Wangensteen and Rice  first described the use of inversion radiography in 1930 to indicate the distance between the gas bubble within the terminal colon and the perineal skin. Many authors [6, 8, 11, 13, 19, 22, 29, 39, 48] then subsequently related the distance from the skin to the underlying abnormality, although the measurement of the distance alone does not provide the essential information required, which is the relationship of the blind-ending rectum to the levator ani and sphincter muscle complex.
An upside-down inversion x-ray is no longer performed, having been replaced by a prone, cross-table lateral examination of the pelvis, with the hips elevated over a bolster . This examination allows for easier positioning of the patient and better delineation of the anatomy. Use of bony landmarks on the pelvic x-ray allows the clinician to relate the gas bubble to the origin of the levator ani and the apex of the levator sphincter muscle complex near the lower point of the ischium.
The pubococcygeal line (PC line) and the ischial (I) point were determined by Stephens [40-43] from dissection of 25 stillborn pelves in documenting the attachment of the levator ani to the pelvic wall both macroscopically and radiologically, with opaque wires marking the attachment of the levator ani. Using these techniques the PC line stretches from the upper border of the symphysis pubis to the sacrococcygeal junction. At the symphyseal end this line is taken as the center of the "boomerang" shape of the os pubis, which corresponds with the upper border of the symphysis. The top of the "boomerang" corresponds with the superior pubic ramus. Ossification of the pubis begins in the second fetal month and extends along the superior pubic ramus medially to the body.
At the coccygeal end, the C point is just caudal to the last or fifth ossific center of the sacrum. The ossification center of this fifth piece normally appears at the 4th month of fetal development. The coccyx, by contrast, does not ossify until 2-5 years after birth. If the caudal segments of the sacrum are deficient, the PC line can be developed by projection from the pubis through the same site on the ischium, which is approximately the junction between the upper quarter and the lower three-quarters. The soft tissues lying at the level of the PC line are the bladder neck, the veru-montanum, the pelvic reflection from the rectum to the prostate, and the external os of the cervix.
In assessing the gas bubble in an ARM, its relation to the PC line defines the essential factor of whether the blind pouch is above or below the attachment of levator ani to the pelvic wall (Fig. 9.3). The ischial line (I line) and I point are related to the ossification center of the ischium, which is a comma shape in the neonate. The I point is demarcated on the x-rays at the inferior end of the ischial comma. The I line is drawn through the I point parallel to the PC line and corresponds to the upper surface of the bulb of the urethra in the male and the upper limit of perineal body and the level of the triangular ligament in the female. The anal pit is normally 1-2 cm caudal to the ossified ischium.
Kelly  has demonstrated the extent of the attachments of the levator ani in dissections of neonatal pelvis. Using wires from the specimens and then taking x-rays, he was able to identify the attachments of the levator ani and the apex of the funnel-shaped muscle complex in the normal neonate. Kelly also dissected the pelves in neonates with rectal and anal anomalies and found that the origin of the levator muscles was constant and followed closely the PC line in both normal patients and those with malformations of the rectum. The triangle bounded by the PC line and the I point denotes the radiographic markings of the levator complex in babies with a significant ARM, with the bowel terminating above the sphincter muscle complex. In Kelly's study group, in those with rectoprostatic urethral fistulae, the wire marking the
Fig. 9.3 Prone, cross-table lateral radiograph of a pelvis demonstrating the pubococcygeal (PC) line between the pubic symphysis and coccyx, and the ischial (I) line running parallel to the PC line at the inferior aspect of the ischium. In the this example the terminal bowel gas extends to the I point in a child with a rectoprostatic urethral fistula
Fig. 9.3 Prone, cross-table lateral radiograph of a pelvis demonstrating the pubococcygeal (PC) line between the pubic symphysis and coccyx, and the ischial (I) line running parallel to the PC line at the inferior aspect of the ischium. In the this example the terminal bowel gas extends to the I point in a child with a rectoprostatic urethral fistula levator ani demonstrates the extent of the puborecta-lis and the bowel opening into the back of the urethra well above the I point. The fistula is at or very close to the PC line. In perineal anomalies the gas bubble on radiology extends well below the I point to within a very short distance from the skin.
A true lateral view of the pelvis with accurate centering on the greater trochanter is essential. It ensures that the ossification centers of the pubic bones are superimposed and readily recognized and that the two ischial bones are accurately superimposed, appearing as one, with the anal dimple and natal cleft outlined with barium or a radiopaque marker correctly aligned. In these circumstances the PC line can be drawn accurately and the visceral anatomy interpreted. The common error, centering on the middle of abdomen, results in an angled projection of the right and left is-chial bones. This causes distortion of the picture and difficult interpretation. The hips should be slightly extended so that the femoral shadows are clear of the pubic ossification centers.
The prone, cross-table lateral x-ray should be delayed for 12-24 h after birth to allow gas to reach the distal rectum. The baby should be placed in the prone position for 3 min before taking the film to allow gas to displace meconium and rise to the termination of the pouch. Barium paste or contrast-soaked gauze placed in the natal cleft is more accurate than a metal marker. A catheter may be placed in the urethra to make delineation of the urethra more obvious, although this is not essential. The greater trochanter should be marked with a marker pen on the upper thigh and the x-ray beam centered on this spot.
Common causes for erroneous interpretation of prone, cross-table lateral x-ray include:
1. Insufficient time for gas to reach the terminal bowel.
2. Meconium plug in the terminal gut may produce an erroneously high shadow if the gas does not displace the meconium.
3. Active contraction of the levator ani/sphincter muscle complex can push the gas shadow higher.
4. Gas escape through a fistula may confuse the x-ray, but clinical diagnosis should then be obvious.
5. Distortion by x-ray magnification resulting in the appearance of a longer gap between the gas within the terminal colon and skin.
6. Inappropriate placement of an anal marker may cause an error of assessment of the exact site of the anus on the skin.
7. Erroneous estimation of level of the lesion inside the sphincter muscle complex may occur if the pelvic floor muscles are relaxed, or if there is a sacral anomaly.
8. Finally, gas in the vagina may be mistaken for gas in the distal bowel.
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Once your pregnancy is over and done with, your baby is happily in your arms, and youre headed back home from the hospital, youll begin to realize that things have only just begun. Over the next few days, weeks, and months, youre going to increasingly notice that your entire life has changed in more ways than you could ever imagine.