Editor-Between submission and publication of the article "Magnetic resonance imaging of male and female genitals during coitus and female sexual arousal" some minor mistakes have come to light that require correction.1

The legend under figure 4 should read: Midsagittal images of the sexual response in a nulliparous woman (experiment 11): (left) at rest; (centre) pre-orgasmic phase; (right) 20 minutes after orgasm.

The reference to figure 4 under ‘Anatomy reveals', line 6, is incorrect, because this figure was not included in the article.

In the table of experiments, the year of the first experiment should be 1992 instead of 1991. There errors do not have any consequences on the results of the study.

Various reactions to the study have led us to the conclusion that certain aspects require further clarification.

Stress during the experiment. Our experiments were not invasive. No instruments were used or whatever. The volunteers (couples/women) were alone. There was no visual contact. They were asked to give a verbal signal when they were ready for the image. The volunteers, especially the women, expressed that the stress was acceptable; they were all very motivated and enjoyed participating in scientific search. This explains why our volunteers agreed to participate in this study. We were told that it had been fun in the MR tube. Therefore, in our opinion, the volunteers were certainly not under more stress, or less aroused than the volunteers in the experiments performed by Masters & Johnson; however, we cannot support this with concrete evidence.2

The length of the penis in erection. The penis reaches maximum erection c.q. length, directly before ejaculation.2 Our volunteers had to lie still while the images were being taken. Maximum erection at the moment an image was taken can therefore practically be ruled out. However, we are talking in terms of a difference in length of a few millimetres, which probably would not have influenced our observations.

The shape of the penis in erection. The images showed that during ‘missionary position' coitus, the penis was not straight, but had the shape of a boomerang. This is in agreement with clinical experience. Every gynaecologist knows that vaginal palpation cannot be performed with the fingers straight. The thousands of images of the enormous penis of the classical god of fertility also provide support for the correctness of our MRI observations. In nearly all cases, the bending was identical. Perhaps in ancient times, there was less tendency towards machismo......3 We think that both Da Vinci (in about 1493) and Dickinson (1933) believed that the penis as a whole was a more or less straight structure. In order to achieve the correct proportions, Da Vinci leaned the woman over backwards (Heijmerinck, personal note, 1999). Dickinson did exactly the same, although not as far. In the upper part of his drawing, the abdominal walls deviate somewhat, which from the perspective of human experience, is unusual. Dickinson did manage to correctly envisage the more caudal position of the male pelvis and the mutual configuration of the transected pubic bones during coitus. This showes that in his time, soft tissues were more difficult to map than hard tissues.

The limited space in the MR tube. With the equipment currently available, it is not possible to establish how much influence the limited spread of the woman's legs had on our MRI observations. The next generation of ‘wide body' MRI equipment will give the volunteers more room and enable us to review the situation.

Contribution of the root of the penis to elongation of the penis in erection. The MR images show that the angle made between the root and pendulous part of the penis in erection lies outside the lower pelvis. Therefore, the root of the penis contributes to the length of the penis in erection. This supports the findings in a Dutch case study "Sexuality following amputation of penis and scrotum": " What is fascinating is that with sexual arousal a kind of erection develops. This swelling has more or less the shape of a volcano and is inconvenient at change of position. That makes us realize that usually during erection not only the deep part of the penis swells, but also contributes to the elongation of the penis! We never gave this a moment's thought".4 This obviously deserves further attention and we therefore plan to obtain MR images of a man following penis amputation.

The retroverted uterus. The woman in experiment 10 and 12 (Figure 3) has a retroverted uterus. In the majority of women, the entire uterus is anteverted, and in some it is retroverted, even when the bladder is empty. This is an anatomical variation. We observed the boomerang shape of the penis in all the experiments, irrespective of the anteverted or retroverted position of the uterus and the depth of penetration. The position of the uterus in anteversion or retroversion did influence the changes we saw during the women's sexual response without coitus: under these conditions we did not observe any elevation of the uterus or lengthening of the anterior vaginal wall in the women with a retroverted uterus (experiment 10). Our observations agree with those made by Masters & Johnson who mentioned that there was no tenting effect in women with a retroverted uterus.2 This means that the elevation of the retroverted uterus in experiment 12 (figure 3) must have been the result of penile insertion and expansion of the bladder as it filled. The elevation of an anteverted uterus and lengthening of the anterior vaginal wall that occurs during sexual arousal without penetration indicates that the uterus becomes elevated due to stretching of the vaginal wall i.c. vasocongestion. Apparently, a retroverted uterus causes such over- stretching of the anterior vaginal wall that no changes occur during sexual arousal, or if changes do occur, they no longer lead to visible lengthening or elevation of the uterus.

The tenting effect. We observed the tenting effect in all our experiments, with the exception of experiment 10 (uterus in retroversion). This confirms the observations made by Masters & Johnson, which have also been recorded on film.2,5 The surprising degree of indentation and stretching of the anterior vaginal wall during coitus in the missionary position, as described by Riley et al., agrees with our findings.6 Our women volunteers in experiments 1, 2, 10-13 mentioned penile contact with the cervix, which could be confirmed by the images. Riley et al. did not observe this phenomenon in the missionary position, they did notice it once in the female superior position.

Palpation of the uterus. Gynaecologists always make sure that the bladder is empty when performing vaginal examination, in order to feel the uterus clearly and prevent misinterpretation. The uterus is a very firm muscle with a tremendous capacity to expand (during pregnancy or because of fibroids), but that takes some time. In our experiments, no notable expansion occurred in such a short time.
Filling of the bladder cannot be the only explanation for the findings of Masters & Johnson (Levin, personal communication, 1999). The increase in uterus size they described returned to normal 10-20 minutes after orgasm, or longer if no orgasm occurred, but it might have contributed to some extent, unless they took specific measures to prevent filling, e.g. by using a catheter. They did not describe this. The only other explanation we can think of is that they palpated the raised uterus that we observed in all our experiments, except for experiment 10 (uterus in RVF). Obviously, it will be worthwhile in future experiments to focus MRI on the uterus. Perhaps some swelling does occur, but with the current overview images, this might be beyond the resolution of the equipment.

Bladder size. We did not realize that the bladder would expand so rapidly in such a relatively short time (45 to 90 minutes). With hindsight, we made the mistake of giving the volunteers a cup of coffee (a diuretic!) before the experiments to help them feel comfortable....... But they were asked to empty their bladder before starting the experiment. Obviously volunteers should not drink in the hours before the experiment. It would also be interesting to study in the MRI what happens to the position of the uterus as the bladder fills, without sexual arousal.

Future research. The main purpose of the BMJ paper was to show the scientific world that it is possible to perform sexological research in a MRI. So far, there have only been a few experiments and the images do not show details. Further research is necessary to increase our knowledge and check our findings. Extensive MRI studies are required with detailed MR images of coitus in other positions and of the penis in erection without coitus.

Willibrord Weijmar Schultz
associate professor of gynaecology

Pek Van Andel
physiologist

University Hospital Groningen, PO Box 30.001, 9700 RB Groningen, The Netherlands
w.c.m.weymar.schultz@oprit.rug.nl

1 Weijmar Schultz WCM, Van Andel P, Sabelis I, Mooyaart E. Magnetic resonance imaging of male and female genitals during coitus and female sexual arousal. BMJ 1999;319:1596-600.

2 Masters WH, Johnson VE. Human sexual response. Boston: Little, Brown, 1966.

3 Bol P. MRI-seks. NRC Handelsblad, 44, 8 January 2000.

4 Gianotten WL, Kirkerls WJ and Haensel SM. Seks na penis- en scrotumamputatie (Sexuality following amputation of penis and scrotum). Tijdschrift voor Seksuologie 1996;20:215-219.

5 Wagner G. Physiological responses of the sexually stimulated female in the laboratory. (16 mm colour film), Institute of Medical Physiology: Copenhagen, Denmark, 1974.

6 Riley AJ, Lees WR, Riley EJ. An ultrasound study of human coitus. In: Bezemer W, Cohen-Kettenis P, Slob K, Van Son-Schoones N, eds. Sex Matters. Amsterdam: Elsevier, 1992: 29-36.