6: Positron Emission Tomography

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6: Positron Emission Tomography. What is the principle of PET imaging? Positron annihilation Electronic collimation coincidence detection. What is really measured by the PET camera?

1 6: Positron Emission Tomography. What is the principle of PET imaging? Positron annihilation Electronic collimation coincidence detection. What is really measured by the PET camera? True, scatter and random coincidences 3. How are the effects attenuation corrected for? 4. What factors can affect resolution? 5. Examples: PET tracers in oncology and neuroscience After this course you are capable of. Describing the essential elements of a PET scan. Distinguish the principle of PET detection from that of SPECT 3. Understand the bases of scatter elimination. 4. Understand the factors affecting spatial resolution in PET What is Positron Emission Tomography? PET Positron Emission tomography: measured are x-rays emitted by annihilation of positrons emitted by exogenous substance (tracer) in body The principle is as emission tomography, but there is one major difference (see later) - Two issues:. How to determine directionality of x-rays?. Absorption is undesirable Injection of positron emitter (tracer) Most widely used tracer for PET Fluoro-deoxy-glucose 6-5

2 What does one want to measure with PET? Annihilation photons Question: Why are two photons are produced? Conservation of linear momentum is not possible with one photon (p=e/c) photons pn + - Energie of photons? h=m e c =5keV (ev = J) Unstable parent nucleus n p NB. Light travels m in 3ns : [m]/3 0 8 [m/s]=3ns Annihilation coincidence detection : two events detected at same time annihilation event along a line (defined by detector) NO need for a collimator 6-6 What is coincidence detection? electronic collimation (i.e. w/o physical collimators) Electronic signal What defines simultaneity (coincidence)? Crystal Photomultiplier Leading edge defines time of detection (sharper, i.e. higher st derivative) Bi 4 Ge 3 O (BGO): ~0ns. Position logic electronics Photomultipliers Light guide Scintillating crystal Collimator PET-camera Elimination of collimator material is a major source of sensitivity increase (why?) 6-7

3 6-. What is really measured with PET? Random coincidences b b True coincidence a ed coincidence Trues T ab Y ab = (A ab + S ab + R ab ) N ab What is measured Normalization (Instrument imperfection) Attenuation Randoms 6-8 Why are Random and ed Events bad? mimic a true coincidence Random emissions from unrelated nuclear transformations interact simultaneously with the detectors At least one annihilation photon is (Compton) scattered Erroneous Line of incidence (LOI) assignment to wrong Radon transform Rate of random coincidences : S and S : count rates on the individual detectors (singles rates) : separation of singles (=coincidence time) R rand S S Reduce randoms by reducing (coincidence interval) Does not work for scattered events (why?) 6-9

4 How can scattered events be distinguished from true coincidence? Energy discrimination & background subtraction Measure E f identify severely scattered photons ENERGY WINDOW ENERGY 5 kev photons WINDOW Emission Transmission Corrected Most scattering is by Compton E f E Ei m ( ec cos f Ei me c ) cos i Counts BGO LSO Some crystals (BGO) only allow 30% energy discrimination Other approaches are needed: Energy (kev) Subtract background (= scatter + randoms) measured in signal void regions theta/ei 5 (kev) How is attenuation correction performed? simpler for PET than SPECT C T *(x) Probability P : S =C T (x)*e -(d-x) P =S /C T *(x) Probability P : S = C T *(x)e -x P =S /C T *(x) Attenuation : Probability of detecting the photon pair PP e x d x e S * T C ( x) d e d S P * P CT Compare to geometric average of SPECT (Lesson 5) 6-

What are the steps in Attenuation Correction for PET? Mass attenuation coefficient μ/ in soft tissue = 0.

693/μ HVL7cm Average path length for the photon pair longer than for a single photon different lines of

coefficient assumed Transmission technique using e.g. Cs source (66keV, why is this good enough?

5 What are the steps in Attenuation Correction for PET? Mass attenuation coefficient μ/ in soft tissue = 0.095cm /g (5keV) HVL 0.693/μ HVL7cm Average path length for the photon pair longer than for a single photon different lines of response attenuate to varying degrees Attenuation correction in practice: Spatially uniform attenuation coefficient assumed Transmission technique using e.g. Cs source (66keV, why is this good enough?) single photon emitter Comparison with blank scan i.e. subject removed e d ( ) e ( x) dx L Correction factor for each Radon transform (μ homogeneous) 6- Why is PET/CT the industry standard? PET-Attenuation correction using CT-Data / (cm/g) 0.3 CT ~70 kev CT + PET = PET/CT 0. Bone 0. Soft tissue PET 5 kev Energy (kev) Bone CT 5 kev Soft tissue scatter & attenuation correction 6-3

6 6-4. Why is Resolution never perfect? Annihilation Range and photon non-collinearity Collinearity: Assumed for Reconstruction Background: At time of annihilation, e-p pair has non-zero kinetic energy conservation of momentum Photon momentum w/zero momentum e-p Range: limits spatial resolution (In air, + range ~ several m) Isotope Half-life (min) Max. Energy (MeV) Range in H O (FWHM, mm) F C.0. 5 O N Ga Rb kev nucleus + x = 0.5 D tan(0.5) e - p e +p p D (detector distance) D (cm) ~ kev x (mm) } x 6-4 How does the detector affect PET spatial resolution? Example: BGO Block Detector Coincidence window: ns Energy resolution: ~ 5% True coincidence count rate R T R T =C* T G. C T * - tissue activity of a voxel. : the intrinsic detector efficiency (-e -x ) 3. G : the geometric efficiency (solid angle defined by the detector surface/4). NB. = 0.9 8% of photon pairs emitted towards detectors produce coincidence This is a reason for the 3cm thick crystals used for PET detection. Crystal thickness Resolution is best at the center 6-5

7 6-5. What are typical PET tracers? Oncology and neuroscience Fluoroethyl-Tyrosine (FET) Amino acid transport Deoxy- fluoro-thymidine (FLT) Oncology Proliferation Fluoromisonidazole (FMISO) Hypoxia C-Methionine Amino acid transport and metabolism H 5 O Blood flow Fluoro-Deoxyglucose (FDG) Glucose metabolism FDOPA Presynaptic dopaminergic function 5 O-Butanol Blood Flow C-Flumazenil Benzodiazepine-receptor mapping Neuroscience FDG or F fluorodeoxyglucose 5 OWater 6-6 X-ray imaging modalities. Overview CT, SPECT, PET Measurement of signal integrated along line of incidence (LOI) (Radon transform).ct: attenuated incident x-ray beam (direction of beam given by source).spect: emitted single photon (need collimation to determine ray direction) 3.PET: annihilation photon pair (directionality by electronic collimation) Apply correction to measured Radon transform (attenuation, scatter, etc.) Backprojection or central slice theorem: Finally an image! CT SPECT PET Projection Encoding Defined by incident x-ray (collimation to reduce scatter) Collimator essential Coincidence detection (electronic collimation) Spatial Resolution (rodent) 00μm-mm (μm) Typical 0mm (Variable and complex) (.5-3 mm) 4.5-5mm at center (mm) Attenuation = measurement variable (Varies with energy) Complex correction (Varies with photon energy) Accurate correction (transmission method) Radionuclides None (contrast agents) Any with h= 60-00keV Positron emitters only 6-0

Radionuclide Imaging MII Positron Emission Tomography (PET)

Radionuclide Imaging MII Positron Emission Tomography (PET) Radionuclide Imaging MII 3073 Positron Emission Tomography (PET) Positron (β + ) emission Positron is an electron with positive charge. Positron-emitting radionuclides are most commonly produced in cyclotron