Biomedical Engineering Reference
In-Depth Information
rections. Currently, systemmanufactures are attempting to reduce metallic content
both in the detector and in the surface coils, in order to minimize this problem.
•
The necessity to adapt the “slow” PET acquisition protocols to the “faster” seg-
mental MRI examination of specific body parts. Traditionally, MRI exams have
been limited to portions of the human body, due to long acquisition times. How-
ever, recent developments have allowed for faster, high-resolution whole-body
MR imaging without the need for patient or surface coil repositioning. The fast
acquisition speed reached today by MRI, allows also for acquisition of dynamic
data sequences and subsequent visualization of dynamic contrast enhancement
[
29
,
30
].
•
Finally, space constraints are a major restriction for PET and MRI devices inte-
gration, since the bore size of the MR scanner is limited. Therefore, a further
challenge was to design PET integration into the MRI system leaving adequate
space for patient comfort.
Currently, a number of papers dealing with the potential value of PET/MRI images
in the clinical practice have been published, e.g. dealing with neurological diseases
[
31
] and several cancer types evaluation [
32
-
36
].
3.3 Quantitative Positron Emission Tomography in Hybrid
Imaging
3.3.1 Data Correction
One important characteristic of PET is that it gives an accurate quantification of the
radioactivity distribution in the human body or a part of interest, after injection of
a metabolic radiotracer. Nevertheless, quantitative use of PET data requires several
accurate corrections to be applied. These corrections are typically applied to the
sinograms as a series of multiplicative factors prior to image reconstruction.
When both photons coming from an annihilation event are detected, a so-called true
coincidence is recorded. Conversely, if some interaction occurs before one or both
photons meet the detector surface, then their direction and energy are changed. The
most important interactions that photons resulting from the positron annihilation
undergo in the human body are Compton scatter and photoelectric absorption. A
scattered coincidence
occurs when at least one of the two photons is diverted by
Compton scattering prior to detection. Since the direction of the photon is changed
during the Compton scattering process, such events increase the likelihood for the
resulting coincidence event to be assigned to the wrong line of response (LOR).
When two photons not arising from the same annihilation event are incident on the
detectors within the same coincidence time window, they are classified as
random
coincidences
. The loss of true coincidence events due to photon absorption within
the patient body or any device components (e.g. patient bed, immobilization devices
