The strengths of in vivo magnetic resonance imaging (MRI) to study environmental adaptational physiology in fish

Adaptational physiology studies how animals cope with their environment, even if this environment is subject to permanent fluctuations such as tidal or seasonal variations. Aquatic organisms are generally more prone to be exposed to osmotic, hypoxic and temperature challenges than terrestrial animals. Some of these challenges are more restraining in an aquatic environment. To date, very few studies have used in vivo magnetic resonance imaging (MRI) to uncover the physiological mechanisms that respond to or compensate for these challenges. This paper provides an overview of what has been accomplished thus far by using MRI to study the environmental physiology of fish. It introduces the reader to the use of small teleost fish such as carp (12 cm, 60 g) and eelpout (25 cm, 50 g) as models for such research and to provide new perceptions into the applicability of MRI tools based on new insights into the nature of MRI contrast. Representative MRI studies have made contributions to the identification of the lack of cell volume repair in stenohaline fish during osmotic stress. They have studied the underlying physiological mechanisms of brain anoxia tolerance in fish and have qualified the role of the cardio-circulatory system in setting thermal tolerance windows of fish.     

Liposomes as fatty acids carriers in isolated rat liver: effect on energy metabolism and on isolated mitochondria activity

The effects of fatty acids (FA)-carrier, egg-lecithin liposomes (LIPO) as alternative to BSA, on ATP, glycogen and glucose contents in isolated perfused liver of fed rats were non-invasively studied using³¹P/¹³C nuclear magnetic resonance (NMR). Oxidative phosohorylation was studied in isolated mitochondria from the same liver consecutively to the NMR experiments. ATP content decreased slowly and ATP turnover was similar during the perfusion with saline solution (KHB) or LIPO. However. LIPO induced an enhancement of respiratory control ratio in isolated mitochondria. Tissue glycogen and glucose content decreased when FA (linoleate or linolenate) were perfused with defatted BSA, (3%) or LIPO (600 mg/l) whereas glucose excretion level was unchanged and lactate excretion tended to increase, reflecting changes in the cytosolic redox state and/or an enhancement of glycolysis. Addition of FA (0.5 or 1.5 mM) to LIPO caused a dramatic fall in liver ATP, a mitochondrial uncoupling and an impairment of the phosphorylation activity. Perfusion with FA (1.5 mM) carried by BSA significantly increased the ATP degradation without change of mitochondrial function. Owing to the higher affinity of BSA than LIPO for FA, these latter could be more easily released from complex LIPO-FA, increasing their uncoupling effect. Hence, the FA concentrations have to be largely decreased from the above currently used concentrations to avoids this effect. It will then be possible to minimize the effector action of FA and to study their more specific metabolic function as fuel. It was concluded that LIPO were appropriate carriers to study the different metabolic effects of FA.     

Effect of opsonins on the uptake of magnetic starch microspheres by rat Kupffer cells

The influence of various sera and proteins on the uptake of a superparamagnetic colloid (magnetic starch microspheres (MSM); particle size, 200 nm; crystal size, 10 nm) by the isolated and perfused rat liver has been studied. It is demonstrated that the capture of MSM is slightly reduced by the addition of rat blood to the protein-free perfusion medium but highly reduced by newborn calf serum (NCS). The SDS-PAGE (sodium dodecyl sulfate-polyacrylamide gel electrophoresis) analysis of proteins adsorbed on the nanoparticles incubated in NCS reveals major coating by albumin and IgG. The addition of bovine IgG to the perfusion fluid reduces the rate of MSM uptake in the same extent that NCS, whereas fetal calf serum that contains only traces of IgG weakly alters the MSM clearance. Finally, complemented and decomplemented NCS exhibit the same influence on the MSM hepatic extraction. It is concluded that although lectins are largely involved in the uptake of MSM administered in the absence of proteins, opsonins receptors are implicated when the perfusion medium contains relevant blood components.     

Preliminary ex vivo 3D microscopy of coronary arteries using a standard 1.5 T MRI scanner and a superconducting RF coil

This paper presents the feasibility of three-dimensional (3D) magnetic resonance (MR) histology of atheromatous coronary lesions in the entire human heart ex vivo using a standard 1.5 T scanner and a 12 mm high-temperature superconducting (HTS) surface coil. The HTS coil was a five-turn transmission-line resonator operated at 77 K, affording a signal-to-noise ratio (SNR) gain of about ninefold as compared to a similar, room-temperature copper coil. Local microscopy at the surface of an explanted, entire heart was achieved by a 3D spoiled gradient echo sequence and assessed by comparison with conventional histology. One hundred and twenty four adjacent cross sections of the coronary artery, with voxels of 59×59×100 m³ and an SNR of about 20, were obtained in 25 min. Consecutive data sets were combined to reconstruct extended views along the artery. Compared to histology, MR microscopy allowed precise nondestructive 3D depiction of the architecture of the atheromatous plaques. This is the first report of microscopic details (less than 10^–3 mm³ voxels) of diseased arteries obtained in an entire human heart preserving the arterial integrity and the spatial geometry of atheroma. This noninvasive microscopy approach using a HTS surface coil might be applied in vivo to study the architecture and components of superficial human structures, using routine MR scanners.     

Quantitative assessment of regional myocardial function in a rat model of myocardial infarction using tagged MRI

We characterized global and regional left ventricular (LV) function during post myocardium infarction (MI) remodeling in rats, which has been incompletely described by previous MRI studies. To assess regional wall motion, four groups of infarcted animals corresponding to 1–2, 3–4, 6–8 and 9–12 weeks post-MI respectively were imaged using a fast gradient echo sequence with a 2D spatial modulation of magnetization (SPAMM) tagging preparation. An additional group was serially imaged (1–2 and 6–7 weeks post-MI) to assess the global function. Regional and global functional parameters of infarcted rats were compared to non-infarcted normal rats. Compared to normal rats, a decrease in ejection fraction (70 ±7 vs. 40 ± 8%, p<0.05) was observed in rats with MI. Maximal and minimal principal stretches (₁, ₂) and strains (E₁, E₂), principal angle () and displacement varied regionally in normal rats but deviated significantly from the normal values in rats with MI particularly in the infarcted and adjacent zones. Not only was strain magnitude reduced segmentally post-MI, but strain direction became more circumferentially oriented, particularly in rats with larger infarctions. We report the first regional myocardial strain values in normal and infarcted rats. These results parallel findings in humans, and provide a unique tool to examine regional mechanical influences on the remodeling process.     

In vivo 13C chemical shift imaging of the human liver

Chemical shift imaging (CSI) was applied to measure natural abundance proton-decoupled¹³C-NMR spectra of the human liver. Large surface coils were designed for¹³C spectra acquisition (16-cm-diameter circular coil) as well as for proton imaging and decoupling (21×20-cm butterfly coil). Such sizes allowed deep observations of the abdomen. A space matrix of 8×4 voxels (4×8 cm each) was defined using 32 phase-encoding steps. Magnetic field gradients were adjusted on multicompartment phantoms to limit contamination between voxels. Spectral maps containing {¹H}-¹³C spectra of liver from healthy volunteers with an acceptable signal-to-noise ratio were recorded within 20 min. Liver spectra exhibited well-defined resonances corresponding to fatty acyl chains, carbonyl groups, and sugars. The (C-l)-glycogen resonance was also detected under such conditions. Such a technique would be of interest in the development of metabolic investigations on the human liverin vivo.     

Application of a micro‐multipoint laser doppler velocimeter for in vivo evaluation of subcutaneous blood flow

We have developed a micro‐multipoint laser Doppler velocimeter (µMLDV), which is the newest version of our improved laser Doppler velocimeter (LDV). This device can potentially be used clinically for the evaluation of the internal diameter of blood vessels, inspection of blood vessel protrusions and diagnosis of melanoma. To date, we have verified the accuracy of this device for three‐dimensional measurement of blood vessels in vivo and measurement of melanoma. The state of blood flow is one of the evaluation criteria for tissue fixing. However, the state of blood flow is difficult to determine visually. If the state of blood flow can be visualized using this system, without damaging the affected area, it is possible to evaluate fixing noninvasively. Therefore, we measured normal blood vessels and cases in which there was no blood flow for comparison and verified the possibility of using the µMLDV as an evaluation device. © 2013 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Improved estimation of tissue hydration and bound water fraction in rat liver tissue

Based on recent improvements in the field, biexponential data from fresh rat liver and monoexponential data from cold storage experiments allow quantification of three distinct relaxation components in liver tissue: bound water (4.2%, R₁=12.0 ± 1.7 s^–1, R₂=440 ± 180 s^–1); structured water (59%, R₁ 3.3 ± 0.07 s^–1, R₂ 24.9 ±1.1 s^–1); and free water (37%, R₁=R₂ 0.4 s^–1). However, only the relaxation rates of the structured water component change with water content: R_1A (s^–1)=6.53 * M_s/M_w – 0.77 (r²=0.911); R_2A (s^–1)=71.15 * M_s/M_w – 3.09 (r²=0.956), respectively. This suggests a slow exchange between bound and structured water in liver cells.     

In vivo diffusion tensor mapping of the brain of squirrel monkey, rat, and mouse using single-shot STEAM MRI

The purpose was to assess the potential of half Fourier diffusion-weighted single-shot STEAM MRI for diffusion tensor mapping of animal brain in vivo. A STEAM sequence with image acquisition times of about 500 ms was implemented at 2.35 T using six gradient orientations and b values of 200, 700, and 1200 s mm^–2. The use of half Fourier phase-encoding increased the signal-to-noise ratio by 45% relative to full Fourier acquisitions. Moreover, STEAM-derived maps of the relative anisotropy and main diffusion direction were completely free of susceptibility-induced signal losses and geometric distortions. Within measuring times of 3 h, the achieved resolution varied from 600×700×1000 m³ for squirrel monkeys to 140×280×720 m³ for mice. While in monkeys the accessible white matter fiber connections were comparable to those reported for humans, detectable fiber structures in mice focused on the corpus callosum, anterior commissure, and hippocampal fimbria. In conclusion diffusion-weighted single-shot STEAM MRI allows for in vivo diffusion tensor mapping of the brain of squirrel monkeys, rats, and mice without motion artifacts and susceptibility distortions.