cameras provide excellent intrinsic linearity and hence superior spatial
resolution in image formation.
Solid-State Digital Cameras
The Digirad Corporation has made commercially available several gamma
cameras using solid-state detectors. Initially CZT detectors were used, but
later the company replaced them with CsI(Tl) detectors. The detectors are
fabricated in modules made of 128 3-mm ×3-mm detector elements. The
cameras are single-head, two-head, or three-head. Each head is comprised
of 32 modules consisting of a total of 4096 elements, resulting in a detector
area of 8 in.×8 in. in a 64 ×64 crystal array. The uniqueness of these cameras
is that they do not use PM tubes for pulse formation and use silicon diodes
instead. No X-,Y-positioning circuit is used, because each CsI(Tl)/silicon
diode element functions as an individual detection system, independent of
other elements, and each event of photon interaction in the crystal is posi-
tioned on the image matrix corresponding to the location of the element
(Early, 2005). This provides an excellent spatial resolution and quality of
the images in the energy range of 60–300 keV.
Various Digirad camera models include Digirad 2020tc, SPECTpack,
Cardius-1 (single-head), Cardius-2 (two-head), and Cardius-3 (three-head).
Appropriate collimators are required for imaging different organs and for
photons of different energies. Many units are small and portable. These
cameras are capable of SPECT studies with the use of a rotating chair
for the patient, which rotates with the patient on it at incremental angles
with respect to the detector.
Questions
- (a) Describe the operational principles of a gamma camera.
(b) The main purpose of a collimator is to limit the field of view of an
imaging device for imaging. True or false?
(c) The purpose of a photomultiplier tube is to convert light photons to
an electron pulse. True or false?
(d) The scattered photons are excluded by the proper choice of a colli-
mator. True or false?
(e) Scattered photons are excluded by the proper choice of discrimina-
tor settings (windows). True or false? - (a) What are the different categories of collimators?
(b) Which collimator is most used in nuclear medicine?
(c) Which types of collimator give image distortion and why? - Describe the function of the X,Y circuit in the gamma camera system.
116 9. Gamma Cameras