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Biomedical subjects

W T Sobol

Publications and source records attributed to W T Sobol.

At least 19 recordsLinked to original sources
PubMed1999

Characterization of the reciprocity law failure in three mammography screen-film systems.

The purpose of this project was to quantify reciprocity law failure (RLF) for mammography screen-film systems. Three widely used screen-film systems were evaluated: the Kodak MinR 2000 system. Fuji UM Mammo Fine screen and Fuji UM MA HC film, and Agfa MR Detail screen and Agfa Mammoray MR5 film. The logit algorithm that linearizes logistic curve shapes was utilized to characterize film sensitometric response. Different values of mammographic phantom thickness, tube current, and kVp were used to vary screen-film exposure rates. RLF was quantified by examining the dependence of logit parameters (maximum and minimum film density, curve shift, and slope) on exposure rate. The shift of the logit curve was found to be a good indicator of the screen-film system speed, while the slope of the logit curve is affected by the RLF. RLF leads to changes in film contrast as well as speed. For the range of exposure rates measured (50-fold), screen-film contrast and speed varied by factors of 2 and 3.5, respectively. Film contrast decreased as exposure rate increased. The greatest changes were observed with the Kodak MinR 2000 screen-film system.

Mammography↗
PubMed1997

Computed model of gadolinium enhanced MRI of breast disease.

OBJECTIVE: Investigation has shown that the most useful MRI finding for the detection of breast cancer is enhancement following in travenous contrast. However, many widely different imaging protocols have been used. The purpose of this study is to explicate factors that affect the signal intensity of breast lesions after intravenous gadolinium. METHODS AND MATERIALS: A computer model was developed using equations based on published data. The effect of gadolinium on breast tissues was calculated using the model with appropriate values for baseline tissue relaxation times, relaxivity of gadolinium at the given field strength and concentration of gadolinium based on published data, for the TR, TE, flip angle and field strength of several published sequences used for enhanced breast MRI. RESULTS: The computer model allows comparison of the performance of different sequences, which can be displayed graphically. These vary in their performance, largely dependent on T1 weighting. Enhancement is also affected by the baseline of the T1 of the lesion and sensitivity of the sequence to gadolinium. Malignant lesions demonstrate greater observed enhancement than predicted when assuming symmetric distribution of contrast, indicating there is greater accumulation of gadolinium, accounting for the differential enhancement between benign and malignant lesions. CONCLUSIONS: MRI sequences vary greatly in their demonstration of enhancement after intravenous gadolinium contrast. Numerical diagnostic criteria such as % signal intensity change must be interpreted with care when using a different sequence than that on which the criterion was developed. There is preferentially greater accumulation of contrast in malignant lesions, whether due to angiogenesis or altered permeability.

Algorithms↗
PubMed1997

Parametrization of mammography normalized average glandular dose tables.

Data from existing tables of normalized glandular doses in mammography were parametrized to determine analytical expressions that match tabulated results within known uncertainties. The parametrization was performed for three different target/filter combinations (molybdenum target-molybdenum filter, molybdenum target-rhodium filter, and rhodium target-rhodium filter) and three different breast compositions (100% adipose, 50% adipose-50% glandular, and 100% glandular). The analytical expressions provide the normalized glandular dose for any breast composition within stated ranges of tabulated input parameters (kVp, half-value layer, and breast thickness). The maximum difference between tabulated and parametrized data is 1.29%, which is smaller than the stated 2% uncertainty in tabulated dose data due to uncertainties in the x-ray spectra models used to generate the tables. Analytical expressions are easily coded to create custom functions that return the normalized glandular dose for the set of input parameters. Examples of implementation are presented in Microsoft Visual Basic for Applications (VBA).

Algorithms↗
PubMed1994

Magnetic resonance contrast agents. Physical basis of relaxation.

Only through a combined understanding of contrast agent properties and fundamental contrast mechanisms, optimal administration and imaging techniques can be applied to contrast-enhanced MR imaging. It is impossible to cover all aspects of the physics of paramagnetic contrast agents in a short review. The basic theories of paramagnetic relaxation enhancement and susceptibility effects, presented in this article, are intended to help readers understand the behavior of existing contrast agents in clinical studies. Additional information about these phenomena can be found in other published reviews, and for a more in-depth coverage, the reader should turn to monographs and scientific papers that discuss the particular topics of interest.

Central Nervous System↗
PubMed1993

Laboratory information system for nuclear magnetic resonance relaxometry in biomedical materials (NMR LIS). I. Data acquisition and experiment control.

A large and comprehensive Laboratory Information System (LIS) was developed for a laboratory that specializes in the studies of nuclear magnetic resonance (NMR) relaxation behavior in biomedical materials. Several programs have been developed (data acquisition and hardware control, data analysis, theoretical modeling, utilities like plotting, FFT, and peak fitting), and several other are under development (data postprocessing, database management, linkage to other systems). This paper describes a software module that was developed to control the hardware (to communicate with the NMR spectrometer pulse programmer, magnet power supply, temperature controller), to achieve efficient data acquisition through a parallel interface to the ADC devices, and to provide a user friendly interface to a complex laboratory system.

Clinical Laboratory Information Systems↗
PubMed1993

Laboratory information system for nuclear magnetic resonance relaxometry in biomedical materials (NMR LIS). II. Data analysis.

Data analysis modules for Nuclear Magnetic Resonance Relaxometry LIS are described. Since the experimental NMR results from biomedical samples often exhibit complex structure that arises from complexity of tissue, the analysis methods focus on search, investigation, and evaluation of complex multiexponential relaxation features. Two algorithms have been implemented: an enhanced spin grouping method that is based on a multiexponential curve stripping algorithm, and an eigenvalue decomposition method, implemented as modified DISCRETE program.

Algorithms↗
PubMed1993

Analysis of variance for 'component stripping' decomposition of multiexponential curves.

An extended analysis of variance is presented for a multiexponential curve fitting procedure, known as 'curve stripping', 'curve peeling', or 'successive subtraction'. In addition to the standard, single variable curves, this analysis includes the two dimensional multiexponential surface analysis. The calculations take into account features required by the graphical user interface that was specifically designed to facilitate the fitting process. The evaluation of the goodness of fit includes an F test (to evaluate the number of components needed to fit the data), and correlation coefficients (to evaluate individual parameters).

Algorithms↗
PubMed1992

A comparison of total body water measurements using whole-body magnetic resonance imaging versus tritium dilution in primates.

Standard techniques conventionally used to assess body composition have various drawbacks which include a requirement for specialized equipment and expertise not widely available, radiation exposure, extensive study time periods, and limited clinical utility. Anthropometric methods, which are more clinically feasible, also involve substantially greater calculation error. This study was designed to compare a newly developed whole-body magnetic resonance (MR) imaging technique with standard tritium dilution (THO) to measure total body water (TBW) in eight primates (Macaca fascicularis). Sedated primates underwent whole-body MR imaging (10-12 contiguous 50-mm slices) and TBW was computed using a previously validated program. After 5 days to allow for primate recovery and stabilization, TBW measurements were repeated using THO methodology. Linear regression analysis of TBWMRI (72.1 +/- 5.7% body wt) compared with TBWTHO (73.8 +/- 3.2% body wt) yielded a Pearson correlation of r = 0.8145, P = 0.02 with a mean squared error of 2.14. Whole-body MR imaging determination of TBW correlates well with THO, requires less time (20 min versus 24 hr), and does not expose the subject to radiation. This study suggests that whole-body MR imaging is an accurate technique for in vivo body composition analysis. As demonstrated by the body size of this investigational primate model, the technique maybe particularly applicable to the study of human infants.

Animals↗
PubMed1991

A complete solution to the model describing Carr-Purcell and Carr-Purcell-Meiboom-Gill experiments in a two-site exchange system.

A general solution for the relaxation decay function observed in both CP and CPMG spin-echo experiments, performed on systems governed by a two-site chemical exchange mechanism, is presented. This solution not only provides the relaxation time constants, but also supplies complete information about relative component sizes. Solutions are analytical, but results are complex enough to warrant computer-assisted calculations. Programming suggestions, based on the extensive tests performed, are provided.

Algorithms↗
PubMed1990

Computer-assisted assessment of the kinetic function of disk-type artificial heart valves.

A simple and accurate computer simulation method was developed to evaluate the geometrical parameters of disk-type artificial heart valves from cinefluorographic recordings. In vitro valve models with preset occluder angles were used to test the accuracy of the method. Linear regression analysis produced slope 1.00055 and intercept 0.0402, with r = 0.999. In vivo reproducibility was tested through repeated measurements, taken several weeks apart, of OMS valves in situ. Regression with slope 1.02 and intercept -0.95, r = 0.95 were found.

Computer Simulation↗
PubMed1990

Pseudolayering of Gd-DTPA in the urinary bladder.

When excreted gadolinium diethylenetriaminepentaacetic acid (DTPA) collects in the bladder of a supine patient during magnetic resonance (MR) imaging, a puzzling pattern of signal intensities is noted. A gradual change in urine signal intensity with progressive addition of Gd-DTPA does not occur; instead, three sharply defined "layers" are seen both on T1- and T2-weighted images within the urine-Gd-DTPA mixture. The physical basis for this triple-layering phenomenon was investigated. A bladder phantom was constructed to reproduce the phenomenon. T1 and T2 relaxivities of urine doped with varying concentrations of Gd-DTPA were measured in vitro; measured signal intensities corresponded closely to predicted intensities. Early urine concentrations of excreted Gd-DTPA may be relatively high (10-40 mmol/L), resulting in extremely short T1 and T2 values (less than 30 msec). These extremely short relaxation times cause an artifactual pseudolayering of signal within the urine-Gd-DTPA mixture.

Contrast Media↗
PubMed1990

Adipose tissue determinations in cadavers--a comparison between cross-sectional planimetry and computed tomography.

The adipose tissue surfaces in 11 slices (+/- 5 cm from the umbilicus) were compared in two cadavers using computed tomography (CT) versus planimetry of band-sawed slices of the corresponding sections. A very close correlation was found with partial correlations of around 0.90. Retroperitoneal fat formed a considerable proportion of the total adipose tissue surface in the slices. The results were similar whether fat was defined as -250 to -50, -190 to -30, or -140 to -40 Hounsfield units. These data indicate that CT measurements agree closely with a direct morphometric method and thus can be used as a 'gold standard' for future development. The fact that fat which is located extraperitoneally, but still intraabdominally, constitutes a significant proportion of the slice surface in the umbilical region indicates that data relating intraabdominal fat measurements to metabolic functions must be interpreted with caution.

Abdomen↗
PubMed1987

NMR spectroscopy of heterogeneous solid-liquid mixtures. Spin grouping and exchange analysis of proton spin relaxation in a tissue.

An attempt to resolve the complicated proton NMR signal of a heterogeneous material is presented. Lung tissue is used to illustrate the approach to this problem, which is based on resolving the total proton magnetization into components according to their relaxation times. This procedure is utilized at both low and high magnetic fields. These results are then correlated and analyzed for the effect of exchange between spin groups. For example, the total proton NMR signal of lung tissue is shown to arise from three interacting proton groups: protons on solid macromolecules, bound water protons, and bulk water protons. The dynamics of water is modeled to satisfy the dispersivity and the values of the relaxation parameters obtained considering the effect of exchange. It is proposed that this approach is generally applicable.

Animals↗
PubMed1986

Modeling of proton spin relaxation in muscle tissue using nuclear magnetic resonance spin grouping and exchange analysis.

NMR spin relaxation experiments performed on healthy mouse muscle tissue at 40 MHz and 293 K are reported. The spin-lattice relaxation experiments were performed using different combinations of selective and nonselective radio frequency pulses. Relaxation experiments in the rotating frame at H1 = 10, 5 and 1 G are also reported. The experimental results were analyzed using the spin-grouping method, which yields the sizes of the resolved magnetization components as well as their T2's and T1's (or T1p's) for the nonexponential relaxation functions. These results were analyzed further for the exchange between different spin groups. It has been found that to explain all of these experimental data it was necessary to use a four-compartment model of the muscle tissue that consists of a lipid spin group, a "solid-like" spin group (mainly proteins), a "bulk water" spin group and a "bound water" spin group. The chemical exchange rate between "bulk" and "bound" water was found to be 29 +/- 9s-1 at room temperature. The exchange rate between the bound water and the solid moderator was estimated to be approximately 500 s-1.

Animals↗
PubMedDate not supplied

On the stationary states in gradient echo imaging.

A formalism is presented that concisely describes the magnetization of a sample subjected to a periodic series of RF pulses. In this formalism, the steady state of the magnetization is shown to be a sum of magnetic substates, each with unique contrast characteristics. When more than one substate contributes to a given image, the substates interfere with each other, producing ghosts and other artifacts. Properly designed gradient protocols can image single substates, producing ghost-free images. The contrast of the image depends largely on the choice of the imaged substate. Analytic solutions for unspoiled, RF spoiled, and gradient spoiled magnetizations are presented.

Artifacts↗