Date post: | 05-Jan-2016 |
Category: |
Documents |
Upload: | arnold-robbins |
View: | 216 times |
Download: | 2 times |
Effect of Flip Angle Error for T1 Bias Correction in MRI Fat Fraction
EstimationMarch 22, 2011
Issac YangSupervisor: Charles McKenzie, PhDDepartment of Medical Biophysics
University of Western Ontario
Background and Motivation
• Non-Alcoholic Fatty Liver Disease (NAFLD) is becoming the most common chronic liver disease– Affects 30% of the Western
population
• Fat depositing in liver cells (steatosis)
• Associated with metabolic syndromes– Obesity, type II diabetes
Normal
Fatty Liver
Background and Motivation
• NAFLD is asymptomatic• Current procedure of detection
is liver biopsy– Invasive procedure
• Complications involves minor bleeding, hospitalization, and (rarely) death
– Steatosis distribution is inhomogeneous• Prone to sampling error
• MRI offers a non-invasive alternative– Unaffected by sampling error
Obtaining Fat Fractions
• An MRI technique called IDEAL was used– Allows production of
separate fat and water signal images.
Fat + Water
Water Fat
IDEAL
Fat Fraction = +
Bias in Fat Fraction
• Signals in each pixel is characterized by the equation
• T1 values for fat is significantly different from T1 of water– Typically: T1f=382ms, T1w=809ms at 3.0 T in vivo
• Difference in T1 values cause unequal weighting of water and fat signals in fat fraction equation
Current Method
• T1 weighting in the signal increases with flip angle
• Current method of mitigating bias is to use a low flip angle.– Decreases signal intensity– Reduced Signal to Noise
Ratio (SNR)
Bias Correction
• Bias can be reduced by performing corrections to the image by using estimated T1 values.
• This method assumes known and uniform flip angle— Untrue for fields of 3.0T and above due to RF pulse
inhomogeneity— Flip angle measurements should be performed to further
reduce error
Method
Fat Image
Water Image
Flip Angle Map
Bias Correction
Corrected Fat Image
Corrected Water Image
Fat Fraction
Map
Bias Correction
T1w Estimate
T1f Estimate
Results
• Flip angle measurements show true flip angle to be 9 degrees on average– Prescribed 12 degrees– 25% lower than expected
• T1 measurements yielded:– T1w=274ms, T1f=181ms– Estimated values: T1w=350ms, T1f=200ms
Flip Angle Map
Correction Results
Discussion
• Measured fat fractions was brought closer to the true fat fraction value for phantoms
Conclusion
• Experiments suggest that including flip angle mapping into imaging protocol would decrease fat fraction bias
Acknowledgement
McKenzie Lab
Dr. Charles McKenzieDr. Trevor WadeDr. Lanette Friesen-Waldner
Curtis WiensBryan AddemanYifan CuiSamantha Flood
Bartha Lab
Dr. Robert BarthaDr. John DrozdJacob Penner
Thank You For Your Attention
Questions?