Eugene Ozhinsky

Magnetic resonance imaging of patellofemoral kinematics with weight-bearing.

BACKGROUND Previous studies of the patellofemoral joint have been limited by the use of invasive techniques, measurements under non-weight-bearing conditions, cadaveric specimens, or computerized models. It has been shown that soft tissue and bone can be accurately quantified with magnetic resonance imaging. The present study was designed to define the relationship between the patellofemoral contact area and patellofemoral kinematics in vivo. METHODS Ten subjects with clinically normal knee joints were scanned with high-resolution magnetic resonance imaging while they pushed a constant weight (133 N) on the foot-plate of a custom-designed load-bearing apparatus. Images were obtained at five positions of flexion between -10 degrees and 60 degrees. Three-dimensional reconstructions were used to measure the patellofemoral cartilage contact area, patellar centroid, patellar medial and inferior translation, patellar medial and inferior tilt, and patellar varus-valgus rotation. All translation and area measurements were normalized on the basis of the interepicondylar distance. Random-effects models of quadratic regressions were used to evaluate the data. RESULTS The mean contact area ranged from 126 mm(2) in extension to 560 mm(2) at 60 degrees of flexion. The patella translated inferiorly to a maximum distance of 34 mm at 60 degrees of flexion and translated medially to a maximum distance of 3.2 mm at 30 degrees of flexion before returning to nearly 0 mm at 60 degrees of flexion. The patella tilted inferiorly to a mean of nearly 35 degrees at 60 degrees of flexion and medially to a maximum of 4.2 degrees at 30 degrees of flexion. By 60 degrees of flexion, the centroid of the contact area had shifted to an inferior and posterior maximum of 20 and 10 mm, respectively. CONCLUSIONS We found that lateral patellar subluxation and tilt occurred in these normal knees at full extension and the patella was reduced into the trochlear groove at 30 degrees of flexion. Therefore, we believe that lateral patellar tilt and subluxation observed during arthroscopy of the extended knee may not represent a pathological condition.

A New Computational Efficient Approach for Trabecular Bone Analysis using Beam Models Generated with Skeletonized Graph Technique

Micro-finite element (FE) analysis is a well established technique for the evaluation of the elastic properties of trabecular bone, but is limited in its application due to the large number of elements that it requires to represent the complex internal structure of the bone. In this paper, we present an alternative FE approach that makes use of a recently developed 3D-Line Skeleton Graph Analysis (LSGA) technique to represent the complex internal structure of trabecular bone as a network of simple straight beam elements in which the beams are assigned geometrical properties of the trabeculae that they represent. Since an enormous reduction of cputime can be obtained with this beam modeling approach, ranging from approximately 1,200 to 3,600 for the problems investigated here, we think that the FE modeling technique that we introduced could potentially constitute an interesting alternative for the evaluation of the elastic mechanical properties of trabecular bone.

Strategies for rapid in vivo 1H and hyperpolarized 13C MR spectroscopic imaging.

In vivo MRSI is an important imaging modality that has been shown in numerous research studies to give biologically relevant information for assessing the underlying mechanisms of disease and for monitoring response to therapy. The increasing availability of high field scanners and multichannel radiofrequency coils has provided the opportunity to acquire in vivo data with significant improvements in sensitivity and signal to noise ratio. These capabilities may be used to shorten acquisition time and provide increased coverage. The ability to acquire rapid, volumetric MRSI data is critical for examining heterogeneity in metabolic profiles and for relating serial changes in metabolism within the same individual during the course of the disease. In this review we discuss the implementation of strategies that use alternative k-space sampling trajectories and parallel imaging methods in order to speed up data acquisition. The impact of such methods is demonstrated using three recent examples of how these methods have been applied. These are to the acquisition of robust 3D (1)H MRSI data within 5-10 min at a field strength of 3 T, to obtaining higher sensitivity for (1)H MRSI at 7 T and to using ultrafast volumetric and dynamic (13)C MRSI for monitoring the changes in signals that occur following the injection of hyperpolarized (13)C agents.

Improved spatial coverage for brain 3D PRESS MRSI by automatic placement of outer‐volume suppression saturation bands

To develop a technique for optimizing coverage of brain 3D 1H magnetic resonance spectroscopic imaging (MRSI) by automatic placement of outer‐volume suppression (OVS) saturation bands (sat bands) and to compare the performance for point‐resolved spectroscopic sequence (PRESS) MRSI protocols with manual and automatic placement of sat bands.

Short-echo three-dimensional H-1 MR spectroscopic imaging of patients with glioma at 7 tesla for characterization of differences in metabolite levels

The purpose of this study was to evaluate the feasibility of using a short echo time, three‐dimensional H‐1 magnetic resonance spectroscopic imaging (MRSI) sequence at 7 Tesla (T) to assess the metabolic signature of lesions for patients with glioma.

Model-based feasibility assessment and evaluation of prostate hyperthermia with a commercial MR-guided endorectal HIFU ablation array

PURPOSE Feasibility of targeted and volumetric hyperthermia (40-45 °C) delivery to the prostate with a commercial MR-guided endorectal ultrasound phased array system, designed specifically for thermal ablation and approved for ablation trials (ExAblate 2100, Insightec Ltd.), was assessed through computer simulations and tissue-equivalent phantom experiments with the intention of fast clinical translation for targeted hyperthermia in conjunction with radiotherapy and chemotherapy. METHODS The simulations included a 3D finite element method based biothermal model, and acoustic field calculations for the ExAblate ERUS phased array (2.3 MHz, 2.3 × 4.0 cm(2), ∼1000 channels) using the rectangular radiator method. Array beamforming strategies were investigated to deliver protracted, continuous-wave hyperthermia to focal prostate cancer targets identified from representative patient cases. Constraints on power densities, sonication durations and switching speeds imposed by ExAblate hardware and software were incorporated in the models. Preliminary experiments included beamformed sonications in tissue mimicking phantoms under MR temperature monitoring at 3 T (GE Discovery MR750W). RESULTS Acoustic intensities considered during simulation were limited to ensure mild hyperthermia (Tmax < 45 °C) and fail-safe operation of the ExAblate array (spatial and time averaged acoustic intensity ISATA < 3.4 W/cm(2)). Tissue volumes with therapeutic temperature levels (T > 41 °C) were estimated. Numerical simulations indicated that T > 41 °C was calculated in 13-23 cm(3) volumes for sonications with planar or diverging beam patterns at 0.9-1.2 W/cm(2), in 4.5-5.8 cm(3) volumes for simultaneous multipoint focus beam patterns at ∼0.7 W/cm(2), and in ∼6.0 cm(3) for curvilinear (cylindrical) beam patterns at 0.75 W/cm(2). Focused heating patterns may be practical for treating focal disease in a single posterior quadrant of the prostate and diffused heating patterns may be useful for heating quadrants, hemigland volumes or even bilateral targets. Treatable volumes may be limited by pubic bone heating. Therapeutic temperatures were estimated for a range of physiological parameters, sonication duty cycles and rectal cooling. Hyperthermia specific phasing patterns were implemented on the ExAblate prostate array and continuous-wave sonications (∼0.88 W/cm(2), 15 min) were performed in tissue-mimicking material with real-time MR-based temperature imaging (PRFS imaging at 3.0 T). Shapes of heating patterns observed during experiments were consistent with simulations. CONCLUSIONS The ExAblate 2100, designed specifically for thermal ablation, can be controlled for delivering continuous hyperthermia in prostate while working within operational constraints.

Quantifying temperature-dependent T1 changes in cortical bone using ultrashort echo-time MRI.

To demonstrate the feasibility of using ultrashort echo‐time MRI to quantify T1 changes in cortical bone due to heating.

Automated Prescription of Oblique Brain 3D MRSI

Two major difficulties encountered in implementing Magnetic Resonance Spectroscopic Imaging (MRSI) in a clinical setting are limited coverage and difficulty in prescription. The goal of this project was to completely automate the process of 3D PRESS MRSI prescription, including placement of the selection box, saturation bands and shim volume, while maximizing the coverage of the brain. The automated prescription technique included acquisition of an anatomical MRI image, optimization of the oblique selection box parameters, optimization of the placement of OVS saturation bands, and loading of the calculated parameters into a customized 3D MRSI pulse sequence. To validate the technique and compare its performance with existing protocols, 3D MRSI data were acquired from 6 exams from 3 healthy volunteers. To assess the performance of the automated 3D MRSI prescription for patients with brain tumors, the data were collected from 16 exams from 8 subjects with gliomas. This technique demonstrated robust coverage of the tumor, high consistency of prescription and very good data quality within the T2 lesion.Two major difficulties encountered in implementing Magnetic Resonance Spectroscopic Imaging (MRSI) in a clinical setting are limited coverage and difficulty in prescription. The goal of this project was to automate completely the process of 3D PRESS MRSI prescription, including placement of the selection box, saturation bands and shim volume, while maximizing the coverage of the brain. The automated prescription technique included acquisition of an anatomical MRI image, optimization of the oblique selection box parameters, optimization of the placement of outer‐volume suppression saturation bands, and loading of the calculated parameters into a customized 3D MRSI pulse sequence. To validate the technique and compare its performance with existing protocols, 3D MRSI data were acquired from six exams from three healthy volunteers. To assess the performance of the automated 3D MRSI prescription for patients with brain tumors, the data were collected from 16 exams from 8 subjects with gliomas. This technique demonstrated robust coverage of the tumor, high consistency of prescription and very good data quality within the T2 lesion. Magn Reson Med 69:920–930, 2013.

Prospective image registration for automated scan prescription of follow-up knee images in quantitative studies

Consistent scan prescription for MRI of the knee is very important for accurate comparison of images in a longitudinal study. However, consistent scan region selection is difficult due to the complexity of the knee joint. We propose a novel method for registering knee images using a mutual information registration algorithm to align images in a baseline and follow-up exam. The output of the registration algorithm, three translations and three Euler angles, is then used to redefine the region to be imaged and acquire an identical oblique imaging volume in the follow-up exam as in the baseline. This algorithm is robust to articulation of the knee and anatomical abnormalities due to disease (e.g., osteophytes). The registration method is performed only on the distal femur and is not affected by the proximal tibia or soft tissues. We have incorporated this approach in a clinical MR system and have demonstrated its utility in automatically obtaining consistent scan regions between baseline and follow-up examinations, thus improving the precision of quantitative evaluation of cartilage. Results show an improvement with prospective registration in the coefficient of variation for cartilage thickness, cartilage volume and T2 relaxation measurements.

Automated prescription of oblique brain 3D magnetic resonance spectroscopic imaging

Two major difficulties encountered in implementing Magnetic Resonance Spectroscopic Imaging (MRSI) in a clinical setting are limited coverage and difficulty in prescription. The goal of this project was to completely automate the process of 3D PRESS MRSI prescription, including placement of the selection box, saturation bands and shim volume, while maximizing the coverage of the brain. The automated prescription technique included acquisition of an anatomical MRI image, optimization of the oblique selection box parameters, optimization of the placement of OVS saturation bands, and loading of the calculated parameters into a customized 3D MRSI pulse sequence. To validate the technique and compare its performance with existing protocols, 3D MRSI data were acquired from 6 exams from 3 healthy volunteers. To assess the performance of the automated 3D MRSI prescription for patients with brain tumors, the data were collected from 16 exams from 8 subjects with gliomas. This technique demonstrated robust coverage of the tumor, high consistency of prescription and very good data quality within the T2 lesion.Two major difficulties encountered in implementing Magnetic Resonance Spectroscopic Imaging (MRSI) in a clinical setting are limited coverage and difficulty in prescription. The goal of this project was to automate completely the process of 3D PRESS MRSI prescription, including placement of the selection box, saturation bands and shim volume, while maximizing the coverage of the brain. The automated prescription technique included acquisition of an anatomical MRI image, optimization of the oblique selection box parameters, optimization of the placement of outer‐volume suppression saturation bands, and loading of the calculated parameters into a customized 3D MRSI pulse sequence. To validate the technique and compare its performance with existing protocols, 3D MRSI data were acquired from six exams from three healthy volunteers. To assess the performance of the automated 3D MRSI prescription for patients with brain tumors, the data were collected from 16 exams from 8 subjects with gliomas. This technique demonstrated robust coverage of the tumor, high consistency of prescription and very good data quality within the T2 lesion. Magn Reson Med 69:920–930, 2013.