MRI Diagnostic Advances Recent studies have shown that congenital anorectal malformations (ARM Anorectal Malformations) are often associated with developmental malformations of the pelvic floor muscles, sacrococcygeal and sacral nerves, and the genitourinary tract, and are early embryonic caudal dysplasia syndromes. Most of these malformations have fistulas that connect to the urinary or genital tracts. The surgical approach varies between the different types of children and the outcome varies. While surgery for low rectoanal malformations alone has excellent results, the presence of concomitant malformations such as those of the kidneys, heart, spine, and spinal cord in higher malformations is often a major cause of fetal or neonatal death and compromises postoperative outcomes. Nowadays, it is gradually recognized that the pelvic floor transverse muscle complex (SMC) plays a decisive role in the postoperative prognosis. Therefore, the ability to comprehensively and accurately grasp the pathological changes in children with anorectal malformations is the key to choosing the treatment and determining the prognosis. Imaging is currently the only means of assessing these pathological changes in ARM prior to surgery. In 1930 Wangensteen and Rice devised the inverted position radiograph to diagnose ARM, and retrograde imaging has an important confirmatory value when a urinary fistula is present. B-mode ultrasound can be used to evaluate the perianal sphincter, but the main drawback is that ultrasound has poor soft tissue resolution, is influenced by the observer’s experience, and makes it difficult to define muscle boundaries. CT scans can clearly show the anatomy of the puborectalis and external sphincter, but only in cross-section. MRI can accurately show the level of ARM atresia, the presence of fistula in multiple planes, clearly show the development of the SMC in the pelvic floor and its relationship with the distal rectum, the sacrococcygeal spine, sacral medulla, etc. and the developmental abnormalities of the genitourinary system, and clarify the postoperative relationship between the new rectum and SMC and the presence of abnormal tissues between them, etc. It is the only imaging method that can detect these abnormalities simultaneously so far. At the same time, MRI scans are extremely helpful in clarifying the developmental process and pathogenesis of ARM. This paper reviews the new advances in MRI research in these aspects in recent years. I. MRI techniques 1. MRI systems: Recent literature shows that MRI examinations of ARM have been performed using magnetic resonance systems with different field strengths, including 0.5T, 0.6T, 1.0T, and 1.5T magnetic resonance machines (MR). Generally low-field MR is inexpensive and has low operating costs, but has long acquisition times and low signal-to-noise ratio (SNR), and is mostly found in the early literature. In contrast, high-field MR reported in the recent literature provides better SNR images, easy acquisition of thin sections, high spatial resolution, and high image quality. Therefore, high field MR should be used as much as possible to examine children with ARM in institutions where it is available, and low field MR should be used to adjust the pixels, matrix and other parameters to obtain reliable images [8]. 2. Scan sequence and parameters: According to recent reports and our experience, T1-weighted images (T1WI) of Spin echo (SE) sequence, Fast spin echo (FSE) weighted images (T2WI) and Short TI inversion recovery (STIR) sequences are required to show all pathological changes in ARM. recovery (STIR) sequences. Traditionally, T1WI mainly shows the anatomical structures, while T2WI mainly reflects the pathological information. Nievelstein et al. applied FSE T2WI sequence can especially improve the detection rate of fistula and show a richer anatomical level of the anal canal, while STIR sequence can severely suppress the fat signal and make the muscle morphology more prominent, which is especially useful for children with muscle dysplasia. 3. Scanning techniques and procedures: no enema before examination, oral or rectal administration of chloral hydrate anesthesia (90~100mg/kg) for infants and children, and transrectal administration of thiopental sodium (30mg/kg) for older children. The anal fossa was marked with cod liver oil pills, a catheter was inserted to drain the bladder, and a 2-3 mm plastic tube was inserted from the fistula into the rectum to facilitate imaging. All children were examined in the supine position with the lower limbs and buttocks positioned. The scanning layer thickness was 3-5 mm, the layer spacing was 0-1.25 mm, and the scanning time was 20-30 minutes. Body phased array coil MRI (head coil for neonates and infants) can clearly show abnormalities in all aspects of anorectal malformations, and the MRI images acquired by phased array coils have a high signal-to-noise ratio, a wide field of view, and good image quality. Anal canal built-in coil MRI is also used for the study of the anal sphincter system, but because the signal intensity decays rapidly at ranges beyond 3 cm from the coil, other lesions of anorectal malformations cannot be displayed. Moreover, it causes varying degrees of patient discomfort, and the large diameter of the coil makes it unsuitable for the examination of pediatric patients and children with congenital anorectal malformations. A transverse TIWI scan (parallel to the pelvic floor) is first performed downward at the superior border of the pubic symphysis, followed by a sagittal TIWI and T2WI scan, which requires that the full length of the anal canal and ureter can be observed through the catheter, the central part of the anal canal is determined, and a coronal scan is performed with it as the center, and finally a transverse T2WI scan is performed from the superior to the inferior border of the anal canal perpendicular to the long axis of the anal canal according to the sagittal and coronal images (perpendicular to the pelvic floor). The sagittal scan should also include the lumbosacral spine and spinal cord and kidneys, and coronal and transverse scans should be performed if there are abnormalities, so that the morphology of the muscle groups in the anal region, the level of atresia and the possible presence of fistulas and associated malformations can be observed and analyzed in three dimensions. II. MRI evaluation of ARM embryonic development Currently, the embryonic developmental process of ARM is mainly studied by surgery and autopsy, while MRI can provide a good in vivo pathomorphological study of ARM. Based on MRI observations, ARM can be classified into ectopic open anus and normally located anomalous anus. Ectopic open anus, formerly known as fistula, can open in the urethra, bladder, or genital tract and is due to dysgenesis of the dorsal portion of the cloaca and cloacal membrane in early embryonic life. With dysplasia or absence of the dorsal portion of the cloacal membrane, the dorsal portion of the cloaca is absent, resulting in a more anterior opening of the hindgut. The position of the opening depends on the degree of development of the dorsal part of the cloaca; in severe dysplasia, the hindgut enters the urogenital sinus in a high position, and in mild dysplasia, the hindgut opens in a low position or in the perineum, so that the fistula combined with ARM is best described as an ectopic opening anus according to the embryologic developmental view. In malformations with high anogenital anomalies not combined with fistulas, the presence of a fibrous cord between the blind end of the rectum and the urogenital system is often found during surgery, and this fibrous cord is a sign of ectopic opening occlusion. Cloacal malformations are due to posterior and anterior cloacal membrane dysplasia, as a direct result of which the hindgut opening is more anterior and the urogenital opening is more posterior, with only the middle cloacal membrane and cloaca present, creating a common opening for the intestinal, urogenital, and genital tracts. It is now believed that the recto-anus formed in early embryos adheres to the wall of the anal canal at the anal opening 7 weeks after fertilization, and the epithelial cells form “emboli”, causing temporary occlusion of the recto-anal canal, and then the occluded anal opening is reopened through apoptosis and other means to form an anal canal. Therefore, the formation of a normally located anus is not due to a fusion disorder between the urorectal septum and the cloacal membrane, but to a defective recanalization of the occluded anal opening in the late embryonic stage. Based on these findings, the new classification of AMR by MRI imaging is: early embryonic developmental defects, which include children with abnormal anal opening with or without fistula (high malformations), and late embryonic developmental defects, which include malformations such as anal stenosis and unbroken anal membranes (low malformations). In assessing the spinal and spinal cord anomalies associated with ARM, Nievelstein suggests that the spinal and spinal cord developmental anomalies are the result of disturbances in the formation of the primitive neural tube in the early embryo, while the spinal cord embolism is the result of disturbances in the degenerative and differentiation processes in the late embryo. It is therefore not difficult to understand that clinically abnormal anal opening malformations are often combined with spinal and spinal cord anomalies, while normally located anomalous anal malformations are combined with spinal cord emboli. As for the rare anomalous anal opening deformity combined with spinal cord embolism, it may be caused by the disruption of the activation process of a specific underlying morphology. With the continuous improvement of MRI technology and the application of high signal-to-noise ratio coils, MRI can visually and clearly display the blind end of the rectum and the related muscular system, thus the degree and type of malformation can be accurately determined, which makes MRI an irreplaceable tool for diagnosing rectoanal malformation. Most cases can show the level of atresia of the malformation by T1-weighted images, while T2-weighted images are more helpful for assessing The T2-weighted images are more helpful in assessing low and intermediate malformations because the high-intensity signal of the rectoanal mucosa allows for better visualization in contrast to the sphincter complex and perineal tissue. Neonatal rectal atresia with blind end meconium is an excellent control for MRI, being high signal on T1WI (due to high mucus or lipid content) and clearly showing the level of atresia in ARM. The literature reports that the level of atresia can be determined by cross-sectional MR imaging in the PC plane and I plane showing the blind end of the rectum. In fact, sagittal and coronal imaging can show the height of atresia more accurately, so transverse imaging is not so important in judging the position of atresia, and only in the case of low-level deformities can it help to determine the exact position of the anal canal and its relationship with the sphincter complex. As for the reference, it is recommended to use the superficial transverse perineal muscle, which is located immediately dorsal to the urogenital diaphragm, and in normal subjects the anal canal is located dorsal to this muscle. When the anal canal is at the same level as this muscle or is located ventral to it, there is a forward ectopic opening of the anal canal and it is outside the sphincter complex. The ectopic opening anal canal used to be called an ARM combined with a fistula, and its morphology varies. To date, however, MRI has been unreliable in showing fistulas, and adequate visualization of fistulas is important for surgical modality development and operation. To address this problem, Taccone et al. suggested that MRI after injection of petroleum jelly (similar in composition to meconium) into the distal colon of the colostomy opening is expected to identify the presence of fistulas prior to reconstruction. It is believed that most of the perianal muscles in medium and high ARM have different degrees of developmental abnormalities, and the developmental status of the puborectalis and external sphincter muscles directly affects the postoperative anal function of the children, so it is important to clarify the developmental status of the muscles before surgery to determine the prognosis. Multilevel imaging by MRI can clearly show the perianal sphincter, coronal T1-weighted images can well show the anal levator muscle (puborectalis) and its relationship with the external sphincter of the anal canal, while cross-sectional views can adequately show the developmental status of the puborectalis and external sphincter of the anal canal. Although the general development of the sphincter complex is easy to determine, there is still a lack of objective criteria for judging the normal thickness of these muscles to date. Vade et al. investigated the normal thickness of the puborectalis muscle with age by CT examination and found that the puborectalis muscle gradually thickens with age and determined that the thickness of this muscle should not be less than 2.5 mm in normal infants less than three months old.