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T Villafana

Publications and source records attributed to T Villafana.

16 recordsLinked to original sources
PubMed1991

Technologic advances in computed tomography.

Technologic improvements in CT continue. As always, emphasis in the past year has been on reducing scan times. Units employing slip-ring technology are now available. This technology makes possible spiral scanning, which promises to further reduce the effects of image-degrading patient motion, as well as opening new possibilities for quantitative and dynamic CT studies. These applications are discussed here and an update on the advances on cine-CT and the Dynamic Spatial Reconstructor unit is provided. Three-dimensional reconstructions obtained at very short scan times promise to be of particular benefit for scanning the bronchial tree. Finally, patient doses of radiation in CT are discussed.

Bone Density↗
PubMed1988

Fundamental physics of magnetic resonance imaging.

Although similar to computerized tomography, in that cross-sectional images are produced, the physical principles underlying magnetic resonance are entirely different. The MRI process, as commonly implemented, involves the excitation of hydrogen nuclei and the analysis of how these nuclei recover to the original equilibrium steady states that they had prior to excitation. This article discusses that process, that is, preparatory alignment, RF excitation, relaxation and signal measurement, and spatial localization.

Fourier Analysis↗
PubMed1987

Screen-film mammographic technique for breast cancer screening.

The values of dose and signal-to-noise ratio (SNR) for many techniques and breast thicknesses were computed and compared with a reference technique and breast thickness to provide a valid basis upon which to select a screening technique for screen-film mammography. The reference consisted of a Min-R/OM1 combination (or its equivalent) exposed through a 4.5-cm thick breast via 28 kV and a molybdenum target-beryllium window tube with a 0.03-mm Mo filter. Radiographs of an improved breast phantom were used to relate computed relative SNR values of techniques to diameters of the smallest calcific and soft-tissue objects demonstrated in mammograms. Without use of a grid, four Mo target/filtration combinations yielded similar computed dose and SNR levels, as did a tungsten target tube operated at 23 kV without a filter. Operation of the latter tube at 27.5 kV with a 0.051-mm rhodium filter should reduce dose by half but also SNR by 22%. However, such operation with a grid should greatly improve imaging moderate to large breasts without increasing dose over reference values.

Breast Neoplasms↗
PubMed1986

The retrolenticular afterglow: an echo enhancement artifact.

A mathematical model for a sonogram containing a circular object with a sonic speed less than that of its surroundings was constructed. The circular object acts as a lens to the ultrasound beam, which explains the refractive artifacts often observed. An unexpected artifact is also predicted, a region of increased echogenicity, which the authors named the retrolenticular afterglow, since it occurs behind a lens-like structure. An experiment was designed to show the appearance of this artifact occurring distal to an ethyl alcohol-filled cylindrical well in a tissue equivalent phantom.

Gallbladder↗
PubMed1979

Rapid computation of diagnostic X-ray bremsstrahlung spectra.

A method is described for rapid and accurate computation of diagnostic x-ray spectra. Accuracy limitations of Kramers' equation are overcome by providing intensity correction factors derived from published measured data. Use of a parameter based on the energy of the Kramers spectrum intensity peak permits deriving master factor curves which are remarkably independent of kVp, waveform and filtration. Computed and measured spectra generally agree to better than +/- 1 keV for beams generated at 100 kVp and below. Possible application of the method to higher energy diagnostic beams is discussed.

Mathematics↗
PubMed1979

A study of mammographic exposure and detail visibility using three systems: Xerox 125, Min-R, and Xonics XERG.

A breast phantom of novel design has been used to measure visibility of simulated calcific and soft-tissue fibrillar details in mammography, as well as to determine the roentgen exposure vs. depth. Exposure data were combined with a model of the breast as compressed during mammography to compute the mean exposure to the ductal parenchyma (MDE). Three different imaging systems were compared over a wide range of x-ray beam energies and breast characteristics. "Dosage" criteria other than the MDE are discussed.

Breast↗
PubMed1978

CT intracranial localization with a new marker system.

Application of a new CT localization system for intracranial lesions in pre- and intraoperative guidance is described. This system consists of air grooves within an acrylic plate for CT examinations and a duplicate plate with wires in place of air grooves for scout radiography. This new device can also be applied in stereotaxic surgery or in radiotherapy planning.

Brain↗
PubMed1978

A device to indicate anatomical level in computed tomography.

A device was designed that makes possible direct anatomical correlation between the computed tomography scan and radiographic scout films. The acrylic-air groove approach used here eliminates the possibility of producing image degrading artifacts as is common with catheter type markers.

Brain↗
PubMed1977

Effect of finite exposure slits in determination of the line spread function and modulation transfer function.

The modulation transfer function of radiologic imaging systems is commonly obtained by determining the line spread function (LSF) of the system and computing its Fourier transform. Ideally, LSF should be obtained with infinitesimally narrow slits. The use of finite slits for obtaining the LSF is analyzed theoretically. An expression for the MTF of a finite rectangular slit is derived. Slit width correction of observed MTF is discussed as well as the correction for any generalized slit configuration.

Computers↗
PubMedDate not supplied

A breast phantom method for evaluating mammography technique.

A new breast phantom has been designed for use in evaluating mammographic system performance. This phantom incorporates simulated calcifications and fibrillar objects in fat, of graded size, to permit measurements of detail visibility. A special methodology has been developed for measuring visible object size to achieve reproducible and clinically relevant results. Materials and construction of the phantom also permit carrying out dosimetry with an appropriate ionization chamber. Dosage and detail visibility measurements are reported for the Xerox 125, Min-R and Xonics systems. In addition to providing information regarding technique and image receptors, these results demonstrate the usefulness of the basic phantom design, and suggest possible improvements.

Breast Neoplasms↗
PubMedDate not supplied

Modulation transfer function of a finite scanning microdensitometer slit.

Inherent in the line-spread function approach in modulation transfer function (MTF) determinations is the use of a microdensitometer to scan the density pattern resulting from the line exposure. In such a procedure, loss of fidelity in the distribution being scanned is expected. This loss is due in part to the use of a finite-width scanning slit. In spite of the central role of the scanning slits in the MTF determination, little if anything has appeared in the literature from which one can quantitate the effect of a scanning slit on the MTF determination. This paper analyzes the loss of response attendant with use of scanning slits of differing widths. Specifically, the MTF of a finite scanning slit is derived. A method for correcting for loss of response, given a specific slit width, is indicated on the basis of the chain-multiplication property of MTFs. The current practice of using a 10-mum width for film-screen combinations is shown to be justified. Results can be generalized to any configuration scan slit. In addition, it may also be applied to any generalized sampling or display-type slit.

Densitometry↗
PubMedDate not supplied

Effect of microdensitometer scan slit misalignment in MTF determinations.

To determine the modulation transfer function (MTF) of a radiographic film or film-screen system, it is common to expose the system to a line source and scan the result with a microdensitometer. The question arises as to how the MTF determination is affected by such factors as microdensitometer slit width and possible misalignment of the slit with the line pattern being scanned. In any experiment, some degree of misalignment can be expected. This paper analyzes the resulting problem and derives an expression for the MTF of slit misalignment and slit width: MTF = [sin[2pif(L/2)]/2pif(L/2)] X (sin[2pif[H sin(theta/2)[[/2pif[Hsin(theta/2)[[), where L is the slit width, H the slit height, theta the angle of misalignment, and f the spatial frequency variable. The left-hand factor of this product is the MTF of a finite slit width, while the right-hand factor is the MTF of misalignment. It is shown that, for radiographic film-screen combinations, small-angle misalignment up to 0.25 degrees is not serious with 1.5-cm-long slits, while such a misalignment may become serious in highbandwidth systems such as nonscreen films.

Densitometry↗
PubMedDate not supplied

Temporal response of microdensitometers.

Microdensitometers have both spatial and temporal finite responses which may lead to degradation in images being analyzed. These responses may be quantitated in terms of spatial and temporal modulation transfer functions (MTFs). The temporal response of microdensitometers is studied here. Specifically, the technique of differentiating temporal step-function responses to determine the temporal MTF is presented. Experimental results illustrating the theory are described using a Baird Atomic microdensitometer.

Absorptiometry, Photon↗