Atherosclerosis, the primary cause of cardiovascular disease, typically presents itself in medium to large arteries such as the aorta, carotids, coronary arteries and peripheral vessels (1). However, visualizing progression of atherosclerosis by evaluating the vessel wall in smaller arteries such as the superficial palmar arch may also provide valuable clinical insights into progression of cardiovascular disease especially in individuals with diabetes or Raynaud’s syndrome (2). Imaging the vessel wall of such smaller caliber arteries requires higher resolution imaging when compared to imaging larger vessels. For this purpose we conducted MR evaluation of the superficial palmar arch using 7T and 3T whole body MR scanners and compared the images acquired with high frequency ultrasound imaging. The superficial palmar arch is the continuation of the ulnar artery as it passes distal to the flexor retinaculum in the hypothenar region of the palm. We chose to focus on the superficial palmar arch because it is easily and consistently visualized using the modalities employed. We hypothesize that 7T MRI will provide better delineation of the vessel wall as compared to 3T and may be a suitable method to visualize small arteries in the hand.To demonstrate feasibility of vessel wall imaging of the superficial palmar arch using 7T and 3T MRI in comparison with very high frequency micro-ultrasound.

Four subjects (ages 22-50 years) were scanned on a micro-ultrasound system with a 40-MHz transducer (Vevo 2100, VisualSonics). The Vevo Imaging Station was used for mounting the transducer and for stabilizing the position of the hand. The subject was seated during the scan with the hand in supine position with slight rotation toward neutral. Padding was also used under the hand and arm for stabilization and comfort purposes (Figure 1A). With this positioning, images in B mode, Power mode, Doppler mode and M mode of the superficial palmar arch were obtained. Images were then subjectively analyzed for image quality. Three readers rated the images on a score of 1-5 with 1 being poorest and 5 being the best. Results from the 3 readers were averaged. Criteria used for subjective evaluation were the overall image quality, visualization of the vessel wall, adequate flow suppression and absence of artifacts. We also obtained measures of average peak Doppler velocity, intima media thickness, wall thickness, lumen diameter, and total vessel diameter.

Subjects’ hands were then imaged on a 3T clinical MR scanner (Siemens Biograph mMR) using an 8 channel special purpose receive only phased array carotid coil. (Figure 1B). Lastly, subjects’ hands were imaged on a 7T clinical MR scanner (Siemens Magnetom 7T Whole Body Scanner) using a custom built 8 channel transmit receive carotid coil. (Figure 1C). Subjects were imaged in a head-first prone position with hand extended above the head. The imaging protocols between 3T and 7T were matched as closely as possible. Total scan time was approximately 20 minutes each. We acquired a localizer, a 3D time-of-flight (TOF) MR angiography sequence followed by a 3D T2 weighted SPACE sequence (3) with 0.6 mm isotropic resolution in all dimensions. MR images were also subjectively analyzed for image quality and visualization of the vessel wall. The imaging criteria used were similar to that for the ultrasound. Furthermore, we also measured wall thickness, lumen and outer diameters from the MR images.

Sample images using the 3 modalities are shown in Figure 2. Subjective results obtained for image quality are shown in Figure 3. Results showed that the 7T MRI has the highest image quality rating followed by the ultrasound and then the 3T MRI. Results of this preliminary study indicated that vessel wall imaging of the superficial palmar arch was feasible with a whole body 7T MRI with subjective evaluations indicating that the image quality obtained at 7T being superior to both 3T MRI and micro-ultrasound. The 3D SPACE sequence with isotropic voxels allowed multi-planar reformatting of images and allows for less operator dependent results as compared to ultrasound imaging. Limitations of this study include a very small sample size and the fact that only healthy individuals without any atherosclerotic disease were examined. Imaging of the superficial palmar arch at the 7T is feasible and could be considered for evaluating atherosclerosis burden and progression in smaller arteries that may be of clinical relevance in specific disease conditions such as diabetes. Future studies need to be performed in diseased individuals and in a larger number of subjects to truly establish clinical feasibility of the approach.