5.D Verification Imaging Flashcards
Radiographic Film:
Dose reaching the image receptor depends on
- Energy
- Patient separation
- Inverse square law
- Everything that affects the scattered component
‒ Field size, thickness, air gap
Quality of film is dependent on
- Latitude
- sensitivity
- speed
- characteristic curve
EPIDS
* Advantage
-the spatial resolution is not limited by the EPID design
-contrast resolution is high but image contrast is limited by MV radiation
-speed (real time)
-remote access
-low profile when not in use
EPIDS
* disadvantage
- radiation damage limits lifetime
- size maybe too small for very large fields.
- increases the backscatter component
EPIDS = How does it work
-ray traced doses are computed into portal imager
-portal dose images are predicted at planning then compared at measured values
-dont match=difficult to relate this back to dose distribution in the patient
What is Transit dosimetry
-darkness of image ===== the dosage at that point
Transit dosimetry:forward projection
portal dose calculated BY TPS at the Epid plane are predicted at planning then compared at measured values from EPID
-IF DONT MATCH=difficult to relate this back to the dose distribution in px
-easy to do
Backprojection
-measured EPID dose is backprojected to any plan of the patient
-more complicated algorithm
Gamma evaluation (1)
Dose difference
- Just subtract the doses for each (x,y) coordinate.
- Very good for homogenous (dose) regions
- Poor where regions have high dose gradients due to misalignment
Gamma evaluation (1)
Distance to agreement (DTA)
Distance between a point on the dose distribution and the nearest point on the calculated distribution with the same dose.
Ghosting
charge trapping in the EPID
-problem for low MU fields
Disadvantages
-Non water/tissue equivalence of amorphous silicon
-Oversensitive to low energy photons
Disadvantages:
Pixels at the edge of the field will have a different low energy profile to those in the middle because of
-Differential beam hardening because of the shape of
the flattening filter
‒ Distance from the patient
‒ Path length in the patient
Disadvantages of Radiographic film
*Changes in processing conditions
* Interfilm emulsion differences
* Artefact caused by air pockets adjacent to the film
Radiographic film is Still useful for checking
- Radiation fields
- Light field coincidence
- Flatness and symmetry
- qualitative radiation distributions
