Biomedical subjects

D C Hunt

Publications and source records attributed to D C Hunt.

16 recordsLinked to original sources

PubMed2006

X-ray imaging with amorphous selenium: pulse height measurements of avalanche gain fluctuations.

Avalanche multiplication in amorphous selenium (a-Se) can provide a large, adjustable gain for active matrix flat panel imagers (AMFPI), enabling quantum noise limited x-ray imaging during both radiography and fluoroscopy. In the case of direct conversion AMFPI, the multiplication factor for each x ray is a function of its depth of interaction, and the resulting variations in gain can reduce the detective quantum efficiency (DQE) of the system. An experimental method was developed to measure gain fluctuations by analyzing images of individual x rays that were obtained using a video camera with an a-Se target operated in avalanche mode. Pulse height spectra (PHS) of the charge produced per x ray were recorded for monoenergetic 30.9, 49.4, and 73.8 keV x-ray sources. The rapid initial decay and long tail of each PHS can be explained by a model in which positive charge dominates the initiation of avalanche. The Swank information factor quantifies the effect of gain fluctuation on DQE and was calculated from the PHS. The information factor was found to be 0.5 for a 25 microm a-Se layer with a maximum gain of approximately 300. Changing the energy of the incident x ray influenced the range of the primary photoelectron and noticeably affected the tail of the experimental PHS, but did not significantly change the avalanche Swank factor.

Dose-Response Relationship, Radiation↗

PubMed2005

Indirect flat-panel detector with avalanche gain: fundamental feasibility investigation for SHARP-AMFPI (scintillator HARP active matrix flat panel imager).

An indirect flat-panel imager (FPI) with avalanche gain is being investigated for low-dose x-ray imaging. It is made by optically coupling a structured x-ray scintillator CsI(Tl) to an amorphous selenium (a-Se) avalanche photoconductor called HARP (high-gain avalanche rushing photoconductor). The final electronic image is read out using an active matrix array of thin film transistors (TFT). We call the proposed detector SHARP-AMFPI (scintillator HARP active matrix flat panel imager). The advantage of the SHARP-AMFPI is its programmable gain, which can be turned on during low dose fluoroscopy to overcome electronic noise, and turned off during high dose radiography to avoid pixel saturation. The purpose of this paper is to investigate the important design considerations for SHARP-AMFPI such as avalanche gain, which depends on both the thickness d(Se) and the applied electric field E(Se) of the HARP layer. To determine the optimal design parameter and operational conditions for HARP, we measured the E(Se) dependence of both avalanche gain and optical quantum efficiency of an 8 microm HARP layer. The results were used in a physical model of HARP as well as a linear cascaded model of the FPI to determine the following x-ray imaging properties in both the avalanche and nonavalanche modes as a function of E(Se): (1) total gain (which is the product of avalanche gain and optical quantum efficiency); (2) linearity; (3) dynamic range; (4) gain nonuniformity resulting from thickness nonuniformity; and (5) effects of direct x-ray interaction in HARP. Our results showed that a HARP layer thickness of 8 microm can provide adequate avalanche gain and sufficient dynamic range for x-ray imaging applications to permit quantum limited operation over the range of exposures needed for radiography and fluoroscopy.

Cesium↗

PubMed2004

Evaluation of the imaging properties of an amorphous selenium-based flat panel detector for digital fluoroscopy.

The imaging performance of an amorphous selenium (a-Se) flat-panel detector for digital fluoroscopy was experimentally evaluated using the spatial frequency dependent modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE). These parameters were investigated at beam qualities and exposures within the range typical of gastrointestinal fluoroscopic imaging (approximately 0.1 - 10 microR, 75 kV). The investigation does not take into consideration the detector cover, which in clinical use will lower the DQE measured here by its percent attenuation. The MTF was found to be less than the expected aperture response and the NPS was not white which together indicate presampling blurring. The cause of this blurring was attributed to charge trapping at the interface between two different layers of the a-Se. The effect on the DQE was also consistent with presampling blur, which reduces the aliasing in the NPS and thereby reduces the spatial frequency dependence of the DQE. (The DQE was independent of spatial frequency from 0.12 to 0.73 mm(-1) due to antialiasing of the NPS.) Moreover, the first zero of the measured MTF and the aperture response appeared at the same spatial frequency (6.66 mm(-1) for a pixel of 150 microm). Hence, the geometric fill factor (77%) was increased to an effective fill factor of 99 +/- 1%. A large scale ( approximately 32 pixels) correlation in the noise due to the configuration of the readout electronics caused increased noise power in the gate line NPS at low spatial frequency (< 0.1 mm(-1)). The DQE (f = 0) was exposure independent over a large range of exposures but became exposure dependent at low exposures due to the electronic noise.

Equipment Design↗

PubMed1998

X-ray imaging using amorphous selenium: determination of Swank factor by pulse height spectroscopy.

The use of photoconductors, especially amorphous selenium (a-Se), in x-ray imaging is currently of interest. A critical performance parameter of an imaging detector is the Swank factor for degradation of the signal to noise ratio, or DQE(0), due to variations in the detector response. The Swank factor is evaluated from measured pulse height spectra generated by the absorption of monoenergetic x-ray photons. The spectra show an additional width over previous theoretical expectations, but the Swank factor is still close to the high values previously predicted theoretically.

Americium↗

PubMed1993

Activation of bean (Phaseolus vulgaris) alpha-amylase inhibitor requires proteolytic processing of the proprotein.

Seeds of the common bean (Phaseolus vulgaris) contain a plant defense protein that inhibits the alpha-amylases of mammals and insects. This alpha-amylase inhibitor (alpha AI) is synthesized as a proprotein on the endoplasmic reticulum and is proteolytically processed after arrival in the protein storage vacuoles to polypeptides of relative molecular weight (M(r)) 15,000 to 18,000. We report two types of evidence that proteolytic processing is linked to activation of the inhibitory activity. First, by surveying seed extracts of wild accessions of P. vulgaris and other species in the genus Phaseolus, we found that antibodies to alpha AI recognize large (M(r) 30,000-35,000) polypeptides as well as typical alpha AI processing products (M(r) 15,000-18,000). Alpha AI activity was found in all extracts that had the typical alpha AI processed polypeptides, but was absent from seed extracts that lacked such polypeptides. Second, we made a mutant alpha AI in which asparagine-77 is changed to aspartic acid-77. This mutation slows down the proteolytic processing of pro-alpha AI when the gene is expressed in tobacco. When pro-alpha AI was separated from mature alpha AI by gel filtration, pro-alpha AI was found not to have alpha-amylase inhibitory activity. We interpret these results to mean that formation of the active inhibitor is causally related to proteolytic processing of the proprotein. We suggest that the polypeptide cleavage removes a conformational constraint on the precursor to produce the biochemically active molecule.

Adsorption↗

PubMed1991

The signal Peptide of a vacuolar protein is necessary and sufficient for the efficient secretion of a cytosolic protein.

A cytosolic pea (Pisum sativum) seed albumin (ALB) and a chimeric protein (PHALB) consisting of the signal peptide and first three amino acids of phytohemagglutinin (PHA) and the amino acid sequence of ALB were expressed in parallel suspension cultures of tobacco (Nicotiana tabacum) cells and their intracellular fates examined. PHALB was efficiently secreted by the cells whereas ALB remained intracellular. These experiments show that the information contained in the signal peptide of a vacuolar protein is both necessary and sufficient for efficient secretion, and define secretion as a default or bulk-flow pathway. Entry into the secretory pathway was accompanied by glycosylation and the efficient conversion of the high mannose glycans into complex glycans indicating that transported glycoproteins do not need specific recognition domains for the modifying enzymes in the Golgi. Tunicamycin depressed the accumulation of the unglycosylated polypeptide in the culture medium much less than the accumulation of other glycoproteins. We interpret this as evidence that glycans on proteins that are not normally glycosylated do not have the same function of stabilizing and protecting the polypeptide as on natural glycoproteins.

Journal Article↗

PubMed1987