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25 Cards in this Set
- Front
- Back
- 3rd side (hint)
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Metabolite |
A substance formed in or necessary for metabolism. |
代谢产物 |
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vriteria |
A principle or standard by which something may be judged or decided |
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Spectrum |
Monovoxel spectrum |
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Any method of spectroscopy, calls for suppression of the water signal (CHESS), and possibly of the fat signal: |
present in large quantityies in the body, these have a masking effect on the metabolites close to their resonance peaks. |
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The basic spectroscopy sequences are PRESS and STEAM. PRESS records a spin echo where whereas STEAM only records a stimuated echo, of weaker intensity. |
Both have an excitation pattern comprised of 3 RF pulses. |
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in single voxel spectroscopy the 3 RF pulses selected the voxel of interest, located at the intersection of the 3 orthogonal planes. |
The recorded echo comes form the voxel submitted to the 3 RF pulses only. |
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In chemical shift imaging (CSI) , the 3 RF pulses select a slice or volume that is spatially encoded by phase gradients. |
There are different methods of accelerating CSI data acquisition. Chemical shift imaging yields multiple spectra of the slice or volume of interest. |
These are represented as a parametric image or studied separately. |
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The resonance spectrum identifies metabolites by: |
1 locating the peaks, determined by chemical shift (ppm) resulting from the shield formed by the electronic cloud of hydrogen nuclei in the molecular. |
2 The same compound being characterized by several peaks ( double, triplet) due to spin- spin coupling ( or J coupling) phenomena: Lac (1.7 and 1.33 ppm). |
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Magnetic resonance spectrometry requires a very homogeneous magnetic field (shimming), a volume voxel and sufficient number of measurements. |
Single voxel spectroscopy Chemical shift imaging |
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MRS focus on the spectroscopy of the hydrogen nucleus. |
The resonance frequency difference is expressed as parts per million or ppm, a value that is independent of the amplitude of the magnetic field. |
The value of the chemical shift thus provides information about the molecular group carrying the hydrogen nuclei. |
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Chemical shift value For a given molecular, the chemical shift in ppm versus the reference molecule is defined by the relationship with: |
. wm: resonance frequency of the studied molecule . wref: resonance frequency of the reference molecule . dm: chemical shift value in ppm |
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Interaction between the atomic nuclei of neighboring chemical groups translates as each peak breaking down into a complex peak (doublet, triplet, multiplet): this is spin-spin coupling. |
The space between these peaks has a fixed frequence value(Hz), called the J-coupling constant, independent of the magnetic field amplitude. |
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abscissa (ab'sisa) |
Tetramethylsilane 四甲基硅烷 |
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The area bellow the peak curve will basically be determined by metabolite concentration. The width of the peak is inversely proportionate to T2* relaxation time. |
SVS: Single voxel spectroscopy Spectroscopic imaging (CSI: Chemical shift imaging) |
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Perturbations |
Anxiety; mental uneasiness |
扰动 |
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Prior to any MRS acquisition, the magnetic field homogenized (shimming) in the region of interest. The bigger the region, the harder it is to homogenize the field throughout. |
The precession frequency of the water must be optimized to adequately suppress the water peak, using selective frequency pulses and dephasing gradients. |
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Spectrum quality is evaluated according to two main criteria: |
1 signal-to-noise ratio (hight of metabolite peaks in relation to background noise) 2 spectral resolution (peak width, which determines whether the different metabolites can be separated). |
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In brain MRS, the principle molecules that can be analyzed are: |
N-acetyl-asparate (NAA) (Molecule present in healthy neurons) at 2.0 ppm |
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Creatine/Phosphocreatine (Cr) (energy metabolism molecules) at 3.0 ppm |
Choline compounds (Cho) (marker in the synthesis and breakdown of cell membranes) at 3.2 ppm |
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Myo-inositol (ml) (only found in glial tissue) at 3.5 ppm |
Glutamine-Glutamate-GABA Complex (Glx) (neurotransmitters) between 2.1 and 2.5 ppm. |
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Lactate (Lac) (anaerobic metabolism): doublet at 1.35 ppm |
Free lipids (lip): wide resonance, doublet at 1.3 and 0.9 ppm. |
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PRESS: Point-RESolved Spectroscopy. STEAM: STimulated Echo Acquisition Mode. CHESS: CHEmical Shift Selective. |
CHESS consists in applying three couples (90o RF pulses + dephasing gradients) in each spatial direction. |
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The bandwidth of these RF pulses is narrow and centered on the resonance frequency of the water peak in order to saturate the water signal and preserve the signal from the other metabolites. |
The analyzed volume is selected by a succession of three selective radiofrequency pulses(accompanied by gradients) in three directions. |
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These pulses determine three orthogonal planes those intersection corresponds to the volume studied. |
orthogonal |
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PRESS: Highest SNR, but more SAR intensive. STEAM: Lower SNR, but less SAR and better for imaging metabolites with shorter T2. |
Advanced MEGA-PRESS |
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