Magnetic Resonance Spectroscopy Techniques:

After the nuclei have been exposed to a uniform magnetic field, they receive a 90 degree RF pulse that rotates them from the z-axis. When this pulse is turned off, the nuclei return to their original position in the x-axis. The time it takes them to return to their original position in the z-axis is governed by their relaxation times. The receiving coil detects the voltage variations at many points in time during this period. This voltage variation is termed Free Induction Decay and may be plotted as an exponential decay function (i.e., intensity versus time) to yield time domain information (i.e, relaxation times). If all the information were used for MRS, the fat and water peaks would be huge and scaling would make the other metabolite peaks invisible. So, fat and water are eliminated. Fat is avoided by placing the voxel for MRS within the brain away from the fat in bone marrow and scalp. Water suppression is accomplished with either a CHESS (CHEmical-Shift Selective) or IR (Inversion Recovery) technique. These suppression techniques are used with a STEAM or PRESS pulse sequences acquisition. A Fourier transform is then applied to the data to separate the signal into individual frequencies. Protons in different molecules resonate at slightly different frequencies because the local electron cloud affects the magnetic field experienced by the proton.

Normal brain H-1 spectrum using PRESS

The STEAM (STimulated Echo Acquisition Mode) pulse sequence uses a 90 degree refocusing pulse to collect the signal like a gradient echo. STEAM can achieve shorter echo times but at the expense of less signal-to-noise. The PRESS (Point REsolved Spectroscopy) sequences refocuses the spins with a 180 degree RF pulse like a spin echo. Two other acronyms require definition. CSI (Chemical Shift Imaging) refers to multi-voxel MRS. SI (Spectroscopic Imaging) displays the data as an image with the signal intensity representing the concentration of a particular metabolite.

Artifacts introduced by magnetic filed inhomogeneities may result in distortion of the line width of the peaks and decreased ability to resolve them Therefore, a homogenous magnetic field is an important prerequisite to obtaining "resolvable" spectra. Shimming the filed in the region of interest to the resonance of water assures the homogeneity of the filed. The water line width should be less than 0.2 ppm after shimming. Spatial localization is achieved by applying static and/or pulsed gradients.

The horizontal axis (abscissa) represents resonance frequency as parts per millions to the total resonance frequency. The sharpness of the peak and line width is affected by:

1. Homogeneity of the external magnetic field
2. Magnetic field inhomogeneity due to susceptibility gradient
3. T2 time of the sample (long T2 causes narrowing of the line).

The MR spectra are analyzed in the following format::

1. Center of the resonance frequency in ppm
2. Peak height
3. Line width at half-height
4. Peak area and shape
5. Composition of the peaks, e.g. single, double, triplet.

The standard conventions to display MR spectra include:

1. Up-field is to the right represents lower frequencies and are shielded.
2. Down-field is to the left represents higher frequencies and are de-shielded.

coronal, sagittal and axial sections for MRS data acquisition

As in MR imaging, the echo time affects the information obtained with MRS. With a short TE of 30 msec, metabolites with both short and long T2 relaxation times are observed. 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 144msec because it inverts lactate at 1.3 ppm.

As a general rule, the single voxel, short TE technique is used to make the initial diagnosis, because the signal-to-noise is high and all metabolites are represented. Multi-voxel, long TE techniques are used to further characterize different regions of a mass and to assess brain parenchyma around or adjacent to the mass. Multi-voxel, long TE techniques are also used to assess response to therapy and to search for tumor recurrence.

The brain metabolites appear at a specific ppm, and each one reflects specific cellular and biochemical processes. NAA is a neuronal marker and decreases with any disease that adversely affects neuronal integrity. Creatine provides a measure of energy stores. Choline is a measure of increased cellular turnover and is elevated in tumors and inflammatory processes. The observable MR metabolites provide powerful information, but unfortunately, many notable metabolites are not represented in brain MR spectra. DNA, RNA, most proteins, enzymes and phospholipids are missing. Some key neurotransmitters such as acetylcholine, dopamine and serotonin are absent.