Metastable CrMnNi steels processed by laser powder bed fusion: experimental assessment of elementary mechanisms contributing to microstructure, properties and residual stress

The complex thermal history imposed by the laser-based powder bed fusion of metals (PBF-LB/M) process is known to promote the evolution of unique microstructures. In the present study, metastable CrMnNi steels with different nickel contents and, thus, different phase stabilities are manufactured by PBF-LB/M. Results clearly reveal that an adequate choice of materials will allow to tailor mechanical properties as well as residual stress states in the as-built material to eventually redundantize any thermal post-treatment. The chemical differences lead to different phase constitutions in as-built conditions and, thus, affect microstructure evolution and elementary deformation mechanisms upon deformation, i.e., twinning and martensitic transformation. Such alloys designed for additive manufacturing (AM) highlight the possibility to tackle well-known challenges in AM such as limited damage tolerance, porosity and detrimental residual stress states without conducting any post treatments, e.g., stress relieve and hot isostatic pressing. From the perspective of robust design of AM components, indeed it seems to be a very effective approach to adapt the material to the process characteristics of AM.

Promoting abnormal grain growth in Fe-based shape memory alloys through compositional adjustments

Iron-based shape memory alloys are promising candidates for large-scale structural applications due to their cost efficiency and the possibility of using conventional processing routes from the steel industry. However, recently developed alloy systems like Fe–Mn–Al–Ni suffer from low recoverability if the grains do not completely cover the sample cross-section. To overcome this issue, here we show that small amounts of titanium added to Fe–Mn–Al–Ni significantly enhance abnormal grain growth due to a considerable refinement of the subgrain sizes, whereas small amounts of chromium lead to a strong inhibition of abnormal grain growth. By tailoring and promoting abnormal grain growth it is possible to obtain very large single crystalline bars. We expect that the findings of the present study regarding the elementary mechanisms of abnormal grain growth and the role of chemical composition can be applied to tailor other alloy systems with similar microstructural features.

Novel iron-based shape memory alloys could be candidates for large-scale structural applications if their grains grew large and long enough. Here, the authors add titanium to an Fe–Mn–Al–Ni shape-memory alloy to promote large grains via compositional tuning of abnormal grain growth.

[Diffusion Weighted Magnetic Resonance Imaging and its Application in Ophthalmology]

The value of diffusion-weighted magnet resonance imaging (DWI-MRI) has been demonstrated for an ever growing range of clinical indications. DWI is sensitive to the diffusion of water molecules and probes their random displacement within tissue. DWI provides both qualitative and quantitative information on tissue characteristics, e.g. tissue cellularity. This review provides an overview of diffusion-weighted imaging and its emerging applications in ophthalmology. The basic physics and technical foundations of DWI are introduced. The emerging applications of DWI are surveyed, particularly in diseases of the eye, orbit and optical nerve.

How bold is blood oxygenation level-dependent (BOLD) magnetic resonance imaging of the kidney? Opportunities, challenges and future directions

Renal tissue hypoperfusion and hypoxia are key elements in the pathophysiology of acute kidney injury and its progression to chronic kidney disease. Yet, in vivo assessment of renal haemodynamics and tissue oxygenation remains a challenge. Many of the established approaches are invasive, hence not applicable in humans. Blood oxygenation level-dependent (BOLD) magnetic resonance imaging (MRI) offers an alternative. BOLD-MRI is non-invasive and indicative of renal tissue oxygenation. Nonetheless, recent (pre-) clinical studies revived the question as to how bold renal BOLD-MRI really is. This review aimed to deliver some answers. It is designed to inspire the renal physiology, nephrology and imaging communities to foster explorations into the assessment of renal oxygenation and haemodynamics by exploiting the powers of MRI. For this purpose, the specifics of renal oxygenation and perfusion are outlined. The fundamentals of BOLD-MRI are summarized. The link between tissue oxygenation and the oxygenation-sensitive MR biomarker T2∗ is outlined. The merits and limitations of renal BOLD-MRI in animal and human studies are surveyed together with their clinical implications. Explorations into detailing the relation between renal T2∗ and renal tissue partial pressure of oxygen (pO2 ) are discussed with a focus on factors confounding the T2∗ vs. tissue pO2 relation. Multi-modality in vivo approaches suitable for detailing the role of the confounding factors that govern T2∗ are considered. A schematic approach describing the link between renal perfusion, oxygenation, tissue compartments and renal T2∗ is proposed. Future directions of MRI assessment of renal oxygenation and perfusion are explored.

[Ultrahigh field MRI in context of neurological diseases]

Ultrahigh field magnetic resonance imaging (UHF-MRI) has recently gained substantial scientific interest. At field strengths of 7 Tesla (T) and higher UHF-MRI provides unprecedented spatial resolution due to an increased signal-to-noise ratio (SNR). The UHF-MRI method has been successfully applied in various neurological disorders. In neuroinflammatory diseases UHF-MRI has already provided a detailed insight into individual pathological disease processes and elucidated differential diagnoses of several disease entities, e.g. multiple sclerosis (MS), neuromyelitis optica (NMO) and Susac's syndrome. The excellent depiction of normal blood vessels, vessel abnormalities and infarct morphology by UHF-MRI can be utilized in vascular diseases. Detailed imaging of the hippocampus in Alzheimer's disease and the substantia nigra in Parkinson's disease as well as sensitivity to iron depositions could be valuable in neurodegenerative diseases. Current UHF-MRI studies still suffer from small sample sizes, selection bias or propensity to image artefacts. In addition, the increasing clinical relevance of 3T-MRI has not been sufficiently appreciated in previous studies. Although UHF-MRI is only available at a small number of medical research centers it could provide a high-end diagnostic tool for healthcare optimization in the foreseeable future. The potential of UHF-MRI still has to be carefully validated by profound prospective research to define its place in future medicine.

[In vivo MR microscopy of the human eye]

MR microscopy using an ultra high-field MR system is a novel non-invasive imaging technique to explore the human eye without optical distortions. This review aims to provide an insight into the technique. Normal MR microscopic anatomy of the human eye in vivo is demonstrated and clinical applications of MR microscopy are discussed.

[Cardiovascular ultrahigh field magnetic resonance imaging : challenges, technical solutions and opportunities]

CLINICAL/METHODICAL ISSUE

This involves high spatial resolution cardiac imaging with ultrahigh magnetic fields (7 T) and clinically acceptable image quality.

STANDARD RADIOLOGICAL METHODS

Cardiovascular magnetic resonance imaging (MRI) at a field strength of 1.5 T using a spatial resolution of (2 × 2 × 6-8) mm(3).

METHODICAL INNOVATIONS

Cardiac MRI at ultrahigh field strength makes use of multitransmit/receive radiofrequency (RF) technology and development of novel technology that utilizes the traits of ultrahigh field MRI.

PERFORMANCE

Enhanced spatial resolution which is superior by a factor of 6-10 to what can be achieved by current clinical cardiac MRI. The relative spatial resolution (pixels per anatomical structure) comes close to what can be accomplished by current cardiac MRI in small rodents.

ACHIEVEMENTS

Feasibility studies demonstrate the gain in spatial resolution at 7.0 T due to the sensitivity advantage inherent to ultrahigh magnetic fields.

PRACTICAL RECOMMENDATIONS

Please stay tuned and please put further weight behind the solution of the remaining technical problems of cardiac MRI at 7.0 T.

Early effects of an x-ray contrast medium on renal T(2) */T(2) MRI as compared to short-term hyperoxia, hypoxia and aortic occlusion in rats

AIM

X-ray contrast media (CM) can cause acute kidney injury (AKI). Medullary hypoxia is pivotal in CM-induced AKI, as indicated by invasively and pin-point measured tissue oxygenation. MRI provides spatially resolved blood oxygenation level-dependent data using T2 * and T2 mapping. We studied CM effects on renal T2 */T2 and benchmarked them against short periods of hyperoxia, hypoxia and aortic occlusion (AO).

METHODS

Rats were equipped with carotid artery catheters (tip towards aorta) and supra-renal aortic occluders. T2 */T2 mapping was performed using a 9.4-T animal scanner. CM (1.5 mL iodixanol) was injected into the thoracic aorta with the animal in the scanner followed by 2 h of T2 */T2 mapping. For T2 */T2 assessment, regions of interest in the cortex (C), outer medulla (OM), inner medulla (IM) and papilla (P) were determined according to morphological features.

RESULTS

Hyperoxia increased T2 * in C (by 17%) and all medullary layers (25-35%). Hypoxia decreased T2 * in C (40%) and all medullary layers (55-60%). AO decreased T2 * in C (18%) and all medullary layers (30-40%). Upon injection of CM, T2 * increased transiently, then decreased, reaching values 10-20% below baseline in C and OM and 30-40% below baseline in IM and P.

CONCLUSION

T2 * mapping corroborates data previously obtained with invasive methods and demonstrates that CM injection affects renal medullary oxygenation. CM-induced T2 * decrease in OM was small vs. hypoxia and aortic occlusion. T2 * decrease obtained for hypoxia was more pronounced than for AO. This indicates that T2 * may not accurately reflect blood oxygenation under certain conditions.

Linking non-invasive parametric MRI with invasive physiological measurements (MR-PHYSIOL): towards a hybrid and integrated approach for investigation of acute kidney injury in rats

Acute kidney injury of various origins shares a common link in the pathophysiological chain of events: imbalance between renal medullary oxygen delivery and oxygen demand. For in vivo assessment of kidney haemodynamics and oxygenation in animals, quantitative but invasive physiological methods are established. A very limited number of studies attempted to link these invasive methods with parametric Magnetic Resonance Imaging (MRI) of the kidney. Moreover, the validity of parametric MRI (pMRI) as a surrogate marker for renal tissue perfusion and renal oxygenation has not been systematically examined yet. For this reason, we set out to combine invasive techniques and non-invasive MRI in an integrated hybrid setup (MR-PHYSIOL) with the ultimate goal to calibrate, monitor and interpret parametric MR and physiological parameters by means of standardized interventions. Here we present a first report on the current status of this multi-modality approach. For this purpose, we first highlight key characteristics of renal perfusion and oxygenation. Second, concepts for in vivo characterization of renal perfusion and oxygenation are surveyed together with the capabilities of MRI for probing blood oxygenation-dependent tissue stages. Practical concerns evoked by the use of strong magnetic fields in MRI and interferences between MRI and invasive physiological probes are discussed. Technical solutions that balance the needs of in vivo physiological measurements together with the constraints dictated by small bore MR scanners are presented. An early implementation of the integrated MR-PHYSIOL approach is demonstrated including brief interventions of hypoxia and hyperoxia.

Corrosion fatigue behavior of a biocompatible ultrafine-grained niobium alloy in simulated body fluid

The present study reports on the corrosion fatigue behavior of ultrafine-grained (UFG) Niobium 2 wt-% Zirconium (NbZr) alloy in simulated body fluid (SBF). The alloy was processed using multipass equal channel angular processing at room temperature, resulting in a favorable combination of high strength and ductility along with superior biocompatibility and excellent corrosion resistance. Electrochemical measurements revealed stable passive behavior in SBF saline solutions, similar to conventional Ti-6Al-4V alloy. High-cycle fatigue tests showed no alteration in the crack initiation behavior due to the SBF environment, and an absence of pitting and corrosion products. More severe test conditions were obtained in the fatigue crack growth experiments in saline environments. Crack growth rates in UFG NbZr were marginally increased in SBF as compared to laboratory air at a constant test frequency of 20 Hz. Upon a 100 fold decrease in the test frequency, slightly higher crack growth rates were observed only in the near-threshold region. Such excellent corrosion and corrosion fatigue properties of UFG NbZr recommend it as an attractive new material for biomedical implants.

Comparison of image quality in magnetic resonance imaging of the knee at 1.5 and 3.0 Tesla using 32-channel receiver coils

We examined to what degree the visualization of anatomic structures in the human knee is improved using 3.0-T magnetic resonance imaging (MRI) and many element RF receive coils as compared to 1.5 T. We imaged 20 knees at 1.5 and 3.0 T using T2-weighted STIR, T2-weighted gradient echo, T1-weighted spin-echo, true-FISP and T2-weighted fast spin echo techniques in conjunction with 32-element RF coil arrays. The 3.0-T examination was considerably faster than its 1.5-T counterpart. A superior subjective visibility at 3.0 T vs 1.5 T was found in 27 of 50 evaluated structures (meniscus, ligaments) with the exception of true-FISP techniques. The 3.0-T examination provided a better visibility (evaluated by blinded consensus-reading by two radiologists) of small structures such as the ligamentum transversum genu. Also, cartilage was better delineated at 3.0 T. A 23% increased average signal-to-noise ratio as assessed using a temporal filter was observed at 3.0 T as compared to 1.5 T. At 3.0 T, imaging of the human knee is faster and results in a subjective visibility of anatomic structures that is superior to and competitive with 1.5 T.

[Acceleration of cardiovascular MRI using parallel imaging: basic principles, practical considerations, clinical applications and future directions]

Cardiovascular Magnetic Resonance (CVMR) imaging has proven to be of clinical value for non-invasive diagnostic imaging of cardiovascular diseases. CVMR requires rapid imaging; however, the speed of conventional MRI is fundamentally limited due to its sequential approach to image acquisition, in which data points are collected one after the other in the presence of sequentially-applied magnetic field gradients and radiofrequency pulses. Parallel MRI uses arrays of radiofrequency coils to acquire multiple data points simultaneously, and thereby to increase imaging speed and efficiency beyond the limits of purely gradient-based approaches. The resulting improvements in imaging speed can be used in various ways, including shortening long examinations, improving spatial resolution and anatomic coverage, improving temporal resolution, enhancing image quality, overcoming physiological constraints, detecting and correcting for physiologic motion, and streamlining work flow. Examples of these strategies will be provided in this review, after some of the fundamentals of parallel imaging methods now in use for cardiovascular MRI are outlined. The emphasis will rest upon basic principles and clinical state-of-the art cardiovascular MRI applications. In addition, practical aspects such as signal-to-noise ratio considerations, tailored parallel imaging protocols and potential artifacts will be discussed, and current trends and future directions will be explored.

Ultra-fast low-angle rapid acquisition and relaxation enhancement (UFLARE) in patients with epilepsy

MRI is an important diagnostic tool in patients with epilepsy, but patient motion during long scans may result in image artefacts. We studied the utility of an ultra-fast MR sequence in patients with epilepsy. Ultra-fast low-angle rapid acquisition and relaxation enhancement (UFLARE) images were acquired for 100 consecutive patients and nine control subjects. Scans were compared with routine T2-weighted spin echo images for signal-to-noise ratio, contrast, and conspicuity, followed by a blind review of lesion detectability. UFLARE scans were also acquired for 15 patients who moved during conventional scans. All UFLARE scans had lower signal-to-noise ratios and lower contrast than the T2-weighted images. Compared with T1- and T2-weighted, PD and FLAIR images, 86% of hippocampal sclerosis (HS), 92% of large but only 24% of small white-matter lesions were detected on the blind review of the UFLARE images. Reduced motion artefacts were seen on the UFLARE images in all 15 patients who moved during the conventional scans, and in three patients UFLARE was the only sequence we were able to obtain. Despite the lower lesion detectability for smaller lesions, the use of an ultra-fast MRI sequence such as UFLARE may be very useful in patients who are not able to co-operate during conventional MRI examinations, if a general anaesthetic is to be avoided.

Acute and chronic changes of the apparent diffusion coefficient in neurological disorders--biophysical mechanisms and possible underlying histopathology

Diffusion-weighted imaging (DWI) of the brain has become a valuable tool for the reliable detection and diagnosis of several neurological disorders. Although DWI is in wide use in daily practice, the underlying biophysical mechanisms that contribute to changes in the apparent diffusion coefficient (ADC) are still under discussion. Alterations in the apparent water diffusion rate reflect pathological changes in the brain tissue state, via changes in the diffusion characteristics of the intra- and extra-cellular water compartments including restricted diffusion, water exchange across permeable boundaries, the concept of the extra-cellular tortuosity and the intra- and extra-cellular volume fraction. A reduction of the ADC has been detected in acute neurological diseases, while disease states associated with dominant acute vasogenic edema formation or chronic tissue destruction usually show elevations of the ADC. Compromise of energy metabolism is likely to contribute to a reduction of the ADC while already minor structural disintegration may contribute to elevations of the ADC.

On the application of susceptibility-weighted ultra-fast low-angle RARE experiments in functional MR imaging

The applicability of displaced, split-echo, and phase-cycled variants of the blood oxygenation level-dependent (BOLD) sensitized ultra-fast low-angle rapid acquisition and relaxation enhancement (UFLARE) technique for the mapping of brain function are examined in functional magnetic resonance imaging (fMRI) experiments at high magnetic field strength (3 T). Activation maps are presented for visual and motor-sensory activation. For the visual studies the range of the stimulation-associated signal intensity changes is 5-7% in voxels containing mainly gray matter and 10-15% in voxels dominated by larger vessels. The motor studies reveal signal changes of 5-10% in the primary motor cortex and in the supplementary motor area. For gray matter, T2* increases from 31.2 +/- 1.5 msec under baseline conditions to 33.0 +/- 1.5 msec during periods of visual stimulation. The results clearly demonstrate that T2*-weighted UFLARE is a robust and reliable method for detection of brain activation. The relative pros and cons of displaced, split-echo, and phase-cycled T2*-sensitized UFLARE versions are discussed for fMRI applications. Since the susceptibility weighting can be freely adjusted from zero upward, the UFLARE variants used are particularly suitable for functional examinations in regions with poor magnetic field homogeneity and at high magnetic field strengths.

Cortical areas and the control of self-determined finger movements: an fMRI study

We investigated cortical areas involved in the control of self-determined finger movements. In a tapping task, subjects tapped with different movement frequencies in two different movement conditions (predetermined vs self-determined). fMRI provided evidence for the involvement of the horizontal and ascending parts of the intraparietal sulcus (IPS), the left superior frontal gyrus and the posterior cingulate gyrus in the control of self-determined finger movements. Higher movement frequency increased the extent of activated area only in the horizontal part of IPS. The results suggest a major role of the IPS in controlling sequences of finger movements. This area probably serves as a region for integration of motor, sensory and sensorimotor feedback information used for movement control.

Temporal and regional changes during focal ischemia in rat brain studied by proton spectroscopic imaging and quantitative diffusion NMR imaging

The early development of focal ischemia after permanent occlusion of the right middle cerebral artery (MCA) was studied in six rats using interleaved measurements by diffusion-weighted NMR imaging (DWI) of water and two variants of proton spectroscopic imaging (SI), multiecho SI (TE: 136, 272, 408 ms) and short TE SI (TE: 20 ms). Measurements on a 4.7-T NMR imaging system were performed between the control phase and approximately 6 h postocclusion. In the center of the ischemic lesion of all rats, the apparent diffusion coefficient (ADC) decreased rapidly to 84.4 +/- 4.2% (mean +/- SD) of the control values approximately 2 min postocclusion. Approximately 6 h postocclusion, the ADC was reduced to 67.1 +/- 5.9%. In contrast, large differences between the animals were observed for the temporal increase of lactate (Lac) in the ipsilateral hemisphere. The maximum Lac signal was reached in four rats after 0.5-1.5 h, and in two rats was not reached even after 6 h postocclusion. Six h postocclusion, SI spectra measured at a TE of 136 ms revealed a decrease in the CH3 signal of N-acetylaspartate (NAA) to 67 +/- 13% of the control values. Differences were observed between the spatial regions of decreased NAA and increased Lac. In the lesions, a T2 relaxation time of Lac of 292 +/- 40 ms, considering a J-coupling constant of 6.9 Hz, was measured. Furthermore, a prolongation of the T2 of the CH3 signal of creatine/phosphocreatine (Cr/PCr) was observed in the lesion, from 163 +/- 22 ms during control to 211 +/- 41 ms approximately 6 h postocclusion. The experiments proved that DWI and proton SI are valuable tools to provide complementary information on processes associated with brain infarcts.

Biexponential diffusion attenuation in various states of brain tissue: implications for diffusion-weighted imaging

Diffusion-weighted single voxel experiments conducted at b-values up to 1 x 10(4) smm-2 yielded biexponential signal attenuation curves for both normal and ischemic brain. The relative fractions of the rapidly and slowly decaying components (f1, f2) are f1 = 0.80 +/- 0.02, f2 = 0.17 +/- 0.02 in healthy adult rat brain and f1 = 0.90 +/- 0.02, f2 = 0.11 +/- 0.01 in normal neonatal rat brain, whereas the corresponding values for the postmortem situation are f1 = 0.69 +/- 0.02, f2 = 0.33 +/- 0.02. It is demonstrated that the changes in f1 and f2 occur simultaneously to those in the extracellular and intracellular space fractions (fex, f(in)) during: (i) cell swelling after total circulatory arrest, and (ii) the recovery from N-methyl-D-aspartate induced excitotoxic brain edema evoked by MK-801, as measured by changes in the electrical impedance. Possible reasons for the discrepancy between the estimated magnitude components and the physiological values are presented and evaluated. Implications of the biexponential signal attenuation curves for diffusion-weighted imaging experiments are discussed.

Status of the neonatal rat brain after NMDA-induced excitotoxic injury as measured by MRI, MRS and metabolic imaging

Intrastriatal injection of the excitotoxin N-methyl-D-aspartate (NMDA) in neonatal rat brain resulted in an acute ipsilateral decrease of the apparent diffusion coefficient (ADC) of brain tissue water, as measured with diffusion-weighted MRI. The early diffusion changes were accompanied by only mild changes in the overall metabolic status as measured by in vivo 1H MRS and 31P MRS and metabolic imaging of brain sections. Minimal decreases in the high-energy phosphate levels and a small hemispheric acidosis were observed in the first 6 h after NMDA administration. In addition, there was very modest lactate accumulation. Twenty-four hours after the induction of the excitotoxic injury the tissue energy status was still only moderately affected, whereas an overall decrease of 1H MRS-detected brain metabolites was found. Treatment with the non-competitive NMDA-antagonist MK-801 given within 90 min after NMDA injection rapidly reversed the NMDA-induced changes in the entire ipsilateral hemisphere. The effect of the competitive NMDA-antagonist D-CPPene was restricted to the cortical areas and was accomplished on a slower time scale. Our results indicate that; (i) early excitotoxicity in the neonatal rat brain does not lead to profound changes in the metabolic status; and (ii) brain tissue water ADC changes are not necessarily associated with a metabolic energy failure.

Adaptation of cellular metabolism to anisosmotic conditions in a glial cell line, as assessed by 13C-NMR spectroscopy

13C-NMR spectroscopy of perchloric acid and lipid extracts of F98 glioma cells showed that volume-regulatory processes under anisosmotic conditions were accompanied by marked alterations in cellular metabolism. Production of alanine, glutamate, and glycine from [U-13C]-glucose is decreased under hypotonic stress and is oppositely increased under hypertonic stress. In contrast, degradation of these molecules is raised under hypotonic conditions and reduced under hypertonic conditions. Furthermore, phospholipid synthesis is decreased under hypertonic stress and increased under hypotonic stress. Obviously, glial metabolism is directed under hypertonic conditions to maintain a high level of small, osmotically active molecules, whereas under hypotonic conditions molecular fragments are increasingly incorporated into the phospholipids and so do not contribute to the osmotic pressure. The latter is evoked by the activation of membrane synthesis process to compensate for stretching and/or damaging of the membranes due to cell swelling.

Interpretation of DW-NMR data: dependence on experimental conditions

This review examines the effect of experimental conditions on the data obtainable from diffusion weighted NMR experiments. The origin and forms of the Stejskal-Tanner experiment are presented, and the relative merits of bipolar to monopolar diffusion weighting gradient pulses are discussed, as are those of spin-echo and stimulated-echo weighting schemes. The short pulse Stejskal-Tanner experiment as required for q-space imaging is described. Criteria for successful diffusion weighted imaging are given, and current strategies for diffusion weighted imaging are evaluated against these. The range of biological objects accessible to diffusion weighted NMR is summarized, together with the associated experimental limitations. In the final section the dependence of diffusion weighted NMR data on diffusion time and b-value range is examined, and the relationship between apparent restricted diffusion and the size of the extracellular space is demonstrated.

Changes in organic solutes, volume, energy state, and metabolism associated with osmotic stress in a glial cell line: a multinuclear NMR study

Diffusion-weighted in vivo 1H-NMR spectroscopy of F98 glioma cells embedded in basement membrane gel threads showed that the initial cell swelling to about 180% of the original volume induced under hypotonic stress was followed by a regulatory volume decrease to nearly 100% of the control volume in Dulbecco's modified Eagle's medium (DMEM) but only to 130% in Krebs-Henseleit buffer (KHB, containing only glucose as a substrate) after 7 h. The initial cell shrinkage to approx. 70% induced by the hypertonic stress was compensated by a regulatory volume increase which after 7 h reached almost 100% of the control value in KHB and 75% in DMEM. 1H-, 13C- and 31P-NMR spectroscopy of perchloric acid extracts showed that these volume regulatory processes were accompanied by pronounced changes in the content of organic osmolytes. Adaptation of intra- to extracellular osmolarity was preferentially mediated by a decrease in the cytosolic taurine level under hypotonic stress and by an intracellular accumulation of amino acids under hypertonic stress. If these solutes were not available in sufficient quantities (as in KHB), the osmolarity of the cytosol was increasingly modified by biosynthesis of products and intermediates of essential metabolic pathways, such as alanine, glutamate and glycerophosphocholine in addition to ethanolamine. The cellular nucleoside triphosphate level measured by in vivo 31P-NMR spectroscopy indicated that the energy state of the cells was more easily sustained under hypotonic than hypertonic conditions.

Health and infarcted brain tissues studied at short diffusion times: the origins of apparent restriction and the reduction in apparent diffusion coefficient

The significance of NMR water diffusion measurements performed at short diffusion times (< 10 ms) for brain tissue is examined. An apparent restriction to diffusion for both healthy and cytotoxically edematous tissue is shown: cytotoxic edema lengthens the diffusion time at which this phenomenon is visible. The dramatic reduction in apparent diffusion coefficient (ADC) observed in the core of cytotoxic edema is explained in terms of the enclosure of extracellular water in non-contiguous pockets in conjunction with the shift of water from the extra-to the intracellular space. The model presented provides an explanation for the ADC reduction without recourse to changes in the cell membrane permeability to water, or unrealistic values for the extra- and intracellular diffusion coefficients.

Detection of apparent restricted diffusion in healthy rat brain at short diffusion times

The application of bipolar diffusion sensitizing gradient pulses to significantly reduce the diffusion time is described. This approach is combined with the rapid U-FLARE imaging sequence. Three diffusion-sensitized types of experiments are compared and their suitability for detecting restricted diffusion is discussed. Experiments using a modification of the diffusion weighting by varying the diffusion time between 1.6 and 6.0 ms obtained nonmonoexponential signal attenuation curves from both healthy brains and postmortem. This behavior is indicative of restricted diffusion, but as it is detectable only at short diffusion times, in contrast to a restriction due to impermeable barriers, we have termed this "apparent restriction."

Incidence of apparent restricted diffusion in three different models of cerebral infarction

High speed magnetic resonance imaging (MRI) and short diffusion times are used to investigate the appearance of restricted diffusion in three different models of cerebral infarction. The models are: the middle cerebral artery occlusion (MCAO) model in the rat, the carotid occlusion model in the gerbil, and the Rose Bengal microvascular occlusion model in the rat. All three were investigated for 16 b-values equally spaced between 10 and 1510 s/mm2 using two distinct experiments. In the ct (constant time) experiment, the diffusion time was held constant at 11.7 ms while the b-value was varied with the gradient strength. In the cg (constant gradient) experiment, the gradient strength was held constant and the b-value increased by varying the diffusion time from 4.4 to 11.7 ms. A monoexponential decay of the signal intensity with b-value in the ct experiment accompanied by nonmonoexponential (NME) decay in the cg experiment is indicative of restricted diffusion. As this phenomenon is detectable only at short diffusion times, it cannot be due to restriction by impermeable membranes, and we have thus termed this apparent restriction. For the MCAO model and the carotid occlusion model, apparent restriction was found both inside the infarct territory and in some regions outside it. No definite evidence for restriction was found for the Rose Bengal model, which was, however, only studied from 24 h post-insult.