Download CT Simulator

Simulation & Acquisition
Todd Pawlicki, Ph.D.
Professor & Vice-Chair
Director, Division of Medical Physics
Objectives
• Discuss the CT simulation process
• Discuss applications of imaging to radiotherapy
Radiation Oncology Workflow
Tx Plan
Consultation
CT Scan
Follow-up
Tx Delivery
Slide courtesy of Jack Yang, PhD
Virtual Sim
(Image Fusion)
Imaging
Plan QA
Acquisition of Patient Data
• 2 Dimensional Process
– Solder wire
– Plaster cast
– Contour plotter
• 3 Dimensional Process
– CT of treatment area
– CT, MR, PET Registration / Fusion
The CT Simulator
• Identical to regular CT
– Scanner, couch, computer console, etc.
• Flat hard top couch
– Same as linac
– Important for position reproducibility
• Laser marking system
• Virtual simulation workstation
– 3D image
• Larger aperture size
– 70 to 90 cm
Slide courtesy of Jack Yang, PhD
Computed Tomography
• The use of CT data in radiation therapy
–
–
–
–
Diagnosis
Tumor / Normal tissue delineation
Treatment planning / beam orientations
Dose calculation
Types of CT Scanners
http://www.asnt.org/publications/Materialseval/basics/may00basics/may00basics.htm
Measured Linear Attn Coefficients
Hounsfield Unit (HU)
-1000 < HU < 1000
 i , j  water 
HU  1000  

 water 
12 bit Internal Computer Number (C)
0 < HU < 4096
 i , j 
C  1000  

 water 
Tissue Types
Sternum
(1030)
Lung
(285)
Soft
Bone
(1220)
Soft Tissue 1
(1050)
Soft Tissue 2
(875)
Hard
Bone
(1810)
Window Width and Level
• Window Width
W
– The range of gray shades in the image
• Window Level
– The middle HU of the window
• Window adjustments are set to highlight
the structures to be viewed
L
Window:
Level:
100
20
W
Window:
Level:
3200
256
L
Window:
Level:
200
20
Slide courtesy of Jack Yang, PhD
DRR – Image Quality
• Scanning slice thickness creates different quality of DRR, thinner
slices produce better images, but need more processing power
• Need balance between large amounts of data and image quality
(scanning protocols need to be established)
3mm Slices
1.2 mm Slices
DRR and Image Quality
CBCT
RTCT
CT Simulator – Pros and Cons
• Small Bore vs. Large Bore
– Patient population with different setup technique
– Conventional simulator availability
– Larger bore creates more scattering dose
• Single Slice vs. Multi Slice
– DRR quality (rendering technique and speed)
– Image quality (Scanning slice thickness)
– 4D CT (respiratory gating)
Slide courtesy of Jack Yang, PhD
CT Simulator – Large Bore (FOV)
70 cm Bore Opening
85 cm Bore Opening
Better for Breast Simulation
Slide courtesy of Jack Yang, PhD
Isocenter Marking
• Isocenter placement could affect the patient treatment
accuracy since it serves as the base line information
Slide courtesy of Jack Yang, PhD
General Immobilization Equipment
Multiple Bags
Vac-Loc Bag
Elekta Stereotactic Body Frame
Civco Wing Board
Medical Intelligence Body Fix System
Immbolization Options
(“Active”)
Immbolization Options
(“Passive”)
• shoulder
constraints
CT Sim Head Immobilization
Achievable variability
in position1-3 mm
Slide courtesy of Jack Yang, PhD
Reproducibility
• Difficult to reproduce a misaligned patient
• Other artifacts can also create setup problems
Head Position
• Optimal position depends on the treatment
volume and delivery type
Example
CT Simulation
Checklist
CT Simulation
Policy & Procedure
CT Simulation – Contrast Issues
• Contrast can be used to help differentiate between
tumors and surrounding healthy tissue
• Using contrast is risky, nursing required to be present
• For heterogeneity-based CT planning, contrast may
create dose distribution errors due to large CT numbers
– Contrast density override should be carefully examined
Slide adapted from Jack Yang, PhD
Simulation – General Issues
CT Used for Dose Calculation
CT Number versus Relative electron density
1.75
Relative electron density
1.50
1.25
1.00
0.75
0.50
0.25
0.00
-900
-600
-300
0
300
CT Number (H)
600
900
1200
CT/MR Anatomy
• Primarily used for target delineation
CT
MR
Bushburg et al. The Essential Physics of Medical Imaging. 2nd Ed, 2002.
T1
Weighted
T2
Weighted
FLAIR
MRSI
• MR spectroscopy and MR imaging methods
– Produce a spectrum identifying different chemical compounds
(metabolites) in various tissues
• Metabolite ratios differentiate between active
tumor, normal tissue, and necrosis
Hunjan et al. IJROBP, 2003.
MRSI
Delorme and Weber. Applications of MRS in the
evaluation of focal malignant brain lesions. Cancer
Imaging, 2006.
FDG PET
(fluorodeoxyglucose PET Imaging)
• Malignant cells compared to healthy cells
– Divide rapidly
– Metabolize glucose at a higher rate
• Attach a positron emitter to a glucose analogue
– Example, fluorine-18
• FDG PET studies show utilization of the
glucose analogue
– Tumor metabolism
PET/CT Images
Registration / Fusion
• Choose fusion
algorithm
• Transform source
image into coordinate
system of destination
image
• Evaluation and
interpretation of the
registered images
Hutton and Braun 2003
Choose Fusion Algorithm
Use Pixel Data
Hutton and Braun 2003
Manual Match
• Interactive tools
Fusion Evaluation
Summary
• CT Simulation is replacing conventional simulation
• Multi-modality imaging is increasingly important
• CT Simulation can improve target and normal tissue
contouring and treatment planning
• TG-66 report provides a very thorough QA process
for CT simulation