KEYWORDS: In vivo imaging, Magnetic resonance imaging, Calibration, Computer programming, Signal to noise ratio, Interference (communication), Brain, Image resolution, 3D acquisition, Neuroimaging
Parallel MRI can achieve increased spatiotemporal resolution in MRI by simultaneously sampling reduced k-space data
with multiple receiver coils. One requirement that different parallel MRI techniques have in common is the need to
determine spatial sensitivity information for the coil array. This is often done by smoothing the raw sensitivities obtained
from low-resolution calibration images, for example via polynomial fitting. However, this sensitivity post-processing
can be both time-consuming and error-prone. Another important factor in Parallel MRI is noise amplification in the
reconstruction, which is due to non-unity transformations in the image reconstruction associated with spatially correlated
coil sensitivity profiles. Generally, regularization approaches, such as Tikhonov and SVD-based methods, are applied to
reduce SNR loss, at the price of introducing residual aliasing. In this work, we present a regularization approach using in
vivo coil sensitivities in parallel MRI to overcome these potential errors into the reconstruction. The mathematical
background of the proposed method is explained, and the technique is demonstrated with phantom images. The
effectiveness of the proposed method is then illustrated clinically in a whole-heart 3D cardiac MR acquisition within a
single breath-hold. The proposed method can not only overcome the sensitivity calibration problem, but also suppress a
substantial portion of reconstruction-related noise without noticeable introduction of residual aliasing artifacts.
Static X-ray computed tomography (CT) volumes are often used as anatomic roadmaps during catheter-based cardiac
interventions performed under X-ray fluoroscopy guidance. These CT volumes provide a high-resolution depiction of
soft-tissue structures, but at only a single point within the cardiac and respiratory cycles. Augmenting these static CT
roadmaps with segmented myocardial borders extracted from live ultrasound (US) provides intra-operative access to
real-time dynamic information about the cardiac anatomy. In this work, using a customized segmentation method based
on a 3D active mesh, endocardial borders of the left ventricle were extracted from US image streams (4D data sets) at a
frame rate of approximately 5 frames per second. The coordinate systems for CT and US modalities were registered
using rigid body registration based on manually selected landmarks, and the segmented endocardial surfaces were
overlaid onto the CT volume. The root-mean squared fiducial registration error was 3.80 mm. The accuracy of the
segmentation was quantitatively evaluated in phantom and human volunteer studies via comparison with manual
tracings on 9 randomly selected frames using a finite-element model (the US image resolutions of the phantom and
volunteer data were 1.3 x 1.1 x 1.3 mm and 0.70 x 0.82 x 0.77 mm, respectively). This comparison yielded 3.70±2.5
mm (approximately 3 pixels) root-mean squared error (RMSE) in a phantom study and 2.58±1.58 mm (approximately 3
pixels) RMSE in a clinical study. The combination of static anatomical roadmap volumes and dynamic intra-operative
anatomic information will enable better guidance and feedback for image-guided minimally invasive cardiac
interventions.
Adaptive filtering of temporally varying X-ray image sequences acquired during endovascular interventions can improve the visual tracking of catheters by radiologists. Existing techniques blur the important parts of image sequences, such as catheter tips, anatomical structures and organs; and they may introduce trailing artifacts. To address this concern, an adaptive filtering process is presented to apply temporal filtering in regions without motion and spatial filtering in regions with motion. The adaptive filtering process is a multi-step procedure. First a normalized motion mask that describes the differences between two successive frames is generated. Secondly each frame is spatially filtered using the specific motion mask to specify different types of filtering in each region. Third an IIR filter is then used to combine the spatially filtered image with the previous output image; the motion mask thus serves as a weighted input mask to determine how much spatial and temporal filtering should be applied. This method results in improving both the stationary and moving fields. The visibility of static anatomical structures and organs increases, while the motion of the catheter tip and motion of anatomical structures and organs remain unblurred and visible during interventional procedures.
With relatively high frame rates and the ability to acquire volume data sets with a stationary transducer, 3D ultrasound systems, based on matrix phased array transducers, provide valuable three-dimensional information, from which quantitative measures of cardiac function can be extracted. Such analyses require segmentation and visual tracking of the left ventricular endocardial border. Due to the large size of the volumetric data sets, manual tracing of the endocardial border is tedious and impractical for clinical applications. Therefore the development of automatic methods for tracking three-dimensional endocardial motion is essential. In this study, we evaluate a four-dimensional optical flow motion tracking algorithm to determine its capability to follow the endocardial border in three dimensional ultrasound data through time. The four-dimensional optical flow method was implemented using three-dimensional correlation. We tested the algorithm on an experimental open-chest dog data set and a clinical data set acquired with a Philips' iE33 three-dimensional ultrasound machine. Initialized with left ventricular endocardial data points obtained from manual tracing at end-diastole, the algorithm automatically tracked these points frame by frame through the whole cardiac cycle.
A finite element surface was fitted through the data points obtained by both optical flow tracking and manual tracing by an experienced observer for quantitative comparison of the results. Parameterization of the finite element surfaces was performed and maps displaying relative differences between the manual and semi-automatic methods were compared.
The results showed good consistency between manual tracing and optical flow estimation on 73% of the entire surface with fewer than 10% difference. In addition, the optical flow motion tracking algorithm greatly reduced processing time (about 94% reduction compared to human involvement per cardiac cycle) for analyzing cardiac function in three-dimensional ultrasound data sets.
Three-dimensional ultrasound machines based on matrix phased-array transducers are gaining predominance for real-time dynamic screening in cardiac and obstetric practice. These transducers array acquire three-dimensional data in spherical coordinates along lines tiled in azimuth and elevation angles at incremental depth. This study aims at evaluating fast filtering and scan conversion algorithms applied in the spherical domain prior to visualization into Cartesian coordinates for visual quality and spatial measurement accuracy.
Fast 3d scan conversion algorithms were implemented and with different order interpolation kernels. Downsizing and smoothing of sampling artifacts were integrated in the scan conversion process. In addition, a denoising scheme for spherical coordinate data with 3d anisotropic diffusion was implemented and applied prior to scan conversion to improve image quality. Reconstruction results under different parameter settings, such as different interpolation kernels, scaling factor, smoothing options, and denoising, are reported. Image quality was evaluated on several data sets via visual inspections and measurements of cylinder objects dimensions. Error measurements of the cylinder's radius, reported in this paper, show that the proposed fast scan conversion algorithm can correctly reconstruct three-dimensional ultrasound in Cartesian coordinates under tuned parameter settings. Denoising via three-dimensional anisotropic diffusion was able to greatly improve the quality of resampled data without affecting the accuracy of spatial information after the modification of the introduction of a variable gradient threshold parameter.
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