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?