Recently gender related dimorphism in the knee has received increased attention in sports medicine and total knee arthroplasty (TKA). Certain soft tissue injuries, particularly ACL injuries, have a higher prevalence rates in young female athletes compared to their male counterparts [1]. Patellofemoral pain, recurrent dislocations and arthritis have been reported to be more common in women [2]. Women have higher incidence of osteoarthritis and account for about two-thirds of TKA patients. Gender specific implant designs have also been reported [3]. However, no study has compared the six-DOF kinematics of the male and female knees during functional activities. Additionally gender differences in the morphology of the knee, particularly the trochlear groove, have not been clearly delineated. Herein, we have examined gender differences in knee morphology and six-DOF kinematics during weight bearing flexion. This was accomplished via combined magnetic resonance (MR) based 3D modeling and dual fluoroscope imaging of the knee.

Intracranial aneurysm rupture is the leading cause of subarachnoid hemorrhage in the United States and is associated with higher fatality rates than ischemic stroke. There is currently no objective diagnostic means to identify aneurysms that are at high risk for rupture despite significant improvements in treatment. Often with no warning signs prior to aneurysm rupture such as the transient ischemic attack in ischemic stroke, the need to develop a method for detecting aneurysm instability is necessary.

Much research has been conducted to obtain detailed and accurate information of blood flow field for clarification and advanced diagnosis of circulatory diseases. In order to reproduce complicated real flow field, we have proposed the concept of integration of measurement and computation, and developed the ultrasonic-measurement-integrated (UMI) simulation [1, 2]. In this study, we propose MR-measurement-integrated (MR-MI) simulation by applying the methodology to the MR (Magnetic Resonance) measurement. In the MR-MI simulation, the difference between the computational result and the PC MRI (Phase-Contrast Magnetic Resonance Imaging) measurement data of the three-dimensional velocity vector field is fed back to the numerical simulation (see Fig. 1). The objective in this study is to investigate the characteristics of MR-MI simulation by numerical experiment dealing with a model steady blood flow field in a cerebral aneurysm.

Atherosclerotic vascular disease leads to changes of the arterial wall and lumen narrowing, and it is the No.1 killer in the western world. Magnetic resonance image (MRI)-based computational models with fluid-structure interactions (FSI) for atherosclerotic plaques have been introduced to perform mechanical analysis to quantify critical flow and stress/strain conditions related to plaque rupture which often leads directly to heart attack or stroke [2]. There are three groups of information needed as model input: plaque morphology, material properties, and flow conditions. Pre shrinkage-stretch process is used to recover the in vivo geometry from zero-stress state with associated initial conditions which are required to achieve building the computational model. An important issue for this process is how to determine zero stress state from in vivo plaque geometry. However, few publications can be found in the current literature about how to quantify human carotid artery shrinkage.

Specimen-specific modeling of the knee can be an effective tool for understanding knee mechanics [1–2]. It can also serve as a design tool for orthopaedic implant design through enhancing understanding of mechanics in the reconstructed knee [3], particularly when used in conjunction with instrumented components that record in vivo joint forces [4]. Techniques for developing specimen-specific computational geometric models of hard tissue and soft tissue are fairly commonplace, using imaging tools such as computed tomography (CT) and magnetic resonance (MR) in conjunction with software tools for image processing. Determination of specimen-specific material properties relies on measuring kinematics of the tissue associated with a defined load, either in vivo or in vitro, selecting an appropriate material model, and estimating values of the parameters of the model that closely match the experimental data. The goal of this work was to utilize inverse finite element (FE) analysis to determine material parameters of ligaments in a specimen-specific model of the knee, using both local and global optimization algorithms.

Abdominal aortic aneurysm stent grafts have been reported to fracture at a rate as high as 8% [1]. Iliac artery deformation induced by cardiac pulsation or musculoskeletal motions has been hypothesized to cause these fractures, but little in vivo data has been available heretofore to test this hypothesis. We have utilized magnetic resonance angiography and image-processing methods to quantify in vivo deformation of the human iliac arteries due to maximal hip flexion.

In the current study we apply the magnetic resonance elastography (MRE) technique to estimate the dynamic shear modulus of mouse brain tissue in vivo . The frequency used (1200 Hz) is well above those reported previously [1]. Estimates of dynamic shear modulus range from 12,600–14,800 N/m 2 at 1200 Hz. These data are strictly relevant only to small oscillations at this specific frequency, but these values are obtained at high frequencies (and thus high deformation rates) and non-invasively throughout the brain. These data complement measurements of nonlinear viscoelastic properties obtained by others at slower rates, either ex vivo or invasively.

Deformation of the human brain was measured in tagged magnetic resonance images (MRI) obtained dynamically during angular acceleration of the skull. This study was undertaken to provide much needed experimental data for the quantitative study of traumatic brain injury (TBI). A custom device was made to impart mild angular acceleration to the skull of a human volunteer inside an MR scanner. Images with a superimposed grid of “tag” lines were obtained using spatial modulation of magnetization (SPAMM) in a fast gradient-echo imaging sequence. Images of the moving brain were obtained dynamically by synchronizing the imaging process with the motion of the head. The deformation of the brain was characterized quantitatively with Lagrangian strain. Strain fields showed reduced strain along the central fissure and to a lesser degree, the central sulcus, suggesting that divisions between regions of the brain may serve to mechanically isolate these regions. Results emphasize the critical role of the brain’s suspension, including the dura mater, falx cerebri, and tentorium membranes, in modulating its deformation.

The quantitative measurement of blood flow using magnetic resonance imaging (MRI) has been a topic of research for over twenty years. At present, blood flow measurements are largely performed using two-dimensional phase contrast magnetic resonance (PC-MR). However, when high acceleration terms are present ( i.e. , valve regurgitation or stenosis), accuracy of conventional (Con) cardiac PC-MR can suffer: when data are acquired at a relatively poor temporal resolution, the data extraction process, which essentially is one of comparison, makes the Con PC-MR data sensitive to temporal acceleration. Here, a novel PC-MR technique termed Self Reference (SR) PC-MR is introduced that overcomes these limitations by eliminating the need for comparing paired data sets.

It has been well-accepted that atherosclerosis initiation and progression correlate positively with low and oscillating flow wall shear stresses. However, this shear stress mechanism cannot fully explain why advanced plaques continue to grow under elevated flow shear stress conditions. Our previous investigations using 3D computational models with fluid-structure interactions (FSI) based on in vivo/ex vivo magnetic resonance images (MRI) of human carotid atherosclerotic plaques indicated that there is a negative correlation between advanced plaque wall thickness and structural maximum principal stress (Stress-P 1 ) in the plaque and a positive correlation between plaque wall thickness and flow shear stress [3].

Correlating In vivo temperatures to the radio-frequency (RF) coil induced total RF power is necessary to ensure human safety in an ultra high field magnetic resonance (MR) application. Thus to ensure human safety in an ultra high field MR head imaging experiment, temperatures were measured as a function of time in the brain and surrounding cutaneous layer of twelve human sized, anesthetized swine (mean animal weight = 52kg, SD = ±6.7kg). In vivo temperatures were correlated to the RF power by developing coil and geometry specific normalized temperatures such that the RF coil induced cranial temperature change could be obtained during an MR exam by measuring only the whole head average specific absorption rate (ASAR) and the duration of the RF deposition. Thus, the feasibility of the thermal characterization of an RF volume head coil was shown. More specifically, a continuous wave (CW) RF was deposited in porcine cranium using a four loop RF head coil at 400 MHz (proton larmor frequency at 9.4T). Temperatures were recorded continuously using an inline probe placed at a predetermined location of 15mm inside the brain and a separate probe in the cutaneous layer. To differentiate the temperature response caused by the RF from that of anesthesia, the temperatures were recorded in four unheated, anesthetized swine for the complete duration of experiments (∼8hours). To study the effect of the spatial distribution of the RF as well as the tissue thermal/electrical properties and blood perfusion, the inline temperature probe was placed at two locations (N = 4 for each location). Results showed that the thermal characterization of an RF coil was possible such that the normalized temperature maps when multiplied by the ASAR and the RF heating time would predict In vivo temperature change during heating. Further, it was shown that at 9.4 T 1) the RF heating caused an inhomogeneous normalized temperature distribution in the brain; and 2) the skin temperature change was an unreliable parameter to assess In vivo temperature change.

Stents implanted to treat renal artery stenosis are vulnerable to stent fracture and thrombosis [1–3]. We hypothesize that the motion of the renal arteries during respiration is a possible cause of stent fracture or in-stent restenosis. However, the respiratory motion of the renal arteries and the kidneys is poorly understood. Using magnetic resonance imaging data we previously quantified the two-dimensional deformation of the renal arteries and demonstrated that respiration-induced kidney motion results in vessel bending near the ostia [4]. In this study we quantified the complex three-dimensional motion of the renal arteries and kidneys over the respiratory cycle using magnetic resonance angiography data and imaged-based modeling methods. We provide quantitative information on anatomic changes to the renal arteries that may provide data to design improved pre-clinical, benchtop tests for renal stents.

An accurate method for noninvasive temperature monitoring in vivo would be invaluable for hyperthermic cancer treatments [1]. These therapies selectively heat tumors with RF or microwave radiation, thereby bringing about enhanced drug delivery (up to 30-fold), inhibited cell repair, and even cell death [2]. Unfortunately, this promising treatment has been stymied by the inadequacy of current methods to map temperature in vivo. Conventional magnetic resonance (MR) temperature maps are easily distorted, and provide only a relative measurement of the temperature. Instead we propose a physically different method based on the detection of water-fat iZQCs (intermolecular Zero Quantum Coherences) which produce an absolute temperature map and are insensitive to other distorting factors [3]. Novel but simple modifications to the pulse sequence allow us to isolate the relevant signal, and early applications demonstrate an order of magnitude improvement of temperature accuracy, both in vitro and in vivo.

MRI is an attractive non-invasive angiographic modality not requiring nephrotoxic contrast agents and damaging exposure to radiation such as traditional angiography and multislice CT. It has previously been shown to provide consistently accurate non-invasive information from the stented regions of peripheral vessels even under stenotic conditions [1–3]. The aim of this study is to show that phase contrast magnetic resonance (PC-MR) can be used to provide accurate quantitative information of blood flow in smaller stented vessels such as the coronaries, using an in-vitro model and a rabbit aorta model. The ultimate goal of the project is to evaluate in-stent restenosis in stented coronary vessels, ultimately describing the degree of stenosis based on blood velocity indices such as translesional pressure gradient and peak systolic velocity ratio (PSVR).

A magnetic resonance (MR) method for measuring the deformation of the brain during angular acceleration was validated in a simple model system. Experimentally-derived shear strain patterns in a cylindrical gel “phantom” under angular acceleration were compared to analogous strain fields predicted by finite element simulation.

Low and oscillatory arterial wall shear stress (WSS) have been shown to have an effect on many factors implicated in atherosclerotic lesion development. The majority of studies on the relationship between low or oscillating WSS and sites of intimal thickening and early atherosclerotic lesion development are based on in-vitro model studies of flow and WSS distribution. These models are based on average vessel geometries with average flow conditions and compared to average pathology distribution of lesions that may obscure the true relationship between WSS and lesion distribution[1]. Recent techniques have been developed using coronary MR angiography to create patient-specific 3D models along with velocity measurements of blood flow using phase contrast magnetic resonance (PCMR). However, these models may lack adequate spatial resolution for accurate, localized calculation of WSS[2]. Current, state-of-art multidetector CT scanners offer improvements in spatial resolution over MRI for creation of 3D vessel models.

Stress fracture in lower extremities occurs commonly among military recruits and athletes during intensive physical training. This injury has a marked impact on the health of military personnel and imposes a significant financial burden [1]. Despites advances in stress fracture studies, its pathogeneses remains poorly understood and early diagnostic tools are lacking. Focal ischemia is a potential initiator of site-specific bone remodeling and may cause stress fractures in human lower extremity [2, 3]. We hypothesize that intensive repetitive loading impairs intramedullary blood flow due to pressurization of the bone marrow cavity, leading to focal ischemia and eventual development of stress fractures. To begin to test our hypothesis, we developed and validated a quantitative, non-invasive method to measure blood flow in vivo. The approach was based on Cine Phase Contrast MRI (CPC-MRI), a dynamic motion measurement and visualization modality that was originally designed for cardiovascular studies. By measuring the phase shift that is induced by pulsatile blood flows in the magnetic resonance signal, cross-sectional images and velocity maps are acquired of the moving fluid. This technique has been adapted to study blood flows in skeletal muscles [4], intracranial flows [5], and muscle mechanics and joint kinematics [6]]. We first performed a flow phantom study to validate the reliability and accuracy of CPC-MRI in measuring flow velocity. We then quantified the effects of brief exercise on blood flows in the lower extremities of human subjects. These non-invasive measurements will help us better understand the interplay between vasculature and skeletal system in various physiological and pathological conditions.

Transposition of the great arteries (TGA) is a congenital heart disease characterized by abnormal spatial arrangement of the two main vessels, with the aorta arising from the pulmonary valve and the main pulmonary artery arising from the aortic valve. TGA surgical repair with the arterial switch operation (ASO) involves physically repositioning the aorta and the pulmonary artery in their correct anatomical location, as well as separately moving the coronary arteries. Following ASO, decreased aortic distensibility and enlarged aortic root have been observed, together with late complications such as coronary artery obstruction, neoaortic valvar insufficiency, and arrhythmia [1]. Clearly, further knowledge of the hemodynamics in the neoaorta following ASO can be helpful in understanding the physiology of repaired-TGA. We suggest that engineering tools can provide access to such knowledge, both experimentally and computationally. 4D flow data from magnetic resonance (MR) imaging can generate excellent maps of velocity streamlines and — to our knowledge — has never been applied to this clinical problem. In addition, 4D MR flow data gathered in-vitro (hence more reproducible and more stable than in-vivo ) can be a resourceful tool for validating a computational fluid dynamics (CFD) model of the same problem. The experimental model, lacking respiration effects and concerns about scanning time, can also be used for exploring the optimal spatial and temporal resolution for improving the quality of the data. Ultimately, we suggest that a synergistic approach (experimental 4D MR flow + CFD study) carried out at a patient-specific level can provide knowledge about the hemodynamics in the neoaorta following ASO. For this purpose, we present two comparisons: (a) TGA anatomy vs. an age-matched healthy subject (b) in-vitro vs. in-silico .

Mechanical characterization of inhomogeneous and/or geometrically complex biological tissues requires precise and accurate determination of strain fields. Digital image correlation is a well established technique for determining strain fields on the surfaces of deforming materials. The technique involves matching patterns between pairs of images to estimate the displacement of certain regions or features on a sample. 2 Image correlation has also been used to track deformations in dynamic magnetic resonance images of heart and brain. 3

The morphology of adult carotid sinus and atherosclerosis are major risk factors for ischemic stroke. It has been reported that the function of carotid sinus is pressure sensing and the regulation of heart rate. Seong and colleagues reported the lifelong development of carotid sinus morphology from infancy to maturity using post-image processed digital subtraction angiograms from 36 patients, ages from newborn to 36 years old [1]. The study unveiled the substantial growth of the internal carotid artery (ICA) with aging and the development of a carotid sinus at the root of the ICA during late adolescence. However, the development of the carotid sinus with associated hemodynamics is not clearly understood yet.