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Biomedical subjects

M J Bronskill

Publications and source records attributed to M J Bronskill.

72 records · Page 4Linked to original sources
PubMed1977

A quantitative measurement of peritoneal drainage in malignant ascites.

With a two-compartment model, a method is described for quantitative determination of peritoneal drainage rates in malignant ascites. Data on twenty-four patients are presented and comparison is made with the qualitative assessment of the integrity of diaphragmatic and mediastinal lymphatics on the basis of lymphoscintigrams. It is concluded that flow rates less than 50 ml/hour are usually associated with abnormalities of the diaphragmatic and mediastinal lymphatics, indicating that tumor permeation of these structures is a significant contributing factor to persistent, intractable ascites in patients with malignant diseases.

Ascites↗
PubMed1977

Compton scatter imaging of transverse sections: corrections for multiple scatter and attenuation.

The electron density (e cm-3) of a tissue sample can be determined by measuring the fluence of photons which it Compton scatters from a narrow incident beam of X- or gamma-radiation. This technique has been applied successfully to diagnostic denistometry (bone, lung), and less successfully to whole body tomography. In this work, the physics of Compton scatter tomography is examined with emphasis on transverse section scanning. The two major limitations of the scatter technique, attenuation of the single scatter photon fluence (of interest), and contamination of this fluence by multiply-scattered photons, are studied and corrective procedures are proposed. The methods described are applied to a simple test phantom; it is concluded that transverse images with an electron density precision of approximately 5% are possible.

Air↗
PubMed1977

Implications of computed tomography for inhomogeneity corrections in photon beam dose calculations.

Patient inhomogeneity information is investigated for use in radiotherapy planning. Absorbed doses measured in a phantom are compared to doses calculated for photon beams using various treatment planning inhomogeneity correction methods. Delineation of inhomogeneities with a spatial resolution of 5 mm and with an electron density accuracy of 2% in usually sufficient to allow doses to be calculated with a mean accuracy of better than 2% for 60Co and 3% for 25-MV x rays if the authors' Equivalent Tissue-Air Ratio Method is used.

Cobalt Radioisotopes↗
PubMed1975

The relationship between mouse arterial partial pressure of oxygen (PaO2) and the effectiveness of localized tumour irradiation.

The effectiveness of localized X-irradiation on the transplantable KHT sarcoma was investigated while the host mice breathed gas mixtures ranging from 5% O2 : 95% N2 to 100% O2. Subcutaneous tumours in the flank of C3H mice anaesthetized with Nembutal (sodium pentobarbitol), Valium (diazepam), or urethane, were irradiated with doses of 1,500 or 2,500 rads and the delay in days for the tumours to grow from 4 to 10 mm in diameter was determined. Measurements of the arterial partial pressure of oxygen (PaO2) were made under conditions identical to the irradiation experiments. For PaO2 values less than 100 mm Hg (animals breathing air), growth delay decreased linearly, while for PaO2 values greater than 100 mm Hg, growth delay increased and reached a plateau at a PaO2 between 200 and 300 mm Hg, (animals breathing congruent to 50% O2). This relationship holds for the two doses and the three anaesthetics studied, and indicates the importance of investigating the Pa02 levels of patients undergoing radiotherapy treatment.

Animals↗
PubMedDate not supplied

Use of magnetic particles for sensitizing MR images to blood flow.

Magnetic resonance (MR) images made with the IVIM (intravoxel incoherent motion) technique for demonstrating tissue microcirculation are limited in sensitivity because of the small volume of blood involved. This limitation may be overcome by incorporating magnetic particles into the flow. The magnetic perturbation caused by the particles extends beyond the walls of the capillary and affects a much larger volume than that of the flowing material. Imaging experiments conducted with an artificial capillary system for renal dialysis, containing large magnetic particles, showed that signal intensity decreased with increasing flow rate through the dialysis bundle and with increasing particle concentration. Predictions of the effect based on a theoretical model of spin dephasing in the field of a magnetic dipole agreed with the experimental data. The results hold promise for development of the technique in vivo.

Humans↗
PubMedDate not supplied

Two-point interference method for suppression of ghost artifacts due to motion.

A two-point interference method is introduced for suppression of ghosting due to motion in magnetic resonance imaging. The method requires only two time-interleaved data acquisitions, without any monitoring of the motion. A postprocessing technique is used to produce a weighted sum of the two acquired images, in which ghosts are suppressed by interference through an automatic regional tuning procedure. The appropriate complex weighting factors are regionally chosen by minimizing the "gradient energy," which is defined as the sum of squared pixel values in the partial-derivative maps. The method was tested in both phantoms and volunteers with a variety of imaging protocols. The level of ghost suppression with the two-point method was found to be comparable to that of the three-point method described previously by the authors.

Artifacts↗
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

MR systems for image-guided therapy.

The use of MRI to guide and monitor interventional procedures requires the merging of surgical and MRI environments. The ideal magnet shape for homogeneity and efficiency is spherical, but this design provides no access. Opening the sphere to provide both patient and surgeon access suggests cylindrical or biplanar magnets. Cylindrical magnets have poor surgical access but provide good imaging capabilities, which can be used in conjunction with a neighboring but distinct surgical environment. Biplanar magnets provide more and better approaches to the patient, but generally with lower field strength. Vertical biplanar systems allows surgical approaches from above but reduce the access of support staff to the patient. A hybrid magnet design, which combines the benefits of both cylindrical and biplanar magnets, can provide increased access with simultaneous approach from two sides of the patient. Application-specific magnets can target a smaller region, leading to compact magnet designs that greatly expand access for both surgical intervention as well as patient support. As the field of interventional MRI matures, the suitability of each design to specific applications will be better understood, leading to more integrated system designs tailored to the needs of image-guided therapy.

Equipment Design↗
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

Gradient coil design considerations for iron core interventional magnets.

The requirements for access and imaging performance compete in the design of open-concept MR magnets and gradient coils. We conducted a theoretical and experimental investigation of gradient coil design using both solid and laminated pole piece construction to determine whether adequate eddy current control can be obtained without shielded gradient coils while maintaining good patient access and high gradient performance. Eddy currents, gradient characteristics, gradient efficiency, and magnet openness are compared and contrasted for various construction options based on a compact, .27 T iron yoke magnet. The resulting pole pieces and gradient coils have high efficiency for an interventional open-configuration magnet while taking up minimal space between the poles for improved patient access.

Equipment Design↗
PubMedDate not supplied

Identification of the periprostatic venous plexus by MR imaging.

T2-weighted images (spin echo 2,000/60) show a high signal intensity rim around the prostate. Our investigations show that this rim is not part of the pelvic fat; it has T1 and T2 relaxation times similar to heparinized blood; it demonstrates little inflow effect; and after an intravenous injection of Gd-diethylenetriamine pentaacetic acid, its signal intensity increases. We conclude that this rim is the periprostatic venous plexus.

Gadolinium↗
PubMedDate not supplied

Signal strength on a 0.15-T magnetic resonance imager.

Signal strength in magnetic resonance (MR) images made on a 0.15-T imager was calculated for spin-echo (SE) and inversion-recovery with echo (ISE) pulse sequences and was compared to measurements of signal strength for aqueous MnCl2 solutions whose T1 and T2 relaxation times encompassed the range of values commonly found for tissues. Although measured signal strength generally agreed with calculated signal strength (correlation coefficient = 0.996 for both SE and ISE), significant reductions in measured strength (greater than 15%) were sometimes observed. Diffusion alone could not account for this discrepancy. Further experiments showed that imaging gradients reduced the effective T2 estimated from the imager. In addition, SE pulse sequences with short repetition times exhibited a reduction in signal strength.

Electron Spin Resonance Spectroscopy↗
PubMedDate not supplied

Noise and filtration in magnetic resonance imaging.

Noise in two-dimensional Fourier transform magnetic resonance images has been investigated using noise power spectra and measurements of standard deviation. The measured effects of averaging, spatial filtering, temporal filtering, and sampling have been compared with theoretical calculations. The noise of unfiltered images is found to be white, as expected, and the choice of the temporal filter and sampling interval affects the noise in a manner predicted by sampling theory. The shapes of the imager's spatial frequency filters are extracted using noise power spectra.

Fourier Analysis↗
PubMedDate not supplied

Phase and sensitivity of receiver coils in magnetic resonance imaging.

Receiver coil response is a major cause of nonuniformities in magnetic resonance images. The spatial dependence of the sensitivity and phase of single-saddle receiver coils has been investigated quantitatively by calculating the H1 field and comparing the results with measurements of a uniform phantom. Agreement between the measurements and calculations is excellent. A method is developed which corrects for both the nonuniform sensitivity and the phase shifts introduced by receiver coils.

Humans↗
PubMedDate not supplied

Receiver operator characteristic (ROC) analysis without truth.

Receiver operator characteristic (ROC) analysis, the preferred method of evaluating diagnostic imaging tests, requires an independent assessment of the true state of disease, which can be difficult to obtain and is often of questionable accuracy. A new method of analysis is described which does not require independent truth data and which can be used when several accurate tests are being compared. This method uses correlative information to estimate the underlying model of multivariate normal distributions of disease-positive and disease-negative patients. The method is shown to give results equivalent to conventional ROC analysis in a comparison of computed tomography, radionuclide scintigraphy, and magnetic resonance imaging for liver metastasis. When independent truth is available, the method can be extended to incorporate truth data or to evaluate the consistency of the truth data with the imaging data.

Calibration↗