Gamma Camera QC Flashcards
What is the difference between quality assurance and quality control and where is it defined?
Quality Assurance: Planned actions to ensure a system or process meets standards.
Quality Control: Specific operations to monitor, maintain, or improve quality, focusing on measurable performance characteristics.
Define in IR(ME)R2017.
What is a critical examination?
This is required if the installation include x-ray equiqment or houses a sealed radiation source.
This is the duty of the installer under IRR17.
Purpose is to check the presence and function of radiation safety features that protect operators
What is Acceptance Testing?
- Ensures equipment meets performance standards agreed at procurement.
- Trigger for final payment and required before patient use.
- Tests assess performance against manufacturer metrics.
- Standards: Typically use NEMA, with IEC standards also applicable.
What is the purpose of Commissioning?
- Establish baseline performance as measured by local routine performance tests
- Establish a QA/QC routine and protocols for the scanner
- Set up and optimise clinical acquisition protocols
What is ‘Routine Testing’?
- Primarily asks – is the equipment in a fit state for clinical use now
- Is there evidence of deterioration that can be addressed proactively
- Needs to be simple and time efficient to perform
- Tests typically undertaken by the equipment user
There are 9 main tests
What are the example QC tests for a Gamma Camera?
- Uniformity (Intrinsic/Extrinsic)
- Spatial Resolution
- Sensitivity
- Energy resolution
- Centre of rotation
- Whole body acquisition function
- Tomographic image quality
- For SPECT/CT - CT tests and SPECT-CT registration
- Absolute quantification
How can the uniformity be tested intrinsically or extrinsically?
Uniformity can be tested as Intrinsic uniformity (collimator off) using a point source at a distance or extrinsic uniformity (with collimator on) using a s flood source (Co-57).
They are assessed visually (qualitatively) or quantified.
What are the factors affecting uniformity?
- PMTs (tuning, gain, PMT drop out)
- Damage to crystal/crystal hydration
- Linearity, energy, uniformity (sensitivity) maps – have these been applied correctly? Do they need updating?
- Temperature fluctuations
- SPECT acquisitions – centre of rotation errors and poor attenuation correction can also cause non-uniformities in the reconstructed image
- Test set up/user error (!)
What is the Integral Uniformity?
Write the equation
Integral uniformity is the difference in contrast between hottest and coldest pixels anywhere in the image.
Integral Uniformity = (cmax - cmin)/(cmax + cmin) x 100%
cmax = counts in hottest pixel
cmin = counts in coldest pixel
What is the Coefficient of Variation?
Write out the equation
Coefficient of variation measures the overall variation in pixel counts across the field of view (standard deviation over the mean).
CoV = 100% x Standard Devation/ Mean
Standard Deviation = √(Σ(Cij-M)^2)/N
Mean = Σ(Cij)/N
What is the differential Uniformity?
Write the equation
Differential uniformity measures change in counts within a local group of 5 pixels.
D = (H-L)/(H+L) x 100%
H = Highest count in 5 adjacent pixels
L = Lowest count in 5 adjacent pixels
What is the spread of Differential Uniformity?
Write the equation
Spread of differential uniformity gives an indication of how rapidly the uniformity varies across the image.
Spread of Differential Uniformity =
√(Σ(Dijh)^2 + Σ(Dijk)^2/N)
Dijh = cij -ci+h,j/cij + ci+h,j
Dijk = cij =ci,j+k/cij + ci,j+k
What is the relationship between the Central Field of View (CFOV) and the Useful Field of View (UFOV)?
CFOV is the same shape as UFOV but with 75% of its linear dimensions.
What determines the Geometric Field of View (GFOV)?
GFOV is determined by the edge of the collimator aperture(s).
How do you measure the sensitivity of a gamma camera?
- Fit required collimator
- Draw up activity (10-20 MBq), note calibrator measurement & time
- Fill sensitivity phantom
- Measure residue and time
- Place source on collimator surface and image for 2 min, noting acquisition time
- Remove source and obtain background image for 2 min
What is the sensitivity calculation?
Sensitivity = (total counts in source image) - (total counts in background) / (decay corrected syringe activity) - (decay corrected residual activity)
How is the spatial resolution tested?
Write the equation for system spatial resolution
Measurements can be made intrinsically or as a system test, qualitatively or quantitatively – IPEM recommend making quantitative assessments of both annually
(Rsystem)^2 = (Rgeo)^2 + (Rintrinsic)^2
How is intrinsic spatial resolution measured?
- Remove collimator and protect crystal surface.
2 . Mark positions of each PMT - Position collimated point sources over centre of a few PMTs
- Set up acquisition using Tc99m energy peak (check peaking) and a matrix to give pixels ≤0.7mm
- Acquire ~2000 counts in max pixel
- Draw wide profiles across the sources in X and Y directions and determine FWHM and FWTM
- Repeat for ~ 12 tubes and again for sources midway between PMTs
How is the system spatial resolution measured?
- Fill pair of capillary tubes with ~200MBq
- Position parallel to each other 100mm apart and camera axis through centre of the field of view.
- Acquire using Tc99m window, matrix to give pixel size ≤1mm for long enough to achieve ~400 counts in maximum pixel
- Measure FWHM and FWTM in terms of pixels
- Use number of pixels between lines to verify pixel size and convert to mm unit
- Repeat for x/y direction, at 0 cm and 10 cm from the collimator and for each detector/collimator available
How is the spatial resolution assessed visually?
Visual assessment of transmission phantoms with bar / quadrant patterns.
What is the added value of the spatial resolution test?
If both the uniformity and spatial resolution fail from visual inspection but a new uniformity map doesn’t resolve the issue then this can indicate an issue with one of the PMTs which requires a full service.
What are the different wholebody testing methods?
- Wholebody System Uniformity
- Wholebody System Spatial Resolution
- Wholebody Count Rate Variation
- Wholebody Exposure Time Correction
Why do the cameras stay stationary at the beginning and end of a wholebody acquisition?
In the stationary position the aquisition windows moves across the face of the camera, this is referred to as the “ramp”.
How is wholebody uniformity tested?
Using a large flat uniform source placed on the bed and acquired using a wholebody aquisition.
