Correction of Partial Volume Effects in Arterial Spin Labeling MRI
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Correction of Partial Volume Effects in Arterial Spin Labeling MRI
By: Tracy SsaliMedical Biophysics 3970zApril 4th 2012Supervisors: Keith St. Lawrence, PhD
Udunna Anazodo, PhD candidate
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Introduction
Brain Tissue consists of Grey Matter (GM) White Matter (WM) and Cerebral Spinal Fluid (CSF)
Perfusion in the brain is indicative of function Irregular flow in grey matter is indicative of
disease state [1]
Arterial Spin Labeling is a novel technique used to measure perfusion in the brain
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Theory – Why ASL MRI?
Established techniques require radioactive exogenous tracers which cannot be used on certain patient populations, and require long clearance times.[2]
ASL MRI uses magnetized water molecules in arterial blood as a tracer to measure tissue perfusion non-invasively
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Theory - ASL MRI
Creating the control image
Creating the tagged image Arterial blood is magnetically labeled
using radiofrequency pulses A delay time is allowed for the blood to
reach the brain When the labeled water interacts with
the magnetic field, it affects the signal being produced
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ASL Image ≈ CBF
Subtract
Adapted from Wolf and Detre Neurother Vol. 4, 346–359, July 2007
Theory – How ASL MRI Works
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Theory - Partial Volume Effects Perfusion images
are taken in quick succession Information from
the labeled blood must be captured before it relaxes
Blood water has a half life of around 1-2s[3]
Pettersen Br J Radiology 2006
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Theory – Partial Volume Effects (Cont’d)
Point spread blurring
Resolution is not fine enough to resolve GM, WM and CSF Voxel size is
approximately 3 x 3 x 3mm [2]
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Theory – Separating the Signals
Partial Volume Effects (PVE) correction Estimates the partial signal contribution
based on the contrast information from anatomical MRI image volume
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Theory – Partial Volume Effects Correction
Kernel Regression Algorithm Based on the size of the kernel the
algorithm assesses a radius around the centre point to reassign a partial volume
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Theory – Signal to Noise Ratio (SNR)
Signal – Mean signal of GM, WM or CSF
Noise – Standard deviation
20
40
Deibler et al AJNR march 2008
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Objective
To measure the signal to noise ratio before and after the partial volume effects correction
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Methods
5 Chronic Regional Pain Syndrome Patients (CRPS)
Image Preprocessing Remove the pixels
representing the skull
Motion correction
Wolf and Detre Neurother Vol. 4, 346–359, July 2007
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Methods (Cont’d)
PVE correction Implemented an In-house written
MATLAB code created by Asllani et al. [2] Images from 5CRPS were processed
using a kernel size of 5 and 9 Kernel filter
Adjust the kernel size from to 5, 7, 9, 11 and 15
1 patient’s data
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K = 5 Results & Discussion
Significant decrease (P<0.05) in the SNR
Variation in the voxels due to the noise could have prevented the code from working as intended
Subject 1
Subject 2
Subject 3
Subject 4
Subject 5
0
0.5
1
1.5
2
SNR Before and After PVE correction
Uncorrected SNR PVE corrected SNR
CRPS Patient
Sig
nal to
Nois
e R
ati
o
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K = 9 Results & Discussion
Inconsistent Results
There is no significant difference (P>0.05)
It is likely that the algorithm is not functioning as intended
Subject 1
Subject 2
Subject 3
Subject 4
Subject 5
00.5
11.5
22.5
3
SNR Before and After PVE correction
Uncorrected SNR PV corrected SNR
CRPS Patient
Sig
nal to
Nois
e R
ati
o
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Subject 5 Results & Discussion
5 7 9 11 150
0.5
1
1.5
2
2.5
3
3.5
Kernel Size vs SNR
Uncorrected Signal Corrected Signal
Kernel Size
Sig
nal to
Nois
e R
ati
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Larger kernel have a greater SNR
Small kernel sizes are sensitive to noise and variance
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Conclusion
The SNR decreased after the PVE correction
Kernel size needs to be chosen carefully
Our in-house implementation of the PVE correction needs to be fine tuned
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References
[1] Tracy, Melzer R. "Arterial Spin Labelling Reveals an Abnormal Cerebral Perfusion Pattern in Parkinson’s Disease." Brain 134.3 (2011): 845-55.
[2] Asllani, Iris, Ajna Borogovac, and Truman R. Brown. "Regression Algorithm Correcting for Partial Volume Effects in Arterial Spin Labeling MRI." Magnetic Resonance in Medicine 60.6 (2008): 1362-371.
[3] Xu, Guofan, Howard A. Rowley, Gaohong Wu, David C. Alsop, Ajit Shankaranarayanan, Maritza Dowling, Bradley T. Christian, Terrence R. Oakes, and Sterling C. Johnson. "Reliability and Precision of Pseudo-continuous Arterial Spin Labeling Perfusion MRI on 3.0 T and Comparison with 15O-water PET in Elderly Subjects at Risk for Alzheimer’s Disease." NMR in Biomedicine 23.3 (2010): 286-93. Print.
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Thank youQuestions?