MR Spectroscopy Basic physics of spectroscopy uses and diagnostic value

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