Page 315 - Biomedical Engineering and Design Handbook Volume 2, Applications

THE PRINCIPLES OF X-RAY COMPUTED TOMOGRAPHY 293

Therefore, a departure from conventional medical x-ray source design is necessary to achieve the
performance required for microtomography applications.

10.4.2 The Microfocal X-Ray Source
The formation of the electron beam is the key to resolution and to the level of photon flux from the
x-ray source. 19 In this respect, the electron source brightness b and focal quality are functions of
E
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the electron energy and the electron current. The principal components that perform this task are
the electron gun and the electron lens. The electron gun consists of an electron emitter, or cathode,
and two beam-forming electrodes. The electron lens can be either electrostatic or electromagnetic.
The latter is usually preferred because the associated focal aberrations are less.
Cathode Condition and Electron Optics. The principal conditions sought for an electron emitter
are high brightness, stable geometry, and long life. To achieve a high-quality focus it is important to
produce a well-defined virtual source at the electron gun. Here, efficient use is made of the peak
cathode brightness of the emitted beam if a single round crossover disk, of effective diameter d , is
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produced by the electron trajectories close to the cathode. Following electron microscopy practice,
the choice of cathode structure generally relates to those based on a refractory metal, usually tungsten,
or a hexaboride compound, usually lanthanum or cerium. In this respect, a 100-mm wire tungsten
(W) hairpin and a lanthanum hexaboride (LaB ) crystal with 16-mm flat tip, both mounted on a stan-
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dard Philips base, offer alternative commercial solutions (Fig. 10.28). The tungsten hairpin is low
cost, is structurally robust, has a high brightness, and can operate in fairly poor vacuum. However, it
suffers from limited lifetime and poor mechanical stability at its high working temperature. The lan-
thanum hexaboride crystal has a very high brightness, good mechanical stability at its relatively low
working temperature, and a very long lifetime (Fig. 10.29). However, it is high in cost, prone to poi-
son unless operated in ultrahigh vacuum, and is structurally delicate.
The shape of the crossover diameter d , which is the virtual electron source, is controlled by the
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geometry of the electrodes that compose the electron gun and the associated voltages (Fig. 10.30).
The size, shape, and location of the electrodes, namely the Wehnelt (grid) and the first anode (extractor),
relate to the diameter d of the Wehnelt aperture, the diameter d of the first anode aperture, and to
W A

FIGURE 10.28 Electron emission cathodes. FIGURE 10.29 Electron emission LaB and W cathodes.
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