Biomedical Engineering Reference
In-Depth Information
are not feasible by CT, such as diffusion and perfusion imaging, and for other meth-
ods using the dynamics of contrast agents to evaluate physiological parameters such
as flow, perfusion and diffusion, which may complement and enhance PET func-
tional information. MRI also provides spectroscopy, that allows for better evaluation
of tissue composition and allows for detection of organ-specific abnormalities and
pathologies by quantifying ratios of concentration of specific molecules.
There are different conceivable options for combining PET and MRI systems. The
easiest method as adopted in the earliest devices available for clinical use, is to place
the two scanners in series in a manner analogous to PET/CT devices. Nowadays,
technical improvements of this configuration, coupled with the excellent results
reached by software fusion in many situations (in particular, for brain and heart
imaging) brought this sequential multimodal technique approach closer to an ideal
simultaneous configuration. In practice, a full integration of the PET system into
the MRI gantry is preferred. Such configuration shows many advantages compared
to the PET/CT systems adopted in the clinical environment, and to the sequential
PET/MRI configuration as well. With a sequential scan, synchronous data acquisition
is not feasible. Any temporal separation of the two study components increases the
likelihood for image misregistration, and affects attenuation correction, caused by
artifacts due to patient movement as well as to the physiological movements occur-
ring internally in the human body, such as gastric emptying or bladder filling. All
this can compromise the accuracy of tissue activity quantitation [
20
-
23
]. Therefore,
fully integrated PET/MRI scanners that may provide accurate temporal correlation
of dynamically acquired datasets from the two imaging modalities remain the final
goal for the healthcare industry. The main problems in the development of a fully
integrated PET/MRI device are:
•
The impossibility for the photomultiplier (PMT)-based PET detectors to work
within or near the magnetic field generated by the MR scanner. The PET system,
and in particular the various hardware components of the PET PMT-based detector,
can reduce the MRI performance by degrading the homogeneity of the MR main
magnetic field and of the radio-frequency field as well. This interference may
cause artifacts in the MR images, i.e. a loss in the MR image quality. Moreover, the
variableMRgradientsmay induce eddy currents in conductivematerials of the PET
detector, which can distort the effective gradient field. On the other hand, the high
magnetic field used in theMRI system excludes the use of PMTs used in traditional
PET scanners, since electrons in the vacuum tube of the PMTs are deflected by
the interaction with the strong MR magnetic field (Lorentz force). Despite this, a
physical integration of PET and MRI devices in a single gantry became possible as
innovative solid-state photo-detectors that are insensitive to the external magnetic
field, became available (e.g. Geiger-mode avalanche photodiodes or silicon PMTs
[
24
-
28
]).
•
Metallic objects (such as surface coils) used to acquire higher quality MR images
interfere with gamma rays from PET, producing attenuation. Surface coil arrange-
ments are needed for better MR image quality, but they contain several metal parts,
that may cause artifacts in the PET images, with subsequent need for adequate cor-
