Biomedical Engineering Reference
In-Depth Information
of the subject we start with a brief explanation of the operation of SPECT and
PET scanners, addressing later the specific problem of motion in these imaging
techniques.
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Basic Principles of the Nuclear Medicine Imaging
Nuclear medicine images can be obtained either by detecting photons that are
emitted from atomic nuclei or by positron annihilation that results in two photons
whose opposite trajectories are (almost) collinear.
When the radioisotopes are gamma emitters, a special collimated detector system
is used to obtain the images. This detector system is known as the gamma camera
and the overall process of imaging is usually called planar scintigraphy or simply
scintigraphy.
The gamma camera can rotate around the subject of study, allowing the ac-
quisition of multiple views from different angles, which after filtering, are used
in the reconstruction algorithm to obtain tomographic images. In this case, the
technique is known as SPECT. For positron emitters, the detector (also of gamma
radiation) does not need a physical collimator because collimation can be performed
electronically, by detecting in coincidence the two 511 keV annihilation photons
that result from the positron annihilation in matter. When these photons, emitted in
opposite directions, are detected in coincidence (within a certain time interval) they
define a line of response (LOR). This technique is called PET and the detector is
known as a PET scanner.
The versatility of these two techniques is based on the combination of specific
radioactive emitters with an appropriate detector which allows the reconstruction of
two or three dimensional images that provide functional information.
Both the gamma camera and the PET scanner can be regarded as passive devices
since the radiation is emitted by radioactive isotopes that are administered to
patients.
Labeling different molecules of biological interest, i.e. molecules that play an
active role in the physiological processes, is a powerful procedure that enables
obtaining important information regarding the various functions of the human body.
In general terms, the nuclear imaging technique involves the administration to the
patient of an adequate radiopharmaceutical and the detection of the gamma photons
that are emitted directly by the radioisotope or that result from the annihilation
of positrons. Thus, the image reveals objectively the spatial distribution of the
radiopharmaceutical to which the doctor lends clinical significance.
There are several technical challenges in nuclear medicine imaging. On one
hand, there are important issues regarding the detection of radiation, namely:
the correlation between the location of the detection and the actual position of
the emission of the photon, the efficiency of the detection, the energy resolution,
and the spatial and temporal resolution. On the other hand, it is crucial to improve
the aspects related to the signal source, which implies the development of new
