7/21/2009
Real--time tracking & Adaptive RT
Real
Real-Time MR
RealMR--guided
Radiotherapy: integration of lowlowfield MR
• Minimize PTV margin in dynamic target
10~20 mm
B. Gino Fallone
Cross Cancer Institute – University of Alberta
Edmonton, AB, Canada
CTV
CTV
Critical
Structure
PTV
State of the art
Critical
Structure
Real-time MR guided RT
Rotating Gantry Concept
Cross Cancer Institute
Viewray
Design Advantages
4. Source of Linac RF is
away from the MRI
avoiding RF interference
5. RF & magnetic shielding
avoids
mutual interferences
Linac-MR.ca
MRI pole
Linac
beam stop
1. Unobstructed Beam
2. Linac is stationary wrt
MRI avoiding MR field
distortions
3. Field strength avoids
distortion of dose
distribution,
susceptibility artifacts
MRI pole
Patient Anxiety
„
Advantages of Low Field MR
„
Open design common with low field systems
reduces incidence of claustrophobia and
increased scan compliance
Low field systems result in reduced vibration
and acoustic noise during switched gradients
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Safety
„
„
„
„
Forces on ferromagnetic implants are considerably
less severe at low field.
RF absorption is much lower (absorbed power goes
upp as B02)), ggreatly
y reducingg risk of burns or tissue
heating.
Longer RF wavelength reduces chance of coupling
(and heating) with conductive (metallic) implants,
lessening risk of contraindication.
Reduced field comes with reduced likelihood of
accident due to projectiles (may be useful in RT)
Quality and Scan Implications
„
„
Cost
„
„
Reduced initial cost.
Greatly reduced maintenance costs for low
field systems that do not utilize cryogens,
especially for systems that use permanent
magnets
Low Field Characteristics for
Image Guidance in RT
„
„
„
„
Susceptibility Artifacts
Reduced geometric distortion due to tissue
susceptibility differences, which increase
linearly with field.
Increased sequence flexibility due to reduced
RF absorption.
SNR Not as critical (as diagnosis & simulation)
Exploit the T1
T1--weighted contrast available at low
fields offering faster imaging compared to T2T2weighted imaging more popular at higher fields
(slower) decreased magnetic field shielding
issues
Decreased image distortion (More important in
RT)
Decreased susceptibility artifacts
Imaging Requirements for
Image--guidance
Image
kVCT TT
0.2 T MR
kV cone‐beam CT
TomoTherapy MVCT
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Image Requirements
Simulation
MAIN ADVANTAGE
Patient Dosimetry Changes due
to Magnetic Fields
„
„
b. Using SNR of Proton density image ∝ main magnetic field
Significant Reduction of curling of electron
trajectories (electron
(electron return effect)
effect) at Low
Magnetic Fields
At Higher Magnetic Field Strengths, electronelectronreturn effects still remain to be considered
SNR ↓ 7.5 from 1.5 T → 0.2 T
6 MV
4 cm
H2O
0T
10 cm Lung
0.2 T
4 cm
H2O
1.5T
•Relative depth doses non-diverging 6 MV beam.
•EGSnrc/black, PENELOPE /gray. The magnetic field points out of the page.
•At 1.5 T substantial increase (~ 40 %) in dose at the water-to-lung
boundary and a decrease (~ 25 %) in the lung-to-water boundary.
•At 0.2 T, slight differences of only ~ 2 % are seen. Dose to air is not
shown.
Parallel--Opposed
Parallel
Clinical-- Monte Carlo Study (Penelope and EGSnrc)
Clinical
4 Field
Brain
Kirkby et al., Med Phys 35, 1019-1027 (2008)
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Patient Studies:
Lung
„
„
Heterogeneous
tissue
RBP 0.2 T
Proof of Concept Achieved
- hot/cold ± 10
10%
%
- notable
t bl inf.
i f shift
hift off
dose
„
Cross Cancer Institute
FC 1.5 T
- hot/cold +30% //-15%
- near soft tissue
tissue--lung
boundaries & beam
edges
- dependence on gantry
angle
Med Phys 35, 1019-1027 (2008)
MRI: 82.5
cm poleto-pole
Head--Unit Rotational Design
Head
5 cm Fe
Lead shielding for 5 cm Fe
linac
350 Ibs
Linac
50 Ibs MLC
50 cm x 75 cm
500 Ibs
Radiation beam
Max field size 20 x 20 cm at isocenter
Beam stop (3 TVL)
20 cm thick x 50 cm x 50 cm
1250 Ibs
Units in mm
Units in mm
Our Measurements (Varian 21EX)
Spectrum Analyzer
RF measurement with Shield
Without Shield
With Shield
NON--SHIELDED
NON
Plot of electric field strengths at various distances from the linac modulator
inside the treatment vault. Also shown is the electric field strength in the
linac control console area.
Phys Med Biol, 54, 2483-2492 (2009)
Lamey et al.
SU-FF-J-76
Lamey et al.,
SU-FF-J-84
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Shielding Calculations
Magnetic Field Measurements
600
Magnetic field (Gauss)
500
without shield
with shield
400
300
200
30
25
20
15
10
5
0
-5
5
60
70
80
90
100
100
0
-100
40
50
60
70
80
90
100
110
Distance from MR magnet isocenter (cm)
Item 1 is a cross section, white horizontal rectangular forms correspond to the MR magnet structure. The
colorwash depicts the magnetic field inside and just outside the magnet structure.
Items 2 and 3 depict the areas to be occupied by the MLC and the linac waveguide, respectively.
Construction
Measurement of magnetic field along the central axis of the linac waveguide with and without
magnetic shield assembly in place. The shield assembly is placed at 65 cm from the magnet
isocenter.
DEC, 2008
CT- IMAGE
acrylic rectangular
cube, 15.95 x 15.95 x
25.4 mm with holes of
di
diameters
t
2
2.52
52 mm,
3.45 mm and 4.78 mm.
SNR: 80
immersed in a 10 mM
solution of CuSO4
within a plastic
container 22.5 mm
inner diameter.
SNR: 61
SAME SNR WITH/WITHOUT
LINAC IRRADIATION
Most Recent
MR- IMAGE
Without Linac
Irradiation
MR-IMAGE
With linac
Irradiation
RF in Magnetic Fields
Millenium MLC Motor
Lamey et al, COMP, Vancouver, BC, July 21-0-24, 2009
One leaf moving
Lamey et al, SU-FF-J-84
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RF in Magnetic Fields
Varian 5252-leaf
Lamey et al, COMP, Vancouver, BC, July 21-0-24, 2009
MR Images with MLC at 70 cm
from Isocentre
UNSHIELDED
STATIONARY
MOVING (13 leafs)
One leaf moving
MR Images with MLC at 70 cm
from Isocentre
SHIELDED
STATIONARY
CCI-- Tumour Tracking
CCI
MOVING (13 leafs)
Radiation-Induced Conductivity
Radiationin RF COILs
RESEARCH IN PROGRESS
Frequency spectrum
Burke et al., SU-EE-A3-3
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Biplanar advantages
(Patient--Dosimetry Changes)
(Patient
„
„
Cylindrical MR only allows MR B0 field to be
perpendicular to direction of centre of photons
(waveguide electrons)
Bi--Planar MR allows perpendicular,
Bi
perpendicular and
PARALLEL orientation of Magnetic Field to
direction of centre of photons (waveguide
electrons
PARALLEL decreases curling of electron
trajectories (electron
(electron return effect)
PERPENDICULAR
PARALLEL
Linac
Linac
„
PERPENDICULAR vs PARALLEL
Bo
Bo
HOMOGENEITY (PPM)
(PPM)
PRIOR to SHIMMING
70 cm gap; 20 cm (D) hole
Hole Compensation
Cross Cancer Institute
PARALLEL
Linac
Pole Plates
Bo
Standard
Rose Ring
Optimized
B0 (T)
0.228
0.202
0.202
ΨΩ (10-11 T·m3)
2296.4
542.0
0.603
PPM30cm DSV
12174
6478
176
PPM40cm DSV
29830
14056
1202
TU-E-BRC-1
MAGNETIC FIELD 0.2T
Examples at 0.5 T
PERPENDICULAR
PERPENDICULAR
PARALLEL
PARALLEL
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Examples at High Fields:
PERPENDICULAR
PARALLEL
RESEARCH IN PROGRESS
Cross Cancer Institute
www.linac-MR.ca
PERPENDICULAR
Linac PARALLEL
Linac
1.0 T – 0 T
Bo
0.5 T
Bo
1.5 T – 0 T
Acknowledgement
„
„
„
Alberta Health Services
Cross Cancer Institute
Alberta Cancer Foundation
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