The brachial plexus is a complex network of nerves innervating the upper limbs. Various pathologies can cause dysfunction of the brachial plexus with the most common being traumatic injuries, obstetrical injuries in newborns and tumors. MR neurography is increasingly used to assess therapeutic measures for brachial plexus injury. The most commonly used MR sequence to image the plexus brachialis is the Short TI Inversion Recovery (STIR) sequence. Recently new methods such as the SHeath signal increased with INKed rest-tissue RARE Imaging (SHINKEI) and Dixon based methods have been introduced(1, 2). Imaging the brachial plexus can be challenging due to the large variations in the B0 and transmit B1 fields in the area of the neck and shoulders (3). These variations can result in poor background tissue and fat suppression as well as reduced nerve signal. This artefact is especially visible posterior to the clavicle bones. In previous research it has been shown that the contrast-to-noise ratio and overall image quality in cardiac and abdominal imaging can be increased using high permittivity (HP) pads (4, 5). Therefore we wanted to study if this approach could also increase the image quality in plexus brachialis imaging.

Participants; In total fifteen participants were scanned with three different pad configurations. The institutional review board approved the study protocol, and written informed consent was obtained from all participants.

Data Acquisition; Experiments were performed on a 3 T Ingenia whole-body scanner (Philips Healthcare, Best, the Netherlands). The body coil was used for transmission and an anterior (16 elements) and posterior (12 elements) array for reception. The entire scan protocol was performed twice: once without the HP pads and once with the HP pads placed between the patient and the receive array. The imaging protocol consisted of a one minute survey used for planning a seven minute diffusion-prepared Dixon scan and a seven minute SHINKEI sequence. B1+ maps were acquired using the actual flip angle method, and B1- maps were derived using a 3D gradient-echo sequence with a flip angle of 5 degrees geometrically aligned with the B1+ map (5).

High permittivity pads; The HP pads were constructed using an aqueous suspension of barium titanate with a 5:1 mass-mass ratio. This results in a very dense mixture with a relative permittivity of approximately 300. From the three different pad configurations tested here the most effective was selected were two HP pads placed around the middle of the clavicle on the anterior side of the body (figure 1). The weights of these HP pads were 1.6 kg each.

Data Processing; Both the Dixon and SHINKEI sequences were reconstructed with a running maximum intensity projection (MIP) averaging over ten slices.The transmit field and receive sensitivities are shown with and without the pads (figure 2). By applying the pads the transmit field increases in the area adjacent to the pads. The receive sensitivity also increases around the area where the pads are placed. In figure 3 the Dixon water image is shown with and without pads. The signal intensity was plotted perpendicular to the brachial plexus on both sides. After application of the pads the signal intensity of the nerves increased for location A from 25 to 50 and location B from 23 to 43. The background signal was similar with and without pads resulting in greater nerve tissue contrast when the pads were applied. Both locations gain in contrast after application of the pads although the contrast on the right hand side of the subject is higher than that on the left.We found a asymmetry in the contrast between the left and right side. Future research should focus on the introduction of asymmetric pads, for example using thicker pads on the left side, to compensate for this difference. Furthermore different geometries and placement of the pads could be studied to potentially reach an even higher contrast. In the current setup the HP pad increase the contrast in the midclavicular area by a factor-of-two.