Erika Ripley, Presenter: Nothing to Disclose

Geoffrey D. Clarke PhD, Abstract Co-Author: Nothing to Disclose

MRI-compatible exercise studies are reported in the literature for measuring phosphorus-31 (31P) metabolites in the vastus lateralis muscle (VL). For these studies the thigh is positioned at an angle with respect to the main magnetic field to allow leg movement, which is a non-optimal orientation for the MRS coil. This study determines the performance degradation in the RF coil due to orientation.

A dual-tuned rigid TX/RX surface coil (Rapid Biomedical, Rimpar, Germany) was used on a 3T MRI system (TIM Trio, Siemens, Malvern, PA) to collect 31P spectra from a leg phantom (15 cm diameter, 4 L plastic cylindrical jug with 10 mM H3PO4) and a small standard (6 mL plastic vial with a 850 mM concentration of methylenediphosphonic acid (MDP)). The standard was positioned on the surface coil and a 1-pulse 31P sequence was performed (TR 8000 ms, 4 NSA, 4 prep scans, BW=3000 Hz). Spectra were taken with the MDP vial at the center of the coil, 5 cm, 8 cm and 10 cm away from the center in the R/L and H/F directions. Measurements were repeated with the leg phantom flat on the table parallel to B0 (0°) and secured to the exercise device at an angle of 38.9° with respect to B0. Spectra were analyzed using jMRUI software to determine the area under each peak. Paired t-tests were used to evaluate the statistical significance (p<0.05) of differences in the MRS signal for the two orientations.

The areas of the MDP peaks were plotted with respect to position. For both orientations, the signal from right to left was symmetric about the center of the coil with the highest signal at ±8 cm. The signals across the coil R/L were reduced by 30% (p=0.015) at 38.9° compared to 0°. Along the z axis, with the phantom flat on the table, the signal was almost symmetric, but spectra obtained at an angle were skewed with the maximum signal at +8 cm and no signal below -5 cm, an overall -56% average difference.

Signal intensity changes from the acquired phantom data suggest that over 60% of the total signal is lost at the 38.9° angle compared to acquisitions with the coil parallel to B0. Also, most of the signal will come from the portion of the VL muscle that is near the knee-end of the coil.

Signal sensitivity profiles that take into account position and coil orientation must be considered for 31P metabolite quantification in exercise studies.

Ripley, E, Clarke, G, Comparison of Signal as a Function of Position for a 1H/31P Surface Coil Used in Exercise Studies.  Radiological Society of North America 2013 Scientific Assembly and Annual Meeting, December 1 - December 6, 2013 ,Chicago IL. http://archive.rsna.org/2013/13017935.html