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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5426_Библиотеки_им_академика_М_И_Перельмана
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F. Calabria et al.
1.5 PET/CT Acquisition Protocols
• Whole Body PET/CT: From the vertex of the
skull to the upper thighs, 60min following the
18
F-FDG administration (300–400 Mbq;
2–3 min per bed position, depending on the
PET scanner). The low-dose CT for the anatomical localization of functional ndings is
sufcient, also in order to reduce radiation
exposure for patients. A full-dose contrastenhanced CT can be normally omitted from
18
F-FDG PET/CT oncologic studies [55]. This
option can be reserved, to ensure the diagnosis
(surgical resectability, disease extension,
peritoneal carcinomatosis, etc.).
• Dynamic PET/CT: During the tracer administration, a dynamic segmental PET scan can be
added to the standard whole body imaging, to
ensure a peculiar nding, in example by avoiding the urinary activity of the bladder in the
late scan or to evaluate the vascular activity
after the bolus injection. In these cases, generally a low-dose CT is associated with PET
imaging for the attenuation correction and to
obtain anatomical landmarks.
• Brain PET/CT: A specic, one bed position
acquisition of the brain is the method of choice
for PET imaging. A low-dose CT of the brain,
with the head of the patient in the center of the
scanner eld of view, is necessary for attenuation correction and anatomical reference.
Thereafter, a 10min PET, following 45–60min
the tracer administration (≈180 MBq), is sufcient for the functional imaging. Lately,
additional brain CT with contrast agent can
follow the standard imaging for a better depiction of encephalic structures or for the detection of brain metastases.
1.6 PET/MRI
The emergence of combined PET/CT scanners
was at the basis of the success of PET imaging
and of hybrid imaging, which allows in a single
session the simultaneous evaluation of metabolic
and morphological data, with an overall diagnostic impact superior to that of both modalities
separately performed. On this basis, the emergence and commercial availability of hybrid
PET/MRI scanners will improve the diagnosis in
specic elds of oncology and neurology in the
next future.
The main technical advantage of PET/MRI,
regarding PET/CT, is the simultaneous acquisition of PET and MRI data, rather than the sequential PET/CT scan (Fig.1.26): in our experience,
we perform the standard PET imaging simultane-
Fig. 1.26 Differences
between the sequential
18
F-FDG brain PET/CT
acquisition protocol and
a proposal of
simultaneous
brain PET/MRI scan, for
imaging dementia
18
F-FDG
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Fig. 1.27 In this dynamic brain 18F-FDG PET are recog-
nizable the arterial phase, rapidly following the tracer
administration, and the venous phases while, 5min after
Fig. 1.28 In the left column (a), axial and coronal
SPECT views of a 65-year-old woman examined for suspicion of Alzheimer disease, showing perfusion decit in
right frontal and parietal regions.
PET/MRI (c) better display a decit of glucose metabolism in the same areas with further hypometabolism in left
18
F-FDG PET (b) and
the injection, the last image shows the beginning of tracer
accumulation in the brain
f
parietal region. Correlative T1-weighted (d) and FLAIR
(e) MRI images display a condition of ventricular enlargement with prevalent atrophy in the right hemisphere.
FDG PET 3D maximum intensity projection of the brain
(f) summarizes the right parieto-frontal decit
18
F-
ously with blood oxygenation level dependent
(BOLD) MRI sequences, which offer the possibility of motion correction during the acquisition
and in the post-processing. As for PET/CT, the
PET/MRI scanner allows the possibility to obtain
dynamic or static PET data (Fig.1.27) but with a
large series of morphological and functional MRI
sequences. T1, T2, and uid attenuation inver-
sion recovery (FLAIR) sequences are the basis
for morphological imaging and represent the substratum for PET/MRI image fusion, especially
for neurodegenerative diseases (Fig. 1.28).
Depending on the clinical indications, other functional MRI sequences can be added to the imaging, in order to deep the knowledge of catabolites
with MR spectroscopy or to assess other functional features, as for diffusion tensor imaging,
which can give additional information to PET/
MRI data (Fig.1.29).
Therefore, concerning simultaneous PET/
MRI system workows, the main issues, also in
comparison to the era of “post-processing PET/
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e
Fig. 1.29 In a patient with cortico-basal degeneration
18
F-FDG PET (a) displays diffuse hypometabolism in the
left hemisphere of the brain and in the right cerebellar
hemisphere (crossed cerebellar diaschisis); T1 weighted
MRI views (b) well show a condition of diffuse atrophy,
MRI fusion imaging” (Fig.1.30) regard capabil-
ity to simultaneously depict the disease for many
points of views, also needing for this reasons the
collaboration of nuclear medicine physicians and
radiologists with skill in this eld, being the
holistic evaluation of hybrid imaging at the basis
of the PET/MRI clinical output.
prevalent on the left hemisphere. PET/MRI (c) summarizes all these ndings, while the analysis of tractography
(d), obtained by diffusion tensor imaging, improves the
display of decreased bers in the left brain hemisphere
and in the right cerebellar hemisphere (white arrows)
Despite these premises, it is also necessary to
state that MRI sequences should be not too long.
In whole body imaging with 18F-FDG, the MRI
displays a better soft tissue contrast than CT,
without using ionizing radiations, which can lead
to better diagnostic accuracy in oncologic studies
(Fig.1.31).
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a
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Fig. 1.30 Example of PET/MRI fusion imaging before
the development of hybrid scanners.
(a) and PET/CT (b) views of a patient with locally
advanced breast cancer. Following the anatomical land-
18
F-FDG axial PET
marks of co-registered CT it is possible to fuse, during the
post-processing, PET and MRI data of the breast (c), only
by using a unique breast coil in the two acquisition steps
abe h
cf
dg
i
Fig. 1.31 18F-FDG PET 3D whole body maximum inten-
sity projection (a) and detail (b) of a patient examined for
staging non-Hodgkin lymphoma of the neck. Correlative
coronal CT (c) and PET/CT (d) views show several right
cervical lymphadenopathies with a further area of uptake
in the contralateral side of the neck (red arrow).
PET 3D maximum intensity projection of the brain and
neck (e), obtained with hybrid PET/MRI scanner, allows
to better display all the cervical lesion and a further subcentimetric left cervical lymph node, eloquently showed
in coronal T1 weighted view (f) and coronal PET/MRI
18
F-FDG
view (g). Finally, axial PET/CT (h) and PET/MRI (i)
details of the neck eloquently show different power resolution limits
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a b
c
Fig. 1.32 The patient was examined for restaging colon
cancer. Beyond abdominal and lung metastases of colon
cancer (maximum PET intensity projection, a), incidentally, an area of focal
right prostate lobe, as evident in MIP (a, arrow) and axial
18
F-FDG uptake was detected in the
PET/MRI will become an important tool in
the prognostic stratication of prostate cancer
patients, due to the possibility to depict in a single whole body imaging tool primary tumor
(Fig.1.32) and distant bone and/or lymph node
metastases [56]. However, rather than 18F-FDG,
other tracers are showing more interesting results
in this specic clinical setting [57].
In neuro-oncology, PET/MRI can provide
simultaneous evaluation of brain tumors, depicting
both functional processes at the basis of neoplastic
proliferation and morphological abnormalities
with the high-power resolution (Fig.
1.33) and the
possibility of multiplanar evaluation [58].
In neurology, the 18F-FDG PET/MRI can play
a specic role in the depiction of functional and
anatomical processes at the basis of the developing of neurodegenerative diseases, due to the
extreme versatility of this tracer in the evaluation
of brain metabolism and the challenge to accurately investigate all the anatomical brain structures, provided by MRI (Fig.1.34).
d
e
PET (b) and PET/CT (c) views. Patient also undergone
MRI of the pelvis, showing a hyperintense node in the
peripheral portion of the right prostate lobe (d), corresponding to the focus of tracer uptake in PET/MRI axial
view (e)
In fact, considering the large amount of neurooncologic amino acid PET tracers (also see
Chaps. 2, 4 and 11) and the limits of 18F-FDG in
neuro-oncological imaging as analog of glucose,
we must also state that this tracer is still the most
useful in the evaluation of all metabolic changes
and disorders that can occur in the human brain.
From 18F-FDG PET/CT imaging, the transitive property holds true also for the study of epilepsy by means of 18F-FDG PET/MRI. In fact,
this diagnostic tool is potentially more useful
than PET/CT in identifying epileptogenic foci
during the ictal phase, with the added value of the
optimal spatial resolution limit of MRI (Fig.
1.35)
[59]. The ultimate goal in neurology is to establish combined PET/MRI as the rst-line imaging
technique to provide in a single session all biomarker information required to increase diagnostic condence toward specic diagnoses, in
dementia, movement disorders, and other neurodegenerative diseases (Fig. 1.36) [60], with
improved patient comfort.
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Fig. 1.33 Multiple brain metastases from melanoma. In
particular, an area of hypometabolism is evident in axial
PET view (a) in left occipital region, in association with
1-cm-wide hyperintense lesion, with surrounding edema,
in corresponding MRI (b) and PET/MRI (c) views.
Similar ndings are displayed in left temporal region in
axial PET (d), MRI (e), and PET/MRI (f) views and in the
left cerebellar lobe in axial PET (g), MRI (h), and PET/
MRI (i) views
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Fig. 1.34 In a patient examined for suspicion of
Alzheimer’s disease with hybrid PET/MRI scanner, axial
PET (a, e), T1 (b, f), and T2 (c, g) weighted MRI images
show hypometabolism in right parietotemporal regions, in
the ipsilateral frontal lobe and in the left temporal lobe,
without meaningful morphological abnormalities. Fused
PET/MRI well summarizes this ndings (d, h)
ab c
Fig. 1.35 Interictal 18F-FDG PET in a young female
patient with temporal lobe epilepsy. Coronal PET and
fused PET/MRI views (a, b, arrows) display hyperme-
tabolism in the left temporal mesial cortex. The corresponding coronal T2 MRI view (c, arrow) shows increased
signal intensity, in the same area
ac
df
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b
e
Fig. 1.36 In a patient with ataxia and chronic headache,
axial SPECT view with
tracer activity in the basal ganglia. The axial
PET view (b) displays hypometabolism in both caudate
nuclei, in association with calcications on T2 weighted
123
I-ioupane (a) shows regular
18
F-FDG
(c) and FLAIR (d) MRI images. Fused PET/MRI view (e)
summarizes these ndings, while correlative CT (f) better
shows diffuse calcications in the basal ganglia. The nal
diagnosis was Fahr’s disease
1.7 Variants andPitfalls
Concerning the molecular processes at the
basis of 18F-FDG cellular uptake, it is also
The bio-distribution of 18F-FDG can vary according to fasting state, level of muscular exertion,
various drugs, and the length of the uptake period
after injection. Incidental pathologies with high
tracer uptake can be found in 25% of PET studies
[61] performed for several indications, while,
being the 18F-FDG the rst and more used PET
tracer, several studies in literature have well
described the possibility of uptake of this tracer
in inammation, benign lesions (Fig. 1.37),
malignancies concomitant to the disease under
study or consequences of surgical procedures
such as chemical pleurodesis [62, 63].
necessary to consider the possibility of uptake in
some physiopathological conditions which are
not strictly related to a specic disease but can
mimic a neoplasm, as in the case of thymic
hyperplasia, occurring in a signicant minority
amount of young adults (Fig.1.38) [64].
Splenosis is a further peculiar condition,
dened as an autotransplantation of the splenic
tissue after splenic rupture or splenectomy; it
occurs most frequently in the peritoneal cavity
and is usually asymptomatic. However, multiple
accessory spleens, showing moderate tracer
uptake, can represent a diagnostic dilemma in
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a
Fig. 1.37 18F-FDG uptake can be incidentally detected in association with pituitary adenoma, as evident in PET 3D
maximum intensity projection of the brain (a) and relative axial (b) and sagittal (c) PET/CT views
a b
c
Fig. 1.38 A case of thymic hyperplasia after chemother-
18
apy.
F-FDG PET 3D whole body maximum intensity
projection (a) of a young female patient following chemotherapy for inguinal non-Hodgkin lymphoma, showing
mild uptake in the upper mediastinum. PET/CT axial view
(b) conrms the uptake in hypodense tissue in the upper
mediastinum, without meaningful contrast enhancement
at CT (c)
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patients examined by 18F-FDG PET/CT for
somatic tumors (Fig.1.39) [65].
However, it is also mandatory to consider
the possibility of faint 18F-FDG uptake in
malignant disease with low rate of glucose
metabolism, as for bronco-alveolar carcinoma
(Fig.1.40) and some kinds of renal cells carcinoma (Fig.1.41). Furthermore, in the urinary
tract, some lesions can be also not easily recognizable due to the high gradient of tracer urinary excretion.
In the eld of neuro-oncological imaging,
some novel radiopharmaceuticals are replacing
the 18F-FDG, due to the lack of specicity of this
tracer and high rate of false negative cases that
can occur, especially when evaluating patients
with brain tumors or suspicion of brain tumor
relapse (Fig.1.42).
Beyond pitfalls linked to the tracer biodistribution, for 18F-FDG PET/CT and PET/MRI,
several studies or reports well described the possibility of technical artifacts, mostly related to the
movement of patient among the two scans
(Fig.1.43), as known for intestinal loops, lungs
during respiration, and head and neck, generally
due to the rotation of the neck of patients from
the PET scan and the CT or MRI scan. To date,
with the development of novel software for PET/
MRI image processing, this frequent artifact can
be satisfactorily managed (Fig.1.44).
a bc
Fig. 1.39 18F-FDG PET maximum intensity projection
(a) and coronal PET/CT view (b) show transplanted kidney in right iliac fossa (circles). In the same patient,
d
e
arrows in axial PET/CT views (c, d, e) show hypodense
lesions with moderate tracer uptake in the abdomen, due
to a condition of splenosis
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