Introduction to Magnetic Resonance Spectroscopy Atiyah Yahya Ph.D., FCCPM, P.Eng. Department of Medical Physics, Cross Cancer Institute Department of Oncology, University of Alberta Edmonton, AB, Canada

Conflicts of Interest None.

Learning Objectives At the end of the session the audience should have a basic understanding of the acquisition of MRS spectra, the challenges involved in interpreting spectra and some MRS quality control tests.

Introduction to NMR Preparation of the nuclear system 1H nuclei z, Bo Mo y Excitation of the nuclear system Mo Bo y x z o = Bo x Absence of magnetic field Presence of a static field Bo RF coil: A resonating LC circuit tuned to o creates an excitation pulse. 90 pulse

Acquisition of the nuclear signal RF coil: A resonating LC circuit tuned to o C C To Receiver M C o spectrum  FT time signal

How can we use NMR spectroscopy? Non-invasive biochemical analysis. Metabolite concentrations in the brain. 1H is the most abundant and most sensitive NMR viable nucleus in the body. NMR spectrum from a rat brain at 9.4 T. Tkac et al. (1999)

Metabolite concentrations are on the order of a few millimoles (mM) About 10-4 times the concentration of water. Water suppression techniques need to be implemented to minimize the water signal. Gradient RF Frequency selective water excitation time

Two important phenomena of spectroscopy Chemical shift Chemical shielding arises because nuclei in different chemical environments are bonded by different electronic orbitals and therefore experience different electronic magnetic fields. The electronic magnetic fields give rise to a shift in the resonance frequency called the “chemical shift”. electron Beff = Bo(1-) nucleus

TMS is tetramethylsilane, (CH3)4Si, and its proton signal is defined as the zero ppm reference. Chemical shift = a = (a - TMS)/o x 106 ppm ppm = parts per million At Bo = 3 T, fo = 127.8 MHz and 1 ppm corresponds to 127.8 Hz. Expressing chemical shifts in ppm makes them field strength independent. The chemical shift frequency difference between peaks increases linearly as function of Bo. Spectral resolution improves with increased Bo.

3 T 9.4 T mI mI Glx Tau + mI Lac Glx + mI Cho Cr mI Cr Lac Glu mI Glx Gln Lac 4 3.5 3 2.5 2 1.5 Chemical shift (ppm)

Scalar coupling (J-coupling) Nuclei also interact with each other, via the electrons in the bonds joining the nuclei, causing splitting in the resonant peaks of the spectrum. Example: C C H J CH3-CH-COOH OH Lactate For lactate (Lac), J is about 6.9 Hz.

Peaks are separated by J Hz CH3-CH-COOH OH Lactate C C H J Peaks are separated by J Hz 4.1 chemical shift (ppm) 1.3 CH CH3

3D-localization using PRESS (Point RESolved Spectroscopy) Acquisition 90x 180y 180y RF gradients time TE1/2 (TE1+TE2)/2 TE2/2 Gx Gy Gz time = slice selection gradients = spoiler gradients = slice selective pulses

STEAM Sequence STimulated Echo Acquisition Mode – another single shot localization MRS sequence. 90x 90x 90x RF gradients time TE/2 TM TE/2 Gx Gy Gz time = slice selection gradients = spoiler gradients = slice selective pulses

Typical brain spectrum 4.0 3.0 2.0 chemical shift (ppm) mI+Tau Cr+PCr Glx mI+Glx mI+Gly Cho NAA+GABA Asp NAA Glx+GABA 3T 8 cm3 volume. Healthy volunteer. PRESS sequence (TE = 30 ms, TR = 3 s)

Quantification Challenges 4.0 3.0 2.0 chemical shift (ppm) mI+Tau Cr+PCr Glx mI+Glx mI+Gly Cho NAA+GABA Asp NAA Glx+GABA Overlap of peaks Macromolecule baseline Spectral fitting software Long TE to enable macromolecule T2 decay Spectral editing

Example of spectral editing: long-TE PRESS for myo-inositol Cr + PCr + GABA Cr + PCr Cho TE1=TE2=15ms mI+Glx mI+Gly Glx mI+Tau Cho Cr + PCr + GABA TE1 = 36 ms TE2 = 160 ms Cr + PCr mI+Gly Glx 4.0 3.0 chemical shift (ppm) Kim et al. Magnetic Resonance in Medicine, 53, p.760, 2005

Response of glutamate to PRESS TE1 = TE2 = 10 ms TE1 = TE2 = 20 ms PQ MN PQ MN A M N P Q Glu O C 1 2 5 3 4 A A TE1 = TE2 = 30 ms TE1 = TE2 = 40 ms TE1 = TE2 = 50 ms TE1 = TE2 = 60 ms 3.75 ppm 2.25 3.75 ppm 2.25

Spectroscopic imaging NAA NAA Cho Cr Cho Cr NAA NAA Cho Cr Cho Cr http://mri.kennedykrieger.org/images/spectro_page2.jpg

Spin echo spectroscopic imaging = slice selective pulses 90x 180y = slice selection gradients time TE/2 TE/2 = spoiler gradients Gz time = phase encode gradients Gx time Gy time

Voxel resolution = 200/12  16.7 mm For example: Field of view (FOV) = 200 mm 12 x 12 phase encodes Voxel resolution = 200/12  16.7 mm Voxel volume = (16.7 x 16.7 x slice thickness) mm3 Acquisition time = 12 x 12 x repetition time 200 mm 200 mm http://www.lahey.org/Images/Radiology/ClickableImages/BrainMRI_Axial.jpg

Examples of metabolites relevant to tumors NAA = N-acetyl aspartate Glx = glutamate + glutamine Cr = creatine Cho = choline Ino = inositol Spectrum from white matter. Spectrum from a low grade oligodendroglioma. Lac Lac = lactate Spectrum from a high grade oligodendroglioma. Rijpkema et al., NMR in Biomedicine, 2003

Quality Control for MRS Use a phantom dedicated for MRS quality control (QC) Use the same setup each time and the same RF coil e.g. head coil Establish baseline values

D-C. Woo et al., Development of a QA phantom and protocol for proton magnetic resonance spectroscopy, Concepts in Magnetic Resonance, v. 35B, p. 168 (2009) D. Drost et al., Proton magnetic resonance spectroscopy in the brain: Report of AAPM MR task group #9, Med. Phys., v. 29, p. 2177 (2002) S. Nicolosi et al., Minimal protocol for MRS quality control and acceptance testing for Philips-Achieva MRS tool, https://arxiv.org/abs/1004.1026, 2010

Example Protocol 3 T, transmit receive head coil, double compartment MRS phantom placed at coil centre 2 x 2 x 2 cm3 PRESS voxel in centre of inner compartment, 1 x 1 x 1 cm3 in outer compartment Spectral width = 2000 Hz, number of samples = 4096, number of averages = 32, echo time = 100 ms, repetition time = 4 s 6 cm 9 cm

Double compartment phantom to verify spatial localization 10 mM creatine (Cr) Double compartment phantom to verify spatial localization 20 mM acetate

10 mM creatine (Cr) 20 mM acetate

Resonance Frequency Record resonance frequency of water, e.g. 127,806,294 Hz Record receiver gain Record chemical shifts of peaks

SNR Record peak area Peak height standard deviation of noise Record peak area

Peak Linewidth (FWHM) 1.6 Hz

Water Suppression Record height of residual peak Or acquire a non-suppressed water spectrum and determine % suppression

Questions? Concluding Remarks Overview of MRS Techniques used in MRS Quantification challenges Spectroscopic imaging QC protocol for MRS Questions?