‘REDUCTION OF SYSTEMATIC ERROR IN MRSI BASED ABSOLUTE BRAIN TEMPERARTURE MAPPING

1. ‘REDUCTION OF SYSTEMATIC ERROR IN MRSI BASED ABSOLUTE BRAIN TEMPERARTURE MAPPING Jehill Parikh1, Michael Thrippleton1 and Ian Marshall1,2 • Brain Research Imaging Centre, Dept. of Clinical Neurosciences, (part of SINAPSE collaboration), 2. Centre of Clinical Brain Science, University of Edinburgh COMPUTER 61 Date 10 May 2012 Time 13.30

2. MR THERMOMETRY • Brain Temperature Stroke, Traumatic Brain injury, Birth Aphasia (neonates), Schizophrenia and Brain Cooling • Proton frequency Shift Linear Temperature coefficient 0.01ppm/oC • MRSI TECHNICALITIES • FREQUENCY SELECTIVE WATER SUPPRESSION (CHESS) • CHEMICAL SHIFT MIS-REGISTRATION T =Tref - k (CSH2O -CSref)

3. METHODS • MRSI DATA ACQUIRED FROM HOMOGENENOUS ROOM TEMPERATURE PHANTOM AND VOLUNTEERS • GE 1.5T SIGNA scanner (GE Healthcare Slough UK) • 2D PRESS-CSI SEQUENCE, TE-144ms Tr-1s, imaging matrix 24*24 (interpolated to 32*32), spectral width=1000Hz, acquisition time 9min 40sec, Axial slice 10 mm, FOV 320,voxel volume-1ml was employed • Automatic first order shimming was used • CHESS based water suppression • 3 fixed Bandwidth (BW) 3 RF pulses • WS level changed by varying the delay τ between the CHESS and PRESS Processing Details Marshall et al, MRI 2006 pg 699-706

4. Systematic error in Temperature maps Default water suppression settings BW=75Hz and τ=48ms, WS factor 90 Temperature estimated using T=37-100(CSwater-CSnaa) A systematic error of 0.6C can be identified in Temperature maps

5. Effect of varying BW of the CHESS pulses BW:75Hz BW:85Hz BW:95Hz BW:105Hz The systematic error in temperature maps decreases by increasing the BW of the RF pulse employed for the CHESS

6. Effect of varying delay τ between CHESS and PRESS τ :48ms τ :68ms τ :78ms τ :88ms τ :58ms Weakening the water suppression by increasing τ reduces the systematic error and temperature maps acquired using weak WS are uniform.

7. No Water Suppression Temperature maps acquired using no water suppression are not uniform The presence of the side bands and baseline variations in non WS data introduces errors in estimate of the frequency of the internal reference (i.e. NAA) using AMARES. Thus it is necessary to employ water suppression

8. Volunteer Scans The systematic error cannot be identified in the temperature maps acquired from volunteers The SD (~1.5-2.240C) of the in-vivo temperature estimate, is higher compared to systematic error (~0.50C) as seen in the phantom data

9. Chemical Shift Mis-registration Small but proportional to chemical shift and different for different metabolites Temperature maps estimated by using different metabolites as internal reference

10. CONCLUSION • Water suppression is necessary for accurate temperature estimation using MRSI data • Systematic error is present temperature maps acquired with strong CHESS based water suppression • Weakening CHESS based water suppression reduced the systematic errors in MRSI temperature maps. • Systematic error in temperature maps cannot be identified in vivo data from volunteers and is likely to be masked by the random in the temperature estimates. • Chemical shift mis-registration has little influence on MRSI temperature mapping.

11. Acknowledgments Radiographers team @ ‘BRIC’ Funding Medical Physics