images

Figure 2 Example of coronal
MIP and coronal slices from
a patient with multiple avid
liver metastases. WB PET
refers to the bed position
acquired during whole body
imaging that has respiratory
blurring and minimal ghosting
(arrows). The full acquisition
from the 15 minute Liver PET
acquisition demonstrates
significant respiratory blurring
and ghosting (dotted and
solid boxes). When respiratory
compensation is applied using
a bellows-derived gating
signal, both the respiratory
blurring and ghosting are
effectively removed.
acquired during end expiration. This approach results in
motion-artifact free images (Figure 2).
Role of hepatobiliary contrast agents
Hepatobiliary agents such as gadoxetate disodium have a
significant portion of biliary excretion, allowing for the acquisition
of a hepatobiliary phase about 15 to 20 minutes after
injection. During the hepatobiliary phase, the hepatic parenchyma
enhances due to biliary excretion of contrast. At the
same time, lesions that do not contain functional hepatocytes,
for example metastases, are markedly hypointense.
This results in increased detection of metastatic lesions
compared to PET or CT imaging.
Another benefit is that the markedly hyperintense liver
parenchyma allows for robust navigation using the liver
dome. Additionally, the contrast of the liver is relatively static
compared to dynamic imaging immediately after injection.
Because of this, high-resolution free-breathing images can
be acquired that further improve the detection of metastatic
foci. Finally, the use of respiratory navigated imaging creates
an MRI image that is acquired during end-expiration, and
therefore fused accurately to the respiratory compensated
PET data.
DOTA-TOC and DOTA-TOC PET/MRI
DOTA-TOC is a somatostatin analog, similar to Octreotide,
used primarily to image patients with neuroendocrine
tumors. It is labeled with Gallium-68, allowing for PET
imaging. Ga-68 DOTA-TOC has a number of benefits over
Octreotide: First, the imaging study is performed one hour
after injection instead of one day later. Second the dose
associated with the exam is roughly half. Third, the study
has better detection sensitivity.
During the past year we started an investigational
new drug approved study to evaluate Ga-68 DOTA-TOC in
patients with somatostatin receptor-positive malignancies. To
date, we have enrolled more than 100 patients in the study.
The availability of this agent through the FDA-approved study
has transformed the way that oncologists stage and restage
patients with neuroendocrine tumors at UCSF.
The liver is the most common site for neuroendocrine
tumor metastasis. This makes for a perfect combination of
DOTA-TOC and PET/MRI. We initially compared DOTA-TOC
PET/CT and PET/MRI in 10 patients to evaluate the benefit
of PET/MRI to PET/CT and found that both hepatobiliaryphase
imaging and diffusion-weighted imaging detected
more hepatic metastasis compared to contrast enhanced
CT, with similar detection sensitivities for extrahepatic disease.
Currently DOTA-TOC PET/MRI is available through a
research protocol at UCSF.
Future work
Future work in liver PET/MRI will focus on two main areas.
First is the improvement in motion correction. Although
respiratory bellows are capable of removing the majority of
respiratory motion associated with breath holds and respiratory
blurring, a significant amount of data remains unused in
the final reconstruction leading to noisy images. Improved
PET reconstruction models taking into account patientspecific
motion need to be implemented to create images
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