a mri simulator from proposal to operation - amos...
TRANSCRIPT
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A MRI Simulator from
Proposal to Operation
Yue Cao, Ph.D.
Departments of Radiation Oncology,
Radiology and Biomedical Engineering
University of Michigan
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Acknowledgments
NIH grants RO1 NS064973 (Cao) RO1 CA132834 (Cao) RO1 EB016079 (Balter) UO1 CA183848 (Cao/Eisbruch) RO1 CA184153 (Eisbruch/Cao) PO1 CA059827 (Ten Haken)
Siemens Research grant (Balter)
Cao 2
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Outline
Advantages of MRI-based simulation
Proposal and planning
MRI safety
QA program
MRI protocol development and workflow for RT
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Advantages of MRI for RT
Soft tissue contrast
Multi-contrast
Functional (Physiological) and metabolic imaging
– Dynamic contrast enhanced (DCE) imaging
– Diffusion weighted imaging (DWI)
– Spectroscopy imaging
Arbitrary slice orientation
Respiratory Motion suppression
Respiratory Motion management (4D MRI)
Cao 4
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MRI Simulation
MRI Simulators have been in use in Radiation Oncology for over 12 years (early systems in FCCC, Hokkaido,…)
Over 100 MRI scanners have been sold for primary/dedicated Radiotherapy use worldwide
Rapid developments are happening to support broad use of MRI as a Simulator
Cao 5
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Superior Soft Tissue Contrast
Cao 6
Post-Gd T1W FLAIR CT
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Superior soft tissue contrast
At high field Cao 7
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Tumor Definition
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Superior Soft Tissue Contrast
Cao 9
CT
MRI
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MRI: Multi-Contrast
Cao 10
T2WI Arterial phase enhancement
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Additional Cancer Sites
Cao 11 Devic, Med Phys, 39: 2011
Rectal
Prostate
GYN
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Cancer Sites Benefit from
MRI Soft Tissue Contrast
Brain primary tumors and metastases
Nasopharyngeal cancers and other HN cancers
Breast cancers
Rectal cancers
GYN cancers
Prostate cancers
Liver cancers
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Proposal and planning
Basic equipment
Specs for a MRI simulator
– Advanced pulse sequences
Site planning
Staff training
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MRI for RT simulation
Wide bore MRI Scanner
Flat table top, immobilization devices and compatible RF coils
Movable lasers
Simulation workstation
RT-sim MRI protocols
QA
Integration in RT workflow
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MRI System at UM
3T wide bore scanner (Siemen SKYRA) with LAP laser system and simulation workstation (Varian)
RF Coils:
– 20 channel head/neck
– 3 18 channel flex coils (“body 18” and “body 18 long”)
– Two small and 1 large 4-channel flex coils
– 2 detachable tabletops with 32-channel posterior spine coils
MRI-compatible immobilization equipment (CIVCO)
Various phantoms and test equipment
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LAP laser
Flat table top 32-channel posterior
Spine RF coil
3-point mask
Head support
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3-point mask – Anterior 18-
channel, posterior spine coil
Sand bag
UM solution Cao 17
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Indexed MRI-compatible table top
and coil holder
Siemens / CIVCO
Bridge to hold coils slightly away from patient
Detach Table Cao 18
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Request for Proposal:
MRI Simulator for RT
System level geometric distortion
B0 field homogeneity in a diameter of 30 cm and 40-50 cm
Gradient non-linearity and correction
Max field of view (50x50x45 cm or larger)
Active shimming (any high order shimming) – Human subject changes the baseline B0 homogeneity
Special pulse sequences and software for RT
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Four-Zone Principle Design
Cao 20
Zone II Zone III
Zone IV
Zone I: Universe
Zone II: Connected to MRI
suite, accessible by
patients and hospital staff
Zone III: Directly connected
to the magnet room,
accessible by screened
patients and trained staff
Field strength < 5 Gauss
Zone IV: Magnet room,
all ferromagnetic materials
excluded, access controlled
by level-2 safety officers
MRI suite ACR recommended 2002
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Site Planning
HDR Suite
CT scanner
Stairway outside department Cao 21
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Special Pulse Sequences for RT
Motion Suppression
– radial sampling, propelled sampling
– triggering sampling
Cao 22
Swollen motion Free HN image Free-breathing liver image
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Special Pulse Sequences for RT
diffusion imaging sequence – High geometric accuracy, high quality, high resolution
Cao 23 EPI RESOLVE
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Special Pulse Sequences for RT
Ultra-short TE pulse sequence for bone imaging
– Available in the quite suite
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Staff Training
Cross-training
– A MRI technologist can be cross-trained to become a RT sim therapist
Hiring a MRI tech prior to the MRI scanner installation and cross-training him/her for CT sim
– A CT sim therapist is cross-trained for MRI sim
We have a limited success on simple protocols
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Challenges (opportunities?) –
imaging of immobilized patients
A number of issues still impact image quality with off-the-shelf coils and standard immobilization materials
– MRI-safe immobilization materials may still be suboptimal for scanning
– Distance from coils to body due to table tops, immobilization equipment,…
– Incompatible configurations for MRI and RT
Cao 26
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Prone Breast scanning using a 4-channel coil
Paulson ES, et al, Med Phys (submitted)
4-channel soft coil
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MR Sim QA at UM
Commissioning
– Distortion across imaging field
– Orientation labeling
Daily
– Uniformity tests
– Rotate through coils
– Disk space, helium level (stable)
Weekly
– ACR phantom imaged (analyzed monthly)
Frequency being determined
– LAP laser phantom
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Geometric accuracy –
phantom measurement
Custom-designed large volume geometric distortion phantom (IMT and UM)
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Characterization of system-level distortion
r <17 cm
Shift < 1 mm
1mm < shift < 1.5 mm
shift <1 mm
Cao 30
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Narrow vs Wide bandwidth
Narrow bandwidth Wide bandwidth
𝑥 = 𝑥′ + ∆𝑥∆𝐵0𝐵𝑊𝑓
B0
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Sources of Geometric Distortion
System-level
– B0 field inhomogeneity
– Gradient non-linearity
Patient-induced
– B0 field inhomogeneity
– Chemical shift effect
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Image taken on a Panorama scanner
Slide courtesy of Henry Ford Hospital Cao 33
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PETRA with 3D Gradient non-linearity
correction
Cao 34
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PETRA without Gradient non-linearity
correction
Cao 35
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Laser QA
Cao 36
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Laser phantom images
axial coronal saggital
Cao 37 Error < 2 mm
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ACR QA phantom
Cao 38
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ACR phantom tests
1. Geometry accuracy
2. High-contrast spatial resolution
3. Slice thickness accuracy
4. Slice position accuracy
5. Image intensity uniformity
6. Percent-signal ghosting
7. Low-contrast object detectability
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Distortion from magnetic
susceptibility differences
Patient-specific magnetic susceptibility can be mapped, and the related distortion can then be calculated and corrected for imaging sequences used
FE
-2mm
1mm
Hesheng Wang, University of Michigan
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Patient-Specific Distortion
Correction and QA
k-space data
(FFT)
Model-based Reconstruction
Algorithm
Complex multi-echo
GRE Image volume Undistorted
field map
Input Processing Output
Corrected
Images QA for
clinical image
Correction Algorithm
Antonis Matakos, UM
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MRI Safety
Form a MRI safety committee
A MD as director, level-2 safety officers (physicists, MRI sim therapists…)
Establish an education Program
All staff in the department has to pass education training annually
Certify level-2 safety officers
Control the access to the MRI suite
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MRI protocol development
and workflow for RT Build a multi-disciplinary team per body site
– Radiation oncologist, MRI/clincial physicist, dosimetrist, sim therapist, (radiologist)
Understand intended use of MRI – Target and OAR delineation
– Tumor invasion, tumor growth since diagnosis
– Tx guidance
– Dose planning, Dose coverage verification
– Image registration, motion management
Develop an imaging protocol and re-visit after a period of time (feedback)
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Dedicated Protocol Development
Optimize the number of sequences to meet multiple needs
Shorten the scan time
Lock shimming after individual patient shimming
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MRI-based HDR using
cylindrical applicators
T1-weighted image (applicator) T2-weighted image (anatomy)
Slide courtesy of Joann Prisciandaro Cao 45
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GYN HDR protocol
Cao 46
Scan time < 6min
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HN protocol
Cao 47
10 min
8 min
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MRI alone simulation: Synthetic CT
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Whole brain setup verification – MV
portal image aligned to MRCT-derived
DRR
MRCT DRR
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Brain Sim MRCT Protocol
Cao 50
7 min
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Example images used for tissue
classification Dixon in-phase (T1W)
T2-weighted
Dixon fat Dixon water
UTE (PETRA) Time of flight
Used for air mask Used for vessel mask
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Summary
RT simulation has different needs than diagnosis radiology for MRI
– More concerns about small distortions, intensity uniformity
– More interest in high spatial resolution
– Large bore size and large FOV
– Short scanning time
– Anatomical extent
– Less critical for initial sensitivity/specificity
Describe these needs in the RFP
and build the workflow in your
program
Cao 52