PowerPoint Presentation

PET/MR Advantages and ChallengesM. Mehdi KhalighiGE HealthcareReview 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 Outline

MRI Review: Spin PrecessionEach 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.3

MRI Review: Spin Precession / AlignmentThe anti parallel has a higher energy state than parallel.4

MRI Review: Spin AlignmentSome spins go to parallel and some to anti parallel randomly. 5

MRI Review: Net MagnetizationWhen they add up, they generate the Net Magnetization vector.The diff. between parallel and anti parallel is 1 ppm.6

MRI Review: Resonance (Excitation)Cant 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). 7

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

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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.9MRI 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 Physics Nobel Prize in 1952 for his work in 1946.He was the first professor for theoretical physics at Stanford.

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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 1984.11

MRI Review: Gradient ConceptFrequency encoding with gradients for spatial resolution.12

MRI Review: 2D Imaging2D imaging and spatial coding of MR signal.A slice is excited and then X and Y axis are encoded with frequency and phase.13

MRI Review: Image ReconstructionThe 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.14

Winding MagnetScience Channel, How stuff works, A program on GE factory in Florence, North Carolina.15

Locking the Winding in EpoxyTreated with epoxy resin.700 Amp

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MRI Hardware OverviewActive Shielding, Supercon Shimming,17

PET Review: AnnihilationA way for molecular imaging. Inject FDG to measure its uptake to find malignant tumors.FDG: fludeoxyglucoseThe 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 metabolism18

PET Review: DetectorThe first part of detector is to detect each 511 keV photon19

PET Review: DetectorThe second part of the detector is to match coincident pair of 511 keV photons.20

PET Review: Detector RingNeed to put the detectors all around the subject to captures all incidents.21

Photoelectric Effect

Compton ScatterPET Review: Photon Interactions in MatterPhotons interact with matter in 2 ways Photoelectric and Compton. Both of them are source of errors.22

PET Review: Positron RangeOther source of error is positron range.23

PET Review: Non-ColinearityAnother source of error is non-colinearity 24

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.25

Courtesy of Dr. Craig Levin (Stanford Univ.)Why PET/MR?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.26

From Dr. Alexander Guimaraes (MGH)PET/MR AdvantagesHigh resolution MR image and poor PET image but highly sensitive.27

From Dr. Alexander Guimaraes (MGH)PET/MR AdvantagesCombining high resolution and high sensitivity 28

PET/MR AdvantagesReconstructing better PET images using MR images as prior info.29

PET/MR AdvantagesSlide from: Dominique Delbeke, Vanderbilt University Medical Center PET/MR is better for soft tissue that PET/CT and has lower radiation.30

System Overview

Minimum material between the patient and detectors. All attenuation has to be accounted for and corrected. More attenuation affects SNR.31System Overview

Split bridge is designed to minimize attenuation. Mechanical stability is a challenge. Motion and vibration is a problem with MR.32

PMT-based ModuleSiPM ModuleAA battery(size comparison)PET DetectorNew 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.33PET 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.34

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.35

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.36Thank You.