A.SHAJITHAA.SHAJITHAB.Sc RD 3B.Sc RD 3rdrd YEAR YEAR

MR SPECTROSCOPYMR SPECTROSCOPY

MR SpectroscopyMR SpectroscopyNoninvasive means of assessing the Noninvasive means of assessing the

biochemical and metabolic processes in biochemical and metabolic processes in intracranial tissues without ionizing intracranial tissues without ionizing radiation.radiation.

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

There are numerous metabolites found in the There are numerous metabolites found in the human brain. human brain. 

Fortunately, only several of them are useful in Fortunately, only several of them are useful in spectroscopic studies. spectroscopic studies. 

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

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

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

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

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

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

NORMAL SPECTRA

The MRS spectrum of major metabolites in The MRS spectrum of major metabolites in a normal braina normal brain

Major Metabolites in the BrainMajor Metabolites in the Brain

Choline

Creatine

Lactate

Glutamine

Lipid

Myo-InositolNAA

N-ACETYLASPARTATE (NAA)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. 

CholineCholine The choline peak receives contribution from The choline peak receives contribution from

glycerophosphocholine, phosphocholine, and glycerophosphocholine, phosphocholine, and phosphatidylcholine. phosphatidylcholine. 

It is the precursor of acetyl choline and It is the precursor of acetyl choline and phosphatidylcholine. phosphatidylcholine. 

Acetylcholine is an important neurotransmitter and the Acetylcholine is an important neurotransmitter and the latter is an integral part of cell membrane synthesis.latter is an integral part of cell membrane synthesis.

Disease processes affecting the cell membrane and myelin Disease processes affecting the cell membrane and myelin

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

injury, neoplasm or acute demyelination diseases. injury, neoplasm or acute demyelination diseases.  Choline is the second largest peak and assigned to 3.2 ppm.Choline is the second largest peak and assigned to 3.2 ppm.

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

phosphate. phosphate.  The phospocreatine supplies phosphate to adenosine The phospocreatine supplies phosphate to adenosine

diphosphate (ADP) to form adenosine triphosphate (ATP) with the diphosphate (ADP) to form adenosine triphosphate (ATP) with the release of creatine. release of creatine. 

The overall level of total creatine in normal brain is fairly The overall level of total creatine in normal brain is fairly constant.constant. ReducedReduced Cr level may be seen in pathologic processes such as 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 Most metastatic tumors to the brain do not produce creatine since

they do not possess creatine kinase. they do not possess creatine kinase.  Cr is the third highest peak and is assigned to 3.03 ppm. It is Cr is the third highest peak and is assigned to 3.03 ppm. It is

usually seen usually seen next to the right of cholinenext to the right of choline..

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

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

Therefore, elevated lactate peak is a sign of hypoxic Therefore, elevated lactate peak is a sign of hypoxic tissue. tissue. 

Low oxygen supply can result from decreased oxygen Low oxygen supply can result from decreased oxygen supply or increased oxygen requirement. supply or increased oxygen requirement. 

The former may be seen in vascular insults, or The former may be seen in vascular insults, or hypoventilation and the latter may be seen in neoplastic hypoventilation and the latter may be seen in neoplastic tissue. tissue. 

Lactate peak occurs at two different Lactate peak occurs at two different locations. locations. The lower field peak (a doublet) occurs at The lower field peak (a doublet) occurs at

approximately 1.32 ppm.approximately 1.32 ppm.   The other peak (a quartet) is seen at 4.1 ppm The other peak (a quartet) is seen at 4.1 ppm

and this is very close to the water peak. and this is very close to the water peak. – usually suppressed during data processing.

Myo-Inositol (mI)Myo-Inositol (mI)

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

Elevated level of mI would be seen where there Elevated level of mI would be seen where there is glial cell proliferation as in gliosis. is glial cell proliferation as in gliosis. 

The main mI peak is assigned to 3.56 ppm and The main mI peak is assigned to 3.56 ppm and additional peak may be seen at 4.06 ppmadditional peak may be seen at 4.06 ppm

LipidsLipids Lipids are composition of triglycerides, phospholipids, Lipids are composition of triglycerides, phospholipids,

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

and myelin.and myelin.   Lipid peak should not be seen unless there is destructive Lipid peak should not be seen unless there is destructive

process of the brain including necrosis, inflammation or process of the brain including necrosis, inflammation or infection. infection. 

Lipids have a very short T1 relaxation time and are Lipids have a very short T1 relaxation time and are normally not seen unless short TEs are utilized. normally not seen unless short TEs are utilized. 

Lipid resonance at 1.2 ppm can sometimes obscure the Lipid resonance at 1.2 ppm can sometimes obscure the lactate peak at 1.32 ppm. lactate peak at 1.32 ppm. 

Fat in the cranium can contaminate the true disease Fat in the cranium can contaminate the true disease process if the voxels are placed too close the cranium.process if the voxels are placed too close the cranium.

Glutamate and Glutamine (Glx)Glutamate and Glutamine (Glx)

Glutamate is an excitatory Glutamate is an excitatory neurotransmitter in mitochondrial neurotransmitter in mitochondrial metabolism. metabolism. 

Glutamine and glutamate resonate closely Glutamine and glutamate resonate closely together. together. 

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

MRS Acquisition modesMRS Acquisition modes

STEAM-Stimulated echo acquisition modeSTEAM-Stimulated echo acquisition modeSingle voxelSingle voxelShort TEShort TE

PRESS-Point resolved spectroscopyPRESS-Point resolved spectroscopyTwice the SNR of STEAMTwice the SNR of STEAMShort and long TE-single voxel possibleShort and long TE-single voxel possible

Single voxelSingle voxel

MULTI VOXEL MULTI VOXEL SPECTROSCOPYSPECTROSCOPY

Multivoxel spectroscopy can be used to obtained Multivoxel spectroscopy can be used to obtained one, two  or three dimensional localization.  one, two  or three dimensional localization.  

The major engine behind multi voxel is The major engine behind multi voxel is CHEMICAL SHIFT IMAGING (CSI).CHEMICAL SHIFT IMAGING (CSI).

This technique is developed to obtain separate This technique is developed to obtain separate images from water and fat bound protons.images from water and fat bound protons.

It is referred as Magnetic Resonance It is referred as Magnetic Resonance Spectroscopic Imaging(MRSI).Spectroscopic Imaging(MRSI).

It obtains simultaneously many voxels and It obtains simultaneously many voxels and spatial distrubution of the metabolities in a spatial distrubution of the metabolities in a single sequence.single sequence.

Large volume of coverage.Large volume of coverage. It is used for complex lesions.It is used for complex lesions. Long acquisition time (6-12 min).Long acquisition time (6-12 min). Time of echo:35 and 144 ms.Time of echo:35 and 144 ms.

Echo timeEcho time

As in MR imaging, the echo time affects the As in MR imaging, the echo time affects the information obtained with MRS. information obtained with MRS. 

Short TE refers to a study in which it varies from Short TE refers to a study in which it varies from 20 to 40 ms.20 to 40 ms.

It has a higher SNR and less signal loss due to It has a higher SNR and less signal loss due to T2 and T1 weighting than long TE.T2 and T1 weighting than long TE.

These short TE properties result in a spectrum These short TE properties result in a spectrum with more metabolites peaks, such as with more metabolites peaks, such as myoinositol and glutamine-glutamate which are myoinositol and glutamine-glutamate which are not detected with long TE .not detected with long TE .

• MRS spectra may also be obtained with long TEs, from 135 to 288 ms.• With a long TE of 270 msec, only metabolites with a long T2 are seen, producing a spectrum with primarily NAA, creatine, and choline.• One other helpful TE is 144 msec because it inverts lactate at 1.3 ppm. • With TE of 270-288 ms there is a lower SNR and the lactate peak is not inverted

Planning of MRSPlanning of MRSThe ROI will be placed at the center of the The ROI will be placed at the center of the enhancing tumor covering the lesion and the enhancing tumor covering the lesion and the normal brain as much as possible but excluding normal brain as much as possible but excluding the subcutaneous fat and sinuses.the subcutaneous fat and sinuses.

Normal MRSNormal MRS

LymphomaLymphoma

Anaplastic Astrocytoma Anaplastic Astrocytoma

Glioma Glioma

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