Topic 12 Flashcards
Gamma Camera Planar Performance Testing
Three levels of Performance Measurements • Manufacturer Characterisation • Acceptance Testing – Performed within a department before clinical use • Quality Assurance / Quality Control – Routine periodic testing to ensure good system performance
Testing Standards
National Electrical Manufacturers Association (NEMA) – American body although accepted worldwide – Lab environment tests to specify equipment performance and allow comparability between systems • Medicines and Healthcare products Regulatory Agency (MHRA) – UK Regulatory Authority – EU (EMEA, CE Mark), US (FDA) • Institute of Physics and Engineering in Medicine – UK Medical Physics Body – Suggests to equipment users tests and their frequency
Commonly Performed Planar Measurements
Intrinsic (Uncollimated) Uniformity • Intrinsic Spatial Resolution • Intrinsic Spatial Linearity • Intrinsic Energy Resolution • Count Rate Performance • System (Collimated) Uniformity • System Spatial Resolution (no scatter) • System Sensitivity
Making Sense of a Measurement
Radionuclide • Energy Window • Scatter Included ? • Collimator (System Measurements) • Areas of Camera Used – Useful Field of View (UFOV) – Central Field of View (CFOV) – 75% of UFOV • Count Rate / Count / Count Density • Pixel/Matrix Size
Uncertainty on Number of counts
The process of Radioactive decay can be described using Poisson statistics • In Poisson statistics – The standard deviation = square root of mean • Example of count uncertainty: – A pixel in a 64 x 64 array has 100 counts – Standard deviation of this measurement is 10 counts (10%) – Moving to 128 x 128 matrix and equivalent count density would give 25 counts in a pixel with s.d. = 5 counts (20%)) • Reduction of pixel size has an adverse effect on uncertainty / noise.
The definition of uniformity
The variability of the observed count distribution in the gamma camera from a uniform source – Intrinsic: Without a collimator – Extrinsic or System: with a collimator attached.
Causes of non-uniformity
Non-uniform detection efficiency across crystal -
different PMT (energy) gains -
positioning non-linearities -
Optical coupling between PMT and crystal Collimator imperfection
Damaged septa
Uneven hole sizes and angulation errors
2 types of Intrinsic uniformity
Integral uniformity and differential uniformity
What is integral uniformity?
Integral uniformity assesses GLOBAL non-uniformities
Integral uniformity, how do you calculate it?
IU = (max-min/max+min) x100
How do you calculate differential uniformity?
DU = (Max-Min/Max+Min) x100
What is differential uniformity used to assess?
REGIONAL non-uniformities
Why can we never 0% uniformity?
because of poisson statistics
What is intrinsic spatial resolution limited by?
Limited by: -statistical fluctuation of light photon distribution PMT, and PMT amplification. - Multiple scattering by gamma photon within the detector
What is intrinsic spatial resolution improved by?
Improved by; - Thinner crystal (less light spread) - More, smaller PMTs - Higher gamma ray energy (more light photons produced) - Signal processing and position calculation algorithms
How can we determine the intrinsic spatial resolution
get a mask with a slit and get FWHM OR FWTM (TENTH MAX)
How can we determine Intrinsic spatial linearity?
Place the mask with slits, and in each row (column) measure deviation of peak count from best line fit. we can do this differentially or integrally.
How can you determine the intrinstic spatial linearity differentially and integrally?
Differential - standard deviation of difference between peak locations and fit (typically < 0.2mm) Integral- maximum difference between peak location and fit (typically < 0.4mm)
Energy resolution calculation again?
( FWHMx100 / peak energy ) %
Better energy resolution allows better discrimination of ______
scattered counts
What is count rate performance
Count rate performance characterizes a scintillation camera’s ability to accurately function at count rates which are near the maximum rate of camera operation. (basically if a gamma ray is being processed a second gamma ray cannot be processed after that in what is known as the “deadtime”)
What types of deadtime are there?
Paralysable ( the gamma ray is ignored and the deadtime is not extended) and non paralysable (deadtime is extended from subsequent gamma ray).
Gamma cameras are paralysable or nonparalysable?
Paralysable
What does this graph show?
The count rate performance
The black line is the ideal line. The grey line is how the gamma camera system responds due to the deadtime effects at the end.
why do we often use collimator in place when we measure the system?
Closer to “Real” imaging
BUT: (have to say what collimator we are using)
Scatter often included
Higher activity sources often needed - sensitivity greatly reduced with collimator
problem with point source to measure System uniformity?
Point source would be seen as a (blurred point) with collimators
Spatial resolution limited by?
- Statistical fluctuation of light photon distribution between PMT, and PMT amplification
- Multiple scattering by gamma photon within detector
What is spatial resolution improved by?
- Thinner crystal (less light spread)
- More, smaller PMTs
- Higher gamma ray energy (more light photons produced)
• Energy Resolution ∝ 1/(E)1/2
- – Signal processing and Position Calculation Algorithms
