Biomedical Engineering Reference
In-Depth Information
8.5.2.2 PET Detectors
Since the basic event in PET is the simultaneous detection of the two 511-keV
γ
rays, PET requires instrumentation and image reconstruction methods that differ
from those used in other medical imaging modalities. Typically, a gamma camera
system is used as the imaging device
(Figure 8.8). Conventional gamma cameras
have been designed with planar detector geometry since the development of Anger-
type position encoding systems. Main components of the PET camera (or detec-
tor) are a large-area scintillator, an array PMTs, and a collimator in front of the
scintillation crystal to localize radiation. The role of the scintillation crystal is to
convert as much energy as possible of the incident gamma rays or photons into
visible radiation, which is converted to an electric current by the PMTs. Photons
deposit energy within the scintillation crystal by a photoelectric or a Compton scat-
tering (described previously in Section 8.3.2) interaction. A small portion (10%) of
the incident photon's energy deposited in the scintillation crystal is converted into
visible light photons ~3-eV energy. Commonly used scintillation crystals include
bismuth germanate crystals (BGO), lutetium oxy-orthosilicate (LOS, that result in
shorter deadtimes and improved countrate responses), GSO, and NaI. Visible light
photons are guided towards the photocathodes of an array of PMTs where they are
converted into electrons, multiplied, and finally converted into an electrical signal
at the anode of each PMT. The amplitudes of the anode signals from each anode are
then examined by either an analog or a digital positioning circuitry to estimate the
position at which a photon interacts with the crystal.
The purpose of the collimator is to mechanically confine the direction of in-
cident photons reaching the scintillation crystal and thereby provide a means to
localize the site of the emitting sources. The collimator is usually made out of a
plate of lead or a similar high atomic number substance such as tungsten in which
a large array of apertures (circular, triangular, square, or hexagonal shaped) are
drilled close to each other with a narrow septal thickness. By physically restricting
the region from which gamma radiation can transmit through a given aperture, the
collimator attached to the scintillation crystal ensures that the position of events
on the scintillation crystal correspond to the position of the 2D projection image
of the object. If the apertures of the collimator are all parallel to each other, the
collimator is then called a parallel hole collimator. Most clinical examinations are
conducted with a parallel hole collimator since it provides the ideal combination of
resolution and sensitivity for most regions of the body with no geometrical distor-
tion. The resolution of scintillation imaging is mainly determined by the intrinsic
resolution of the camera and the resolution of the collimator, and the net resolution
is obtained from a quadrature addition of these two factors. In some applications,
the apertures may be angulated to form converging or diverging hole collimators,
providing certain advantages in imaging relatively small regions (converging col-
limator) or relatively large regions (diverging collimator) with appropriate mag-
nification of the images. Alternatively, just one or a limited number of holes may
be used to form a pin-hole collimator, which is particularly useful in imaging very
small regions or small organs such as the thyroid gland.
Tomographs consist of a large number of detectors arranged in many rings
around the subject. A linear combination of energy information from each PMT is
