3 Introduction to Brain Imaging
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absence of impurities, and its safety for injection in humans. These steps involve
a range of processes, including high-performance liquid chromatography and thinlayer and gas chromatography. Following preparation and passing the quality control steps, the PRP is aliquoted into separate doses for individual patients. The most
common PRPs used for human imaging include F-18-labelled fluorodeoxyglucose
(FDG), O-15-labelled H2O, N-13-labelled ammonia, and C-11-labelled Pittsburgh
compound B, and these PRPs are used for imaging glucose metabolism, tissue
perfusion, blood flow, and cerebral amyloid deposition, respectively [50].
The PET scanner detects 511 keV gamma photons produced as a result of the
annihilation of positrons emitted by the PRP, when the positron interacts with
negatively charged electrons in the tissue environment. Positrons and electrons are
antimatter and matter, respectively, and their interaction results in their annihilation
and conversion of their mass into energy, according to the E = mc 2 rule, thereby
leading to the release of two 511 keV gamma photons in opposite directions from
the site of annihilation. The law of conservation of momentum governs the release of
two gamma photons in two opposite directions because as the positron and electron
interact and annihilate, they tend to come to rest. There is, however, some residual
momentum (in other words, the positron and electron do not completely come to
rest), so the two gamma photons are released at a slight angle to one another.
This phenomenon has implications for the resolution of the PET scanner. Scanners
with smaller axial diameters have a better spatial resolution than their larger bore
counterparts because increasing distance from the site of radioactive decay leads
to greater errors in its assessment. Further, the speed of emitted positrons varies
with individual PET radioisotopes and determines the distance travelled before
interaction with an electron, their subsequent annihilation, and release of photons.
This phenomenon also affects the PET resolution. For example, positrons emitted
by O-15 decay have higher momentum, travel longer distances before annihilation,
and hence lead to poorer spatial resolution, as compared to positrons emitted
by F-18 decay. The typical spatial resolution of clinically used PET scanners is
approximately 4–4.5 mm. Higher-resolution PET scanners dedicated for human
brain imaging are available with a resolution of 2.5 mm. Micro-PET scanners used
for animal imaging have a much better resolution (up to 1.2 mm) given their small
axial diameters [48, 50]. This is superior to the resolution of single-photon emission
computed tomography (SPECT) cameras that typically have a resolution of 6–
10 mm.
Image reconstruction algorithms generate a two- and three-dimensional map of
the distribution of PRP concentration, which is subsequently utilised for visual
interpretation as a PET image and/or for further quantitative analyses.
PET imaging procedure involves positioning of the human or animal subject
in a PET scanner. PRP is injected prior to or after subject positioning, depending
on individual scanning protocols. Subjects may be scanned at variable intervals
after PRP injection, for example, approximately 45 minutes after [F-18] FDG
injection to assess glucose metabolism in a ‘static’ image; or the scanning may
begin simultaneously with the PRP injection, when ‘kinetic’ data is needed, for
53
absence of impurities, and its safety for injection in humans. These steps involve
a range of processes, including high-performance liquid chromatography and thinlayer and gas chromatography. Following preparation and passing the quality control steps, the PRP is aliquoted into separate doses for individual patients. The most
common PRPs used for human imaging include F-18-labelled fluorodeoxyglucose
(FDG), O-15-labelled H2O, N-13-labelled ammonia, and C-11-labelled Pittsburgh
compound B, and these PRPs are used for imaging glucose metabolism, tissue
perfusion, blood flow, and cerebral amyloid deposition, respectively [50].
The PET scanner detects 511 keV gamma photons produced as a result of the
annihilation of positrons emitted by the PRP, when the positron interacts with
negatively charged electrons in the tissue environment. Positrons and electrons are
antimatter and matter, respectively, and their interaction results in their annihilation
and conversion of their mass into energy, according to the E = mc 2 rule, thereby
leading to the release of two 511 keV gamma photons in opposite directions from
the site of annihilation. The law of conservation of momentum governs the release of
two gamma photons in two opposite directions because as the positron and electron
interact and annihilate, they tend to come to rest. There is, however, some residual
momentum (in other words, the positron and electron do not completely come to
rest), so the two gamma photons are released at a slight angle to one another.
This phenomenon has implications for the resolution of the PET scanner. Scanners
with smaller axial diameters have a better spatial resolution than their larger bore
counterparts because increasing distance from the site of radioactive decay leads
to greater errors in its assessment. Further, the speed of emitted positrons varies
with individual PET radioisotopes and determines the distance travelled before
interaction with an electron, their subsequent annihilation, and release of photons.
This phenomenon also affects the PET resolution. For example, positrons emitted
by O-15 decay have higher momentum, travel longer distances before annihilation,
and hence lead to poorer spatial resolution, as compared to positrons emitted
by F-18 decay. The typical spatial resolution of clinically used PET scanners is
approximately 4–4.5 mm. Higher-resolution PET scanners dedicated for human
brain imaging are available with a resolution of 2.5 mm. Micro-PET scanners used
for animal imaging have a much better resolution (up to 1.2 mm) given their small
axial diameters [48, 50]. This is superior to the resolution of single-photon emission
computed tomography (SPECT) cameras that typically have a resolution of 6–
10 mm.
Image reconstruction algorithms generate a two- and three-dimensional map of
the distribution of PRP concentration, which is subsequently utilised for visual
interpretation as a PET image and/or for further quantitative analyses.
PET imaging procedure involves positioning of the human or animal subject
in a PET scanner. PRP is injected prior to or after subject positioning, depending
on individual scanning protocols. Subjects may be scanned at variable intervals
after PRP injection, for example, approximately 45 minutes after [F-18] FDG
injection to assess glucose metabolism in a ‘static’ image; or the scanning may
begin simultaneously with the PRP injection, when ‘kinetic’ data is needed, for