Parallel-hole collimators are classified as high-resolution, all-purpose,
and high-sensitivity type, or low-energy, medium-energy, or high-energy
type, depending on the resolution and sensitivity they provide in imaging.
High-sensitivity collimators are made with smaller thickness than all-
purpose collimators, whereas high-resolution collimators are thickest of all.
These characteristics are discussed in detail in Chapter 10.
Currently, ultra-high-energy collimators useful for 511-keV photons are
commercially available and used for single photon emission computed
tomography (SPECT) imaging with^18 F-fluorodeoxyglucose.
Several collimators are available that are designed for some specific pur-
poses.Fan-beam collimators are designed with holes that converge in one
dimension but are parallel to each other in the other dimension. These col-
limators are primarily used for imaging smaller objects and hence magnify
the images.Cone-beam collimators are similar to fan-beam collimators and
magnify the images except that the holes are designed such that they con-
verge in two dimensions.
In earlier collimators, the holes were originally circular, but current
designs have square, hexagonal, or even triangular holes with uniform thick-
ness of lead around the opening. These collimators provide better spatial
resolution than the circular-hole ones.
Photomultiplier Tube
As in scintillation counters, PM tubes are essential in gamma cameras for
converting the light photons in the NaI(Tl) detector to a pulse. Instead of
one PM tube, an array of PM tubes (19 to 107) are mounted in a hexago-
nal fashion to the back of the detector with optical grease, or in some
instances, using lucite light pipes between the detector and the PM tubes.
In modern gamma cameras, square or hexagonal PM tubes are used for
better packing. The output of each PM tube is used to define the X,Y coor-
dinates of the point of interaction of the g-ray in the detector by the use of
an X-,Y-positioning circuit (see later) and also is summed up by a summing
circuit to form a pulse known as theZ pulse.TheZ pulse is then subjected
to pulse-height analysis and is accepted if it falls within the range of selected
energies.
X-,Y-Positioning Circuit
Each pulse arising out of the g-ray interaction in the NaI(Tl) detector is
projected at an X,Y location on the image corresponding to the X,Y loca-
tion of the point of interaction of the g-ray. This is accomplished by an X-,
Y-positioning circuit in conjunction with the PM tubes and a summing
circuit. Figure 9.4 illustrates the principles of X,Y positioning of pulses
arising from g-ray interactions in the detector employing seven PM tubes.
All PM tubes are connected through capacitors to four output leads rep-
112 9. Gamma Cameras