Magnetic resonance imaging diagnosis of Takayasu arteritis¹

Abstract

Conventional angiography has been used for diagnosis of Takayasu arteritis (TA). In early stage of TA, subtle inflammatory wall thickening of the aorta and its branches without significant stenosis may be the only morphologic change of TA. Magnetic resonance (MR) imaging appears to be useful in the diagnosis of early stage TA by visualizing concentric wall thickening of the vessels. T2-weighted images may show bright signal of edema in and around the inflamed vessel. Contrast media-enhanced MR shows enhanced wall of vessels even in chronic stage, which may indicate the activity of TA at the tissue level. MR angiography allows us noninvasive assessment of the vascular tree and it may replace conventional angiography.

Introduction

Takayasu arteritis (TA) is an inflammatory arterial disease that involves large and medium-sized arteries. TA has early systemic or prepulseless stage and late occlusive or pulseless stage. The diagnosis of TA is often difficult in early stage because of nonspecific manifestation of the disease. TA generally has a progressive course with wax and wane of symptoms. Early treatment with immunosuppressive agents may affect the prognosis of the disease [1].

The major diagnostic criteria for TA have been stenosis of subclavian arteries, claudications of extremities, and arteriographic abnormalities that are late manifestations of TA 2, 3, 4.

Noninvasive diagnostic imaging modalities such as CT, magnetic resonance (MR) imaging, and ultrasonography have the capability of depicting early mural change of the aorta and its branches in TA 5, 6, 7, 8, 9, 10. Recently CT angiographic study showed the potential role of CT in the evaluation of TA 6, 7. MR has the same capability for visualization of inflammatory vascular thickening in the early stage of TA 9, 10. MR angiography with or without contrast media provides angiographic images noninvasively [8]. In this review, we describe MR findings of TA and the role of MR in the diagnosis of TA.

Angiography has been the principal imaging modality for TA 11, 12, 13, 14, 15. Angiography visualizes only the lumen of the vascular tree and, therefore abnormalities of the arch vessels and aorta are usually found in rather advanced cases. Recently CT angiography and MR have been utilized for evaluation of wall thickness of the aorta. These cross-sectional imaging modalities appear to be useful in early stage of the disease with thickened aortic wall without significant aortic branch stenosis.

Electrocardiography-gated T1-weighted axial and oblique sagittal images (echo time=10–11 ms, 256×192 matrix, 2 averaging) parallel to the aortic arch are obtained for the thoracic aorta using a 1.5 tesla scanner (General Electric Medical Co., Wisconsin, USA). T1-weighted axial scan is also performed for the abdominal aorta. For better visualization of aortic mural changes, thin slices (4–5 mm thickness) and smaller field of view (14 cm×14 cm to 20 cm×20 cm) are adopted. Flow compensation technique may eliminate the flow-related artifact in the aortic lumen. T2-weighted images (repetition time=3–4 R–R intervals, echo time=96–102) are also acquired to look for aortic wall edema or atherosclerotic change.

For determination of the degree of aortic wall enhancement and MR angiography, 0.2–0.4 ml/kg gadopentetate dimeglumine (Gd-DTPA or Magnevist; Schering, Berlin, Germany) is injected intravenously (total amount: 15–30 ml with an injection speed of 2 ml/s). The thoracic aorta and proximal neck vessels, renal arteries and visceral branches are imaged by using fast gradient echo 3-dimensional MR angiography (repetition time/echo time=6/1.1 ms, flip angle=40°, FOV=36×36 cm, 3 mm slice thickness with 1.6 mm overlapping, 256×126 matrix, 1 averaging) sequentially in two sessions. Then T1-weighted imaging of thoracic aorta is performed immediately, 5–10 min, and 11–20 min after contrast media injection. The degree of aortic wall enhancement is measured by using small ROIs (1–7 mm²). The signal intensity of myocardium and paraspinal back muscles is also measured for comparison with aortic wall signal intensity.

T1-weighted images show a diffusely thickened aortic wall (Fig. 1). Aortic wall thickness is barely measurable in the normal aorta. Aortic wall thickness is variable (3 to 10 mm) and more pronounced in mid to distal ascending aorta, distal thoracic aorta, and abdominal aorta. Aortic wall thickening in TA is always in a concentric pattern. On axial sections, ascending aorta is frequently enlarged and there may be also thickened aortic cusps and enlarged left ventricle. T1-weighted images in oblique sagittal planes parallel to the aortic arch are useful in depicting stenosis of proximal segments of neck vessels. In patients with TA, pericardial effusion can be associated and there may be complicated fluid or granulation tissue with increased signal in the pericardial sac. Lung lesions such as tuberculosis which can be associated with TA may be detected on T1-weighted images. On high resolution T1-weighted images, the innermost layer of the aortic wall is thicker and brighter than middle low intensity layer (Fig. 2). The outermost layer is also bright and not discriminated from the surrounding fat. The inner layer seems to be thickened tunica intima and media. The middle low signal layer is a part of tunica media and adventitia. The outer bright layer is adventitia and surrounding fat.

On T2-weighted images, aortic wall and periaortic tissue may appear bright due to inflammatory edema in acute phase. With use of gadolinium-DTPA as a contrast medium, increased vascularity in the thickened aortic wall is reflected in the enhanced aortic wall on T1-weighted images. Increased signal intensity in the aorta compared with that of myocardium may suggest significant disease activity.

MR angiography using various techniques can be utilized in noninvasive evaluation of aortic branches and peripheral arteries (Fig. 3). Recently contrast enhanced MR angiography techniques have been introduced. Among them, fast gradient 3-dimensional MR angiography (3-D MRA) technique with breathhold is very quick (within 20–30 s) and provides high quality angiographic images of the aorta and peripheral arteries. Both renal arteries and proximal segments of neck vessels can be evaluated for stenosis with 3-D MRA.

Cine MR technique can be used for evaluation of aortic regurgitation and left ventricular function. Cine MR may show decreased compliance or distensibility of the fibrotic aortic wall during a cardiac cycle.