physics : simulations Flashcards
compton effect
photons that lost some energy in the collisions with electrons in the target material and then scattered at lower energy.
photoelectric effect
electrically charged particles released from material when the material absorbs electromagnetic radiation
Interactions in KV
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
interactions in MV
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
MV vs KV
MV is more penetrating, since compton effect dominates, the image quality is worse.
scatter with MV and KV
MV= high energy photons = ↑scatter
.Scatter causes a decreased contrast and blurring
KV= lower energy photons =↓ scatter
-what is a CT SIMULATOR
-what does it consist of
-real time, CT linked , 3D planning system
-CT scanner and multi-image display and a tx planning device for real-time visual optimization.
Considerations of a CT simulation scan
-tx position reproducibility
-accurate tumour localisation
-CT number accuracy - dose calculation
-patient movement
-dose
small bores
standard for diagnostic ct
ADVANTAGE=image quality improves bc xray travels less further
large bore
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
features of large bore
lower contrast resolution
increases image noise(artefact)
increases patient dose
what is DRR (digitally reconstructed radiograph)
-a plain film radiograph created from CT images
-interpolated by 3d CT data
-can be made in any direction (similar to conventional sim)
why is DRR used
creates a fixed relationship between internal and external anatomy allowing localization of the tumour.
advantages of DRR
image can include contours, block and MLC shapes
disadvantages
-not always as good as quality as simulation radiograph
how are DRR created
summing CT intensities from rays from each pixel to the simulated x ray source
Steps in producing a DRR
-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
what is isotropic imaging
same resolution in every direction, diagnostic doesnt worry about this bc they dont need DRR.
advantages of isotropic imaging
-better spatial accuracy
-better tumour definition
-reduced partial volume artefact
-equal qulaity DRR from every direction
disadvantages of isotropic imaging
-more slices
-tf more data and slower imaging and storage
-more slices may also increase dose
DICOM stands for
digital communication in medicine.
its is a protocol that allows communication in medicine
what is a simulator
anything that is c-arm shaped
LINAC
used plain radiograph that had either orthogonal images or fluoroscopy
CT sim
donut shaped
is NOT simulator
just like tomography and MRI
uses 3CDT or 4DCT
Simulator :production of image for verfication
radiograph taken
film put in place or a digital image
contains beams eye view w field defining wires in place
CT sim: production of image for verfication
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
Consideration of CT sim : CT numbers
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
what laser class are laser in treatment room
2M = safe unless view under optical instruments
CT data represents
linear attenuation coeffecient.
This gives us information on how much attenuation of a particular material per m eg 3.1m^-1
factors that affect LAC
-material
-energy of beam
↑energy = beam more penetrating =↓attentaution per m
voxels of CT scan
512x512 voxels that are each calculated by LAC
CT scanis high quality due to
high spatial resolution and contrast resolution(due to KV image)
spatial resolution is
distance between voxels
which is DRR not always good quality
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
