Radiation imaging Flashcards
Radiation imaging
uses low-level radioactive chemicals → designed to absorbed by different types of tissues
- detection of hotspots
radiographic imaging
- externally produced radation passes through tissue
non- radioactive imaging
diagnostic ultrasound imaging
- high frequency sound send into body, detecetion of refelcet sound waves
MRI
- nuclear magnetic resonance NMR, to image nuclei of atoms inside body → magnetic nuclei absorb & re-emit elecromagnetic radiation in a magnetic field
Basic principles of radiation imaging
- radioactive trager injected into blood
- radiation is detected and converted into electrical impulses
- electrical impulses are converted into images
radioactive tracer
chemical compound, atoms replaced by a radioisotope
→ can be used to explore specific chemical reactions/ bodily functions
e.g. glucose in tumor diagnostics
radiation theroy
- radioactive emission
- protons and neutros
- energy states
- radioactive emission = spotaneous disintegration of atoms → atoms decay over time → eject particles
- protons & neutrons experience short range nuclear forces → interplay of these forces induced different arrangement of particles within the nucleus
- energy states: ground state (most stable) & excited state (precedes radiactive decay)
radiation emittion
alpha radiation: alpha particles, (+)charge, 2 protons and 2 neutrons, penetration: not even paper (big particles)
beta radiation: (-) charged electrons or (+) charged particles, penetration: not through wood
gamma radiation: pure electromagnetic radiation with zero mass & charge, penetrate through all (concrete)
Notation
- nucleons: protons and neutrons
- Atomic mass A (above): total number of nucleons
- Atomic number Z (lower): total number of protons
radioactive elements
Elements with an atomic number Z >83 are radioactive an spontaneously decay into other elements
Atomic mass units AMU
mass of nuclear particels expressed in AMU using carbon 12 as reference
- gram atomic mass: weight in grams of isotope equivalent to atomic mass
- avogadro’s number 6.023 x 10^23 atoms/gm
alpha-decay
- kinetic energy is liberated
- mass equates to energy
- mass of atom is less than sum of individual components
- mass defect is manifestes as the energy required to bind nucleus together
beta- decay
negatron decay (beta - )
- nucleus same number of nucleons, but total atomic number increases by 1
- neutron transformed → 1 proton + 1 electron
- energy carried away by neutrino (zero change)
positron decay (beta+)
- decreasing the atomic number by 1
radioactive deay characteristics
- unaffected by changes in pressure, temperature, chemical combination
- rate of decay (lamda) constant → same number of disintegrations per time
- half life T1/2 → time required for half of the nuclei to decay (see notes for equation)
gamma decay
- accompanies beta decay → protons and neutrons left in high energy state
- photons of energy emitted to achieve a more stable configuration
nuclear instrumentation
- scintillation
- photomultiplier
- collimator
scintillation
flash of light, which is emitted when a substance is struck by an emitted particle/gamma ray
→ amount of light emitted is proportional to the energy of the colliding particle
photomultiplier
- oscilloscope
measures light intesitiy & converts it to electrical impulse
- electrical impulses are amplified and processed to provide information about incident radiation
→ using an oscilloscope
this gives information about: energy/intensity of the incident radiation, acitivty of a specific radionuclide
- no spatial/positional information
work flow nuclear instrumentation
indicent photon into scintillator → produciton of light photon → light photon detected by photocathode → photoelectric effect (photon into electron) → ejection of electron through focusing electrode into multiple dynode → amplification
at the dynode every incident of electron is doubled → incident photons are rare
collimator
filters a stream of rays → those travelling parallel to specific direction allowed through
- Pinhole collimator: allows only one point of entry → very time intensive
→ multihole is preferred
anhnihilation
- positrons emitted though decay only travel short distances (1-2mm) in tissues
- annihilation creates a pair of photons (gamma ray) that travel 180° to one another
Basic concept of PET system
- annihilation of positron and electreon → postronium → decay into gamma ray
- simultaneous detection of gamma rays of the two detectors means that this two gamma rays are coinciding → computer tomography produces an image
error sources
- true conincidence: no error
- scattering process: one gamma ray is scattered → location of event interpreted somewhere else
- concurrent events: two events simultaneously → solution: exclude all simultanoues events
- secondary photon: three gamma rays
combined MRI/PET imaging
- whole body PET using F-FDG (fluodeoxyglucose) → does not provide spatial resoltution → MRI
- PET provides functional information & MRI provides anatomical information