Band-selective chemical exchange saturation transfer imaging with hyperpolarized xenon-based molecular sensors

RF Heating Effects in CEST NMR with Hyperpolarized 129Xe Considering Different Spin Exchange Kinetics and Saturation Schemes

Chemical exchange saturation transfer (CEST) improves the sensitivity of NMR but depending on the spin exchange kinetics, it can require substantial RF energy deposition to label magnetization. Potential side effects like RF-induced heating may occur and must be monitored. Here, we explore the parameter space considering not only undesired heating but also efficient CEST build-up (depolarization rate), spectral resolution (line width), and subsequent effects like changes in chemical shifts of CEST responses. We present a systematic study to compare conventional block pulse with shaped-pulse saturation and quantify how the effective average saturation power impacts these parameters. Pulse shape and nominal excitation bandwidth, however, turned out to have little impact on acquired z-spectra and temperature changes. This study illustrates how different exchange kinetics define different regimes of suitable RF power within the dynamic range of fully saturable magnetization from hyperpolarized 129Xe. Temperature-related changes in the resonance frequency of bound spins were also quantified for the two Xe hosts CB6 and CrA-ma and put into context for typically used CEST acquisition parameters, including the stability of the magnetic field.

Hyperpolarized Xenon-129 Chemical Exchange Saturation Transfer (HyperCEST) Molecular Imaging: Achievements and Future Challenges

Molecular magnetic resonance imaging (MRI) is an emerging field that is set to revolutionize our perspective of disease diagnosis, treatment efficacy monitoring, and precision medicine in full concordance with personalized medicine. A wide range of hyperpolarized (HP) 129Xe biosensors have been recently developed, demonstrating their potential applications in molecular settings, and achieving notable success within in vitro studies. The favorable nuclear magnetic resonance properties of 129Xe, coupled with its non-toxic nature, high solubility in biological tissues, and capacity to dissolve in blood and diffuse across membranes, highlight its superior role for applications in molecular MRI settings. The incorporation of reporters that combine signal enhancement from both hyperpolarized 129Xe and chemical exchange saturation transfer holds the potential to address the primary limitation of low sensitivity observed in conventional MRI. This review provides a summary of the various applications of HP 129Xe biosensors developed over the last decade, specifically highlighting their use in MRI. Moreover, this paper addresses the evolution of in vivo applications of HP 129Xe, discussing its potential transition into clinical settings.

Radiofrequency labeling strategies in chemical exchange saturation transfer MRI

Chemical exchange saturation transfer (CEST) MRI has generated great interest for molecular imaging applications because it can image low-concentration solute molecules in vivo with enhanced sensitivity. CEST effects are detected indirectly through a reduction in the bulk water signal after repeated perturbation of the solute proton magnetization using one or more radiofrequency (RF) irradiation pulses. The parameters used for these RF pulses-frequency offset, duration, shape, strength, phase, and interpulse spacing-determine molecular specificity and detection sensitivity, thus their judicious selection is critical for successful CEST MRI scans. This review article describes the effects of applying RF pulses on spin systems and compares conventional saturation-based RF labeling with more recent excitation-based approaches that provide spectral editing capabilities for selectively detecting molecules of interest and obtaining maximal contrast.

Investigating the impact of RF saturation-pulse parameters on compartment-selective gas-phase depolarization with xenon polarization transfer contrast MRI

PURPOSE

To demonstrate the utility of continuous-wave (CW) saturation pulses in xenon-polarization transfer contrast (XTC) MRI and MRS, to investigate the selectivity of CW pulses applied to dissolved-phase resonances, and to develop a correction method for measurement biases from saturation of the nontargeted dissolved-phase compartment.

METHODS

Studies were performed in six healthy Sprague-Dawley rats over a series of end-exhale breath holds. Discrete saturation schemes included a series of 30 Gaussian pulses (8 ms FWHM), spaced 25 ms apart; CW saturation schemes included single block pulses, with variable flip angle and duration. In XTC imaging, saturation pulses were applied on both dissolved-phase resonance frequencies and off-resonance, to correct for other sources of signal loss and compromised selectivity. In spectroscopy experiments, saturation pulses were applied at a set of 19 frequencies spread out between 185 and 200 ppm to map out modified z-spectra.

RESULTS

Both modified z-spectra and imaging results showed that CW RF pulses offer sufficient depolarization and improved selectivity for generating contrast between presaturation and postsaturation acquisitions. A comparison of results obtained using a variety of saturation parameters confirms that saturation pulses applied at higher powers exhibit increased cross-contamination between dissolved-phase resonances.

CONCLUSION

Using CW RF saturation pulses in XTC contrast preparation, with the proposed correction method, offers a potentially more selective alternative to traditional discrete saturation. The suppression of the red blood cell contribution to the gas-phase depolarization opens the door to a novel way of quantifying exchange time between alveolar volume and hemoglobin.

Paramagnetic Shifts and Guest Exchange Kinetics in ConFe4-n Metal-Organic Capsules

We investigate the magnetic resonance properties and exchange kinetics of guest molecules in a series of hetero-bimetallic capsules, [Co_nFe_4-nL₆]^4- (n = 1-3), where L^2- = 4,4'-bis[(2-pyridinylmethylene)amino]-[1,1'-biphenyl]-2,2'-disulfonate. H bond networks between capsule sulfonates and guanidinium cations promote the crystallization of [Co_nFe_4-nL₆]^4-. The following four isostructural crystals are reported: two guest-free forms, (C(NH₂)₃)₄[Co_1.8Fe_2.2L₆]·69H₂O (1) and (C(NH₂)₃)₄[Co_2.7Fe_1.3L₆]·73H₂O (2), and two Xe- and CFCl₃-encapsulated forms, (C(NH₂)₃)₄[(Xe)_0.8Co_1.8Fe_2.2L₆]·69H₂O (3) and (C(NH₂)₃)₄[(CFCl₃)Co_2.0Fe_2.0L₆]·73H₂O (4), respectively. Structural analyses reveal that Xe induces negligible structural changes in 3, while the angles between neighboring phenyl groups expand by ca. 3° to accommodate the much larger guest, CFCl₃, in 4. These guest-encapsulated [Co_nFe_4-nL₆]^4- molecules reveal ¹²⁹Xe and ¹⁹F chemical shift changes of ca. -22 and -10 ppm at 298 K, respectively, per substitution of low-spin Fe^II by high-spin Co^II. Likewise, the temperature dependence of the ¹²⁹Xe and ¹⁹F NMR resonances increases by 0.1 and 0.06 ppm/K, respectively, with each additional paramagnetic Co^II center. The optimal temperature for hyperpolarized (hp) ¹²⁹Xe chemical exchange saturation transfer (hyper-CEST) with [Co_nFe_4-nL₆]^4- capsules was found to be inversely proportional to the number of Co^II centers, n, which is consistent with the Xe chemical exchange accelerating as the portals expand. The systematic study was facilitated by the tunability of the [M₄L₆]^4- capsules, further highlighting these metal-organic systems for developing responsive sensors with highly shifted ¹²⁹Xe resonances.

129Xe NMR-based sensors: biological applications and recent methods

Molecular systems that target analytes of interest and host spin-hyperpolarized xenon lead to powerful ¹²⁹Xe NMR-based sensors.

An Expanded Palette of Xenon-129 NMR Biosensors

Molecular imaging holds considerable promise for elucidating biological processes in normal physiology as well as disease states, by determining the location and relative concentration of specific molecules of interest. Proton-based magnetic resonance imaging (¹H MRI) is nonionizing and provides good spatial resolution for clinical imaging but lacks sensitivity for imaging low-abundance (i.e., submicromolar) molecular markers of disease or environments with low proton densities. To address these limitations, hyperpolarized (hp) ¹²⁹Xe NMR spectroscopy and MRI have emerged as attractive complementary methodologies. Hyperpolarized xenon is nontoxic and can be readily delivered to patients via inhalation or injection, and improved xenon hyperpolarization technology makes it feasible to image the lungs and brain for clinical applications.

In order to target hp ¹²⁹Xe to biomolecular targets of interest, the concept of “xenon biosensing” was first proposed by a Berkeley team in 2001. The development of xenon biosensors has since focused on modifying organic host molecules (e.g., cryptophanes) via diverse conjugation chemistries and has brought about numerous sensing applications including the detection of peptides, proteins, oligonucleotides, metal ions, chemical modifications, and enzyme activity. Moreover, the large (∼300 ppm) chemical shift window for hp ¹²⁹Xe bound to host molecules in water makes possible the simultaneous identification of multiple species in solution, that is, multiplexing. Beyond hyperpolarization, a 10⁶-fold signal enhancement can be achieved through a technique known as hyperpolarized ¹²⁹Xe chemical exchange saturation transfer (hyper-CEST), which shows great potential to meet the sensitivity requirement in many applications.

This Account highlights an expanded palette of hyper-CEST biosensors, which now includes cryptophane and cucurbit[6]uril (CB[6]) small-molecule hosts, as well as genetically encoded gas vesicles and single proteins. In 2015, we reported picomolar detection of commercially available CB[6] via hyper-CEST. Inspired by the versatile host–guest chemistry of CB[6], our lab and others developed “turn-on” strategies for CB[6]-hyper-CEST biosensing, demonstrating detection of protein analytes in complex media and specific chemical events. CB[6] is starting to be employed for in vivo imaging applications. We also recently determined that TEM-1 β-lactamase can function as a single-protein reporter for hyper-CEST and observed useful saturation contrast for β-lactamase expressed in bacterial and mammalian cells. These newly developed small-molecule and genetically encoded xenon biosensors offer significant potential to extend the scope of hp ¹²⁹Xe toward molecular MRI.

Design and optimization of pulsed Chemical Exchange Saturation Transfer MRI using a multiobjective genetic algorithm

Pulsed Chemical Exchange Saturation Transfer (CEST) MRI experimental parameters and RF saturation pulse shapes were optimized using a multiobjective genetic algorithm. The optimization was carried out for RF saturation duty cycles of 50% and 90%, and results were compared to continuous wave saturation and Gaussian waveform. In both simulation and phantom experiments, continuous wave saturation performed the best, followed by parameters and shapes optimized by the genetic algorithm and then followed by Gaussian waveform. We have successfully demonstrated that the genetic algorithm is able to optimize pulse CEST parameters and that the results are translatable to clinical scanners.

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.

Ultrafast Z-Spectroscopy for 129Xe NMR-Based Sensors

When working with hyperpolarized species, it is often difficult to maintain a stable level of magnetization over consecutive experiments, which renders their detection at the trace level cumbersome, even when combined with chemical exchange saturation transfer (CEST). We report herein the use of ultra-fast Z-spectroscopy as a powerful means to detect low concentrations of 129Xe NMR-based sensors and to measure the in-out xenon exchange. Modifications of the original sequence enable a multiplexed detection of several sensors, as well as the extraction of the exchange buildup rate constant in a single-shot fashion.

Nuts and bolts of chemical exchange saturation transfer MRI

Chemical exchange saturation transfer (CEST) has emerged as a novel MRI contrast mechanism that is well suited for molecular imaging studies. This new mechanism can be used to detect small amounts of contrast agent through the saturation of rapidly exchanging protons on these agents, allowing a wide range of applications. CEST technology has a number of indispensable features, such as the possibility of simultaneous detection of multiple 'colors' of agents and of changes in their environment (e.g. pH, metabolites, etc.) through MR contrast. Currently, a large number of new imaging schemes and techniques are being developed to improve the temporal resolution and specificity and to correct for the influence of B0 and B1 inhomogeneities. In this review, the techniques developed over the last decade are summarized with the different imaging strategies and post-processing methods discussed from a practical point of view, including the description of their relative merits for the detection of CEST agents. The goal of the present work is to provide the reader with a fundamental understanding of the techniques developed, and to provide guidance to help refine future applications of this technology. This review is organized into three main sections ('Basics of CEST contrast', 'Implementation' and 'Post-processing'), and also includes a brief Introduction and Summary. The 'Basics of CEST contrast' section contains a description of the relevant background theory for saturation transfer and frequency-labeled transfer, and a brief discussion of methods to determine exchange rates. The 'Implementation' section contains a description of the practical considerations in conducting CEST MRI studies, including the choice of magnetic field, pulse sequence, saturation pulse, imaging scheme, and strategies to separate magnetization transfer and CEST. The 'Post-processing' section contains a description of the typical image processing employed for B0 /B1 correction, Z-spectral interpolation, frequency-selective detection and improvement of CEST contrast maps.

Utilizing a water-soluble cryptophane with fast xenon exchange rates for picomolar sensitivity NMR measurements

Hyperpolarized (129)Xe chemical exchange saturation transfer ((129)Xe Hyper-CEST) NMR is a powerful technique for the ultrasensitive, indirect detection of Xe host molecules (e.g., cryptophane-A). Irradiation at the appropriate Xe-cryptophane resonant radio frequency results in relaxation of the bound hyperpolarized (129)Xe and rapid accumulation of depolarized (129)Xe in bulk solution. The cryptophane effectively "catalyzes" this process by providing a unique molecular environment for spin depolarization to occur, while allowing xenon exchange with the bulk solution during the hyperpolarized lifetime (T(1) ≈ 1 min). Following this scheme, a triacetic acid cryptophane-A derivative (TAAC) was indirectly detected at 1.4 picomolar concentration at 320 K in aqueous solution, which is the record for a single-unit xenon host. To investigate this sensitivity enhancement, the xenon binding kinetics of TAAC in water was studied by NMR exchange lifetime measurement. At 297 K, k(on) ≈ 1.5 × 10(6) M(-1) s(-1) and k(off) = 45 s(-1), which represent the fastest Xe association and dissociation rates measured for a high-affinity, water-soluble xenon host molecule near rt. NMR line width measurements provided similar exchange rates at rt, which we assign to solvent-Xe exchange in TAAC. At 320 K, k(off) was estimated to be 1.1 × 10(3) s(-1). In Hyper-CEST NMR experiments, the rate of (129)Xe depolarization achieved by 14 pM TAAC in the presence of radio frequency (RF) pulses was calculated to be 0.17 μM·s(-1). On a per cryptophane basis, this equates to 1.2 × 10(4)(129)Xe atoms s(-1) (or 4.6 × 10(4) Xe atoms s(-1), all Xe isotopes), which is more than an order of magnitude faster than k(off), the directly measurable Xe-TAAC exchange rate. This compels us to consider multiple Xe exchange processes for cryptophane-mediated bulk (129)Xe depolarization, which provide at least 10(7)-fold sensitivity enhancements over directly detected hyperpolarized (129)Xe NMR signals.