brian welch mr clinical science 7t phase stability november 18, 2009 7t phase stability and sparse...

Brian WelchMR Clinical Science7T Phase StabilityNovember 18, 2009

7T Phase Stability and Sparse Spokes

2INTERNAL USE ONLY MR Clinical Science, Brian Welch, November 18, 2009

Outline

• Background

• Sparse Spokes Phase Instability Document Prepared by Vanderbilt

• Surrogate Phase Stability Scan (dynamic 3D FFE B0 maps)

• Cleveland stability results collected 2009.10.06

• Vanderbilt stability results collected 2009.11.17

• Previous Vanderbilt stability results collected 2009.07.07

• 0th – 3rd order shim component fitting results to 2009.07.07 data

• Discussion

• Take home points


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, 23.04 ° could be accrued from a 4 Hz off-resonance.


Sparse Spokes Phase Instability Document Prepared by Vanderbilt (June 4, 2009)

(click above to open embedded PDF)


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 26.25 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

ExamCard Zip File

(click above to open embedded ExamCard zip file)


Cleveland stability results collected 2009.10.06AUTO SHIM PBVOL SHIM

driftfrom10 to14 HzOver30

Mins

driftfrom38 to42 HzOver30

mins

~0.025 ppm/hour ~0.025 ppm/hour


Vanderbilt stability results collected 2009.11.17AUTO SHIM PBVOL SHIM

driftfrom

-28.8 to-27.6 Hz

Over30

mins

driftfrom

41.5 to43.5 Hz

Over30

mins

~0.0125 ppm/hour ~0.0125 ppm/hour


Previous Vanderbilt stability results collected 2009.07.07

~1 Hz phase jumps

30 min scan

60 min scan

15 min scanwith dynamicstabilization

Coarse resolution of dyn stab causes jumps of ~1Hz


0th – 3rd order shim component fitting results to 2009.07.07 data(thanks Saikat)

Z0

0

1

2

3

4

5

1 8 15 22 29 36 43 50 57 64 71 78 85 92 99

Dynamic

Hz

Z0

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1 9 17 25 33 41 49 57 65 73 81 89 97

Dynamic

Hz/

cm

Z

X

Y

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

1 9 17 25 33 41 49 57 65 73 81 89 97

Dynamic

Hz/

cm^

2

Z2

ZX

X2-Y2

ZY

2XY

• Most of the field change is the 0th order component

• Contribution from higher order components is small – cannot explain the spatial phase differences in the Vanderbilt PDF document


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 0.025 ppm/hour, ~1/2 the specification from Magnex of 0.05 ppm/hour

• VUIIS 7T exhibits a drift of approximately 0.0125 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 0th 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


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 ~0.0125 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 0th 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

brian welch mr clinical science 7t phase stability november 18, 2009 7t phase stability and sparse...

Documents