physics : simulations

Question 1

compton effect

Answer 1

photons that lost some energy in the collisions with electrons in the target material and then scattered at lower energy.

Question 2

photoelectric effect

Answer 2

electrically charged particles released from material when the material absorbs electromagnetic radiation

Question 3

Interactions in KV

Answer 3

PE is more likely w KV. The probability of the beam having PE is proportional to z cubed of the tissue.

A small difference in Z = big difference in Z cubed,
tf large difference in contrast

Question 4

interactions in MV

Answer 4

the probability of the beam having CE is proportional to electron density.

difference in e density is quite small
tf image is slightly washed out

Question 5

MV vs KV

Answer 5

MV is more penetrating, since compton effect dominates, the image quality is worse.

Question 6

scatter with MV and KV

Answer 6

MV= high energy photons = ↑scatter
.Scatter causes a decreased contrast and blurring
KV= lower energy photons =↓ scatter

Question 7

-what is a CT SIMULATOR
-what does it consist of

Answer 7

-real time, CT linked , 3D planning system
-CT scanner and multi-image display and a tx planning device for real-time visual optimization.

Question 8

Considerations of a CT simulation scan

Answer 8

-tx position reproducibility
-accurate tumour localisation
-CT number accuracy - dose calculation
-patient movement
-dose

Question 9

small bores

Answer 9

standard for diagnostic ct

ADVANTAGE=image quality improves bc xray travels less further

Question 10

large bore

Answer 10

xray source has to go further to get to patient.
Inverse square law, double the distance so quarter the dose.
This means more intensity is needed.
Usually lower quality

Question 11

features of large bore

Answer 11

lower contrast resolution
increases image noise(artefact)
increases patient dose

Question 12

what is DRR (digitally reconstructed radiograph)

Answer 12

-a plain film radiograph created from CT images
-interpolated by 3d CT data
-can be made in any direction (similar to conventional sim)

Question 13

why is DRR used

Answer 13

creates a fixed relationship between internal and external anatomy allowing localization of the tumour.

Question 14

advantages of DRR

Answer 14

image can include contours, block and MLC shapes

Question 15

disadvantages

Answer 15

-not always as good as quality as simulation radiograph

Question 16

how are DRR created

Answer 16

summing CT intensities from rays from each pixel to the simulated x ray source

Question 17

Steps in producing a DRR

Answer 17

-choice of virtual source position
-definition of image plane
-ray tracing from virtual source to image plane
-determinated of CT value of each voxel
-summation of CT values along the ray line

Question 18

what is isotropic imaging

Answer 18

same resolution in every direction, diagnostic doesnt worry about this bc they dont need DRR.

Question 19

advantages of isotropic imaging

Answer 19

-better spatial accuracy
-better tumour definition
-reduced partial volume artefact
-equal qulaity DRR from every direction

Question 20

disadvantages of isotropic imaging

Answer 20

-more slices
-tf more data and slower imaging and storage
-more slices may also increase dose

Question 21

DICOM stands for

Answer 21

digital communication in medicine.
its is a protocol that allows communication in medicine

Question 22

what is a simulator

Answer 22

anything that is c-arm shaped
LINAC
used plain radiograph that had either orthogonal images or fluoroscopy

Question 23

CT sim

Answer 23

donut shaped
is NOT simulator
just like tomography and MRI
uses 3CDT or 4DCT

Question 24

Simulator :production of image for verfication

Answer 24

radiograph taken
film put in place or a digital image
contains beams eye view w field defining wires in place

Question 25

CT sim: production of image for verfication

Answer 25

3D image is produced
since we cant have the same volumetric image on a day to day basis …. DDR is made
this is created by the verification images of the CT scan

Question 26

Consideration of CT sim : CT numbers

Answer 26

they need to be accurate to it can be fed into TPS so that the electron density can calculate where the scatter will go tf giving good dose distribution

Question 27

what laser class are laser in treatment room

Answer 27

2M = safe unless view under optical instruments

Question 28

CT data represents

Answer 28

linear attenuation coeffecient.
This gives us information on how much attenuation of a particular material per m eg 3.1m^-1

Question 29

factors that affect LAC

Answer 29

-material
-energy of beam

↑energy = beam more penetrating =↓attentaution per m

Question 30

voxels of CT scan

Answer 30

512x512 voxels that are each calculated by LAC

Question 31

CT scanis high quality due to

Answer 31

high spatial resolution and contrast resolution(due to KV image)

Question 32

spatial resolution is

Answer 32

distance between voxels

Question 33

which is DRR not always good quality

Answer 33

this depends on slice thickness
if slice thickness is not the same as resolution the z direction will not equal to x and y direction
this is why you want isoptropic imaging