Perturbative measurement of magnetoresistance.

In this paper, we report the development of a novel technique in which the magnetoresistance of nanostructures is perturbatively measured by transversally modulating the DC magnetic field. It measures the electrical transport counterpart of the transverse magnetic AC-susceptibility. The technique was developed in a conventional four-probe configuration in which a small DC current flows through the sample and a small transverse AC-field perturbates the equilibrium position of the sample magnetization. Lock-in detection, in-phase with the AC-perturbation, is used to measure the voltage signal between the two inner electrodes of the four-probe station. We successfully demonstrated that this signal is proportional to the product of the first derivative of sample resistance with respect to the equilibrium position of the magnetization times the second derivative of the energy with respect to the equilibrium position of the magnetization. These dependences add new features to the technique investigated here that were not captured by the investigations previously reported on the same subject. To show the effective use of the technique, we discuss its application in measuring magnetic properties of thin magnetic films in the non-saturated regime.

Radiographic features of periodontal bone defects: evaluation of digitized images.

OBJECTIVES: Radiographic evaluation of periodontal conditions is a difficult process because of the many limitations inherent to this complementary examination and the lack of image specificity for different types of bone defects. The objectives of this study are to utilize digitized imaging resources to describe the radiographic features of different types of periodontal bone defects produced artificially in dry mandibles. METHODS: 14 dry mandibles were utilized, in which periodontal bone defects were produced. Digital photographs and conventional radiographs of each site were obtained in a standardized manner, before and after producing these defects. The radiographs were then digitized, displayed on the screen and evaluated by three examiners. RESULTS: Most of the bone defects presented distinct imaging characteristics that allowed them to be identified, with the exception of one-, two- and three-wall infrabony defects. The defects that were most easily interpreted were horizontal and vertical defects and the most difficult were defects in the radicular septum. CONCLUSIONS: Despite the importance of imaging diagnoses in reaching clinical decisions regarding treatment, such diagnoses for different types of periodontal bone defects are extremely difficult to make. In the present study, the utilization of digital tools for interpreting digitized radiographic images facilitated the process.

Morphometric methods to evaluate craniofacial growth: study in rabbits.

OBJECTIVES: The purpose of this study was to evaluate the reliability of direct and indirect measurement methods, with a view to standardizing morphometry instruments. METHODS: Dry skulls of 30 New Zealand rabbits were used. Measurements were made with a transparent flexible plastic ruler, an EKG calliper and a digital calliper, as well as with computerized tomography and lateral radiographs for direct and indirect analysis of distances and angles. RESULTS: The different instruments studied showed only partial agreement. CONCLUSIONS: A standardized and reliable direct or indirect morphometry method for the craniofacial skeleton of rabbits still has not been determined.

Relative microelastic mapping of living cells by atomic force microscopy.

The spatial and temporal changes of the mechanical properties of living cells reflect complex underlying physiological processes. Following these changes should provide valuable insight into the biological importance of cellular mechanics and their regulation. The tip of an atomic force microscope (AFM) can be used to indent soft samples, and the force versus indentation measurement provides information about the local viscoelasticity. By collecting force-distance curves on a time scale where viscous contributions are small, the forces measured are dominated by the elastic properties of the sample. We have developed an experimental approach, using atomic force microscopy, called force integration to equal limits (FIEL) mapping, to produce robust, internally quantitative maps of relative elasticity. FIEL mapping has the advantage of essentially being independent of the tip-sample contact point and the cantilever spring constant. FIEL maps of living Madine-Darby canine kidney (MDCK) cells show that elasticity is uncoupled from topography and reveal a number of unexpected features. These results present a mode of high-resolution visualization in which the contrast is based on the mechanical properties of the sample.

Solid-state DNA sizing by atomic force microscopy.

Atomic force microscopy (AFM) allows rapid, accurate, and reproducible visualization of DNA adsorbed onto solid supports. The images reflect the lengths of the DNA molecules in the sample. Here we propose a solid-state DNA sizing (SSDS) method based on AFM as an analytical method for high-throughput applications such as finger-printing, restriction mapping, +/- screening, and genotyping. For this process, the sample is first deposited onto a solid support by adsorption from solution. It is then dried and imaged under ambient conditions by AFM. The resulting images are subjected to automated determination of the lengths of the DNA molecules on the surface. The result is a histogram of sizes that is similar to densitometric scans of DNA samples separated on gels. A direct comparison of SSDS with agarose gel electrophoresis for +/- screening shows that it produces equivalent results. Advantages of SSDS include reduced sample size (i.e., lower reagent costs), rapid analysis of single samples, and potential for full automation using available technology. The high sensitivity of the method also allows the number of polymerase chain reaction cycles to be reduced to 15 or less. Because the high signal-to-noise ratio of the AFM allows for direct visualization of DNA-binding proteins, different DNA conformations, restriction enzymes, and other DNA modifications, there is potential for dramatically improving the information content in this type of analysis.