There are multiple definitions:

1. percentile tracking filter(English)

   • FIR
   • Medical imaging, Signal analysis
   • https://staff.f…tilefilter.html

2. perfusable tissue fraction(English)

   • CFR, PTI, PET, ATF
   • Coronary circulation
   • http://www.turk…_h2o_heart.html

3. phase transfer function(English)

   • the change in phase position as a function of spatial frequency
   • CTF, MTF, OTF
   • Microscopy, Optics and vision
   • https://www.oly…atomy/mtfintro/

4. preterm infant formula(English)

   • FHM, MOM, PDHM
   • Nutrition, Neonatal situation
   • https://doi.org…058.2014.958460