Stationary intraoral digital tomosynthesis using a carbon nanotube X-ray source array

OBJECTIVES

Intraoral dental tomosynthesis and closely related tuned-aperture CT (TACT) are low-dose three-dimensional (3D) imaging modalities that have shown improved detection of multiple dental diseases. Clinical interest in implementing these technologies waned owing to their time-consuming nature. Recently developed carbon nanotube (CNT) X-ray sources allow rapid multi-image acquisition without mechanical motion, making tomosynthesis a clinically viable technique. The objective of this investigation was to evaluate the feasibility of and produce high-quality images from a digital tomosynthesis system employing CNT X-ray technology.

METHODS

A test-bed stationary intraoral tomosynthesis unit was constructed using a CNT X-ray source array and a digital intraoral sensor. The source-to-image distance was modified to make the system comparable in image resolution to current two-dimensional intraoral radiography imaging systems. Anthropomorphic phantoms containing teeth with simulated and real caries lesions were imaged using a dose comparable to D-speed film dose with a rectangular collimation. Images were reconstructed and analysed.

RESULTS

Tomosynthesis images of the phantom and teeth specimen demonstrated perceived image quality equivalent or superior to standard digital images with the added benefit of 3D information. The ability to "scroll" through slices in a buccal-lingual direction significantly improved visualization of anatomical details. In addition, the subjective visibility of dental caries was increased.

CONCLUSIONS

Feasibility of the stationary intraoral tomosynthesis is demonstrated. The results show clinical promise and suitability for more robust observer and clinical studies.

A first generation compact microbeam radiation therapy system based on carbon nanotube X-ray technology

We have developed a compact microbeam radiation therapy device using carbon nanotube cathodes to create a linear array of narrow focal line segments on a tungsten anode and a custom collimator assembly to select a slice of the resulting wedge-shaped radiation pattern. Effective focal line width was measured to be 131 μm, resulting in a microbeam width of ∼300 μm. The instantaneous dose rate was projected to be 2 Gy/s at full-power. Peak to valley dose ratio was measured to be >17 when a 1.4 mm microbeam separation was employed. Finally, multiple microbeams were delivered to a mouse with beam paths verified through histology.

WE-C-217BCD-03: Restricted Data Set Reconstruction Based on Respiration Quality to Improve Prospectively Gated in Vivo Micro-CT of Mice

PURPOSE

Micro-CT is commonly employed for lung imaging of mice; prospective gating allows for in-vivo imaging of free-breathing subjects. While this technique is successfully executed for healthy animals, results are less consistent for some disease models whose symptoms include irregular or unstable respiration. The purpose of this work is to repair the quality of high-blur images that arise from respiration instability using a retrospective method of motion reduction which identifies the individual x-ray projection images contributing most to the motion blur. Reconstructions were performed after the exclusion of these projections (the so-called restricted set).

METHODS

Sixteen mice were imaged using field emission cone beam micro-CT and prospective gating with a bellows-type respiration sensor. The scanner was operated in step-and-shoot mode; 400 projection images were acquired per scan. An algorithm was developed to analyze the respiration trace file and segment the individual breath corresponding to each projection image. We tested three different criteria to define a bad breath shape (correlation, mean breath height, or mode breath height), and restricted data set reconstructions were performed using each of these criteria to exclude projections corresponding to bad breaths. Each restricted set was compared against the full unrestricted data set image; the slope perpendicular to the diaphragm was used as a quantitative assessment of motion blur.

RESULTS

All image sets saw a reduction in motion blur with at least one restriction technique. In 22 of 27 images, improvement was measured regardless of the removal criterion. Five percent total projection removal is optimal; a more aggressive correction increases the likelihood of under-sampling artifacts.

CONCLUSIONS

Removing a subset of bad projections from otherwise complete image sets measurably decreases motion blur in respiratory-gated imaging. An approach based on breath height generally provides the best results. The technique is applicable to a variety of imaging modalities.

SU-D-217A-01: A High-Resolution in Vivo Molecular Imaging Technique Based on X- Ray Fluorescence

PURPOSE

Traditional molecular imaging techniques such as PET/SPECT have many limitations, including relatively low spatial resolution, short lifetime of radioisotope probes, limited availability due to reliance on cyclotron, relatively high dose, and lack of effective molecular probes for certain tumor cells. In this work we demonstrate the feasibility of a novel x- ray fluorescence molecular imaging (XFMI) technique using high-power carbon nanotube (CNT) x-ray array technology. The XFMI overcomes some limitations and will be a significant advance in molecular imaging technology for cancer drug development and cancer biology research.

METHODS

A testing chamber was constructed containing two Amptek energy-resolving detectors analyzing a copper collimated fluorescence beam placed opposite each other symmetric about the sample and perpendicular to the primary incoming x-ray beam, also using the copper collimator. Different concentrations of indium trichloride and iodine were tested to determine the minimum detectable concentration (MDC) for each.

RESULTS

The MDC for indium was found to be 80 ug/mL and 100 ug/mL for iodine at 50 kVp, 30mAs (5 minutes imaging time). This is on the order of magnitude of the MDCs determined at large synchrotron facilities using XFI. Higher concentrations above 1mg/mL of both elements were detectable at 1.5 mAs (15 seconds).

CONCLUSIONS

These results show that it is possible to not only measure low concentrations of the above elements, but also distinguish between similar L-alpha peaks. This allows for future work of obtaining 2D and 3D imaging to determine element types and concentrations diffused in different parts of the body. Carolina Center of Cancer Nanotechnology, University Cancer Research Fund.

Design and feasibility studies of a stationary digital breast tomosynthesis system

Studies have shown that digital breast tomosynthesis (DBT) can improve breast cancer diagnosis by reconstructing 3D images. However, DBT scanners based on rotation gantry prolong the imaging time and reduce spatial resolution due to motion comparing with the regular two-view mammography. To obtain three dimension reconstruction images and maintain the high image quality of conventional mammography, we proposed a prototype stationary digital breast tomosynthesis system (s-DBT). The proposed s-DBT system acquires projection images without mechanical movement. The core component of the s-DBT system is a specially designed spatially distributed multi-beam x-ray tube based on the carbon nanotube field emission x-ray technology. The multi-beam x-ray source array enables collection of all projection images from different viewing angles without mechanical motion. Preliminary results show the s-DBT system can achieve a scan time comparable to the regular two-view mammography, and improve the spatial resolution comparing with rotating gantry DBT.

Stationary digital breast tomosynthesis with distributed field emission X-ray tube

Tomosynthesis requires projection images from different viewing angles. Using a distributed x-ray source this can be achieved without mechanical motion of the source with the potential for faster image acquisition speed. A distributed x-ray tube has been designed and manufactured specifically for breast tomosynthesis. The x-ray tube consists of 31 field emission x-ray sources with an angular range of 30°. The total dose is up to 100mAs with an energy range between 27 and 45 kVp. We discuss the source geometry and results from the characterization of the first prototype. The x-ray tube uses field emission cathodes based on carbon nanotubes (CNT) as electron source. Prior to the manufacturing of the sealed x-ray tube extensive testing on the field emission cathodes has been performed to verify the requirements for commercial tomosynthesis systems in terms of emission current, focal spot size and tube lifetime.

Aligned Carbon Nanotubes Via Microwave Plasma Enhanced Chemical Vapor Deposition

Aligned multi-wall carbon nanotubes have been grown on silicon substrates by microwave plasma enhanced chemical vapor deposition using methane/ammonia mixtures. The concentration ratio of methane/ammonia in addition to substrate temperature was varied. The morphology, structure and alignment of carbon nanotubes were studied by scanning electron microscopy and transmission electron microscopy. Both concentric hollow and bamboo-type multi-wall carbon nanotubes were observed. Growth rate, size distribution, alignment, morphology, and structure of carbon nanotubes changed with methane/ammonia ratio and growth temperature. Preliminary results on field emission properties are also presented.

Electronic Structures of Single-Walled Carbon Nanotubes Studied by NMR

An individual single-walled carbon nanotube (SWNT) has been shown [1-4] to exhibit remarkable electronic properties which depend on its diameter and chirality. In this work, the 13C nuclear magnetic resonance technique is used to measure quantitatively the electronic density-of-state (DOS) at the Fermi level in a bulk SWNT sample. Two types of 13C nuclear spins are observed with drastically different nuclear spin-lattice relaxation time (Tl). About onethird of the 13C nuclear spins with shorter Tl, are identified to reside at the metallic SWNTs and two-thirds can be associated with the semiconducting SWNTs. For the metallic SWNTs, the DOS at the Feimi level is measured quantitatively, which is about 0.022 states/(eV·atom·spin). The measured electronic DOS at the Fermi level agrees with the theoretical prediction for metallic tubes. This study also found that the semiconducting SWNTs in bundles, in fact, possess weak metallic characters. This indicates that tube-tube interactions within SWNT bundles could change the electronic properties.

Fabrication and Field Emission Properties of Carbon Nanotube Cathodes

A variety of carbon nanotube films have been fabricated and tested as cold cathodes. A spray deposition technique was developed for processing as-grown bulk nanotubes, both single-walled and multi-walled, into films of randomly oriented nanotubes. Films of randomly oriented multi-walled nanotubes were grown using thermal chemical vapor deposition, and arrays of well-aligned multi-walled nanotubes have been fabricated using a microwave plasma enhanced chemical vapor deposition technique. The emission current-voltage (I-V) characteristics of these nanotube cathodes have been measured. Both multi-walled (random and aligned) and single-walled carbon nanotubes exhibit low turn-on fields (∼ 2 V/μm to generate 1 nA) and threshold fields (< 5 V/μm to generate 10 mA/cm2). Significantly, these cathodes were capable of operation at very large current densities (> 1A/cm2), making them candidates for application in a variety of vacuum microelectronic devices.

Distributed source x-ray tube technology for tomosynthesis imaging

Tomosynthesis imaging requires projection images from different viewing angles. Conventional systems use a moving xray source to acquire the individual projections. Using a stationary distributed x-ray source with a number of sources that equals the number of required projections, this can be achieved without any mechanical motion. Advantages are a potentially faster image acquisition speed, higher spatial and temporal resolution and simple system design. We present distributed x-ray sources based on carbon nanotube (CNT) field emission cathodes. The field emission cathodes deliver the electrons required for x-ray production. CNT emitters feature a stable emission at high current density, a cold emission, excellent temporal control of the emitted electrons and good configurability. We discuss the use of stationary sources for two applications: (i) a linear tube for stationary digital breast tomosynthesis (sDBT), and (ii) a square tube for on-board tomosynthesis image-guided radiation therapy (IGRT). Results from high energy distributed sources up to 160kVp are also presented.

A dynamic micro-CT scanner based on a carbon nanotube field emission x-ray source

Current commercial micro-CT scanners have the capability of imaging objects ex vivo with high spatial resolution, but performing in vivo micro-CT on free-breathing small animals is still challenging because their physiological motions are non-periodic and much faster than those of humans. In this paper, we present a prototype physiologically gated micro-computed tomography (micro-CT) scanner based on a carbon nanotube field emission micro-focus x-ray source. The novel x-ray source allows x-ray pulses and imaging sequences to be readily synchronized and gated to non-periodic physiological signals from small animals. The system performance is evaluated using phantoms and sacrificed and anesthetized mice. Prospective respiratory-gated micro-CT images of anesthetized free-breathing mice were collected using this scanner at 50 ms temporal resolution and 6.2 lp mm(-1) at 10% system MTF. The high spatial and temporal resolutions of the micro-CT scanner make it well suited for high-resolution imaging of free-breathing small animals.