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Biomedical subjects

D B Plewes

Publications and source records attributed to D B Plewes.

At least 55 records · Page 3Linked to original sources
PubMed1986

Pulsed-injection method for blood flow velocity measurement in intraarterial digital subtraction angiography.

The pulsed-injection method for measuring the velocity of blood flow in intraarterial digital subtraction angiography is described. With this technique, contrast material is injected at a pulsing frequency as high as 15 Hz, so that two or more boluses can be imaged simultaneously. The velocity of flow is determined by measuring the spacing between the boluses and multiplying it by the pulsing frequency. Results of tests with phantoms correlate well with flow measurements obtained with a graduated cylinder for velocities ranging from 8 to 60 cm/sec. The potential of the method for time-dependent velocity measurement has been demonstrated with simulated pulsatile flows.

Angiography↗
PubMed1985

Chest equalization radiography.

The conventional chest radiograph has a number of limitations as a diagnostic method of evaluating chest disease. The wide variation in attenuation between the lung and mediastinum frequently results in images with low contrast over the thicker portions. This, coupled with the presence of intense body-scattered radiation that must be adequately removed, makes chest radiography one of the most challenging diagnostic procedures in conventional radiography. Scanning equalization radiography (SER) is an alternative method of chest imaging that overcomes these limitations without employing other highly complex or expensive solutions such as digital imaging. Recent clinical studies have shown that diagnostic efficacy is improved with SER, and that SER offers the potential of significantly reducing the frequency of diagnostic errors and repeated examinations while producing cost savings.

Bone Diseases↗
PubMed1985

Quantitative digital subtraction angiography: two scanning techniques for correction of scattered radiation and veiling glare. Work in progress.

Scattered radiation and veiling glare in digital subtraction angiography degrade contrast signals in a nonlinear and nonuniform way. This effect prohibits direct use of image data for accurate iodine measurement or energy subtraction imaging. Two techniques based on the use of scanning lead bars were proposed to measure the spatial distribution of scattered radiation and veiling glare in the unprocessed image data. With either technique, signals behind the lead bars were used to estimate the scatter-glare component and remove it from the image data. The scanning-lead-bar techniques can be used to reduce the scatter-glare component by a factor of up to 10. The scatter-glare correction leads to the recovery of degraded contrast signals and achieves reasonable linearity, uniformity, and consistency in the contrast measurement.

Angiography↗
PubMed1985

Exposure equalization radiography of the chest: clinical comparison of slit and raster scanning techniques.

Exposure equalization radiography systems with scanning slit and raster geometries were constructed and tested with 75 patients. The scanning equalization radiography (SER) technique uses a detector, placed behind the patient, connected in a feedback loop to a microprocessor-controlled x-ray source. The detector monitors the transmitted radiation, and in turn the x-ray output is varied to equalize the radiographic film density over the entire image. The clinical evaluation of these systems included 25 posteroanterior (PA) chest radiographs by an SER slit-geometry system (5.0-sec scan time), 25 PA chest radiographs by an SER raster-geometry system with an 8.8-sec scan time, and 25 PA chest radiographs by an SER raster-geometry system with a 4.7-sec scan time. These SER radiographs were compared to conventional radiographs of the same patients by two radiologists. The observers noted that the SER slit radiographs had seriously overexposed areas in 80% of instances, and that any potential gains from this system were offset by the overexposure problems. The radiographs obtained by the SER raster technique with a 4.7-sec scan time showed more uniform and adequate exposure in 80% of instances and better visualization of normal anatomic detail in the lung (52%) and mediastinum (84%) than conventional radiographs. The radiographs obtained by the SER raster technique with an 8.8-sec scan time showed fewer peripheral lung markings in 15 of 25 cases, presumably due to motion. In all other respects, the images were similar in quality to the SER raster 4.7-sec radiographs.

Female↗
PubMed1985

Improved chest disease detection with scanning equalization radiography.

The efficacy of chest disease detection with scanning equalization radiography (SER) was evaluated in a clinical study of 95 patients: 51 normals and 44 with abnormal chest radiographs. A conventional and an SER image of each patient were interpreted independently by four radiologists. The increased numbers of true positives (3%) and true negatives (9%) when the SER images were interpreted were statistically significant. There was also a reduced number of false positives (7%) with SER. This improved disease detection was noted by each of the radiologists and led to more frequent agreement (11%) of the correct interpretation among the radiologists.

Diagnostic Errors↗
PubMed1982

A scanning equalization system for improved chest radiography.

The wide variation in absorber thickness characteristic of the posteroanterior projection of the chest routinely results in film underexposure and attendant suboptimal contrast in the mediastinal, retrocardiac, and diaphragmatic regions. We propose a special scanning system to deliver a more uniform film exposure over the image by modulating the intensity of a vertically oriented narrow fan (43 x 2 cm) of radiation, swept laterally across the patient's chest. Measurements of patient attenuation obtained with a detector placed behind the film cassette are used to adjust the x-ray tube output so as to maintain a constant film exposure during the course of the scan. The scan is completed in 5 seconds and results in images of significantly improved mediastinal penetration and contrast compared with those of conventional chest radiographs.

Humans↗
PubMed1979

Detectability of spherical objects by computed tomography.

A Lucite phantom was constructed with voids cut in long cylinders, short cylinders and spheres in order to quantitate the effect of partial volume averaging on detectability. Typical scans of varying contrast revealed that the long cylinders cinsistently can be detected to a smaller size than either short cylinders or spheres of the same diameter. The authors feel that low contrast detectability must involve the influence of beam profile on partial volume contrast losses for spherical objects.

Physical Phenomena↗
PubMedDate not supplied

TE interleaving: new multisection imaging technique.

A new pulse sequence that increases the number of sections that can be acquired with a T2-weighted multisection imaging prescription is presented. The sequence achieves a higher efficiency in section acquisition per unit time by interleaving excitation and data collection for up to three separate sections during the long TE (greater than or equal to 80 msec) of a T2-weighted spin-echo pulse sequence. The necessary modifications of the section-select and readout gradient waveforms for the implementation of TE-interleaved (TEI) sequences, with and without flow compensation, are described. Secondary- and stimulated-echo artifacts are present on some TEI images, but these can be controlled with appropriate spoiler gradients. The signal-to-noise ratio for contiguous sections is slightly reduced compared with that of a standard multisection pulse sequence. These observations indicate that the effects of radio-frequency section interference are enhanced by the TEI sequence.

Brain↗
PubMedDate not supplied

MR imaging of blood vessels with an intravascular coil.

A method for producing high-resolution magnetic resonance (MR) images of blood vessel walls is described. The authors review a theoretical analysis of receiver-coil design and present a coil well suited for intravascular MR imaging. The design is based on two coaxial solenoids separated by a gap region and with current driven in opposite directions. Placement of this receiver coil within the vascular space is shown to provide a substantial increase in sensitivity over that external surface coils. Experimental verification of these predictions was obtained in a vessel phantom in which a 13-cm surface coil was compared with a 3.5-mm-diameter opposed-solenoid intravascular coil. This intravascular coil had a cylindric region of high sensitivity that offered a 10-fold improvement in signal-to-noise ratio over that of an external coil near the vessel wall. The performance of this coil was also tested in the jugular vein of a swine.

Animals↗
PubMedDate not supplied

Mechanisms of flow-induced signal loss in MR angiography.

Mechanisms of signal loss in magnetic resonance angiography were studied with a stenotic flow phantom. The results indicate that while signal loss induced by mean fluid motions is localized about the stenosis, the fluctuating component of fluid motion induces signal loss over a much larger region, primarily distal to the stenosis. For both motion components, use of gradient moment nulling (GMN) above first order was found to be an ineffective means of reducing signal loss. In contrast, shortened gradient durations were found to reduce signal loss substantially. However, though a zeroth-order gradient is generally of the shortest duration, use of a slightly longer, first-order gradient was found to be the most robust means of reducing signal loss.

Blood Flow Velocity↗
PubMedDate not supplied

Why fat is bright in RARE and fast spin-echo imaging.

Fast spin-echo (FSE) sequences are becoming popular for T2-weighted clinical imaging because they result in a severalfold reduction in imaging time and because they provide conventional spin-echo contrast for most tissues. Fat, however, has been observed to have anomalously high signal intensity on FSE images. The present study shows that the brighter fat results from the multiple 180 degrees refocusing pulses, which eliminate diffusion-mediated susceptibility dephasing and suppress J-coupling modulation of the echo train.

Adipose Tissue↗
PubMedDate not supplied

MR measurements of pulsatile pressure gradients.

A magnetic resonance (MR) imaging method for evaluating pulsatile pressure gradients in laminar blood flow is presented. The technique is based on an evaluation of fluid shear and inertial forces from cardiac-gated phase-contrast velocity measurements. The technique was experimentally validated by comparing MR and manometer pressure gradient measurements performed in a pulsatile flow phantom. Analyses of random noise propagation and sampling error were performed to determine the precision and accuracy of the method. The results indicate that a precision of 0.01-0.03 mmHg/cm and an accuracy of better than 8% can be achieved by using standard clinical pulse sequences in tubes exceeding 6 mm in diameter. The authors conclude that MR measurement of pressure gradients is feasible and that additional hemodynamic information may be derived from conventional phase-contrast imaging studies.

Algorithms↗
PubMedDate not supplied

Dynamic spin-echo imaging: theoretical assessment and implementation.

A spin-echo method for obtaining dynamic magnetic resonance (MR) images is described. The method combines the RARE (rapid acquisition with relaxation enhancement) pulse sequence with a data acquisition scheme in which only a centric fraction of the raw data is sampled to increase the time resolution of the dynamic images. The missing high-resolution data are supplied from reference images. By these means, an effective time resolution of about 10 seconds per image is achieved, which is suitable for diagnostic assessment of contrast enhancement procedures. A promising clinical application is contrast-enhanced MR imaging of the pituitary. Although the resolution of small objects in this size range is potentially degraded, this shortcoming is compensated for with use of variable refocusing flip angles. In the context of pituitary imaging, the centric 30%-40% of the raw data is shown to be the optimal fraction to acquire for the low-resolution dynamic images. Ten patients with previous history of pituitary disease have been imaged with dynamic and conventional spin-echo techniques. In six of these patients, an equivalent diagnosis was reached with dynamic and conventional images, while in two, only the dynamic images showed the lesion; in the final two patients, only the conventional images showed the lesion.

Adenoma↗
PubMedDate not supplied

Nonsusceptibility artifacts due to metallic objects in MR imaging.

The authors investigated eddy current artifacts due to metallic objects within the magnetic resonance imaging field. The problem was simplified by using a circular copper loop as a model for the more complex eddy current pathways present in a metallic implant. With this simple geometry, the authors show that radio-frequency (RF)-induced eddy currents in the metal produce a significant local artifact. However, no appreciable artifacts due to the switching magnetic field gradients were observed. A detailed quantitative analysis of the mechanism of RF-induced eddy current artifact due to the the copper loop was performed. The artifact was demonstrated experimentally to result from perturbations of the transmit and receive sensitivities of the RF coil. Theoretical calculations of these perturbations showed excellent agreement with experimental results. With an understanding of the artifact mechanism, methods for correcting the RF-induced eddy current artifact were applied.

Artifacts↗
PubMedDate not supplied

Errors in quantitative dynamic three-dimensional keyhole MR imaging of the breast.

Three-dimensional (3D) keyhole magnetic resonance (MR) imaging has been proposed as a means of providing dynamic monitoring of contrast agent uptake by breast lesions, with complete breast coverage and high spatial and temporal resolution. The 3D keyhole technique dynamically samples the central regions of k-space in both phase-encoding directions and provides high-frequency data from a precontrast acquisition. Errors due to data truncation with two-dimensional and 3D region-of-interest measurements are estimated from a numerical simulation of various implementations of the 3D keyhole technique. Errors were found to increase with increasing temporal resolution and reduced object size. Errors of 75% are possible for objects with a diameter approaching 1 pixel when a 3D keyhole implementation that samples 50% of phase-encoding data in each direction is used. Preliminary clinical images with this approach illustrate artifacts consistent with inadequate k-space sampling.

Artifacts↗
PubMedDate not supplied

MR measurement of time-dependent blood pressure variations.

An MR imaging method for measuring intravascular pressure variations is introduced. The technique is based on estimates of vascular compliance and vessel distension, which are obtained from a correlation of spatial and temporal velocity derivatives and measurements of the velocity gradient in the direction of flow, respectively. The accuracy of the technique was determined in vitro through a comparison of MR and transducer pressure measurements obtained in distensible vessel phantoms undergoing pulsatile flow. Results indicated that a root-mean-square error of 4-12% can be expected in phantoms covering a physiological range of compliance. In vivo feasibility was demonstrated by thoracic aorta pressure measurements, which produced pressure waveforms with an expected shape and magnitude.

Adult↗