Clinical Utility of PET-MRI in neuro-oncologyAmmar Chaudhry, MD

Maryam Gul, MD

Jared Dunkin, MD

Robert Peyster, MD

Dinko Franceschi, MD

Robert Mtthews, MD

Lev Bangiyev, MD

Correspondence: ammarchaudhry84@gmail.comeEdE-70

Disclosures• NONE

Objectives1. 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 provides functional data

Relatively high sensitivity

Quantitative technique

Target specific tracers

- Molecular targets

- Flexibility of tracer, design and application

- Excellent signal to background ratio

PET- Limitations - Poor anatomical delineations

- Lesser spatial resolution

- Uses radiation

- Imaging time 30-40 min

MRI- Advantages

- MRI provides anatomical data with high anatomical resolution

- High soft tissue contrast

- No ionizing radiation

- Measures several metabolic parameters

- Helps in motion correction*

- Helps in partial volume correction*

*Catana - JNM 2012, 53, 1916-1925: MR sequence provide high temporal resolution motion estimates

MRI- Limitations

- Lower molar sensitivity for different metabolites

- Limitation with absolute quantification

- Long imaging times with multiple sequences

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 detection forms the basis of PET imaging

• Data collected over 3600 around the patient are used to reconstruct the image of radioactivity distribution in the slice of interest

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

PET: Coincidence Detection PET

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

- Side-by-side comparison

- Fusion of separate studies

- Co-Registration

- SPECT-CT

- PET-CT

- Now PET-MRI

Hybrid Imaging: Challenges

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

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

• PET/CT +MR: Trimodality Imaging

• Two Separate modalities

• PET-CT and MRI

• Images are not compromised

• Images Fused with specialized software

PET-CT MRI

Philips: PET-MRI Philips Ingenuity-TF PET-MRI Sequential PET and MR imaging Two separate gantries almost 10

ft apart High quality MRI High quality PET Data merged and analyzed by

specialized software

PET MRI

Siemens: PET-MRI Siemens Biograph mMR PET-MRI housed in one device – a

PET ring detector fits 3T magnet Whole body integrated PET-MRI

scanner Integral unit Simultaneous PET/MRI scans Reduces imaging time Limitations of attenuation

correction

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: mMR

Siemens website

Detector Assembly in mMR

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 µ-maps

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

• ~ 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

WHOLE-BODY MRI

• Simultaneous image acquisition with PET

• Routine protocol includes whole body axial T2, axial T1 TSE, axial DWI Most cases (esp. tumor imaging)-

obtain axial whole-body T1FS post contrast

+/- sagittal STIR for spine cases

• Image acquisition time approx. 40 minutes

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.

Case: 78 year old male with memory loss

• PET-MRI reveals diffusely dreased cortical metabolic activity most prominent in bilateral parietotemporal region, most consistent with Alzheimer’s Dementia

Case: 58 year old male with subacute memory loss, visual hallucinations and difficulty swallowing

PET-MRI: Markedly abnormal FDG distribution with prominent hypometabolism is involving bilateral parietal and occipital lobes also predominantly posterior and lateral aspect of the temporal lobes. Mild to moderate hypometabolism is noted in the left frontal lobe and posterior cingulate. The ventricles, sulci, fissures are prominent for patient's age; Findings most consistent with Lewy Body Dementia

Case : Patient with seizures, had negative routine MRI; however, on PET-MRI, + right frontal lobe hypometabolic focus likely the source of seizure

Case: Patient with history of GBM, with worsening headaches. Conventional MRI suggestive of post radiation changes. + on PET-MRI,

consistent with recurrence.

Case: GBM recurrence- initially thought to have post radiation changes. Additional views demonstrate satellite

lesion

Case: GBM recurrence with + satellite lesions

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 post-contrast 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: 58 year old smoker with dysphagia

MRI reveals T2 hyperintense lesion in the right palatine tonsil. PET-MRI reveals corresponding increased FDG avidity in the lesion, concerning for neoplasm.

PET-MRI also reveals additional T2 hyperintense FDG avid right level II lymph node, concerning for metastasis.Lymph node was biopsied confirming diagnosis of metastatic squamous cell carcinoma

Case: 86 year old female with growing left jaw mass

MRI: T1 hypointense, T2 hyperintense lesion in the left submandibular region with mild peripheral enhancement

PET-MRI reveals cystic lesion with increased peripheral radiotracer activity

Biopsy was performed

Dx: Basal cell carcinoma

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 PresentationPlease send correspondence to: Ammarchaudhry84@gmail.com

Thank you