Ramya Rajaram

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.

Design and characterization of a spatially distributed multibeam field emission x-ray source for stationary digital breast tomosynthesis

Digital breast tomosynthesis (DBT) is a limited angle computed tomography technique that can distinguish tumors from its overlying breast tissues and has potentials for detection of cancers at a smaller size and earlier stage. Current prototype DBT scanners are based on the regular full-field digital mammography systems and require partial isocentric motion of an x-ray tube over certain angular range to record the projection views. This prolongs the scanning time and, in turn, degrades the imaging quality due to motion blur. To mitigate the above limitations, the concept of a stationary DBT (s-DBT) scanner has been recently proposed based on the newly developed spatially distributed multibeam field emission x-ray (MBFEX) source technique using the carbon nanotube. The purpose of this article is to evaluate the performance of the 25-beam MBFEX source array that has been designed and fabricated for the s-DBT system. The s-DBT system records all the projection images by electronically activating the multiple x-ray beams from different viewing angles without any mechanical motion. The configuration of the MBFEX source is close to the published values from the Siemens Mammomat system. The key issues including the x-ray flux, focal spot size, spatial resolution, scanning time, beam-to-beam consistency, and reliability are evaluated using the standard procedures. In this article, the authors describe the design and performance of a distributed x-ray source array specifically designed for the s-DBT system. They evaluate the emission current, current variation, lifetime, and focal spot sizes of the source array. An emission current of up to 18 mA was obtained at 0.5 x 0.3 mm effective focal spot size. The experimentally measured focal spot sizes are comparable to that of a typical commercial mammography tube without motion blurring. Trade-off between the system spatial resolution, x-ray flux, and scanning time are also discussed. Projection images of a breast phantom were collected using the x-ray source array from 25 different viewing angles without motion. These preliminary results demonstrate the feasibility of the proposed s-DBT scanner. The technology has the potential to increase the resolution and reduce the imaging time for DBT. With the present design of 25 views, they demonstrated experimentally the feasibility of achieving 11 s scanning time at full detector resolution with 0.5 x 0.3 mm source resolution without motion blur. The flexibility in configuration of the x-ray source array will also allow system designers to consider imaging geometries that are difficult to achieve with the conventional single-source rotating approach.

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

Three-Dimensional Tomosynthesis Reconstruction from 1D and 2D X-ray Source Arrays

We study the effects of geometric design on the reconstruction of 3D images from an X-ray tomosynthesis system using microfabricated discrete X-ray sources. Carbon-nanotube-based field-emission X-ray sources can be fabricated in arrays; however, little is known about the effects of the geometry of such a system on reconstruction of tomosynthesis data. We produced simulated X-ray projection data for several source array geometries including a 1 times 11 array, and a 3 times 11 array. The phantom simulates a mammography task, with seven 400 mum spheres embedded in a uniformly-absorbing background. Data from both geometries was reconstructed using the ordered-subset convex (OSC) algorithm specially implemented for these array geometries. Reconstruction was performed on an 800 times 800 (lateral) times 20 (depth) array of noncubic voxels of size 200 mum times 200 mum (lateral) times 2.5 mm (depth). Contrast, resolution, and noise measurements on the reconstructed spheres were used to compare results from the different geometries. The reconstruction from the 2D array had higher contrast than that from the 1D array, but the 2D array image also suffered from higher levels of noise for the same total exposure. The contrast-to-noise ratio for the 2D array, was 5-17% higher than the 1D array. Other studies showed that staggering a 2D array is beneficial, and that increasing the density of sources is detrimental after a certain optimal density is reached We conclude that, in a tomosynthesis system such as that described here, a 2D source array will outperform a 1D array in terms of contrast-to-noise and resolution, while also increasing the usable field-of-view, and that further studies encompassing the effects of scatter will be needed to optimize such systems.

A dynamic micro-CT scanner with a stationary mouse bed using a compact carbon nanotube field emission x-ray tube

In this paper we report the development of a high resolution dynamic micro-computed tomography (CT) scanner with a stationary mouse bed using a compact carbon nanotube (CNT) x-ray tube. The scanner comprises a rotating x-ray tube and detector pair and a stationary and a horizontally positioned small animal bed. The system is optimized for in vivo mouse cardiac imaging. Its performance is evaluated with CT scans of phantoms and free-breathing mice. The modulation transfer function (MTF) at 10% is 5 lp/mm. At single frame acquisition, mouse cardiac micro-CT at 20msec temporal resolution has been demonstrated by prospectively gating the imaging acquisitions to both respiration and cardiac signals.

Respiratory-gated micro-CT using a carbon nanotube based micro-focus field emission x-ray source

A prototype physiologically gated micro-computed tomography (micro-CT) system based on a field emission micro-focus x-ray source has been developed for in vivo imaging of small animal models. The novel x-ray source can generate radiation with a programmable waveform that can be readily synchronized and gated with non-periodic physiological signals. The system performance is evaluated using phantoms and sacrificed and anesthetized mouse models. Prospective respiratory-gated CT images of anesthetized free-breathing mice are collected using this scanner at 100msec temporal resolution and 10 lp/mm of 10% system MTF.

Image reconstruction for a stationary digital breast tomosynthesis system

We have designed and built a stationary digital breast tomosynthesis (DBT) system containing a carbon nanotube based field emission x-ray source array to examine the possibility of obtaining a reduced scan time and improved image quality compared to conventional DBT systems. There are 25 individually addressable x-ray sources in our linear source array that are evenly angularly spaced to cover an angle of 48°. The sources are turned on sequentially during imaging and there is no motion of either the source or the detector. We present here an iterative reconstruction method based on a modified Ordered-Subset Convex (MOSC) algorithm that was employed for the reconstruction of images from the new DBT system. Using this algorithm based on a maximum-likelihood model, we reconstruct on non-cubic voxels for increased computational efficiency resulting in high in-plane resolution in the images. We have applied the reconstruction technique on simulated and phantom data from the system. Even without the use of the subsets, the reconstruction of an experimental 9-beam system with 960×768 pixels took less than 6 minutes (10 iterations). The projection images of a simulated mammography accreditation phantom were reconstructed using MOSC and a Simultaneous Algebraic Reconstruction technique (SART) and the results from the comparison between the two algorithms allow us to conclude that the MOSC is capable of delivering excellent image quality when used in tomosynthesis image reconstruction.

SU-FF-I-40: A Novel Gantry-Free DBT System Using a Stationary Multi-Beam Field Emission X-Ray Source Array Based On Carbon Nanotubes (CNTs)

Purpose: To test the feasibility of a novel gantry‐free digital breast tomosynthesis (DBT) system using a stationary multi‐beam field emission X‐ray source array based on carbon nanotubes(CNTs) and to compare the system performance with the conventional devices.Method and Materials: Two tomosynthesisimagingsystems have been built: a compact model and a full scale model, which contain 9 and 25 individual X‐ray pixels, respectively. The compact model can only image a partial breast phantom due to the limited span of the X‐ray source array and the small FOV of the detector. The full scale system is capable of full field digital mammography by utilizing a detector with 20‐cm FOV. The geometry of the full scale system is also comparable to the conventional DBT devices (refer to the supporting material). The system geometry, such as the source to detector distance (SDD) and X‐ray source position, is calibrated. The slice images at different depths are reconstructed using ordered subset convex (maximum likelihood) method. The system performance is evaluated by measuring parameters such as MTF and SNR. Results: By eliminating the rotary gantry, the system design is simplified and the issue if image blurring due to x‐ray source motion is removed. The total scan time can potentially be further shortened with a faster detector readout speed. Conclusion: By eliminating the rotary gantry, the system noise and equipment cost of the tomosynthesisimagingsystem are reduced. The total scan time can be further shortened with faster detector readout speed. The novel stationary tomosysthesis system shows great potential in clinical imaging.

WE-D-304A-09: Characterization of Multi-Beam Field Emission X-Ray Source for Stationary Digital Breast Tomosynthesis

Purpose: The current prototype digital breast tomosynthesis (DBT) scanners are based on the regular full‐field digital mammography systems and require partial isocentric motion of a mammography x‐ray tube over certain angular range to record the projection views needed for reconstruction. This prolongs the scanning time and in turn degrades the imaging quality due to motion blur. We are developing a stationary DBT (s‐DBT) scanner to mitigate the above limitations. Method and Materials: The proposed s‐DBT system is based on the carbon nanotube multi‐pixel field emission x‐ray (MBFEX) technology demonstrated by our group. The pixilated and spatially distributed MBFEX source can generate x‐ray radiation from multiple views without any mechanical motion of the source, detector, or object. This enables the design of tomography systems with great flexibility in source configuration and imaging sequence. It further enables multiplexing imaging — simultaneously collection of multiple images using one detector.Results and Conclusions: To demonstrate the feasibility of the s‐DBT scanner, we have designed and constructed a proof‐of‐concept full‐field s‐DBT system. The configuration of the system, in terms of angular coverage, number of views, and dose, etc., closely resembles the Siemens DBT scanner for a more realistic comparison. In the Siemens DBT scanner, the x‐ray tube moves along an arc with the exposure points evenly distributed along the rotation route. Our s‐DBT scanner is designed with the MBFEX pixels positioned in a straight line parallel to the detector plane. We will report a detailed study on the design and the performance characteristics of the MBFEX source. In particular evaluation on key parameters including the x‐ray flux, the lifetime of the x‐ray source, effective x‐ray focal spot size, variation between different source, system spatial resolution, and x‐ray energy spectrum will be reported. These preliminary results demonstrate the feasibility of the proposed s‐DBT scanner.