It is appropriate to mention that whereas the FWHM of the LSF does
not account for the scatter and septal penetration of g-radiations, the MTF
takes these two factors into consideration and provides a complete descrip-
tion of the spatial resolution of a system. Furthermore, individual com-
ponents of an imaging system may have separate MTFs, and they are
combined to give the overall MTF, as follows:
MTF =MTF 1 ×MTF 2 ×MTF 3... (10.6)Problem 10.1
The MTFs at a certain spatial frequency of the detector, PM tubes, and PHA
of a gamma camera are 0.8, 0.6, and 0.7, respectively. What is the overall
MTF of the camera?
Answer
MTF =0.8 ×0.6 ×0.7
=0.34Sensitivity
Sensitivity of a gamma camera is defined as the number of counts per unit
time detected by the device for each unit of activity present in a source. It
is normally expressed in counts per second per microcurie (cps/mCi). Sen-
sitivity depends on the geometric efficiency of the collimator, the intrinsic
photopeak efficiency of the detector, PHA discriminator settings, and the
dead time of the system. Intrinsic photopeak efficiency of a detector
(Chapter 8), PHA discriminator settings (Chapters 8 and 9), and the dead
time (Chapter 8) have been discussed previously. Briefly,
(a) the intrinsic photopeak efficiency of a detector decreases with increas-
ing photon energy and with increasing source-to-detector distance (see
Fig. 8.9A) but increases with the thickness of the detector (see Fig. 8.8).
Most modern cameras have 0.95 cm thick NaI(Tl) crystals. The photo-
peak efficiency of these crystals is about 90% for 140-keV photons of
99mTc and about 30% for 364-keV photons of (^131) I.
(b) A narrow window setting on the PHA reduces the counts measured
and therefore compromises the counting efficiency.
(c) Counts are lost when counting a high-activity sample using a
device with a long dead time, and hence the counting efficiency is
reduced.
Sensitivity of a gamma camera is most affected by the collimator effi-
ciency, which is described next.
Sensitivity 127