Gamma camera Flashcards
Why is patient positioning important ?
Patient detector distance affects image resolution
Drooping couch can cause artefacts
Increased motion artefacts if patient is uncomfortable
How does a collimator work ?
Sets acceptance angle of a GC
What are the measures of collimator performance ?
Sensitivity,
Resolution,
Magnification
Septal penetration
What is the equation for geometric resolution ?
Rg = D + (D/L)(Z+G)
D = width gap betw collimator bars L = length of collimator holes Z = Collimator to source distance G = Collimator to mid crystal distance
What is collimator sensitivity dependent on?
Trade off between resolution and sensitivity
Geometric efficiency depends upon:
Fraction of gamma rays reaching each hole
Number of holes allowing gamma rays through
What is the equation for minimum septal thickness?
T = 2DW/L-W W > 3/u
D = gap width
W=path length through septa
L = Length of holes
Pinhole collimator resolution
RGeff = D + (D / L+G)Z
What are the requirements of a scintillation crystal ?
High efficiency for stopping gamma rays
High probability of photoelectric absorption rather than Compton scattering
High conversion efficiency of absorbed energy to light photons
Transparent to its own emissions
Good matching between wavelength of emitted light and photomultiplier response
Short scintillation time
Mechanically robust
How does doping effect scintillation crystals?
Promotes fluorescence over phosphorescence
Creates electron traps, alters band gap structure
y = 350-500nm, peak 410nm eff 10% pulse dur 0.25us
What is a light guide?
Optical interface between crystal and the PMTs
Usually glass or quartz with a refractive index matched to the scintillator
What do the PMTs, pre-amps and ADC do ?
Convert scintillated light to electrons and amplify
pre-amps: amps before signal can degrade
ADC: coverts analogue signal to digital
What is the principle of pulse arithmetic?
Signals from each PMT fed through resistors with values which increase in proportion to PMT position.
Using matched resistors for energy output gives the signal for energy discrimination.
No normalisation for energy deposited so signals are larger for higher energy events
How is energy windowing used by a gamma camera?
- Used to discriminate between scattered and primary photons
- Scatter reduces energy
- Apply a window (typically ± 10%)
Which factors affect an energy peak?
Non uniformity in components responses
Impact of reflections and gap in system
PMT efficiency - variations in pmt output over time
Why is gain stabilisation required?
- changes in high voltage
- temperature changes
- changes in magnetic field
- ageing PM tubes
What is gain stabilisation ?
Real-time correction:
Reference LED pulses light on the photocathode
PMT output is redirected and compared to a ref voltage.
Now a microprocessor task
What are some energy correction issues ?
- varying PMT response across faces
- gaps between tubes
- poorer light collection at crystal edges
fixed window could lead to under acceptance
Example energy corrections
Hexagonal PMTs inc fill factor
Pixel-wise corrected applied to energy signals based on an energy correction map
What is linearity distortion?
Misplacement of counts due to their position across a PMT
Sensitivity peaks in the middle and is lowest between detectors
How does linearity distortion effect images?
Distorts bar phantom image
Causes deviations of a few mm, can misplace small features
Can cause variable magnification
How is linearity corrected?
Use a correction matrices, applied similarly to an energy correction.
Obtain a base image by screening detector with a lead mask on top and illum with a flood source
Improves uniformity but not underlying variations
How does interaction depth effect an image?
- Gamma rays interaction depth varies randomly
- Inc spread if further from PMT
- Distortion inc with inc thickness
Causes blurring and loss of resolution
How to correct for interaction depth?
Standard corrections assume a single depth of interaction
have to review the level of signal spread to estimate a correction
What is a sensitivity (uniformity) Map?
Used to correct for:
Small residual effects due to minor imperfections in tuning, gain stabilisation, energy and linearity corrections
Sensitivity variations due to variable crystal stopping power (due to variable thickness or density) or collimator imperfections
Applies a scaling factor to each pixel
Created from a high count flood image (120Mcts)
What are the fundamental uncertainties of energy and spatial resolution ?
Fluctuations in signal caused by:
Number of scintillation photons emitted by crystal Number of photons collected by each PMT Electron emission by the photocathode Secondary emission from dynodes High voltage supply Pre-amplifier noise
Poisson stats apply - random
