Nuclear medicine Pet/Spect Chapters 18 to 22

Activity Number of radioactive atoms undergoing nuclear transformation per unit time. Change in radioactive atoms N in time dt Number of radioactive atoms decreases with time (- minus sign)

Activity Expressed in Curie –3.7x10 10 disintegrations per second dps Becquerel discovers natural radioactive materials in 1896 the SI unit for radioactivity is the Becquerel. 1 becquerel = 1dps

Nuclear medicine Therapeutic and diagnostic use of radioactive substances First artificial radioactive material produced by the Curies 1934  “Radioactivity,” “Radioactive

Definitions: Nuclide Nuclide: Specie of atoms characterized by its number of neutron and protons Isotopes Isotones Isobars (…)

Definitions: Nuclide Isotopes are families of nucleide with same proton number but different neutron number. Nuclides of same atomic number Z but different A  same element A Z X A mass number, total # of protons and neutrons Z atomic number (z# protons)

Definitions: Nuclide Radionuclide: Nuclide with measurable decay rate A Radionuclide can be produced in a nuclear reactor by adding neutrons to nucleides 59 Co + neurtron -> 60 Co

Radioactive Decay Disintegration of unstable atomic nucleus Number of atoms decaying per unit time is related to the number of unstable atoms N through the decay constant ( )

Radioactive Decay Radioactive decay is a random process. When an atom undergoes radioactive decay -> radiation is emitted Fundamental decay equation (Number of radioactive atoms at time t -> N t

Radioactive Decay Father and daughter. Is Y is not stable will undergo more splitting (more daughters) Father Daughter

Radioactive Decay Processes

Alpha decay Spontaneous nuclear emission of  particles  particles identical to helium nucleus -2 protons 2 neutrons  particles -> 4 times as heavy as proton carries twice the charge of proton

Alpha decay Occurs with heavy nuclides Followed by  and characteristic X ray emission Emitted with energies 2-10MeV NOT USED IN MEDICAL IMAGING

Positron emission  + Decay caused by nuclear instability caused by too few neutrons Low N/Z ratio neutrons/protons A proton is converted into a neutron – with ejection of a positron and a neutrino

Positron emission  + Decrease of protons by 1 atom is transformed into a new element with atomic # Z-1 The N/Z ratio is increased so “daughter” is more stable than parent

Positron emission  + Fluorin oxygen

Positron emission  + Fluorin oxygen

Positron emission  + Positron travels through materials loosing some kinetic energy When they come to rest react violently with their antiparticle -> Electron The entire rest mass of both is converted into energy and emitted in opposite direction –Annihilation radiation used in PET

Annihilation radiation Positron interacts with electron->annihilation Entire mass of e and   is converted into two 511keV photons 511keV energy equivalent of rest mass of electron

 - decay Happens to radionuclide that has excess number of neutron compared to proton A negatron is identical to an electron Antineutrino neutral atomic subparticle

Electron captive  Alternative to positron decay for nuclide with few neutrons Nucleus capture an electron from an orbital (K or L)

Electron captive  Nucleus capture an electron from an orbital (K or L) Converts protons into a neutron ->eject neutrino Atomic number is decreased by one – new element

Electron captive  As the electron is captured a vacancy is formed Vacancy filled by higher level electron with Xray emission Used in studies of myocardial perfusion

Isomeric transition During a radioactive decay a daughter is formed but she is unstable As the daughter rearrange herself to seek stability a  ray is emitted

Principle of radionuclide imaging Introduce radioactive substance into body Allow for distribution and uptake/metabolism of compound  Functional Imaging! Detect regional variations of radioactivity as indication of presence or absence of specific physiologic function Detection by “gamma camera” or detector array (Image reconstruction)

Radioactive nuclide Produced into a cyclotron Tagged to a neutral body (glucose/water/ammonia) Administered through injection Scan time min

Positron Emission Tomography  Tomography ?

Positron emission  + Fluorin oxygen

Cancer detection Examine changes due to cancer therapy –Biochemical changes Heart scarring & heart muscle malfunction Brain scan for memory loss –Brain tumors, seizures Lymphoma melanoma PET Positron emission tomography

Principles Uses annihilation coincidence detection (ACD) Simultaneous acquisition of 45 slices over a 16 cm distance Based on Fluorine 18 fluorodexyglucose (FDG)

PET Ring of detectors surrounds the patient Obtains two projection at opposite directions Patient is injected with a 18 fluorine fluorodeoxyglucose (FDG)

Pet principle Ring of detectors

Annihilation radiation Positron travel short distances in solids and liquids before annihilation Annihilation COINCIDENCE -> photons reach detectors, we collect the photons that happen almost at the same time –coincidence? I don’t think so! Detector 1 Detector 2

True coincidence Detector 1 Detector 2

Random coincidence Emission from different nuclear transformation interact with same detector Detector 1 Detector 2

Scatter coincidence One or both photons are scattered and don’t have a simple line trajectory Detector 1 Detector 2 False coincidence

Total signal is the sum of the coincidences C total = C true +C scattered+ C random

PET noise sources Noise sources: –Accidental (random) coincidences –Scattered coincidences Signal-to-noise ratio given by ratio of true coincidences to noise events Overall count rate for detector pair ( i,j ):

Pet detectors NAI (TI) Sodium iodide doped with thallium BGO bismuth germanate LSO lutetium oxyorthosilicate

PET resolution Modern PET ~ 2-3 mm resolution (1.3 mm) MRI PET

PET evolution

SPECT Single photon emission computed tomography  rays and x-ray emitting nuclides in patient

SPECT cnt One or more camera heads rotating about the patient In cardiac -180 o rotations In brain o rotations It is cheaper than MRI and PET

SPECT cnt projections Technetium is the isothope Decays with  ray emission Filtered back projection to reconstruct an image of a solid

Typical studies Bone scan Myocardial perfusion Brain Tumor

Scintillation (Anger) camera 1.Enclosure 2.Shielding 3.Collimator 4.NI(Tl) Crystal 5.PMT Imaging of radionuclide distribution in 2D Replaced “Rectilinear Scanner”, faster, increased efficiency, dynamic imaging (uptake/washout) Application in SPECT and PET One large crystal (38-50 cm-dia.) coupled to array of PMT

Anger logic Position encoding example: PMTs 6,11,12 each register 1/3 of total Photocurrent, i.e.: I 6 = I 11 = I 12 = 1/3 I p Total induced photo current (I p ) is obtained through summing all current outputs Intrinsic resolution ~ 4 mm

L d Collimators Purpose: Image formation (acts as “optic”) Parallel collimator Simplest, most common 1:1 magnification Resolution Geometric efficiency Tradeoff: Resolution  Efficiency A open A unit

Collimator types Tradeoff between resolution and field-of view (FOV) for different types: Converging:  resolution,  FOV Diverging:  resolution,  FOV Pinhole (~ mm): High resolution of small organs at close distances Diverging L d d Converging L d

SPECT applications Brain: –Perfusion (stroke, epilepsy, schizophrenia, dementia [Alzheimer]) –Tumors Heart: –Coronary artery disease –Myocardial infarcts Respiratory Liver Kidney