Multiple beam field emission x-ray source

As was described in Chapter 1, x-ray imaging applications such as computed tomography and tomosynthesis require the acquisition of multiple two- dimensional projection views of the object/patient that are then reconstructed to give back three-dimensional object information. Most of the commercial tomographic scanners have a design where a single x-ray source (and a corresponding detector) rotates about the stationary object. The mechanical movement involved in such a source design greatly affects total scan time and image quality.

While other novel systems have been tried and tested, like the electron beam CT [1], they have not been altogether successful because of issues of size and cost. In addition, the angular range offered by these systems is often limited. The other alternative is to use many spatially distributed cathodes that then combined with their corresponding anodes can act as separate and possibly individually addressable x-ray sources. As was discussed in Chapter 1, carbon nanotubes (CNTs) are excellent field emitters and it is also easy to miniaturize the CNT based x-ray sources. Based on this idea and the development of a

single CNT cathode based system, we were able to build a multi-beam x-ray source.

The multi-beam field emission x-ray (MBFEX) source consists of five CNT cathodes, focusing stages and a molybdenum anode target. The whole set-up was arranged in a vacuum chamber with a beryllium window. The set-up of the linear MBFEX source based system is shown in figure 3.1. Multiple metal-oxide semiconductor field effect transistors (MOSFETs) were used to individually control the x-ray sources. This preliminary system was characterized by measuring the field emission current from the sources in triode mode and by focal spot size measurements [2].

Figure 3.1 The multi-beam field emission x-ray system and schematic of a single source (below) (a) The system has five cathodes, focusing electrodes and a molybdenum target. The cathode current was uniformly 1 mA with varying gate voltages across the sources and the system was operated at 40 kVp. (b) A single x-ray source consists of a 1.5 mm cathode on a metal disk, a 150

µm dielectric spacer, an extraction gate, focusing electrode and a metal anode target [2].

Further, the MBFEX system was then used to obtain five projection images of a blade from a surgical scalpel placed behind a metal rod [2]. Imaging

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was done at 40 kVp using a digital x-ray detector (Hamamatsu C7921) running at 16 frames/second. From these images, it could be seen that while the blade and the metal rod appeared as one in the central projection view, the other projection views were able to separate out the two objects. The acquisition time for the images in this system was only determined by the x-ray exposure time as there is no mechanical movement and electrical switching time is really negligible only. Another set of nine MBFEX sources over a larger angular range was used to demonstrate potential tomosynthesis applications. This system comprised of nine x-ray sources, a flat-panel detector, and a computer to synchronize the x-ray source and detector and to save the projection images. A schematic of the nine- beam system together with three of the nine projection views acquired of a commercial full size stereotactic needle biopsy tissue equivalent phantom are shown in figure 3.2.

Figure 3.2 The nine-beam imaging system and projection images.

A schematic of the nine-beam imaging system with three of the nine projection images acquired of a mammography phantom showing some of the mass-like objects.

Stereotactic needle biopsy tissue equivalent breast phantom detector Multi-beam field emission x-ray sources

At that time, due to the limited field of view afforded by the small detector, only a portion of the breast phantom containing the masses and calcifications was imaged. The nine sources were spaced 1.14 cm apart and the source to detector distance was about 19.3 cm. The total angular coverage is therefore about 25°. About 200 pixels in the depth direction fr om the nine projection images of the phantom (1000 x 200 x 9) were reconstructed with non-cubic voxels of 1 mm x 0.1 mm x 0.1 mm using an appropriate algorithm [3]. The reconstructed slices through the phantom are shown in figure 3.3.

Figure 3.3 Reconstructed slices of the breast phantom obtained from the preliminary system. The slices through the breast phantom (from two different regions of interest) show how the overlapping masses in the raw projection images are resolved at their true depths. The vertical lines in the reconstructed slices are artifacts due to blank scan inconsistencies [3].

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Thus the above imaging test using stationary MBFEX sources successfully established the great potential for tomosynthesis. It was evident that the system could be greatly improved – in terms of source stability, angular coverage and detector specifications – so as to make it a full-field stationary digital breast tomosynthesis (DBT) system. It is essential to set a full-scale system up in order to make a fair comparison to other commercial systems.

3.2 Stationary digital breast tomosynthesis system – Argus