QA Flashcards
Why perform QA?
3 reasons
potential adverse effects if you didnt
Scanner is complicated, underperformance leads to adverse effects for image quality. QA routine is established to:
–Ensure scanner is performing within accepted (manufactured) criteria
–Benchmark performance
–Ensure safe operation
Questions regarding diagnosis, longer scans, repeat scans, reduced throughput.
Required qualities for a test object
- Need to be tested under routine conditions (TR,TE micmicking clinical imaging, similar to tissue relaxation times)
- High temp stability (relaxation is temp dependant)
- Minimal susceptability effects - introdces distortion
SNR
What 5 things do SNR depend on in scanner?
Which components normally causes a problem with SNR?
2 methods to measure SNR
What happens to noise distribution in image?
SNR depends on: proton density, transverse relaxation, longitudinal relaxation, flip angle, reciever sensitivity.
Poor SNR normally caused by: poor RF calibration/amplifier issues, poor RF coil performance, preamp problem.
- Uniform flood field, 2 images, place 1 central and 4 compas ROIS, measure mean in 1st image. subtract the 2 images, measure SNR in each. SNR = root(2) * mean /(average of SDnoise)
- Single image, measure mean across 5 ROIS, background ROI outside object. SNR = 0.66*mean signal/SD of noise.
Image is the magnitude of the signal, noise distribution goes from gaussian to rayleigh
Uniformity
Why do we care?
How do you measure it?
What components can knack it?
Need uniform signal across FOV to ensure what we are seeing is pathology.
Test with uniform flood field, measure intensity profile across image, measure flatness of it. Record SD of signal over largest area of object (80% inner radius).
Problems are most likely caused by RF coil (un-equal channels) or shim problem (non-uniform B).
Ghosting/Stability
Why do we care?
Components that knack it
How do you assess?
Phase encoding requires that there is no movement and that there is stability in performance (RF amplitude, phase, grad strength, shim noise). Instabilities cause phase encoding artefacts, signal being recorded at wrong point in k-space.
Normally its RF amplifier, gradient amplifier noise, shim noise
Assess visually, but use loner TE to enhance effects of instability and make it easier to see.
Scan Geometry
Why do we care?
Components that knack it
How do you measure?
Image needs to be same shape and size as object (important for surgery plans).
Often knacked by gradient amplifier problem, or shim problem
Test an object with well-defined geometric structure e.g. grids, compare distances on image viewer.
Slice Offset
How do you measure?
Measurement of slice through overlapping wedges. Location of gap in slice tells you if the position is right.
