A clear understanding of the first pass dynamics of contrast agents in the vascular system is crucial in
synchronizing data acquisition of 3D MR angiography (MRA) with arrival of the contrast bolus in the
vessels of interest. We implemented a computational model to simulate contrast dynamics in the vessels
using the theory of linear time-invariant systems. The algorithm calculates a patient-specific impulse
response for the contrast concentration from time-resolved images following a small test bolus injection. This
is performed for a specific region of interest and through deconvolution of the intensity curve using the long
division method. Since high spatial resolution 3D MRA is not
time-resolved, the method was validated on
time-resolved arterial contrast enhancement in Multi Slice CT angiography. For 20 patients, the timing of the
contrast enhancement of the main bolus was predicted by our algorithm from the response to the test bolus,
and then for each case the predicted time of maximum intensity was compared to the corresponding time in
the actual scan which resulted in an acceptable agreement. Furthermore, as a qualitative validation, the
algorithm's predictions of the timing of the carotid MRA in 20 patients with high quality MRA were
correlated with the actual timing of those studies. We conclude that the above algorithm can be used as a
practical clinical tool to eliminate guesswork and to replace empiric formulae by a priori computation of
patient-specific timing of data acquisition for MR angiography.
Circumferential strain of the left ventricle reflects myocardial contractility and is considered a key index of
cardiac function. It is also an important parameter in the quantitative evaluation of heart failure.
Circumferential compression encoding, CIRCOME, is a novel method in cardiac MRI to evaluate this strain
non-invasively and quickly. This strain encoding technique avoids the explicit measurement of the
displacement field and does not require calculation of strain through spatial differentiation. CIRCOME
bypasses these two time-consuming and noise sensitive steps by directly using the frequency domain (k-space)
information from radially tagged myocardium, before and after deformation. It uses the ring-shaped
crown region of the k-space, generated by the taglines, to reconstruct circumferentially compression-weighted
images of the heart before and after deformation. CIRCOME then calculates the circumferential
strain through relative changes in the compression level of corresponding regions before and after
deformation. This technique can be implemented in 3D as well as 2D and may be employed to estimate the
overall global or regional circumferential strain. The main parameters that affect the accuracy of this method
are spatial resolution, signal to noise ratio, eccentricity of the center of radial taglines their fading and their
density. Also, a variety of possible image reconstruction and filtering options may influence the accuracy of
the method. This study describes the pulse sequence, algorithm, influencing factors and limiting criteria for
CIRCOME and provides the simulated results.
The study here, suggests a macroscopic structure for the Left Ventricle (LV), based on the heart kinematics which is
obtained through imaging. The measurement of the heart muscle deformation using the Displacement ENcoding with
Stimulated Echoes (DENSE) MRI, which describes the heart kinematics in the Lagrangian frame work, is used to
determine the high resolution patterns of true myocardial strain. Subsequently, the tangential Shortening Index (SI) and
the thickening of the LV wall are calculated for each data point. Considering the heart as a positive-displacement pump,
the contribution of each segment of LV in the heart function, can be determined by the SI and thickening of the wall in
the same portion. Hence the SI isosurfaces show the extent and spatial distribution of the heart activity and reveals its
macro structure. The structure and function of the heart are, therefore, related which in turn results in a macroscopic
model for the LV. In particular, it was observed that the heart functionality is not uniformly distributed in the LV, and
the regions with greater effect on the pumping process, form a band which wraps around the heart. These results, which
are supported by the established histological evidence, may be considered as a landmark in connecting the structure and
function of the heart through imaging. Furthermore, the compatibility of this model with microscopic observations
about the fiber direction is investigated. This method may be used for planning as well as post evaluation of the
Proc. SPIE. 6143, Medical Imaging 2006: Physiology, Function, and Structure from Medical Images
KEYWORDS: Signal to noise ratio, Data modeling, Blood, Magnetic resonance imaging, Computing systems, Computer simulations, Iterative methods, Electroluminescent displays, Solid modeling, In vitro testing
In the past, several methods based on iterative solution of pressure-Poisson equation have been developed for measurement of pressure from phase-contrast magnetic resonance (PC-MR) data. We have developed a novel non-iterative harmonics-based orthogonal
projection method which can keep the pressures measured based on the Navier-Stokes equation independent of the path of integration. The gradient of pressure calculated with Navier-Stokes equation is expanded with a series of orthogonal basis functions, and is subsequently projected onto an integrable subspace. Before the projection step however, a scheme is devised to eliminate the
discontinuity at the vessel boundaries.
The approach was applied to noise-added velocities obtained for both
steady and pulsatile stenotic flows from computational fluid
dynamics (CFD) simulations and compared with pressures independently obtained by CFD. Additionally, MR velocity data for steady flows measured in in-vitro phantom models with different degree of stenoses and different flow rates were used to test the algorithm and results were compared with CFD simulations.
Confocal microscopy enables us to track myocytes in the embryonic zebrafish heart. The Zeiss LSM 5 Live high speed confocal microscope has been used to take optical sections (at 3 μm intervals and 151 frames per second) through a fluorescently labeled zebrafish heart at two developmental stages (26 and 34 hours post fertilization (hpf)). This data provides unique information allowing us to conjecture on the morphology and biomechanics of the developing vertebrate heart. Nevertheless, the myocytes, whose positions could be determined in a reliable manner, were located sparsely and mostly in one side of the heart tube. This difficulty was overcome using computational methods, that give longitudinal, radial and circumferential displacements of the myocytes as well as their contractile behavior. Applied strain analysis has shown that in the early embryonic heart tube, only the caudal region (near the in-flow) and another point in the middle of the tube can be active; the rest appears to be mostly passive. This statement is based on the delay between major strain and displacement which a material point experiences. Wave-like propagation of all three components of the displacement, especially in the circumferential direction, as well as the almost-periodic changes of the maximum strain support the hypothesis of helical muscle structure embedded in the tube. Changes of geometry in the embryonic heart after several hours are used to verify speculations about the structure based on the earlier images and aforementioned methods.
Inhomogeneity of static magnetic field, induced by object susceptibility, is unavoidable in magnetic resonance imaging (MRI). This inhomogeneity generates distortions in both image geometry and its intensity. Based on node magnetic voltage values, a fast Finite Difference Method (FDM) is developed for susceptibility-induced mapping of magnetic field inhomogeneity and applied to simulated MRI data. Its accuracy and speed of convergence are evaluated by comparing the method to Finite Elements Method (FEM), which had been validated experimentally. Effects of inhomogeneity on Spin Echo (SE) MRI are simulated using the proposed field calculation method. Also, a pixel based (direct) method as well as a grid based (indirect) method for removing the effects are developed. The fast execution of the algorithm stems from the multi-resolution nature of the proposed method. The main advantage of the proposed method is that it does not need any data except for the image itself. Efficiency of both correction methods in distortion removal is investigated.
In this paper a very fast algorithm for non-invasive intra-vascular pressure measurement using Phase-Contrast MRI data is introduced. A noise robust finite difference numerical method has been developed to quickly calculate the relative static pressure in steady flow by using the Navier-Stokes equation in axisymmetric geometries. We show that in such geometries, pressure calculation may be performed in less than two minutes and by using a single PC MR slice containing the axis of symmetry, in turn resulting in dramatic reductions in the acquisition time. The method has been validated under a variety of additive noise conditions with computational fluid dynamics (CFD) flow simulations within phantom geometries. Effects of noise, resolution, and velocity (flow) are discussed.