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Application of
Positron Emission
Tomography-MR
Imaging in
Neuroradiology
Correspondence:
[email protected]
Ammar Chaudhry, MD
Maryam Gul, MD
Lev Bangiyev, DO
Robert Matthews, MD
Robert Peysterd, MD
Abstract No:
eEdE-29
Submission Number:
2753
Disclosures
•
NONE
Objectives
1.
Physical principles and techniques of PET-MRI: review image acquisition and post
processing
2. Utility of PET-MRI in neuro-oncology: role in initial tumor diagnosis, treatment
planning and post-treatment follow-up
3. Discuss key imaging findings that help exclude mimics resulting from
neurodegenerative disorders and inflammatory conditions.
4. Review common pearls and Pitfalls of PET-MRI.
5. Future of PET- MRI: Discuss current challenges facing PET- MRI in neuroradiology
Why PET-MRI?
•
Two Major Reasons:
 High anatomical resolution and tissue contrast
 particularly with soft tissue like brain, breast, liver, and pelvis
 bone malignancy
 Radiation dose reduction
 particularly for pediatric population and
 for adults who have multiple scans during their treatment
PET- Advantages
PET- Limitations
- Poor anatomical delineations
PET provides functional data
Relatively high sensitivity
Quantitative technique
Target specific tracers
- Molecular targets
- Flexibility of tracer, design and application
- Excellent signal to background ratio
- Lesser spatial resolution
- Uses radiation
- Imaging time 30-40 min
MRI- Advantages
- MRI provides anatomical data with
high anatomical resolution
- High soft tissue contrast
MRI- Limitations
- Lower molar sensitivity for
different metabolites
- No ionizing radiation
- Limitation with absolute
quantification
- Measures several metabolic
parameters
- Long imaging times with
multiple sequences
- Helps in motion correction*
- Helps in partial volume correction*
*Catana - JNM 2012, 53, 1916-1925: MR sequence provide high temporal resolution motion estimates
PET-MRI
- Complementary to each other
- Overcome each others limitations
- Synergistic effects
- Excellent tool in molecular imaging
PET: Principles
• PET
imaging is based on the principle of detection of
the two 511 keV annihilation radiations that originate
from β+ -emitting sources
• Two
photons of 511 keV are detected by two detectors
connected in coincidence
• Coincidence
• Data
detection forms the basis of PET imaging
collected over 3600 around the patient are used
to reconstruct the image of radioactivity distribution in
the slice of interest
PET: Coincidence
Detection
PET
PET- Imaging
•
PET had been the most rapidly growing area of medical
imaging by which molecular medicine is incorporated in to
medical practice, beginning with its use in early detection,
treatment planning, and monitoring of patients with cancer
•
The great advantage of PET over other types of imaging
modalities is that PET can in principle provide quantitative
information about the physiological processes occurring in
vivo
•
Therefore, it is very important to ensure that PET data are of
the highest accuracy and precision
Basic MR Components and
Principle
MRI system consists of the following components:
1. A Large Magnet
2. Several Coils:
 Shim coils for homogeneous magnetic field
 A RF coil to transmitting the radio signal to the body part to be
imaged
 A receiver coil for detecting the returning radio signals
 Gradient coils for providing spatial localization of the signals
3. Computer System
Basic Factors Involved in MRI Signal
Intensity
• Proton density:
 Proton density is the concentration of protons in the tissue in the
form of water and macromolecules i.e. proteins, fat
• T1 and T2 relaxation times:
 The T1 and T2 relaxation times define the way that the protons
revert back to their resting states after the initial
RF pulse
• Flow:
 The most common usage of flow is to produce angiography
Hybrid Imaging:
Rationale
- Independent studies
Hybrid Imaging:
Challenges

- Side-by-side comparison
- Fusion of separate studies
- Co-Registration
Technical Challenges in PET-MRI

Conventional PET uses PMTs sensitive to magnetic field

MRI uses high frequency, high RF that interfere with PET

MRI FOV should not be obstructed with material of high
magnetic susceptibility

Adequate shielding for PET detectors
- SPECT-CT
- PET-CT
- Now PET-MRI

Operational Challenges in PET-MRI

MRI-PET or PET-MRI or mMR- PET

Radiologists vs. Nuclear Physicians and bringing people
together

Cross education of each modality

Operational Issues (Licensing!!)

Changes in a way to teach/practice NM
Differences in Current PET/MRI
Technology
Three Imaging Companies:
- GE Healthcare Solutions:
PET/CT + MR Trimodality Imaging
- Philips Medical Systems:
Ingenuity-TF PET/MRI
- Siemens Medical Solutions:
Biograph mMR Technology
GE: PET-MRI
Philips: PET-MRI Siemens: PET-MRI

Philips Ingenuity-TF PET-MRI

Siemens Biograph mMR
•
PET/CT +MR: Trimodality Imaging

Sequential PET and MR imaging

•
Two Separate modalities

Two separate gantries almost 10 ft
apart
PET-MRI housed in one device – a
PET ring detector fits 3T magnet

Whole body integrated PET-MRI
scanner
•
PET-CT and MRI

High quality MRI
•
Images are not compromised

High quality PET

Integral unit

Data merged and analyzed by
specialized software

Simultaneous PET/MRI scans

Reduces imaging time

Limitations of attenuation
correction
•
Images Fused with specialized
software
PET
PET-CT
MRI
MRI
PET-MRI
PHOTOMULTIPLIER TUBES
The photomultiplier tubes used in conventional PET
scanners are very sensitive to magnetic fields.
New technology based PMTs:
- Silicon based
- Avalanche photo-diodes
“Avalanche Photo-Diode Detector (APD)”
Structural
Design:in
mMR
Detector
Assembly
mMR
Siemens website
Attenuation
Correction
•
Conventional transmission is not possible
•
Derivation of attenuation data from MRI is
different and is not density correlated with
material density
•
MRI data cannot be linearly transferred to CT
•
MR based attenuation correction provides
biased information in comparison to CT
based
Attenuation
Correction in mMR

Sophisticated approach for Attenuation
Correction:

Image segmentation

ATLAS based approach

Dedicated MRI sequences which generate
rich signals

Predicts a Pseudo CT pattern, creates µmaps

Generates a whole body ATLAS/Pattern
recognized Attenuation Correction
PET-MRI Imaging Protocol
PET- PROTOCOL
•
Shallow free breathing
•
4-5 bed positions from mid-thighs
cephalad
•
WHOLE-BODY MRI
•
Simultaneous image acquisition with
PET
•
Routine protocol includes whole body
axial T2, axial T1 TSE, axial DWI
~ 10 minutes per bed position
•
Axial field of vies = 25.8 cm
•
Reconstruction algorithm





3D AW OSEM
3 iterations
21 subsets
Zoom I
Gaussian smooth of 4mm FWHM
 Most cases (esp. tumor imaging)- obtain
axial whole-body T1FS post contrast
 +/- sagittal STIR for spine cases
•
Image acquisition time approx. 40
minutes
64 year old male with lung cancer
presents with worsening headaches
A 1.8 cm left frontal lobe mass at the gray-white junction with mass effect and surrounding vasogenic
edema with corresponding increased FDG uptake, compatible with metastasis
64 year old male with lung cancer
presents with worsening headaches
An additional 2.3 cm right temporal lobe mass at the gray-white junction with mass effect and
surrounding vasogenic edema with corresponding increased FDG uptake, consistent with metastasis
49 year old male with CKD and GBM
status post resection
Postsurgical changes are seen in the right frontal lobe
without evidence of abnormal PET uptake to suggest
recurrence or residual disease
57 year old male with patient with history of GBM, with
worsening headaches
Conventional MRI demonstrates nodular foci of enhancement with corresponding increased FDG uptake
on PET-MRI, consistent with recurrence.
Additional views demonstrate satellite lesions which did not
demonstrate increased post-contrast enhancement
Case: 40 year old with seizure. MRI reviews enhancing lesion with restricted
diffusion with +FDG avidity
Bx: +Left temporal lobe glioma
Case: 66 y/o female with
worsening headaches,
altered mental status and
paresthesias
CT w/o contrast: Hyperdense brainstem mass with perilesional edema was concerning for acute hemorrhage
MRI: Heterogenous T1 Hypointense T2/FLAIR hyperintense heterogenous lesion demonstrating postcontrast
enhancement and areas of susceptibility artifact
PET-MRI: reveals hypermetabolic lesion in the brain stem with 29.7 SUV most compatible with a neoplastic process.
Biospy revealed metastatic renal cell carcinoma
Case: 56 year old female with breast cancer presents with
headaches
Pre-Treatment MRI reveal
leptomeningeal T2/FLAIR
hyperintense lesion with postcontrast enhancement. There is
marked associated FDG-PET
activity, suggestive of breast
cancer leptomeningeal
metastasis,
Post-Treatment: On routine MRI, there is residual FLAIR
activity with mild post-contrast enhancement. However, on
PET-MRI, there is no residual activity, confirming successful
treatment.
Case: 56 year old female with breast cancer. PET MRI
performed for surveillance
MRI reveal T1 isointense T2 hyperintense (relative to skeletal muscle) lesion in the left parotid gland. No additional
soft tissue abnormality was seen on whole-body MRI.
PET-MRI images reveal increased FDG avidity in the left parotid lesion.
Routine MRI appearance were suggestive of pleomorphic adenoma, however, the lesion was biopsied due to increase
FDG avidity and confirmed diagnosis of benign mixed parotid tumor.
CASE: 54 YEAR OLD MALE WITH SEIZURE
Anti-LGI1+ Paraneoplastic syndrome
Case: 23 year old male with seizures, gait instability, visual
hallucinations, and memory loss
• CT with contrast and MRI
Brain were unremarkable.
• PET-MRI: reveals abnormal
metabolic acitivity in left
frontotemproparietal lobes,
bilateral occipital lobes
including visual cortex, and
cerebellum (left worse than
right).
• PET-MRI findings raised
suspicion for Limbic
encephalitis. Patient tested
positive for anti-NMDA
antibodies and was treated with
IVIG followed by a tapering
dose of corticosteroids.
CONCLUSION
•
PET-MRI is an emerging hybrid imaging modality offering detailed functional and structural imaging
with promising clinical applications especially in the field of oncology, infectious and inflammatory
conditions.
•
Familiarity with the technical and clinical aspects of PET-MRI along with knowledge of common
pearls and pitfalls of PET-MRI will aid in better integration and relevant usage of this modality in
clinical practice.
•
Future applications include:
 Differentiate between neoplastic and non-neoplastic conditions using advanced MRI technique (DWI, DKI, etc)
in conjunction with functional information obtained from PET
 Role in initial tumor diagnosis, treatment planning and post-treatment follow-up
 Evaluation of neurodegenerative disorders
End
Presentation
Please send correspondence to: [email protected]
Thank you