PET/MR Advantages and Challenges M. Mehdi Khalighi GE Healthcare

Outline Review Magnetic Resonance Imaging (MRI) Introduction to Magnetic Resonance and Spectroscopy (Pippa Storey) Review Positron Emission Tomography (PET) PET: Physics, Instrumentation and Scanners (Michael Phelps) Why hybrid PET/MR? Challenges of making hybrid PET/MR

MRI Review: Spin Precession Each hydrogen atom has a spin. Acts like a little magnet. There are lots of H atoms in body (2 for each water molecule) also in fat. MR is good for soft tissue but not so much for bone. 2 things happen to a spin in magnetic field, alignment and precession.

MRI Review: Spin Precession / Alignment The anti parallel has a higher energy state than parallel.

MRI Review: Spin Alignment Some spins go to parallel and some to anti parallel randomly.

MRI Review: Net Magnetization When they add up, they generate the Net Magnetization vector. The diff. between parallel and anti parallel is 1 ppm.

MRI Review: Resonance (Excitation) Can’t measure/image M in equilibrium state. Need to move it away from equilibrium to the transverse plane. This is done by another rotating field called B1 (radio frequency waveform).

MRI Review: Resonance (Excitation) The frequency of RF should match the spin precession (larmour freq) freq for resonance to happen.

MRI Review: Free Induction Decay (FID) After RF is cut off, M goes back to equilibrium. It spins with the Larmour freq and the signal is recorded.

MRI Review: Relaxation The transvers magnetization decays and the longitudinal magnetization recovers back to equilibrium. Different proton in different molecules have different T1 and T2. Felix Bloch Physic’s Nobel Prize in 1952 for his work in 1946. He was the first professor for theoretical physics at Stanford.

MRI Review: Gradient Concept Paul Lauterbur 1929-2007 Peter Mansfield 1933- We needed a way to create spatial resolution. Using gradient to change magnetic field was proposed by P. Lauterbur. It took about 30 years to go from a voxel to an image. Lauterbur and Mansfield gor Nobe prize in Phsiology and Medicine in 2003 !! Why did it take so long? The first MR machine was built in late 70s. GE first commercial machine was 30 years ago 1984.

MRI Review: Gradient Concept Frequency encoding with gradients for spatial resolution.

MRI Review: 2D Imaging 2D imaging and spatial coding of MR signal. A slice is excited and then X and Y axis are encoded with frequency and phase.

MRI Review: Image Reconstruction The raw image captured is in frequency domain and has to be converted back to image space. Options to speed up: Accelerated readout, Parallel imaging and compress sensing.

Winding Magnet Science Channel, How stuff works, A program on GE factory in Florence, North Carolina.

Locking the Winding in Epoxy Treated with epoxy resin. 700 Amp

MRI Hardware Overview Active Shielding, Supercon Shimming,

PET Review: Annihilation A way for molecular imaging. Inject FDG to measure it’s uptake to find malignant tumors. FDG: fludeoxyglucose The uptake of 18F-FDG by tissues is a marker for the tissue uptake of glucose, which in turn is closely correlated with certain types of tissue metabolism

PET Review: Detector The first part of detector is to detect each 511 keV photon

PET Review: Detector The second part of the detector is to match coincident pair of 511 keV photons.

PET Review: Detector Ring Need to put the detectors all around the subject to captures all incidents.

PET Review: Photon Interactions in Matter Compton Scatter Photoelectric Effect Photons interact with matter in 2 ways Photoelectric and Compton. Both of them are source of errors.

PET Review: Positron Range Other source of error is positron range.

PET Review: Non-Colinearity Another source of error is non-colinearity

PET Review: Coincidence Event Types We are interested in true coincidence but random and scattered events limit the image quality. So generally PET images are poor in resolution.

Why PET/MR? Courtesy of Dr. Craig Levin (Stanford Univ.) PET/MR is better than PET MR. Combination of PET/MR provides more info to Radiologists. PET images can be reconstructed better using MR attenuation map. Courtesy of Dr. Craig Levin (Stanford Univ.)

PET/MR Advantages From Dr. Alexander Guimaraes (MGH) High resolution MR image and poor PET image but highly sensitive.

PET/MR Advantages From Dr. Alexander Guimaraes (MGH) Combining high resolution and high sensitivity

PET/MR Advantages Reconstructing better PET images using MR images as prior info.

PET/MR Advantages PET/MR is better for soft tissue that PET/CT and has lower radiation. Slide from: Dominique Delbeke, Vanderbilt University Medical Center

System Overview Minimum material between the patient and detectors. All attenuation has to be accounted for and corrected. More attenuation affects SNR.

System Overview Split bridge is designed to minimize attenuation. Mechanical stability is a challenge. Motion and vibration is a problem with MR.

PET Detector SiPM Module PMT-based Module AA battery (size comparison) New detectors are smaller and more efficient. Easier to fit in tight magnet. Magnet real state is extremely expensive. Disturb the magnetic field less since they have less metal. Magnet shimming is expensive and creates problem.

PET Tube 28 modules. Each module is water cooled and gets powered. Fiber optic connection. Body coil RF shield gets closer to the rungs in the middle. This is challenging. Modules are shielded because RF will interfere with them. Temperature stability is important.

The very first PET/MR images. Patient was injected with 10mCi of FDG for PET/CT and scanned roughly 2hrs later in PET/MR. Stunning images. According to radiologist: Simply beautiful.

We might be able to reduce the dose with the new PET/MR. First not CT radiation. Very important for Pediatric. Might be able to do more than one PET/MR in a year for patients follow up.

Thank You.