Magnetic resonance spectroscopy has been used to measure the concentration of GABA and GSH in healthy volunteers. They used two different spectral editing methods for finding this concentration. One method is called MEGA-PRESS where you send in specific pulse frequencies that either effect GABA or GSH and give the concentration of one of the two biochemical substances. HERMES use another technique where one can get the concentration of both GABA and GSH at the same time.
For this assignment, we will look at a collection of datasets from HERMES and MEGA-PRESS, and plot histograms with error bars using C++. The goal will be to see if the two methods give a good result with a small deviation. The dataset used in this assignment is from a bachelor thesis "MR measurements of GABA and GSH with new spectroscopy method: HERMES" written by Lars Sandnes and Tobias Heggeli.
4. Discussion and Conclusion
When one need information about an object, a picture may not always suffice. Images can be useful for identifying what something may be, but spectroscopy gives us the understanding of what an object actually consists of. Spectroscopy is by definition the study of how electromagnetic radiation interacts with matter. It was first used to study visible light from prisms and later to look at absorption lines from gasses, but in the 1950s one could by using a strong magnet, coils and radio waves detect changes in the frequency of the atomic nucleus due to their chemical bonds. This led to magnetic resonance spectroscopy, which is the MR-technique that presents information about biochemical compounds in an object. MRS has because of this become the clinical assessment of conditions such as epilepsy, multiple sclerosis and cancer (Jansen, Backes, Nicolay, & Kooi, 2006).
Figure 1.1: An example of sectroscopy;
Analysis of white light by dispersing
it with a prism.
Magnetic Resonance Imaging is an imaging technique that uses a strong magnetic field, around 1.5-3Tesla, that make a nucleus with a spin different to 0, like protons, align parallel to the exterior magnetic field. When the protons align with the outer magnetic field they start to resonate with a frequency that is called the Larmor frequency. The Larmor frequency is given by the equation below.
Figure 1.2: A spinning proton precesses
around an applied magnetic field.
Figure 1.3: T1- and T2-relaxation after a 90 degree rf pulse.
Magnetic resonance Spectroscopy (MRS) is similar to MRI, but instead of creating a picture we get a spectrum of the biochemical information in tissue. This allows for examination of individual molecules within a voxel, which means that one can study the biochemistry of disease processes without invasive procedures like biopsies (Brown and Semelaka, 2003, p. 181). MRS is based on the fact that different molecules are surrounded by different electron clouds that affect the resonance frequency of the protons in the molecule. This change in frequency is called a chemical shift and is measured in parts per million (ppm). The chemical shift values we get from the protons are measured up against a reference frequency of tetramethylsilane (TMS) that then has a chemical shift of 0. One can then measure the chemical shift of a tissue from the formula:
Chemical shift = (f-fref)*10^6/frefWhere fref is the reference frequency and f the resonance frequency to the proton in question. This makes it possible to separate substances from each other and make a spectrum. Figure 1.4 shows how an MR spectrum may look like. Each top represents one or more molecules, and the area under the graph give the consentrtion of the molecule.
Figure 1.4: A typicle MR spectrum form a voxel.
This experiment where performed at Haukeland university hospital. The MR-machine used was a 3Tesla GE Discovery MR750 with 32 channel head coils to scan ten healthy volunteers ageing 23+-3 years. Single voxel spectroscopy, with dimensions 25mm*33mm*25mm, was positioned on the left side of the anterior cingulate cortex (ACC) in each participant.
Figure 2.1: T1-weighted picture of voxel in front of the left cortex
The result from the scanning is presented in table 3.1 and table 3.2. The first table show the concentration of GABA and GSH compared with Cr in the voxel, and their respective mean values. An error in the scanning of GABA and GSH with H2O as a reference frequency lead to some higher valus of GABA and GSH, but the values where still calculated and their result is found in table 3.2.
Table 3.1: Results from GABA/Cr and GSH/Cr for
measurements with MEGAPRESS and HERMES
Table 3.2: Results from GABA/H2O and GSH/H2O for
measurements with MEGAPRESS and HERMES
To find the concentration of GABA and GSH one would use a program that fit the curves of the collected data with a reference dataset. The area under the graph would give the concentration. It is possible to calculate how well the reference dataset fit together with the collected data and estimate the error between them. The calculations of errors are presented in table 3.3.
Table 3.3: Fit error between collected data and reference
data baced on the creatine levels
The histograms in figure 3.1 shows the values of the dataset from table 3.1 and table 3.2 together with the uncertainty of each value taken from table 3.3 and the results based on the H2O-levels, which is not added in a table above because of an error in the sampling. This is to better illustrate the uncertainty of each value for this assignment. To find the uncertainty of each value the fit error had to be calculated for all volunteers and put into text files. The histograms were then created using c++ scripts that opened each text file.
Figure 3.1: Comparing the results from GABA/Cr, GABA/H2O, GSH/Cr and GSH/H2O for meassurements with MEGAPRESS and HERMES
Here are the scripts used for this project:root_GABA_Cr.C
Here are the text files:GABA_Cr.txt