3D Motion Estimation of Left Ventricular Dynamics Using MRI and Track-to-Track Fusion.

OBJECTIVE: This study investigates the estimation of three dimensional (3D) left ventricular (LV) motion using the fusion of different two dimensional (2D) cine magnetic resonance (CMR) sequences acquired during routine imaging sessions. Although standard clinical cine CMR data is inherently 2D, the actual underlying LV dynamics lies in 3D space and cannot be captured entirely using single 2D CMR image sequences. By utilizing the image information from various short-axis and long-axis image sequences, the proposed method intends to estimate the dynamic state vectors consisting of the position and velocity information of the myocardial borders in 3D space. METHOD: The proposed method comprises two main components: tracking myocardial points in 2D CMR sequences and fusion of multiple trajectories correspond to the tracked points. The tracking which yields the set of corresponding temporal points representing the myocardial points is performed using a diffeomorphic nonrigid image registration approach. The trajectories obtained from each cine CMR sequence is then fused with the corresponding trajectories from other CMR views using an unscented Kalman smoother (UKS) and a track-to-track fusion algorithm. RESULTS: We evaluated the proposed method by comparing the results against CMR imaging with myocardial tagging. We report a quantitative performance analysis by projecting the state vector estimates we obtained onto 2D tagged CMR images acquired from the same subjects and comparing them against harmonic phase estimates. The proposed algorithm yielded a competitive performance with a mean root mean square error of 1.3±0.5 pixels (1.8±0.6 mm) evaluated over 118 image sequences acquired from 30 subjects. CONCLUSION: This study demonstrates that fusing the information from short and long-axis views of CMR improves the accuracy of cardiac tissue motion estimation. Clinical Impact: The proposed method demonstrates that the fusion of tissue tracking information from long and short-axis views improves the binary classification of the automated regional function assessment.

A realistic phantom for validating MRI-based synthetic CT images of the human skull.

PURPOSE: To introduce a new realistic human skull phantom for the validation of synthetic CT images of cortical bone from ultra-short echo-time (UTE) sequences. METHODS: A human skull of an adult female was utilized as a realistic representation of skull cortical bone. The skull was stabilized in a special acrylic container and was filled with contrast agents that have T1 and T2 relaxation times similar to human brain. The phantom was MR scanned at 3T with UTE and T2 -weighted sequences, followed by CT. A clustering approach was developed to extract the cortical bone signal from MR images. T2∗ maps of the skull were calculated. Synthetic CT images of the bone were compared to cortical bone signal extracted from CT images and confounding factors, such as registration errors, were analyzed. RESULTS: Dice similarity coefficient (DSC) of UTE-detected cortical bone was 0.84 and gradually decreased with decreasing number of spokes. DSC did not significantly depend on echo-time. Registration errors were found to be significant confounding factors, with 25% decrease in DSC for consistent 2 mm error at each axis. CONCLUSION: This work introduced a new realistic human skull phantom, specifically for the evaluation and analysis of synthetic CT images of cortical bone.

Metal artefacts in MRI-guided brachytherapy of cervical cancer.

The importance of assessing the metal-induced artefacts in magnetic resonance imaging (MRI)-guided brachytherapy is growing along with the increasing interest of integrating MRI into the treatment procedure of cervical cancer. Examples of metal objects in use include intracavitary cervical applicators and interstitial needles. The induced artefacts increase the uncertainties in the clinical workflow and can be a potential obstacle for the accurate delivery of the treatment. Overcoming this problem necessitates a good understanding of its originating sources. Several efforts are recorded in the literature to quantify the extent of such artefacts, in phantoms and in clinical practice. Here, we elaborate on the origin of metal-induced artefacts in the light of brachytherapy applications, while summarizing recent efforts that have been made to assess and overcome the induced distortions.

Quantitative MRI assessment of a novel direction modulated brachytherapy tandem applicator for cervical cancer at 1.5T.

BACKGROUND AND PURPOSE: The purpose of this work is to quantitatively investigate the artifacts and image distortions induced in the MR images by a recently proposed direction modulated brachytherapy (DMBT) tandem applicator prototype. This new MRI-compatible applicator allows better sparing of organs-at-risk (OAR) for cervical cancer patients, while providing conformal dose distributions to target volumes. MATERIALS AND METHODS: Specific phantom and tools were designed and manufactured for this study. The phantom was filled with a tissue-like solution and MR images were acquired with clinical protocols as per GEC-ESTRO recommendations. Images were obtained at 6 different orientations that mimic possible clinical settings and full-width-at-half-maximum (FWHM) was recorded at multiple locations/angles. The accuracy of detecting the centerline of the tandem was assessed using a novel radial-fiducials mount. RESULTS: FWHM from all line profiles at all angles and all orientations was 6.14±0.7mm (compared to 6mm of the actual DMBT tandem diameter). The in-plane spatial-shift observed at para-axial and para-sagittal views was less than 0.5mm. CONCLUSIONS: This work demonstrated that the novel DMBT tandem applicator prototype has minimal artifact in T2-weighted images employed in clinical practice, suggesting the applicator might be a good candidate for MRI-guided adaptive brachytherapy.

Fat quantification using an interleaved bipolar acquisition.

PURPOSE: To demonstrate a new multigradient echo bipolar acquisition sequence for fat quantification. THEORY AND METHODS: A multiecho bipolar acquisition is used such that the even echoes have opposite polarity to the odd echoes. In addition, the readout gradients alternate their polarities every other phase-encode line. Each echo, therefore, consists of phase-encode lines with both positive and negative polarities. Phase-encodes acquired with the same polarity are grouped together, and parallel imaging reconstruction is used to obtain two full k-space maps with opposite readout polarities at all the echoes. By complex averaging, the inconsistent phase errors between odd and even echoes are removed and water/fat separation techniques used with conventional unipolar sequences can be performed. RESULTS: Phantoms and in vivo experiments demonstrated accurate fat fraction and increased signal to noise ratio efficiency compared with the established unipolar acquisition. Artefacts due to phase and magnitude errors of bipolar acquisition were eliminated in all experiments. CONCLUSION: The interleaved bipolar sequence is an efficient technique for fat quantification. It demonstrated accurate fat measurements in a shorter scan time compared with the standard unipolar sequence.

Validation of volumetric and single-slice MRI adipose analysis using a novel fully automated segmentation method.

PURPOSE: To validate a fully automated adipose segmentation method with magnetic resonance imaging (MRI) fat fraction abdominal imaging. We hypothesized that this method is suitable for segmentation of subcutaneous adipose tissue (SAT) and intra-abdominal adipose tissue (IAAT) in a wide population range, easy to use, works with a variety of hardware setups, and is highly repeatable. MATERIALS AND METHODS: Analysis was performed comparing precision and analysis time of manual and automated segmentation of single-slice imaging, and volumetric imaging (78-88 slices). Volumetric and single-slice data were acquired in a variety of cohorts (body mass index [BMI] 15.6-41.76) including healthy adult volunteers, adolescent volunteers, and subjects with nonalcoholic fatty liver disease and lipodystrophies. A subset of healthy volunteers was analyzed for repeatability in the measurements. RESULTS: The fully automated segmentation was found to have excellent agreement with manual segmentation with no substantial bias across all study cohorts. Repeatability tests showed a mean coefficient of variation of 1.2 ± 0.6% for SAT, and 2.7 ± 2.2% for IAAT. Analysis with automated segmentation was rapid, requiring 2 seconds per slice compared with 8 minutes per slice with manual segmentation. CONCLUSION: We demonstrate the ability to accurately and rapidly segment regional adipose tissue using fat fraction maps across a wide population range, with varying hardware setups and acquisition methods.

Max-IDEAL: a max-flow based approach for IDEAL water/fat separation.

PURPOSE: To propose a novel approach to water/fat separation using a unique smoothness constraint. THEORY AND METHODS: Chemical-shift based water/fat separation is an established noninvasive imaging tool for the visualization of body fat in various anatomies. Nevertheless, B0 magnetic field inhomogeneities can hamper the water/fat separation process. In this work, B0 variations are mapped using a convex-relaxed labeling model which produces a coarse estimate of the field map, while considering T2* decay during the labeling process. Fat and water components are subsequently resolved using T2*-IDEAL. An adaptive spatial filtering (ASF) was introduced to improve the robustness of the estimate. The method was tested on cardiac and abdominal datasets from healthy volunteers and nonalcoholic fatty liver disease (NAFLD) patients. RESULTS: Out of 168 cardiac and abdominal images, only 1 case has shown water/fat swaps that can hinder the clinical interpretation of the underlying anatomy. CONCLUSION: This work demonstrates a new water/fat separation approach that prevents the occurrence of water/fat swaps, by means of a unique smoothness constraint. Incorporating T2* effect in the labeling procedure and including the ASF processing enhance the robustness of the proposed approach and permit the procedure to handle abrupt B0 variations within the field of view.

A convex relaxation approach to fat/water separation with minimum label description.

While Magnetic Resonance Imaging is capable of separating water and fat components in the body, mapping of magnetic field inhomogeneities is essential for the successful application of this process. In this study, we address the problem of field map estimation using a convex-relaxed max-flow method. We propose a novel two-stage approach that leads to the global optimum of the proposed problem. The first stage minimizes the signal residuals via a convex-relaxed minimum description length (MDL)-based approach. The MDL-based labeling model penalizes the total number of appearing labels, which helps to avoid field map errors when abrupt changes in field homogeneity exist. By exploringthe whole range of possible frequency offsets, this stage ensures limiting the estimated field offset within certain boundaries where the global minimum resides. The second stage employs the output of the labeling model in a commonly used gradient-descent based method (known as IDEAL) to converge to the exact global minimum, i.e. the required value of the field offset. Experimental results for cardiac imaging, where challenging field inhomogeneities exist, showed that our method significantly outperforms over a widely-used technique for fat/water separation in terms of robustness and efficiency.