BASIC PHYSICAL PRINCIPLES OF MR SPECTROSCOPY

The resonant frequencies of nuclei are at the lower end of the electromagnetic spectrum between FM radio and radar. The resonant frequencies of protons range between about 10 MHz at 0.3 T to about 300 MHz on a 7 T magnet. The advantages of higher field strength are higher signal-to-noise and better separation of the metabolite peaks. In a proton spectrum at 1.5 T, the metabolites are spread out between 63,000,000 and 64,000,000 Hertz. Rather than use these large numbers, some very smart person decided to express the resonant frequencies in parts per million (ppm), and he/she positioned NAA at 2.0 ppm and let the other metabolites fall into their proper positions on the spectral line. Then, for unknown reasons, he/she reversed the ppm scale so that it reads from right to left. 
For MR imaging, the total signal from all the protons in each voxel is used to make the image. If all the signal were used for MRS, the fat and water peaks would be huge and scaling would make the other metabolite peaks invisible. Since we aren't interested in fat and water anyway, the 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 sequence 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.
The STEAM (STimulated Echo Acquisition Mode) pulse sequence uses a 90o 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) sequence refocuses the spins with a 180o 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.
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 144 msec 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 that are commonly seen on the MR spectrum are listed on the right. Each metabolite appears 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 integ-rity. 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. Either their concentrations are too low, or the molecules are invisible to MRS.
Normal MR spectra obtained from gray matter and white matter are shown on the right. The predominant metabolites, displayed from right to left, are NAA, creatine, choline, and myo-inositol. The primary difference between the two spectra is that gray matter has more creatine.Hunter's angle is the line formed by the metabolites on the white matter spectrum. The common way to analyze clinical spectra is to look at metabolite ratios, namely NAA/Cr, NAA/Cho, and Cho/Cr. Normal and abnormal values are shown in the chart to the right. By including a known reference solution when acquiring the MR spectral data, absolute concentrations of metabolites can be calculated.