COMPARISON BETWEEN THE CLINICAL

APPLICATONS OF PET/CT AND PET/MRI

BYDR. WALID REZK, M.D.

LECTURER OF RADIODIAGNOSISAIN SHAMS UNIVERSITY

PRINCIPLE OF PET-CT FDG PET is a strictly functional modality with anatomical

landmarks. Unless anatomical correlation is available to delineate

normal structures, pathological sites of FDG accumulation can easily be confused with normal physiological uptake, leading to false-positive or false-negative findings.

Coregistration of PET scans with CT using a combined PET-CT scanner improves the overall sensitivity and specificity of information provided by PET or CT alone .

PET-CT provides more precise anatomical definition for both the physiological and pathological uptake seen at FDG PET

PET/CT MACHINE

ADVANTAGES OF PET-CT Accurate localization of small areas of increased

radiotracer activity that would have been difficult or not possible to localize on PET images alone .

Diffrentiating between structures that normally show high metabolic activity from those with abnormally increased activity.

PET-CT combines the advantages of the excellent functional information provided by PET and the excellent spatial and contrast resolution of CT

Finally, attenuation correction for quantitative or semi quantitative assessment of data is possible by using the CT data.

LIMITATIONS AND ARTIFACTS OF PET-CT In PET/CT, the images are acquired

sequentially (PET after CT). This can cause image artifacts due to respiration motion, bowel movement, or patient movement between the 2 scans

Attenuation correction artifacts due to highly attenuating objects in the path of the CT beam, such as hip prostheses, pacemakers, dental devices, and contrast-enhanced vessels

PRINCIPLE OF PET-MRI

A NEW PAGE ADDED TO HISTORY OF IMAGING….PET / MRI approved by FDA in June 2011.

In these five years: The technology continue to struggle

with learning how to integrate the system with workflow.

Comparing the utility of PET/MRI versus PET/CT.

Determining which organs and disease processes are best imaged with PET/MRI

TYPES OF PET/MR SCANNERS

A. Sequential Scanners: PET and MR gantries arranged along axis with patient handling system between. This implementation does not allow for simultaneous data acquisition and, therefore, results in longer examination times.

TYPES OF PET/MR SCANNERS

B. Simultaneous Scanners: Integrated PET and MR allows simultaneous PET/MRI data acquisition

ADVANTAGES OF INTEGRATED PET/MR

Excellent registration of PET and MR images

No time required to shuttle patientLess physical space needed

BENEFITS OF PET/MRExcellent human anatomical information.

Superior soft tissue characterization.

Better temporal resolution.

Less Radiation Hazards.

MRI allows for additional techniques - such as angiography, functional MRI ,diffusion ,spectroscopy and perfusion techniques within one single examination.

DISADVANTAGES OF PET/MR Longer acquisition time Incapable with metal artifacts (pacers or

implants) Claustrophobia CT superior for lung imaging

QUANTITATIVE COMPARISON OF PET/CT AND PET/MR IMAGING

Regarding quantitative analysis of tracer uptake by means of SUV-based analysis, significantly lower SUVs have been observed for data acquired on the PET/MR than on the PET/CT scanner for suspected lesions and the normal different organs assessed.

The decrease of mean SUV in suspected lesions is much less pronounced than the decrease in the normal system organs.

Consequently, in PET/MR the lesion-to-background contrast is higher than PET/CT, thus increasing the conspicuity of the lesions.

MAIN CLINICAL APPLICATIONS OF BOTH PET/CT AND

PET/MRI

Diagnose malignant tumors Select and monitor therapy Detect recurrent tumors before they

can be seen on CT or other imaging modalities

Detect metastasis

A) Oncologic applications

BENEFITIAL ROLE OF PET/MRI OVER PET/CT IN ONCOLOGICAL USES: Highly accurate in T-staging of tumors for

which MRI is routinely employed, such as head and neck cancer and primary bone and soft tissue tumors.

For N-staging, the performance will probably be similar to PET/CT although use of the novel lymphotropic superparamagnetic nanoparticles may provide new opportunities for detection of nodal micrometastases that is not possible with PET/CT.

For M-staging, higher accuracy for detection of lesions particularly in the brain, liver, and bone.

PET/MRI is expected to supersede PET/CT in imaging cancers which are anatomically better defined by MRI compared to CT, due to its superior soft-tissue contrast.

These include: Brain Head And Neck Breast Liver Musculoskeletal System Urogenital Tumors

While PET/CT is likely to remain the preferred imaging modality for lung and mediastinal nodal disease.

APPLIED CASES

BRAIN METASTASIS

Large left-hemisphere metastasis (black arrow) is visible on (from left to right) axial contrast-enhanced CT, 18F-FDG PET, PET/CT, axial contrast-enhanced MRI, and PET/MRI, whereas smaller metastasis of left frontal lobe (white arrow) is visible solely on MRI and PET/MRI. Location of this metastasis directly adjacent to highly 18F-FDG–avid cortex leads to problems with diagnosing this lesion on 18F-FDG PET scan.

LIVER METASTASES A: Coronal T2 weighted

STIR MR, B+E: FDG-PET and C: fused PET/ MR illustrating several metastases to the liver, but only some of them PET positive. D-F: Transaxial DWI MRI and FDG-PET illustrating some but not complete overlap between metastases with high signal intensity on DWI MRI and FDG-PET. Finally G-I: illustrates transaxial CT and PET from the PET/CT scanner, with hardly any metastases being visible on the liver on CT-images (not contrast enhanced). 

CERVICAL CANCER

A: CT-scan, B: FDG-PET scan performed on the PET/CT scanner, C: Fused PET/CT image –note the mismatch between the bladder on CT and PET due to the difference in uptake time, D: MR-scan, T2 weighted, E: PET scan acquired simultaneous with MR, F: fused PET/MR image –note the perfect fit with bladder on MR and PET.

CANCER PROSTATE 71 year old

patient with biopsy proven prostate cancer, referred for PET/CT for primary staging.

PET/CT revealed adcanced disease (primary prostate cancer, iliacal and pararectal lymph node metastases) (A 1-3) CT scan, (B 1-3) PET scan, (C 1-3) PET/CT fused images.

CANCER PROSTATE

A 72-year old patient with prostate cancer. (A) ADC map shows low signal intensity in the central gland (arrow), which is confirmed by 18F-choline PET/MRI (arrow in B) to be metabolically active central gland tumor.

LUNG METASTASIS

Axial CT and axial PET/CT images show the large metastasis in the left lung.

The lesion is only detectable in co-registered PET/MRI images but undetectable on the MRI.

PET/MRI IN A PATIENT WITH NON-SMALL CELL LUNG CANCER

The contrast-enhanced T1-weighted coronal image (a) is fused with the ADC (b), FDG-PET (c) and DCE (d).

Multiparametric PET/MRI allows for a more holistic understanding of tumor and tumor stroma, with better cancer characterization and therapy control.

B) NON-ONCOLOGIC APPLICATIONS

Localize epileptogenic focus in patients with epilepsy

Differentiate Alzheimer’s disease from multi-infarct dementia or depression

Analyze Parkinson’s diseaseEvaluate extent of stroke and

recovery following therapy

1) NEUROLOGICAL APPLICATIONS:

ALZHEIMHER’S

Axial FLAIR (A) cerebral atrophy; Axial T1wMPRAGE (D) Reveals hippocampal atrophy; Axial fused PET MRI (B and E) Sagittal (C) and coronal PET (F) Reveal bilateral fronto-temporal and parietal hypometabolism (white arrowhead). Note well preserved glucose metabolism in sensorimotor cortex and occipital lobes (white arrow) on sagittal PET images consistent with established AD.

EPILEPSY

Simultaneous PET/MRI study in epilepsy patient. From left to right are shown axial FDG PET, high-resolution MRI scan, and fusion image. Distinct hypometabolism is visible in polar region of left temporal lobe, typically corresponding to epileptogenic focus. 

Detect presence of coronary artery disease

Assess the extent of damage from heart disease (is the patient a bypass candidate?)

Determine which patients will benefit from cardiac transplantation

2) CARDIAC APPLICATIONS:

CARDIAC PET CT IMAGING The stress

images show a severe perfusion defect throughout the anterolateral wall that is completely reversible at rest

CARDIAC PET MRI IMAGING

A patient with acute myocardial infarction due to occlusion of LAD. Late gadolinium-enhancement reveals transmural septal infarction (a, arrows) that corresponds to a clearly reduced FDG uptake (b).

a

b

TAKE HOME MESSAGE

“With PET/CT, we’ve been able to localize things. Now we can give more precise location with MRI than with CT. MRI has the capability of doing functional imaging, combined with physiology-based imaging of PET. We’ve doubled the amount of things we can see with this.”

“Since MRI provides more biological and functional data than CT — without radiation —the system is a huge advantage for pediatric patients and those needing multiple scans”

“PET/MRI may not replace PET/CT for older adults who have cancers easily worked up by PET/CT, but selected applications like pediatric malignancies, pelvic malignancies, head and neck cancer, those areas are going to be extremely promising.”

THANK YOU