Brian Welch MR Clinical Science 7T Phase Stability November 18, T Phase Stability and Sparse Spokes

INTERNAL USE ONLY MR Clinical Science, Brian Welch, November 18, Outline Background Sparse Spokes Phase Instability Document Prepared by Vanderbilt Surrogate Phase Stability Scan (dynamic 3D FFE B0 maps) Cleveland stability results collected Vanderbilt stability results collected Previous Vanderbilt stability results collected th – 3 rd order shim component fitting results to data Discussion Take home points

INTERNAL USE ONLY MR Clinical Science, Brian Welch, November 18, Background A “sparse spokes” RF pulse is comprised of a series of slice-selective Gaussian subpulses with unique phase encoding before each subpulse to move to different (kx,ky) locations in excitation k-space Eddy currents from the large kz slice selective gradients activated for each subpulse cause different 2D spatial phase accrual patterns to exist for each subpulse. The phase accrual values must be measured for accurate optimization of the amplitude, phase and kx, ky location for each subpulse. A typical sparse spokes with 25 subpulses is 16ms in duration Currently the B0 field mapping protocol used to measure the phase accrual observed at each of the 24 (25-1) subpulse positions takes ~15 minutes Observed phase instabilities between successive sparse spokes phase accrual calibrations performed 16 minutes apart has been as high as 20 ° The Magnex specification for the static field drift stability of the 7T scanner is 0.05ppm/hour (15Hz/hour at f0=300MHz, i.e. up to 4 Hz in 16 minutes) A 4 Hz off-resonance can cause 1.44 ° phase accrual in 1 ms. So over a 16 ms pulse, ° could be accrued from a 4 Hz off-resonance.

INTERNAL USE ONLY MR Clinical Science, Brian Welch, November 18, Sparse Spokes Phase Instability Document Prepared by Vanderbilt (June 4, 2009) (click above to open embedded PDF)

INTERNAL USE ONLY MR Clinical Science, Brian Welch, November 18, Surrogate Phase Stability Scan (dynamic 3D FFE B0 maps) 3D FFE 3 mm x 3 mm x 3 mm voxels, 80 x 60 x 5 matrix TR ms, TE 2.9 (shortest) WFS = 2.0 pixels, ~500 Hz/pixel dynamic acquisition time ~19 s B0 map delta TE = 10 ms (maximum value) –delta_phase [rad] = 2 *  * delta_f [Hz] * delta_TE [s] –phase wraps will occur at , corresponds to   /(2 *  * 10e -3 s) =  50 Hz 100 dynamics, 0 dummy scans Tested AUTO vs PBVOL shim Scanned 3L mineral oil phantom (click above to open embedded ExamCard zip file)

INTERNAL USE ONLY MR Clinical Science, Brian Welch, November 18, Cleveland stability results collected AUTO SHIMPBVOL SHIM drift from 10 to 14 Hz Over 30 Mins drift from 38 to 42 Hz Over 30 mins ~0.025 ppm/hour

INTERNAL USE ONLY MR Clinical Science, Brian Welch, November 18, Vanderbilt stability results collected AUTO SHIMPBVOL SHIM drift from to Hz Over 30 mins drift from 41.5 to 43.5 Hz Over 30 mins ~ ppm/hour

INTERNAL USE ONLY MR Clinical Science, Brian Welch, November 18, Previous Vanderbilt stability results collected ~1 Hz phase jumps 30 min scan 60 min scan 15 min scan with dynamic stabilization Coarse resolution of dyn stab causes jumps of ~1Hz

INTERNAL USE ONLY MR Clinical Science, Brian Welch, November 18, th – 3 rd order shim component fitting results to data (thanks Saikat) Most of the field change is the 0 th order component Contribution from higher order components is small – cannot explain the spatial phase differences in the Vanderbilt PDF document

INTERNAL USE ONLY MR Clinical Science, Brian Welch, November 18, Discussion Cleveland 7T data and recent VUIIS 7T data do not show the phase stability jumps observed on the VUIIS 7T back in July Cleveland 7T exhibits a drift of approximately ppm/hour, ~1/2 the specification from Magnex of 0.05 ppm/hour VUIIS 7T exhibits a drift of approximately ppm/hour, ~1/4 the specification from Magnex of 0.05 ppm/hour Field drifts of magnitude up to 0.05 ppm/hour could cause up to 20 ° phase difference over the course of a 16 ms RF pulse for scans repeated ~15 minutes apart The sparse spokes calibration procedure should be faster or should include interleaved reference scans (repeated acquistions of the 0 th spoke) to account for field drift during calibration Field drift alone cannot explain the spatial pattern of the drift presented in the VUIIS PDF document

INTERNAL USE ONLY MR Clinical Science, Brian Welch, November 18, Take home points The static field does drift (as expected) on both the Cleveland 7T and VUIIS 7T The drift is within the specification from Magnex The VUIIS 7T appears to have ½ the drift of the Cleveland 7T The VUIIS 7T drift of ~ ppm/hour could explain ~5 ° of phase errors over the length of 16 ms pulse for measurements taken ~15 mins apart Data collected on the VUIIS 7T showed phase jumps that are not present in the Cleveland data nor in data recently collected on the VUIIS 7T High order fits to the dynamic field maps collected on the VUIIS 7T in July show primarily a 0 th order (spatial DC) effect The field drift can only explain part of the observed phase differences observed between sparse spoke calibration scans with respect to both the magnitude (up to 20 °) and spatial frequency (not just a DC effect) However, static field drifts on the order of the Magnex spec of.05 ppm/hour could at least explain phase difference of the magnitude observed so far (~20°) Regardless of the source of the phase changes, the sparse spokes calibration process should be made faster and more robust