CT QA Flashcards
What are 5 causes of radiation incidents?`
- work without alertness and awareness
- lack of procedures and checks
- lack of qualified or well trained staff
- gaps and ambiguties in funcionts or
personnel - absence of safety culture
What are the daily CT QA tests?
- alignment of lasers to gantry plane
- CT number - water
- image noise
- spatial accuracy
What are the monthly CT QA tests?
- orientation of lasers
- spacing of wall lasers for patient localisation lasers to imaging scan plane
- orientation of CT table relative to imaging scan plane
- accuracy and reproducibility of digital inidcators for vertical and longitudinal motion
- CT numer accuracy for different materials
- 3D spacial accuracy
What is spatial accuracy and why is it important?
- planning relies on accurate reproducible representation of the patient dimensions and shape for skin, internal organs contouring
- image distortion can lead to misleading and cuase inappropriate dose dumping in the wrong area
- geometric miss of target
How is spatial accuracy measured?
- verified by CT cannning a phantom of known dimensions
What is the tolerance for spaital accuracy?
- 1mm
- difference is CT scanned image of object dimension - actual object dimension
What is the definition of laser QA?
- alignment of the gantry lasers with the centre of the imaging plane
What is the tolerance for laser QA?
- 2mm
How are daily laser QA completed?
- alignment tool or QA phantom design to asses the laser geometry and accuracy
What is a limitation of a cube laser test?
- ony check position of the intersection of the alsers indicating centre and not check the skew and tilt
What is the impact of lasers out of tolerance?
- affect ability to identify skin marks required for reproducible patient position for setup
How do you check for noise on a CT?
- phantoms that automatically measure noise
- should perform for head and body phantoms
What is the impact of noise being out of tolerance?
- image quality directly affects the ability to identify and delineate target volume and surrounding critical structures
- cause suboptimal images which may cause inadequate delineation of target and OAR
What is the impact of water HU being out of tolerance?
- CT calibrated to give 0 HU for water and the relationship between the relative electron density to CT numbers are mapped in the TPS
What does a deviation of HU value indicate?
- equipment fault in beam hardening
- image reconstruction software issues
