Otto Zhou

Stationary scanning x-ray source based on carbon nanotube field emitters

We report a field emission x-ray source that can generate a scanning x-ray beam to image an object from multiple projection angles without mechanical motion. The key component of the device is a gated carbon nanotube field emission cathode with an array of electron emitting pixels that are individually addressable via a metal–oxide–semiconductor field effect transistor-based electronic circuit. The characteristics of this x-ray source are measured and its imaging capability is demonstrated. The device can potentially lead to a fast data acquisition rate for laminography and tomosynthesis with a simplified experimental setup.

Carbon nanotube based microfocus field emission x-ray source for microcomputed tomography

Microcomputed tomography is now widely used for in vivo small animal imaging for cancer studies. Achieving high imaging quality of live objects requires the x-ray source to have both high spatial and temporal resolutions. Preliminary studies have shown that carbon nanotube (CNT) based field emission x-ray source has significant intrinsic advantages over the conventional thermionic x-ray tube including better temporal resolution and programmability. Here we report the design and characterization of a CNT based field emission x-ray source that also affords a high spatial resolution. The device uses modified asymmetric Einzel lenses for electron focusing and an elliptical shaped CNT cathode patterned by photolithography. Stable and small isotropic x-ray focal spot sizes were obtained.

Multiplexing radiography using a carbon nanotube based x-ray source

Speed and temporal resolution are critical for tomographic imaging of objects in rapid motion. Current x-ray scanners record images sequentially in the time domain. The serial approach limits their performance and demands increasingly high x-ray peak power and gantry speed. We have developed a multipixel carbon nanotube based field emission x-ray source that produces spatially and temporally modulated radiations. Using this device we show the feasibility of multiplexing radiography that enables simultaneous collection of multiple projection images using frequency multiplexing. A drastic increase of the speed and reduction of the x-ray peak power are achieved without compromising the imaging quality.

Stationary digital breast tomosynthesis system with a multi-beam field emission x-ray source array

A stationary digital breast tomosynthesis (DBT) system using a carbon nanotube based multi-beam field emission x-ray (MBFEX) source has been designed. The purpose is to investigate the feasibility of reducing the total imaging time, simplifying the system design, and potentially improving the image quality comparing to the conventional DBT scanners. The MBFEX source consists of 25 individually programmable x-ray pixels which are evenly angular spaced covering a 48° field of view. The device acquires the projection images by electronically switching on and off the individual x-ray pixels without mechanical motion of either the x-ray source or the detector. The designs of the x-ray source and the imaging system are presented. Some preliminary results are discussed.

Tomosynthesis reconstruction from multi-beam X-ray sources

We investigate methods for reconstructing tomosynthesis data using arrays of microfabricated X-ray sources and area CCD detectors. Tomosynthesis is a 3D imaging technique for limited-angle tomography that uses multiple radiographic images taken from an X-ray source placed at several positions to estimate a 3D distribution of X-ray attenuation. In our implementation, the moving X-ray source is replaced with multiple carbon nanotube field-emission X-ray sources fabricated on a single wafer. The sources are individually addressable, so the motion of the single X-ray source in traditional tomosynthesis is replaced by sequential sampling of the individual sources. Reconstruction is performed using the ordered-subsets convex (OSC) algorithm with pre-computed geometry factors and an image shearing technique for efficiency. Because of the limited resolution of tomosynthesis along the primary direction of projection, reconstructions are computed on non-cubic voxels for relatively thick slabs. We demonstrate the capabilities of the reconstruction technique on simulated data and breast phantom data from a prototype device. Reconstructions for an 11-beam system with 1000times1000 pixels per projection onto a 800times800times20 grid require 55 minutes ( 10 iterations) on a 3.2 GHz workstation with 2 GB memory. We conclude that this implementation of the OSC algorithm is effective for reconstructing tomosynthesis datasets

Microwave plasma enhanced chemical vapor deposition growth of few-walled carbon nanotubes using catalyst derived from an iron-containing block copolymer precursor

The microwave plasma enhanced chemical vapor deposition (MPECVD) method is now commonly used for directional and conformal growth of carbon nanotubes (CNTs) on supporting substrates. One of the shortcomings of the current process is the lack of control of the diameter and diameter distribution of the CNTs due to difficulties in synthesizing well-dispersed catalysts. Recently, block copolymer derived catalysts have been developed which offer the potential of fine control of both the size of and the spacing between the metal clusters. In this paper we report the successful growth of CNTs with narrow diameter distribution using polystyrene-block-polyferrocenylethylmethylsilane (PS-b-PFEMS) as the catalyst precursor. The study shows that higher growth pressure leads to better CNT growth. Besides the pressure, the effects on the growth of CNTs of the growth parameters, such as temperature and precursor gas ratio, are also studied.

Multiplexing radiography based on carbon nanotube field emission x- ray technology

State-of-the-art tomographic imaging technique is based upon of simple serial imaging scheme. The tomographic scanners collect the projection images sequentially in the time domain, by a step-and-shoot process using a single-pixel x-ray source. The inefficient serial data collection scheme severely limits the data collection speed, which is critical for imaging of objects in rapid motion such as for diagnosis of cardiovascular diseases, CT fluoroscopy, and airport luggage inspection. Further improvement of the speed demands an increasingly high x-ray peak workload and gantry rotation speed, both of which have approached the engineering limits. Multiplexing technique, which has been widely adopted in communication devices and in certain analytical instruments, holds the promise to significantly increase the data throughput. It however, has not been applied to x-ray radiography, mainly due to limitations of the current x-ray source technology. Here we report a method for frequency multiplexing radiography (FMR) based on the frequency multiplexing principle and the carbon nanotube field emission x-ray technology. We show the feasibility of multiplexing radiography that enables simultaneous collection of multiple projection images. It has the potential to significantly increase the imaging speed for tomographic imaging without compromising the imaging quality.

Fabrication and Characterization of Individually Controlled Multi- Pixel Carbon Nanotube Cathode Array Chip for Micro-RT Application for Cancer Research

We report here the development of a new carbon nanotube (CNT) field emission multi-pixel cathode array chip, a vital component for the multi-pixel beam x-ray micro-radiotherapy (micro-RT) system under development in our group for cancer research. The CNT field emission cathode array chip has up to 25 (5 × 5) individually addressable cathode pixels, each 1 mm in diameter and with center-to-center distance of 2 mm. The fabrication is a two-step process: first a Cr/Cu electrical contact was fabricated on Si substrates with a 5 μm SiO(2) dielectric layer using photolithography; and second the CNTs were selectively deposited on 1 mm-diameter predefined Cr/Cu contact dots by using a combined photolithography/electrophoresis deposition technique. The electron pixel beams produced from the multi-pixel array chips are uniform and individually controllable. Each pixel beam is expected to generate a dose rate in the order of 100 cGy/min based on our Monte Carlo simulations.