He Zhu PhD, Abstract Co-Author: Nothing to Disclose

Peter Blakeley Barker DPhil, Presenter: Speakers Bureau, Koninklijke Philips Electronics NV

Michael Schär PHD, Abstract Co-Author: Employee, Koninklijke Philips Electronics NV, Cleveland, OH

Joseph Gillen BS, Abstract Co-Author: Nothing to Disclose

Brian Timothy Welch BS, Abstract Co-Author: Nothing to Disclose

Malcolm Avison PhD, Abstract Co-Author: Research grant, Serono SA

John C. Gore PhD, Abstract Co-Author: Nothing to Disclose

Peter C.M. van Zijl PhD, Abstract Co-Author: Speaker, Koninklijke Philips Electronics NV

et al, Abstract Co-Author: Nothing to Disclose

Magnetic resonance spectroscopic imaging (MRSI) at 7T promises superior resolution and sensitivity (SNR) compared to lower field strengths. However, various technical challenges exist at 7T, including obtaining sufficient homogeneity of the magnetic (B0) and transmit RF (B1) fields, and balancing the need for high bandwidth pulses (to minimize chemical shift displacement artifacts) with acceptable specific absorption rate (SAR) in humans. This abstract describes approaches to addressing these limitations.

Experiments were performed on 5 normal volunteers using a 7T Achieva (Philips Healthcare Inc.) system located at Vanderbilt University, equipped with a 16-channel receiver array/quadrature volume transmit coil (Nova Medical, Wilmington, MA). The maximum transmit field was limited to 10 μT. To limit SAR, a 90˚-90˚ (Hahn) spin-echo sequence was used with frequency-modulated pulses (‘fremex05’, J. Murdoch, Philips Healthcare) of 11.67 ms duration and 3.56 kHz bandwidth (giving a choline-NAA shift displacement of 10%). VAPOR water suppression and 6 OVS lipid suppression pulses were used. Three axial 15 mm slices were recorded with TR/TE 6000/51.8 ms, 26x32 matrix size, SENSE acceleration factor = 4, FOV 186x230 mm, nominal voxel size = 0.77 cm3, 12.6 min scan time. Prior to data acquisition, a rapid dual-field mapping technique was used to optimize B0 and transmit B1 field homogeneity.

Average transmit B1 was found to be 68% of nominal, resulting in an average RF scaling correction factor of 1.54. Using shim coils up to 2nd order, the average peak-to-peak B0 homogeneity was 259 ± 19 Hz (= 0.87 ± 0.06 ppm, mean ± st. dev.) over the 3 MRSI slices, a 34% improvement over linear shimming alone. Following these adjustments, good quality MRSI data were obtained in all 5 subjects.

The 90˚-90˚ sequence, combined with rapid mapping of B0 and B1 fields, allows good quality MRSI data to be recorded with acceptable SAR. While only half the magnetization is refocused compared to the more commonly used 90˚-180˚ sequence, the 90˚-90˚ sequence provides more slices slices, shorter TR, and smaller chemical shift displacements.

Multi-slice high field MRSI, with appropriate sequence design, can be performed with high bandwidth pulses, while maintaining safe SAR levels for studies of human brain metabolism.

Zhu, H, Barker, P, Schär, M, Gillen, J, Welch, B, Avison, M, Gore, J, van Zijl, P, et al, , Multislice Proton Magnetic Resonance Spectroscopic Imaging of the Human Brain at 7 Tesla Using a 90°-90° Spin Echo Sequence.  Radiological Society of North America 2008 Scientific Assembly and Annual Meeting, February 18 - February 20, 2008 ,Chicago IL. http://archive.rsna.org/2008/6015288.html