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L Axel

Publications and source records attributed to L Axel.

At least 19 recordsLinked to original sources
PubMed1992

Fully three-dimensional reconstruction from data collected on concentric cubes in Fourier space: implementation and a sample application to MRI.

An algorithm is proposed for rapid and accurate reconstruction from data collected in Fourier space at points arranged on a grid of concentric cubes. The Fourier transform of the object to be reconstructed is decomposed into the sum of three functions by subdividing its domain into three non-overlapping mutually orthogonal double pyramids. Each of the three functions is zero-valued outside one of the double pyramids and has values inside that double pyramid which are the same as those of Fourier transform of the object to be reconstructed at the same points. Inverse Fourier transforms of these individual functions can be calculated using the chirp z-transform. The outputs of these inverse transforms for the three functions are estimates of their values at points of the same rectangular grid. The function to be reconstructed is estimated for this grid by adding together the three inverse transforms. The whole process has computational complexity of the same order as required for the 3D fast Fourier transform and so (for medically relevant sizes of the data set) it is faster than backprojection into the same size rectangular grid. The design of the algorithm ensures that no interpolations are needed, in contrast to methods involving backprojection with their unavoidable interpolations. As an application, a 3D data collection method for MRI has been designed which directly samples the Fourier transform of the object to be reconstructed on concentric cubes as needed for the algorithm.

Algorithms↗
PubMed1992

Efficient method for selecting cardiac magnetic resonance image locations.

Selecting locations for long- and short-axis cardiac magnetic resonance images (MRIs) conventionally requires two preliminary image sets, one to find the plane of the left ventricular (LV) axis and a second to find the axis within that plane. By specifying the LV axis from landmarks on the initial image set, it is possible to go directly to short-axis images without the second set of preliminary images, thus improving efficiency and shortening the duration of the examination.

Heart↗
PubMed1992

Regional heart wall motion: two-dimensional analysis and functional imaging with MR imaging.

Analysis of the transmural distribution or nonradial components of myocardial motion has previously been possible only with use of invasive techniques such as implantation of radiopaque markers. Magnetic tagging of the heart wall in conjunction with magnetic resonance imaging allows noninvasive regional analysis of within-wall motion, including its separation into components of rigid body motion and deformation. The results of this analysis can be displayed as functional images. This provides a new tool for the study of heart wall motion that should be of use for both basic physiologic and clinical research applications.

Computer Simulation↗
PubMed1992

Three-dimensional motion and deformation of the heart wall: estimation with spatial modulation of magnetization--a model-based approach.

A method for in vivo estimation of the three-dimensional (3D) motion and deformation of the heart from tagged magnetic resonance images of the myocardium is presented. The method is based on a 3D deformable model fitted to the motion of tagged points in two views (short and long axes), which results in a comprehensive kinematic model of the dynamic geometry of the left ventricle. The method was applied to data obtained in four healthy volunteers, and data were pooled according to position within the model. Analytic modeling demonstrated that the calculated strain field was relatively invariant to the type of smoothing constraint applied; the greatest error was in the circumferential-radial shear strain. Displacement, torsion, and strain extracted from the model agreed with previous results of two-dimensional MR imaging analyses and 3D studies involving the implantation of radiopaque beads in canine myocardium. The model-driven approach provides an accurate and flexible technique for noninvasive estimation of 3D in vivo deformation of the human heart.

Adult↗
PubMed1991

Evaluation of aortic regurgitation by cardiac cine magnetic resonance imaging: planar analysis and comparison to Doppler echocardiography.

Cine magnetic resonance imaging (MRI) displays cardiac flow in cine loop fashion on multiple tomographic sections. Since laminar flow is easily distinguished from turbulent flow, cine MRI may be uniquely suited to the study of valvular regurgitation: the entire cardiac volume can be sampled and the regurgitant jet at the valve plane can be depicted. We therefore assessed aortic regurgitation (AR) by cine MRI in 35 patients and 11 normal volunteers and compared results to pulsed (n = 32) or color flow Doppler (n = 14). The extent of the flow disturbance was estimated for both cine MRI and Doppler by indexing the size of the maximal, single plane regurgitant jet area (JA) to the left ventricular (LV) area. Cine MRI JA/LV ratio compared well with pulsed (r = 0.81) and color flow (r = 0.88) Doppler; classification as mild (less than 20%), moderate (20-40%), and severe (greater than 40%) AR by both methods was identical in 43 of 46 cases with no differences of more than one grade. Overall sensitivity and specificity of cine MRI, compared to Doppler, were 94 and 95%, respectively. Cine MRI also depicted the regurgitant jet at the valve plane in 11 patients. Thus planar analysis of cine MRI images in patients with AR provides a semiquantitative assessment of the AR flow disturbance which is similar to Doppler but, in addition, can image the entire cardiac volume and the regurgitant jet at the valve plane.

Adult↗
PubMed1991

Circumferential myocardial shortening in the normal human left ventricle. Assessment by magnetic resonance imaging using spatial modulation of magnetization.

BACKGROUND: Conventional cardiac imaging methods do not depict true segmental myocardial shortening, since they cannot determine segment length between fixed points in the myocardium. METHODS AND RESULTS: We used electrocardiographically gated magnetic resonance imaging with spatial modulation of magnetization to noninvasively "tag" the myocardium with dark stripes at uniform 7-mm intervals center to center at end diastole. We then determined end-systolic stripe separation and thereby calculated circumferential shortening. When end systole was not reached in the first image series, a second temporally overlapped series starting in late systole was used to determine late-systolic shortening. Septal, anterior, lateral, and inferior segments were assessed at endocardium, midwall, and epicardium on five midventricular short-axis sections each in 10 normal volunteers. A transmural gradient in circumferential shortening was observed, with the percentage of endocardial segment shortening consistently greater than epicardial segment shortening (epicardial, 22 +/- 5%; midwall, 30 +/- 6%; and endocardial, 44 +/- 6%; p less than 0.0001 by analysis of variance). Circumferential shortening varied from apex to base with slices closer to the base of the left ventricle showing less shortening at the midwall (28 +/- 9%) and endocardium (39 +/- 6%) than more apical slices at the midwall (34 +/- 13%) and endocardium (49 +/- 9%) (p less than 0.05 and p less than 0.01, respectively, by analysis of variance). CONCLUSIONS: Transmural and longitudinal heterogeneity of circumferential shortening is present in the normal human left ventricle. Magnetic resonance imaging with spatial modulation of magnetization is a powerful new tool for assessment of circumferential shortening and provides information unobtainable with conventional imaging methods.

Adult↗
PubMed1990

Evaluation of mitral regurgitation by cine magnetic resonance imaging.

We used cine magnetic resonance imaging (MRI) to assess mitral regurgitation (MR) in 40 patients with coronary and/or valvular disease and 10 normal subjects and compared results to pulsed (n = 30) or color flow Doppler mapping (n = 20). Mitral regurgitation produced a dynamic signal void in the left atrium in systole in 15 of 16 patients with MR by pulsed Doppler and in an additional 15 of 16 patients whose MR was demonstrated by color flow Doppler. There were no false positives (sensitivity 94%, specificity 100% for both). The ratio of single-plane, maximal jet area to left atrial area was used to grade MR severity with mild defined as less than 20%, moderate between 20 and 40% and severe greater than 40%. Cine MRI classification was identical to pulsed Doppler echocardiography in 26 of 30 patients and to color flow Doppler in 16 of 20 patients with no differences of greater than 1 grade. Cine MRI consistently depicted smaller flow disturbances than pulsed Doppler (slope = 0.65) or color flow Doppler (slope = 0.60). Nonetheless, the cine MRI area ratio correlated well with pulsed Doppler (r = 0.78) and with color flow Doppler (r = 0.74). Thus, planar analysis of cine MRI in patients with MR of varying severity gave results that were similar to Doppler echocardiography. At present, for routine clinical assessment of MR, the benefits of cine MRI may be limited to patients in whom transthoracic Doppler echocardiography is not adequate.

Adult↗
PubMed1990

Hydrogen ultrathin phase-encoded spectroscopy (HUPSPEC).

This paper describes a new clinical spectroscopy pulse program, hydrogen ultrathin phase-encoded spectroscopy. This sequence combines high spatial resolution with magnitude hydrogen spectroscopy. A linear volume is spatially frequency encoded with a conventional readout gradient and phase encoded spectrally by incrementing the timing of acquisition. The pulse sequence is implemented on a whole-body MRI scanner and supports several standard scanner options, including autoprescanning, offset of the center of field of view in the frequency-encoding direction, and oblique imaging. Some preliminary experimental experience is reported, demonstrating the possibility of observing the spectral linewidths of fat and water in marrow, and of observing the spectral linewidths of fat and water in marrow, and of observing multiline spectra. Combination of the technique with water suppression methods in a sequence called WASHUP is also discussed.

Adult↗
PubMed1990

Pulmonary vascular cine MR imaging: a noninvasive approach to dynamic imaging of the pulmonary circulation.

Cine gradient-recalled magnetic resonance (MR) imaging, which has flow sensitivity and high temporal resolution, may potentially yield both morphologic and dynamic flow-related information in the pulmonary vasculature. The authors used this modality to evaluate pulmonary vessels in 12 healthy subjects and in 14 patients with a variety of cardiopulmonary disorders. Normal pulmonary arteries and veins were characterized by distinctive signal intensity and diameter variations as well as motion of the vessels during the cardiac cycle. Patients with pulmonary arterial hypertension demonstrated loss of the normal pulsatile systolic increase and diastolic decline in velocity-related signal intensity and in diameter of the proximal pulmonary arteries. Disorders of pulmonary venous signal and diameter profiles during the cardiac cycle, which show a characteristic biphasic pattern in healthy subjects, were identified in five patients with mitral valvular disease. These initial results indicate that cine MR imaging techniques hold promise in the evaluation of pathophysiologic conditions in the pulmonary circulation.

Adult↗
PubMed1989

The correction of nonuniform signal intensity profiles in magnetic resonance imaging.

The increasing use of digital image data in Radiology has opened the door to the routine use of numerical image-enhancement techniques. Of course, numerical image processing cannot put information into the image which is not already there. However, if some means can be found to separate diagnostic image information from noise or artifact, the diagnostic information can be extracted with post-processing. The diagnostic quality of an exam may be enhanced by such numerical manipulations, even though technically, the information content of the digitized image is reduced.

Algorithms↗
PubMed1989

Use of Gd-DTPA and fast gradient-echo and spin-echo MR imaging to demonstrate renal function in the rabbit.

The paramagnetic magnetic resonance (MR) imaging contrast agent gadolinium diethylenetriaminepentaacetic acid (DTPA) is freely filtered at the glomerulus and is neither secreted nor reabsorbed by the renal tubules. Fast MR imaging techniques, either gradient-echo or spin-echo, can be used to document the passage of Gd-DTPA through the renal tubules, as reflected by alteration in the MR signal intensity within the different anatomic regions of the kidney. Gradient-echo (repetition time of 35 msec, echo time of 7 msec, flip angles of 10 degrees-100 degrees) and spin-echo (repetition time of 35 msec, echo time of 8 msec) pulse sequences were used to acquire 20 consecutive images, one every 12 seconds, of the rabbit kidney. Both pulse sequences depicted the time course of Gd-DTPA distribution through the kidney but with distinctly different patterns of MR signal change. These dynamic MR images provide an MR nephrogram that directly demonstrates renal morphology and indirectly reflects the functional status of the renal vasculature, renal perfusion, and tubular concentrating ability.

Animals↗
PubMed1989

Pulmonary vasculature: high-resolution MR imaging. Work in progress.

High-resolution magnetic resonance (MR) imaging of the lung has the potential to depict not only small pulmonary vascular structures but also pulmonary blood flow. Five healthy volunteers were examined to assess the effects of (a) the use of two 5-inch surface coils located on the anterior and posterior chest walls, with a 24-cm field of view and a matrix of 256 X 256; (b) spin-echo acquisition with electrocardiographic (ECG) gating during systole or diastole; and (c) gradient recalled acquisition in a steady state (GRASS) with breath holding. These techniques yielded images showing small peripheral pulmonary vascular structures. Most subsegmental vessels and sixth- and seventh-order branches could be traced, especially near the coils. GRASS images obtained with dual surface coils and breath holding depicted fifth- and sixth-order branches. Preliminary results indicated that small pulmonary vessels can be imaged with MR with a combination of high-resolution techniques and ECG gating in diastole. The sensitivity and reproducibility of these techniques in demonstrating pulmonary vasculature warrant further investigation.

Adult↗
PubMed1989

MR imaging of motion with spatial modulation of magnetization.

A novel magnetic resonance imaging technique provides direct imaging of motion by spatially modulating the degree of magnetization prior to imaging. The preimaging pulse sequence consists of a radio-frequency (RF) pulse to produce transverse magnetization, a magnetic field gradient to "wrap" the phase along the direction of the gradient, and a second RF pulse to mix the modulated transverse magnetization with the longitudinal magnetization. The resulting images show periodic stripes due to the modulation. Motion between the time of striping and image formation is directly demonstrated as a corresponding displacement of the stripes. This technique can be used to study heart wall motion, to distinguish slowly moving blood from thrombus, and to study the flow of blood and cerebrospinal fluid.

Adult↗
PubMed1989

Prostatic carcinoma and benign prostatic hyperplasia: correlation of high-resolution MR and histopathologic findings.

High-resolution magnetic resonance (MR) imaging of 24 fresh radical prostatectomy specimens was performed on an experimental 1.9-T system. Direct correlation between the findings in 7-micron-thick macrosections and their corresponding MR images was possible. Fourteen patients had macroscopic evidence of cancer. In all 14 cases, the carcinoma nodules appeared as areas of low signal intensity on images obtained with a repetition time of 2,500 msec and an echo time of 80 msec. Ten of 14 nodules had well-defined margins and consisted of densely packed glandular elements, which displaced the surrounding normal glandular material of higher signal intensity. Ten specimens displayed benign prostatic hyperplasia (BPH). The MR characteristics of this entity were quite variable but relatively predictable, depending on the distribution and size of the glandular elements, as well as the composition of the surrounding stroma. In BPH, the changes began in the central portion of the gland. The areas of highest signal intensity corresponded to dilated glandular elements (cystic ectasia), while the areas of lowest signal intensity corresponded to collagen (scar) and fibromuscular stroma. Nodules of mixed glandular BPH and fibromuscular BPH were found to have signal intensities similar to those of well-differentiated nodules of prostatic adenocarcinoma.

Aged↗
PubMed1989

Heart wall motion: improved method of spatial modulation of magnetization for MR imaging.

A previously reported method of using magnetic resonance (MR) imaging to study heart wall motion involves a pair of nonselective radio-frequency (RF) pulses, separated by a magnetic field gradient pulse, prior to imaging; this produces images with a regular pattern of stripes that move with the heart wall and that have a sinusoidal intensity profile. It is demonstrated in this study that the substitution of more RF pulses, with their relative amplitudes distributed according to the binomial sequence, results in sharper stripes. This permits the use of a two-dimensional grid of stripes for more detailed studies of heart wall motion and provides a unique method of analyzing regional ventricular myocardial strain.

Heart↗
PubMed1989

Noninvasive determination of coronary artery bypass graft patency by cine magnetic resonance imaging.

Cine magnetic resonance imaging (MRI) is a gradient-recalled, retrospectively gated, fast-scan technique that depicts laminar flowing blood as bright signal and has been proposed as a useful method for determination of coronary artery bypass graft (CABG) patency. Therefore, we performed a blinded prospective study to assess the value of cine MRI determination of CABG patency in 20 patients with 45 CABG proximal anastomoses who were undergoing repeat angiography. Ten normal subjects served as controls to define normal intrathoracic vascular patterns. There were 21 left anterior descending (LAD) grafts, of which four were left internal mammary (LIMA), 12 left circumflex (Cx), and 12 right coronary (RCA) grafts. After localizing spin-echo coronal images were obtained, multiple axial multislice interleaved cine MRI acquisitions, each consisting of two to four 5-10-mm-thick slices at eight to 24 frames per cardiac cycle, were obtained from the superior main pulmonary artery to the inferior left ventricle. Each acquisition took 5-8 minutes with a subsequent 5-10 minutes of computer image reconstruction. Total study time per patient was 50-75 minutes. Known to cine MRI interpreters were the original surgical CABG insertions but not the angiographic findings. A graft was called patent if a bright graft flow signal, not corresponding to a normal vessel, was identified on multiple frames at multiple levels abutting the great vessels or epicardial surface of the heart. Angiographically, there were 33 patent grafts, of which 29 were identified as patent by cine MRI (sensitivity, 88%).(ABSTRACT TRUNCATED AT 250 WORDS)

Aged↗