Biomedical subjects

I R Medina

Publications and source records attributed to I R Medina.

3 recordsLinked to original sources

PubMed2001

Low-reflection-coefficient liquid interfaces for system characterization.

The use of liquid brominated hydrocarbons to form a planar reflecting interface with water is described. Gravity-based planar reflecting surfaces with known reflection coefficients can be used in system characterization for quantitative ultrasonics, and a set of surfaces with a range of reflection coefficients allows calibration of the output power and receiver gain of ultrasonic imaging systems. The substances reported here are immiscible in water and form interfaces with water, resulting in a broad range of acoustic reflection coefficients. Reflection coefficients were measured at temperatures from 18-24 degrees C for "pure" substances and for mixtures of two brominated hydrocarbons. Results show that reflection coefficients are weakly dependent on temperature and that, at a specific temperature, a significant range of arbitrarily small reflection coefficients is available, in the case of the mixtures, by the appropriate choice of weight-percents of the two brominated hydrocarbons.

Hydrocarbons, Brominated↗

PubMed1994

Performance testing of transrectal US scanners.

PURPOSE: To test the performance of transrectal ultrasound (US) scanners in the detection of low-contrast focal lesions as a function of depth, size, and contrast. MATERIALS AND METHODS: US images of two phantoms were analyzed by three observers. One phantom was most suitable for US units allowing scanning in the transverse or longitudinal planes, while the other was suitable for any scanning situation. Both phantoms were made of tissue-mimicking material and had six sections. Five sections contained different sizes of low-contrast simulated lesions, which were of different echogenicities and were randomly distributed to a depth of 8 cm. The sixth section contained only background material and served as a reference. A resolution zone, corresponding to the depth range over which a given type of lesion was detectable, was determined for each of the five lesion-containing sections and a figure of merit equal to a weighted sum of resolution zone lengths computed. RESULTS: Data from 12 different scanner configurations were analyzed; a broad range of figures of merit were found. Agreement was found among the three observers regarding relative performance of the scanner configurations. CONCLUSION: The phantoms are useful for comparing performances of different transrectal US configurations.

Evaluation Studies as Topic↗

PubMed1988

Stretch-activated ion channels modulate the resting membrane potential during early embryogenesis.

By using the patch-clamp technique, stretch-activated ionic channels were found in the membrane of cleaving freshwater fish embryos at the early stages of embryogenesis (2-256 cells). The application of negative pressure to the pipette increased the frequency of activation and the duration of bursts. This type of channel has a preferential K+ selectivity. When bathed on both membrane surfaces with 140 mM KCl the channel conductance was 71 pS. The kinetic behaviour did not depend markedly on either membrane potential (in the range from -70 to +70 mV) or calcium concentration on the cytoplasmic side of the membrane. On continuous recording, the probability of the channel being open was found to change periodically over a 5- to 20-fold range for different cells. These variations correlated with changes in resting potential and membrane conductance during the cell cycle. These results suggest that the oscillation of resting potential within the cell cycle is associated with the operation of stretch-activated ion channels.

Animals↗