306 7.8 Biomedical Physics Tools
Improvements to detection for use in dynamic functional imaging can be made with
contrast rearrangements in a similar way to MRI. A good example of CT/CAT functioning
imaging is in diagnosing gut disorders. These investigations involve the patient swallowing a
suitable x-ray contrast reagent (e.g., a barium meal) prior to scanning.
7.8.3 SINGLE-PHOTON EMISSION CT AND POSITRON EMISSION TOMOGRAPHY
Nuclear imaging involves the use of gamma ray detection instead of x-rays, which are emitted
following the radioactive decay of a radionuclide (also known as tracer or radioisotope), which
can be introduced into the human body to bind to specific biomolecules. They are valuable
functional imaging technologies. Single-photon emission CT (SPECT) works on similar 2D
scanning and 3D reconstruction principles to CAT/CT and MRI scanning. Although there
are several different radionuclides that can be used, including iodine-123, iodine-131, and
indium-111, by far, the most commonly used is technetium-99m. This has a half-life of ~6 h
and has been applied to various diagnostic investigations, including scanning of glands, the
brain and general nerve tissue, white blood cell distributions, the heart and the bone, with a
spatial resolution of ~1 cm.
There is an issue with the global availability of technetium-99m and, in fact, with a variety
of other less commonly used radionuclides applied to biomedicine, referred to as the techne
tium crisis; in 2009 two key nuclear research reactors, in the Netherlands and Canada, were
closed down, and these were responsible for generating ca. two-thirds of the global supply
of molybdenum-99, which decays to form technetium-99m. There are other technologies
being investigated to plug this enormous gap in supply, for example, using potentially cheaper
linear particle accelerators, but at the time of writing, the sustainable and reliable supply of
technetium-99m in particular seems uncertain.
Positron emission tomography (PET) works on similar gamma ray detection principles
to SPECT, but instead utilizes positron radionuclide emitters to bring about gamma ray
emission. Positrons are the antimatter equivalent of electrons that can be emitted from
the radioactive decay of certain radionuclides, the most commonly used being carbon-11,
nitrogen-13, oxygen-15, fluorine-18, and rubidium-82 (all of which decay with relatively
short half-lives in the range ~1–100 min to emit positrons), which can be introduced into
the human body to bind to specific biomolecules in a similar way to radionuclides used in
SPECT. Emitted positrons, however, will annihilate rapidly upon interaction with an elec
tron in the surrounding matter, resulting in the emission of two gamma ray photons whose
directions of propagation are oriented at 180° to each other. This straight line of coinci
dence is particularly useful, since by detecting these two gamma rays simultaneously (in
practice requiring a detector sampling time precision of <10−9 s), it is possible to determine
very accurately the line of response for the source of the positrons, since this line itself is
oriented randomly, and so by intersecting several such lines, the source of the emission
in 3D space can be determined, with a spatial resolution better than that of SPECT by a
factor of ~2.
Time-of-flight PET (TOF PET) determines the difference δt in the arrival times of
the two gamma ray photons generated from positron-electron annihilation produced at
a 180° orientation. Multiple detectors are placed in a ring around the biological tissue
sample (which can be a live human, since this is a very valuable new medical imaging
technology), which has been doped with a positron emitter inside the scanner. To pin
point the source of emission from the sample, with coincident signals detected at a rate
of a few hundred MBq for typical doped samples (where 1 Bq is the SI unit of radio
activity corresponding to 1 disintegration per second) and sampled at GHz rates, the
spatial displacement x is c.δt/2 where δt is the detection of coincidence timing resolution
and c is the speed of light.
The rate of random coincidences k2 from two identical gamma ray detectors oriented
at 180° from each other, each with a random single detector rate k1 during a sample time
interval Δt is