R2Star

Algorithms

The signal obtained from T2* decay is described by one of the following relations:

where:

S = Signal Intensity

S0 = Constant

TE = Echo Time

T2* = Transverse Relaxation Rate

where:

C = Constant offset

Taking the logarithm of the first equation on both sides converts it into a linear equation:

which is of the standard form:

y = mx + c

where:

m => T₂*^-1= R₂*

c => ln(A)

The R2Star protocol provides two curve fitting options: 2-parameter and 3-parameter.

The 2-parameter algorithm is similar to T2Map in that it performs a least-squares linear fit of the log-transformed signal intensity data plotted against TE.

Success of the fitting algorithm is governed by comparison of the resultant chi-squared value with a reference threshold derived from the user-specified confidence level.

The 3-parameter algorithm is a non-linear least-squares approach that works directly on the original input signal. The fit is initialized using a trial 2-parameter run with an initial setting of C=0, after which the non-linear fit is performed.

Constraints:

The R2Star algorithm is designed to work best with 8 or more echoes. Acquisition with less than 4 echoes cannot be processed.

R2Star measurement units

The R2Star functional maps have the following units of measurement.

READY View protocols that use R2Star scan data

• R2*
• MR Liver