Isotropic 3D black blood MRI of abdominal aortic aneurysm wall and intraluminal thrombus

Abstract

Introduction

The aortic wall and intraluminal thrombus (ILT) have been increasingly studied as potential markers of progressive disease with abdominal aortic aneurysms (AAAs). Our goal was to develop a high resolution, 3D black blood MR technique for AAA wall and ILT imaging within a clinically acceptable scan time.

Methods

Twenty two patients with AAAs (maximal diameter 4.3 ± 1.0 cm), along with five healthy volunteers, were imaged at 3 T with a 3D T₁-weighted fast-spin-echo sequence using variable flip angle trains (SPACE) with a preparation pulse (DANTE) for suppressing blood signal. Volunteers and ten patients were also scanned with SPACE alone for comparison purposes. The signal to noise ratio (SNR) and the aortic wall/ILT to lumen contrast to noise ratio (CNR) were measured. Qualitative image scores (1–4 scale) assessing the inner lumen and outer wall boundaries of AAA were performed by two blinded reviewers. In patients with ILT, the ratio of ILT signal intensity (ILT_SI) over psoas muscle SI (Muscle_SI) was calculated, and the signal heterogeneity of ILT was quantified as standard deviation (SD) over the mean.

Results

All subjects were imaged successfully with an average scan time of 7.8 ± 0.7 minutes. The DANTE preparation pulse for blood suppression substantially reduced flow artifacts in SPACE with lower lumen SNR (8.8 vs. 21.4, p < 0.001) and improved the wall/ILT to lumen CNR (9.9 vs. 6.3, p < 0.001) in patients. Qualitative assessment showed improved visualization of lumen boundaries (73% higher scores on average, p = 0.01) and comparable visualization of outer wall boundary (p > 0.05). ILT was present in ten patients, with relatively high signal and a wide SD (average ILT_SI/Muscle_SI 1.42 ± 0.48 (range 0.75–2.11)) and with SD/mean of 27.7% ± 6.6% (range 19.6%–39.4%).

Conclusion

High resolution, 3D black blood MRI of AAAs can be achieved in a clinical accepted scan time with reduction of flow artifacts using the DANTE preparation pulse. Signal characteristics of ILT can be quantified and may be used for improved patient-specific risk stratification.

Introduction

Abdominal aortic aneurysm (AAA) disease is a common condition among men > 60 years old, and is associated with a high rate of morbidity and mortality. Current management of patients with AAAs is based on the maximal diameter of the aneurysm, and intervention is recommended when the AAA has a diameter larger than 5.5 cm [1]. While straightforward and practical, diameter measurements alone do not always predict the progression and rupture of AAAs. A considerable fraction of small AAAs (< 5.5 cm) rupture, whereas many larger AAAs never rupture [2]. The annual rupture risk of small AAAs ranges 0.7% to ~ 10% [2]. It has been reported that imaging the pathology within the aneurysm wall and intraluminal thrombus (ILT) may play a key role in providing a more complete, patient-specific risk stratification [3].

Two-dimensional (2D) ultrasound (US) is the most widely used tool of AAA screening within the general population [4]. However, it is limited by inter-operator variability and is not able to capture the three-dimensional (3D) anatomy of an AAA as it is not a cross-sectional modality. Computed tomography (CT) is also frequently used, but it requires radiation and iodinated contrast, and so would not be the preferred modality for serial monitoring.

MRI is a promising tool for AAA monitoring because it is a cross-sectional modality that does not require radiation and has excellent soft tissue contrast that allows characterization of the aortic wall and ILT. Previous MRI studies of the AAA wall, however, had limited coverage, coarse through-plane resolution (4 mm–5 mm), and degraded image quality due to flow artifacts [5], [6].

High resolution, 3D black blood techniques using fast-spin-echo (FSE) sequences with variable flip angle trains (SPACE, Siemens Medical Systems) is advantageous in aorta wall/ILT imaging given its high scan efficiency enabled by the use of long echo trains [7] and its intrinsic black blood effect [8]. It has been successfully used for aorta wall imaging in healthy volunteers and patients with cardiovascular or atherosclerotic disease with high isotropic resolution (1.1 mm³) [9], [10], [11]. However, blood suppression is less effective in the presence of slow or complex flow, and flow artifacts have been observed even in healthy volunteers with normal aorta geometry [9].

In patients with AAA disease, complex flow is common [3] and may lead to strong flow artifacts. Novel blood suppression techniques including flow-sensitive dephasing (FSD) [12] and Delay Alternating with Nutation for Tailored Excitation (DANTE) [13] can be used to improve the blood suppression in SPACE and they have been successfully applied in carotid plaque imaging [12], [14]. These techniques have not been evaluated in patients with AAA disease. Previous 3D MRI studies of aorta wall are also limited by the long scan times needed (~ 22 min) [9], [10], which are poorly tolerated by patients.

ILT is commonly present in AAAs with diameter greater than 4 cm [15]. A previous study suggested that AAAs with fresh ILT could progress two times faster compared with those without fresh ILT [5]. Identification of ILT and its composition may improve the risk evaluation of AAA.

This study aims to: 1) optimize the SPACE acquisition such that it can achieve good image quality and be acquired within a clinically acceptable scan time; 2) evaluate the blood suppression performance of SPACE with and without DANTE preparation for aorta wall/ILT imaging in healthy volunteers and patients with AAA disease; and, 3) evaluate whether a T₁-weighted SPACE protocol would be able to characterize thrombus components within the AAA.