Biomedical subjects

T Villafana

Publications and source records attributed to T Villafana.

23 records · Page 2Linked to original sources

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↗

PubMedDate not supplied

Measurements of diagnostic x-ray backscatter by a novel ion chamber method.

There is a major gap in backscatter information for diagnostic x-ray beams. Such information is increasingly needed for dose measurements and calculations, as well as for designing devices and techniques. We have therefore carried out measurements on both low Z materials and metals, using an ion chamber method designed specifically for the purpose. Lucite and two D.R. White tissue substitutes were studied extensively (BR 12 "average breast" and MS 11 "water"). Measured percent backscatter (BS) was greatest for Lucite and least for MS 11, with BR 12 in between. Backscatter buildup is rapid: 50% of full backscatter is achieved with 6 mm thickness for all three materials using mammographic beams and with about 12 mm using general diagnostic beams. A simple relationship between BS and field area permits close estimates of BS values for fields for which measured data is not available. Among metals tested, copper exhibited greatest backscatter (39% BS maximum), aluminum least, and lead in between--information of potential importance in cassette design and similar applications.

Mammography↗