High-Resolution Magnetic Resonance Neurography at 7T: A Pilot Study of Hand Innervation

The emergence of 7T clinical MRI technology has sparked our interest in its ability to discern the complex structures of the hand. Our primary objective was to assess the sensory and motor nerve structures of the hand, specifically nerves and Pacinian corpuscles, with the dual purpose of aiding diagnostic endeavors and supporting reconstructive surgical procedures. Ethical approval was obtained to carry out 7T MRI scans on a cohort of volunteers. Four volunteers assumed a prone position, with their hands (N = 8) positioned in a "superman" posture. To immobilize and maintain the hand in a strictly horizontal position, it was affixed to a plastic plate. Passive B0 shimming was implemented. Once high-resolution 3D images had been acquired using a multi-transmit head coil, advanced post-processing techniques were used to meticulously delineate the nerve fiber networks and mechanoreceptors. Across all participants, digital nerves were consistently located on the phalanges area, on average, between 2.5 and 3.5 mm beneath the skin, except within flexion folds where the nerve was approximately 1.8 mm from the surface. On the phalanges area, the mean distance from digital nerves to joints was approximately 1.5 mm. The nerves of the fingers were closer to the bone than to the surface of the skin. Furthermore, Pacinian corpuscles exhibited a notable clustering primarily within the metacarpal zone, situated on the palmar aspect. Our study yielded promising results, successfully reconstructing and meticulously describing the anatomy of nerve fibers spanning from the carpus to the digital nerve division, alongside the identification of Pacinian corpuscles, in four healthy volunteers (eight hands).

MR microscopy of the developing upper extremity of the chicken in ovo using 7 Tesla MRI

MR microscopy (MRM) is known as ultra-high-field (UHF) magnetic resonance imaging with an in-plane spatial resolution of <100 μm, yields highly resolved non-invasive anatomical imaging and allows longitudinal assessment of embryonic avian development. The aim of the present study was to evaluate the feasibility of in vivo anatomical MRI assessment of the developing upper extremity of the chicken. Thirty-eight fertilized chicken eggs were examined at 7 Tesla acquiring high-resolution T2-weighted images with an in-plane resolution of 74 × 74 μm. To reduce motion artefacts, the eggs were moderately cooled before and during MRI. Development of the upper extremity was anatomically and quantitatively assessed. Chondrification and ossification on MRI were correlated with histological examination. MRM allowed the identification of the embryo from stage D5 onwards. First chondrification of the upper extremity was visible at stage D7, and the differentiation of the forearm was possible from stage D9 throughout the developmental period with excellent correlation to histology. MRM also allowed the differentiation between cortical and medullary bone as well as the detection of chondrified areas. UHF MRM allows the in vivo and in ovo evaluation of the upper limb during embryonic development and provides non-invasive longitudinal anatomical information. This technique allows longitudinal studies of the same embryo during the developmental period and may therefore provide further insights into the development of the upper extremity. With improved coil technique and increasing availability of UHF MR systems, there is great potential regarding several research topics in experimental musculoskeletal radiology.

Nail Anatomy, Nail Psoriasis, and Nail Extensor Enthesitis Theory: What Is the Link?

The concept of the nail unit as a musculoskeletal appendage has become popular in the dermatological and rheumatological literature. However, an exact knowledge of the characteristics of mesenchyme surrounding the nail such as the composition of the collagen and elastic fibers and their arrangement is fundamental before one can propose a constitutive model. To the best of my knowledge, there is no study in the literature concerning the elastic network of the ligamentous connective tissue of the base of the distal phalanx. This study by means of elastic stains demonstrates that the so-called superficial, deep, and lateral laminae of the extensor tendon correspond to 3 different microanatomic structures: the nail dermis and its fibrous root, the subcutaneous proximal nail fold, and the periosteum. The complex fascial and adipose connective tissue of the proximal nail fold surrounds the matrical dermis and could viewed as a kind of suspensory system for the proximal nail unit. Such suspensory system protects the nail matrix epithelium from the biomechanical strain of the extensor mechanism. Near the ulnar and radial edge of the base of the phalanx, focal interconnections between the fibrous root of the apex of the matrix and the periosteum through a fascia-like structure are visible. In its most lateral zone, the matrical horns lie on a thick dermis connected to the interosseous ligament. Such lateral laminar system serves as anchoring ropes for the vault of the nail plate. The nail-extensor enthesitis theory relies on an oversimplified anatomy because the nail unit is an epidermal appendage with a specialized connection with the lateral periosteum, but not a musculoskeletal appendage. Finally, I would like to emphasize, the practical importance of recognizing the matrical hypoderm. In nail surgery, the interface between the matrical nail dermis and its submatrical loose connective tissue is potentially a new, relatively superficial, surgical cleaving plane, beside the classical deep surgical procedure usually extending to the periosteum. Recognition of this additional cleaving plane leads to an optimal nail tangential biopsy.

A three-dimensional visco-hyperelastic FE model for simulating the mechanical dynamic response of preloaded phalanges

This study lays the groundwork for a multi-scale strategy that will lead to a better understanding and prediction of the effects of vibration on the digital arterial network. This is accomplished by modelling the mechanical and biological factors that could disturb the basal vasoconstriction balance in the fingertip. The first stage of this novel approach involved building and validating an original dissipative constitutive law for the fingertip soft tissue for the purpose of finite element modelling of the mechanical response of preloaded phalanges in vibration. This visco-hyperelastic constitutive law was established by means of a two-stage procedure for combining a classical pure static nonlinear law with an original dissipative model. First, the parameters of an Ogden-Hill pure static nonlinear constitutive law were identified using a constrained optimisation algorithm. Second, an original viscous dissipation model was proposed in the spectral domain. This model is based on the linearization of the nonlinear quasi-linear viscoelasticity law and the use of a viscoelastic relaxation modulus, expressed as a continuous distribution of relaxation spectra suitable for living tissues. The experimental data used to fit this model were the static and dynamic stiffnesses of preloaded fingertips acquired from a group of 20 subjects. The relative errors between the measured and simulated stiffnesses were less than 5% in the static procedure and approximately 8% using dynamic analysis. The computed mechanical pressure and maximal tangential stress within the fingertip were high in the soft tissues close to the vibration excitation and also in the bones and interphalangeal cartilages far from the vibration source. Mechanical power was only dissipated significantly in the immediate vicinity of the contact area between the probe and the finger. The main contribution of this study was to implement and identify the parameters of a new spectral dissipative law for fingertip soft tissues. This work may apply in occupational health for modifying the vibration dose assessment or for the follow-up and screening of connective tissue diseases.

In vivo MRI of the human finger at 7 T

Purpose

To demonstrate a dedicated setup for ultrahigh resolution MR imaging of the human finger in vivo.

Methods

A radiofrequency coil was designed for optimized signal homogeneity and sensitivity in the finger at ultrahigh magnetic field strength (7 T), providing high measurement sensitivity. Imaging sequences (2D turbo‐spin echo (TSE) and 3D magnetization‐prepared rapid acquisition gradient echo (MPRAGE)) were adapted for high spatial resolution and good contrast of different tissues in the finger, while keeping acquisition time below 10 minutes. Data was postprocessed to display finger structures in three dimensions.

Results

3D MPRAGE data with isotropic resolution of 200 µm, along with 2D TSE images with in‐plane resolutions of 58 × 78 µm² and 100 × 97 µm², allowed clear identification of various anatomical features such as bone and bone marrow, tendons and annular ligaments, cartilage, arteries and veins, nerves, and Pacinian corpuscles.

Conclusion

Using this dedicated finger coil at 7 T, together with adapted acquisition sequences, it is possible to depict the internal structures of the human finger in vivo within patient‐compatible measurement time. It may serve as a tool for diagnosis and treatment monitoring in pathologies ranging from inflammatory or erosive joint diseases to injuries of tendons and ligaments to nervous or vascular disorders in the finger. Magn Reson Med 79:588–592, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

MR microscopy of the human fetal upper extremity - a proof-of-principle study

BACKGROUND

Current knowledge of the human fetal and embryonic development relies on early descriptive studies of humans and from experimental studies of laboratory animals and embryos. Taking the upper extremity as an example, this study explores the potential of magnetic resonance microscopy (MRM) for the assessment of the development of the fetal upper extremity and discusses its correlation with histological findings.

METHODS

Ex vivo MRM at 7.1 T (Clin Scan, Bruker Biospin, Germany) was performed in 10 human specimens at 8 to 12 weeks of gestational age (GA). In-plane resolution was 20 μm with a slice thickness of 70 μm. MRM was followed by histological work-up of the specimens. MRM images were then correlated with conventional histology with a focus on the presence of chondrification and ossification.

RESULTS

Ossification of the upper human extremity is detectable at 8 weeks GA in the humerus and the long bones of the forearm. There is excellent correlation for location and size of ossification between MRM and conventional histology. MRM imaging is in accordance with historical studies.

CONCLUSION

Ex vivo MRM for the non-invasive assessment of the embryonic and fetal development of the upper human extremity is feasible. It may provide an accurate complementary tool for the evaluation of embryological development.