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Figure 11.15. Images in SE T1 "black blood" (a) and in steady-state free precessing (SSFP) "white blood" (b) in parasagittal plane of thoracic aorta in a patient with type B aortic dissection. In the "black blood" image, the false lumen appears slightly hyperintense due to slow flow. In the "white blood" image, the communication site is easily recognized.

Figure 11.16. Image in SE T1 "black blood" in a patient with type A aortic dissection in short axis plane of the vessel. The intimal flap is evident in ascending aorta.

agent inside the vessel strongly shortens the T1 of the flowing blood, inducing a strong increase of contrast-to-noise ratio. With this sequence, while the

Figure 11.17. (a) Figure of the short axis of the ascending aorta in a patient with type A aortic dissection. (b) Flow measurement of true and false lumen by the phase contrast method.

Figure 11.17. (a) Figure of the short axis of the ascending aorta in a patient with type A aortic dissection. (b) Flow measurement of true and false lumen by the phase contrast method.

presence of contrast induces a marked increase of signal within the vessel, the signal deriving from the stationary tissue is practically null. The acquisition itself is performed with a three-dimensional approach, and the result is a luminogram of the vessel (Figure 11.19a-e). There are several advantages to using this technique, such as a lower sensitivity to turbulences and to local slowing of flow. The major technical problem arises from the necessity of es-

Figure 11.18. Image by three-dimensional TOF of thoracic aorta. The shades effect is evident on the vessel profile, reducing the diagnostic accuracy.

tablishing a synchronism between the arrival of the contrast agent (when the effect on T1 of the highest) and the start of the acquisition. Practically, the operator has to decide the delay (in seconds) between the start of the contrast injection and the start of the acquisition signal. To reach this goal, there are several different approaches. The most often used are (1) the so-called best guess, where the expected time of circulation is presumed; (2) the monitoring system adopting detection algorithms, capable of online processing of the signal intensity within a defined volume positioned inside the targeted vessel and automatically starting the acquisition of the signal at the arrival of the contrast agent; and (3) the fluoroscopic imaging, where the operator is asked to visually monitor the vessel while ultrafast images (less then 100 ms) are continuously acquired, until the arrival of the contrast agent and then starting the signal acquisition. The latter seems to be the more efficient when the necessary expertise is reached inside the MRI Lab66,69-71.

With modern equipment, the simple acquisition of 3D CEMRA takes no more than 10 min. Only if the examination is made complex by the acquisition of adjunctive images (such as the fast SE for morphology or PC images to calculate the flow inside the vessel), an extra 10 min are needed.

An MRI examination for the follow-up of an aortic dissection should be a comprehensive morphologic evaluation of the lumen, the vascular wall, and

Figure 11.19. Three-dimensional CEMRA of type B aortic dissection. (a) The original maximum intensity projection. (b) The same case after volume rendering filtering. (c) The same case, analysis of the single partition to evidence the connection site between false and true lumen. (d) Multiplanar volumetric reconstruction, the involvement of right renal artery is evident. (e) The same case, the multistation acquisition procedure allows to follow the dissection along the whole vessel to the iliac arteries.

Figure 11.19. Three-dimensional CEMRA of type B aortic dissection. (a) The original maximum intensity projection. (b) The same case after volume rendering filtering. (c) The same case, analysis of the single partition to evidence the connection site between false and true lumen. (d) Multiplanar volumetric reconstruction, the involvement of right renal artery is evident. (e) The same case, the multistation acquisition procedure allows to follow the dissection along the whole vessel to the iliac arteries.

the perivascular findings (Figure 11.20). The presence of complications (such as intramural bleeding), the measurement of flow in the true and false lumen, and the comparative evaluation of results with those already available from previous studies have to be reported.

The identification of connection sites (proximal and distal) is not always possible, but it necessary to underline that in 13% of cases the intimal rupture is not even detectable at autopsy. In 65% of cases, dissection starts in the ascending aorta, in 20% in the descending aorta, in 10% at the arc level, and in 5% in the abdominal aorta. However, the disease has a dynamic evolution either spontaneously or following the therapy. Massive thrombotic phenomena can take place within the lumen mainly if excluded by the surgical intervention, and in some cases proximal and distal extension of the dissective process can be observed.

The incidence of valve regurgitation in type A aortic dissection is variable, ranging from 16% to 67% in published series. MRI can easily assess the in

Figure 11.19. (Continued)

Figure 11.19. (Continued)

volvement of the valve and the worsening of an already existing regurgitation on a native valve. Usually, the presence of a valvular prosthesis does not reduce the possibility of the MRI examination; however, the presence of a valve with metallic components can significantly reduce the image quality (Figure 11.21).

Cerebral ischemia has been reported in 3-5% of cases due to involvement of supraortic vessels or embolism. CEMRA images of the thoracic aorta are always comprehensive for the origin and proximal segments of supraortic vessels. Eventually, an ad hoc study can be planned if it is necessary to evaluate the entire course of the vessels.

Figure 11.20. Image in gradient echo in short axis of the thoracic aorta. The true lumen and the thrombotic material partially filling the false lumen are evident.
Figure 11.21. Image in 3D CEMRA in a patient with aortic prosthesis MRI compatible (nitilol).

Myocardial infarction due to the involvement of a coronary artery (1-2%) and ischemic lesions of spinal medulla due to involvement of spinal arteries are rare events21,69 that require a specific diagnostic approach, where MRI may play a role both in the acute phase and in the follow-up. In fact, it has been shown that MRA can identify the spinal arteries and, in particular, the artery of Adamkiewicz, helping the surgical plan of patients who are candidates for descending aorta repair. However, data on this issue are limited to small cohorts of patients, and the task is sometimes hindered by the wide anatomical variability of the spinal vessels and in particular of the artery of Adamkiewicz82.

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