MR SPECTROSCOPY
MR SPECTROSCOPY
MR SPECTROSCOPY
It’s a technique for defining
in vivo
chemical environment
and allowing analysis of neurophysiology and
neurochemistry
Quantitative noninvasive assay of metabolites like NAA,
choline, creatine, lipids, lactates, etc.
MRI
-
Data collected are analyzed in time domain to
generate anatomic image
MRS-
Time domain converted to frequency domain to
form distribution of intensities of chemical groups vs
their larmor frequencies
The spatial resolution for MRS is a thousand times
larger than what is typically achieved for MRI
The homogeneity required for MRI is about 0.5 ppm
whereas for MRS it is about 0.1 ppm. The process of
making the magnetic field homogeneous is called as
shimming.
Basic Principles
•
Magnetic resonance frequency of an atomic nucleus
is altered by its chemical environment, which causes
a small change in the resonance frequency called
chemical shift
•
Different metabolites resonate at different
frequencies along the horizontal / chemical shift axis.
•
Metabolite signal is identified on the horizontal axis
by it’s chemical shift.
J-coupling
In addition to chemical shifts, the spectrum is also
modulated by J-coupling
J-coupling is the result of an internal indirect interaction of
two spins via the intervening electron structure of the
molecule. The coupling strength is measured in Hertz and
is independent of the external B 0 field strength.
J-coupling results in a modulation of signal intensity
depending on sequence
type, particularly the
echo time (TE).
Echo Time
The most important parameter is the echo time (TE) in mrs.
MR spectroscopy can be separated into long TE (typically TE
> 135 ms) and short TE (TE » 35 ms) methods.
Long echo times simplifies spectra because the number of
detectable peaks is reduced and the remaining peaks are
more readily identifi ed.
Short TE MRS allows the detection of an increased number
of metabolites and has a signal-to-noise advantage over long
TE.
Data Acquisition
Single-Voxel Magnetic Resonance Spectroscopy
Single-voxel (SV) MRS measures the MR signal of a
single selected region of interest whereas signal outside
this area is suppressed.
Manufacturers generally provide PRESS (Point
Resolved Spectroscopy), STEAM (Stimulated Echo
Acquisition Mode) , and ISIS (Image Selected In Vivo
Spectroscopy)
These sequences differ in how radiofrequency pulses
and gradient pulses are arranged in order to achieve
localization.
STEAM
90----90----90
Short TE
Demo short T2
Metabolites
Less SNR
PRESS
90----180----180
Long TE
Less sensitive to short
T2 Metabolites
More SNR
ISIS: Image Selected In vivo Spectroscopy
-
Three frequency selective inversion pulses are applied in
presence of the orthogonal gradients. Fourth non-
selective pulse is used for the observation of signal.
-
ISIS is used in the 31P spectroscopy.
CSI: Chemical Shift Imaging /
Magnetic Resonance
Spectroscopic Imaging (MRSI)
-
CSI is used for the multivoxel spectroscopy, where a large
area is covered and divided into multiple voxels.
Slice selection can be achieved with a selective RF pulse as
for MR imaging
CSI is an excellent technique to obtain metabolic maps
Steps in MRS Acquisition
Patient positioning
Shimming-
Optimization of the magnetic field homogeneity is
done over the entire volume detected by the receiver
coil.
Acquisition of MR images for localization-
Images are obtained in all three planes for placement
of a voxel.
Selection of MRS parameters-
TR and TE are important parameters.
Improved SNR is obtained at a longer TR.
-
Longer TE- choline, creatine, NAA and lactate are visible.
Short TE- lipid, glutamate, glutamine, GABA and inositol
Selection of VOI (volume of interest)-
SVS can be used for local or diffuse diseases.
MVS is used in irregularly shaped large lesion.
-
Water suppression-
Water peak is suppressed so that smaller metabolite
peaks are visible. Water peak suppression is done by
CHESS (Chemical shift selective spectroscopy)
technique.
MRS data collection-
On modern machines, 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
Tetramethylsilane (TMS).
Interpretation-
The area under the peak of any metabolite is directly
proportional to the number of spins contributing to the
peak.
Look at peak separation
Identify the Normal & Abnormal Metabolites
Assess Metabolite Ratios using Cr as internal reference
Role of MRS
Evaluation of neoplasms
Infection
Infarction & Hypoxia
Epilepsy
Metabolic diseases
Neurodegenerative diseases
Thank You
MR Spectroscopy is a technique for analyzing in vivo chemical environments to study neurophysiology and neurochemistry. This quantitative noninvasive assay allows for the identification of various metabolites such as NAA, choline, creatine, lipids, and lactates. Data collected are analyzed in the time domain to generate anatomic images, with the spatial resolution for MRS being significantly higher than MRI. The process of making the magnetic field homogeneous, known as shimming, is crucial for the accuracy of the results. Understanding the basic principles of MR spectroscopy involving chemical shifts and J-coupling is essential for interpreting the spectra. The echo time (TE) parameter plays a crucial role in MRS, with long TE and short TE methods offering distinct advantages in spectral simplification and peak detection. Data acquisition through single-voxel Magnetic Resonance Spectroscopy further enhances the capabilities of this valuable imaging tool.
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Presentation Transcript
MR SPECTROSCOPY It s a technique for defining in vivo chemical environment and allowing analysis of neurophysiology and neurochemistry Quantitative noninvasive assay of metabolites like NAA, choline, creatine, lipids, lactates, etc.
MRI- Data collected are analyzed in time domain to generate anatomic image MRS- Time domain converted to frequency domain to form distribution of intensities of chemical groups vs their larmor frequencies The spatial resolution for MRS is a thousand times larger than what is typically achieved for MRI The homogeneity required for MRI is about 0.5 ppm whereas for MRS it is about 0.1 ppm. The process of making the magnetic field homogeneous is called as shimming.
Basic Principles Magnetic resonance frequency of an atomic nucleus is altered by its chemical environment, which causes a small change in the resonance frequency called chemical shift Different metabolites resonate at different frequencies along the horizontal / chemical shift axis. Metabolite signal is identified on the horizontal axis by it s chemical shift.
J-coupling In addition to chemical shifts, the spectrum is also modulated by J-coupling J-coupling is the result of an internal indirect interaction of two spins via the intervening electron structure of the molecule. The coupling strength is measured in Hertz and is independent of the external B 0 field strength. J-coupling results in a modulation of signal intensity depending on sequence type, particularly the echo time (TE).
Echo Time The most important parameter is the echo time (TE) in mrs. MR spectroscopy can be separated into long TE (typically TE > 135 ms) and short TE (TE 35 ms) methods. Long echo times simplifies spectra because the number of detectable peaks is reduced and the remaining peaks are more readily identifi ed. Short TE MRS allows the detection of an increased number of metabolites and has a signal-to-noise advantage over long TE.
Data Acquisition Single-Voxel Magnetic Resonance Spectroscopy Single-voxel (SV) MRS measures the MR signal of a single selected region of interest whereas signal outside this area is suppressed. Manufacturers generally provide PRESS (Point Resolved Spectroscopy), STEAM (Stimulated Echo Acquisition Mode) , and ISIS (Image Selected In Vivo Spectroscopy) These sequences differ in how radiofrequency pulses and gradient pulses are arranged in order to achieve localization.
PRESS STEAM 90----180----180 90----90----90 Short TE Demo short T2 Metabolites Less SNR Long TE Less sensitive to short T2 Metabolites More SNR ISIS: Image Selected In vivo Spectroscopy -Three frequency selective inversion pulses are applied in presence of the orthogonal gradients. Fourth non- selective pulse is used for the observation of signal. -ISIS is used in the 31P spectroscopy.
CSI: Chemical Shift Imaging /Magnetic Resonance Spectroscopic Imaging (MRSI) - CSI is used for the multivoxel spectroscopy, where a large area is covered and divided into multiple voxels. Slice selection can be achieved with a selective RF pulse as for MR imaging CSI is an excellent technique to obtain metabolic maps
Steps in MRS Acquisition Patient positioning Shimming- Optimization of the magnetic field homogeneity is done over the entire volume detected by the receiver coil. Acquisition of MR images for localization- Images are obtained in all three planes for placement of a voxel.
Selection of MRS parameters- TR and TE are important parameters. Improved SNR is obtained at a longer TR. - Longer TE- choline, creatine, NAA and lactate are visible. Short TE- lipid, glutamate, glutamine, GABA and inositol Selection of VOI (volume of interest)- SVS can be used for local or diffuse diseases. MVS is used in irregularly shaped large lesion. -
Water suppression- Water peak is suppressed so that smaller metabolite peaks are visible. Water peak suppression is done by CHESS (Chemical shift selective spectroscopy) technique. MRS data collection- On modern machines, 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 Tetramethylsilane (TMS).
Interpretation- The area under the peak of any metabolite is directly proportional to the number of spins contributing to the peak. Look at peak separation Identify the Normal & Abnormal Metabolites Assess Metabolite Ratios using Cr as internal reference
Chemical compound Chemical shift (ppm) Comments Lipids (lip) 0.9 1.4 An indicator of necrosis , myelin sheath disruption Lactate (Lac) Indicates anaerobic glycolysis. 1.3 N-Acetylaspartate (NAA ) Marker of neuronal and axonal viability 2.0 Glutamate (Glu) Glutamine (Gln) Main ammonia 2.1 2.5 3.0, 3.9 Cerebral metabolism marker Creatine/ Phosphocreatine Choline (Cho) Reflects cellular proliferation 3.2 Myo-inositol (ml) Glial marker; Cell volume regulator 3.6
Role of MRS Evaluation of neoplasms Infection Infarction & Hypoxia Epilepsy Metabolic diseases Neurodegenerative diseases
