Wk07 Image Guided RT Flashcards
What is the definition of image registration
The process of determining the geometric transformation that relates identical points in two image series: a moving dataset (study A) and a stationary source dataset (Study B)
What is the definition of image fusion?
The combined display of the mapped date from the moving dataset with the stationary dataset.
What are the 3 common ways of image transformation?
- Rigid transformation
- translations and rotations - Affine transformation
- rigid transformation + scaling and sheering - Deformable registration
- objects are deformed, translated and rotated to match both images
What are the application of deformable registration?
- Correct difference between MR and CT flexion angle of cervical spine
- Deform contours for adaptive radiotherapy
What should be aware when perform deformable registration?
- carefully select the ROI, especially when the scan range are different
- deformable registration should not be performed for post-op cases or cases with bolus
- care should taken on images with substantially different tumour sizes.
Name 2 automatic registration algorithms
- Intensity-based
- Geometry-based
Due to vastly different intensity pattern, ___________________ is useful in multi-modality image registration.
Mutual information
Daily imaging can reduce _____________________error, regular imaging can reduce __________________error.
Systematic and random ; systematic
What are the special features of a CT simulator compared to a normal diagnostic CT?
- a big bore to cater treatment positions
- laser system
- accurate CT numbers for dose calculations
- flat couch top
What report guide the QA of CT simulators?
Report NO. 083 (AAPM)
CT simulators need regular QA on:
- geometrical accuracy of images
- accuracy of CT numbers
- accuracy of CT lasers
- Image quality (noise, contrast, resolution)
- accuracy of table motion
MR simulation can replace CT simulation if MR simulation can?
- can be used for delineation of all OARs and targets accurately
- can be used for radiation dose calculation
- can simulate treatment position accurately
What are the features of a MR simulator?
- large bore size
- laser system
- flat couch top
- special receiver coils and accessories dedicated for RT simulation
- dedicated pulse sequence —> to generate density map for dose calculations
_______________ mimic simulation CT for dose calculation and _________________ for setup verification.
synthetic CT ; generate DRR
What report or guidelines control the QA for MR simulation?
AAPM task group 284: MRI simulation in RT
AAPM report No.100: acceptance and QA for MRI
During MRI QA, physicist need to ensure:
- geometrical accuracy of images
- accuracy of synthetic CT numbers
- accuracy of simulation lasers
- appropriate zoning
- fringe fields
- integrity of RF shielding
- image quality
- accuracy of table motion
What phantom is used for MRI QA?
ACR phantom
What are the definition of different MRI zones? (Zone 1-4)
Zone 1: areas that are freely accessible to the general public
Zone 2: interface between publicly accessible and controlled area (eg. Px waiting area)
Zone 3: area which personnel or ferromagnetic equipment can result in serious injury or death (eg. Control room)
Zone 4: the MR scanner room itself
What is the definition of MR safe, MR unsafe and MR conditional?
MR safe = an item poses no known hazards in all MR environments
MR unsafe = an item is known pose hazards in all MR environment
MR conditional = an item pose no known hazards in specified MR environments with specified conditions of use. (Eg. An item is safe near a 1.5T magnet, but not for 3.0T)
Pacemaker may be affected by magnetic field exceeding ______________.
5 gauss
What is the earth magnetic field?
0.5G
What are the 2 functions of radiofrequency (RF) shielding?
- To prevent extraneous electromagnetic radiation from contaminating or distorting the MR signal
- To prevent electromagnetic radiation generated by the scanner from causing interference to nearby medical devices
What is ray tracing in constructing DRR?
Tracing ray lines from a virtual source position through the CT date of the patient t a virtual film plane
What are the difference between CBCT images and normal CT images?
- CBCT have higher noise level due to increase amount of scatter received
- CBCT are more susceptible to patient motion due to longer acquisition time
- Limited field of view (FOV)
How to get a larger FOV for CBCT?
Use half-fan mode
Larger FOV but more noise
Why it is important to perform QA on treatment imaging equipments?
- to ensure the centre of these images corresponds to the treatment isocenter
- to ensure that the image quality are of adequate quality for treatment verification
What phantom is used for CBCT QA?
Catphan 600
What s a typical dose for a stereotactic radio surgery?
> 10Gy
In the old days, _____________ are used to immobilise patient for SRS.
Invasive frame-based system
A SRS frameless system requires:
- a RT delivery system that can deliver radiation with high precision to the target
- an image guided system for accurate localisation of treatment target
- a rigid immobilisation system to reduce intra-fractions motion of patients
- a tracking system to track the position during the treatment
What test is recommended periodically to ensure reproducibly of isocenter for frame-based system
Winston / Lutz test
Winston-Lutz test is used to verify ___________ & _________.
The accuracy of laser and the accuracy of radiation isocenter
What are common immobilisation system for SBRT?
Bodyfix, Pro-Lok
What is 4DCT?
It is an add-on feature to a CT simulator that uses infra-red camera and detector to capture the breathing pattern of the patient.
What is MIP, MinIP and AVG in 4DCT images?
MIP = maximum intensity projections
Assign each voxel the value of the highest value voxel at that location
(For ITV delineation)
MinIP = minimum intensity projections
Assign each voxel the value of the lowest value voxel at that location
AVG = averaged CT
Voxel-by-voxel numerical averaging over all the breathing phases
(For dose calculation)
The active breathing control system (ABC) uses an ___________________ to assist breathhold.
External ventilation valve
What are the features of the Synchrony system?
- incorporated into cyberknife and tomotherapy system
- make use of kv x ray sources
- continual detect and correct tumour and patient movement without interrupting the treatment
What is calypso verification system?
A RF based system to monitor the position of the prostate during RT
What are implanted into the patient to localise prostate position using Calypso 4D tracking system?
Three transponders, transponders are excited sequentially at each of three unique resonant frequencies.
What are the benefits of DIBH?
- increase distance between the heart and treatment target —> better sparing of heart
- lung volume increase, better sparing of lung
What are the sources of geometric distortion in MR?
Magnet inhomogenelty
Gradient field nonlinearity
Patient susceptibility
What is entropy?
A function which characterised the unpredictability of a random variable
What is mutual information?
It is the reduction in the uncertainty of one variable after knowing the other
What the image are co-registered, the mutual information is __________________
Maximised
