Quantification of breast tissue density: correlation between single-sided portable NMR and micro-CT measurements

Mammographic density (MD) is a strong independent risk factor for breast cancer. Traditional screening for MD using X-ray mammography involves ionising radiation, which is not suitable for young women, those with previous radiation exposure, or those having undergone a partial mastectomy. Therefore, alternative approaches for MD screening that do not involve ionising radiation will be important as the clinical use of MD increases, and as more frequent MD testing becomes desirable for research purposes. We have previously demonstrated the potential utility of spin relaxation-based, single-sided portable-NMR measurements for the purpose of MD quantification. We present here a more refined analysis by quantifying breast tissue density in excised samples on a continuous scale (0% to 100% fibroglandular tissue content) using micro-CT (μCT), and comparing the results to spin-relaxation and diffusion portable-NMR measurements of the same samples. μCT analysis of mammary tissues containing high- and low-MD (HMD and LMD, respectively) regions had Hounsfield Unit (HU) histograms with a bimodal pattern, with HMD regions exhibiting significantly higher HU values than LMD regions. Quantitative MD (%HMD) values obtained using μCT exhibited an excellent correlation with portable-NMR results, namely longitudinal spin-relaxation time constants (T) and the relative tissue water content obtained from portable-NMR diffusion measurements (R = 0.92, p

Diffusion-sensitive magnetic resonance spectroscopy and imaging in biomedical sciences

We present a mini-review of the development and contemporary applications of diffusion-sensitive\ud nuclear magnetic resonance (NMR) techniques in biomedical sciences. Molecular diffusion is a fundamental physical phenomenon present in all biological systems. Due to the connection between experimentally measured diffusion metrics and the microscopic environment sensed by the diffusing molecules, diffusion measurements can be used for characterisation of molecular size, molecular binding and association, and the morphology of biological tissues. The emergence of magnetic resonance was instrumental to the development of biomedical applications of diffusion. We discuss the fundamental physical principles of diffusion NMR spectroscopy and diffusion MR imaging. The emphasis is placed on conceptual understanding, historical evolution and practical applications rather than complex technical details. Mathematical description of diffusion is presented to the extent that it is required for the basic understanding of the concepts. We present a wide range of spectroscopic and imaging applications of diffusion magnetic resonance, including colloidal drug delivery vehicles; protein association; characterisation of cell morphology; neural fibre tractography; cardiac imaging; and the imaging of load-bearing connective tissues. This paper is intended as an accessible introduction into the exciting and growing field of diffusion magnetic resonance

Studies of Nuclear Magnetic Relaxation Processes in Paramagnetic Metalloporphyrin Complexes

Temperature dependence of Nuclear Magnetic Resonance (NMR) chemical shifts and longitudinal and transverse relaxation times (T1 and T2) was studied for the pyrrole protons in a number of six-coordinate S=1/2 iron(III) tetraphenylporphyrin (TPP) and tetramesitylporphyrin (TMP) complexes in the temperature range 190---310 K. In all complexes, temperature behavior of the chemical shifts and relaxation times is consistent with the presence of a low spin - high spin exchange caused by the dissociation of one axial ligand. In symmetric sterically hindered complexes, cyclic exchange induced by the synchronous rotation of axial ligands is also present. In all complexes, T2s are considerably shorter than T1s. Relaxation times in the TMP complexes are generally longer than corresponding values for the TPP complexes. Estimate of the electronic T1 is given and mechanisms of nuclear relaxation are discussed. The rate of NOE buildup for a pair of pyrrole protons in [TMPFe(2-MeImH)2]+ was measured; it is consistent with the Stokes rotational correlation time. A method is proposed to predict the detectability and optimum detection conditions of NOE between a pair of structurally rigid protons in similar complexes. Contrary to previous studies, no NOE is detected between pyrrole protons of two unsymmetrically substituted bis-N-methylimidazole Fe(III) TPP complexes. Two NMR approaches were utilized to measure the rate constant of axial ligand rotation in the TMP complex. Saturation transfer measurements yield overestimated rate constant. The measurement based on the temperature dependence of the T2s (ΔH‡ = 48 ± 1 kJ/mol, ΔS‡ = -10 ± 6 J/K mol) is consistent with previous studies. Modified MM2 potentials were also used to study the rotation of axial ligands in [TMPFe(1,2-Me2Im)2]+ and [TPPFe(1-MeIm)2]+. Adiabatic potential energy surfaces (PES) for rotation of axial ligands were constructed for both complexes. Synchronous rotation of the axial ligands (ΔH‡ = 48 kJ/mol) is highly preferable in the TMP complex. For the TPP complex, the enthalpy barriers to synchronous and asynchronous rotation are 3.3 and 5.4 kJ/mol, respectively. The relationship between the orientation of axial ligands, distortion of metallo¬porphy¬rin core from planarity, and the bulkiness of axial ligands and porphyrin substituents is discussed

Hydrated Collagen: Where Physical Chemistry, Medical Imaging, and Bioengineering Meet

It is well-known that collagen is the most abundant protein in the human body; however, what is not often appreciated is its fascinating physical chemistry and molecular physics. In this Perspective, we aim to expose some of the physicochemical phenomena associated with the hydration of collagen and to examine the role collagen's hydration water plays in determining its biological function as well as applications ranging from radiology to bioengineering. The main focus is on the Magic-Angle Effect, a phenomenon observed in Nuclear Magnetic Resonance (NMR) spectroscopy and Magnetic Resonance Imaging (MRI) of anisotropic collagenous tissues such as articular cartilage and tendon. While the effect has been known in NMR and MRI for decades, its exact molecular mechanism remains a topic of debate and continuing research in scientific literature. We survey some of the latest research aiming to develop a comprehensive molecular-level model of the Magic-Angle Effect. We also touch on other fields where understanding of collagen hydration is important, particularly nanomechanics and mechanobiology, biomaterials, and piezoelectric sensors.</p

Toroid cavity detectors for high-resolution NMR spectroscopy and rotating frame imaging: Capabilities and limitations

The capabilities of toroid cavity detectors for simultaneous rotating frame imaging and NMR spectroscopy have been investigated by means of experiments and computer simulations. The following problems are described: (a) magnetic field inhomogeneity and subsequent loss of chemical shift resolution resulting from bulk magnetic susceptibility effects, (b) image distortions resulting from off-resonance excitation and saturation effects, and (c) distortion of lineshapes and images resulting from radiation damping. Also, special features of signal analysis including truncation effects and the propagation of noise are discussed. B0 inhomogeneity resulting from susceptibility mismatch is a serious problem for applications requiring high spectral resolution. Image distortions resulting from off-resonance excitation are not serious within the rather narrow spectral range permitted by the RF pulse lengths required to read out the image. Incomplete relaxation effects are easily recognized and can be avoided. Also, radiation damping produces unexpectedly small effects because of selfcancellation of magnetization and short free induction decay times. The results are encouraging, but with present designs only modest spectral resolution can be achieved

Rate Constants and Thermodynamic Parameters of Rotation of Axial Ligands in a Bisligated Ferric Tetramesitylporphyrinate Complex Measured from the Temperature Dependence of 1H Transverse Relaxation Rates

The rate constant of the four-site cyclic chemical exchange between pyrrole protons in tetramesitylporphyrinatoiron(III)bis(2-methylimidazole) has been measured in the temperature range 236-255 K from 1H transverse relaxation times (T2) observed at the 500 MHz field strength. The values of the rate constant were obtained through the simulation of the observed T2's under the assumption that their values are dominated by the rate of the chemical exchange. The measurement of the exchange rate constant was also attempted at a lower temperature (230 K), but the use of extrapolated intrinsic T2's at that temperature introduces a significant error into the simulated rate constant. The thermodynamic activation parameters of the cyclic exchange were determined as ∆H‡ = 48 ± 1 kJ/mol and ∆S‡ = -10 ± 6 J/K⋅mol. The value of k varied from 30 to 270 s-1 in the temperature range 236-255 K. The near-zero exchange activation entropy is consistent with two previous studies but contradicts another study based on saturation-transfer measurements previously carried out by the authors of this work. A comparison of all available studies shows that the saturation-transfer experiments overestimate the value of the exchange rate constant by up to a factor of 2. It is shown that equilibrium with the high-spin form of the porphyrin complex does not affect the observed T2's in the temperature range used in this study but may do so at temperatures above 270 K. It is also shown that neither the linearity of the Curie plot nor the behavior of the T1 versus temperature plot should be considered indicators of the absence of the low-spin ⇌ high-spin equilibrium

MAS NMR measurements of intact articular bovine cartilage

Articular cartilage (AC), an avascular connective tissue lining articulating surfaces of the long bones, comprises extracellular biopolymers. In functionally compromised states such as osteoarthritis, thinned or lost AC causes reduced mobility and increased health-care costs. Understanding of the characteristics responsible for the load bearing efficiency of AC and the factors leading to its degradation are incomplete. DTI shows the structural alignment of collagen in AC [1] and T2 relaxation measurements suggest that the average director of reorientational motion of water molecules depends on the degree of alignment of collagen in AC [2]. Information on the nature of the chemical interactions involved in functional AC is lacking. The need for AC structural integrity makes solid state NMR an ideal tool to study this tissue. We examined the contribution of water in different functional ‘compartments’ using 1H-MAS, 13C-MAS and 13C-CPMAS NMR of bovine patellar cartilage incubated in D2O. 1H-MAS spectra signal intensity was reduced due to H/D exchange without a measureable redistribution of relative signal intensity. Chemical shift anisotropy was estimated by lineshape analysis of multiple peaks in the 1H-MAS spinning sidebands. These asymmetrical sidebands suggested the presence of multiple water species in AC. Therefore, water was added in small aliquots to D2O saturated AC and the influence of H2O and D2O on organic components was studied with 13C-MAS-NMR and 13C-CPMAS-NMR. Signal intensity in 13C-MAS spectra showed no change in relative signal intensity throughout the spectrum. In 13C-CPMAS spectra, displacement of water by D2O resulted in a loss of signal in the aliphatic region due to a reduction in proton availability for cross-polarization. These results complement dehydration studies of cartilage using osmotic manipulation [3] and demonstrate components of cartilage that are in contact with mobile water

Investigations of Rotation of Axial Ligands in Six-Coordinate Low-Spin Iron(III) Tetraphenylporphyrinates: Measurement of Rate Constants from Saturation Transfer Experiments and Comparison to Molecular Mechanics Calculations

Saturation transfer experiments have been utilized to measure the rate of axial ligand rotation in (tetramesitylporphyrinato)iron(III) bis(2-methylimidazole), [(TMP)Fe(2-MeImH)2]+. Saturation transfer peak intensities of four distinct pyrrole protons have been measured at a series of temperatures. Derivation of analytical expressions for steady-state peak intensities in the case of cyclic four-site exchange allowed the determination of the exchange rate constant. Previously measured longitudinal relaxation rate constants of the pyrrole protons of [(TMP)Fe(2-MeImH)2]+ have been used for rate constant determination. The temperature dependence of the rates has allowed estimation of the enthalpy barriers and entropy of this rotation. Modified MM2 potentials have also been used to study the rotation of axial ligands in [(TMP)Fe(1,2-Me2Im)2]+ and (tetraphenylporphyrinato)iron(III) bis(1-methylimidazole), [(TPP)Fe(1-MeIm)2]+. The "adiabatic" potential energy surfaces (PES) for rotation of axial ligands (minima achieved in all degrees of freedom except for constrained internal rotation coordinates for the two axial ligands) have been constructed for both complexes by combining a Ramachadran-type dihedral drive with geometry minimization or Monte Carlo single minimum analysis with subsequent geometry minimization. The PES of the TMP-hindered imidazole complex indicates that the preferable mode of rotation is synchronous clockwise or counterclockwise rotation of the two axial ligands, with an enthalpy barrier to such rotation of approximately 48 kJ/mol. For the TPP-nonhindered imidazole complex, enthalpy barriers to synchronous and asynchronous rotation were found to be 3.3 and 5.4 kJ/mol, respectively, thus prompting the assumption that no particular mode of rotation is highly preferable in that complex. The rotational enthalpy barrier for the TMP-hindered imidazole complex was found to be consistent with experimental measurements of the current (59 kJ/mol) and previous work (50-54 kJ/mol) (Shokhirev, N. V.; Shokhireva, T. Kh.; Polam, J. R.; Watson, C. T.; Raffii, K.; Simonis, U.; Walker, F. A. J. Phys. Chem. A 1997, 101, 0000. Nakamura, M.; Groves, J. T. Tetrahedron 1988, 44, 3225). The relationship between the orientation of axial ligands, the distortion of the metalloporphyrin core from planarity, and the bulkiness of axial ligands and porphyrin substituents is discussed