CT Simulator Flashcards
Which two researcher invested in CT?
Godfrey Hounsfield and Dr. Allan Cormack
When was the first CT of brain taken?
1971
What were some characteristics of the first CT scanners?
Low Resolution, Long acquisition times and Long data to image conversion times
Describe the 1st generation CT Scanners?
- Pencil Beam system
- Parallel beam geometry
- Excellent scatter rejection
- Each Projection measured
- 160 Parallel Rays
Describe 2nd Generation CT scanners.
- Narrow Fan Beam (30 detectors)
- Reduced # of exposure for acquisition
- More scatter is detected than “Pencil” beam
Describe a 3rd Generation CT Scanner.
- Rotate/Rotate (Translation eliminated)
- Wider fan beam, enough to cover patient
- Reconstruction takes seconds
- More scatter detected than 2nd generation
Describe 4th generation CT Scanners.
- Rotate/Stationary
- Similar scatter to 3rd generation
- Fixed array of detectors
What is helical scanning?
- 3rd and 4th generation with continuous motion of source
- Slip rings provide electrical connection to x-ray tube (and detectors in 3rd generation)
- Table continuously move with source and detectors rotating
What are hounsfield units?
- CT Numbers that relate to linear attenuation coefficients
- Range from 3000 to -1000
What is Hounsfield equation?
HU= (Ux-Uwater)/ (Uwater-Uair) x 1000
Water has a hounsfield value of?
0
Bone has a Hounsfield value of ?
+1000
Air has a Hounsfield value of?
-1000
What is principle of Dual energy CT?
Materials exhibit different attenuation at different mean energies
What are the two interactions that photon have with matter?
Compton and Photoelectric effect
What happens to the atom to undergo photoelectric effect?
photon has sufficient energy to overcome the binding energy of the K-shell
What is the K-Edge?
The K-shell threshold of a substance, that is unique to every substance
What is dual spiral?
Two spiral scans at low and high kV
What is slow Kv Switching?
Switch kV level typically once per gantry rotation
What is fast kV switching?
Switch kV level every millisecond
What is dual source?
Simultaneous scan with two tubes
What is split filter?
Beam of one sources “sees” two different filters
What type of detectors do dual energy CT use?
Sandwich type, two layers per channel
What is the purpose of the top layer of the detector?
Detection of lower energy quanta
What is the purpose of the bottom layer of the detector?
Detection of higher energy quanta
What two examples of applying Dual Energy CT?
Metal Artifact Reduction, Certain structure exclusion with manipulating energy dependences
What is required when working with metal artifact?
The artifact must be contours and assigned a proper density for accurate dose distrubtion
What are the two used metal artifact reduction algorithms?
Normalized Metal Artifact Reduction (NMAR) AND Empirical Beam Hardening and Scatter Correction (EBHSC)
What type of table top do CT Simulators utilize?
Flat patient to replicate the table on the linac
What is the contiguous slice thickness and Increment?
2mm & 2mm, which exhibits acceptable quality and acceptable dose
What is 50% overlap slice thickness and increment?
2mm slice & 1mm increment, best quality and highest dose
What is a 100% gap slice thickness and increment?
2mm slice and 4mm increment, worst quality and lowest dose
What is partial volume averaging?
When object is partially out of the scan plane or smaller than the spatial resolution. The visible portions can be averages to estimate a hounsfield
What is slice thickness for non-IMRT?
5mm
What is slice thickness for IMRT/VMAT?
2-3mm
What is the slice thickness for SBRT/SRS?
1-1.5mm
As slice thickness increase, what happens to DRR Resolution?
Declines
As slice thickness increases, what happen to partial volume averaging?
Often difficult to visualize certain structures, making PVA difficult
As slice thickness increase, what happens to signal to noise ratio ?
Improves
What is the name of the system used for patient positioning.
XYZ Coordination system
The X plane utilizes which plane ?
Transverse plane (L or R)
The y plane utilizes which plane ?
Longitudinal, extending from superior to inferior
The z plane utilizes which plane ?
The Coronal, extending from superior to posters
What is the purpose of the XYZ system?
To replicate the position relative to the linear accelerator
What is localization?
Delineation of treatment target and placement of isocenter relative to target
