Imaging and Planning Flashcards
What is the influence of voltage on diagnostic x-rays? of current?
1) kVp is the maximum x-ray energy; increasing the kVp will increase the exposure and penetration, but decrease contrast. This is because higher energy electrons will yield higher energy photons = more penetrating = more pass through tissue instead of interacting with it (less contrast) and
2) The product of time and current (mAs), this increases the # x-ray produced per unit time, increasing the exposure (no effect on energy, therefore no effect on penetration or contrast)
How many Hounsfield Units is:
1) Air
2) Water
3) Bone
Water is the normalization point of HU
1) -1000
2) 0
3) +1800
Why does the craniocaudal resolution of a CB and regular CT differ?
The regular (fanbeam) CT is designed to have high resolution 1mm slices in the axial plane. It ‘skips’ tissue to get to the next slice (e.g. 1mm slices, 5mm apart). Thus, the c-c resolution is limited by only scanning 1 of every 6mm. With a CBCT, the incident exposure is not 1mm thick, but at whatever the aperture is, so a thickness may be more like 10 cm. Thus, more information is gathered. Because there is so much more tissue to scatter however, the images are of poorer quality
How does the c-c resolution compare between a CT and MRI?
The MRI has much better resolution in the c-c direction as images can be acquired in any direction (not just axially)
Explain windowing and leveling on CT scans
CT scan assign a HU value between -1000 and +1000, with 0 being water-equivalent.
- The “window” refers to the range of HUs you want to see (eg. only show gradations of gray for 100 units).
- The “intensity” refers to the center of the range.
So for a level of 50 and a window of 100, all HUs between 0-100 will be shown as gradations of gray. <0 would be black and >100 would be white
What is TCP (tumor control probability)?
As you apply a radiobiological formula to a DVH, you can develop an algorithm to determine the chance this plan will control a tumor. Same thing for NTCP (normal tissue complication probability)
How does a pencil beam algorithm work?
It assumes the scattering angles of electrons are small, and a beam the size of a pencil should account for dose deposition; wide scattering is considered minimal. This algorithm thus fails with inhomogeneities
How does convolution/superposition work?
This is summation of two kernels: a primary photon beam dose and a separate dose from scatter. The primary dose is treated like a pencil beam approach whereas the scatter dose is ‘superimposed’ on this.
What is collapsed cone convolution/superposition?
The beams-eye view is divided into rays, each of which 3D is a ‘cone’ of dose. The cones are collapsed to lines, and calculated for dose. Thus, a ‘sphere’ of dose is broken into cones, which can each be tabulated
