physics: physics for treatment planning Flashcards
electron density map tells us about
electron denisty in patient and scatter.
if we know where the electron density is high or low, will we know where the scatter will occur
voxel
adds thickness to a pixel giving us the third dimension
Hounsfield Units (HU)
-what is it
-aka
-measures attentuation of xrays in a substance
-CT number or pixel value
formula for HU
HU= 1000 x µx-µw / µw-µa
µ=linear attenutaion coefficent
x=unknown
w=water
a=air
-Water is assigned a value of
-Air is assigned a value of
-Bone then receives a value of
- 0 HU
- -1000 HU
-+1000HU
hounsfield represents
the standardisation of CT pixel greyscales values
10 Hounsfield unit (HU) corresponds to
1% of the LAC difference between air and water
electron density and HU
-ED doesnt follow linearly w HU
-theres two linear portions and discontinuity between them (bone)
cause of discontinuity
change in atomic number (causes a change in the proportion of PE to CE interactions)
Electron density is the goal for treatment planning imaging.
why?
-am of scatter for high energy photon beams depends on the compton effect
treatment planning algorithim use HU data for
correct dose distribution for tissue inhomogenity
linear attenuation coefficient definition
am. of attentuation to be expected per m of material
depends on= substance it goes thru and beam energy ( if beam more penetrating , the LAC decreases)
Relative Levels of Radiation Dose:
CXR =
CT Head =
CT abdomen /pelvis
-0.02mSv (the smallest there is)
-2mSv
-10mSv
CT has two purposes in treatment planning
-resolution (contrast and spatial) for delineation of the tumour
-quantative values for electron densities to calculate the potential scatter
what are window and level used for
to adjust the display of CT scans.
The window is the range of HU and the level in the midpoint of window range
window width determines
window level determines
-contrast of image
-brightness of image
human eye can only differiate
small amount of hounbsfield units (around 16)
in order to calculate the HU range…
electron density is compared to water , which has a electron density of 1.
bone has a higher electron density than water therefore it will have a higher HU range.
soft tissue has a lower electron density than water tf it will have a lower HU range
The overall process
- patient has CT scan (acquisition of data)
- using simlutaneous equations , to calculate LACs for every voxel
-use HU formula to convert the LACs into HU which can then produce an experimental graph
-Graph then converts HU into electron energies which is used in TPS to calculate dose distribution
what is pitch
patient couch movement per rotation divided by slice thickness
contrast resolution… how is it detereminded
between two objects which are similar levels of greyscale using LAC
spatial resolution
determine the distance. between 2 objects
