A Novel Nanomaterial Coil for High Resolution Prostate Imaging

Introduction Recently an emerging RF coil technology based on a new nanostructured material was introduced [1]. The nanomaterial can be macroscopically configured in a ribbon or string form and is comprised of a dense array of carbon nanotubes. The macroscopic configuration is mechanically robust and is held together densely by van der Waals forces. It was demonstrated in [1] that this material holds significant promise for the construction of high performance MR imaging coils. In contrast to metallic conductors, the bulk carbon nanotube material shows negligible skin effect over the frequency range of interest for MRI applications, so that with a sufficiently large packing density the resistance of a given length of nanomaterial can be made smaller than that expected for a metallic conductor of similar geometry. Furthermore, ballistic charge transport within an individual carbon nanotube results in a kinetic inductance due to higher effective charge carrier inertia, leading to larger electrical inductance. The combination of increased inductance and reduced resistance can be used to build receive coils with enhanced SNR. Here this is explicitly demonstrated in a prototype prostate imaging coil that was built to fit into a commercially available endorectal probe for prostate imaging and biopsy. Materials and Methods Carbon nanotubes were grown in a furnace process and drawn into a continuous, macroscopic nanomaterial yarn or wire form by a spinning process. Four strips of the nanomaterial, each about 8 cm in length and about 300 microns in diameter, were attached to metal electrodes and formed into conductor strips to construct a long rectangular two-turn coil with capacitors placed between successive strips. The coil had approximate dimensions of 1.6 cm x 7.6 cm and was attached to a circuit board with suitable tuning and matching circuitry that included an active PIN diode-based block for detuning during transmit. A preamplifier proximal to the circuit board amplified the signal from the coil before it was routed to the scanner. The coil was designed to be capable of easy insertion into a small profile, rigid endorectal prostate imaging coil housing (Sentinelle Dual-Channel Endo Coil Array, Sentinelle Medical Inc., Toronto, Canada), Figure 1. The single-channel prototype nanomaterial-based imaging coil was tested on a 1.5T GE Signa scanner in a prostate phantom consisting of MnCl2-doped saline and designed with a 35 mm diameter plastic tube through the center of the phantom for insertion of imaging coils. The new nanomaterial-based coil was compared to imaging results acquired with a single-channel Medrad endorectal imaging coil (eCoil ®, Medrad, Indianola, PA, USA), and with a standard Sentinelle dual-channel endo coil array placed in the same phantom. All three coils were oriented towards the bottom of the phantom, with the smaller diameter standard Sentinelle dual-channel endo coil array and the nanomaterial coil laying on the bottom of the plastic tube inside the phantom. The same spin echo scan sequence was used in each case (SE, TE = 14 ms, TR = 450 ms, FOV = 20 cm x 20 cm, slice thickness 5 mm, 122 Hz pixel bandwidth). Results SNR contour maps were constructed for the nanomaterial-based imaging coil (nanocoil), the Medrad eCoil ®, and the Sentinelle dual-channel endo coil array, in each case using an axial image slice through the phantom located approximately at the middle of the length of the coil (the axial image slice is shown in Figure 2 for the nanocoil). While the single channel nanocoil was somewhat longer than either of the commercial coils, Figure 3 illustrates that it yielded significantly higher SNR numbers than the Medrad eCoil ®, and SNR profiles very similar to the Sentinelle dual-channel endo coil array.