Biomedical Engineering Reference
In-Depth Information
annihilation event along the line of response (LOR) connecting the two activated
detectors. Raw data from a PET detector are typically stored in a special diagram
known as a sinogram. The image reconstruction process starts with such raw data
and produces cross-sectional images that represent the radioactivity distribution into
the human body.
SPECT is a more conventional technique also based on the use of a radioactive tracer
material, but, in contrast with PET, the tracers used in SPECT emits gamma rays
(
γ
-rays) that are measured directly by using gamma-cameras, which acquire two-
dimensional images (projections) from multiple angles. Reconstruction algorithms
are subsequently used to obtain three-dimensional images from the acquired two-
dimensional datasets.
Although SPECT scanners are significantly less expensive than PET devices, a higher
resolution can be obtained with PET event detection techniques. Moreover, positron-
emitting isotopes used in PET are of much shorter half-life than conventional gamma
emitting isotopes used in SPECT and hence expose the patient to much lower dose
of radiation.
3.2 Hybrid Imaging: Concepts and Technical Evolution
During the last decades a growing interest arose around hybrid, or multimodal,
imaging techniques combining two different imaging devices in one scanner, e.g.
a CT and a PET scanner. An increasing amount of studies have demonstrated that
multimodal imaging can provide unique and important information in diagnostic and
follow-up treatment for different types of pathologies. Such hybrid devices open a
whole new scope of diagnostic perspectives and investigative capabilities that goes
far beyond the traditional anatomical evaluation of the human body. The ability to
obtain functional and metabolic parameters concurrent with anatomical data for the
whole human body or for a single organ, has not only opened new perspectives for
diagnosis and follow-up of diseases, but also for a better understanding of physio-
logical and biological functions of different parts of the human body.
3.2.1 SPECT/CT
In 1966, David Kuhl demonstrated for the first time the importance of acquiring mor-
phological information together with molecular and functional information derived
from scanning a radionuclide distribution within the human body. He experimentally
introduced a
241
Am radionuclide source in the center hole of a collimated detector
and used the opposite detector to simultaneously image the emitted photons from this
