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Biomedical subjects

N J Pelc

Publications and source records attributed to N J Pelc.

At least 19 recordsLinked to original sources
PubMed1995

Cine spiral imaging.

Interleaved spiral scanning of k-space is an efficient and fast method for imaging dynamic processes. In this article, a cine version of interleaved spiral imaging is presented. The method is shown to overcome the "lightning-flash" artifacts of the conventional triggered (gated) method. Compared with the segmented k-space 2DFT method, it achieves better temporal resolution in a comparable or shorter scan time. Preliminary human studies show that the method is a promising tool for imaging dynamic processes.

Humans↗
PubMed1995

Flow quantification and analysis methods.

The sensitivity of nuclear magnetic resonance to motion can be harnessed to produce quantitative measurements of low flow velocity and volume flow rate. MR flow quantification is noninvasive, can be performed in arbitrary locations and in any image orientation, and is fairly rapid. This article discusses the basic techniques with particular attention to phase contrast quantitation of volume flow rate, especially the determinants of accuracy and reproducibility in these measurements.

Blood Flow Velocity↗
PubMed1994

Evaluation of myocardial motion tracking with cine-phase contrast magnetic resonance imaging.

RATIONALE AND OBJECTIVES: The accuracy of myocardial motion measurements, computed from cine-phase contrast (cine-PC) magnetic resonance (MR) velocity data, was compared with directly visualized motion of MR signal voids caused by implanted tantalum markers in anesthetized dogs. METHODS: Magnetic resonance imaging (MRI) data were electrocardiogram-gated and divided into 16 phases per cardiac cycle. Myocardial trajectories as a function of time in the cardiac cycle were measured using both methods for four to seven markers in each of eight animals. RESULTS: The peak observed in-plane excursion was 4.0 +/- 2.1 mm. The average deviation between displacements derived from velocity data versus displacements visualized directly was 1.1 +/- 0.7 mm (27.5% of the peak displacement). The difference was less if three separate MR scans were used to measure each velocity component in the cine-PC method. This improvement is probably caused by improved temporal resolution. CONCLUSIONS: Cine-PC MRI offers a noninvasive method for accurate quantification of myocardial motion.

Animals↗
PubMed1994

Quantitative magnetic resonance flow imaging.

Time-of-flight and phase shift methods have both been used for vascular imaging with magnetic resonance. Phase methods, and phase contrast in particular, are well suited to quantitative measurements of velocity and volume flow rate. The most robust methods for measuring flow encode through-plane velocity into phase shift and compute flow by integrating the measured velocity over the vessel lumen. The accuracy of the flow data can be degraded by the effects of acceleration and eddy currents and by partial volume effects, including the effects of finite slice thickness and resolution, pulsatile waveforms, motion, and chemical shift. The reproducibility depends on the signal-to-noise of the data and the strength of the flow encoding and can be degraded by inconsistent definition of the vessel boundary. The adjustable flow sensitivity inherent in this method is a particular asset, allowing phase contrast flow measurement to operate over a dynamic range exceeding 10(5). Recently developed rapid imaging methods are helpful in applications that would be compromised by respiratory motion. With care, excellent quantitative data can be quickly obtained in vivo, and the resulting flow information is valuable for the diagnosis and management of a variety of conditions.

Humans↗
PubMed1992

Determination of cerebral blood flow with a phase-contrast cine MR imaging technique: evaluation of normal subjects and patients with arteriovenous malformations.

This study evaluated a phase-contrast cine magnetic resonance (MR) imaging technique capable of simultaneously allowing determination of velocity and volume flow rate (VFR) in both carotid arteries and the basilar artery. Forty patients were studied; 24 were neurologically normal, and 16 had intracerebral arteriovenous malformations (AVMs). In the normal group, mean basilar flow was significantly less than mean carotid flow. Mean velocity and VFR showed a significant decline with age in the basilar artery. Carotid artery flow and total cerebral blood flow did not decline with age. In the AVM patients, flow and velocity measurements were significantly elevated in all three arteries. Flow in the carotid artery ipsilateral to the AVM was significantly greater than flow in the contralateral carotid artery. VFR increased in all three arteries with increasing AVM volume. Four patients underwent partial embolization, and a corresponding decrease in flow was observed. Phase-contrast cine MR imaging provides rapid, simultaneous, noninvasive velocity and VFR measurement in the major intracranial arteries.

Adolescent↗
PubMed1992

Brain motion: measurement with phase-contrast MR imaging.

Brain motion during the cardiac cycle was measured prospectively in 10 healthy volunteers by using a phase-contrast cine magnetic resonance (MR) pulse sequence. The major cerebral lobes, diencephalon, brain stem, cerebellum, cerebellar tonsils, and spinal cord were studied. The overall pattern of brain motion showed caudal motion of the central structures (diencephalon, brain stem, and cerebellar tonsils) shortly after carotid systole, with concurrent cephalic motion of the major cerebral lobes and posterior cerebellar hemisphere. Peak brain displacement was in the range of 0.1-0.5 mm for all the structures except the cerebellar tonsils, which had greater displacement (0.4 mm +/- 0.16 [mean +/- standard error of mean]). Caudal motion of the central structures did not occur simultaneously but progressed in a caudal-to-rostral and posterior-to-anterior sequence, being seen first in the cerebellar tonsils and then later in the diencephalon (hypothalamus). Caudal motion of the low brain stem and cerebellar tonsil was simultaneous with caudal motion of cerebrospinal fluid in the cervical subarachnoid space. Oscillatory flow in the aqueduct was delayed compared with brain stem motion.

Brain↗
PubMed1992

Arterial and venous blood flow: noninvasive quantitation with MR imaging.

Quantitative measurements of arterial and venous blood flow were obtained with phase-contrast cine magnetic resonance (MR) imaging and compared with such measurements obtained by means of implanted ultrasound (US) blood flow probes in anesthetized dogs. The US flowmeter was enabled during a portion of each MR imaging sequence to allow virtually simultaneous data acquisition with the two techniques. MR imaging data were gated by means of electrocardiography and divided into 16 phases per cardiac cycle. The rates of portal venous blood flow measured with MR imaging and averaged across the cardiac cycle (710 mL/min +/- 230 [standard deviation]) correlated well with those measured with the flowmeter and averaged in like fashion (751 mL/min +/- 238) (r = .995, slope = 1.053). The correspondence in arterial blood flow was almost as good. No statistically significant difference existed between the paired measurements of blood flow obtained with MR imaging and the implanted probe. It is concluded that, as a noninvasive means of accurate quantification of blood flow, phase-contrast MR imaging may be especially useful in deep blood vessels in humans.

Animals↗
PubMed1991

Evaluation of acute renal failure with magnetic resonance imaging using gradient-echo and Gd-DTPA.

Detection of acute renal failure (ARF) using fast-scan magnetic resonance imaging (MRI) with Gd-DTPA was studied in a dog model. ARF was produced in five dogs by infusion of norepinephrine (0.75 micrograms/kg/min) into the renal arteries for 40 minutes. MRI was performed 1 hour later and compared with baseline (pre-ARF) MRI. There was no significant difference in the ratios of signal intensity-vs.-time curves from 0 to 35 seconds after injection of Gd-DTPA. However, a difference between the outer and inner medulla was significant in the time period of 5 to 20 minutes after Gd-DTPA injection. These later signal intensity differences by fast-scan (gradient-echo) technique may be useful in the evaluation of ARF.

Acute Kidney Injury↗
PubMed1991

Normal flow patterns of intracranial and spinal cerebrospinal fluid defined with phase-contrast cine MR imaging.

A phase-contrast cine magnetic resonance (MR) imaging technique was used to study normal dynamics of cerebrospinal fluid (CSF) in 10 healthy volunteers and four patients with normal MR images. This pulse sequence yielded 16 quantitative flow-encoded images per cardiac cycle (peripheral gating). Flow encoding depicted craniocaudal flow as high signal intensity and caudo-cranial flow as low signal intensity. Sagittal and axial images of the head, cervical spine, and lumbar spine were obtained, and strategic sites were analyzed for quantitative CSF flow. The onset of CSF systole in the subarachnoid space was synchronous with the onset of systole in the carotid artery. CSF systole and diastole at the foramen of Monro and aqueduct were essentially simultaneous. The systolic and diastolic components were different in the subarachnoid space, where systole occupied approximately 40% and diastole 60% of the cardiac cycle, compared with the ventricular system, where they were equal. This difference results in systole in the intracranial and spinal subarachnoid spaces preceding that in the ventricular system; the same is true for diastole. The fourth ventricle and cisterna magna serve as mixing chambers. The high-velocity flow in the cervical spine and essentially no flow in the distal lumbar sac indicate that a portion of the capacitance necessary in this essentially closed system resides in the distal spinal canal.

Adult↗
PubMed1991

Phase contrast cine magnetic resonance imaging.

Phase contrast cine magnetic resonance imaging (MRI) combines the flow-dependent contrast of phase contrast MRI with the ability of cardiac cine imaging to produce images throughout the cardiac cycle. Two pulse sequence types are used for sensitivity to flow in one direction, whereas four are needed for sensitivity in all directions. Several alternatives for synchronization of the data to the cardiac cycle exist. Retrospectively interpolated methods can image the entire cardiac cycle efficiently. Rapid interleaving of the various sequence types ensures immunity to motion misregistration. The technique produces images in which contrast is related to flow velocity as well as magnitude images such as those of conventional cine MRI. The data can be interpreted qualitatively to demonstrate the presence, magnitude, and direction of flow, and quantitatively to provide estimates of flow velocity, volume flow rate, and displaced volumes. Phase contrast cine MRI is helpful in the diagnosis of aortic dissections, in the study of flow distributions in large vessels such as pulmonary arteries, as well as in smaller vessels such as carotid and basilar arteries, and in the evaluation of complex anatomical variants. Future developments are expected to reduce imaging time and expand the quantitative applications.

Cardiovascular System↗
PubMed1990

Analysis of T2 limitations and off-resonance effects on spatial resolution and artifacts in echo-planar imaging.

Several aspects of blipped echo-planar imaging (EPI) are treated mathematically. An expression relating the necessary readout gradient strength and sampling time to the spatial resolution and readout duration is derived. It is shown how the net spatial resolution may be limited by the object's T2 characteristics and B0 field homogeneity, irrespective of the number of sampled points. Additionally, off-resonance effects result in a loss of spatial resolution and image distortion to a considerably greater degree than in conventional two-dimensional Fourier transform imaging. The extent of these effects is directly related to the time required to acquire the data matrix, and is therefore amplified when EPI is implemented on a standard commercial whole-body system which because of limited gradient performance uses necessarily longer sampling durations. Specific hardware modifications to a standard commercial imager are considered to allow successful EPI implementation. EPI image characteristics are compared quantitatively with those of conventional methods.

Magnetic Resonance Imaging↗
PubMed1990

Volume MR angiography: methods to achieve very short echo times.

Angiographic displays of cerebral vessels can be generated with single-excitation three-dimensional magnetic resonance imaging. Differentiation of true stenosis from artifactual signal loss, due to dephasing effects from fast or nonconstant blood flow and field inhomogeneities, poses a significant clinical problem that can be largely resolved with the use of very short echo times (TEs). A three-dimensional imaging technique was developed that allows TEs of 3.1 msec without and 4.5 msec with first-order flow compensation gradients. The short TEs were achieved with short asymmetric radio-frequency pulses, gradients of minimal duration, and fractional echoes. Significantly improved images of normal tortuous vessels with fast flow were obtained. With this method, accuracy in depicting the vessel lumen and confidence in the findings were markedly increased.

Cerebral Arteries↗
PubMed1990

Rapid MR imaging of blood flow with a phase-sensitive, limited-flip-angle, gradient recalled pulse sequence: preliminary experience.

To assess blood flow rapidly, a limited-flip-angle, gradient recalled pulse sequence was modified to acquire two views at the same phase-encoding step in successive repetitions. One view is obtained with first-moment flow compensation, while the second view is obtained with selectable flow encoding (non-zero first moment) along one direction. Blood flowing along the encoded direction acquires a phase difference between the two views, resulting in signal dependent on both direction and speed of flow. Stationary tissues undergo no phase change. Therefore, the phase shift between the two views produces an image that spatially renders flow direction and velocity. With a 24-msec repetition time, a 256 X 128 matrix, and two excitations, data acquisition is completed in 13 seconds per location (both a magnitude image and a flow image are produced at each location). Images generated with flow phantoms confirmed the accuracy of this method. Preliminary clinical evidence in 23 human subjects suggests that this method is useful in evaluating portal hypertension, distinguishing arterial from venous flow, distinguishing between slow flow and clot, and confirming the presence of clot. This method appears to be a fast, easy way to assess blood flow in large vessels.

Adolescent↗
PubMed1989

Flow-compensated limited flip angle MR angiography.

A method of subtraction angiography that has an acquisition time of 8 s per slice is described. Flow-compensated and uncompensated measurements are acquired in an interleaved fashion using limited flip angles and gradient refocusing. Magnitude images are reconstructed and subtracted to generate the angiogram. Results were generated in vivo in the imaging of the carotid bifurcation of several human volunteers. Susceptibility and inhomogeneity induced artifacts are prominent in thick slices, but can be greatly reduced by imaging several thin slices and adding them together. Thin slices do not require dephasing gradients to reduce the dynamic range, and there is no signal cancellation in overlapping vessels. The method is ideal for acquiring scout angiograms, and with averaging may produce images of diagnostic quality.

Blood Flow Velocity↗
PubMed1987

Rapid calculation of T1 using variable flip angle gradient refocused imaging.

We present a method for rapid measurement of T1 relaxation times using gradient refocused images at limited flip angles and short repetition times. This "variable nutation" techniques was investigated using a T1 phantom. There was a high correlation between measurements obtained with the variable nutation and partial saturation techniques. The ability of this method to create calculated T1 images is also demonstrated. We conclude that the variable nutation method may allow measurement of T1 relaxation times with a significant reduction in acquisition time compared to partial saturation techniques.

Humans↗
PubMed1987

A multichannel PWM telemetry system for kinematic gait analysis.

A multichannel biotelemetry system using pulse-width modulation-frequency modulation (PWM-FM) is described in detail for laboratory construction. Its application in a kinematic gait-analysis system is demonstrated, employing minimally encumbering electrogoniometry and foot-contact switches. The triaxial electrogoniometers sense rotational joint motion, and four foot-switches under the sole of each foot provide information on placement and temporal contact. Signals from the multiple sensors are amplified, encoded by pulse-width modulation, and transmitted at an FM radio frequency of 107 MHz. Received data are decoded and then sampled by a minicomputer for analysis. Results from a comparative study of kinematic gait in five normal subjects and five children with cerebral palsy demonstrate system effectiveness in providing quantitative data and various intrasubject and intersubject gait differences. Factors reviewed in the analysis include swing and stance times; cadence; hip-joint motion in sagittal, coronal, and transverse planes; and sequence of foot placement.

Analog-Digital Conversion↗
PubMed1985

An attenuated projector-backprojector for iterative SPECT reconstruction.

A new ray-driven projector-backprojector which can easily be adapted for hardware implementation is described and simulated in software. The projector-backprojector discretely models the attenuated Radon transform of a source distributed within an attenuating medium as line integrals of discrete pixels, obtained using the standard sampling technique of averaging the emission source or attenuation distribution over small square regions. Attenuation factors are calculated for each pixel during the projection and backprojection operations instead of using precalculated values. The calculation of the factors requires a specification of the attenuation distribution, estimated either from an assumed constant distribution and an approximate body outline or from transmission measurements. The distribution of attenuation coefficients is stored in memory for efficient access during the projection and backprojection operations. The reconstruction of the source distribution is obtained by using a conjugate gradient or SIRT type iterative algorithm which requires one projection and one backprojection operation for each iteration.

Heart↗