physics: physics for treatment planning

Question 1

electron density map tells us about

Answer 1

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

Question 2

voxel

Answer 2

adds thickness to a pixel giving us the third dimension

Question 3

Hounsfield Units (HU)
-what is it
-aka

Answer 3

-measures attentuation of xrays in a substance
-CT number or pixel value

Question 4

formula for HU

Answer 4

HU= 1000 x µx-µw / µw-µa
µ=linear attenutaion coefficent
x=unknown
w=water
a=air

Question 5

-Water is assigned a value of
-Air is assigned a value of
-Bone then receives a value of

Answer 5

• 0 HU
• -1000 HU
  -+1000HU

Question 6

hounsfield represents

Answer 6

the standardisation of CT pixel greyscales values

Question 7

10 Hounsfield unit (HU) corresponds to

Answer 7

1% of the LAC difference between air and water

Question 8

electron density and HU

Answer 8

-ED doesnt follow linearly w HU
-theres two linear portions and discontinuity between them (bone)

Question 9

cause of discontinuity

Answer 9

change in atomic number (causes a change in the proportion of PE to CE interactions)

Question 10

Electron density is the goal for treatment planning imaging.
why?

Answer 10

-am of scatter for high energy photon beams depends on the compton effect

Question 11

treatment planning algorithim use HU data for

Answer 11

correct dose distribution for tissue inhomogenity

Question 12

linear attenuation coefficient definition

Answer 12

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)

Question 13

Relative Levels of Radiation Dose:
CXR =
CT Head =
CT abdomen /pelvis

Answer 13

-0.02mSv (the smallest there is)
-2mSv
-10mSv

Question 14

CT has two purposes in treatment planning

Answer 14

-resolution (contrast and spatial) for delineation of the tumour
-quantative values for electron densities to calculate the potential scatter

Question 15

what are window and level used for

Answer 15

to adjust the display of CT scans.
The window is the range of HU and the level in the midpoint of window range

Question 16

window width determines
window level determines

Answer 16

-contrast of image
-brightness of image

Question 17

human eye can only differiate

Answer 17

small amount of hounbsfield units (around 16)

Question 18

in order to calculate the HU range…

Answer 18

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

Question 19

The overall process

Answer 19

• 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

Question 19

what is pitch

Answer 19

patient couch movement per rotation divided by slice thickness

Question 20

contrast resolution… how is it detereminded

Answer 20

between two objects which are similar levels of greyscale using LAC

Question 21

spatial resolution

Answer 21

determine the distance. between 2 objects