Image Acquisition in NM Flashcards
Differences between PET and other modalities>
- Administering radioactive pharmaceutical designed to look art function of body
- Emission not transmission scan
- Physiiological not anatomical
4 main Components of gamma camera
- Collimator - gives sharp image accepts only alligned gammas
- Detection crystal - gamma ray converted into glash of green ligh
- PMTs - detect green light & convert to electronic signal
- Electronics & Computer - work out position and energy
Components of gamma camera
Collimator
Construction
What are they characterised by?
Lead 25mm thick, thickness and diamater of holes depends onneed for res or sensitivity Allows each point int he image to correspond to a pointin the source.
Sensitivity - number of counts for stated acticivity
Reslution - sharpness of image, measured as FWHM
Components of gamma camera
Parallel hole Collimator
How are res and sensitivity affected by increasing:
hole diameter
hole length
septal thickness
patient-colimator distance
hole diameter
- Res decreases
- Sens increases
hole length
- Res increases (smaller angle of acceptance)
- Sens decreases (fewer photons accepted)
septal thickness
- Res No effect
- Sens decrease (letting less gammas through)
patient-colimator distance
- Res decrease (closer is better, angle of acceptance)
- Sens no effect
Components of gamma camera
Scintillation crystal
Purpose
Material
Purpose is to absorb gammas and convert them into detectable light photons.
Material: thallium doped Sodium Iodide NaI(Tl).
- High density and atomic number makes it good absorber of gammas, signal is proportional to energy depositied in crystal.
- High ouput of photons per keV gives good energy res.
- Short lifetime of excited state makes for small dead time and high count rate.
- Transparrent
- -ve: fragile
- -ve: hygyroscopic: sealed in aluminium but moisture wrecks the crystal
Components of gamma camera: Scintillation crystal
4 methods of energy deposition
- PE absorption (complete absorption 140kEv)
- Compton Scatter (energy is deposited but some some energy escapes with photon 49kev)
- Compton _ PE absoption (secondary interaction 140kev)
- Multiple compton scatter (more energy deposited but not the full amount)
Components of gamma camera: Scintillation crystal
Describe the scintillation process
- Incoming gamma rays with energy greater than band gap of material
- excites electrons from valence into conduction band
- Fall back down & emit light
Components of gamma camera: Scintillation crystal
What is crystal thickness a trade-off between?
Increase thiclness = better stopping efficency = better sensitivity
increae thickness = worse spatial resolution (more potential for multiple scatters and less photons for tubes nearest the event)
Components of gamma camera: PMT
Describe
What is QE?
- Light in through glass window
- semi-transparrent photo-cathode produces electrons
- Electrons through focussing grids and onto dynodes ( 200-300V), avalanche effect = measureable signal.
- Overall gain 10^7
Energy of photoelectron = (hv -w) eV
QE = #photoelectrons/#incident photons typicaly 20%
Signal Analysis:
How do you get gamma energy?
How do you get position?
energy = sum all signals in PMTS
position = relative signals of PMTs
Components of gamma camera: PHA
How does it work?
- Energy window around isotope of interest to window out scatter
- 15-20 % window
- multiple peaks from one isotope
Energy resolution:
definition
how to measure
dependence on y-energy
Shape of the photopeak sue to statistical variation in #photons and #photoelectrons produced.
Measure FWHM
Improves with 1/sqrt(E) Think about this poission -> 1/sqrtN & EproportionalN
Camera is peaked every morning
How does gamma camera calculate x-y position?
What do correction circuits & signal thresholding do?
- Amount of light recieved by PMT is related to proximity to interaction site
- Weighted average for x-y position
Correction circuits correct for spatial distortion and for drift of energy cal
Signal thresholding includes on PM tubes above background noise, increases accuracy and reduces dead time.
Image Acquisition
What is meant by a static image?
How do you quanitfy it?
Activity distribution is static, wait a few minutes/hours for image. Series of views from differetnt angles to max image quality.
Wuantify with geometric mean
C1 = sqrt(CA1*CP1)
C2 = sqrt(CA2*CP2)
Mean of counts between anteripr and posteripr detector
Image Acquisition:Bone Imaging
- Radiopharm
- Activity
- Procedure
- Factors
- Radiopharm - Tc99-m MDP
- Activity - 600MBq
- Procedure - IV injection, wait 2-3 hours, hi res collimator, staic images
- Factors - blood flow, bone crystal, calcium content
