trevor thang department of medical biophysics, western university quality assurance of mri for...
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Trevor ThangDepartment of Medical Biophysics, Western University
QUALITY ASSURANCE OF MRI FOR ADAPTIVE RADIOTHERAPY:
PRELIMINARY INVESTIGATIONS
Six Week Project – MEDBIO 3970ZApril 04, 2012
Supervisor: Dr. Eugene Wong, Physics and Astronomy
Rob Bartha, Charles McKenzie, Aaron Ward, Glenn Bauman
• Dr. Cesare Romagnoli
• Dr. Blaine Chronik
• Trevor Szekere
• Eli Gibson
• Funding supported in part by
• OCAIRO, ORF
• CIHR-STP
Quality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations
ACKNOWLEDGEMENTS
MOTIVATION
• CANCER – Endogenous Disease
• Radiotherapy – High Energy• X-Rays, Gamma Rays, etc.
• Result: • Multiple irradiations over long periods of time
• New Variables
Quality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations
MOTIVATION
• Adaptive Radiotherapy• Most common: Adapt patient to Treatment Plan
• Geometric Information from linear accelerator on-board CT imaging
• Ultimate Goal: Adapt radiotherapy based on treatment response
Quality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations
MOTIVATION
• To access the Tumor’s Response to Therapy:• Employ functional imaging
• Compare baseline and mid-treatment images
• Adapt treatment based on changes in the images
• But …
Quality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations
MOTIVATION
Are the Changes in the Images:
Biological?
or
Inconsistent Imaging?
Quality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations
OBJECTIVES
• Develop Patient-Specific Quality Assurance Metrics
• For MR Spectroscopy
Since MRS data is labour intensive to generate and analyze …
Can Volumetric Distortions in DW-EP Images be used as surrogate for MRS quality assessments?
BACKGROUNDQuality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations
MR SPECTROSCOPYMETHODS
• MRS analyzed using fitMAN developed by Bartha’s group• For 11 patients,
1. Load Patient Raw Data2. Exponential Filter in frequency domain to reduce noise3. Remove the Residual Water Signal4. Lipid Signal Rephased
Quality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations
MAGNETIC RESONANCE SPECTROSCOPY
• Different peaks correspond to protons in different chemical environment
• Inhomogeneous Magnetic Fields produce Large FWHM
BACKGROUNDQuality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations
• Spatial Information Encoded with the Use of Magnetic Gradients• Gx – Frequency Encoding, Gy – Phase Encoding, Gz – Slice Encoding
3
• T2w Images Unsusceptible
• 180° Refocusing Pulse
• Gold Standard
VOLUMETRIC DISTORTION IN DW-EPI
BACKGROUNDQuality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations
Inhomogeneous Field↓
Improper Spatial Encoding↓
Deformation
3 MR Pulse Sequences: What Every Radiologist Wants to Know but Is Afraid to Ask. Bitar
METHOD
• Patient Data
• From London’s CIHR funded multi-parametric MR of prostate cancer patients imaged prior to prostatectomy• Identified 11 patients with single voxel MRS data
• Employed deformed volume in Diffusion weighted Echo Planar Imaging (DW-EPI) as a gauge of B0 uniformity
• Correlate deformed Volume with FWHM MRS data.
Quality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations
PROSTATE SEGMENTATION ON PINNACLE
AX T2w – FSE• Unsusceptible due to
180° Refocusing Pulse
DW-EPI• Susceptible due to lack of
180° Refocusing Pulse
METHODSQuality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations
Patient A Patient B
VOLUMETRIC PERCENT DIFFERENCE EXAMPLESMETHODS
21% 6.8% 4.1% 2.5%
Quality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations
RESULTS
Quality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations
Residual Water as a Surrogate for Main Magnetic Field Uniformity
RESULTS
FWHM (Hz) = 1.2 * %Diff + 4.4 R² = 0.82
N = 11
Quality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations
0 2 4 6 8 10 12 14 16 18 200
5
10
15
20
25
FWHM (Hz)
Volu
met
ric P
erce
nt C
hang
e (%
)
Lipid as a Surrogate for Main Magnetic Field Uniformity
RESULTS
FWHM (Hz) = 1.9 * %Diff + 20 R² = 0.63
N = 11
Quality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations
10 20 30 40 50 60 700
5
10
15
20
25
FWHM (Hz)
Volu
met
ric P
erce
nt C
hang
e (%
)
Lipid as a Surrogate for Main Magnetic Field Uniformity
RESULTS
FWHM (Hz) = 1.5 * %Diff + 24 R² = 0.87
N = 10
Quality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations
10 20 30 40 50 60 700
5
10
15
20
25
FWHM (Hz)
Volu
met
ric P
erce
nt C
hang
e (%
)
VOLUMETRIC DEFORMATION THRESHOLD
• Acceptable FWHM Lipid MRS ≤ 35 Hz at 3T
• Issues:• Volumetric contours have uncertainties.• Other Metrics to Measure Distortion
• Surface Area-to-Volume Ratio
• Volumetric Overlap
RESULTSQuality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations
NEXT STEPS
• ROC Analysis with more patient data
• Validate Volume Threshold Values
• Determine Other Direct Methods detecting Inhomogenous B 0
• Phase Difference Method 4
• Bandwidth-Difference (∆BW)5
Downfall: These all come with the cost of increased patient scanning time.
Quality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations
4 Magnetic Resonance Imaging Quality Control Manual (American College of Radiology), Weinreb, 19915 Routine Testing of Magnetic Field Homogeneity on clinical MRI Systems. Hua-Hsuan Chen, 2006
SUMMARY
• Employed deformation in DW-EPI as surrogates for B0 homogeneity, correlating this to the FWHM of MRS
• Potential Indicator of Quality of MRS data :
• DW-EPI Volumetric Percent Change ≤ 7%
• If this threshold is exceeded, the data should not be used.
• In the present 11 patient cohort, 4 were deemed unacceptable
• Analyze the citrate and choline peaks for the remaining 7 patients.
• Will Validate correlation with double the patient data.
Quality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations