Biomedical subjects

David R Pickens

Publications and source records attributed to David R Pickens.

8 recordsLinked to original sources

PubMed2005

Characterization of the phase-contrast radiography edge-enhancement effect in a cabinet x-ray system.

The purpose of this study was to demonstrate that a commercially available cabinet x-ray system is capable of phase-contrast radiography (PC-R) and to evaluate the effect of different system parameters on the degree of edge enhancement. An acrylic plastic edge phantom was imaged at different tube potentials (25-60 kV) and in different geometries (variable object-to-detector distances, R(2), at a constant source-to-detector distance, R(1) + R(2)). In addition, the effect of noise on the perceived edge enhancement was studied as a function of exposure time. Our results show that a modest degree of phase contrast can be achieved in an unmodified cabinet x-ray system. In addition, the particular system evaluated allowed low-noise PC-R images to be obtained with short (6 s or less) exposures. These results suggest that with appropriate geometric choices PC-R is already available to a wide range of research scientists for use in both small-animal and human-specimen experiments.

Equipment Design↗

PubMed2005

Development of computer-generated phantoms for FMRI software evaluation.

Functional magnetic resonance imaging (FMRI) is a major tool for the evaluation of brain function and architecture. It is widely used by physicians, neuroscientists, psychologists and others. In order to process the data collected using FMRI, it is necessary to use post-acquisition processing software that employs motion correction and statistical modeling capabilities. These types of programs permit the user to extract the information about areas of brain activations that have occurred during a study. How well a particular motion-correction technique works and what effect it has on statistical processing are difficult to evaluate, since the level of activation present is not known a priori. This paper provides a description of the construction of a software phantom for use with FMRI post-acquisition processing tools with the properties that it is based on real subject data, has known locations and levels of activation, has known amounts of rigid body motion and noise added, and can be used to evaluate a processing system as if it were a real data set. Versions of the software phantom are available for downloading at the website: .

Humans↗

PubMed2005

Experimental validation of the Wigner distributions theory of phase-contrast imaging.

Recently, a new theory of phase-contrast imaging has been proposed by Wu and Liu [Med. Phys. 31, 2378-2384 (2004)]. This theory, based upon Wigner distributions, provides a much stronger foundation for the evaluation of phase-contrast imaging systems than did the prior theories based upon Fresnel-Kirchhoff diffraction theory. In this paper, we compare results of measurements made in our laboratory of phase contrast for different geometries and tube voltages to the predictions of the Wu and Liu model. In our previous publications, we have used an empirical measurement (the edge enhancement index) to parametrize the degree of phase-contrast effects in an image. While the Wu and Liu model itself does not predict image contrast, it does measure the degree of phase contrast that the system can image for a given spatial frequency. We have found that our previously published experimental results relating phase-contrast effects to geometry and x-ray tube voltage are consistent with the predictions of the Wu and Liu model.

Algorithms↗

PubMed2005

Extracting data from a DICOM file.

DICOM v3.0 is a vendor-independent standard for digital medical images that describes a file format and network protocol for the exchange of images between computer systems. When simply viewing a DICOM file, it is not necessary for the user to understand the details of the entire DICOM standard. However, understanding parts of the standard is essential when DICOM files are read and processed by a user-generated program. This paper offers an overview of information a user needs to write a program for extracting the image and acquisition parameters from a DICOM file.

Computer Communication Networks↗

PubMed2003

Experimental model for functional magnetic resonance imaging of somatic sensory cortex in the unanesthetized rat.

Functional magnetic resonance imaging (fMRI) has evolved into a method widely used to map neural activation in the human brain. fMRI is a method for recording blood oxygen level-dependent (BOLD) signals. These signals change with local cerebral blood flow coupled to neural activity. However, the relationship between BOLD signals and neural function is poorly understood and requires the development of animal models. Here we use an unanesthetized rat preparation to study BOLD responses to whisker stimulation in somatic sensory barrel cortex. Five rats were trained to tolerate restraint in a holder and fMRI noise with positive reinforcement. For maximal immobilization, the head was fastened to the holder with nuts screwed on threaded bolts attached to the head. On scanning day, residual stress was alleviated with injections of diazepam, and the rats were restrained in the holder and transferred into the scanner. After >75 min to allow the tranquilization to abate, structural images were acquired from three coronal brain slices. Subsequently, functional images were taken utilizing 4-min epochs without stimulation alternated with equivalent epochs during which the right caudal whiskers were stimulated with three air puffs/s. After 4 weeks, fMRI could be repeated in four rats. In seven of the nine functional runs, head motion was minimal and whisker stimulation resulted in a statistically significant (P </= 0.05) increase in BOLD signal in barrel cortex predominantly on the contralateral side. The results provide encouragement that long-term fMRI studies on cerebral function in unanesthetized rats may be feasible with our procedure.

Animals↗

PubMed2003

Brain fMRI activation associated with self-paced finger tapping in chronic alcohol-dependent patients.

BACKGROUND: Fine and gross motor dysfunction in chronic alcoholic patients is prevalent, but not extensively studied. Brain autopsy studies of brain regions involved in motor movements indicate cerebellum and frontal lobes are particularly sensitive to alcohol-induced damage, in contrast to motor cortex. METHODS: Using functional magnetic resonance imaging (fMRI), we compared the pattern of activation of the cerebral cortex and cerebellum during repetitive, self-paced dominant (DH) and nondominant (NDH) index finger tapping in eight uncomplicated alcohol-dependent patients after approximately 2 weeks of abstinence and in nine normal controls. RESULTS: Whereas alcoholic patients tapped significantly more slowly than normal controls, a greater percentage of pixels were activated in the ipsilateral cortex during DH tapping. Furthermore, alcoholics tapped significantly less efficiently (tapping rate divided by percent pixels activated [weighted by pixel intensity] in a given region of interest [ROI]) than normal controls in every ROI examined while using DH, but only in ipsilateral hemi-cerebellum using NDH. Finally, the alcohol-dependent patients did not demonstrate the greater mean pixel activation, percentage activated pixels, and lesser activation efficiency in the ipsilateral cortex during NDH compared to DH tapping that was observed in the normal control group. CONCLUSIONS: These findings are compatible with motor inefficiency and compensatory alterations of cortical-cerebellar circuits. Further studies are needed to determine whether these deficits recover with prolonged abstinence and how they relate to cognitive inefficiency throughout the clinical course of alcoholism.

Adult↗

PubMed2003

Dual focal-spot imaging for phase extraction in phase-contrast radiography.

The purpose of this study was to evaluate dual focal spot imaging as a method for extracting the phase component from a phase-contrast radiography image. All measurements were performed using a microfocus tungsten-target x-ray tube with an adjustable focal-spot size (0.01 mm to 0.045 mm). For each object, high-resolution digital radiographs were obtained with two different focal spot sizes to produce matched image pairs in which all other geometric variables as well as total exposure and tube kVp were held constant. For each image pair, a phase extraction was performed using pixel-wise division. The phase-extracted image resulted in an image similar to the standard image processing tool commonly referred to as "unsharp masking" but with the additional edge-enhancement produced by phase-contrast effects. The phase-extracted image illustrates the differences between the two images whose imaging parameters differ only in focal spot size. The resulting image shows effects from both phase contrast as well as geometric unsharpness. In weakly attenuating materials the phase-contrast effect predominates, while in strongly attenuating materials the phase effects are so small that they are not detectable. The phase-extracted image in the strongly attenuating object reflects differences in geometric unsharpness. The degree of phase extraction depends strongly on the size of the smallest focal spot used. This technique of dual-focal spot phase-contrast radiography provides a simple technique for phase-component (edge) extraction in phase-contrast radiography. In strongly attenuating materials the phase-component is overwhelmed by differences in geometric unsharpness. In these cases the technique provides a form of unsharp masking which also accentuates the edges. Thus, the two effects are complimentary and may be useful in the detection of small objects.

Algorithms↗

PubMed2003

Quantification of the effect of system and object parameters on edge enhancement in phase-contrast radiography.

The purpose of this study was to evaluate the effects of system parameters (focal spot size, tube voltage, geometry, detector resolution, and image noise) and object characteristics (edge gradient/ shape, composition, thickness, and overlying attenuating material) upon the edge enhancement effect in phase-contrast radiography. Each variable of interest was adjusted and images of a 3 mm lucite phantom were obtained with the other variables remaining constant. A microfocus x-ray source coupled to a CCD camera with an intensifying screen was used to acquire the digital images. Two parameters of image analysis were used to quantify the effects. The edge enhancement index (EEI) was used to measure the absolute degree of edge enhancement, while the edge enhancement to noise ratio (EE/N) was used to measure the conspicuity of the edge enhancement relative to image noise. Little effect on EEI was seen from tube voltage, object thickness, overlying attenuating material, while focal spot size and system geometry demonstrated measurable effects upon the degree of edge enhancement. It was also shown that while the edge enhancement effect over straight edges is highly dependent upon how the edge aligns with the x-ray beam, rounded edges, which better model biological objects, do not suffer from this dependence and the EEI reaches its maximal level at any alignment. Decreasing detector resolution diminished the EEI slightly, but even with pixel sizes of 0.360 x 0.360 mm edge enhancement effects were readily visible. The effect of image noise on EE/N was evaluated using different exposure times showing an expected improvement with longer exposure time with EE/N approaching a plateau at 5 min. Many of the parameters that will go into the design of a future PC-R imaging system have been quantified in terms of their effect on the degree of edge enhancement in the acquired image. These results, taken together, indicate that either a specimen or even clinical breast imaging system could be created with currently available technology. The major limitation to a clinical system would be the low x-ray flux from the microfocal x-ray source.

Pattern Recognition, Automated↗