Biomedical Engineering Reference
In-Depth Information
If the x-ray projection data are collected at a sufficient
number of angles, a matrix of values of the attenuation
coefficient m for different
dxdy
cells can be calculated by
a simple back-projection technique, which yields the
two-dimensional distribution m(
x, y
) over the whole
tissue slice. In displaying the variation of the attenuation
coefficient pictorially in shades of gray, one creates an
image that shows the various anatomical features of the
tissue slice. In practice, the back projection is calculated
by Fourier transforming the projection data into (spatial)
frequency space.
The first type of CTscanners used a single collimated
x-ray source and two detectors for data to be collected
from two contiguous tissue slices. The source and each
detector mapped out projection data in a translate-rotate
geometry, one x-ray path at a time. The efforts to collect
the data faster soon led to the successive generation of
fan-beam scanners shown in
Figures 6.2-20
b,
6.2-20
c,
and
6.2-20
d.
Figure 6.2-20
b shows an array of detectors
that move in a translate-rotate configuration;
Figure
6.2-20
c shows a bank of detectors that move in a purely
rotational geometry; finally
Figure 6.2-20
d shows a ring
array of stationary detectors. Using fan-beam geometries,
a whole tissue slice can be imaged in a few seconds. By
combining these geometries with the patient gantry
moving continuously along its long axis, one can get cross-
sectional images of many slices of the patient in mini-
mum time, a procedure called spiral scanning.
Although the evolution of x-ray scanning geometries
greatly shortened the time required to acquire images,
none of the three fan-beam generations permitted data
acquisition in a time (less than 0.1 s) short enough to
capture images of the heart and other blood-perfused
organs without significant degradation caused by motion.
Fifth-generation scanners employ an electron-beam gun
that generates x-ray beams in different directions by
scanning over a stationary concave metal target. The
resulting scan times are only a few milliseconds.
X-Ray Tube
Antiscatter Grid
Image Receptor
Data Acquisition
Image Storage and DSP
Display
Data Storage
Hard Copy
Figure 6.2-21 Digital radiography system.
and photostimulatable phosphors with laser-scanning
readout.
A digital radiographic unit (
Figure 6.2-21
), the x-ray
source, and receptor are computer controlled to provide
digital images that can be displayed in real time on video
screens. Digital images can be stored on magnetic media.
Digital image storage and display are used routinely in
x-ray CT and magnetic resonance imaging.
6.2.6 The endoscope
6.2.5.2 Digital imaging
The future of diagnostic devices using photonics looks
promising with a large amount of research focusing on
early cancer detection. Photonic components for the
detection of cancer and other diseases keep getting
smaller and cheaper, making many of these instruments
fit inside the body. One such device, the endoscope, has
been used for many years to diagnose and treat gastro-
intestinal problems, and research is still being performed
to improve this instrument.
The endoscope uses a CCD or optical fibers to form
images that are transmitted to a monitor. Endoscopes not
only let doctors see inside, but also include an instrument
that can take a biopsy, and some can even dye the area
for x-ray imaging. By guiding an endoscope into the gas-
trointestinal tract, doctors can view lesions or sources of
The combination of intensifying screen and photographic
film has many advantages for capturing and recording
many x-ray images. This approach is simple, inexpensive,
and it yields excellent spatial resolution. This approach is
limited to a narrow range of acceptable exposures and
offers little flexibility for image processing or data com-
pression. Film images are bulky to store, and they must
be transported from place to place. Digital imaging
methods overcome these limitations, but they are more
expensive and more complex. Digital methods employ
a variety of approaches for x-ray detection and mea-
surements: fluorescent crystals with photomultipliers,
semiconductor detectors, channel electron multipliers,
