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Biomedical subjects

M D Harpen

Publications and source records attributed to M D Harpen.

At least 19 recordsLinked to original sources
PubMed1998

Dynamic magnetic resonance imaging in evaluating suspicious breast lesions: correlation with pathologic findings.

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.

Breast↗
PubMed1998

An analysis of the assumptions and their significance in the determination of required shielding of CT installations.

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.

Biophysical Phenomena↗
PubMed1998

An introduction to wavelet theory and application for the radiological physicist.

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.

Biophysical Phenomena↗
PubMed1997

Ethnic factors in the variability of primary vesico-ureteral reflux with age.

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.

Adolescent↗
PubMed1994

The theory of shielded loop resonators.

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.

Electric Impedance↗
PubMed1993

Equivalent circuit for birdcage resonators.

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.

Humans↗
PubMed1993

Radiative losses of a birdcage resonator.

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.

Electricity↗
PubMed1993

Cylindrical coils near self-resonance.

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.

Magnetic Resonance Imaging↗
PubMed1992

A new approach to split resonator design.

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.

Equipment Design↗
PubMed1991

Analysis of the interaction between an MRI coil and a heterogeneous sample.

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.

Magnetic Resonance Imaging↗
PubMed1990

Magnetic resonance imaging of bone marrow in sickle cell disease: clinical, hematologic, and pathologic correlations.

A longitudinal, prospective, controlled evaluation of magnetic resonance images (MRI) of long bones in sickle cell patients was undertaken simultaneously with assessment of clinical status and hematologic parameters, including dense erythrocytes. MRI of bone marrow in sickle cell patients during steady states appeared patchy and were markedly different from those in matched controls (P approximately 0). Patients with severe patchiness were older than those with mild or moderate patchiness (P less than .03). Sixty-nine MRI were performed during 28 painful episodes occurring in 14 subjects with sickle cell disease (SCD). Increased signals on intermediate and T2-weighted images were detected in 35.7% of painful episodes. These abnormalities were distinct and not observed to occur spontaneously during the steady-state examinations (P approximately 0). Bone marrow infarcts were confirmed by biopsy in two instances and autopsy in one instance. Dense red cells decreased by 40.81% of baseline during pain crises (P = .00005), more remarkably in those who had pain in the lower extremities (P = .0145). Patients with change in MRI during pain crises had a greater percentage change in the dense cells than those without the change in MRI (69.7% v 31.3%, P = .0120).

Anemia, Sickle Cell↗
PubMed1989

Eddy current distributions in cylindrical samples: effect on equivalent sample resistance.

We present a general technique for the determination of eddy current distributions within an irregularly shaped conducting sample in the uniform field of an NMR RF coil. Also presented is a general expression for the sample-induced coil resistance. The technique is applied specifically to a conducting cylindrical solid. Unlike previous descriptions of cylindrical samples in solenoidal coils where the induction is parallel to the axis of the cylinder and eddy current streamlines are circular, we treat the case where the induction is perpendicular to the cylindrical axis and where consequently eddy current streamlines take on an irregular shape.

Electric Conductivity↗
PubMed1988

Influence of skin depth on NMR coil impedance.

A derivation of the change in electrical impedance of an NMR surface coil produced by inductive coupling with a conducting medium is presented. The effect of skin depth is treated explicitly. Theoretical predictions of coil sample resistance and frequency offset are compared with those observed experimentally in a variety of circular surface coils and sample conductivities.

Electric Conductivity↗