A simplified sequence for observing deoxymyoglobin signals in vivo: myoglobin excitation with dynamic unexcitation and saturation of water and fat (MEDUSA)

Musculoskeletal spectroscopy

Magnetic resonance spectroscopy (MRS) of skeletal muscle has been successfully applied by physiologists over several decades, particularly for studies of high-energy phosphates (by (31)P-MRS) and glycogen (by (13)C-MRS). Unfortunately, the observation of these heteronuclei requires equipment that is typically not available on clinical MR scanners, such as broadband capability and a second channel for decoupling and nuclear Overhauser enhancement (NOE). On the other hand, (1)H-MR spectra of skeletal muscle can be acquired on many routine MR systems and also provide a wealth of physiological information. In particular, studies of intramyocellular lipids (IMCL) attract physiologists and endocrinologists because IMCL levels are related to insulin resistance and thus can lead to a better understanding of major health problems in industrial countries. The combination of (1)H-, (13)C-, and (31)P-MRS gives access to the major long- and short-term energy sources of skeletal muscle. This review summarizes the technical aspects and unique MR-methodological features of the different nuclei. It reviews clinical studies that employed MRS of one or more nuclei, or combinations of MRS with other MR modalities. It also illustrates that MR spectra contain additional physiological information that is not yet used in routine clinical applications.

Muscle blood flow and oxygenation measured by NMR imaging and spectroscopy

Tissue perfusion and oxygenation in many organs can be evaluated by various NMR techniques. This review focuses on the specificities, limitations and adaptations of the NMR tools available to investigate perfusion and oxygenation in the skeletal muscle of humans and animal models. A description of how they may be used simultaneously is provided as well. 1H NMR spectroscopy of myoglobin (Mb) monitors intramyocytic oxygenation. It measures the level of deoxy-Mb, from which Mb concentration, Mb desaturation/resaturation rates, muscle oxygenation changes and intracellular partial oxygen pressure (pO2) can be calculated. Positive and negative blood oxygen level-dependent (BOLD) contrasts exist in skeletal muscle. BOLD contrasts primarily reflect changes in capillary-venous oxygenation, but are also directly or indirectly dependent on muscle blood volume, perfusion, vascular network architecture and angulation, relative to the main magnetic field. Arterial spin labelling (ASL) techniques, having high spatial and temporal resolution, are the methods of choice to quantify and map skeletal muscle perfusion non-invasively. Limitations of ASL are poor contrast-to-noise ratio and sensitivity to movement; however, with the introduction of specific adaptations, it has been proven possible to measure skeletal muscle perfusion at both rest and during exercise. The possibility of combining these NMR measurements with others into a single dynamic protocol is most interesting. The 'multiparametric functional (mpf) NMR' concept can be extended to include the evaluation of muscle energy metabolism simultaneously with 31P NMR or with lactate double quantum filtered 1H NMR spectroscopy, an approach which would make NMR an exceptional tool for non-invasive investigations of integrative physiology and biochemistry in skeletal muscle in vivo.

Mitochondrial NADH as the bellwether of tissue O2 delivery

It is proposed that the redox state of mitochondrial NADH will complement blood gas analysis for measuring the health and welfare of human tissues. Use of arterial oxygen saturation levels (SaO2), especially as assayed by the Nellcor instrument, has spread almost everywhere in medicine despite the fact that hypoxia of internal organs, liver, kidney, brain, pancreas, etc. is not well indicated by peripheral digital oxygenation. Indeed, there is an implied liability in the failure to infer central oxygenation from peripheral values. Near infrared (NIR) sensing of deep tissue saturation of hemoglobin (StO2) requires multi-wavelength, multi-site measurement of both absorption and scattering properties by time or frequency domain NIR methods. Corrections for underlying water and lipid absorptions can be made so that the correct value for, and saturation oh hemoglobin are obtained. Nevertheless, the significance of blood oxygen saturation, even localized to particular organs, can be questioned from the standpoint of what is the critical value of the desaturation from which the tissue can recover; for example, in the case of cortical neurons where stroke, compression ischemia, etc. cause O2 lack, this value becomes of significant clinical importance in both the brain and the spinal chord. These approaches are actively pursued and the possibility of subsurface redox state measurement in human tissues may eventually emerge as the quantitative metric of tissue metabolic state and of hypoxic stress. The great flexibility and versatility of the fast, economical and "tetherless" nature of opto-electronic technology is appropriate to the manifold challenges of neuronal function as currently measured by intrinsic signals and soon to be studiable by extrinsic signals of metabolism and electrophysiological functions.

Effect of mild carboxy-hemoglobin on exercising skeletal muscle: intravascular and intracellular evidence

We studied muscle blood flow, muscle oxygen uptake (VO(2)), net muscle CO uptake, Mb saturation, and intracellular bioenergetics during incremental single leg knee-extensor exercise in five healthy young subjects in conditions of normoxia, hypoxia (H; 11% O(2)), normoxia + CO (CO(norm)), and 100% O(2) + CO (CO(hyper)). Maximum work rates and maximal oxygen uptake (VO(2 max)) were equally reduced by approximately 14% in H, CO(norm), and CO(hyper). The reduction in arterial oxygen content (Ca(O(2))) (approximately 20%) resulted in an elevated blood flow (Q) in the CO and H trials. Net muscle CO uptake was attenuated in the CO trials. Suprasystolic cuff measurements of the deoxy-Mb signal were not different in terms of the rate of signal rise or maximum signal attained with and without CO. At maximal exercise, calculated mean capillary PO(2) was most reduced in H and resulted in the lowest Mb-associated PO(2). Reductions in ATP, PCr, and pH during H, CO(norm), and CO(hyper) occurred earlier during progressive exercise than in normoxia. Thus the effects of reduced Ca(O(2)) due to mild CO poisoning are similar to H.

Quantitative (1)H magnetic resonance spectroscopy of myoglobin de- and reoxygenation in skeletal muscle: reproducibility and effects of location and disease

1H-magnetic resonance spectroscopy ((1)H-MRS) of deoxymyoglobin (DMb) provides a means to noninvasively monitor the oxygenation state of human skeletal muscle in work and disease. As shown in this work, it also offers the opportunity to measure the absolute tissue content of DMb, the basic oxygen consumption of resting muscle, and the reperfusion characteristics after release of a pressure cuff. The methodology to determine these tissue properties simultaneously at two positions along the calf is presented. The obtained values are in agreement with invasive determinations. The reproducibility of the (1)H-MRS measurements is established for healthy controls and patients with peripheral arterial disease (PAD). A location dependence in axial direction, as well as differences between controls and patients are demonstrated for all parameters. The reoxygenation time in particular is expected to provide a means to quantitatively monitor therapies aimed at improving muscular perfusion in these patients.

Impaired muscle oxygen transfer in patients with chronic renal failure

We hypothesized that impaired O2 transport plays a role in limiting exercise in patients with chronic renal failure (CRF). Six CRF patients (25 +/- 6 yr) and six controls (24 +/- 6 yr) were examined twice during incremental single-leg isolated quadriceps exercise. Leg O2 delivery (QO2(leg)) and leg O2 uptake (VO2(leg)) were obtained when subjects breathed gas of three inspired O2 fractions (FI(O2)) (0.13, 0.21, and 1.0). On a different day, myoglobin O2 saturation and muscle bioenergetics were measured by proton and phosphorus magnetic resonance spectroscopy. CRF patients, but not controls, showed O2 supply dependency of peak VO2 (VO2(peak)) by a proportional relationship between peak VO2(leg) at each inspired O2 fraction (0.59 +/- 0.20, 0.47 +/- 0.10, 0.43 +/- 0.10 l/min, respectively) and 1) work rate (933 +/- 372, 733 +/- 163, 667 +/- 207 g), 2) QO(2leg) (0.80 +/- 0.20, 0.64 +/- 0.10, 0.59 +/- 0.10 l/min), and 3) cell PO2 (6.3 +/- 5.4, 1.7 +/- 1.3, 1.2 +/- 0.7 mmHg). CRF patients breathing 100% O2 and controls breathing 21% O2 had similar peak QO2(leg) (0.80 +/- 0.20 vs. 0.79 +/- 0.10 l/min) and similar peak VO2(leg) (0.59 +/- 0.20 vs. 0.57 +/- 0.10 l/min). However, mean capillary PO2 (47.9 +/- 4.0 vs. 38.2 +/- 4.6 mmHg) and the capillary-to-myocite gradient (40.7 +/- 6.2 vs. 34.4 +/- 4.0 mmHg) were both higher in CRF patients than in controls (P < 0.03 each). We conclude that low muscle O2 conductance, but not limited mitochondrial oxidative capacity, plays a role in limiting exercise tolerance in these patients.

Deciphering the mysteries of myoglobin in striated muscle

Myoglobin (Mb) is a large protein that reversibly binds oxygen in the muscle cell and is thought to be critical for O2 supply to the mitochondria during exercise. The role of Mb in aerobic function is evaluated based on the physical properties of Mb as an O2 carrier and experimental evidence of Mb function in vivo. This role depends on the reversible binding of O2 by Mb depending on PO2, which results in: (1) storage of O2; (2) buffering of PO2 in the cell to prevent mitochondrial anoxia; and (3) parallel diffusion of O2 (so-called, 'facilitated diffusion'). The storage role is well established in diving mammals and buffering of cell PO2 above anoxic levels is shown here by in vivo magnetic resonance spectroscopy (MRS). However, the quantitative role of Mb in 'facilitated' or parallel diffusion of O2 is controversial. Evidence in support of this role is from MRS analyses, which reveal rapid Mb desaturation with exercise, and from the proportionality of Mb content of a muscle to the O2 diffusion limitation. Recent experiments with myoglobin knockout mice demonstrating high levels of aerobic function in normal and myoglobin-free mice argue against a link between Mb and oxidative phosphorylation. Thus, the current evidence supports the role of Mb in the physical diffusion of O2; however, the unimpaired aerobic function of Mb knockout mice indicates that this role may not be critical to O2 supply in active muscle.

Molecular aspects of magnetic resonance imaging and spectroscopy

Magnetic resonance imaging (MRI) is a well known diagnostic tool in radiology that produces unsurpassed images of the human body, in particular of soft tissue. However, the medical community is often not aware that MRI is an important yet limited segment of magnetic resonance (MR) or nuclear magnetic resonance (NMR) as this method is called in basic science. The tremendous morphological information of MR images sometimes conceal the fact that MR signals in general contain much more information, especially on processes on the molecular level. NMR is successfully used in physics, chemistry, and biology to explore and characterize chemical reactions, molecular conformations, biochemical pathways, solid state material, and many other applications that elucidate invisible characteristics of matter and tissue. In medical applications, knowledge of the molecular background of MRI and in particular MR spectroscopy (MRS) is an inevitable basis to understand molecular phenomenon leading to macroscopic effects visible in diagnostic images or spectra. This review shall provide the necessary background to comprehend molecular aspects of magnetic resonance applications in medicine. An introduction into the physical basics aims at an understanding of some of the molecular mechanisms without extended mathematical treatment. The MR typical terminology is explained such that reading of original MR publications could be facilitated for non-MR experts. Applications in MRI and MRS are intended to illustrate the consequences of molecular effects on images and spectra.

Evidence of muscle BOLD effect revealed by simultaneous interleaved gradient-echo NMRI and myoglobin NMRS during leg ischemia

The purpose of this work was to investigate the temporal relationship between intensity changes in T2*-weighted NMR images and tissue oxygen content, measured by myoglobin proton NMR spectroscopy, in the skeletal muscle. During an ischemic stress test, the calf muscles of five healthy volunteers were studied at 3 Tesla. An interleaved NMRI-NMRS sequence was used, which made it possible to record T2*-weighted images and myoglobin spectra simultaneously. During ischemia, rapid changes in muscle signal intensity were observed on T2*-weighted images, which immediately preceded myoglobin desaturation. Bearing in mind the respective P50 of hemoglobin and myoglobin, this observation clearly favored the hypothesis that hemoglobin desaturation was responsible for the changes in T2*. This interpretation was further supported by the temporal coincidence between the experimental NMR data and a model of hemoglobin desaturation solely derived from physiological considerations.