ABSTRACT
As its name suggests, single photon emission computed tomography (SPECT)
relies on the three-dimensional reconstruction of gamma emitter distributions in
the brain. Without the advantage of the special geometry of positron annihilation
into two photons travelling in opposite directions, SPECT has to use collimators
to limit the field of view of the photon detectors. This implies that a significant
amount of the radiation is absorbed by the collimator walls and does not
contribute to the reconstructed image. Consequently, the sensitivity of SPECT is
reduced compared with PET. Other differences between PET and SPECT derive
from the different radioactive half-life of the gamma and positron emitters: short
half-lives of positron-emitters make several repeat examinations within a
short period possible, while longer half-lives of gamma-emitters allow for the
examination of metabolic or pharmacological processes that require longer
periods to establish. Further, the replacement of carbon atoms of the ligand with
the positron emitter 11C can generate radio-ligands with identical pharmacologi-
cal properties, whereas substitution with the gamma-emitters 123Iodine or 99mTechnetium is likely to change the ligand’s chemical properties. Conse-
quently, the generation of new SPECT ligands has proceeded at a disappointingly
slow rate, partially because of the need to establish their pharmacological
properties after synthesis.
