MRS

Introduction

• There are numerous metabolites found in the human brain.

• Fortunately, only several of them occur in significant quantities and are useful in proton spectroscopic studies.

• There is evidence that the normal metabolites in the brain vary with according to the patient's age.

• The changes are most noticeable during the first three years of life.

• Most of the metabolites are involved in energy metabolism.

• Presented here is a diagram of the major biochemical pathways in the energy production.

MR SPECTROSCOPY

• Detection of frequency dependent signals from individual metabolites

• Interpretation is based on identity of chemical and concentration

• Baseline normal spectra - constant • Concentration of each metabolites alter in a

reproducible pattern - Abnormal spectra = DISEASE PATTERN

METABOLITES

• Cho - Cell membrane turn over• Cr - Energy marker -Reference• NAA - Neuronal cell marker• mI - Osmolyte• Lactate - Anerobic state - NOT SEEN

IN NORMAL BRAIN

NAA Regional Variations

• NAA peak – Highest due to N acetyl group• Marker of neuronal / axonal viability and

density• Evenly distributed in Cerebral hemisphere • Less in hippocampus and cerebellum• NAA - Neuronal marker • Decreases with loss of neuronal integrity. 

Creatine

• Energy stores. • Cr 1 - 3.0 ppm • Cr 2 - 3.9 ppm • Marker of intact brain Energy Metabolism.• Reference for interpretation of ratio.• Higher in grey matter than white matter• Higher in thalamus and cerebellum

Choline

• Cho - 3.2 ppm • Present in Cell membrane• Cell membrane turnover• Choline released during disease from pool• Choline - Increased with increased cellular

turnover• Elevated in tumors and inflammation

Choline Regional Variations

• Slightly higher in white matter than gray• Higher in thalamus and cerebellum• More choline in pons and terminal zones

of myelination

Myo-Inositol

• Cell Volume Regulator - Osmolyte • mI - 3.5 ppm ; 4.0 ppm.• Present in astrocytes• Astrocyte /glial marker - Product of myelin

degradation

Lactate

• Accelerated glycolysis /Anaerobic glycolysis• Lac - 1.3 ppm – Doublet• Inverts with TE 144 or 135 ms• Normal in preterm / term infants & CSF

contamination

Lipids• Lip 1 - 0.9 ppm; Lip 2 - 1.3 - 1.4 ppm• Broad based • Sign of brain injury• Normally Bound - Not seen• Seen when there is cell death and cell membrane

destruction• Indicates necrosis and / or disruption of myelin• Difficult to differentiate from macromolecules• Non significant lipid – from scalp contamination

Glutamate & Glutamine (Glx)

• Neurotransmitters• Beta, Gamma Glx - 2.0 - 2.5 ppm• Alfa Glx - 3.6 - 3.8 ppm

NORMAL H BRAIN SPECTRA

• A spectrum of the metabolites is plotted on a two dimensional graph. – The horizontal axis represents the frequencies (chemical

shifts) and the vertical axis represents the concentration of the metabolites.

• The frequencies are plotted with reference to a stable compound. – The reference compound most often used is

tetramethylsilane (TMS). The chemical shifts are now expressed as parts per million (ppm).

• This approach allows for consistent spectra regardless of the field strength.

• It also provides a standardization of the spectrum . – TMS is assigned a chemical shift of zero ppm and it lies to

the far most right hand . – The far left end is occupied by water approximately 4.7

ppm. – The area under a peak is contributed by the concentration

of that metabolites. – Therefore, higher or wider peak results from higher

concentration.

• The 1H-MRS spectrum of major metabolites in a normal brain is shown in Fig 1 below:

Major Metabolites in the Brain

• NAA• Choline• Creatine• Lactate• Glutamine• Lipid• Myo-Inositol

N-ACETYLASPARTATE (NAA)• NAA is the marker of neuronal density and viability.

– It is present in both gray and white matter and the difference in concentration is not clinically significant.

– NAA is detected by the its N-acetyl methyl group. – Its concentration appears to decrease with any brain insults such as

infection, ischemic injury, neoplasm, and demyelination process. • NAA is not in found in tumors outside the central nervous

system (CNS) such as meningioma. • NAA is the tallest peak in the proton MR spectrum and it is

assigned at 2.0ppm. Additional smaller peaks may be seen at 2.6 and 2.5 ppm.

• Elevation of NAA is rare and may be found in hyperosmolar state and axonal recovery.

Choline• The choline peak receives contribution from

glycerophosphocholine, phosphocholine, and phosphatidylcholine. – It is the precursor of acetyl choline and

phosphatidylcholine. – Acetylcholine is an important neurotransmitter and the

latter is an integral part of cell membrane synthesis. • Disease processes affecting the cell membrane and myelin can

lead to the release of phosphatidylcholine. • Thus, elevation of choline can be seen during ischemic injury,

neoplasm or acute demyelination diseases. • Many brain tumors will lead to elevated choline peak,

presumably associated with their increased cellularity and compression of surrounding brain tissue.

• Choline is the second largest peak and assigned to 3.2 ppm.

Creatine (Cr)• The Cr peak receives contribution mainly from creatine, and creatine

phosphate. – Note that phospocreatine supplies phosphate to adenosine diphosphate (ADP)

to form adenosine triphosphate (ATP) with the release of creatine. • The overall level of total creatine in normal brain is fairly constant.

– Reduced Cr level may be seen in pathologic processes such as neoplasm, ischemic injury, infection or some systemic diseases.

– Most metastatic tumors to the brain do not produce creatine since they do not possess creatine kinase.

– Therefore, metastatic tumors should be suspected if there is an absence of a Cr peak in the proton spectrum.

• Cr is the third highest peak and is assigned to 3.03 ppm. It is usually seen next to the right of choline. An additional peak occurs near 4 ppm but is usually suppressed with water.

Lactate• Lactate has a molecular structure of CH3-COH2-CO2. • Lactate levels in the brain are normally are very low or

absent. When oxygen supply is depleted, the brain switches to anaerobic respiration for which one end product is lactate.

• Therefore, elevated lactate peak is a sign of hypoxic tissue. – Low oxygen supply can result from decreased oxygen supply or

increased oxygen requirement. – The former may be seen in vascular insults, or hypoventilation and the

latter may be seen in neoplastic tissue.

Lactate• Resonance of lactate consists of two distinct peak (doublet)

due to J-J coupling. • Lactate peak also occurs at two different locations.

– The lower field peak (a doublet) occurs at approximately 1.32 ppm.– The other peak (a quartet) is seen at 4.1 ppm and this is very close to

the water peak. • usually suppressed during data processing.

• The lactate peak at lower frequency field show a peak inversion for different TE's. – This property serves as an excellent confirmation for the presence of

lactate. The lactate peak is above the baseline at TE of 270 ms and below the baseline at TE of 136 ms. The inversion arises from the weak J-J coupling from the CH3 and CH protons. The coupling constant J is 7Hz.

Myo-Inositol (mI)

• Myo-Inositol is a glucose-like metabolite and it involves primarily in hormone-sensitive neuroreception. It is found mainly in astrocytes and helps to regulate cell volume.

• Elevated level of mI would be seen where there is glial cell proliferation as in gliosis. – Depressed level of mI would be seen processes causing

glial cell destruction , as in neoplasm, infection or ischemic injury. The main mI peak is assigned to 3.56 ppm and additional peak may be seen at 4.06 ppm.

Lipids• Lipids are often in composition of triglycerides, phospholipids,

and fatty acids. • These substances are incorporated into cell membranes and

myelin. – Lipid peak should not be seen unless there is destructive process of the

brain including necrosis, inflammation or infection. – Lipids have a very short T1 relaxation time and are normally not seen

unless short TEs are utilized. • The proton lipid peaks occur at several frequencies including 0.8, 1.2, 1.5

and 6.0 ppm. • Lipid resonance at 1.2 ppm can sometimes obscure the lactate peak at

1.32 ppm. • Fat in the cranium can contaminate the true disease process if the voxels

are placed too close the cranium.

Glutamate and Glutamine (Glx)

• Glutamate is an excitatory neurotransmitter in mitochondrial metabolism.

• Glutamine and glutamate resonate closely together.

• Their sum is often designated as Glx and is assigned between 2.1 and 2.5 ppm.

• Glutamine is astrocyte marker• Glutamate – Neurotransmiter - neurotoxin in

excess amount.• Main ammonia intake route• Elevated - In hypoxia, ischemia, recovering

brain.• Its not a grave prognostic finding like lactate

T2/FLAIR For ROI

MRS acquisition modes

• STEAM -Stimulated echo acquisition mode• Single voxel • Short TE

• PRESS -Point resolved spectroscopy• Twice the SNR of STEAM• Short and long TE - single voxel possible

PROBE - Single voxel proton MRS

• Fully automated prescan, scan• shimming• water suppression• 2 -6 minutes Complete acquisition• Short TE (PRESS, STEAM) and• long TE (PRESS)

Single voxel proton MRS

Multi voxel MRS

• Variable voxel sizes• More than one lesion• Control from normal

Acquisition Parameters

TR - 1500 msTE - 35 msNAV - 64Voxel Size - 2 x 2 x 2 cmVoxel Location:-

Cingulate gyrus -GMParietal -WM

Chemical Shift Imaging in TumorsSpatial distribution of metabolites

METABOLITES

SHORT TE 35

• mI• LACTATE• LIPIDS• GLUTAMATE / GLUTAMINE

BOTH SHORT 35 AND LONG TE 144

• NAA• CREATINE• CHOLINE• LACTATE signal lowered

Lactate Vs Lipid

• Lactate is doublet• Inversion below the baseline at 144 ms• Persists at TE 270 ms.

• Lipid peak is broad• Has a shoulder to left• Suppressed at TE 270 ms

Lactate

TE 35 TE 144

TE 270

LIP-LACTATE

Tumor Biochemistry

• Understood by identifying important metabolites and quantifying them.

• Comparing with normal and benign tissues, we can understand metabolite markers and grade them.

Alteration of metabolites in Brain Tumors

• Decreased or absence of N – Acetyl Aspartate (NAA) (Non-neuronal and NAA is only found in neurons)

• Decreased Creatine• Increased Choline• Appearance of Lactate (Anaerobic glycolysis)• Myo-Inositol may distinguish hemangiopericytomas from

meningiomas• Glutamine and Glutamate are prominent in meningiomas

FAQ

• Is it a tumour • GBM/ Metz/ Abscess?• Grade?• Survival?• ? Oligodentroglioma?

Tumour?

• D/D -Stroke, Focal cortical dysplasia, Herpes and Neoplasm

• ^ Cho – Neoplasm• Always exclude Demylination - ^ Cho

GBM/Metz/Abscess

• Multivoxel PRESS sequence with intermediate TE -for elevation of Cho in enhancing rim and in peri-lesional T2 hyperintensity

• If Cho is elevated in both areas - GBM• Elevated in rim; N –Around - Metz• Detection of peptides and amino acids in Short

TE - Pyogenic abscess

Grade

• Cho/NAA ratio - Most sensitive index for tumor cell density and proliferation.

• Marker of tumor infiltration• High Cho/NAA and Cho/Cr - Fast

growing and high grade neoplasm

Prognosis

• High Creatine levels in grade II gliomas- malignant transformation and poor survival

• High Cho -Pediatric brain tumors

Oligo dentro gliomaSky-rocketing Choline - high cellular density

MR perfusion:Increased- rCBV- high capillary density low level of angiogenesis

Low-grade astrocytoma with elevated lactate

High grade glial tumors

TE 35

TE 135

57 yr M + LOC

After 20 days

Key points

• High Cho - High tumor cell density & high vascular proliferation.

• Low Cho and elevation of lipids - Necrosis.• Cho higher enhancing rim -may be the faster growing

side of the tumor. • Vasogenic edema -Normal Cho and slightly decreased

NAA.

Spectra of active demyelination indistinguishable from gliomas.MR perfusion may be helpful.

41y focal seizure

Tumefactive multiple sclerosis

• Alanine (Ala) doublet at 1.4 ppm• Elevation of Cho• Presence of Lac at 1.3 ppm.• Absent NAA - Non-neural origin.

• Ala -30–40% of Meningioma• Mobile lipid and high Cho -

aggressive lesions

43 y Focal deficit

Recurrent astrocytoma24 y post Rx

Recurrent astrocytoma

• Normal -1 Infiltrative -2• Solid -3 Early necrotic -4• All with high Cho/ Cr >1.7• High Cho, very low NAA and no lipid -Solid

tumor. • Small Lac without lipid may be early indicator

of transformation to high grade.

• 51y M, GBM Rx RT.

• Reduced Cho, NAA and Cr relative to normal brain indicates necrosis.

Stroke

• Localized decreased NAA - few hours of ischemia.• Very low or absent – chronic infarcts.• Lac is elevated in acute stroke due to anaerobic glycolysis in

ischemic brain.• Creatine and Choline may change in acute and chronic stroke. • Lipid - Reflect necrosis.• MRS is added value to diffusion and perfusion

Occlusion of the left ICA /MCA @ 24 h

Left - Elevated Lac and near absent NAA

Occlusion of Right ICA

With in 24 hr

Lac- ElevatedNAA- Preserved

Follow-up @ 1 wk

absence of NAALac in infarct region. High Cho in peri infarct WM

Follow-up @ 5 m

Reduced NAAhigh Cho in WMNo LacLipid + in infarcted right basal ganglia.

Pyogenic meningitis

large amount of amino acids -1, lactate -2, alanine -3, acetate -4,acetoacetate -5

Pyogenic abscess

135 TEInv Of AA,0.9 ppm, Lac, 1.33 ppm, and Ala, 1.47 ppm peaks

TUBERCULOMA

• Similar on MRI.• Tuberculous abscesses - only Lac and lipid signals @

0.9 and 1.3 ppm; No amino acids • Lipid peaks –In Both tuberculoma and pyogenic

abscess. • Amino acid signals helps to discriminate pyogenic

from tuberculous abscess

• @ 135 TE Inv of AA- 0.9 ppm

Tuberculous vs Pyogenic abscesses

Tuberculous abscessesSTEAM

TE 35, only Lip and Lac at 1.3 ppm.

TE 135 spectrum, phase reversal & reduction in signal

HSE

Findings are due to interstitial edema; MRS - Non-specific.

Fungal abscess

• TE 135 proton MR spectrum from core of abscess - inverted AA and Lac peaks.

• Multiple signal (*) @ 3.6–3.8 ppm- trehalose

Hydatid cyst:

MRS with TE 35 - Lac at 1.33 ppm,acetate at 1.92 ppm, and succinate at 2.4 ppm;

@TE 135 - Lac and Ala at 1.5 ppm show phase reversal while Ace and Suc show normal phase.

Neurocysticercosis

• Spectroscopy may be of value in the large cysticercus cyst without visible scolex, where differential diagnosis includes brain abscess and cystic metastases.

• In vivo MRS shows acetate, succinate and Lac.

• Presence of Cr depending on whether the lesion is in the vesicular or colloid stage

HIV encephalopathy

• Reductions in NAA and increases in Cho, mI in both lesional and normal appearing brain tissues.

• Toxoplasmosis Vs lymphoma– Toxoplasmosis -Very large lipid signals– Lymphoma -Large lipid (smaller than toxo)

-High Cho (not seen in toxo).

D/D

Demyelination

Multiple sclerosis - Axonal damage - Decreased NAA

Demyelination - Increased mI, Cho.

Acute MS plaques - Decreased NAA and Cr in large plaques

Increased mI, Cho and Lac

Chronic plaque - Cr and Lac return quickly to normal,

Cho - months to return to normal

NAA -may or may not recover to normal.

Tumefactive demyelination may be similar to neoplasm (elevated Cho, Lac, decreased NAA) –Perfusion useful.

Monophasic Acute Disseminated Encephalo Myelitis - Mild, reversible NAA reductions without changes in other metabolites - Good prognosis.

Multiple sclerosis

• Long TE spectra in acute and chronic MS lesions.– Both - Elevated Cho and reduced NAA– Only acute lesion - Elevated lactate

• Short TE spectra from acute lesion and normal brain for comparison– Increased mI, choline, and lipids, slightly

decreased Cr and NAA.

Acute Vs Chronic Plaque

• During acute phase- focal increases in Cho and Lac and decreases in NAA, Cr.

• 15 m later - Reduction of lesion and normalization of Cho, Cr, and Lactate. NAA- partial recovery.

Seizure disorders• Help in localize and characterize epileptogenic

foci.• Helps in lateralizing in temporal lobe epilepsy• @35TE: Decreased NAA, Increased Cho and

mI - Gliosis• MRS may help to characterize epileptogenic

lesions visible on MRI (aggressive vs. indolent neoplasia, dysplasia)

TLE

Ipsilat MRS:

Reduced NAA signal and increased Cho and mI signals- Gliosis

Helpful in identification of seizure focus in refractory pts with normal MR

Contralateral MRS:

Reversible with time - transient neuronal dysfunction.

Bilateral metabolic changes, associated with poor post-op seizure outcome

Aging and dementia

• Aging - Cho and Cr increase and NAA stable• AD – Reduced NAA and High mI• NAA/Cr and mI/Cr ratios correlate with

cognitive function in AD, and this correlation is more significant with NAA/mI ratios.

• WM NAA/Cr is lower in VaD –than AD

• Progression of AD - Regional elevation of mI/Cr levels in prodromal AD

• mI/Cr and NAA/Cr - useful for predicting and monitoring prodromal AD.

AD

Neurodegenerative diseases

• Neuronal dysfunction & cell death.• Metabolite changes in idiopathic Parkinson’s

disease are inconsistent.• Multiple system atrophy -reduction in NAA

and NAA/Cr ratio when compared with IPD.• Lactate increased in Huntington’s disease.

Traumatic brain injury

• Conventional CT and MR – major role• High lactate levels - Poor outcome.• Visible Lac in normal appearing brain

soon after injury - Poor outcome• Fall in NAA - Continue for months after

the initial insult.

3 Mon after

injury

Hypoxic brain injury

• Loss of NAA, increase in Lac and glutamine and decrease in Cr

• High Lac; low NAA and Cr -Bad prognosis.

In severe HIE distinction

between poor prognosis

and good prognosis is

made on basis of:

(1) excess Lac

(2) decreased NAA

(3) loss of Cr

Pediatric white matter disease

• Reduced axonal integrity - Reduced NAA

• Demyelination -High cho and mI• Hypomyelination or Gliosis -low

Cho, normal NAA

MLD

Loss of NAA and elevation of mI

Adrenoleukodystrophy

• Most common leukodystrophy in children• Zones- demylination, inflamm, gliosis• MRI often precede clinical symptoms showing

symmetrical WM lesions in parietal and occipital regions.

• MRS - Onset of demyelination and extent of WM damage, information for Hemo Stem Cell Transplant.

Inborn errors of metabolism

• Canavan and Salla disease show an elevated NAA• Maple syrup urine disease -Branched-chain amino

acids at 0.9 ppm.• Phenylketonuria -Small phenylalanine signal at

7.36ppm (i.e. downfield of water)• Non-ketotic hyperglycinemia -Glycine at 3.55 ppm

(use long TE to distinguish from mI)

Canavan’s disease- AR

• Deficiency of aspartoacylase an enzyme that deacetylates NAA, Increased free acetate

• Hypotonia and macrocephaly• Symmetrical confluent subcortical WM T2

prolongation & Centripetal spread• Bilateral involvement of globi pallidi, thalami,

cerebellum and brainstem

NAA is elevated in posterior sub cortical WM (1) that is hyper intense on T2 image;

NAA is near normal levels in (2) is relatively spared by signal abnormality

Maple syrup urine disease

• Deficiency of branched-chain -keto acid dehydrogenase, catalyzing essential branched-chain amino acids (BCAA) isoleucine, leucine and valine.

• Hypertonia and hypotonia, irregular respiration and apnea• Diffusion restriction compatible with cytotoxic edema in pons,

midbrain, pallidi, thalami, cerebellar, and periventricular WM• Abnormal peak at 0.9 ppm due to accumulation of lactate

(Lac) and loss of NAA• Prognostic value & monitor response to Rx / diet• TE 136 ms- avoids lipids

Elevation of Lac (1.3 ppm) and of the methyl group of BCAA/BCKA (0.9 ppm).

After therapy at day 12 the two abnormal peaks have disappeared.

Phenylketonuria

• Phe hydroxylase def• Periatrial and periventricular WM symmetrical

hyperintensities.• Calcifications bilaterally in the globi pallidi

and frontal subcortical regions• Elevated Phe signal at 7.36 ppm

Phe signal at 7.36 ppm

Non-ketotic hyperglycinemia

• Highly elevated glycine in the CSF and absence of ketoacidosis

• Large glycine peak at 3.55 ppm• Long TE is necessary to distinguish glycine

resonance from that of mI at 3.56 ppm which is normally high in neonates

abnormal elevation of Gly at 3.55 ppm with a Gly/Cr ~ 1.

Progressive decrease of Gly during treatment with a protein restriction diet

Mitochondrial encephalopathy

• Diffuse symmetrical hyperintensity and volume loss in WM

• mild Lac accumulation in WM, with moderate NAA and mild Cho and Cr signal losses

Multivoxel TE 136 ms at centrum semiovale level

Thank You