Publications

The ability to resolve complex fiber populations in muscular tissues is important for relating tissue structure with mechanical function. To address this issue in the case of tongue, we employed diffusion spectrum imaging (DSI), an MRI method for determining three-dimensional myoarchitecture where myofiber populations are variably aligned. By specifically varying gradient field strength, molecular displacement in a tissue can be determined by Fourier-transforming the echo intensity against gradient strength at fixed gradient pulse spacing. The displacement profiles are visualized by graphing three-dimensional isocontour icons for each voxel, with the isocontour shape and size representing the magnitude and direction of the constituting fiber populations. To validate this method, we simulated a DSI experiment within the constraints of arbitrary crossing fibers, and determined that DSI accurately depicts the angular relationships between these fibers. Considering the fiber relationships in the whole bovine tongue, we compared the images obtained by DSI with those obtained by diffusion tensor imaging in an anterior slice of the lingual core, a region known to possess extensive fiber crossing. In contrast to diffusion tensor imaging, which depicts the anterior core solely as a region with low anisotropy due to the presence of mixed-orientation fiber populations, DSI shows two distinct fiber populations, with an explicit orthogonal relationship to each other. In imaging the whole lingual tissue, we discerned arrays of crossing and noncrossing fibers involving the intrinsic and extrinsic muscles, which merged at regions of interface. We conclude that DSI has the capacity to determine three-dimensional fiber orientation in structurally complex muscular tissues.

The "Neurobiological Correlates of Acupuncture" Conference was convened November 17-18, 2005 in Bethesda, Maryland. The conference was sponsored by the National Center for Complementary and Alternative Medicine (NCCAM) of the National Institutes of Health (NIH), U.S. Department of Health and Human Services (DHHS). Its goals were to encourage exchange of ideas regarding the direction of neuroimaging in acupuncture research as well as to discuss some of the challenges in this field. The use of neuroimaging, a relatively recent advance in the study of acupuncture, holds the promise of localizing and characterizing brain activity associated with acupuncture interventions in real time and in a minimally invasive way. Among the main challenges to research into the biological mechanisms of acupuncture are the multiple treatment variables and the difficulties of selecting appropriate experimental controls. Despite these challenges, numerous findings from acupuncture neuroimaging experiments were presented and discussed at the conference on topics related to possible signaling networks, sham acupuncture controls, acupoint specificity, acupuncture analgesia, acupuncture-associated brain response, and the potential for using neuroimaging in conjunction with translational and clinical acupuncture research. Future directions in acupuncture neuroimaging research, as recommended by conference participants, should focus on (1) continuing exploration of acupuncture signaling networks; (2) establishing standards and recommendations for performing and reporting acupuncture neuroimaging results; (3) enabling data sharing in the acupuncture neuroimaging community; (4) gaining a better understanding of placebo and control groups in acupuncture neuroimaging experiments; and (5) developing biomarkers that relate to physiologically and/or clinically relevant acupuncture responses to neuroimaging results.

The anatomy of the mammalian tongue consists of an intricate array of variably aligned and extensively interwoven muscle fibers. As a result, it is particularly difficult to resolve the relationship between the tongue's microscopic anatomy and tissue-scale mechanical function. In order to address this question, we employed a method, diffusion spectrum imaging (DSI) with tractography, for displaying the macroscopic orientational properties of the tissue's constituting myofibers. DSI measures spatially variant proton displacement for a given 3D imaging segment (voxel), reflecting the principal orientation(s) of its myofibers. Tractography uses the angular similarity displayed by the principal fiber populations of multiple adjacent voxels to generate tract-like structures. DSI with tractography thus defines a unique set of tracts based on the net orientational behavior of the myofiber populations at different positions in the tissue. By this approach, we demonstrate a novel myoarchitectural pattern for the bovine tongue, consisting of short and orthogonally aligned crossing fiber tracts in the intrinsic core region, and longer, parallel-aligned fiber tracts on the tissue margins and in the regions of extrinsic fiber insertion. The identification of locally aligned myofiber populations by DSI with tractography allows us to reconsider lingual anatomy, not in conventional microscopic terms, but as a set of heterogeneously aligned and macroscopically resolved myofiber tracts. We postulate that the properties associated with these myofiber tracts predict the mechanical behavior of the tissue and thus constitute a method to relate structure and function for anatomically complex muscular tissues.

Past neuroimaging studies of acupuncture have demonstrated variable results for important brainstem nuclei. We have employed cardiac-gated fMRI with T1-variability correction to study the processing of acupuncture by the human brain. Furthermore, our imaging experiments collected simultaneous ECG data in order to correlate heart rate variability (HRV) with fMRI signal intensity. Subjects experienced one of three stimulations over a 31.5 minute fMRI run: (1) electro-acupuncture at 2Hz/15Hz over the acupoint ST-36 (2) electro-acupuncture at a sham non-acupoint, or (3) sensory control tapping over ST-36. The ECG was analyzed with power spectral methods for low frequency and high frequency components, which reflect the balance in the autonomic nervous system. The HRV data was then correlated with the time-varying fMRI signal intensity. Our data suggests that fMRI activity in the hypothalamus, the dorsal raphe nucleus, the periaqueductal gray, and the rostroventral medulla showed significant correlation with LF/HF ratio calculated from simultaneous HRV data. The correlation of time-varying fMRI response with physiological parameters may provide insight into connections between acupuncture modulation of the autonomic nervous system and neuroprocessing.

The myoarchitecture of the tongue is believed to consist of a complex network of interwoven fibers, which function together to produce a near limitless array of functional deformations. These deformations contribute mechanically to speech production and to oral cavity food handling during swallowing. We have previously imaged the 3D myoarchitecture of the mammalian tongue in excised tissue with diffusion tensor MRI, a technique which derives the 3D orientation of intramural fibers as a function of the extent to which a direction-specific MR signal attenuates under diffusion-encoding magnetic gradients. The resulting 3D diffusion tensor defines the relative orientations of the myofiber populations within a region of tissue. In this study, we have extended the use of this method to assess lingual myoarchitecture in normal human subjects in vivo. Subjects were imaged using a diffusion-sensitive stimulated-echo pulse sequence with single-shot echo-planar spatial encoding in the midsagittal plane. Differences in lingual fiber orientation were manifested by graduated changes in fiber direction throughout the tissue, without clear anatomical demarcations between regions of the tissue. The anterior tissue was composed generally of orthogonally oriented fibers surrounded by an axially oriented ring of tissue, whereas the posterior portion of the tissue was composed mostly of fibers projecting in the superior and posterior directions. The bulk of the tissue displayed a highly homogeneous, vertically oriented set of fibers, including the anteroinferior region of the tissue and extending nearly to the superior surface. Further analysis of the tissue in terms of diffusion anisotropy demonstrated that the tissue could be represented by varying degrees of anisotropy, with a tendency toward high anisotropy in the dorsal and anteroventral periphery and low anisotropy in the central region of the tissue. These findings demonstrate that the muscular anatomy of the tongue can be displayed as a continuous array of structural units, or tensors, representing fibers of varying orientations throughout the tissue.

The human tongue is a structurally complex and extremely flexible organ. In order to better understand the mechanical basis for lingual deformations, we modeled a primitive movement of the tongue, sagittal tongue bending. We hypothesized that sagittal bending is a synergistic deformation derived from co-contraction of the longitudinalis and transversus muscles. Our model of tongue bending was based on classical bimetal strip theory, in which curvature is produced when one muscle layer contracts more so than another. Contraction was modulated via mismatched thermal expansion coefficients and temperature change (to simulate muscular contraction). Our results demonstrated that synergistic contraction produced curvature and strain results which were in better correspondence to empirical results derived from tagging MRI than were the results of contraction of the longitudinalis muscle alone. This fundamental reliance of tongue bending on the synergistic contraction of its intrinsic fibers supports the muscular hydrostat theory of tongue function.

The myoarchitecture of the tongue is comprised of a complex array of muscle fiber bundles, which form the structural basis for lingual deformations during speech and swallowing. We used magnetic resonance imaging of the water diffusion tensor to display the primary and secondary fiber architectural attributes of the excised bovine tongue. Fiber orientation mapping provides a subdivision of the tongue into its principal intrinsic and extrinsic muscular components. The anterior tongue consists of a central region of orthogonally oriented intrinsic fibers surrounded by an axially oriented muscular sheath. The posterior tongue consists principally of a central region of extrinsic fibers, originating at the inferior surface and projecting in a fan-like manner in the superior, lateral, and posterior directions, and lateral populations of extrinsic fibers directed posterior-inferior and posterior-superior. Analysis of cross-fiber anisotropy indicates a basic contrast of design between the extrinsic and the intrinsic fibers. Whereas the extrinsic muscles exhibit a uniaxial architecture typical of skeletal muscle, the intrinsic core muscles, comprised of the verticalis and the transversus muscles, show strong cross-fiber anisotropy. This pattern is consistent with the theory that the tongue's core functions as a muscular hydrostat in that conjoint contraction of the transverse and vertical fibers enable the tissue to expand at right angles to these fibers. These findings suggest that three-dimensional analysis of diffusion tensor magnetic resonance imaging provides a structural basis for understanding the micromechanics of the mammalian tongue.

In clinical practice, the assessment of lung mechanics is limited to a global physiological evaluation, which measures, in the aggregate, the contributions of the pulmonary parenchyma, pleura, and chest wall. In this study, we used an MR imaging methodology which applies two-dimensional bands of inverted magnetization directly onto the pulmonary parenchyma, thus allowing for the quantification of local pulmonary tissue deformation, or strain, throughout inhalation. Our results showed that the magnitude of strain was maximal at the base and apex of the lung, but was curtailed at the hilum, the anatomical site of the poorly mobile bronchial and vascular insertions. In-plane shear strain mapping showed mostly positive shear strain, predominant at the apex throughout inhalation, and increasing with expanding lung volume. Anisotropy mapping showed that superior-inferior axial strain was greater than medial-lateral axial strain at the apex and base, while the opposite was true for the middle lung field. This study demonstrates that localized pulmonary deformation can be measured in vivo with tagging MRI, and quantified by applying finite strain definitions from continuum mechanics.

The determination of principal fiber directions in structurally heterogeneous biological tissue substantially contributes to an understanding of its mechanical function in vivo. In this study we have depicted structural heterogeneity through the model of the mammalian tongue, a tissue comprised of a network of highly interwoven fibers responsible for producing numerous variations of shape and position. In order to characterize the three-dimensional-resolved microscopic myoarchitecture of the intrinsic musculature of the tongue, we viewed its fiber orientation at microscopic and macroscopic length scales using NMR (diffusion tensor MRI) and optical (two-photon microscopy) imaging methods. Diffusion tensor imaging (DTI) of the excised core region of the porcine tongue resulted in an array of 3D diffusion tensors, in which the leading eigenvector corresponded to the principal fiber orientation at each location in the tissue. Excised axially oriented lingual core tissues (fresh or paraffin-embedded) were also imaged with a mode-locked Ti-Sapphire laser, (76 MHz repetition rate, 150 femtosecond pulse width), allowing for the visualization of individual myofibers at in situ orientation. Fiber orientation was assessed by computing the 3D autocorrelation of discrete image volumes, and deriving the minimal eigenvector of the center voxel Hessian matrix. DTI of the fibers, comprising the intrinsic core of the tongue, demonstrated directional heterogeneity, with two distinct populations of fibers oriented orthogonal to each other and in-plane to the axial perspective. Microscopic analysis defined this structural heterogeneity as discrete regions of in-plane parallel fibers, with an angular separation of ~80 degrees, thereby recapitulating the macroscopic angular relationship. This analysis, conceived at two different length scales, demonstrates that the lingual core is a spatially complex tissue, composed of repeating orthogonally oriented and in-plane fiber patches, which are capable of jointly producing hydrostatic elongation and displacement.

Our goal was to quantify intramural mechanics in the tongue through an assessment of local strain during the physiological phases of swallowing. Subjects were imaged with an ultrafast gradient echo magnetic resonance imaging (MRI) pulse sequence after the application of supersaturated magnetized bands in the x and y directions. Local strain was defined through deformation of discrete triangular elements defined by these bands and was depicted graphically either as color-coded two-dimensional strain maps or as three-dimensional octahedra whose axes correspond to the principal strains for each element. During early accommodation, the anterior tongue showed positive strain (expansive) in the anterior-posterior direction (x), whereas the middle tongue showed negative strain (contractile) in the superior-inferior direction (y). During late accommodation, the anterior tongue displayed increased positive x-direction and y-direction strain, whereas the posterior tongue displayed increased negative y-direction strain. These findings were consistent with contraction of the anterior-located intrinsic muscles and the posterior-located genioglossus and hyoglossus muscles. During propulsion, posterior displacement of the tongue was principally associated with positive strain directed in the x and y directions. These findings were consistent with posterior passive stretch in the midline due to contraction of the laterally inserted styloglossus muscle, as well as contraction of the posterior located transversus muscle. We conclude that MRI of lingual deformation during swallowing resolves the synergistic contractions of the intrinsic and extrinsic muscle groups.

Contraction of the tongue musculature during speech and swallowing is associated with characteristic patterns of tissue deformation. In order to quantify local deformation (strain) in the human tongue, we used a non-invasive NMR tagging technique that represents tissue as discrete deforming elements. Subjects were studied with a fast gradient echo pulse sequence (TR,TE 2.3/0.8 ms, slice thickness 10 mm, and effective spatial resolution 1.3x1.3 mm). Individual elements were defined by selectively supersaturating bands of magnetic spills in resting tongue tissue along the antero-posterior and superior inferior directions of the mid-sagittal plane, resulting in a rectilinear square grid. Axial and shear strains relative to the rest condition were determined for each clement and represented by two-dimensional surface strain maps. During forward protrusion, the anterior tongue underwent positive antero posterior strain (elongation) (maximum 200%) and symmetrical negative medial lateral and superior inferior strain (contraction). During sagittal curl directed to the hard palate, the tongue exhibited positive asymmetrical antero posterior strain (maximum 160%) that increased radially as a function of distance from the center of curvature (r = 0.9216, p<0.0005), and commensurate negative strain in the medial lateral direction. Similarly, the magnitude of anterior posterior strain during left-directed tongue curl was proportional to the distance from the curved inner surface (r = O.8978, p<0.0005). We conclude that the regulation of tongue position for the motions studied was related to regional activation of the intrinsic lingual musculature.