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2.01 ppm, is considered a neuronal marker and is present only in

neurons, axons and dendrites. Choline resonates at 3.2ppm and is

involved in membrane synthesis and degradation, whereas creatine

resonates at 3.0ppm and is involved in energy metabolism. Glutamate

and glutamine metabolites resonate closely at 3.75ppm and between

2.1 and 2.5 ppm, respectively.

Glutamate is an excitatory neurotransmitter that plays a role in

mitochondrial metabolism; glutamine is involved in detoxification and

regulation of neurotransmitter activity. Myo-inositol is a pentose sugar

that resonates at 3.5-3.6ppm and is part of the inositol triphosphate

intracellular second messenger system. It is considered an astroglial cell

marker since it is predominantly found in glial cells.

Glutathione resonates at 3.77ppm; it is an antioxidant and is essential

for maintaining red blood cell structure and maintaining haemoglobin in

a ferrous state. In pathological conditions, these metabolites may be

found in abnormal concentrations (absent, lower, or higher

concentrations), and other metabolites (e.g. lipids or lactate) that aren’t

typically present in a healthy brain may be detected.

Fig. 2 shows single voxel short echo time MRS performed on the basal

ganglia region of a seven-year-old golden retriever with chronic liver

disease and hepatic encephalopathy. In particular, the high peak for the

glutamine-glutamate complex and the lower peak for myo-inositol in this

dog differ when compared with results shown in Fig. 1.

Dog’s brain

At the Clinic of Diagnostic Imaging at Vetsuisse Zürich, active work

employing

1

H MRS has been developed for the last three years. The first

step was to define an optimised protocol and to assess the metabolite

concentration in the brain of healthy dogs. Single voxel short echo time

1

H MRS was performed on ten healthy Beagle dogs in different regions

and the y-axis represents the signal intensity.

Tissue concentration of a metabolite is related

to the integrated amplitude of the MRS

(magnetic resonance spectroscopy) signal it

generates, which is the area under the

1

H MRS

(proton magnetic resonance spectroscopy)

signal curve. Both peak area and height are

contributing factors for determining the

concentration of each metabolite. In relation

to Fig. 1, Cho is choline; Cr, creatine; Glx,

glutamine-glutamate complex; MI, myo-inositol;

and NAA, N-acetylaspartate.

1

H MRS

MRS is a non-invasive technique that

provides specific biochemical information on

numerous intracellular metabolites.

1

H MRS

uses measurements of signals emitted by

proton nuclei because of their high magnetic

sensitivity and presence in all tissues of the

human or animal body. Instead of a

morphological image of the brain, the result

of MRS can be visualised as a graph of signal

intensity with respect to its frequency.

Proton signals of different metabolites, or even

different protons of one molecule, can occur at

different positions (frequencies) within the MRS

spectrum. The shift of peaks in their relationship

to one another on the frequency axis is called

chemical shift. Instead of a frequency scale,

which is dependent on the magnetic field’s

strength, a parts per million (ppm) is commonly

used to describe the position of the spectral

peaks on the x-axis.

Each metabolite has a characteristic set of

chemical shift values in its signal. The

concentration of each metabolite is related to

the corresponding signal amplitude, which is

the area under the curve. The relative

concentration of the metabolites can be

measured by numerical integration (normally

using metabolite ratios) or by sophisticated

software such as LCModel, which is the one

used in the Diagnostic Research Unit at the

Vetsuisse Faculty of Zürich.

Glutamate and glutathione

Proton MRS can detect several metabolites

such as

N-

acetylaspartate, choline, creatine,

glutamate, glutamine, myo-inositol, and

glutathione in the brain of clinically normal

subjects.

N-

acetylaspartate, which resonates at

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105

M E D I C A L T E C H N O L O G Y & R E S E A R C H

Fig. 1 Single voxel

short echo time

magnetic resonance

spectroscopy

performed on the

basal ganglia of a

healthy three-year-old

Beagle dog