At high field (7 T) spectral editing of γ-aminobutyric acid with MEGA-point-resolved spectroscopy is inefficient due to the large chemical shift displacement error. In this article, a new pulse sequence is designed which has minimal chemical shift displacement error to perform an efficient spectral editing of the γ-aminobutyric acid 3.0 ppm resonance at 7 T. The sequence consists of the conventional MEGA editing pulses and a semi-localized by adiabatic selective refocusing sequence. Phantom and in vivo measurements demonstrated an efficient detection of γ-aminobutyric acid. Using ECG triggering, excellent in vivo performance of the MEGA-semi-localized by adiabatic selective refocusing (MEGA-sLASER) provided well-resolved γ-aminobutyric acid signals in 27 mL volumes in the human brain at an echo time of 74 ms within a relatively short acquisition time (5 min). Furthermore, the high efficiency of the MEGA-sLASER was demonstrated by acquiring small volumes (8 mL) at an echo time of 74 ms, as well as long echo time measurements (222 ms in 27 mL volume).
Deuterium metabolic imaging (DMI) is a novel MR-based method to spatially map metabolism of deuterated substrates such as [6,6'-2 H 2 ]-glucose in vivo. Compared with traditional 13 C-MR-based metabolic studies, the MR sensitivity of DMI is high due to the larger 2 H magnetic moment and favorable T 1 and T 2 relaxation times.Here, the magnetic field dependence of DMI sensitivity and transmit efficiency is studied on phantoms and rat brain postmortem at 4, 9.4 and 11.7 T. The sensitivity and spectral resolution on human brain in vivo are investigated at 4 and 7 T before and after an oral dose of [6,6'-2 H 2 ]-glucose. For small animal surface coils (Ø 30 mm), the experimentally measured sensitivity and transmit efficiency scale with the magnetic field to a power of +1.75 and −0.30, respectively. These are in excellent agreement with theoretical predictions made from the principle of reciprocity for a coil noise-dominant regime. For larger human surface coils (Ø 80 mm), the sensitivity scales as a +1.65 power. The spectral resolution increases linearly due to nearconstant linewidths. With optimal multireceiver arrays the acquisition of DMI at a nominal 1 mL spatial resolution is feasible at 7 T. K E Y W O R D Sdeuterium metabolic imaging, magnetic field dependence, resolution, sensitivity | INTRODUCTIONDeuterium metabolic imaging (DMI) is a novel MR-based method to spatially map metabolism. 1 DMI lies in the category of stable isotope methods, in which an enriched substrate isotope is followed over time as it appears in downstream metabolic products. Common stable isotope methods include 13 C MR spectroscopy (MRS), 2 inverse 1 H-[ 13 C] MRS 3 and hyperpolarized 13 C MRS. 4,5 DMI is characterized by its technical simplicity and robustness as well as the relatively high sensitivity due to the larger magnetic moment and short T 1 relaxation time constants. The low natural abundance of 2 H of 0.0115% 6 leads to low-intensity water and lipid signals, thus eliminating the need for water and lipid suppression. To maximize sensitivity, the 2 H signal is excited by a single RF pulse, after which spatial localization is achieved with short 3D phase-encoding blips before FID acquisition. The robustness of DMI is further enhanced by the low sensitivity to magnetic field inhomogeneity due to the low 2 H Larmor frequency.
γ-Aminobutyric acid (GABA) and lactate are metabolites which are present in the brain. These metabolites can be indicators of psychiatric disorders or tumor hypoxia, respectively. The measurement of these weakly coupled spin systems can be performed using MRS editing techniques; however, at high field strength, this can be challenging. This is due to the low available B1 (+) field at high fields, which results in narrow-bandwidth refocusing pulses and, consequently, in large chemical shift displacement artifacts. In addition, as a result of the increased chemical shift displacement artifacts and chemical shift dispersion, the efficiency of the MRS method is reduced, even when using adiabatic refocusing pulses. To overcome this limitation, frequency offset corrected inversion (FOCI) pulses have been suggested as a mean to substantially increase the bandwidth of adiabatic pulses. In this study, a Mescher-Garwood semi-localization by adiabatic selection and refocusing (MEGA-sLASER) editing sequence with refocusing FOCI pulses is presented for the measurement of GABA and lactate in the human brain. Metabolite detection efficiencies were improved by 20% and 75% for GABA and lactate, respectively, when compared with editing techniques that employ adiabatic radiofrequency refocusing pulses. The highly efficient MEGA-sLASER sequence with refocusing FOCI pulses is an ideal and robust MRS editing technique for the measurement of weakly coupled metabolites at high field strengths.
1Higher magnetic field strengths like 7 T and above are desirable for MR spectroscopy given the increased spectral resolution and signal to noise ratio. At these field strengths, substantial nonuniformities in B 1 1/2 and radiofrequency power deposition become apparent. In this investigation, we propose an improvement on a conventionally used endorectal coil, through the addition of a second element (stripline). Both elements are used as transceivers. In the center of the prostate, approximately 40% signal to noise ratio increase is achieved. In fact, the signal to noise ratio gain obtained with the quadrature configuration locally can be even greater than 40% when compared to the single loop configuration. This is due to the natural asymmetry of the B 1 1/2 fields at high frequencies, which causes destructive and constructive interference patterns. Global specific absorption rate is reduced by almost a factor of 2 as expected. Furthermore, approximately a 4-fold decrease in local specific absorption rate is observed when normalized to the B 1 values in the center of the prostate. Because of the 4-fold local specific absorption rate decrease obtained with the dual channel setup for the same reference B 1 value (20 mT at 3.5 cm depth into the prostate) as compared to the single loop, the transmission power B 1 duty cycle can be increased by a factor 4. Consequently, when using the two-element endorectal coil, the radiofrequency power deposition is significantly reduced and radiofrequency intense sequences with adiabatic pulses can be safely applied at 7 T for 1 H magnetic resonance spectroscopy and MRI in the prostate. Altogether, in vivo 1 H magnetic resonance spectroscopic imaging of prostate cancer with a fully adiabatic sequence operated at a minimum B 1 1 of 20 mT shows insensitivity to the nonuniform transmit field, while remaining within local specific absorption rate guidelines of 10 W/kg. Magn Reson Med 68:311-318, 2012. V C 2011 Wiley Periodicals, Inc.
An endorectal coil and an eight-element microstrip array were compared for prostate imaging at 7 T. An extensive radiofrequency safety assessment was performed with the use of finite difference time domain simulations to determine safe scan parameters. These simulations showed that the endorectal coil can deliver substantially more B(1)(+) to the prostate than can the microstrip array within the specific absorption rate safety guidelines. However, the B(1)(+) field of the endorectal coil is very inhomogeneous, which makes the use of adiabatic pulses compulsory for T(1) - or T(2) -weighted imaging. As a consequence, a full prostate examination is only possible in a feasible amount of time when the microstrip array is used for T(1) - and T(2) -weighted imaging, whereas the endorectal coil is required for spectroscopic imaging. The pulse parameters were optimised within the specific absorption rate guidelines and thereafter used to provide a good illustration of the possibilities of prostate imaging at 7 T.
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