Thoracic outlet syndrome (TOS) is a condition arising from compression of the subclavian vessel and/or brachial plexus as the structures travel from the thoracic outlet to the axilla^1,2. MRI is increasingly used in the diagnosis of vascular TOS. 3D contrast-enhanced MRA has recently been utilized in diagnosing vascular complications resulting from TOS^2,3. Furthermore, 3D MRA using provocative arm positions is helpful to determine the presence and degree of vascular compression and associated complications in the thoracic outlet³. Non-contrast MRA, on the other hand, might be a promising method to diagnose and follow-up of TOS because it is more noninvasive method compared to CE-MRA.

In this study, we propose a new non0contrast MRA technique based on gradient echo DIXON sequence with flow-independent relaxation-enhanced MRA method (Relaxation-Enhanced Angiography without Contrast and Triggering: REACT). The purpose of this study was to optimize the imaging parameters for REACT in the subclavian area and to evaluate the feasibility of this sequences.

Theory and pulse sequence: REACT sequence consists of improved mDIXON gradient echo sequence (mDIXON XD), non-volume-selective short tau inversion recovery (STIR) pulse and T2 preparation (T2prep) prep-pulse (Fig.1). First, mDIXON XD provides robust fat suppression even if the large FOV is applied (Fig.1a). Besides, STIR pulse is applied for increasing the contrast between blood vessels and background signals such as muscle4. Moreover, T2prep pulse is adopted to differentiate between arteries and veins by utilizing the differences of their T2 relaxation times⁵. Consequently, REACT would provide flow-independent relaxation-based MRA while keeping high robustness for uniform fat suppression in the subclavian area.

Experiments: A total of six volunteers were examined on 3.0T whole-body clinical systems (Ingenia, Philips Healthcare). The study was approved by the local IRB, and written informed consent was obtained from all subjects.

(1) Parameter optimization: Theoretically, arteriovenous contrast depends on the preparation time (prep-time) of T2prep pulse as aforementioned. Hence, we investigated the optimal prep-time by changing the prep-times (30, 50, 70, and 90ms). To quantitatively compare the effect of respective prep-times, we measured the contrast-ratio (CR) between subclavian arteries and veins (CR_Artery-Vein) and between artery and muscle (CR_{Artery-Muscle}). Also images of respective prep-time were visually assessed and compared.

(2) Feasibility evaluation: To demonstrate the feasibility of REACT sequence, we compared visually the image quality with conventional time-of-flight (TOF) MRA and balanced steady-state free- precession (bSSFP) sequence in regards to overall SNR, the arteriovenous contrast and the presence of artifacts. Finally, feasibility of REACT MRA with provocative arm positions was evaluated.

Imaging parameters for REACT were; Coronal, voxel size=1.0mm³, T2 prep-time=50ms, shot interval=3000ms, TR=3.9ms, TE1/TE2=1.38/2.5ms, flip angle=13°, turbo factor=100 and total acquisition time=2m49s. Imaging parameters for TOF-MRA were; Axial, voxel size=1.0mm³, TR=42ms, TE=3.5ms, flip angle=18°, and total acquisition time=8m11s. Imaging parameters for conventional bSSFP were; Coronal, voxel size=1.0mm³, shot interval=2000ms, TR=4.0ms, TE=2.0ms, flip angle=60°, turbo factor=77, SPIR fat suppression and total acquisition time=3m37s.

Figure 2 shows the results of parameter optimization, both the CR_Artery-Vein and CR_{Artery-Muscle} was increased as prep-time increased. Longer prep-time might be better for differentiating the arteries and veins; however, it caused failure water-fat separation in some cases. Hence, prep-time of 50ms would be optimal within the current sequence.

Figure 3 shows the comparison of REACT and conventional TOF-MRA/bSSFP sequences. REACT based MRA clearly shows the structure of both sides of subclavian arteries/veins more distally compared with other methods. bSSFP caused banding artifacts and fat suppression failure. In contrast, REACT could provide more robust and stable MRA without any artifacts and failed fat suppression. Moreover, REACT stably enables the scan with arm-abduction position. Representative images are shown in Figure 4. Thus, REACT has a great clinical potential for evaluation of TOS without contrast enhancement.