IIP : Imaging for Target Volume Definition Flashcards
What is Target Volume Definition?
defining what we are going to treat
what is GTV ( gross target volume)
the gross tumour thats clearly present
good quality of imaging allows GTV to be outlined accurately
this means
less normal tissue irradiation
less normal tissue complications
The ability to identify a tumour and its extent depends on
contrast and the surrounding normal tissue
why is CT is vital in RT treatment planning
- electron density is needed for dose calculations due to densities of different tissues
- delineation of OAR and target volumes can be in 4D or 3d
CT provides
Excellent visualisation of tissues with different xray attenuations values eg bone and tissue
not very good visualisation of tissues with same xray attenuations values eg tumour and healthy soft tissue
Improving TVD and OAR localisation:
increase quality by removing artefacts , add contrast , reducing motion
combining different modalities w CT eg PET, MRI, ultrasound
CT Images recieved
in axial and reconstructed in sagittal and coronal plane
Each acquired image is
2D matrix pixels in the x and y axis but has measurement of depth (depth is in slice thickness)
each pixel has a depth in 3D aka a voxel
voxel is given a grey shade during image reconstruction
for maximum image resolution
slice thickness should be small as possible
Number of slices ↑ Patient dose ↑
RT planning scan: Range from 1.25mm - 5mm
Partial Volume Effect
occurs when tissues of widely different absorption are encompassed on the same CT voxel producing a beam attenuation proportional to the average value of these tissues.
Detrimental effect of slice thickness on image resolution
↑ slice thickness
↓ image resolution
Partial Volume Effect: Example
thick slice
borders are less defined and attenuation values are difficult to recognise
slice thickness contains normal tissue below and above nodes
Partial Volume Effect: Example
thin slice
border and attenuation value more accurate
node occupies whole slice thickness
delineation and measurement are more accurate but
ALARA
Grey Scales –CT Images
CT produce 256 grey shades (32 to human eye)
represent different tissues and their electron densities and attenuation
The mapped greyscale value is called
represents
Hounsfield units
standardisation of Ct value greyscale
Density of tissue can be extrapolated from….
CT units of attenuation (hounsfield units)
scale is ranging +3000HU to -1000HU
0 HU being water
CT colours
more dense structures (radio opaque) display as white
less dense structures(radiolucent) as darker
CT Images
what does black mean
what does white mean
-low attenuation …. air or lung tissue
•air above diaphragm = respiratory tract
•air below diaphragm = digestive tract
-high attenuation - bone, cartilage , metal
CT colours
what does grey mean
-medium attenuation
eg organs, muscles , lymph nodes of body fluids (soft tissues)
5 Basic Densities in X-ray attenuation
Material. Appearance
- gas black
- fat dark grey
- soft tissue / fluid light grey
- bone/Calcification white
- metal intense white
image manipulation
setting a window to a particular HU range so you can view those tissues in that range
“Windowing” grey scale bar to adjust contrast and intensity refining of slice
CT:TVD
advantages
- electron densities generated in CT can be in dose calculations for treatment planning
- reasonable image quality
- widely available
- images can be manipulated
CT:TVD
disadvantages
ionising
only gives structural info
no info on tissue type and functional activity
other modalities required for this …eg PET and MRI
