This reminds me that many years ago but after I got a J Pouch I was sent for an abdominal ultrasound examination. I specifically told the radiology technician "tell the radiologist I have a J pouch." Either that message was not communicated or not understood. A few days later I got the report and it was talking about what sounded like a very large tumor in my abdomen (with measurements) and I became scared and sent it to my gastroenterologist. He called me up and was laughing. I asked him why he was laughing and he said, "the radiologist measured your J Pouch and called it a tumor." He thought it was unbelievable and asked me what the communication was with the radiologist.
It's a good question about what the TSA scanning machines show but I have been through them many times with no problems or questions about metal or pouches.
I did one time get hassled about bringing bottled water in a carry on knapsack. I told the TSA agent I did so for medical reasons because on longer flights I feared dehydration and DVTs (which can happen with dehydration on longer flights) . Initially he hassled me but then when he saw my medications bag which was a one gallon ziplock bag filled with prescriptions, he said, "with all those Meds I am tending to believe you do have a medical condition and I am going to let you bring them on board."