Gradient ascent pulse engineering for rapid exchange saturation transfer

The Proton Resonance Enhancement for CEST imaging and Shift Exchange (PRECISE) family of RF pulse shapes for Chemical Exchange Saturation Transfer MRI

Purpose

To optimize a 100 msec pulse for producing CEST MRI contrast and evaluate in mice.

Methods

A gradient ascent algorithm was employed to generate a family of 100 point, 100 msec pulses for use in CEST pulse trains ('PRECISE'). Gradient ascent optimizations were performed for exchange rates (k ca ) = 500 s -1 , 1,500 s -1 , 2,500 s -1 , 3,500 s -1 and 4,500 s -1 and offsets (Δω) = 9.6, 7.8, 4.2 and 2.0 ppm. 7 PRECISE pulse shapes were tested on an 11.7 T scanner using a phantom containing three representative CEST agents with peak saturation B 1 = 4 μT. The pulse producing the most contrast in phantoms was then evaluated for CEST MRI pH mapping of the kidneys in healthy mice after iopamidol administration.

Results

The most promising pulse in terms of contrast performance across all three phantoms was the 9.6 ppm, 2500 s -1 optimized pulse with ∼2.7 x improvement over Gaussian and ∼1.3x's over Fermi pulses. This pulse also displayed a large improvement in contrast over the Gaussian pulse after administration of iopamidol in live mice.

Conclusion

A new 100 msec pulse was developed based on gradient ascent optimizations which produced better contrast compared to standard Gaussian and Fermi pulses in phantoms. This shape also showed a substantial improvement for CEST MRI pH mapping in live mice over the Gaussian shape and appears promising for a wide range of CEST applications.

Optimal control pulses for the 1.2-GHz (28.2-T) NMR spectrometers

The ability to measure nuclear magnetic resonance (NMR) spectra with a large sample volume is crucial for concentration-limited biological samples to attain adequate signal-to-noise (S/N) ratio. The possibility to measure with a 5-mm cryoprobe is currently absent at the 1.2-GHz NMR instruments due to the exceedingly high radio frequency power demands, which is four times compared to 600-MHz instruments. Here, we overcome the high-power demands by designing optimal control (OC) pulses with up to 20 times lower power requirements than currently necessary at a 1.2-GHz spectrometer. We show that multidimensional biomolecular NMR experiments constructed using these OC pulses can bestow improvement in the S/N ratio of up to 26%. With the expected power limitations of a 5-mm cryoprobe, we observe an enhancement in the S/N ratio of more than 240% using our OC sequences. This motivates the development of a cryoprobe with a larger volume than the current 3-mm cryoprobes.

A new approach to Z-spectrum acquisition: prospective baseline enhancement (PROBE) for CEST/Nuclear Overhauser Effect

PURPOSE

To develop a prospective baseline enhancement that compensates for intermingled background effects in Z-spectra to achieve sensitivity enhancement of peaks related to CEST and nuclear Overhauser effect.

METHODS

An MRI sequence-specific compensation of background effects is achieved through variation of the pulsed saturation power, ω 1 , max , with the chemical shift, δ . After a "scout acquisition" of a standard Z-spectrum, the background is modeled through an appropriate spin system. Subsequently, an optimization procedure yields ω 1 , m a x ( δ ) values that compensate for background contributions yielding a flat baseline. Contributions from metabolites not considered in the optimization procedure are enhanced as distinct perturbations to the baseline. For experimental verification, mapping of the lactate concentration in the presence of cross-linked bovine serum albumin was performed in phantoms at 7 T. As proof of concept, explorative experiments were performed in healthy human subjects at 3 T.

RESULTS

Nuisance contributions from direct water saturation, macromolecular magnetization transfer, and exchanging background protons were successfully removed from the Z-spectrum in phantoms and in brain tissue. The lactate methyl, methine, and hydroxyl peaks were readily observable in vitro. The peak areas correlated linearly with known concentrations. Improvement of the detection limit was achieved by a sparse distribution of saturation frequencies, allowing for more efficient signal averaging.

CONCLUSION

An optimization framework for high-resolution metabolite mapping by means of CEST/nuclear Overhauser effect was developed. It offers full flexibility to select spin-pool moieties, whose influence on the Z-spectrum will be compensated. Deviations from this background model will provide a contrast at the respective offset frequencies.

CEST, ASL, and magnetization transfer contrast: How similar pulse sequences detect different phenomena

Chemical exchange saturation transfer (CEST), arterial spin labeling (ASL), and magnetization transfer contrast (MTC) methods generate different contrasts for MRI. However, they share many similarities in terms of pulse sequences and mechanistic principles. They all use RF pulse preparation schemes to label the longitudinal magnetization of certain proton pools and follow the delivery and transfer of this magnetic label to a water proton pool in a tissue region of interest, where it accumulates and can be detected using any imaging sequence. Due to the versatility of MRI, differences in spectral, spatial or motional selectivity of these schemes can be exploited to achieve pool specificity, such as for arterial water protons in ASL, protons on solute molecules in CEST, and protons on semi-solid cell structures in MTC. Timing of these sequences can be used to optimize for the rate of a particular delivery and/or exchange transfer process, for instance, between different tissue compartments (ASL) or between tissue molecules (CEST/MTC). In this review, magnetic labeling strategies for ASL and the corresponding CEST and MTC pulse sequences are compared, including continuous labeling, single-pulse labeling, and multi-pulse labeling. Insight into the similarities and differences among these techniques is important not only to comprehend the mechanisms and confounds of the contrasts they generate, but also to stimulate the development of new MRI techniques to improve these contrasts or to reduce their interference. This, in turn, should benefit many possible applications in the fields of physiological and molecular imaging and spectroscopy.

An optimal control approach to design entire relaxation dispersion experiments

A general approach is introduced to optimize experiments for the analysis of spin systems in the presence of chemical exchange. Rather than optimizing individual pulse sequence elements, such as refocusing pulses, entire relaxation dispersion sequences are optimized in the form of a single shaped pulse. This is achieved by defining a performance index that is only based on the remaining signal after the relaxation dispersion sequence for a range of exchange, relaxation, offset, and rf inhomogeneity parameters. The approach is demonstrated by optimizing energy-limited broadband relaxation dispersion sequences that closely approach the overall effect of ideal CPMG sequences. As illustrated both theoretically and experimentally, significant improvements are found compared to standard amplitude or energy-limited CPMG sequences.

Continuous-wave saturation considerations for efficient xenon depolarization

The combination of hyperpolarized Xe with chemical exchange saturation transfer (Hyper-CEST) is a powerful NMR technique to detect highly dilute concentrations of Xe binding sites using RF saturation pulses. Crucially, that combination of saturation pulse strength and duration that generates the maximal Hyper-CEST effect is a priori unknown. In contrast to CEST in proton MRI, where the system reaches a steady-state for long saturation times, Hyper-CEST has an optimal saturation time, i.e. saturating for shorter or longer reduces the Hyper-CEST effect. Here, we derive expressions for this optimal saturation pulse length. We also found that a pulse strength, B1, corresponding to five times the Xe exchange rate, k(BA) (i.e. B1 = 5 k(BA)/γ with the gyromagnetic ratio of (129)Xe, γ), generates directly and without further optimization 96% of the maximal Hyper-CEST contrast while preserving spectral selectivity. As a measure that optimizes the amplitude and the width of the Hyper-CEST response simultaneously, we found an optimal saturation pulse strength corresponding to √2 times the Xe exchange rate, i.e. B1=√2k(BA)/γ. When extremely low host concentration is detected, then the expression for the optimum saturation time simplifies as it approaches the longitudinal relaxation time of free Xe.