2 research outputs found

    MR imaging of left-ventricular function : novel image acquisition and analysis techniques.

    Get PDF
    Many cardiac diseases, such as myocardial ischemia, secondary to coronary artery disease, may be identified and localized through the analysis of cardiac deformations. Early efforts for quantifying ventricular wall motion used surgical implantation and tracking of radiopaque markers with X-ray imaging in canine hearts [1]. Such techniques are invasive and affect the regional motion pattern of the ventricular wall during the marker tracking process and, clearly are not feasible clinically. Noninvasive imaging techniques are vital and have been widely applied to the clinic. MRI is a noninvasive imaging technique with the capability to monitor and assess the progression of cardiovascular diseases (CVD) so that effective procedures for the care and treatment of patients can be developed by physicians and researchers. It is capable of providing 3D analysis of global and regional cardiac function with great accuracy and reproducibility. In the past few years, numerous efforts have been devoted to cardiac motion recovery and deformation analysis from MR imaging sequences. In order to assess cardiac function, there are two categories of indices that are used: global and regional indices. Global indices include ejection fraction, cavity volume, and myocardial mass [2]. They are important indices for cardiac disease diagnosis. However, these global indices are not specific for regional analysis. A quantitative assessment of regional parameters may prove beneficial for the diagnosis of disease and evaluation of severity and the quantification of treatment [3]. Local measures, such as wall deformation and strain in all regions of the heart, can provide objective regional quantification of ventricular wall function and relate to the location and extent of ischemic injury. This dissertation is concerned with the development of novel MR imaging techniques and image postprocessing algorithms to analyze left ventricular deformations. A novel pulse sequence, termed Orthogonal CSPAMM (OCSPAMM), has been proposed which results in the same acquisition time as SPAMM for 2D deformation estimation while keeping the main advantages of CSPAMM [4,5]: i.e., maintaining tag contrast through-out the ECG cycle. Different from CSPAMM, in OCSPAMM the second tagging pulse orientation is rotated 90 degrees relative to the first one so that motion information can be obtained simultaneously in two directions. This reduces the acquisition time by a factor of two as compared to the traditional CSPAMM, in which two separate imaging sequences are applied per acquisition. With the application of OCSPAMM, the effect of tag fading encountered in SPAMM tagging due to Tl relaxation is mitigated and tag deformations can be visualized for the entire cardiac cycle, including diastolic phases. A multilevel B-spline fitting method (MBS) has been proposed which incorporates phase-based displacement information for accurate calculation of 2D motion and strain from tagged MRI [6, 7]. The proposed method combines the advantages of continuity and smoothness of MBS, and makes use of phase information derived from tagged MR images. Compared to previous 2D B-spline-based deformation analysis methods, MBS has the following advantages: 1) It can simultaneously achieve a smooth deformation while accurately approximating the given data set; 2) Computationally, it is very fast; and 3) It can produce more accurate deformation results. Since the tag intersections (intersections between two tag lines) can be extracted accurately and are more or less distributed evenly over the myocardium, MBS has proven effective for 2D cardiac motion tracking. To derive phase-based displacements, 2D HARP and SinMod analysis techniques [8,9] were employed. By producing virtual tags from HARP /SinMod and calculating intersections of virtual tag lines, more data points are obtained. In the reference frame, virtual tag lines are the isoparametric curves of an undeformed 2D B-spline model. In subsequent frames, the locations of intersections of virtual tag lines over the myocardium are updated with phase-based displacement. The advantage of the technique is that in acquiring denser myocardial displacements, it uses both real and virtual tag line intersections. It is fast and more accurate than 2D HARP and SinMod tracking. A novel 3D sine wave modeling (3D SinMod) approach for automatic analysis of 3D cardiac deformations has been proposed [10]. An accelerated 3D complementary spatial modulation of magnetization (CSPAMM) tagging technique [11] was used to acquire complete 3D+t tagged MR data sets of the whole heart (3 dynamic CSPAMM tagged MRI volume with tags in different orientations), in-vivo, in 54 heart beats and within 3 breath-holds. In 3D SinMod, the intensity distribution around each pixel is modeled as a cosine wave front. The principle behind 3D SinMod tracking is that both phase and frequency for each voxel are determined directly from the frequency analysis and the displacement is calculated from the quotient of phase difference and local frequency. The deformation fields clearly demonstrate longitudinal shortening during systole. The contraction of the LV base towards the apex as well as the torsional motion between basal and apical slices is clearly observable from the displacements. 3D SinMod can automatically process the image data to derive measures of motion, deformations, and strains between consecutive pair of tagged volumes in 17 seconds. Therefore, comprehensive 4D imaging and postprocessing for determination of ventricular function is now possible in under 10 minutes. For validation of 3D SinMod, 7 3D+t CSPAMM data sets of healthy subjects have been processed. Comparison of mid-wall contour deformations and circumferential shortening results by 3D SinMod showed good agreement with those by 3D HARP. Tag lines tracked by the proposed technique were also compared with manually delineated ones. The average errors calculated for the systolic phase of the cardiac cycles were in the sub-pixel range

    4D ultra-short TE (UTE) phase-contrast MRI for assessing stenotic flow and hemodynamics.

    Get PDF
    Phase-contrast (PC) MRI is a non-invasive technique to assess cardiovascular blood flow. However, this technique is not accurate in the case of atherosclerotic disease and vascular and valvular stenosis due to intravoxel dephasing secondary to disturbed blood flow, flow recirculation, and turbulence distal to the narrowing, resulting in flow-related artifacts. Previous studies have shown that reducing the echo time (TE) decreases the errors associated with phase incoherence due to random motions as observed in unsteady and turbulent flows. As part of this dissertation, a novel 3-D cine Ultra-Short (UTE)-PC imaging method has been developed, and implemented to measure the blood velocity using a UTE center-out radial k-space trajectory with short TE time compared to standard PC MRI sequences. 3D UTE characterizes flow in one direction in a 3D volume, resulting in a single component of the flow velocities. In order to obtain a comprehensive flow assessment in three directions, the 3D UTE sequence needs to be repeated three times, which can be inefficient and time consuming. 4-D flow MRI has been recently used for quantitative flow assessment and visualization of complex flow patterns resulting in more anatomical information and comprehensive assessment of blood flow. With 4D flow MRI method, all the flow information in three direction in a 3D volume though the time can be achieved as part of a single scan. In this dissertation, a novel 4D UTE flow MRI technique has also been designed and implemented which is capable of deriving the three orthogonal components of the velocity field in the flow in a single scan, while achieving very short echo times. In flow phantom studies, comprehensive investigation of several different flow rates revealed significant improvement in flow quantification and reduction of flow artifacts when compared to conventional 4D flow. Furthermore, a reduced TE 4D Spiral flow MRI method has also been implemented which reduces scan times when compared to conventional 4D flow MRI (as well as 4D UTE flow). Despite reduction of scan time as well as TE relative to conventional 4D flow, the achieved TE with the 4D spiral technique is indeed longer than 4D UTE flow. In order to assess clinical feasibility and in order to perform further validation of 4D UTE flow, in an IRB-approved study, twelve aortic stenosis (AS) patients underwent Doppler Ultrasound, conventional 4D flow, and 4D UTE flow scans for a 3 way comparison. 4D UTE flow displayed good correlation with Doppler Ultrasound in patients with moderately severe aortic stenosis, though with the added benefit of not having confounding factors encountered in Doppler Ultrasound (e.g., angle dependence, 2D measurement, and difficulty in locating a proper acoustic window). The proposed 4D UTE flow permits 4D visualization of flow and true 3D measurement of all flow quantities, not possible with Doppler. Further investigations will be required to test the technique in patients with severe or critical aortic stenosis wherein conventional 4D flow will be less accurate due to intravoxel dephasing and spin incoherence
    corecore