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Biomedical subjects

L Axel

Publications and source records attributed to L Axel.

154 records · Page 9Linked to original sources
PubMedDate not supplied

Pulmonary MR angiography utilizing phased-array surface coils.

Magnetic resonance angiography of the pulmonary vasculature was evaluated in 12 subjects using breath-hold gradient echo scans and surface coils at 1.5 T. Flow-compensated GRASS, spoiled GRASS (SPGR), and WARP-SPGR sequences were utilized. Comparisons were made among flip angles of 10-60 degrees, slice thicknesses of 3-10 mm, and body coil as well as Helmholtz pair and phased-array multiple coils. With 30-40 contiguous slices encompassing the lung, intrathoracic vasculature was segmented using a UNIX/X-windows based package dubbed VIDA. Three-dimensional anatomy was visualized by a brightest voxel projection algorithm, following reduction of chest wall pixel intensities by an operator-interactive module. Both SPGR (30 degrees flip angle, 4 mm slice thickness) and WARPSPGR (15 degrees flip angle, 5 mm slice thickness) in combination with phased-array multiple coils provided the most satisfactory images, based upon observations by three radiologists and signal-to-noise ratio measurements. The MR angiograms visualized vessels as distal as sixth to seventh order branches. The technique was successfully applied to three patients with pulmonary embolism. The results of this study demonstrate that the pulmonary vascular tree can be imaged by MR angiography combining a high resolution technique utilizing phased-array multiple coils, fast gradient echo sequences with breath-holding, and postprocessing of the volumetric image data. The technique is attractive since it is noninvasive and provides a full three-dimensional portrayal of the pulmonary vasculature.

Humans↗
PubMedDate not supplied

Potential problems with selective pulses in NMR imaging systems.

Most nuclear magnetic resonance (NMR) imaging systems require pulses whose frequency spectrum is shaped so as to selectively excite a given plane in the presence of a magnetic field gradient. We demonstrate by both computer simulation and experiment that linear Fourier transform theory is not a reliable guide to the uniformity of flip angle in the slice. We show by simulation that the nonuniformity can have serious implications for the measurement of relaxation time T1 if selective 180 degrees pulses are used; the exact results depend also on the details of data analysis and criteria for adjusting the rephasing gradients. We describe an experiment and a phantom in which the axial nonuniformity can be demonstrated on clinical NMR imaging machines.

Humans↗
PubMedDate not supplied

Noise performance of surface coils for magnetic resonance imaging at 1.5 T.

In this paper we analyze the signal-to-noise ratio (SNR) for surface coil magnetic resonance imaging at 1.5 T. We have applied the treatment of Hoult and Lauterbur to determine the factors that most affect coil performance. We have imaged lossy phantoms with 8-, 10-, and 14-cm-diam circular surface coils and compared the results to body and head coil images. Surface coils can improve SNR by a factor of 4 or more for regions close to the surface. Surface coils are effective for regions up to 6 cm deep in the head and about 12 cm deep in the body. Nonuniformity of image intensity is a necessary requirement for improved SNR in surface coils. Coil losses make only a small contribution to image noise compared to tissue losses at 1.5 T. Surface coils need not be placed in close contact with the patient at 1.5 T.

Humans↗
PubMedDate not supplied

Correction of phase wrapping in magnetic resonance imaging.

In phase reconstruction MR imaging, e.g., for velocity measurement, phase shifts beyond +/- pi radians will "wrap around" to smaller apparent phases. Such large phase shifts could arise either due to large background (non-flow-related) phase variations or due to large velocity-induced phase shifts. For sufficiently smooth phase variation, such discontinuities can be automatically recognized and corrected, thus restoring the correct phase values and extending the effective dynamic range of such phase imaging techniques.

Magnetic Resonance Imaging↗
PubMedDate not supplied

Gadolinium-enhanced ultrafast three-dimensional spoiled gradient-echo MR imaging of the abdominal aorta and visceral and iliac vessels.

Gadolinium-enhanced ultrafast three-dimensional (3D) spoiled gradient-echo magnetic resonance (MR) imaging is a noninvasive method for evaluating the abdominal aorta and the visceral and iliac vessels. With an enhanced gradient system, 20-48 sections can be obtained during a single 18-32-second patient breath hold. The 3D volume obtained from a single acquisition can be reformatted at a workstation and the vascular anatomy viewed in any projection and with a variety of section thicknesses, making this MR imaging technique particularly useful in the evaluation of aberrant arteries, vascular stenoses, aneurysms, and dissection flaps. Gadolinium-enhanced ultrafast 3D spoiled gradient-echo MR imaging has been used instead of or in addition to conventional contrast material-enhanced MR angiography in patients who have chronic symptoms of mesenteric ischemia, who have abdominal aortic aneurysms, or who are at risk for iodinated contrast material-related renal dysfunction. This technique shows great promise for accurate and noninvasive evaluation of the abdominal aorta and the visceral and iliac vessels.

Adult↗
PubMedDate not supplied

Acute tubular necrosis: use of gadolinium-DTPA and fast MR imaging to evaluate renal function in the rabbit.

Sequential fast magnetic resonance (MR) images (repetition time = 33 ms, echo time = 7 ms, alpha = 22 degrees, one image every 12 s) were acquired using gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA) (10 or 100 mumol/kg) to study perfusion and concentrating ability in normal rabbit kidneys and in kidneys with HgCl2-induced acute tubular necrosis (ATN). In normal rabbits receiving 100 mumol Gd-DTPA/kg a concentric region of decreased MR signal was observed. In sequential images the dark ring pattern migrated centripetally through the kidney moving from the corticomedullary junction to the inner medulla. The decrease in MR signal intensity occurred as a consequence of T2 relaxation (magnetic susceptibility) due to high concentration of Gd-DTPA within the tubules. This suggests that the dark ring pattern may serve as a qualitative feature indicative of the ability of the kidneys to concentrate. With the onset of HgCl2-induced ATN the pattern of enhancement due to Gd-DTPA administration changed markedly. Although the kidneys with ATN did continue to be perfused, the concentric dark ring pattern seen in normal kidneys receiving 100 mumol Gd-DTPA/kg was not observed. These results suggest that Gd-DTPA and fast imaging MR may provide a method of assessing perfusion and concentrating ability within the healthy or diseased kidney.

Acute Kidney Injury↗
PubMedDate not supplied

Model-driven visualization of coronary arteries.

In a joint project between the Department of Computer and Information Sciences and the Department of Radiology, we are applying techniques of artificial intelligence to improve clinical performance in coronary arteries. Specifically, we are investigating how images from intravenous digital subtraction angiography (DSA) can be enhanced so that their efficacy for lesion detection and quantitation becomes comparable with that of the more dangerous procedure of selective coronary arteriography. The enhancement techniques (which include algorithms for 3-dimensional vessel detection, reconstruction and display, as well as for accurate lumen-size estimation) are based on models of (i) the 3-dimensional topological structure of the coronary arterial tree, (ii) myocardial dynamics, and (iii) the X-ray imaging process involved in producing digital subtraction angiograms. The evaluation of these model-driven visualization techniques is done by the standard psychophysical method of Receiver Operating Characteristic (ROC) analysis applied to observer performance tests on images from an animal coronary atherosclerosis model.

Analog-Digital Conversion↗