Evaluation of volume selection methods in in vivo MRS. Design of a new test phantom

MRS quality assessment in a multicentre study on MRS-based classification of brain tumours

This paper reports on quality assessment of MRS in the European Union-funded multicentre project INTERPRET (International Network for Pattern Recognition of Tumours Using Magnetic Resonance; http://azizu.uab.es/INTERPRET), which has developed brain tumour classification software using in vivo proton MR spectra. The quality assessment consisted of both MR system quality assurance (SQA) and quality control (QC) of spectral data acquired from patients and healthy volunteers. The system performance of the MR spectrometers at all participating centres was checked bimonthly by a short measurement protocol using a specially designed INTERPRET phantom. In addition, a more extended SQA protocol was performed yearly and after each hardware or software upgrade. To compare the system performance for in vivo measurements, each centre acquired MR spectra from the brain of five healthy volunteers. All MR systems fulfilled generally accepted minimal system performance for brain MRS during the entire data acquisition period. The QC procedure of the MR spectra in the database comprised automatic determination of the signal-to-noise ratio (SNR) in a water-suppressed spectrum and of the line width of the water resonance (water band width, WBW) in the corresponding non-suppressed spectrum. Values of SNR > 10 and WBW < 8 Hz at 1.5 T were determined empirically as conservative threshold levels required for spectra to be of acceptable quality. These thresholds only hold for SNR and WBW values using the definitions and data processing described in this article. A final QC check consisted of visual inspection of each clinically validated water-suppressed metabolite spectrum by two, or, in the case of disagreement, three, experienced MR spectroscopists, to detect artefacts such as large baseline distortions, exceptionally broadened metabolite peaks, insufficient removal of the water line, large phase errors, and signals originating from outside the voxel. In the end, 10% of 889 spectra with completed spectroscopic judgement were discarded.

Spatial localization in nuclear magnetic resonance spectroscopy

The ability to select a discrete region within the body for signal acquisition is a fundamental requirement of in vivo NMR spectroscopy. Ideally, it should be possible to tailor the selected volume to coincide exactly with the lesion or tissue of interest, without loss of signal from within this volume or contamination with extraneous signals. Many techniques have been developed over the past 25 years employing a combination of RF coil properties, static magnetic field gradients and pulse sequence design in an attempt to meet these goals. This review presents a comprehensive survey of these techniques, their various advantages and disadvantages, and implications for clinical applications. Particular emphasis is placed on the reliability of the techniques in terms of signal loss, contamination and the effect of nuclear relaxation and J-coupling. The survey includes techniques based on RF coil and pulse design alone, those using static magnetic field gradients, and magnetic resonance spectroscopic imaging. Although there is an emphasis on techniques currently in widespread use (PRESS, STEAM, ISIS and MRSI), the review also includes earlier techniques, in order to provide historical context, and techniques that are promising for future use in clinical and biomedical applications.

The magnitude of signal errors introduced by ISIS in quantitative 31P MRS

It is well known that the quality of a quantitative 31P MRS measurement relies largely on the performance of the volume selection method, and that image selected in vivo spectroscopy (ISIS) suffers from contaminating signal caused mostly by T1 smearing. However, these signal errors and their magnitude are seldom addressed in clinical studies. The aim of this study was therefore to investigate the magnitude of signal errors in 31P MRS when using ISIS. The results from the measurements with a homogeneous head phantom are as follows: at low TR/T1 ratios the contamination increases rapidly, especially for small (<27 cm3) VOI sizes; at TR/T1=1, the signal from a 27 cm3 VOI was 20% too high, and from an 8 cm3 VOI 150% too high. The signal obtained from different VOI positions varied between 80 and 127%. The signal varied linearly with the 31P concentration in the object. However, a too high signal was obtained when the concentration was lower in the region of interest (inner container) than in the rest of the phantom. The agreement between the simulations and measurements shows that the results of this study are generally applicable to the measurement geometry and the ISIS experiment order rather than being specific for the MR system studied. The errors obtained both experimentally and in computer simulations are too large to be ignored in clinical studies using the ISIS pulse sequence.

A two-compartment phantom for VOI profile measurements in small-bore 31P MR spectroscopy

A two-compartment gel phantom for VOI profile measurements in volume-selective 31P spectroscopy in small-bore units is presented. The phantom is cylindrical with two compartments divided by a very thin (30 microm) polyethene film. This thin film permits measurements with a minimum of susceptibility influences from the partition wall. The phantom was used for evaluation of the volume selection method ISIS (image-selected in vivo spectroscopy). The position of the phantom was fixed in the magnet during the measurements, while the volume of interest (VOI) was moved stepwise over the border. The signal from the two compartments was measured for each position and the data were evaluated following differentiation. We have found this phantom suitable for VOI profile measurements of ISIS in small-bore systems. The phantom forms a useful complement to recommended phantoms for small bore-spectroscopy.

Signal profile measurements for evaluation of the volume-selection performance of ISIS

High-resolution signal profiles obtained with a test phantom were used in this study to evaluate the volume-selection performance of an implementation of ISIS (Image Selected In vivo Spectroscopy). The phantom simulated the brain with regard to volume and loading of coil. A remotely controlled, movable signal source inside the phantom was filled with orthophosphoric acid. Signal profiles of the volume of interest (VOI) were measured in three perpendicular directions. Special interest was focused on the transition zones, the position of the profiles, and the effects of off-resonance and T1 smearing. The transition zones were on average 5.6 mm wide and the full width at half maximum (FWHM) was 35 mm for a VOI of 40 x 40 x 40 mm3. The positions of the centre of the signal profiles were x = 3.2, y = -0.7 and z = 3.3 mm off-centre. The deviation of the volume position could be explained by off-resonance effects during imaging and spectroscopy. These data illustrate the importance of detailed knowledge of the volume-selection performance when attempting precision measurements using image-guided in vivo MRS.