This morning I got a question in e-mail, asking if I'd heard of a particular paper. Of course I had, it's a very fun bit of research...and then I realized I'd never mentioned it on the weblog before. I guess it's because it's focused entirely on the phylum Chordata, specifically one rather peculiar species—Homo sapiens. I probably just assumed nobody would be interested, because there aren't any arthropods or molluscs in it.

The paper is all about visualizing the arrangement of organs during coitus. People have tried to figure out how the pieces all fit together internally using cadavers and their imagination, by using a speculum and poking around with their fingers, and by clever tools, like hollow glass tubes shaped like a penis. This paper tries something different: the investigators had people have sex in an MRI tube, and snapped a few pictures while they were at it.

This wasn't easy. If any of you have seen an MRI tube, they tend to be small and cramped, difficult for claustrophobics to handle, and a tight enough squeeze for one person, let alone two. The methods section is the most interesting I've ever read.

The participants (pairs of men and women) were recruited by personal invitation and through a local scientific television programme. Respondents were invited to participate if they met the following criteria: older than 18 years, intact uterus and ovaries, and a small to average weight/height index. The experimental procedure was explained in a letter sent to respondents along with an informed consent form. Participants were assured confidentiality, privacy, anonymity, and the possibility of withdrawing from the study at any time. After written informed consent had been obtained, the participants were invited to come for a scan when the equipment was available on a Saturday.

The tube in which the couple would have intercourse stood in a room next to a control room where the searchers were sitting behind the scanning console and screen. An improvised curtain covered the window between the two rooms, so the intercom was the only means of communication. Imaging was first done in a 1.5 Tesla Philips magnet system (Gyroscan S15) and later in a 1.5 Tesla magnet system from Siemens Vision. To increase the space in the tube, the table was removed: the internal diameter of the tube is then 50 cm. The participants were asked to lie with pelvises near the marked centre of the tube and not to move during imaging. After a preview, 10 mm thick sagittal images were taken with a half-Fourier acquisition single shot turbo SE T2 weighted pulse sequence (HASTE). The echo time was 64 ms, with a repetition time of 4.4 ms. With this fast acquisition technique, 11 slices of relatively good quality were obtained within 14 seconds.

The volunteers were shown the equipment in the two rooms, and personal and gynaecological histories were taken. The experimental procedure was explained, and all investigators left the imaging room. After a preliminary image for positioning the true pelvis of the woman was taken, the first image was taken with her lying on her back (image 1). Then the male was asked to climb into the tube and begin face to face coitus in the superior position (image 2). After this shot—successful or not—the man was asked to leave the tube and the woman was asked to stimulate her clitoris manually and to inform the researchers by intercom when she had reached the preorgasmic stage. Then she stopped the autostimulation for a third image (image 3). After that image was taken the woman restarted the stimulation to achieve an orgasm. Twenty minutes after the orgasm, the fourth image was taken (image 4). At the end of the experiment, the images were evaluated in the presence of the participants.

Science porn! Maybe the stuff about 1.5 Tesla magnets and T2 weighted imaging gets in the way of the romance, but it lets us cut straight to the action. Unfortunately, it sounds like action was hard to come by—while the ladies were willing, the guys seemed to have, umm, flopped. The researchers say the experiment was only possible thanks to the availability of a drug called sildenafil (aka Viagra), with one exception.

We did not foresee that the men would have more problems with sexual performance (maintaining their erection) than the women in the scanner. All the women had a complete sexual response, but they described their orgasm as superficial. Only the first couple was able to perform coitus adequately without sildenafil (experiments 1 and 2). The reason might be that they were the only participants in the real sense: involved in the research right from the beginning because of their scientific curiosity, knowledge of the body, and artistic commitment. And as amateur street acrobats they are trained and used to performing under stress.

Dang. My experimental animals are fish, Danio rerio. I wonder if I could switch my lab over to studying small, curious street acrobats?

Anyway, here is a sample of the results. We're looking at the pelvic region of the two people rotated into an upright position; the woman is on the left, the man on the right. You'll just have to stare at it a bit and let your imagination go to work, and you'll figure out what's where.

Midsagittal image of the anatomy of sexual intercourse. P=penis, Ur=urethra, Pe=perineum, U=uterus, S=symphysis, B=bladder, I=intestine, L5=lumbar 5, Sc=scrotum

I have to admit, though, that although it sounds like an experiment that was a great deal of fun, they didn't learn anything too surprising. They notice that the penis is more contorted than expected, although I had to wonder if that wasn't just a consequence of the difficulty of having sex in a narrow tube. Intromission didn't seem to do much to the female anatomy (no surprise, again), but one interesting observation was that the female reproductive tract does jigger itself around in curious ways during arousal and orgasm. Here are photos of the female pelvis at rest, just before and a short time after orgasm.

Midsagittal images of sexual response in a multiparous woman: (left) at rest; (centre) pre-orgasmic phase; (right) 20 minutes after orgasm

What you should be able to see is that the uterus is progressively displace upwards, stretching the anterior wall of the vagina. So you see, guys, while all looks no different from the outside, strange and subtle things are shifting around on the inside. All we need to know for sure is to take an MRI of her brain during orgasm, or of her pelvis within 20 minutes or so.

Schultz WW, van Andel P, Sabelis I, Mooyaart E (1999) Magnetic resonance imaging of male and female genitals during coitus and female sexual arousal. BMJ 319:1596-1600.