H., I have to admit that I really would find another doctor. You can't get a good MRI without being very still, which is next to impossible for a child. Years ago my niece had a pediatrician who refused to have her sedated for a catheter in her urethra, and that was such a nightmare. My S-I-L found a different doctor to help treat the underlying condition, and that doctor was appalled that the first ped would subject a young child to that. You don't have to put up with that, you know. An MRI machine is scary to adults, much less children, with the loud knocking noises and even the fear of being closed in for an "undetermined" amount of time.
Before accepting the diagnosis of constipation, read up on the kind of foods she should be eating so that you treat constipation naturally, and pay attention to her stools.
What Tara Lynne is sending you on the link is scary, but you need to be informed if that could possibly be what's wrong. Best to get a second opinion.
Best of luck to you and your DD.