Biomedical Engineering Reference
In-Depth Information
preferential clinical tool for the detection and identification of lesions or structures in
clinical applications requiring detailed anatomical and biological characterization.
Currently, vendors no longer offer PET-only scanners, such that all PET scanners
now come combined with CT [
13
]. Nowadays, the acquisition time for a combined
PET/CT examination has greatly reduced compared to the first devices. This enabled
a more efficient use of short-lived PET tracers while providing better patient comfort
and convenience, more timely access to this technology, and, moreover, a greater
amortization of costs. At the same time, developments in CT systems and computer
processing has resulted in a substantial improvement of image quality and analysis,
with a subsequent gain in terms of value and reliability of diagnostic information
provided to clinicians [
14
]. The usual protocol for a PET/CT scan consists of a low-
dose CT scan, acquired without contrast medium (CM), followed by the PET scan,
and, if needed, followed by a dedicated full-organ or region-focused CM-enhanced
CT [
15
].
3.2.3 PET/MRI
Despite the clinical success of PET/CT, there are some debated points regarding the
use of CT as morphological imaging technique within a multimodal imaging device.
In particular, CT adds a significant contribution to the final amount of radiation dose
that the patient receives during the examination, and moreover, it provides relatively
poor soft-tissues discrimination, even when the scan is acquired with contrast media.
The desire to overcome these limitations, supported by the great success of hybrid
PET/CT imaging in the clinical and medical research environment, encouraged the
development of other multimodal imaging techniques, such as PET/MRI.
It is interesting to observe how the idea of combining PET and MRI arose around
the same time that PET/CT was conceptualized. PET/MRI was firstly applied to
small animal imaging studies in the early 1990s [
16
,
17
], with a major difficulty
represented by the interference between the high magnetic field of MRI and the
sensitive electronic components of the PET scanners. After this first approach, the
first clinical device for human brain PET/MRI imaging was introduced in 2006 using
solid state PET detectors that are less sensitive to interference from magnetic field
than conventional photomultipliers used in PET scanners [
18
].
MRI does not show the same limitations as for CT, since it does not involve any
ionizing radiation and provides superior soft-tissue imaging compared to CT, in par-
ticular if innovative and specific MRI contrast agents are used [
19
]. This fact, in
particular, indicates MRI as a natural and excellent alternative to CT when imaging
the brain. Another great advantage of MRI with respect to CT is that it offers a much
broader variety of data acquisition techniques than CT that may adapt to a high vari-
ety of clinical needs. MRI allows for the use of contrast agents that that have less
toxicity than contrast agents used for CT and enables an additional enhancement of
soft-tissue contrast. Moreover, MRI allows for advanced functional techniques that
