Synopsis
PET and MR are two imaging modalities that complement each other, and by
combining the two, both the anatomical depiction of MRI, and the high molecular
sensitivity of PET can be exploited. With truly integrated PET/MR systems,
PET and MR images can be acquired simultaneously in one imaging session, saving
time and securing minimal need for registration between images. Within
oncology, PET/MR could be a viable option in cancers where MR is the preferred
imaging modality and where PET/CT currently has a limited role in the clinic.
Introduction
Multiparametric
MRI (mpMRI) is a powerful tool within oncologic imaging with excellent
anatomical details (T1 and T2 weighted images), and the ability to image
function (DWI and DCE-MRI) and metabolism (MRSI) [1]. There are, however, situations where the
addition of positron emission tomography (PET) is useful. The two modalities
complement each other, and by combining PET and MRI, both the anatomical
depiction of MRI, and the high molecular sensitivity of PET can be
exploited. With truly integrated PET/MR
systems [2, 3], PET and MR images can be acquired simultaneously
in one imaging session, saving time and securing minimal need for registration
between images.Basics of positron emission tomography
Positron
emission tomography (PET) is an imaging modality where a tracer, a compound labeled with a positron-emitting radioisotope, is
used to image biochemical processes in the body. PET is considered a quantitative imaging
modality since it has the ability to measure local tracer concentration. There
are different approaches for evaluation of PET images, including qualitative
analysis by visual assessment, semi-quantitative analysis by standardized
uptake value (SUV) measurements, and absolute quantification of tracer kinetics by
pharmacokinetic modeling. Within oncology, SUV is the most commonly used index
for quantification of tumor uptake. SUV is a unit-less quantity defined as the activity
concentration in a volume of interest divided by the injected activity. The
injected activity is typically normalized to patient body weight and decay
corrected to the PET acquisition start time [4]. SUV is simple to calculate and, in contrast to
absolute quantitative approaches, it does not require blood sampling and dynamic
scanning.Challenges in quantitative PET/MR
The signal in
a PET image arise from two 511 keV photons emitted in opposite direction as a
result of positron decay of the radioisotope. Quantification based on PET
images requires that the PET data are corrected for measurement errors, and of
particular importance in PET/MR is the attenuation correction. Since the photons
traverse tissue (or hardware components) with different electron density and
thickness on their way to the PET detector, the photons are attenuated differently. In PET/CT it is quite
straight forward to transform CT transmission images into maps of attenuation
coefficients at 511 keV. In PET/MR on the other hand, there is no direct link
to electron density, and other approaches for attenuation correction has to be pursued [5]. The most common method for
creating tissue attenuation maps is the Dixon technique, where the body is
divided into four tissue classes (air, lung, soft tissue and fat), and an
attenuation map is created based on this segmentation [6]. The main disadvantage with this
technique is that one are not able to segment out bone as a separate
compartment, leading to possible underestimation of the attenuation. Other techniques,
such as ultrashort echo time (UTE) sequences [7] or atlas based segmentation [8] has been proposed to include bone
in the attenuation maps. Although the attenuation correction was a major
concern in the initial phase of hybrid PET/MR scanners, the community now seems
to agree that attenuation correction with the currently available methods is acceptable
[9].Applications in oncology
18F-FDG PET/CT has been a huge success in
oncological imaging and oncology was therefore also identified as a focus area for PET/MRI. Indeed, it has been shown that PET/MR performs equally well as
PET/CT in most types of cancer [10]. However, PET/MR is more expensive and is
mainly explored as an alternative to PET/CT in cancers where MR is the
preferred imaging modality, such as breast, liver and pelvic cancers[11]. In lung cancer, PET/MR has been suggested to
reduce the spatial mis-registration compared to PET/CT [12]. PET/MR of prostate cancer has gained much interest
over the last years, and it has been shown that prostate cancer recurrence was
detected more accurately with Ga-PSMA PET/MRI than PET/CT [13] . Also in pediatric imaging, PET/MR would be
preferred over PET/CT due to lower radiation exposure [14].Acknowledgements
No acknowledgement found.References
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