Biomedical Engineering Reference
In-Depth Information
Chapter 3
Hybrid Imaging: From Anatomy to Function
David García Juan, Sara Trombella and Osman Ratib
3.1 The Revolution of Molecular Imaging
Imaging radioactive tracers in the human body is where the origin of Nuclear
Medicine applications emerged, allowing for recording the physiological behavior of
organs and diseases in vivo and leading to a revolution in medicine, known today as
“molecular imaging”. While this technique of imaging does not show the molecules
themselves, it allows for following infinitesimal traces of some radio-labeled mole-
cules in the body. Different types of radioactive emissions can be observed and used
for varied purposes. Electromagnetic radiation exists in a wide range of frequencies
or energies starting at the long wave radio band, and arriving at the short ultra-violet
(UV), x and gamma
(
γ
)
radiation via microwaves, infrared (IR) and visible light.
Positron emission tomography (PET) and single-photon emission computed
tomography (SPECT) are nuclear medicine techniques based on the introduction into
the human body of a radioactive labeled substance, with the subsequent acquisition of
images mapping the distribution of this agent into the whole body or a specific organ
of interest. Such substances (radiotracers) are analogues to bio-markers of specific
molecules or biological pathways. Therefore, they participate in the physiological
processes of the subject, and their bio-distribution quantitatively reflects cellular and
molecular behaviors.
The physical process that enables the PET image production is the radioactive
beta plus decay (
β
+
-decay) of the administered radionuclide with the emission of a
positron that annihilates with an electron within a short distance into the human body,
producing a couple of opposite-sided photons. The two photons are captured using
a ring of detectors around the patient and they are revealed
in coincidence
. When
two photons are detected within a short time-window they are assigned to a unique
