Associated congenital cardiovascular anomalies must also be characterized during the study. MRA allows easy visualization of the anomaly from different orientations and is an excellent way of communicating these unusual findings to colleagues.

Indications

CMR can:

CMR cannot: