Image-Based Tissue Distribution Modeling for Skeletal Muscle Quality Characterization.

The identification and characterization of regional body tissues is essential to understand changes that occur with aging and age-related metabolic diseases such as diabetes and obesity and how these diseases affect trajectories of health and functional status. Imaging technologies are frequently used to derive volumetric, area, and density measurements of different tissues. Despite the significance and direct applicability of automated tissue quantification and characterization techniques, these topics have remained relatively underexplored in the medical image analysis literature. We present a method for identification and characterization of muscle and adipose tissue in the midthigh region using MRI. We propose an image-based muscle quality prediction technique that estimates tissue-specific probability density models and their eigenstructures in the joint domain of water- and fat-suppressed voxel signal intensities along with volumetric and intensity-based tissue characteristics computed during the quantification stage. We evaluated the predictive capability of our approach against reference biomechanical muscle quality (MQ) measurements using statistical tests and classification performance experiments. The reference standard for MQ is defined as the ratio of muscle strength to muscle mass. The results show promise for the development of noninvasive image-based MQ descriptors.

Characterization of skin abnormalities in a mouse model of osteogenesis imperfecta using high resolution magnetic resonance imaging and Fourier transform infrared imaging spectroscopy.

Evaluation of the skin phenotype in osteogenesis imperfecta (OI) typically involves biochemical measurements, such as histologic or biochemical assessment of the collagen produced from biopsy-derived dermal fibroblasts. As an alternative, the current study utilized non-invasive magnetic resonance imaging (MRI) microscopy and optical spectroscopy to define biophysical characteristics of skin in an animal model of OI. MRI of skin harvested from control, homozygous oim/oim and heterozygous oim/+ mice demonstrated several differences in anatomic and biophysical properties. Fourier transform infrared imaging spectroscopy (FT-IRIS) was used to interpret observed MRI signal characteristics in terms of chemical composition. Differences between wild-type and OI mouse skin included the appearance of a collagen-depleted lower dermal layer containing prominent hair follicles in the oim/oim mice, accounting for 55% of skin thickness in these. The MRI magnetization transfer rate was lower by 50% in this layer as compared to the upper dermis, consistent with lower collagen content. The MRI transverse relaxation time, T2, was greater by 30% in the dermis of the oim/oim mice compared to controls, consistent with a more highly hydrated collagen network. Similarly, an FT-IRIS-defined measure of collagen integrity was 30% lower in the oim/oim mice. We conclude that characterization of phenotypic differences between the skin of OI and wild-type mice by MRI and FT-IRIS is feasible, and that these techniques provide powerful complementary approaches for the analysis of the skin phenotype in animal models of disease.

Effects of knee injection on skeletal muscle metabolism and contractile force in rats.

OBJECTIVE: We tested the hypothesis that intrusion of the knee joint capsule alters quadriceps muscle metabolism and function independently from the damage induced to knee cartilage. METHODS: Adult rats were separated into four groups: intraarticular injections of saline (SAL; n=9); intraarticular injections of papain, a model for osteoarthritis (PIA; n=7); sham injections (SHAM; n=8); and controls (CTL; n=5). 31P magnetic resonance spectroscopy (31P-MRS) was performed after 2 weeks. Spectra were obtained from the left quadriceps: two at baseline, eight during electrical stimulation with simultaneous measurement of contractile force, and 15 during recovery. 31P-MRS data were presented as the ratio of inorganic phosphate (Pi) to phosphocreatine (PCr), concentrations of PCr [PCr], intramuscular pH, and the rates and time constants of PCr breakdown during stimulation and PCr recovery. Intramuscular cytokine concentrations were measured within the quadriceps. Histologic slides of the knees were scored for severity of cartilage damage. RESULTS: The interventional groups produced values of Pi/PCr ratio, [PCr], contractile force and pH that were significantly different from CTL. These changes in muscle function were accompanied by higher concentrations of interleukin-1 observed with PIA and SAL. We did not observe any effect of cartilage damage on muscle function or metabolism. CONCLUSIONS: Knee joint intrusion alters quadriceps muscle metabolism with accelerated depletion of energy stores and fatigue during stimulation. This study demonstrates that needle intrusion into the knee joint results in muscle dysfunction, independently from the extent of cartilage damage.

The lever-coil: a simple, inexpensive sensor for respiratory and cardiac motion in MRI experiments.

Thoracic and abdominal magnetic resonance imaging studies generally require some type of compensation for respiratory and cardiac motions in order to yield artifact-free images with good signal-to-noise ratio. Most techniques for respiratory compensation require the use of a non-NMR sensing device to monitor the subject's chest motion, while cardiac motion compensation generally requires the use of ECG leads within the magnet. An inductive pickup coil placed on the subject's chest is perhaps the simplest and least expensive means of monitoring respiration in a MR scanner. However, due to inductive coupling between the pickup coil, radio frequency resonator and gradient set, this arrangement often results in both NMR and respiratory signal artifacts and can also present a burn hazard to the subject depending on the placement and orientation of the pickup coil. Moreover, the presence of a pickup coil on the chest can degrade local magnetic field homogeneity and thus degrade image quality. Similar problems arise when ECG leads must be connected to the subject for cardiac monitoring and gating. To preserve the benefits of the simple pickup coil while circumventing these limitations, a "lever-coil" sensor is presented in which a pickup coil is mechanically coupled to the subject but is not located within the resonator or gradient coil. This results in much lower mutual inductance between the pickup coil and the resonator or gradients. The optimization of the geometry of the apparatus is discussed and lever-coil signal traces are shown which demonstrate the sensor's ability to simultaneously detect both respiratory and cardiac motion in mice. Finally, respiratory-gated and cardiac-triggered spin echo images of the rat abdomen and mouse heart are presented to demonstrate the utility of the lever-coil sensor.

Pitfalls in the measurement of metabolite concentrations using the one-pulse experiment in in Vivo NMR: commentary on "On neglecting chemical exchange effects when correcting in vivo (31)P MRS data for partial saturation".

In an article in a previous issue of the Journal of Magnetic Resonance, Ouwerkerk and Bottomley (J. Magn. Reson. 148, pp. 425--435, 2001) show that even in the presence of chemical exchange, the dependence of saturation factors on repetition time in the one-pulse experiment is approximately monoexponential. They conclude from this fact that the effect of chemical exchange on the use of saturation factors when correcting for partial saturation is negligible. We take issue with this conclusion and demonstrate that because saturation factors in the presence of chemical exchange are strongly dependent upon all of the chemical parameters of the system, that is, upon all T(1)'s and M(0)'s of resonances in the exchange network and upon the reaction rates themselves, it is problematic to apply saturation factor corrections in situations in which any of these parameters may change. The error criterion we establish reflects actual errors in quantitation, rather than departures from monoexponentiality.

31P NMR spectroscopy of developing cartilage produced from chick chondrocytes in a hollow-fiber bioreactor.

31P NMR was used to measure the concentrations and spin-lattice relaxation times of phosphorus-containing metabolites in neocartilage developing in an NMR-compatible hollow-fiber bioreactor over four weeks. Separate studies were performed for tissue developing from chondrocytes taken from the proximal and the distal sternum of the chick embryo. The metabolite ratio beta-ATP/Pi did not change significantly with development (proximal: beta-ATP/Pi = 0.38+/- 0.12 at one week, beta-ATP/Pi = 0.44+/-0.07 at four weeks, P< 0.63; distal: beta-ATP/Pi = 0.39+/-0.05 at one week, beta-ATP/Pi = 0.66+/- 0.26 at four weeks, P<0.28). ATP spin-lattice relaxation times were found to be comparable to those in muscle and brain tissue (proximal: T(1)(beta-ATP) = 0.5+/-0.06 sec at one week, T(1)(beta-ATP) = 0.4+/- 0.01 sec at four weeks; distal: T(1)(beta-ATP) = 0.3+/-0.12 sec at one week, T(1)(beta-ATP) = 0.4+/-0.04 sec at four weeks). A large increase in the spin-lattice relaxation time of inorganic phosphate, from 1.2+/-0.13 sec to 3.8+/-0.04 sec (P<0.0001) over four weeks of growth, was observed in tissue developing from chondrocytes harvested from the proximal sternum. No comparable increase in T(1)(Pi) was found in tissue developing from chondrocytes harvested from the distal portion of the sternum, which ossifies later in vivo.

Adenovirus-mediated VEGF(121) gene transfer stimulates angiogenesis in normoperfused skeletal muscle and preserves tissue perfusion after induction of ischemia.

BACKGROUND: Administration of angiogenic factors stimulates neovascularization in ischemic tissues. However, there is no evidence that angiogenesis can be induced in normoperfused skeletal muscles. We tested the hypothesis that adenovirus-mediated intramuscular (IM) gene transfer of the 121-amino-acid form of vascular endothelial growth factor (AdCMV.VEGF(121)) could stimulate neovascularization in nonischemic skeletal muscle and consequently attenuate the hemodynamic deficit secondary to surgically induced ischemia. METHODS AND RESULTS: Rabbits and rats received IM injections of AdCMV.VEGF(121), AdCMV.Null, or saline in the thigh, 4 weeks (rabbits) or 2 weeks (rats) before femoral artery removal in the injected limb. In unoperated rats, at the site of injection of AdCMV.VEGF(121), we found 96% and 29% increases in length density of arterioles and capillaries, respectively. Increased tissue perfusion (TP) to the ischemic limb in the AdCMV.VEGF(121) group was documented, as early as day 1 after surgery, by improved blood flow to the ischemic gastrocnemius muscle measured by radioactive microspheres (AdCMV.VEGF(121)=5.69+/-0.40, AdCMV.Null=2.97+/-0.50, and saline=2.78+/-0.43 mL x min(-1) x 100 g(-1), P<0.001), more angiographically recognizable collateral vessels (angioscore) (AdCMV. VEGF(121)=50.58+/-1.48, AdCMV.Null=29.08+/-4.22, saline=11.83+/-1.90, P<0.0001), and improvement of the bioenergetic reserve of the gastrocnemius muscle as assessed by (31)P NMR spectroscopy. Follow-up studies showed that superior TP to the ischemic limb in the AdCMV.VEGF(121) group persisted until it was equalized by spontaneous collateral vessel development in untreated animals. CONCLUSIONS: IM administration of AdCMV.VEGF(121) stimulates angiogenesis in normoperfused skeletal muscles, and the newly formed vessels preserve TP after induction of ischemia.

The relationship between creatine kinase kinetics and exercise intensity in human forearm is unchanged by age.

Using (31)P magnetic resonance spectroscopy, creatine kinase (CK) reaction kinetics was assessed in the forearm flexor digitorum profundus muscle of healthy young (n = 11, age 34.7 +/- 5 yr) and older (n = 20, age 73.5 +/- 8 yr) subjects at rest, intermittent exercise at 20% maximum voluntary contraction (MVC), and 40% MVC. Exercise resulted in a significant increase in the average ratio of inorganic phosphate (P(i)) to phosphocreatine (PCr) from resting values of 0.073 +/- 0.031 (young) and 0.082 +/- 0.037 (older) to 0. 268 +/- 0.140 (young, P < 0.01) and 0.452 +/- 0.387 (older, P < 0. 01) at 40% MVC. At 40% MVC, intracellular pH decreased significantly, from resting values of 7.08 +/- 0.08 (young) and 7.08 +/- 0.11 (older) to 6.84 +/- 0.19 (young, P < 0.05) and to 6.75 +/- 0.25 (older, P < 0.05). Average values of the pseudo-first-order reaction rate k((PCr-->ATP)) at rest were 0.07 +/- 0.04 s(-1) in the young and 0.07 +/- 0.03 s(-1) in the older group. At both exercise levels, the reaction rate constant increased compared with the resting value, but only the difference between the resting value and the 20% MVC value, which showed an 86% higher reaction rate constant in both groups, reached statistical significance (P < 0.05). No difference in the reaction rate constant between the young and older groups was observed at either exercise level. As with k((PCr-->ATP)), the average phosphorus flux through the CK reaction increased during exercise at 20% MVC (P < 0.05 in the older group) but decreased toward resting values at 40% MVC in both groups. The data in our study suggest that normal aging does not significantly affect the metabolic processes associated with the CK reaction.

Measurement of spin-lattice relaxation times and concentrations in systems with chemical exchange using the one-pulse sequence: breakdown of the Ernst model for partial saturation in nuclear magnetic resonance spectroscopy.

A fundamental problem in Fourier transform NMR spectroscopy is the calculation of observed resonance amplitudes for a repetitively pulsed sample, as first analyzed by Ernst and Anderson in 1966. Applications include determination of spin-lattice relaxation times (T(1)'s) by progressive saturation and correction for partial saturation in order to determine the concentrations of the chemical constituents of a spectrum. Accordingly, the Ernst and Anderson formalism has been used in innumerable studies of chemical and, more recently, physiological systems. However, that formalism implicitly assumes that no chemical exchange occurs. Here, we present an analysis of N sites in an arbitrary chemical exchange network, explicitly focusing on the intermediate exchange rate regime in which the spin-lattice relaxation rates and the chemical exchange rates are comparable in magnitude. As a special case of particular importance, detailed results are provided for a system with three sites undergoing mutual exchange. Specific properties of the N-site network are then detailed. We find that (i) the Ernst and Anderson analysis describing the response of a system to repetitive pulsing is inapplicable to systems with chemical exchange and can result in large errors in T(1) and concentration measurements; (ii) T(1)'s for systems with arbitrary exchange networks may still be correctly determined from a one-pulse experiment using the Ernst formula, provided that a short interpulse delay time and a large flip angle are used; (iii) chemical concentrations for exchanging systems may be correctly determined from a one-pulse experiment either by using a short interpulse delay time with a large flip angle, as for measuring T(1)'s, and correcting for partial saturation by use of the Ernst formula, or directly by using a long interpulse delay time to avoid saturation; (iv) there is a significant signal-to-noise penalty for performing one-pulse experiments under conditions which permit accurate measurements of T(1)'s and chemical concentrations. The present results are analogous to but are much more general than those that we have previously derived for systems with two exchanging sites. These considerations have implications for the design and interpretation of one-pulse experiments for all systems exhibiting chemical exchange in the intermediate exchange regime, including virtually all physiologic samples.

Cartilage formation in a hollow fiber bioreactor studied by proton magnetic resonance microscopy.

The ideal in vitro system for investigating the regulation of cartilage formation and maintenance would allow for three-dimensional tissue growth, a wide range of biochemical interventions, and non-destructive evaluation. We have developed a hollow fiber bioreactor (HFBR) system which meets these criteria. After injection with embryonic chick sternal chondrocytes, neocartilage is elaborated around the hollow fibers, reaching a thickness of up to a millimeter after four weeks of growth. This process was monitored over time with nuclear magnetic resonance (NMR) microimaging and correlative biochemical and histologic analyses. Tissue volume and cellularity increased greatly during development. This was accompanied by changes in magnetic resonance properties consistent with increased macromolecular content. Further, tissue heterogeneity, observed as regional variations in cell size in histologic sections, was also observed in quantitative NMR images.