Biomedical subjects

D B Plewes

Publications and source records attributed to D B Plewes.

71 records · Page 4Linked to original sources

PubMedDate not supplied

A velocity correlation method for measuring vascular compliance using MR imaging.

A method for estimating vascular compliance using MR velocity imaging is presented. The technique combines an analysis of pulse propagation, based on spatially averaged equations of continuity and momentum, together with phase-contrast velocity measurements to estimate the compliance from a correlation of second-order spatial and temporal velocity derivatives. The technique can be applied in the presence of reflected flow waves and uses velocity data acquired throughout the entire cardiac cycle. The accuracy of the technique was assessed in distensible vessel phantoms spanning a physiological range of compliance through a comparison with compliance estimates obtained using high-resolution MR imaging and pressure transducers. The mean error of all measurements was found to be 0.04 +/- 0.02% per mm Hg, with the relative errors ranging from 1.2% to 46%. Error was found to decrease as the temporal sampling rate and/or image plane separation were increased. This suggests that an accurate hemodynamic evaluation of a vessel's elastic properties is feasible with MR velocity imaging techniques.

Blood Flow Velocity↗

PubMedDate not supplied

Changes in fibroglandular volume and water content of breast tissue during the menstrual cycle observed by MR imaging at 1.5 T.

MR imaging at 1.5 T was used to investigate variations in breast parenchyma during the menstrual cycle. Seven subjects were examined twice weekly over at least one menstrual cycle. A three-point Dixon technique (TE = 19 msec, TR = 2000 msec) provided images of fat, water, and static magnetic field (Bo), from which two quantitative whole breast parameters were calculated: the mean relative volumetric water content, , and the mean volumetric fibroglandular fraction, . Four of seven subjects showed unequivocal cyclic variations in and consistent with expected histologic changes; and values were elevated during menses and reduced in mid-cycle. The maximum deviation measured for each of the four subjects was < or = 10% in and units. These variations probably do not influence significantly the clinical interpretation of unenhanced MR breast images. Quantitative measurements of breast parenchyma, however, should recognize these effects.

Adipose Tissue↗

PubMedDate not supplied

Visualizing tissue compliance with MR imaging.

We propose a method for visualizing the mechanical properties of tissue based on the use of periodic mechanical compression in conjunction with phase-contrast MR imaging. A specialized mechanical transducer was used to provide programmable compression pulses to the surface of compliant phantoms. These compression pulses were synchronized to a spin-echo sequence with motion-sensitizing gradients to generate phase information reflecting spin displacement throughout the phantom. This sequence was tested with two agarose gel phantoms. The first was a cylinder containing three parallel layers of varying compliance and the second was composed of a semirigid sphere suspended in a uniform layer of decreased elastic modulus. Images showed complex patterns of motion throughout the phantom, which correlated with expected motion behavior of the phantom structures. This indicates that the biomechanical properties of tissues may be elucidated through the use of motion-sensitized MR imaging and suggests that a form of image contrast relating to tissue elasticity may be feasible.

Compliance↗

PubMedDate not supplied

Limitations of the keyhole technique for quantitative dynamic contrast-enhanced breast MRI.

The effect of keyhole data acquisition on quantitative analysis of dynamic MRI was examined. Experiments were performed retrospectively on raw data obtained from clinical dynamic contrast-enhanced breast imaging procedures. The effects of keyhole phase-encoding acquisition and type of reconstruction algorithm on the accuracy of derived quantitative parameters was assessed. Results indicate that the minimum keyhole size used should be restricted by the approximate minimum size of the expected lesions. Furthermore, reconstruction algorithms that offer improved image resolution do not circumvent this restriction.

Algorithms↗

PubMedDate not supplied

MRI monitoring of interstitial microwave-induced heating and thermal lesions in rabbit brain in vivo.

The purpose of this experiment was to use MRI to monitor microwave heating and thermal damage of brain tissue in vivo. Interstitial microwave antennas were implanted into the cerebral hemispheres of seven anesthetized rabbits. Variable power of 30 to 100 W was applied for periods of 5 to 15 minutes and tissue temperature was monitored continuously. MR images were obtained throughout the procedure at 20-second intervals, using a spoiled gradient-echo sequence, without significant artifact. Magnitude, phase, and complex difference images all demonstrated temperature-related signal changes during heating. The findings were better visualized on the phase and complex difference images. Phase difference image analysis revealed an approximately linear relationship between phase change and temperature. Post-treatment thermal lesions measured up to 2.0 cm in size on pathologic specimens and exhibited a zonal pattern on spin-echo MR images.

Animals↗

PubMedDate not supplied

Magnetic resonance imaging of shear wave propagation in excised tissue.

The propagation of shear waves in ex vivo tissue samples, agar/gel phantoms, and human volunteers was investigated. A moving coil apparatus was constructed to generate low acoustic frequency shear perturbations of 50 to 400 Hz. Oscillating gradients phase-locked with the shear stimulus were used to generate a series of phase contrast images of the shear waves at different time-points throughout the wave cycle. Quantitative measurements of wave velocity and attenuation were obtained to evaluate the effects of temperature, frequency, and tissue anisotropy. Results of these experiments demonstrate significant variation in shear wave behavior with tissue type, whereas frequency and anisotropic behavior was mixed. Temperature-dependent behavior related mainly to the presence of fat. Propagation velocities ranged from 1 to 5 m/sec, and attenuation coefficients of from 1 to 3 nepers/unit wavelength, depending on tissue type. These results confirm the potential of elastic imaging attributable to the intrinsic variability of elastic properties observed in normal tissue, although some difficulty may be experienced in clinical implementation because of viscous attenuation in fat.

Adipose Tissue↗

PubMedDate not supplied

Fat/water quantitation and differential relaxation time measurement using chemical shift imaging technique.

The use of chemical shift imaging for fat and water quantitation and differential measurement of relaxation times for the fat and water component is demonstrated using a hybrid technique. The efficacy of the imaging technique for fat and water quantitation has been tested by comparing the results of imaging to the results of volumetric measurements in phantoms with oil and water homogeneously mixed, fat extraction in ground meat of different grades, and biopsy in preliminary clinical studies. Good correlation is found between the fat and water content measured by imaging and that measured by other means except for the inability to differentiate unsaturated fat protons from water protons. Longitudinal (T1) and transverse (T2) relaxation times for water and fat are also shown to be measurable independently when fat and water signal are suppressed accordingly. The independently measured relaxation times correspond closely to those of the pure samples except that unsaturated protons give decreased water relaxation estimates.

Body Water↗

PubMedDate not supplied

A scanning system for chest radiography with regional exposure control: theoretical considerations.

Conventional chest radiography is limited by the presence of scattered radiation and the small useful exposure range of radiographic film. A computer-assisted scanning system to minimize these two effects is outlined. The system uses a small beam of radiation swept over the patient's chest in a raster pattern to expose a conventional film cassette, while a slit collimator scanning between the patient and the film serves to reject scattered photons. A microcomputer measures beam attenuation by the patient with a detector placed behind the film which in turn automatically adjusts the x-ray tube output to minimize excursions in film exposure as the beam scans. A formalism which relates the patient transmission and film exposure distribution is developed and a system transfer function is given. It is shown that such a system operates as a spatial filter which attenuates film contrast for structures of spatial frequency less than the inverse scanning beam width. By manipulating the software parameters of the feedback network, it is possible to alter this filter and produce radiographs with low spatial frequency enhancement, attenuation, or contrast inversion.

Radiography, Thoracic↗

PubMedDate not supplied

A scanning system for chest radiography with regional exposure control: practical implementation.

An experimental scanning apparatus for chest radiography is described which offers good scatter rejection and regional manipulation of film exposure. The apparatus is capable of studying two scanning geometries that uses either a fan of radiation swept laterally over the film in 5s or a spot of radiation scanned over two dimensions in a raster pattern in 8.8s. The manipulation of tube output during the scan is achieved by pulse width modulation with film exposure monitored by a fluorescence detector placed behind the film cassette. Measurement of the scatter rejection properties of the system show that it is superior to that of a 10:1 grid when used with 120-kVp radiation. Phantom images with both geometries demonstrate the capability of the system to improve uniformity of film exposure and soft tissue contrast throughout the mediastinal, diaphragmatic, and retrocardiac areas. The relative characteristics of the one- versus two-dimension scan geometries are given. Clinical raster images formed with the 8.8-s scan time show signs of motion unsharpness which becomes insignificant when the scan time is reduced to 4.5 s.

Radiography, Thoracic↗

PubMedDate not supplied

Exposure artifacts in raster scanned equalization radiography.

The image artifacts characteristic of a scanning chest radiographic system are reviewed. The technique employs a pulsed beam of radiation swept in an overlapping raster pattern that can result in severe ripple and scan line artifacts with improper scanning parameters. A one-dimensional treatment of the scanner geometry shows that the artifacts can be eliminated when the beam width is an integral multiple of interpulse spacing. An extension to a two-dimensional analysis indicates that with the collimator geometries employed, artifact-free images are not possible with a fixed x-ray frequency but can be achieved when a variable frequency source is used. A treatment of the sensitivity for artifact formation shows that with proper choice of scanning parameters sizable errors in beam width can be tolerated without significant artifact formation.

Humans↗

PubMedDate not supplied

Grid controlled x-ray tube switching time: implications for rapid exposure control.

The transient response properties of a conventional grid controlled x-ray source and the implications for rapid exposure control are studied. The use of a cathode cable with a length of only 1 m to connect the x-ray tube and the grid control electronics results in a switching time of 19 microseconds. The phototiming artifacts from the delayed radiation delivered during this transition time can be severe for short pulses. An analog correction technique that anticipates the magnitude of the delayed radiation is used to overcome this limitation and allows accurate phototiming over a wide range of pulse widths ranging down to 20 microseconds.

Humans↗

PubMedDate not supplied

Effects of scattered radiation and veiling glare in dual-energy tissue-bone imaging: a theoretical analysis.

Dual-energy subtraction imaging allows tissue and bone structures to be separated from each other and attenuating thicknesses measured. Potential applications include chest imaging, bone mineral measurement, angiography, and mammography. However, intrinsic to most x-ray detectors is the acceptance of scattered radiation as part of the image signal. Added to that is the veiling glare component when an image intensifier is used. Together, they result in erroneous transmission measurement and degrade the accuracy of energy subtraction processing. In this paper, the effects of scattered radiation and veiling glare on energy subtraction images are examined theoretically. A model is derived and used to compute the effects on the thickness signals, image contrast, and image noise as a function of the scatter glare to primary ratios. The ratios were measured on a point-by-point basis for a Rando chest phantom. For 96% of the image field studied, the thickness signals may be subject to an error ranging from 0 to -22.5 cm for tissue and 0 to 5.2 cm for bone. The image contrast in the tissue image may be reduced by a factor ranging from 1 to 59. The percentage of the uncanceled bone signals ranges from -52% to 52%. The contrast-to-noise ratio may be reduced by a factor ranging from 1 to 18.

Bone and Bones↗

PubMedDate not supplied

Maximizing film contrast for scanning equalization radiography.

The results of a numerical simulation indicate that scanning equalization radiography (SER) provides the opportunity to use film/screen combinations of significantly higher contrast than currently practical in conventional chest radiography. These calculations which attempt to maximize overall image contrast while minimizing areas of over and under exposure demonstrate that optimum film gradient varies inversely with scanning beam area. Furthermore, we show that in an SER system which we have optimized for chest imaging that the peak film gamma gradient can be increased from that typical of conventional films (3.0-3.5) to approximately 7 with advantage.

Computer Simulation↗

PubMedDate not supplied

Mammographic scanning equalization radiography.

It is well recognized that variations in breast thickness and parenchymal composition can produce a range of exposure which exceeds the latitude of high contrast mammographic film/screen combinations. Optimal imaging of the dense breast is desired since 30%-60% of women present with dense breasts, and they are believed to be at the highest relative risk of developing breast cancer. The application of scanning equalization radiography to mammography has been investigated through the construction and characterization of a prototype mammographic scanning equalization radiography (MSER) system, designed to image mammographic phantoms. The MSER system exposes a Min-R/MRH cassette by raster scanning a 2.0 x 1.6 cm beam of pulsed x-rays across the cassette. A scanning detector behind the cassette measures the local x-ray transmission of the breast. Feedback of the transmission information is used to modulate the duration of each x-ray pulse, to equalize the film exposure. The effective dynamic range of the MSER system is 25 times greater than that of conventional mammography. Artifact-free images of mammographic phantoms show that MSER effectively overcomes the latitude limitations of film/screen mammography, enabling high contrast imaging over a wide range of object x-ray transmission. Anthropomorphic phantom images show that MSER offers up to a sixfold increase in film contrast in the normally underexposed regions of conventional mammograms. Characterization of the entrance exposure shows that there is not a significant difference in exposure between MSER and conventional mammographic techniques, suggesting that both would pose comparable risk to the patient. Calculations show that the construction of a clinical multiple beam MSER system is feasible with minor changes to existing technology.

Adipose Tissue↗

PubMedDate not supplied

Observer performance and dose efficiency of mammographic scanning equalization radiography.

The detection of fibrils, microcalcifications, and low contrast lesions is vital to the detection of early breast cancer. It has been shown through sensitometric measures and anthropomorphic phantom images that mammographic scanning equalization radiography (MSER) overcomes the latitude limitations of conventional mammographic techniques. MSER increases image quality by regional modulation of the entrance exposure to suit the local variations in x-ray transmission within the patient. In order to assess the effect of equalization on the detection of breast lesions, we have compared observer performance in MSER and conventional imaging techniques. The observation tasks were the threshold visualization of fibrils, microcalcifications, and low contrast discs (simulating lesions), located on a uniform background. The performance of the observers was determined for a range of background x-ray transmission simulating the range of transmission generated by variations in breast composition and thickness. For the conventional images, the threshold visible diameter of the fibrils, microcalcifications, and low contrast discs, increased as the x-ray transmission of the phantom changed from that for which the film was optimally exposed. For the MSER images, the performance of the observers was almost independent of the background transmission of the object since MSER ensures that the film is optimally exposed for a large range of object transmission. Even with significant changes in object x-ray transmission, only minor changes in fibril, microcalcification, and disc detection were observed. Utilizing the results of the contrast-detail experiment, a dose efficiency comparison of conventional and MSER imaging techniques was performed. The dose efficiency analysis showed that MSER varied the incident exposure so as to maintain consistent performance of the observer, over the entire breast. These results suggest that MSER would improve the ability of radiologists to detect early breast cancer in women presenting with mammographically dense breasts, in a very dose efficient manner.

Biophysical Phenomena↗

PubMedDate not supplied

Dual cervical thoracic coil for spine magnetic resonance imaging.

The need for repositioning of surface coils and patients in MR examinations of the cervical and thoracic spine prolongs examination time. A new receiver design is proposed which overcomes this problem. The device is composed of two actively decoupled receiver coils mounted on the frame of a Philadelphia collar. These coils may be used separately to image either the thoracic or cervical spine or together to produce larger field-of-view images of the combined region. Signal-to-noise ratios of the separate cervical and thoracic spine images are not degraded as a result of mounting the receivers together. The full cervical and thoracic region is shown to be imaged at a signal-to-noise ratio significantly higher than that afforded by the body coil. A retrospective review of our case load suggests that a time saving could be achieved in approximately 1/3 of spine examinations by using this coil.

Humans↗

PubMedDate not supplied

Technical aspects on magnetic resonance imaging of the spine at 1.5 tesla.

Technical aspects on surface coil magnetic resonance imaging of the spine using a superconducting system with a field strength of 1.5 tesla are described. By using a flat surface coil instead of the body coil the image quality was markedly improved and the signal-to-noise ratio (S/N) was increased approximately 2.6 times. Small voxels resulted in low S/N. The best image quality was achieved with a slice thickness of 5 mm, a field of view of 20 to 24 cm and a matrix of 256 X 256. Interleaved slices provided superior image quality compared with contiguous slices at the expense of acquisition time. For sagittal images the phase encoding gradient should be in the cranio-caudal direction to minimize motion artifacts. To obtain T1 and T2 images of high quality, spin echo pulse sequences with TR 600/TE 20 ms and TR 2000/TE 40 to 80 ms are useful.

Epidural Space↗