Best way to detect ball speed

Hi all,

I'm designing an electronic board (based on iMX ARM processor) to detect and show the speed of a baseball ball in competitions or bullpen sessions. 

I'm wondering which module is suitable to achieve the request, considering that the speed range should be 50-100 mph, the distance between plate and mound is 18m and that in front of the sensor there will also probably be one or two people (with lower/no speed, of course). 

Available ports are USB, SPI, UART.

Thanks a lot, 

regards