Non-identifiability of the Rayleigh damping material model in magnetic resonance elastography

Magnetic Resonance Elastography (MRE) is an emerging imaging modality for quantifying soft tissue elasticity deduced from displacement measurements within the tissue obtained by phase sensitive Magnetic Resonance Imaging (MRI) techniques. MRE has potential to detect a range of pathologies, diseases and cancer formations, especially tumors. The mechanical model commonly used in MRE is linear viscoelasticity (VE). An alternative Rayleigh damping (RD) model for soft tissue attenuation is used with a subspace-based nonlinear inversion (SNLI) algorithm to reconstruct viscoelastic properties, energy attenuation mechanisms and concomitant damping behavior of the tissue-simulating phantoms. This research performs a thorough evaluation of the RD model in MRE focusing on unique identification of RD parameters, μIμI and ρIρI. Results show the non-identifiability of the RD model at a single input frequency based on a structural analysis with a series of supporting experimental phantom results. The estimated real shear modulus values (μRμR) were substantially correct in characterising various material types and correlated well with the expected stiffness contrast of the physical phantoms. However, estimated RD parameters displayed consistent poor reconstruction accuracy leading to unpredictable trends in parameter behaviour. To overcome this issue, two alternative approaches were developed: (1) simultaneous multi-frequency inversion; and (2) parametric-based reconstruction. Overall, the RD model estimates the real shear shear modulus (μRμR) well, but identifying damping parameters (μIμI and ρIρI) is not possible without an alternative approach

Combined soft and skeletal tissue modelling of normal and dysmorphic midface postnatal development

BACKGROUND: Midface hypoplasia as exemplified by Treacher Collins Syndrome (TCS) can impair appearance and function. Reconstruction involves multiple invasive surgeries with variable long-term outcomes. This study aims to describe normal and dysmorphic midface postnatal development through combined modelling of skeletal and soft tissues and to develop a surgical evaluation tool. MATERIALS AND METHODS: Midface skeletal and soft tissue surfaces were extracted from computed tomography scans of 52 control and 14 TCS children, then analysed using dense surface modelling. The model was used to describe midface growth, morphology, and asymmetry, then evaluate postoperative outcomes. RESULTS: Parameters responsible for the greatest variation in midface size and shape showed differences between TCS and controls with close alignment between skeletal and soft tissue models. TCS children exhibited midface dysmorphology and hypoplasia when compared with controls. Asymmetry was also significantly higher in TCS midfaces. Combined modelling was used to evaluate the impact of surgery in one TCS individual who showed normalisation immediately after surgery but reversion towards TCS dysmorphology after 1 year. CONCLUSION: This is the first quantitative analysis of postnatal midface development using combined modelling of skeletal and soft tissues. We also provide an approach for evaluation of surgical outcomes, laying the foundations for future development of a preoperative planning tool

Biomechanical Characerization and Evaluation of Conservative Clubfoot Correction

Congential talipes equinovarus, or clubfoot, affects approximately 200,000 newborns worldwide each year and presents with equinovarus of the hindfoot, as well as cavus and adduction of the midfoot. In addition to bone malformation and displacement, soft tissue contractures encapsulate the medial and posterior aspects of the affected foot. The Ponseti method is a conservative treatment that progressively repositions the clubfoot through weekly casting, followed by bracing. Concerns exist regarding the variability in outcomes, resistance to treatment, and risk of relapse, which occur in approximately 10% of the population. Potential factors contributing to variability and resistant clubfoot include cast material performance, as well as biomechanics of medial soft tissue of the clubfoot. There are no clinical guidelines for clubfoot correction based upon mechanical response of commonly used casting materials, nor the mechanics of the medial fibrotic clubfoot tissue. Untreated or under-corrected clubfoot can result in abnormal gait, pain, and further foot deformity. The purpose of this research was to investigate the biomechanics of conservative clubfoot correction through: i) a kinematic assessment of the creep behavior of three common cast materials used during conservative correction, ii) development and validation of a benchtop system for the mechanical evaluation of miniature soft tissue specimens, and iii) performing a mechanical analysis to model the behavior of medial fibrotic mass tissue (MFMT) from children with clubfoot. Utilizing a model to simulate clubfoot correction, creep rotation was found to be dependent on cast material with maximum values for plaster-of-Paris (θ ≈ 2.1 deg). Reducing cast creep may result in a more efficient correction. Utilizing nylon monofilament, the benchtop system was validated against a commercial system (MTS). Versatility was demonstrated with quasistatic and viscoelastic protocols performed on PTFE tape and rabbit ligament, respectively. Clubfoot MFMT underwent a quasistatic and viscoelastic protocol, including requisite preconditioning as well as stress relaxation. Major findings include high specimen variability, less relaxation than reported for normal deltoid ligaments, and estimated QLV model parameters with R2 \u3e 0.8 for 16 specimens. Results from this research provide mechanical insight into the correction process that may lead to individualized, evidence-based clubfoot care. Future directions include in vivo analysis of tissue properties and mechanical-genetic correlation

Virtual reality training and assessment in laparoscopic rectum surgery

Background: Virtual-reality (VR) based simulation techniques offer an efficient and low cost alternative to conventional surgery training. This article describes a VR training and assessment system in laparoscopic rectum surgery. Methods: To give a realistic visual performance of interaction between membrane tissue and surgery tools, a generalized cylinder based collision detection and a multi-layer mass-spring model are presented. A dynamic assessment model is also designed for hierarchy training evaluation. Results: With this simulator, trainees can operate on the virtual rectum with both visual and haptic sensation feedback simultaneously. The system also offers surgeons instructions in real time when improper manipulation happens. The simulator has been tested and evaluated by ten subjects. Conclusions: This prototype system has been verified by colorectal surgeons through a pilot study. They believe the visual performance and the tactile feedback are realistic. It exhibits the potential to effectively improve the surgical skills of trainee surgeons and significantly shorten their learning curve. © 2014 John Wiley & Sons, Ltd