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

Quantitative cardiac 31P spectroscopy at 3 Tesla using adiabatic pulses

Cardiac phosphorus magnetic resonance spectroscopy (MRS) with surface coils promises better quantification at 3 Tesla (T) from improved signal-to-noise ratios and spectral resolution compared with 1.5 T. However, Bloch equation and field analyses at 3T show that for efficient quantitative MRS protocols using small-angle adiabatic (BIR4/BIRP) pulses the excitation-field is limited by radiofrequency (RF) power requirements and power deposition. When BIR4/BIRP pulse duration is increased to reduce power levels, T2-decay can introduce flip-angle dependent errors in the steady-state magnetization, causing errors in saturation corrections for metabolite quantification and in T1s measured by varying the flip-angle. A new dual-repetition-time (2TR) T1 method using frequency-sign-cycled adiabatic-half-passage pulses is introduced to alleviate power requirements, and avoid the problem related to T2 relaxation during the RF pulse. The 2TR method is validated against inversion-recovery in phantoms using a practical transmit/receive coil set designed for phosphorus MRS of the heart at depths of 9-10 cm with 4 kW of pulse power. The T1s of phosphocreatine (PCr) and adenosine triphosphate (gamma-ATP) in the calf-muscle (n=9) at 3 T are 6.8+/-0.3 s and 5.4+/-0.6 s, respectively. For heart (n=10) the values are 5.8+/-0.5 s (PCr) and 3.1+/-0.6 s (gamma-ATP). The 2TR protocol measurements agreed with those obtained by conventional methods to within 10%.

The performance of volume selection sequences for in vivo NMR spectroscopy: implications for quantitative MRS

Previous work has demonstrated that deficiencies in volume selection sequences used in magnetic resonance spectroscopy may compromise the quality of the spectra obtained. In this paper, further studies on the ISIS and PRESS sequences are presented. Under conditions of partial saturation, ISIS can exhibit serious contamination with extraneous signal, particularly when a small volume of interest (VOI) is selected. ISIS protocols should therefore use VOIs that are large relative to the target volume, and repetition times that are as long as practicable. In PRESS, contamination is found to be minimised by using a VOI that is small relative to the target volume, and to be independent of repetition time. PRESS performance is also independent of echo time, except when very short echo times are used. These results are consistent with previously published work on ISIS and PRESS, and it is now possible to establish generic features of these sequences and to understand the implications for quantitative spectroscopy. T(1)-weighting of contamination in ISIS can compromise both relative and absolute quantification techniques in several respects. Contamination in PRESS is largely independent of relaxation times and would be easier to model and correct for in the context of quantitative spectroscopy.

Point-wise measurements of MRS volume selection performance are insensitive to magnetic susceptibility effects of phantom materials

The purpose was to analyse magnetic susceptibility effects on accuracy of point-wise measurements of signal profiles in the assessment of MRS volume selection performance. An existing phantom design consisting of a sphere with a movable signal source was used for the investigation. The influence from the phantom on magnetic field homogeneity was measured with phase sensitive 1H imaging and 31P spectroscopy on a 1.5 T whole body MR system. The susceptibility effects for such a phantom design can be separated in 1/ A variation in the background magnetic field, which is caused by the stationary structures and has a significant influence on spatial accuracy. 2/ A magnetic field distortion, which is caused by the movable signal source and has very little influence on accuracy. The spatial inaccuracy due to susceptibility effects in this phantom, was 0.03 mm for positions of the signal source covering a 40-mm VOI. Susceptibility effects from the movable signal source were substantial but had very little influence on spatial accuracy. Still, improvements of this phantom design are possible. Point-wise measurements using a phantom with a movable signal source is inherently insensitive to susceptibility effects from the signal source and permits accurate signal profile measurements of high spatial (sub-mm) resolution.

A simple and realistic tissue-equivalent breast phantom for MRI

A simple and inexpensive breast phantom is described for use in magnetic resonance imaging. The work demonstrates the similarity of the phantom materials to in vivo breast tissue in terms of T(1) relaxation times. The phantom is also qualitatively compared with images acquired from a patient with a primary breast lesion.J. Magn. Reson. Imaging 10:968-971, 1999.

Quality assessment of localization technique performance in small volume in vivo 1H MR spectroscopy

A new phantom and evaluation method for experimental evaluation of 1H-magnetic resonance spectroscopy single volume localization techniques regarding signal contamination (C), defined as the part of the signal originating outside the volume of interest, is presented. The quality assessment method is based on a spherical phantom with an oil/water interface in order to reduce susceptibility effects, and applied for stimulated-echo acquisition method (STEAM) and spin-echo (SE) sequences, echo times of 270, 135, and 10 ms, and cubic volumes of interest (VOI) of 1(3), 1.5(3), 2(3), 2.5(3), and 3(3) cm3. To be able to mimic measurements of the contamination in three dimensions the physical gradients representing the three orthogonal directions for slice selection were shifted in the pulse sequences. Contamination values in one dimension differed between 6.5% and 8.4% in SE sequences, and between 0.7% and 13.8% in STEAM sequences. In STEAM sequences a decrease of C with increasing VOI size was observed while SE sequences showed comparable C values for the different VOI sizes tested. The total contamination in three dimensions were 19% and 18% in SE and STEAM sequences with a TE of 270 ms, and 7% in a STEAM sequence with a TE of 10 ms, respectively. The presented evaluation method is easily applied to the new phantom and showed high reproducibility.

Performance of 2D 1H spectroscopic imaging of the brain: some practical considerations regarding the measurement procedure

This paper deals with some of the practical considerations in the planning and performance of chemical shift imaging (MRSI or CSI) of the brain. It contains some aspects of 1) the imaging procedure (MRI), i.e., suggestions of an imaging protocol useful for the spectroscopic planning, 2) the planning of the spectroscopic volume, i.e., size and position, 3) evaluation and judgment of the preparation results, and 4) evaluation of the MRSI images. The paper also contains suggestions of developmental work and quality assessment to be done before patient studies are begun. Examples are given for MRSI studies of temporal lobe epilepsy. Several of the aspects described are obvious for the experienced spectroscopist but may be useful in the initiation of MRSI. The goal of this paper was to share our experiences of how to achieve high quality MRSI, experiences that we would had been grateful for in our prelude of MRSI experiments.