Biomedical subjects
Publications and source records attributed to M D Harpen.
An adaptation of linear acoustic scattering theory is used to describe the comet tail artifact which is observed in the diagnostic ultrasound imaging of small spherical masses. The calculated backscattered pressure wave is shown to correspond well with the appearance of the artifact in a variety of clinical images.
We describe the use of a graphical mathematical spreadsheet programming environment which can be used to simulate the acquisition and reconstruction processes of an x-ray computed tomography (CT) machine. The simulation is used to study the effect of photon counting statistics on the noise in the reconstructed image. Finally we describe and evaluate a novel technique for noise reduction using a nonlinear wavelet filter in which the filter thresholds are calculated individually from the "measured" projection data. This filter is shown to compare favorably to threshold filters based on global estimates of noise variance.
Presented is a simple model describing the dependence of image noise in computed tomography on the x-ray beam profile. The model is used to derive the x-ray profile which minimizes total image noise at constant integral patient dose. The profile may be produced with a bow-tie-type beam shaping filter. Results of the analysis are validated using a computer simulation of computed tomography (CT) acquisition and reconstruction.
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BACKGROUND: We used dynamic magnetic resonance imaging (MRI) to study breast lesions in 13 women. METHODS: We observed differences in contrast uptake between benignancy and malignancy in 14 suspicious breast lesions. Three-dimensional (3D) gradient echo sequences were obtained before and after administration of gadolinium-based contrast medium (0.16 mmol/kg). The percentage of signal increase in lesions was measured in a series of five 90-second sequences, and time-enhancement patterns were correlated with pathologic diagnoses. RESULTS: Seven benign lesions and three breast cancer recurrences showed less than 185% signal increase at 90 seconds after contrast administration. Three new breast cancers and one recently biopsied benign lesion showed more than 185% signal increase at 90 seconds. CONCLUSION: Using this MRI technique, we can discriminate between new breast cancers (more than 185% early signal increase) and breast cancer recurrence and/or benign lesions (less than 185% early signal increase) but cannot distinguish recurrent from benign lesions.
In this investigation we propose a simple model for the spatial distribution of scattered photons from a CT unit. The model is used to determine the shielding required for a CT installation. A mathematical based spreadsheet which carries out the calculation is given and is also used to assess the significance of the various assumptions used in CT barrier calculations.
The wavelet transform, part of a rapidly advancing new area of mathematics, has become an important technique for image compression, noise suppression, and feature extraction. As a result, the radiological physicist can expect to be confronted with elements of wavelet theory as diagnostic radiology advances into teleradiology, PACS, and computer aided feature extraction and diagnosis. With this in mind we present a primer on wavelet theory geared specifically for the radiological physicist. The mathematical treatment is free of the details of mathematical rigor, which are found in most other treatments of the subject and which are of little interest to physicists, yet is sufficient to convey a reasonably deep working knowledge of wavelet theory.
OBJECTIVE: To determine any ethnic difference in the rate of spontaneous decrease in the incidence of primary vesico-ureteral reflux (VUR). MATERIALS AND METHODS: Voiding cystourethrograms on 738 patients with urinary tract infection were reviewed, and the incidence of primary vesico-urethral reflux was correlated with age, gender and ethnic origin. RESULTS: There was a gradual decrease in the incidence of primary VUR in whites between 0-10 years, while the incidence of primary VUR in blacks was very low and remained the same. After 10 years of age, no difference was present. CONCLUSION: The possibility of delayed maturation of the antireflux mechanism in whites is considered as a possible explanation, and the possibility of an ethnic basis for this is suggested.
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We present a mathematical model for shielded loop resonators. The model implies equivalent circuits for coaxial as well as flat Faraday shielded resonators under both balanced and unbalanced termination conditions. Expressions for impedance derived from the model are shown to agree well with observation using two test experimental resonators in a variety of experimental conditions.
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We present an equivalent circuit analysis for both low pass and high pass birdcage resonators loaded with lossy samples. In a generalization of the method of Hoult and Lauterbur (J. Magn. Reson. 34, 425 (1979)), we also derive circuit component values by application of the laws of electrodynamics. Measured resonance spectra, quality factors, and feed point impedances in a test resonator are shown to be in agreement with those predicted by the proposed model.
We present a derivation of the losses in a birdcage resonator due to radiation. We also present an expression for the radiation limited Q. It is shown that in head coil imaging at 63 MHz radiative losses may account for 20% of the total loss with a radiation limited Q on the order of 150. The results are shown to be consistent with those reported in the recent literature.
We present a model of birdcage resonator operation when the size of the resonator approaches a quarter wave length resulting in significant phase shifts along the current conducting elements. In the model resonators are treated as generalized transmission lines. Line termination determines whether the resonator is of the low pass or high pass type. The model correctly describes the dependence of resonant frequency on resonant mode number as well as the distribution of current along the length of the resonator.
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Using a novel application of the method of images we show how a plane array of resonant elements can be substituted for the conducting plane in a semicylindrical or split resonator. In an experimental low-pass split resonator-plane array system used as a magnetic resonance imaging receive coil at 0.5 T sensitivity values are shown to be uniform (approximately +/- 3%) across the field of view.
A theoretical analysis of the interaction between a magnetic resonance imaging radio frequency coil and a heterogeneous sample consisting of two conducting regions separated by a non-conducting membrane is presented. The effect of the capacitive coupling between the two regions is treated. Expressions are obtained for the power dissipation in the sample and the electrical impedance of the coil and an equivalent electrical circuit which models the interaction is presented. The sample power dissipation in this case lacks the usual quadratic dependence on frequency observed with homogeneous samples. Results of the analysis are shown to agree well with observation in an experimental coil-sample system.