MRS is a non-invasive MRI technique that detects biochemicals in tissues and provides information to diagnose and monitor diseases. It detects small metabolites that are suppressed in standard MRI. Different metabolites including NAA, Cr, Cho, ml, Lac, Glx and lipids provide information about neuronal integrity, cell membrane turnover, hypoxia and other disease states when their levels are abnormal. MRS acquisition and interpretation requires high magnetic field homogeneity and specialized pulse sequences like STEAM and PRESS. Metabolite levels are assessed relative to Cr and compared to normal values.

1. Rahman Ud Din
Lecturer Medical Imaging
NWIHS

2.  MRS an exciting application of MRI
 Non-invasive technique
 Used to assess various metabolites or
biochemicals from the body tissues
 This metabolites information used to
diagnose, monitor and follow-up diseases
 1H, 13C, 19F, 23Na and 31P (theoretical MRS
can be done with)
 1H & 31P (mostly clinical used MRS)
 We will discuss 1H spectroscopy

3.  For the brain in particular,
 MRS has been a powerful research tool
 and provide additional clinical information
for several disease
 such as brain tumors,
 metabolic disorders, and
 systemic diseases

4.  Principles are same as MRI
 But few differences are
 In MRI, images are reconstructed from all protons
in the tissue, water and fats dominate only
 Contrary, MRS detect small metabolites of
clinical interest (resonate of frequency b/w
water and fats)
 Small metabolites are detected when large signal
are suppressed

5.  How are small metabolites from the tissue
detected?
 Chemical shift
 Chemical environment or electron cloud around
protons
 Protons in water, fat and other compound precess at
different rate
 Such change in frequency is called chemical shift
 Frequency of p+ in metabolites=chemical shift=position of
metabolites peaks
 Measured in ppm
 Chemical shift is proportional to B°, need high Tesla
MRI
 Can be performed on 0.5T or above
 1.5T or above required for spectral separation & ↑SNR

6.  Magnetic Field Homogeneity
 MR field must be homogeneous
 MRS require more homogeneous field than MRI
because smaller chemical shift needs to detect
 In inhomogeneous field, smaller chemical shifts
can be misinterpreted and incorrect
concentration may be recorded
 Homogeneity required for MRI is about 0.5 ppm
where for MRS it is about 0.1 ppm
 The process of making the magnetic field
homogeneous is called ???!!!!!
 Shimming

7.  No Frequency Encoding Gradient in MRS
 Similar to MRI, localisation is done by slice
selection and phase encoding gradients
 The frequency encoding gradient is not used in
MRS to preserve optimal homogeneity and
chemical shift information
 The spin-spin or J-coupling
 Spins (p+) with a small difference of precessional
frequency, for example spins within a molecule,
interact with each other
 This is facilitated by electrons around a nuclei
 This spin-spin interaction modifies the resonant
frequency of the spins involved in it
 J-coupling causes fusion of peaks on spectral map
e.g. doublet of lactate at 1.3ppm

10.  In early days, localisation done on surface coil
 Area covered by surface coil was VOI
 From VOI metabolites information obtained
 In current practice, four methods used
 STEAM
 PRESS
 ISIS and
 CSI (MRSI)
 First three are single voxel MRS tech:
 CSI used for multivoxel MRS technique

11.  STEAM
 Stimulated Echo Acquisition Method
 VOI excited by 90 degree pulses in three
orthogonal planes
 Echo stimulated signal is weak
 STEAM used for short TE(20-30 ms) spectroscopy
 PRESS
 Point Resolved Spectroscopy
 One 90 degree pulse and two 180 degree pulses
are applied along three orthogonal planes
 The signal is strong with better SNR, PRESS used
for long TE (135, 270 ms) spectroscopy

12.  ISIS
 Image Selected In vivo Spectroscopy
 Three frequency selective inversion pulses
applied
 In the presence of the orthogonal gradients
 Fourth non-invasive pulse is used for the
observation of signal
 ISIS is used in 31P spectroscopy
 CSI Chemical Shift Imaging
 Multivoxel Spectroscopy
 Large area divided into multiple voxels
 Also called MRSI (imaging and spectroscopy)
 Localisation is done by phase encoding in one,
two or three directions (1D, 2D or 3D spectro..)

13.  Patient positioning
 Global Shimming
 Acquisition of MR images for localisation
 Selection of MRS measurement and parameters
 Improved SNR at long TR
 TEs 20-30 ms, 135-145 ms and 270 ms
 TEs longer than 135 ms, peaks of major metabolites
i.e. Ch, Cr, NAA and lactate visible
 Other metabolites peaks are suppressed (short T2)
 Selection of VOI
 SVS (single voxel spectro) local or diffuse diseases
 CSI used large lesion

14.  Local Shimming
 Optimisation of homogeneity over selected VOI
 Good shim results in narrower metabolite peaks
 Better spectral resolution
 Good SNR
 Full width at half height of H2O used as an
indicator of shim
 A local shim of 4-10 Hz is desirable
 Water suppression
 Water peak is suppressed so that smaller
metabolite peaks are visible
 Water peak suppression is done by CHESS
(chemical shift selective spectroscopy) tech

15.  MRS data collection
 Modern machine in use, SVS takes 3-6 minutes and CSI
takes up to 12 minutes for data acquisition
 Data processing and display
 Acquired data is processed to get spectrum and spectral
maps
 Zero point of spectrum is set in the software itself by
frequency of a particular compound called
Tetramethysilane (TMS)
 Interpretation
 Area under peak of any metabolite is directly
proportional to the number of spins
 Absolute values for each varies with age and population
 Interpretation based on ratios of metabolites and
comparison with normal side

17.  NAA: N-Acetylaspartate
 Peak position: 2.02 ppm
 There is some contribution from NAAG and
glutamate to the NAA peak
 It is a neuronal marker and any insult to the brain
causing neuronal loss or degeneration causes
reduction in NAA
 It is absent in tissues/lesions with no neurons e.g.
metastasis and meningioma
 NAA is reduced in: hypoxia, infraction,
Alzheimer’s, herpes, encephalitis, hydrocephalus,
Alexander’s disease, epilepsy, some neoplasms,
stroke, NPH, close head trauma,
 NAA increased in: Canavan’s disease

18.  Cr: Creatine
 Peak position: 3.0 ppm. Second peak at 3.94 ppm
 Cr peak  creatine, CrPO4, GABA (Gamma-
aminobutyric acid), Lysine and glutathione
 Cr serves as high energy phosphate and as a buffer
in ATP/ADP reservoir. Increase in amount with age
 Cr increased in: hypometabolic states and in trauma
 Cr reduced in: hypermetabolic states, hypoxia,
stroke and some tumor
 Cr remain stable in many disease hence serves as
reference or control peak for the comparison

19.  Cho: Choline
 Peak position: 3.22 ppm
 Choline is phospholipids of the cell membrane
 Precursor of acetyl choline and phosphatidylcholine
 Indicator of cell membrane integrity
 Cho increases with increased cell membrane
synthesis and increased cell turnover
 Cho is increased in: chronic hypoxia, epilepsy,
Alzheimer’s, gliomas and some other tumors,
trauma, infraction, hyperosmolar states, diabetes
mellitus
 Cho is reduced in: hepatic encephalopathy and
stroke

20.  ml: Myo-inositol
 Peak position: 3.56 ppm, second peak at 4.1 ppm
 MI acts as an osmolyte and is marker of gliosis
 Involved in hormone sensitive neuroreception and is
precursor of glucuronic acid
 It is the dominent peak in newborn babies and
decrease with age
 MI is increased in: Alzheimer’s, frontal lobe
dementias, diabetes and hyperosmolar states
 MI decreased in: hepatic and hypoxic
encephalopathy, stroke, tumor, osmotic pontine
myelinolysis and hyponatremia

21.  Lac: Lactate
 Peak position: 1.3 pm
 Doublet
 Inverted at TE of 135 ms with PRESS, upright at
other TEs on PRESS and at all the TEs with STEAM
sequences
 Not seen in normal brain spectrum
 Elevated in hypoxia, tumor, mitochondrial
encephalopathy, intracranial hemorrahage, stroke,
hypoventilation, Canavan’s disease, Alexander’s
disease and hydrocephalus

22.  Glx: Glutamate (Glu) and Glutamine (Gln)
 Peak position: 2-2.45 ppm for beta and gamma Glx
 Second peak of alpha Glx at 3.6-3.8 ppm
 Glu is excitatory neurotransmitter and GABA is
product of Glu
 It has role in detoxification and regulation of
neurotransmitter activities
 Glx peak is elevated in head injury, hepatic
encephalopathy and hypoxia

23.  Lipids
 Peak position: 0.9, 1.3, 1.5 ppm
 Not seen in normal brain spectrum
 Seen in acute destruction of myelin
 Increased in high grade tumors (reflects necrosis),
stroke and MS lesions
 Aminoacids
 Alanine (at 1.3-1.4 ppm), Valine (0.9 ppm) and
leucine (3.6 ppm)
 Usually multiplets visualised at short TE
 Inverted at 135 ms
 Alanine is seen in the meningioma whereas valine
and leucine are marker of abcess

24.  Glucose
 When present, seen next to Cho peak on its left
side
 May increases in diabetes, parenteral feeding and
hepatic encephalopathy
 GABA
 Peak position: 1.9 and 2.3 ppm
 Used for monitoring of vigabatrin therapy given in
children with myoclonic jerks