mri-a : magnetic resonance spectroscopy › sigmi › 2015 › data › 151230_groupe.pdf ·...
TRANSCRIPT
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)
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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