What does it mean to get a Prescription for Radiology?

I had my period April 3 and i just took a pregnancy test yesterday which said Im positive. I went to the clinic today and took another pregnancy test. And the nurse say it doesnt make sense that im positive to be pregnant so early now. She said i would be aleast 3 weeks pregnant now but i should go for a sonogram to make shure there is a baby inside. =P but what does it mean to get a prescription for radiology and HOW do i the prescription? Cause i tried to make an appointment for a sonogram but the staff suggested me to get a prescription so I dont have to pay so much. I dont understand these things... =[