Biomedical Engineering Reference
In-Depth Information
8.5 Imaging Devices
8.5.1 X-Ray Imaging
The penetrating power of X-rays makes them ideal for medical diagnostics. How-
ever, X-ray photons have energies, which can cause ionization and therefore are
biologically hazardous if the absorbed dose is not kept below certain minimums.
The amount of absorption depends on the tissue composition. Dense bone will ab-
sorb more than soft tissues, such as muscle, fat, and blood (see Section 8.3.3). The
amount of deflection depends on the density of electrons in the tissues. Tissues with
high electron densities cause more X-ray scattering than those of a lower density.
Thus, the X-ray image will appear brighter for bone or metal, as less photons reach
the X-ray film after encountering bone or metal rather than tissue. As the emitted
X-rays (accelerated electrons) pass through the patient, they are deflected by the
tissues and recorded by a detector on the opposite side. The positives of imaging
using X-rays is a high resolution as small as 0.1 mm and the ease of use. Its limita-
tion are an inability to discriminate tissues of similar densities and rendering only
2D structures.
An X-ray CT does not use film to detect the transmitted rays (see Section
8.4.3). Instead, the photons are collected by an electronic device, which converts
the X-ray photons into an electric current. In a CT scanner, the X-rays enter crystal
scintillators and are converted to flashes of light. Since the presence of more de-
tectors improves the resolution of the image, multidetector technology has made
remarkable progress. A “single slice” CT has a row of these detectors positioned
opposite to the X-ray tube and arranged to intercept the fan of X-rays produced
by the tube. In some cases, the detector row rotates with the X-ray tube; in others
there is a complete ring of stationary detectors. A “multislice” (or multidetector)
CT has several rows of small scintillator detectors.
Two basic designs for CT detectors are the ionization chamber, and scintillation
with photodiodes. Ion chambers are built on the ability of X-rays to ionize gases
such as Xe
2
, N
2
, and Ar
2
. An electric field attracts electron and ions, and measured
current is proportional to X-ray intensity. A scintillation detector converts X-rays
into light, which is detected by photodiodes. This arrangement is more sensitive
than ion chambers. The requirements for X-ray CT scintillators are different than
planar imaging, and should have: a low afterglow; high stability (chemical, tem-
perature, and radiation damage); high density (
6 g/cm
3
); an emission wavelength
well matched to photodiode readout (500-1,000 nm); and high luminous efficiency
(
>
15,000 photons/MeV). Commonly used scintillators are made of cesium iodide
(CsI) and cadmium tungstate, as well as rare earth oxides and oxysulfides such
as cadmium tungstate (CdWO
4
) and doped gadolinium oxisulfide. Detectors are
made of either single crystals or ceramic bars, and their surfaces painted with a
metallic paint for light reflection. CT detectors do not count individual photons,
but integrate the energy deposited by many photons.
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