The patient Flashcards
What terms are used to describe the initial X-ray beam and the one leaving the patient?
- Primary beam exits the tube
- Remnant beam exits the patient
How do the intensities vary for the primary and remnant beams?
Primary beam has uniform intensity
Remnant beams intensity varies according to the characteristics of tissue
Intensity reduced by absorption & scattering
What effects the level of attenuation for different body parts?
- Thickness of the body part
- Density of tissue
- The effective atomic number of tissue (Zeff)
What is the main source of scattered radiation? What type of scattering occurs? What problems will arise from this scatter?
- The patient
- Compton scattering
- Dose implication for staff
- Image quality problems
- Scattered photons will hit IR in places not representative of anatomy
What is ‘off focus radiation’ and what does it cause?
Off focus radiation is where photons are created from a point in the tube housing other than the focal spot, they can come off in any direction and can lead to a ghosting appearance
List ways of removing scatter from images to improve quality and contrast
- Anti Scatter Grids
- Bucky
- Air Gap Technique
- Collimation and Field Size
- Compression (expiration)
- kVp selection
- Lead strips on the Image Receptor may help absorb patient scatter to improve images
What are anti scatter grids? How do they work? What compromises have to be made to use them?
Made up of thin Lead/Aluminium strips spaced by less attenuating material (carbon based)
Protects IR from scattered photons improving image quality
Photons not travelling in the direction of the primary beam are absorbed. Some primary beam is also absorbed so an increase in exposure (and patient dose) is required (up to 2-6x mAs depending on grid ratio)
What is the ‘grid ratio’ when talking about anti scatter grids?
Height of strips/interspace width
What is the ‘grid frequency’ when talking about anti scatter grids?
Number of strips per centimetre
What are some consideration associated with using anti scatter grids?
- Will require an increase in exposure
- Centring important
- CR perpendicular to grid
- SID important for focused grids
- Landscape or Portrait Grid
Who was the Bucky named after? What is the Bucky? What does a Bucky do? What types of Bucky are there?
- Gustav Bucky
- A vibrating grid
- Removes grid lines on image
- Erect and table Buckys
Discuss the air gap technique, pros and cons?
- Leaving a gap between the patient and the IR means some scatter will project away from IR
- More exposure required, therefore more patient dose
- Some loss of sharpness
- Magnification occurs
- Geometric penumbra increases (small focal spot recommended)
Benefits of collimation and field size
- Field Size controlled by collimation diaphragm
- Reduces patient dose as we don’t irradiate unrequired parts of the body
- Reduces scatter from adjacent tissue and therefore noise over areas of interest on our image Win/Win!
State 3 ways in which you can reduce scatter on an image
- Patient Compression (expiration): Less tissue irradiated = less scatter
- kVp selection: Related to the probability of Compton scatter hitting the IR, has a patient dose effect
- Lead strips on the Image Receptor may help absorb patient scatter to improve image
What 3 negatives does this image show with not having a point source?
- Geometric Penumbra
- Magnification
- Distortion
