I found this on another medical board. It's pretty good. It was written by a guy named Kaz. I copy it here for your enjoyment:
"I was at the medical clinic to have an ultrasound examination of my kidneys. A young [no more than 20] and very attractive girl had led me to a small booth and asked me to wait until the examination room was ready. A short time later she led me to the room, which was a dimly lit room with an examination table beside the ultrasound equipment. She told me to loosen my pants and lay on the table and then she left the room.
She returned and asked me to lower my pants to expose my lower belly. I was very surprised [pleasantly] because I had thought she was an assistant and not the technician. I had been there a few months earlier for a hernia ultrasound scan and the technician had been a male who I guess I was expecting would be again.
She spread lubrication gel onto my belly and started to scan the area while making adjustments to the equipment. I was able to see the images on the monitor and asked her what they were. She described what we were seeing and we developed a conversation that was relaxed and comfortable. I asked her if she would scan the hernia operation area because I would like to see the repair patch that I was told had been inserted. She said she would.
When she was finished, she asked me to pull my pants down a bit further to expose the area where the hernia operation was. I don't wear underwear and I noticed a warm smile come to her lips as she watched me struggle to pull my pants down further. She spread gel over the area and identified various details of the images as she explored the region with the scanner. It was fascinating and I don't know why, but suddenly I asked her if she had ever scanned a testicle. I guess it was the comfortable and intimate atmosphere we were in. She said yes and would I like to see that too. Of course I said yes and I proceeded to pull my pants down further to expose my equipment. It was soft and neither of us seemed to be embarrassed.
She looked at me and a warm smile was on her face and in her eyes as she tenderly grasped my right testicle and began spreading gel all over. She started scanning and we both watched the image silently. She then began scanning below and behind my sac as she very gently held my balls up with her left hand. I noticed a smile come to her lips as she sensed my penis grow to a full erection and then she turned and spread gel on it.
She looked at me in the eyes and without a word began massaging my very erect cock with both hands. I don't know where the scanner went and didn't care. She held the shaft in her right hand and very slowly and smoothly massaged the head with her left. Her left hand fingers traced the rim and rubbed the area where it all comes together under the opening at the end, with just the right pressure. She knew what she was doing. She continued to massage with both her hands and fingers while silently we both looked into each others eyes. An image that will forever be with me! She was not in any hurry and in fact delayed me from cumming by applying firm pressure around the rim of my cockhead with her finger and thumb when she sensed that I was close. Not many girls know this trick from my experience and I was ecstatic.
Alas, all good things must come to an end and what an ending. She stroked, massaged and manipulated me to the most entense orgasm of my life. I don't think females really know what an orgasm feels like to a man but I'll try to describe this one. My body was already warm, sensitive and dancing with delightful tingles. Somewhere deep in my abdomen, midway between my anus and belly button, a warm, fluid sensation spread in all directions as it intensified. Her hands and fingers seemed to be everywhere and moving with a steady rhythm. The expression on her face was calm, smiling and aware of what was about to happen. She continued and every sense and feeling within my body moved and converged to the base of my penis. At the same time, the lower portion of the head screamed with concentrated, intense pleasure that seemed to be ignited by an electrical pulse that had traveled along and within the entire shaft. And then came an involuntary explosion in my body that raced outward and through the entire length of my cock. The sperm flew and most of it landed and trailed down from my neck. The rest had cleared my head with several drops on my face. She continued to stroke and I continued to explode in convulsions for what seemed like 5 minutes. Who knows. I have not ejaculated like that since I was a teenager."