T to Z convertor for higher T value and degrees of freedom

Hi Guys,

I have 3 T score maps generated during regression analysis using SPM8, the three T score maps have different degrees of freedom (Df 361,623 and 821). The typical T score value is 20. Since the T distribution is dependent on Df, I would like to generate a score from this T score which is not dependent on Df. So I am looking for a script or algorithm to convert T score to Z score (which is Df independent). Please can anyone suggest a method to pursue this problem.

Regards