When measured in the carcass, 12-13th rib fat thickness and ribeye area are used to calculate U.S.D.A. Yield Grade.  It is known that these two measurements, most importantly fat thickness, account for an important amount of the variation in carcass cutability.  Fat thickness and ribeye area are moderately to highly heritable, suggesting that differences found between animals would be expected to be passed on to their offspring.
 
 

Real-time Ultrasound Image
Collected for Measurement of
Ribeye Area and 12-13th Back Thickness

Research studies at Iowa Sate University have found a relatively high correlation (75%) between ultrasound-predicted percent fat in the live animal and actual percent fat in the carcass ribeye.  Thus, collecting ultrasound data is a much cheaper and more efficient means of evaluating carcass traits on breeding animals relative to progeny testing