QUALITY ASSURANCE OF MRI FOR
ADAPTIVE RADIOTHERAPY:
PRELIMINARY INVESTIGATIONS
Six Week Project – MEDBIO 3970Z
April 04, 2012
Trevor Thang
Department of Medical Biophysics, Western University
Supervisor: Dr. Eugene Wong, Physics and Astronomy
Rob Bartha, Charles McKenzie, Aaron Ward, Glenn Bauman
Quality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations
ACKNOWLEDGEMENTS
• 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
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
BACKGROUND
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?
Quality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations
METHODS
MR SPECTROSCOPY
• MRS analyzed using fitMAN developed by Bartha’s group
• For 11 patients,
1.
2.
3.
4.
Load Patient Raw Data
Exponential Filter in frequency domain to reduce noise
Remove the Residual Water Signal
Lipid Signal Rephased
Quality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations
BACKGROUND
MAGNETIC RESONANCE SPECTROSCOPY
• Different peaks correspond to protons in different
chemical environment
• Inhomogeneous Magnetic Fields produce Large FWHM
Quality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations
BACKGROUND
VOLUMETRIC DISTORTION IN DW-EPI
• Spatial Information Encoded with the Use of Magnetic Gradients
• Gx – Frequency Encoding, G y – Phase Encoding, G z – Slice Encoding
Inhomogeneous Field
↓
Improper Spatial Encoding
↓
Deformation 3
• T2w Images Unsusceptible
• 180° Refocusing Pulse
• Gold Standard
3
MR Pulse Sequences: What Every Radiologist Wants to Know but Is Afraid to Ask. Bitar
Quality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations
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 B 0 uniformity
• Correlate deformed Volume with FWHM MRS data.
Quality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations
METHODS
PROSTATE SEGMENTATION ON PINNACLE
AX T2w – FSE
• Unsusceptible due to
180° Refocusing Pulse
DW-EPI
• Susceptible due to lack of
180° Refocusing Pulse
Patient A
Patient B
Quality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations
METHODS
VOLUMETRIC PERCENT DIFFERENCE EXAMPLES
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
RESULTS
Residual Water as a Surrogate for Main Magnetic Field Uniformity
Volumetric Percent Change (%)
25
N = 11
20
15
10
5
0
0
5
10
FWHM (Hz)
FWHM (Hz) = 1.2 * %Diff + 4.4
15
20
R² = 0.82
Quality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations
RESULTS
Lipid as a Surrogate for Main Magnetic Field Uniformity
Volumetric Percent Change (%)
25
N = 11
20
15
10
5
0
10
20
30
40
FWHM (Hz)
FWHM (Hz) = 1.9 * %Diff + 20
50
60
R² = 0.63
70
Quality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations
RESULTS
Lipid as a Surrogate for Main Magnetic Field Uniformity
Volumetric Percent Change (%)
25
N = 10
20
15
10
5
0
10
20
30
40
50
60
70
FWHM (Hz)
FWHM (Hz) = 1.5 * %Diff + 24
R² = 0.87
Quality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations
RESULTS
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
Quality 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 B0
• Phase Difference Method ∆𝜑
4
• Bandwidth-Difference (∆BW) 5
Downfall: These all come with the cost of increased patient
scanning time.
Magnetic Resonance Imaging Quality Control Manual (American College of Radiology), Weinreb, 1991
5 Routine Testing of Magnetic Field Homogeneity on clinical MRI Systems. Hua-Hsuan Chen, 2006
4
Quality Assurance of MRI for Adaptive Radiotherapy: Preliminary Investigations
SUMMARY
• Employed deformation in DW-EPI as surrogates for B 0
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.