MR Data for Treatment Planning and
Stereotactic Procedures: Sources of
Distortion, Protocol Optimization,
and Spatial Accuracy Assessment
(Preview of TG117 Report)
Debra H. Brinkmann
Mayo Clinic,
Rochester MN
Acknowledgements
„ TGTG-117 “Use of MRI Data in Treatment
Planning and Stereotactic Procedures –
Spatial Accuracy and Quality Control
Procedures”
Procedures”
„
Deb Brinkmann, chair
„
Kiaran McGee
„
Ed Jackson
„
R. Jason Stafford
„
Steve Goetsch
Outline
„ MR in Treatment Planning
„ Spatial accuracy of MR images
„ Impact of Scanner (strength / configuration)
„ Impact of Pulse Sequence / Parameters
„ Distortion assessment / QC
MR in
Tx Planning
Overview – MR in RTP
Overview – MR in RTP
„ Advances in planning and delivery Æ
„ MR - Functional / biological information
„ Further improve target definition / extent
„ Target Ï severity with Ï dose
necessitate improved target delineation
„
MR – soft tissue contrast
Chang, 2006 MedPhys
Khoo, 2006 BJR
1
Overview – MR in RTP
Overview – MR in RTP
„ Image registration
„ Spatial Distortions
„
To correlate MRMR-delineated structures to CT
„ Can be ≥ 1cm
„ ScannerScanner-Dependent Distortions
„ External Magnetic Field – Inhomogeneity, Environmental
„ Gradients – Nonlinearities, Scale Factor Errors, Eddy
Î
Currents
„
52 cm
MR narrow
Kessler, 2006 BJR
70 cm
MR wide
RF – Slice Profile
„ Patient/ObjectPatient/Object-Induced Distortions
„ Chemical Shift
„ Magnetic Susceptibility
80 – 85 cm
CTsim
MR basics
MR basics
„ MR signals depend on Larmor equation:
„ In general:
„
ω = γ B0
„
RF pulse is applied together with a gradient to
generate a signal from a particular subsub-volume
„
Signals are spatially encoded using gradients
for image localization
„
Complex MR signals are detected
„ Localization depends on linear relationship
b/w space & resonance frequency
„
Established by linear gradients
„
ω = γ ( B 0 + G × r)
„
„
Imaginary
Magnitude
Magnitude and phase components
φ
Real
Reconstruction to create an MR image
MR basics
„ Perturbed MR signal (magnitude, phase) affects:
„ Presence of artifacts
„ Image quality
„ Interference with relationship between space
and resonance frequency:
„
Leads to geometric distortions
„
„
„
Shifting
Warping
Intensity gradients
x = ω/γ - B0
Gx
Spatial
Accuracy
2
Spatial Accuracy
Spatial Accuracy
„ ScannerScanner-dependent distortions
„ External magnetic field inhomogeneities:
inhomogeneities:
„ Result of:
„ Desire high uniformity over entire volume
„ For linear relationship b/w space & frequency
„
Design compromises
„
Imperfections
„
Drifting / failure of specific components
„ Constant
„
Across multiple imaging sessions
„
Allows for regular testing
„ Perfect uniformity not technically feasible
„ Imperfections change resonant frequency
„ Result spatial misregistration
Spatial Accuracy
Spatial Accuracy
„ Shimming to improve B0 homogeneity:
„ External magnetic field inhomogeneities:
inhomogeneities:
„ Passive shims (pieces of metal in bore)
„ Installed during initial calibration
„ Homogeneity maintained over finite
„ Active shims (superconducting shim circuits)
„ Adjusted at calibration, preventative maintenance
„ Resistive shims (linear, nonnon-linear shim circuits)
„ Adjusted during imaging
„ Shim imaging volume vs. whole field
„ NonNon-linear efficacy - application dependent
Spatial Accuracy
„ Environmental Magnetism:
Magnetism:
„
Vendor specification: DSV (diameter of spherical
volume)
„
Imaging outside of DSV subject to distortions
„
Shift depends on strength of applied gradient
x = ω/γ - B0
Gx
Spatial Accuracy
With Garbage Truck
„ Electromagnetic shielding
„ Environmental Magnetism:
„ Importance of QC:
used in site design
„ Stray magnetism can affect
MR acquisition
„ Example:
„ Garbage truck near scanner
„ Iron in truck magnetized
„ Affected B0 homogeneity
„ Result: severe distortion
spherical volume
„
More subtle effects might not be apparent
„
QC procedures needed to catch such errors
„
Without Garbage Truck
Example – detected impact from steel beams for
construction placed near MR suite
Courtesy of Kiaran McGee
3
Spatial Accuracy
Spatial Accuracy
„ Gradient Nonlinearities:
Nonlinearities:
„ Gradient Nonlinearities:
Nonlinearities:
„ Spatial encoding achieved with gradients
„ Linearly mapping position with frequency
„ Conventional 2D sequences:
„ “pinpin-cushion”
cushion” or “barrel”
barrel” effect inin-plane
„ “potato chip”
chip” effect on image plane
„ “bowbow-tie”
tie” effect on slice thickness
„ Deviations from linearity due to:
„ deviations in rise time
„ peak amplitude
„ physical design
„ Deviations Ï with distance from isocenter
„ primary source of distortion over majority of
imaging volume Baldwin, Med Phys 2007
Sumanaweera, Neurosurg 1994
Spatial Accuracy
Spatial Accuracy
„ Gradient Nonlinearities
„ Example: warping inin-plane
„ Gradient Nonlinearity Corrections:
Corrections:
Baldwin,
Med Phys
2007
„
Example: warping along slice select direction
„ Vendors provide inin-plane corrections
„ Assume distortion to gradient amplitude is
constant
„ Quantitate distortion with phantom
„
Apply to reconstructed images after patient
data acquired
„ Some but not all vendors provide
corrections along slice encoding axis
Wang, Med Phys 2004
Gradient Field Nonlinearity Effects - In-Plane
Distortion
Slice Plane at Isocenter
– 20 cm FOV
– White circles: with gradient nonlinearity correction
– Black circles: without gradient
non-linearity correction
– Maximum error w/o correction:
~ 4.5 mm at ±10 cm from
isocenter
– Maximum error w/ correction:
< 1 mm at ±10 cm from
isocenter
Gradient Field Nonlinearity Effects - In-Plane
Distortion
Slice Plane at 20 cm
from Isocenter
– 20 cm FOV
– White circles: with gradient nonlinearity correction
– Black circles: without gradient
non-linearity correction
– Maximum error w/o correction:
~ 5.5 cm at ±10 cm from iso
– Maximum error w/ correction:
< 2 mm at ±10 cm from iso
MDACC MR Research
MDACC MR Research
Courtesy of Ed Jackson, MDACC
Courtesy of Ed Jackson, MDACC
4
Gradient Field Nonlinearity Effects - Slice
Distortion
Spatial Accuracy
„ UltraUltra-wide short bore
Large
gradient
coil
Small
gradient
coil
Effects are
exacerbated with
ultrashort bore
magnets!
125 Hz/pixel
2D vendor corrections
(c)
490 Hz/pixel
2D vendor corrections
(b)
490 Hz/pixel
no vendor corrections
(a)
Frequency
Slice Profile
Phantom
Phase
gradient corrections do not correct for
distortions due to resonance offsets
MDACC MR Research
Courtesy of Ed Jackson, MDACC
Courtesy of R. Jason Stafford, MDACC
Spatial Accuracy
Spatial Accuracy
„ Gradient Nonlinearities
„ Example: pelvis with vs. without corrections
„ Reported distortion values
„ Wang et al, MRI 2004 (five 1.5T systems)
„
„
5-26mm uncorrected 240x240x240mm3
3-12mm 2D vendor corrections applied
240x240x240mm3
„
CT
MR, no corrections
Chen,
Med Phys
2006
Baldwin et al, Med Phys 2004 (3T system)
„
Doran et al, PMB 2005 (1.5T system)
„
Chen et al, Med Phys 2006 (0.23T system)
„
„
„
MR, GDC corrections MR, GDC + additional
point-by-point corrections
<2.5mm 2D vendor corrections applied 95mm sphere
„
„
<7mm uncorrected 260x260x200mm3
25mm uncorrected 320x200x340mm3
<5mm 2D vendor corrections applied 360x360x200mm3
<5mm 2D vendor corrections applied 480x480x200mm3
Spatial Accuracy
Spatial Accuracy
„ Eddy Currents:
Currents:
„ RF nonnon-uniformity:
uniformity:
„ Generated in conducting materials…
materials…
„ Metal dewer,
dewer, gradient coils, RF coils
„ RF energy generates a detectable signal
„ Delivered as a pulse (RF pulse)
„ … exposed to time varying magnetic field
„ Faraday’
Faraday’s law of induction
„ Changing gradient fields
„ Creates perturbing magnetic field
„ Lenz’
Lenz’s law
„ Result – spatial distortion
„ Vendors provide some correction method
„ Design criteria for RF waveform
„ Exact shape variable
„ May not be designed to maintain uniform signal
„
Issue for advanced techniques
„ Gradient linearity also impacts slice profile
„ Evaluate RF profile to verify width
5
Spatial Accuracy
Spatial Accuracy
„ Patient/objectPatient/object-induced distortions
„ Chemical shift (of the first kind):
kind):
„ Result of:
„ Composition of patient or object
„ Produces shift in resonant frequency
„ E.g. 220 Hz decrease for 1.5 T ( ω0 = 63.8 MHz)
„ Misregistration when BW / pixel < chemical shift
„ Manifest along frequency encoding direction
„ Unique:
„ Can vary dramatically b/w patients
„ Must be considered for each imaging situation
Spatial Accuracy
„ Produces banding
„ Banding – only lipid signal shifted for voxels
with mixed composition
Spatial Accuracy
Chemical Shift of the first kind
„ Magnetic Susceptibility:
Susceptibility:
„ Relates net magnetization to the applied
magnetic field
BW/pixel:
16 Hz
BW/pixel:
31 Hz
BW/pixel:
61 Hz
BW/pixel:
122 Hz
BW/pixel:
244 Hz
BW/pixel:
488 Hz
Spatial Accuracy
„
Materials in magnetic field become magnetized
„
Creates changes in magnetic field at interfaces
„ Complex, depending on many factors:
„ Susceptibility difference across interface
„ Shape and orientation of the interface with B0
„ Strength and polarity of gradients
Spatial Accuracy
Magnetic Susceptibility
„ Magnetic Susceptibility:
Susceptibility:
„ Perturbations produce distortion, signal loss
„ Relatively small for soft tissues
„ Undetectable for many applications
„ Difference b/w tissue & air: ~ 9 x 10-6* Î
BW/pixel:
16 Hz
BW/pixel:
31 Hz
BW/pixel:
61 Hz
BW/pixel:
122 Hz
BW/pixel:
244 Hz
BW/pixel:
488 Hz
distortions
„
„
AirAir-tissue interfaces for air cavities
External surface of patient
*Schenk, Med Phys 1996
6
Spatial Accuracy
„ Magnetic
Susceptibility:
Susceptibility:
Impact of
Scanner
„ Difference between
metal and tissue is
large
„ E.g., titanium
„
~ 20 times larger than
soft tissue*
tissue
Courtesy of Kiaran McGee
*Schenk, Med Phys 1996
Impact of Scanner
Impact of Scanner
„ Design includes compromises & tradetrade-offs
„ No single design for all performance spec’
spec’s
„ Systems optimized to meet subset of applications
„ Field strength options:
options:
„ MR data for use in RTP
„ Each scanner used should be characterized
„ LOW field strength advantages:
„ Ð patientpatient-induced distortions
„ Ð artifacts from metal objects (e.g. brachytherapy)
„ The physicist needs
„ Understanding of distortion sources
„ Ability to quantitate image distortion
„
„
0.2 - 1.0 T (“Open”
Open”, permanent or resistive magnet)
1.5, 3.0 T (Cylindrical bore, superconducting magnet)
„ HIGH field strength advantages:
„ Ï signalsignal-toto-noise (image quality)
„ Ï resolution for metabolites (MRS)
Impact of Scanner
Impact of Scanner
„ OPEN magnet:
magnet:
„ NARROW, CYLINDRICAL LONG bore:
bore:
„ High performance systems (e.g. cardiac)
„ High resolution imaging (e.g. CNS)
„ Advantages
„ Flexibility in patient positioning
„ Increased patient access
„ Drawbacks
„ Significant external field
inhomogeneities
„ Ï scannerscanner-dependent
distortions
www.gehealthcare.com
GE 0.7T OpenSpeed
„ Advantages
„ Increased field homogeneity
„ Ð scannerscanner-dependent distortions
„ Drawbacks
„ Narrow bore
„ Ð patient comfort
www.medgear.org
Phillips 1.0T Panorama
MR simulator
7
Impact of Scanner
„ WIDE, SHORT bore:
bore:
Impact of
Sequence
„ Advantages
„ Increased magnet aperture
„
„
Treatment position
Ð claustrophobia
www.medical.siemens.com
Siemens 1.5T Espree
Wide / short bore
„ Drawbacks
„ Sacrifice performance, field homogeneity, field
strength
„ Decreased homogeneity Î increased distortion
Impact of Sequence
Impact of Sequence Parameters
„ Sensitivity to distortion:
distortion:
„ 3D vs. 2D sequences:
sequences:
„ Gradient echo sequences
„ Most sensitive to distortion sources
„ Inhomogeneity effects accumulate throughout
acquisition
„ For standard rectilinear imaging, spatial
„ Conventional spin echo sequences
„ 180°
180° refocusing pulse reduces distortion
„ Fast spin echo sequences
„ Least sensitive to offoff-resonance effects
„ Multiple 180°
180° refocusing pulses and short TEs
distortion due to resonance offsets:
„
„
Manifest along the frequency encoding axis
Phase encoding direction not affected
„ 3D acquisitions use phase encoding
along slice encoding direction
„
Less distortion for 3D vs. equivalent 2D
acquisitions
Impact of Sequence
Impact of Sequence Parameters
„ Advanced acquisition techniques: Majority
rely on “echo planar imaging”
imaging” or EPI*
EPI
„ EPI techniques collect a train of echoes
„ Bandwidth per pixel:
pixel:
„
„
„
„
„
Uninterrupted accumulation of phase
Very sensitive to field inhomogeneities and
eddy current effects
PE: severe shifting or compression of objects
FE: shearing of object
Object induced inhomogeneities
„
„
„ Ï BW minimizes resonance offsets
„ Field inhomogeneity,
inhomogeneity, chemical shift, magnetic
susceptibility
„ If Δf > BW/pixel, shift will result
„ Magnitude depends on pixel dimensions
„ TradeTrade-off: Ï BW will Ð SNR
„ SNR ∝ (voxel vol. × √ Ny × NEX / BW)
considerable local distortions
distortions Ï with increasing field strength
*Mansfield, Br J Radiol 1978
BW/pixel:
16 Hz
BW/pixel:
488 Hz
8
Impact of Sequence Parameters
Impact of Sequence Parameters
„ BW / pixel for PatientPatient-Induced Distortions:
Distortions:
„ Distortions Ï with increasing field strength
„ Spatial resolution:
resolution:
„
Distortions Ï with decreasing BW/pixel
„
Δf = γ⋅B
9.0ppm* for MS)
γ⋅B0⋅∂ppm, (3.5ppm for CS, 9.0ppm*
*Schenk, Med Phys 1996
# pixels = Δf / (BW/pixel)
„
„ Magnitude depends on pixel dimensions
„
dimension of shift
(depends on pixel dimensions)
Chemical Shift Artifact
Maximum Susceptibility Artifact
0
0.2 T
1.5 T
3.0 T
-10
-15
# pixels (+/-)
# pixels
„
-20
40
0.2 T
1.5 T
3.0 T
30
20
50
100
150
200
250
300
10
0
Bandwidth (Hz/pixel)
50
100
150
200
250
300
Bandwidth (Hz/pixel)
Impact of Sequence Parameters
Scenario #
1
2
3
4
5
6
7
8
1.5
1.5
1.5
1.5
3.0
3.0
3.0
3.0
0.5
T
215
72
Hz
250
250
220
320
250
250
320
320
260
mm
64000
32000
64000
32000
64000
96000
84000
20000
Hz
Num Freq Enc Points
256
256
256
320
256
256
320
320
256
Num Phase Steps
256
256
256
320
256
256
320
320
256
BW per pixel
125
250
125
200
125
250
300
263
78
1
1
2
1
1
1
0.5
0.5
4
chemical shift
1.68
0.84
1.48
1.07
3.35
1.68
1.43
1.63
0.93
mm
susceptibility shift
2.26
1.13
1.99
1.44
4.51
2.26
1.93
2.20
1.25
mm
relative SNR
100
71
110
93
200
141
107
114
91.2
%
relative acquisition time
100
100
200
125
100
100
62.5
62.5
400
%
resolution (frequency)
0.98
0.98
0.86
1.00
0.98
0.98
1.00
1.00
1.02
mm
resolution (phase)
0.98
0.98
0.86
1.00
0.98
0.98
1.00
1.00
1.02
mm
BW (total FOV)
NEX
215
215
215
429
429
429
429
Impact of Sequence Parameters
„ Lipid Suppression:
Suppression:
Reference
32000
FOV
„ TradeTrade-off: Ï resolution will Ð SNR
„ SNR ∝ (voxel vol. × √ Ny × NEX / BW)
0
0
delta_fat_water
ÐFOV will Ï resolution
50
-5
B0
Typical pixel resolution .75 – 1.5 mm/pixel
„ Higher resolution reduces physical
Hz
„ Lipid signal can be nulled
„ If not clinically relevant
„ To eliminate chemical shift effects
„
Both kinds of chemical shift artifacts
„ Several imaging techniques
„ Spectral saturation
„ Inversion recovery
„ Dixon technique
Impact of Sequence Parameters
Impact of Sequence
„ Lipid Suppression:
Suppression:
„ Lipid Suppression:
Suppression:
„ Spectrally selective saturation pulses
„ Uses RF pulse
to saturate spins
precessing at
resonant freqfrequency of fat
„ Affected by poor
shimming:
„ Inversion recovery
techniques (STIR)
„
„
Incomplete fat saturation
Inadvertent suppression
of water
Courtesy of
Kiaran
McGee
T1, FSE
STIR
„ Dixon technique
„ Requires multiple data
acquisitions (two TE)
„ Lipid & tissue signals in
and 180°
180° out of phase
„ Add / subtract for lipidlipidand tissuetissue-only images
IP
OP
Water-only
Fat-only
J Ma,
JMRI
2006
9
Impact of Sequence Parameters
„ Frequency Encoding Direction:
Direction:
„ Resonance offsets manifest along
frequency encoding direction
„ Can manipulate to visualize such
distortions (in(in-plane)
„ Repeating scan with reversed gradient
„ Repeating scan swapping frequency & phase
„ Cost – additional scan
Distortion
Assessment /
QC
Assessment / QC
Assessment / QC
„ Current Guidance
„ ACR Weekly QC protocol
„ Geometric accuracy criteria
„ ≤ 2mm over 148mm x 190mm
„ AAPM and ACR have published
acceptance test and QC documents
„
Do not address necessary QC program and
image acquisition optimization goals when MRI
data used for procedures in which spatial
accuracy is critical
Courtesy of Kiaran McGee
Assessment / QC
Assessment / QC
„ Works in Progress
„ Assessment/QC for new application
„ TGTG-117: Use of MRI Data in Treatment
Identify New
Application
Requirements
Planning and Stereotactic Procedures –
Spatial Accuracy and QC Procedures
„
„
„
Review physical bases for spatial accuracy
limitations in MRI
Provide guidance with examples for reducing
or eliminating the effects of distortion
Propose QC tests for systems used for
applications requiring high spatial accuracy
Inventory
Equipment &
Resources
Quantitate
System Performance
(Baselines)
Initiate Scanner/
Application Upgrade
No
Yes
Modify/
Upgrade
Application or
Scanner?
No
Review feasibility
of pursuing
application
Report Findings to
Supervising Physician
Performance
Adequate for
Application?
Yes
Establish
Routine
Distortion QC
10
Assessment / QC
Identify New
Application
Requirements
Assessment / QC
Inventory
Equipment &
Resources
Quantitate
System Performance
(Baselines)
Identify New
Application
Requirements
Inventory
Equipment &
Resources
Quantitate
System Performance
(Baselines)
„ Step 1: Identify Application Requirements
„ Step 2: Inventory Equipment and Resources
„ Volumetric coverage (FOV, craniocranio-caudal extent)
„
Scanner capabilities
„ Spatial resolution (voxel dimensions)
„
„ Spatial accuracy (tolerance, volume)
„
„ Bore diameter, RF coils (tx position)
„
applicators
Assessment / QC
Identify New
Application
Requirements
„
„
„ Pulse sequence(s)
sequence(s) (contrast)
Phantoms for geometric distortion assessment
„
„ MR compatibility of immobilization,
Identify magnet, gradients, coils, sequences needed
to achieve required performance specifications
Existing scanner? Upgrades needed? New system?
Acceptance / commissioning tests
Routine QC
Analysis tools (IT support for programming, networking)
Assessment / QC
Inventory
Equipment &
Resources
Quantitate
System Performance
(Baselines)
Identify New
Application
Requirements
Inventory
Equipment &
Resources
Quantitate
System Performance
(Baselines)
„ Step 3: Quantitate System Performance
„ Step 3: Quantitate System Performance
„ Baselines (Acceptance / Commissioning)
„ Existing tests (ACR, AAPM)
„ Purpose: maintain diagnostic image quality
„ Characterize scanner subsystems
„ External magnetic field (homogeneity)
„ Gradients (linearity, correction algorithms, eddy current
„
„
„
Do not assess spatial fidelity over RTP volumes
Can be used with modifications
compensation…
compensation…)
„
RF (slice profile)
„
Assessment / QC
Identify New
Application
Requirements
Assess over the desired imaging volume
Using application specific imaging parameters
Assessment / QC
Inventory
Equipment &
Resources
Quantitate
System Performance
(Baselines)
Identify New
Application
Requirements
Inventory
Equipment &
Resources
Quantitate
System Performance
(Baselines)
„ Step 3: Quantitate System Performance
„ Application requirements identified
„ Application Specific Protocol Optimization
„ 2D vs. 3D
„ System characterized
„ Over volume of interest
„
FOV, inin-plane resolution
„
Gradient echo vs. Spin echo
„
Signal suppression techniques
Initiate Scanner/
Application Upgrade
No
Performance
Adequate for
Application?
Yes
Modify/
Upgrade
Application or
Scanner?
Review feasibility
of pursuing
application
No
„ Step 4: Performance Adequate?
„ If not:
Report Findings to
Supervising Physician
„
„
Modify application? Upgrade scanner?
ƒ Initiate modification/upgrade & rere-evaluate
Or report unacceptable accuracy
11
Quantitate
System Performance
(Baselines)
Assessment / QC
Establish
Routine
Distortion QC
Yes
Quantitate
System Performance
(Baselines)
Assessment / QC
Establish
Routine
Distortion QC
Performance
Adequate for
Application?
Yes
„ Step 5: Establish QC program
„ Establishing a QC program
„ Measure scanner dependent distortions
„ Distortion phantom for routine QC
„ Define the QC phantom requirements
„
Phantom of known geometry
„
Verify constancy of spatial fidelity
„
„
„
„
Drifting, failure
„
Environmental magnetism
„
„
Over the volume of interest
Quantitate
System Performance
(Baselines)
Assessment / QC
Yes
„ Establishing a QC program
„ QC testing
„
Establish imaging protocol
„
Determine testing frequency
„
Develop analysis tools
„
„
Automated evaluation
„
Automated reporting
Volume to test over
Fiducial spacing
Fiducial resolution (in(in-plane ≥ 5 pixels)
Dimension compatibility (RF coil, stereotactic frames,
immobilization)
„
Establish
Routine
Distortion QC
Performance
Adequate for
Application?
Establish procedures when QC fails
Identify / modify / develop applicationapplication-specific
QC phantom
Summary
Performance
Adequate for
Application?
„ Spatial fidelity in MR images
„ Source of geometric distortions
„ Impact of scanner characterization
„ Impact of image acquisition parameters
„ Vendor supplied correction methods
„ Importance of assessing distortions
„ Over the volume of interest
„ With the same parameters to be used clinically
„ Importance of appropriate MR QC program
12