pet_spect

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Imaging: PET and SPECT Positron Emission Tomography Single Photon Emission Computed Tomography

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PET and SPECT Properties of ideal imaging nuclides, biological, chemical , physical Production of radionuclides Nuclear fission Charged particle bombardment The Tc-99m Generator Chemistry Chelators vs organic chemistry Delivery strategies Blood brain barrier Metabolic pathways Chemical affinity Clinical applications Tumor imaging and staging Cardiac imaging Gene therapy Brain function Dopamine pathways, addiction

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Imaging Image removed.

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Radionuclides SI unit is the Becquerel (Bq) 1Bq =1dps (disintegration per second ) old unit is the Curie (Ci ) 1Ci = 3.7 X1010 dps Activity (A) =rate of decay No =number of active nuclei at time t = 0 N(t) is the number of active nuclei at time ‘t’ ?is the decay constant ?=0.693/T(T=half-life) dN/dt =-?N(t) N(t) =Noe-?t A(t) =Aoe -?t

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Effective Half-Life Physical half-life, TP [radioactive decay] Biological half-life, TB [clearance from the body]

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Effective Half-Life E.g., for an isotope with a 6-hr half life attached to various carrier molecules with different biological half-lives. TP TB TE 6 hr 1 hr 0.86 hr 6 hr 6 hr 3 hr 6 hr 60 hr 5.5 hr 6 hr 600 hr 5.9 hr

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Effective Half-Life Assume 106 Bq localized in atumor site, vary T

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Effective Half-Life Assume 1010 atoms of radionuclide localized in a tumor site, vary T

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Production of Radionuclides Reactor production, Nuclear fission •Heavy nuclides (A>230) capture a neutron; tend to fission •Daughter nuclides of ~half the parent mass are produced •Possible to purify nuclides carrier free (chemically different) •Nuclides generally neutron rich and decay by ß- emission

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Production of Radionuclides Image removed.

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Production of Radionuclides Image removed.

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Production of Radionuclides Cyclotron production: Charged particle bombardment • Accelerates charged particles to high energies • Nuclear reactions have threshold energies • The product is different than the target • Nuclides can be produced carrier-free

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Production of Radionuclides Image removed.

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Properties of the ideal diagnostic radiopharmaceutical 1. Pure gamma emitter 2. 100 < gamma energy < 250 keV. 3. Effective half-life = 1.5 X test duration. 4. High target:nontarget ratio. 5. Minimal radiation dose to patient and Nuclear Medicine personnel 6. Patient Safety 7. Chemical Reactivity 8. Inexpensive, readily available radiopharmaceutical. 9. Simple preparation and quality control if manufactured in house.

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Properties of the ideal diagnostic radiopharmaceutical One nuclide comes close to being the ideal gamma-emitting nuclide Technetium-99m (99mTc) •Half-life = 6hr •Almost a pure ?ray emitter •E = 140 keV •can be obtained at high specific activity and carrier free

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Nuclides 99mTc is a decay product of the fission product 99Mo Image removed. 99mTc

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Table of the nuclides

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Decay scheme for 99mTc 99Mo decays to 99mTc by ß- emission (99Mo: T= 67 hrs) 99mTc excited nuclear state decays by ?emission (140 keV) to ground state 99Tc (99mTc: T=6hrs) 99Tc (ground state) decays by ß- emission to 99 Ru (stable isotope) (99Tc: T=2x105 years) Image removed.

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Radioactive equilibrium Parent N1 decays to daughter N2, both are radioactive. Special Case: Transient equilibrium

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Radioactive Decay Example 99Mo (T = 67 hrs) 99mTc (T = 6 hrs) Image removed. Fig. 4.5 in Turner J. E. Atoms, Radiation, and Radiation Protection, 2nd ed. New York: Wiley-Interscience, 1995.

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The 99mTc Generator 99Mo is adsorbed on an alumina column as ammonium molybdate (NH4MoO4) 99Mo (T =67 hrs) decays (by ß- decay) to 99mTc (T = 6hrs) 99MoO4 ion becomes the 99mTcO4 (pertechnetate) ion (chemically different) 99mTcO4 has a much lower binding affinity for the alumina and can be selectively eluted by passing physiological saline through the column. Image removed.

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Chelators Image removed. Technetium Pentetate bond structure Image removed. 99mTc Mertiatide bond structure DTPA EDTA ethylenediaminetetraacetate

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Chelators Image removed.

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Production of Radionuclides Cyclotron production •Products are proton rich, neutron deficient •Decay by ß+ decay •Positron emitters Image removed

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Chart of the Nuclides

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Cyclotron Production Targets O-15: 14N(d,n)15O; deuterons on natural N2 gas; 15O2 directly or C15O2, by mixing 5% carrier CO2 gas. C-11: 14N(p,a)11C; protons on natural N2 gas: including 2% O2 produces 11CO2 N-13: 16O(p,a)13N; protons on distilled water F-18: 18O(p,n)18F; protons on 18O-enriched water (H218O),. Fluoride is recovered as an aqueous solution. For nucleophilic substitution. F-18: 20Ne(d,a)18F; deuterons on neon gas. For electrophilic substitutions.

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PET Radiopharmaceuticals Image removed.

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PET Radiopharmaceuticals • 11CO2 from the target is converted into a highly reactive methylating agent: 11CH3I or 11CH3Tf • Elapsed time is 12 minutes.. • The radiochemical yield, based on 11CO2 is about 90%. • Specific activities of more than 6 Ci/µmol (220 GBq/µmol) can be obtained. • 11C-Methylation of various precursors is performed in the second reaction vessel within a few minutes. • After methylation, the reaction product is separated via a semi preparative Radio-HPLC, purified via a solid phase extraction unit, followed by formulation of the radiotracer as an injectable saline solution.

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Delivery strategies Blood brain barrier Metabolic pathways Biological affinity Late 19th century German chemist Paul Ehrlich demonstrates that certain dyes injected i.v. do not stain the brain. The same dyes, when injected into the cerebral spinal fluid, stain the brain and spinal cord, but no other tissues. Image removed.

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The Blood-Brain Barrier Function Provide neurons with their exact nutritional requirements. Glucose • Sole source of energy (adult brain consumes ~100 g of glucose/day) • Neurons need a steady supply at an exact concentration The BBB is selective • Glucose and other nutrients are transported through • Proteins, complex carbohydrates, all other foreign compounds are excluded. • Ion concentrations are tightly regulated Image removed.

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Drug Delivery Tumors do not have a blood tumor barrier Image removed.

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Delivery Strategies: Metabolic pathways

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Delivery Strategies: Metabolic pathways Glu ?G6P? F6P?FBP •FDG is transported into the cells •FDG is phosphorylated to FDG-6P (charged molecules cannot diffuse out) •FDG is NOT a substrate for the enzyme that catalyzes the next step in glycolysis.. Image removed

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Mapping Human Brain Function 18F-FDG PET scans show different patterns of glucose metabolism related to various tasks. Image removed.

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FDG in Oncology • FDG transport into tumors occurs at a higher rate than in the surrounding normal tissues. • FDG is de-phosphorylated and can then leave the cell. • The dephosphorylation occurs at a slower rate in tumors. Applications of FDG •Locating unknown primaries •Differentiation of tumor from normal tissue •Pre-operative staging of disease (lung, breast, colorectal, melanoma, H&N, pancreas) •Recurrence vs necrosis •Recurrence vs post-operative changes (limitations with FDG) •Monitoring response to therapy

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Delivery Strategies: Metabolic pathways PET can provide highly specific metabolic information. • FDG, MET, FLT are incorporated via transporters • Uptake is indicative of tumor grade. 11C-methionine •specific for tumor •avoids high brain background problem seen with FDG •no significant uptake in chronic inflammatory or radiogenic lesions •MET better than FDG in low-grade gliomas

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Functional imaging of gliomas Imaging objectives • Location and relation to surrounding brain activity • Biological activity = malignancy • Response to therapy Image removed.

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Tumor recurrence vs post-radiotherapy changes FDG uptake indicates Recurrence Left: MRI Center: PET Right: fused image Image removed.

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Functional Imaging Tumor vs functional brain 11C-MET + MRI delineates tumor (GREEN) [15O]H2O PET delineates function (blood flow) Stimulation of brain regions causes increased blood flow (RED) finger tapping (A) verb generation (B) Pre-surgical analysis to guide surgery. Tumors cause swelling and deformation of brain anatomy: mapping function is critical. Intra-operative electrical stimulation causes aphasia: correlated well with area mapped by [15O]H2O PET. Information can be displayed in neuro-navigation software during surgery.. Image removed

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Recurrent tumor vs necrosis Image removed. MRI (right) indicates necrosis 11C-MET (left) shows tumor recurrence

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Image correlation with different modalities High-grade glioma: three- Dimensional determination of ? Localization ? Extent ? Metabolism Top: MRI Middle: 11C-MET Bottom: 18FDG [Note lower ipsilateral glucose metabolism.] Image removed.

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Bone scanning Bone scans are the second most frequent nuclear medicine procedure. Clinical uses: •Detection of primary and metastatic bone tumors •Evaluation of unexplained bone pain •Diagnosis of stress fractures or other musculoskeletal injuries or disorders. E.g., Prostate cancer: •Incidence is rising •Most common cause of death in males in many western countries •Of prostate deaths, 85% have mets in bone •60% of new cases have mets •Bone metastases are painful and debilitating •Diagnosis of bone mets is part of the staging process that determines treatment Breast cancer: •Bone is the most common site of metastasis •8% of all cases develop bone mets •70% of advanced cases experience bone mets

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Bone Bone is a living tissue comprised of a crystalline matrix of hydroxyapatite Ca5(PO4)3OH in a collagen matrix. Osteoblasts: responsible for new bone formation, repair of damaged sites, lay down new crystalline hydroxyapatite. Osteoclasts: responsible for bone resorption, dissolve bone. Osteoclasts are more active in metastatic tumor sites.

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Delivery Strategy Pyrophosphate Normal metabolite from ATP hydrolysis Source of phosphate in bone. Bisphosphonates •have an affinity for the hydroxyapatite component of bone •are incorporated into the crystalline matrix during bone remodeling or repair. •are used to slow or prevent bone density loss leading to osteoporosis

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Bone Scans Normal pediatric bone image Image removed.

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Bone scans SCHAPHOID fracture •48 y. o. woman presenting with with painful wrist 2 weeks after fall onto outstretched hand. •X rays normal •Blood flow (13NH3) increased to the left wrist (top) •Left scaphoid fracture revealed on 99mTc-MDP image (bottom) Image removed.

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Active metastatic disease 41 y.o. male with lung carcinoma presents with pain in upper right humerus, 2-3 months of bilateral rib pain, 3 weeks of left knee pain. Scan shows multiple focal sites of abnormal tracer uptake •Right humerus •Multiple ribs •Left femur •Sacral and lumbar vertebrae Image removed.

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Coronary artery disease Use PET and/or SPECT imaging to assess information on: ?perfusion ?metabolism ?distinguish viable from non-viable myocardium.

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Cardiac Imaging Image removed.

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The Cardiac Stress Test Exercise causes •Increased HR, contractility, BP •Increased O2 demand •Coronary vasodilation Increased myocardial blood flow Image removed.

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Gene Therapy Image removed.

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Gene Therapy Use of PET to confirm vector gene expression Specific retention of FIAU PET signal at 68 hrs (left) indicates phosphorylation by HSV TK. Same area shows necrosis after treatment with ganciclovir (right). Image removed.

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PET in studies of substance abuse Drugs of abuse • Why are they pleasurable? • What brain changes reinforce usage and lead to addiction?

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Brain Function Changes in specific components of this system present in various disease states. Parkinsons Disease aging substance abuse depression. Image removed.

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Brain Function Quantitative PET •Signal intensity in regions of interest is monitored as a function of time. •Concurrent sampling of arterial blood allows correlation of signal to blood concentration. •Pharmacologic doses of antagonist block PET tracer uptake. Image removed. Image removed.

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Drug Addiction •Cocaine: one of the most reinforcing drugs of abuse •Cocaine binds to the DA reuptake transporter (DAT) •DAT blockade results in increased DA concentrations. Effect is greatest in brain regions rich in DA neurons (e.g., basal ganglia). Image removed.

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Drug Addiction Control 1 week de-tox 3 months de-tox Image removed. FDG PET: Low frontal metabolism may underlie the loss of control in cocaine addiction.

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Drug Addiction Image removed.

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Cocaine and methylphenidate (Ritalin) Image removed. 11C-cocaine 11C-methylphenidate • show identical distribution • highest in basal ganglia (highest DAT concentrations) • binding to the same receptors • cold cocaine blocks 11C-methylphenidate uptake • cold methylphenidate blocks 11C-cocaine uptake

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Cocaine and methylphenidate (Ritalin) Slow on-rate of oral methylphenidate does not produce a high Peak DAT blockade i.v. cocaine: 4-6 min i.v. methylphenidate: 8-10 min oral methylphenidate 60 min Slow off-rate for methylphenydate does not lead to “binging” behavior. Second dose would not produce a high. Image removed.