Thermal heterogeneity within aqueous materials quantified by 1H NMR spectroscopy: Multiparametric validation in silico and in vitro

Contactless Thermometry by MRI and MRS: Advanced Methods for Thermotherapy and Biomaterials

Control of temperature variation is of primordial importance in particular areas of biomedicine. In this context, medical treatments such as hyperthermia and cryotherapy, and also the development and use of hydrogel-based biomaterials, are of particular concern. To enable accurate temperature measurement without perturbing or even destroying the biological tissue or material to be monitored, contactless thermometry methods are preferred. Among these, the most suitable are based on magnetic resonance imaging and spectroscopy (MRI, MRS). Here, we address the latest developments in this field as well as their current and anticipated practical applications. We highlight recent progress aimed at rendering MR thermometry faster and more reproducible, versatile, and sophisticated and provide our perspective on how these new techniques broaden the range of applications in medical treatments and biomaterial development by enabling insight into finer details of thermal behavior. Thus, these methods facilitate optimization of clinical and industrial heating and cooling protocols.

Multiparametric statistical quantification of pH heterogeneity by 1 H MRS and MRSI of extracellular pH markers: Proof of principle

Acid production and transport in numerous biological tissues and medical conditions are active areas of research. Heterogeneity of pH within a given homogeneous-appearing tissue volume has been reported, but none of the conventional methods currently available for measuring tissue pH provides quantitative parameters describing the frequency of occurrence of pH values within such a volume. We have previously presented a multiparametric noninvasive in vivo approach, providing at least 10 different statistical descriptors of pH heterogeneity based on a novel type of line shape analysis developed for pH-sensitive ³¹ P MRS resonances. However, this method suffers from lack of sensitivity, thus making rapid and spatially resolved measurements difficult. We present here the proof of principle of a new, more sensitive approach to statistical characterization of extracellular pH heterogeneity based on ¹ H MRS, with the potential of being combined with spatial resolution. We experimentally study a range of test solutions of a reporter molecule that has previously been shown to possess a ¹ H MRS resonance whose chemical shift varies with pH, including when injected intravenously into experimental animals (imidazole ethoxycarbonylpropionic acid, [IEPA]). Statistical pH heterogeneity descriptors are determined for phantoms mimicking tissue pH heterogeneity. To this end, the pH-sensitive ¹ H MRS resonance is transformed into a pH curve. Subsequently, the digital points of this pH profile are used to build a histogram using dedicated algorithms. The following descriptors are computed from this histogram: weighted mean pH and median pH, pH standard deviation, pH range, pH mode(s), pH kurtosis, pH skewness and pH entropy. Our new method is also validated by analyzing previously published in vivo MRSI spectra. The proof of principle provided in this work should form the basis of further in vivo studies in physiology and medicine, eg in cancer research, but also in other fields such as kidney and muscle research.

A method for multiparametric statistical quantification of the heterogeneity of free Na+ concentration by 19 F MR spectroscopy: Proof of principle in silico and in vitro

Sodium(I) (Na⁺ ) is one of the most important cations in mammalian tissues. Since Na⁺ plays a key role in basic cell function, noninvasive methods for measuring intracellular concentrations of free sodium ions in biological tissue have been developed on the basis of ¹⁹ F NMR spectroscopy. However, intracellular Na⁺ levels are often not uniform throughout a tissue volume (or voxel) being measured. In such cases, [Na⁺ ] heterogeneity is not reflected in results obtained by the classical technique, and may even result in biased average values. For this reason, we have designed an approach for quantifying [Na⁺ ] heterogeneity. First, the ¹⁹ F MRS resonance from FCrown-1 serving as a "Na⁺ probe" is transformed into a [Na⁺ ] curve. Then the digital points of the resulting [Na⁺ ] profile are used to construct a histogram with specially developed algorithms. From each [Na⁺ ] histogram, at least eight quantitative parameters describing the underlying statistical [Na⁺ ] distribution were computed: weighted median, weighted mean, standard deviation, range, mode(s), kurtosis, skewness, and entropy. In addition to our new paradigm, we present a first validation based on (i) computer simulations and (ii) experimentally obtained ¹⁹ F MR spectra of model solutions. This basic proof of principle warrants future in vivo experiments, in particular because of its ability to provide quantitative information complementary to that made available by commonly used ²³ Na MRI: (i) multiparametric statistical characterization of [Na⁺ ] distributions; (ii) total [Na⁺ ] heterogeneity analysis not intrinsically limited by the size of any MRI voxels; and (iii) analysis of unequivocally intracellular [Na⁺ ], as opposed to measurement of a combination of intra- and extracellular [Na⁺ ].

Multiparametric quantification of the heterogeneity of free Ca2+ concentration by 19F MR spectroscopy

For biological tissue that is heterogeneous with respect to free intracellular Ca²⁺ concentration ([Ca²⁺]_i), the lineshape of the ¹⁹F MRS resonance of injected [Ca²⁺]-sensitive 4-FBAPTA or BAPTA-FF reflects the statistical distribution of [Ca²⁺]_i values. While conventional ¹⁹F MRS of these fluorinated Ca²⁺ reporter molecules only provides one [Ca²⁺]_i value per spectrum, our specially designed lineshape analysis reveals at least eight quantitative statistical parameters (descriptors) characterizing the [Ca²⁺]_i distribution within the observed tissue volume. To this end, the [Ca²⁺]-sensitive ¹⁹F MRS resonance is transformed into a [Ca²⁺]_i curve. Subsequently, the digital points of this [Ca²⁺]_i profile are used to build a histogram using dedicated algorithms. The following statistical descriptors are computed from this histogram: weighted mean and median, standard deviation, range, mode(s), kurtosis, skewness, and entropy. Our new method is thoroughly validated through in silico and experimental models. The potential of combining statistical [Ca²⁺] information with spatial resolution is demonstrated by simulated statistical CSI maps. This proof of principle should form the basis of future in vivo studies in physiology and medicine, notably in heart and muscle research.

Multiparametric quantification of heterogeneity of metal ion concentrations, as demonstrated for [Mg2+] by way of 31P MRS

Magnesium(II) is the second most abundant intracellular cation in mammals. Non-invasive ³¹P MRS is currently used to measure intracellular free Mg²⁺ levels in studies of magnesium deficiency disorders. However, this technique only provides one [Mg²⁺] value for a given tissue volume (or voxel), based on the chemical shift of the ATP-β (or NTP-β) resonance. We present here an approach for quantifying tissue heterogeneity in regard to [Mg²⁺], by way of multiple ³¹P MRS-derived descriptors characterizing the statistical intra-volume distribution of free [Mg²⁺] values. Our novel paradigm exploits the fact that the lineshape of the ATP-β ³¹P MRS resonance reflects the statistical distribution of [Mg²⁺] values within the observed volume (or voxel). Appropriate lineshape analysis reveals multiple quantitative statistical parameters (descriptors) characterizing the [Mg²⁺] distribution. First, the ATP-β ³¹P MRS resonance is transformed into a [Mg²⁺] curve that is used to construct a histogram with our specially developed algorithms. From this histogram, at least eight [Mg²⁺] descriptors are computed: weighted mean concentration and median concentration, standard deviation of concentration, range of concentration, concentration mode(s), concentration kurtosis, concentration skewness, and concentration entropy. Comprehensive evaluation based on in silico and experimental models demonstrates the validity of this new method. This basic feasibility study should open new avenues for future in vivo studies in physiology and medicine.