KAIST 의료영상 연구회 학생 발표: E팀

MRI-A : Magnetic resonance spectroscopy (박현욱, 정범석 교수님 연구실)

Contents

• Acquisition of functional data • Functional task based on prior study by Prof. Jung

• MRS data acquisition • Regions of interest (ROI) & target metabolites • Acquisition steps • Data quality: mean linewidths per ROI

Introduction to MR Spectroscopy

• What is MR Spectroscopy (MRS) • Perform in-vivo NMR spectroscopy using clinical MRI scanners • Get chemical information within the targeted region

• Single-Voxel Spectroscopy (SVS)

• Detailed spectrum of a single region • No spatial information

• Multi-Voxel Spectroscopy (MRSI)

• Spatially varying spectrum • Lower spectral resolution

• Differences with MR Imaging

• No frequency encoding: FID signal

𝐹𝐹𝐹𝐹𝐹𝐹 Fourier Transform

𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆

Technical issues of MRS

• Proton MR spectroscopy • Acquisition of proton in organic compounds (CH) • Water & Fat signals not desirable

• Overview of issues • VOI placement, shimming and water suppression • MRS pulse sequences • Post-processing of MRS data • Data fitting & analysis

VOI placement • Volume of interest (VOI): target volume for data acquisition

• AutoVOI: registration-based VOI placement tool

• MATLAB based implementation, uses ITK, FSL-BET, etc. • Calculation time ~1min

Testing at KAIST

Shimming and water suppression

• Proton spectroscopy • Resonance signal from H+ mostly in water (MRI) • Signal ratio between water & metabolites (~10,000:1)

• Water suppression

• Selectively excite water and then de-phase it so its signal is not visible in the spectra

• Metabolite signal only visible after suppression

• Importance of good shimming • Narrow-band suppression pulse: accurate targeting of water • Measure of shimming: FWHM of water signal (less than 10Hz) • FAST(EST)MAP shimming: automated shimming method

Water suppression simulation (VAPOR, Time)

• Longitudinal Magnetization vs. Time (assume transverse is zero, FA = 90deg)

7

β 1.778 β β β

1.778 β 1.778 β

1.585 β

β

𝟒𝟒𝟒𝟒𝟒𝟒 = 10 log10 𝑥𝑥2 0.4 = log10 𝑥𝑥2 100.4 = 𝑥𝑥2 = 2.512 𝑥𝑥 = 2.512 = 𝟏𝟏.𝟓𝟓𝟓𝟓𝟓𝟓

𝟓𝟓𝟒𝟒𝟒𝟒 = 10 log10 𝑥𝑥2 0.5 = log10 𝑥𝑥2 100.5 = 𝑥𝑥2 = 3.162 𝑥𝑥 = 3.162 = 𝟏𝟏.𝟕𝟕𝟕𝟕𝟓𝟓

VAPOR water suppression simulation (T1)

• Longitudinal Magnetization vs. T1 (FA = 90deg) • WET & VAPOR: Improved WS performance in varying T1 against

single or 3 identical CHESS pulses

8

VAPOR water suppression simulation (B1)

• Longitudinal Magnetization vs. B1 (T1 = 1500 ms) • VAPOR: Improved WS performance in varying B1 against others

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MRS Phantom experiments @ 7T

• Dotted line: default WET suppression by Siemens

10

Data fitting & analysis

• Simulation of MR spectra • Define metabolite & pulse

sequences (Hamiltonian) • Solve for Schrodinger

equation

• Availability of simulation tools

• VeSPA (pyGamma): Simulate NMR & MRS spectrum

Acquisition of functional data

1) Trauma-specific Stimuli (Modified emotional Stroop task)

2) Facial Stimuli (Gender discrimination task)

Event related design Block design

씨발

의자

전쟁

3s

….

SAD NEU CONT NEU

20s

HAP

8 stimuli (2s stim+0.5s rest)

NEU

The group differences in left VLPFC functional connectivity with the left amygdala and left hippocampus.

1) Trauma-specific Stimuli (Modified emotional Stroop task)

The overall brain connectivity from (A) right amygdala and (B) left amygdala during negative emotion processing. X, Y, and Z represents X, Y, and Z coordinate of MNI space. Cluster-level threshold was applied in fMRI analysis

2) Facial Stimuli (Gender discrimination task)

3) Volume change in Hippocampus subfield

MRS Overview

• Regions of interest • Medial prefrontal cortex, (20x15x20mm3) • Hippocampus in both right and left hemispheres

(13x27x12mm3)

• Target neurotransmitter • Glutamate / Glutamine • GABA

MRS data acquisition

• Shimming with FAST(EST)MAP • Flip angle calibrations • VAPOR water suppression calibrations • Sequence parameters

• Semi-LASER: 90-deg. sinc pulse (2000ms) + 180-deg. HS adiabatic refocusing pulse (4000ms)

• TR = 4000ms, TE = 28ms, ADC BW 6000Hz, 2048 samples • VAPOR water suppression with OVS (fat-sup.) pulses

• Reference data acquisition

• Water reference • ECC reference (water sup RF OFF, but gradients ON) • Water T2 decay reference (for CSF quantization)

MRS data acquisition

• About 20 mins per ROI (1hr MRS scan) • 15 subjects since November • All datasets under 10Hz linewidth

• Mean signal quality

MPFC LHC RHC Water peak linewidth

7.72(+0.92)Hz 8.25(+0.62)Hz 8.49(+0.75)Hz

MRS data analysis