How is it determined if a3 mm echogenic focus without posterior shadowing in mid-portion is a kidney stone?

May not be stone: You likely have had a renal ultrasound. A kidney stone usually does create shadowing because it does not allow transmission of sound waves. A "twinkle" artifact is also associated with a stone. Other structures can be echogenic, including calcium deposits in blood vessels, fat in the renal sinus, and fat containing benign masses. CT scan without contrast is most sensitive to detect stones.