| Affiliation |
(AGL) Chairman, Department of Ophthalmology, The Methodist Hospital, Houston, Texas; Professor of Ophthalmology, Weill Cornell Medicine, New York City, New York; (ZF) Class of 2020, Baylor College of Medicine, Houston, Texas |
| Transcript |
We'll be talking about PET scan: Positron Emission Tomography. A tomogram is a slice. A tomogram of an emission of a particle - subatomic particle - called a positron. Positron is the positive antimatter equivalent to the negative electrons, and so what we're doing with the PET scan is, instead of imaging the patient by delivering radiation, or putting them in a magnetic field, or hitting them with ultrasound or light, we are giving the patient a radioactive substance that will decay and release a positron to get back to its lower energy state. And for PET that is F18DG: fluorine deoxyglucose. So we can bind the radioactive fluorine 18 to the glucose molecule, and the sugar molecule will go wherever there's metabolic activity, because the sugar goes where the metabolic need is. And so when we're doing a PET scan, we are doing a functional imaging study rather than a structural imaging study. A functional imaging study tells us about function or metabolism or perfusion or brain activity, but a structural study is like a CAT scan or an MRI or an x-ray. So this is a functional study rather than a structural one. So once we inject the FDG18 the glucose will travel to the places where there's metabolic activity, the fluorine will emit a positron and when the positron is emitted, it will hit its antimatter particle, and when the positive and negative meet, an annihilation event occurs with complete obliteration of the positron and the electrons and the mass will be converted to energy. So even though we have a subatomic annihilation event, because the amount of energy that's released by that is so great, two photons will be released and those photons of energy can be imaged. The photons are emitted and so we can tell where the photon came from. This PET scan then has to be paired with a structural imaging study, so that we know where what structure it came from, and so a CT scan and a PET scan are overlaying so that we can see where the positron came from that emitted the photon, paired against the CT scan so we can tell what it came from. So we have to pair the structural and the functional imaging study so that we can see where the positron was emitted. So back to the clinic ,when would we use this PET scan? We're going to be using it when we're trying to find something with high metabolic activity like a tumor. So it's often used for staging the metastatic please like lymphoma, or lung cancer, because we can find a highly metabolic tumor which is wanting to eat the glucose, and then when the positron is released the annihilation event and we can go find the tumor. Or if we're looking for a place for a biopsy, like a lymph node in sarcoidosis, in a patient who has uveitis, but we don't go biopsy their eyes, so we're go find a lymph node. PET scan might show it while our structural imaging study, like a chest x-ray, or CT chest might show nothing. And we can use it for hypometabolism, when the metabolism is too low, and the way that would come to us is if you have a homonymous hemianopia, or cortical visual loss, you did the structural imaging study - the cat scan or MRI - and it showed nothing, but then we do the PET scan and there's no metabolism in the occipital lobe. That would be like the visual variant of Alzheimer's or posterior cortical atrophy, where there's no activity in the brain even though there's no tumor there, and no stroke because they have Alzheimer's dementia. So we can use PET scan to look for metabolic lesions (high or low), tumors (lymphomas), or sarcoidosis, or low (Alzheimer's dementia, posterior cortical atrophy). It is a completely different imaging modality than structural imaging studies because the patient is the emitter, and what they're emitting is a subatomic particle called a positron, and that is the basis of the PET scan. |
