Introduction

The midgut plays a crucial role in the development of the digestive system, undergoing key embryological processes such as physiological herniation, rotation, and fixation within the abdominal cavity. Variations in these processes can lead to congenital anomalies with significant clinical implications, including malrotation, volvulus, omphalocele, gastroschisis, Meckel’s diverticulum, and intestinal duplications.

Diagnostic imaging is essential in the evaluation of the midgut, enabling early identification and characterization of these anomalies. Modalities such as plain radiography, contrast-enhanced studies, Doppler ultrasound, computed tomography (CT), and magnetic resonance imaging (MRI) serve as fundamental tools in clinical practice.

Embryology of the Midgut and Hindgut

Midgut

The midgut develops into the distal small intestine, cecum, appendix, ascending colon, and right two-thirds of the transverse colon, supplied by the superior mesenteric artery (SMA).

By the sixth week, rapid intestinal growth causes physiological umbilical herniation. The midgut loop, with a proximal limb (small intestine) and a distal limb (cecum and ascending colon), undergoes a 90° counterclockwise rotation outside the abdomen, followed by an additional 180° rotation during retraction in the tenth week. The jejunum returns first, while the cecum and ascending colon settle in the lower right quadrant.

The duodenum and pancreas become retroperitoneal, while the ascending colon loses its mesentery. The cecum and appendix appear in the sixth week, with a retrocecal appendix in 64% of cases.

Fig 1: Diagrams of midgut development (1 and 2). Normal process of midgut herniation and rotation during embryological development, showing the midgut before herniation (A), growth and umbilical herniation (B1-B3), and return to the abdominal cavity with final rotation (C). Image from: Development of Abdominal Organs. (2023, March 29). Lecturio. https://app.lecturio.com/#/article/3869

Hindgut

The hindgut forms the left third of the transverse colon, descending colon, sigmoid colon, rectum, and upper anal canal, supplied by the inferior mesenteric artery (IMA). The descending colon becomes retroperitoneal, while the sigmoid colon retains its mesentery.

Cloaca and Anal Canal

By the seventh week, the urorectal septum divides the cloaca into the rectum and anal canal dorsally and the urogenital sinus ventrally. The anal membrane ruptures in the eighth week.

The anal canal has a dual origin:

• The upper two-thirds (endoderm-derived) are supplied by the superior rectal artery and autonomically innervated.
• The lower third (ectoderm-derived) is supplied by the inferior rectal arteries with somatic innervation.

The pectinate line marks this transition (1).

Anatomy of the Jejunum, Ileum, and Large Intestine

The jejunum and ileum form the intraperitoneal portion of the small intestine, extending from the duodenojejunal flexure to the ileocecal junction. They measure approximately 6-7 meters, with the jejunum primarily in the upper left quadrant and the ileum in the lower right quadrant.

• Vascular supply: The superior mesenteric artery (SMA) provides jejunal and ileal branches, while venous drainage occurs via the superior mesenteric vein (SMV), contributing to the portal vein.
• Lymphatic drainage follows a sequential pathway from lacteals to superior mesenteric lymph nodes.
• Innervation: The superior mesenteric plexus, with sympathetic fibers from T8-T10 and parasympathetic fibers from the vagus nerve.

Fig 2: Anatomy and functions of the small intestine. Text based on: Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2013). Clinically Oriented Anatomy. Lippincott Williams & Wilkins.

Fig 3: Anatomy and functions of the small intestine. Image taken from: The Small and Large Intestines | Anatomy and Physiology II. https://courses.lumenlearning.com/suny-ap2/chapter/the-small-and-large-intestines/ Text based on: Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2013). Clinically Oriented Anatomy. Lippincott Williams & Wilkins.

The large intestine extends from the ileocecal valve to the rectum, including the cecum, appendix, ascending, transverse, descending, and sigmoid colon, rectum, and anal canal. It is characterized by teniae coli, haustra, and omental appendices, with a primary function of water absorption and fecal formation.

• Vascular supply: the SMA and inferior mesenteric artery (IMA). Venous drainage occurs via the SMV and inferior mesenteric vein (IMV).
• Lymphatic drainage: Varies by region, reaching superior or inferior mesenteric lymph nodes.
• Innervation: The superior and inferior mesenteric plexuses, with sympathetic fibers reducing motility and parasympathetic fibers enhancing it (2). 

Fig 4: Anatomy and functions of the large bowel. Text based on: Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2013). Clinically Oriented Anatomy. Lippincott Williams & Wilkins.

Fig 5: Anatomy of the large intestine. Text based on and image taken from: IMAIOS. (2012, March 20). Anatomy of the Abdomen and Digestive System: Illustrations. https://www-imaios-com.ez.urosario.edu.co/es/e-anatomy/abdomen-y-pelvis/sistema-digestivo

Congenital Pathology of the Midgut

Congenital anomalies of the gastrointestinal tract encompass a wide spectrum of malformations, some evident at birth and others diagnosed later in childhood or adulthood. Diagnostic imaging plays a key role in their evaluation, with plain radiography, contrast studies, ultrasound (US), computed tomography (CT), and magnetic resonance imaging (MRI) being essential tools.

Classification of Neonatal Anomalies

Structural:

• Embryologic maldevelopment: midgut malrotation, duplication cysts, anorectal atresia.
• Intrauterine vascular complications: jejunoileal atresia, colonic atresia/stenosis.

Functional:

• Meconium ileus.

Mixed (structural-functional):

• Midgut volvulus (secondary to malrotation), colonic aganglionosis (3).

Duodenal Atresia

A congenital anomaly (1:6,000–1:10,000 live births) caused by failed duodenal recanalization (weeks 5–6 gestation). It is 10 times more common than pyloric atresia and frequently associated with trisomy 21 (≈33%) and other congenital anomalies (>50%). A genetic component is suspected but not confirmed. Morphological types include web, fibrous cord, or complete gap.

Jejunal Atresia

Results from fetal intestinal ischemia, leading to vomiting and abdominal distension. The “apple-peel” variant involves severe intestinal shortening and poor prognosis.

Ileal Atresia

Causes distal obstruction with absent meconium passage. Classified as:

• Type 1: Obstructing membrane.
• Type 2: Fibrous cord between segments.
• Type 3: Complete atresia with mesenteric defect.
• Type 4: Multiple atresias.

Fig 6: Characteristics of duodenal and jejunoileal atresia. Anteroposterior abdominal radiograph of a newborn with jejunal atresia, showing dilation of intestinal loops in the upper abdomen. Text based on and image taken from: Hamm, B., & Ros, P. R. (2013). Abdominal Imaging. Springer.

Fig 7: Upper gastrointestinal contrast study in a 5-week-old female patient, frontal (A) and oblique (B) projections. Contrast passage into the duodenum is observed, with dilation mainly in the second and third portions and no contrast passage into the fourth portion due to duodenal atresia. Case courtesy of Fundación Cardio Infantil-LaCardio, Department of Radiology, 2019.

Meconium Ileus

A common manifestation of cystic fibrosis, caused by thick meconium impaction in the distal ileum, leading to abdominal distension and bilious vomiting. Complications include volvulus, perforation, and meconium peritonitis.

Fig 8: Sonographic findings in meconium ileus. Ultrasound of a newborn with meconium ileus associated with cystic fibrosis, showing dilated bowel loops with echogenic content. Text based on and image taken from: Hamm, B., & Ros, P. R. (2013). Abdominal Imaging. Springer.

Meconium Peritonitis

A chemical peritoneal inflammation due to intrauterine bowel perforation, often from ileal atresia or meconium ileus. Imaging may show fibrosis and calcifications. In some cases, a meconium cyst forms, encapsulating inflammatory fluid.

Fig 9: Characteristics of meconium peritonitis. Text based on and image taken from: Hamm, B., & Ros, P. R. (2013). Abdominal Imaging. Springer.

Fig 10: Anteroposterior (A) and lateral (B) abdominal radiographs demonstrating a meconium cyst in a newborn with meconium peritonitis due to intestinal perforation secondary to meconium ileus obstruction. Image taken from: Hamm, B., & Ros, P. R. (2013). Abdominal Imaging. Springer.

Malrotation

Failure of normal midgut rotation during embryonic development results in:

• Non-rotation: small bowel on the right, colon on the left.
• Incomplete rotation: Failure of final duodenojejunal loop rotation.
• Reverse rotation: transverse colon posterior and duodenum anterior to the superior mesenteric artery (SMA).

Fig 11: Diagram illustrating the types of intestinal malrotation. (A) Non-rotation. (B) Incomplete rotation. (C) Reverse rotation. Modified from: Berrocal, T., Gayá, F., & De Pablo, L. (2005). Embryological, clinical, and radiological aspects of intestinal malrotation. Radiología, 47(5), 237–251. https://doi.org/10.1016/s0033-8338(05)72843-5.

Fig 12: Upper gastrointestinal contrast study in an adolescent patient, anteroposterior projection. The contrast medium is observed passing through the esophagus, stomach, and duodenum, with the latter located on the right side of the abdomen. The third portion does not cross the midline, a finding consistent with intestinal malrotation. Case courtesy of Fundación Cardio Infantil-LaCardio, Department of Radiology, 2023.

Fig 13: (A, B, C, and D) Axial contrast-enhanced abdominal CT images (oral and IV contrast) of a 14-year-old female patient with intestinal malrotation. The third portion of the duodenum (green arrows) is located anterior to the pancreatic head instead of its usual retroperitoneal position, failing to cross the midline to position the Treitz angle to the left of the lumbar vertebrae. Additionally, a swirling pattern of intestinal loops is observed. Case courtesy of Fundación Cardio Infantil - LaCardio, Department of Radiology, 2022.

Volvulus

A twisting of the intestine around its mesenteric axis, compromising vascular supply. Presents as acute abdominal pain and bilious vomiting, especially in neonates. It may be associated with Ladd’s bands, peritoneal structures that can cause obstruction even without volvulus.

Fig 14: Coronal reconstruction of the abdominal CT of the same patient from the previous figure shows the swirling of the mesenteric vessels (red arrow), forming the “whirlpool sign,” consistent with intestinal malrotation complicated by midgut volvulus. Case courtesy of Fundación Cardio Infantil-LaCardio, Department of Radiology, 2022.

Fig 15: (A) Color Doppler ultrasound in an infant demonstrating the superior mesenteric vein rotating around the superior mesenteric artery, forming the “whirlpool sign.” (B) Abdominal ultrasound in a young patient showing an abnormal position of the mesenteric vessels, with the vein (blue arrow) located to the left of the artery (red arrow). Text adapted and images taken from: Hamm, B., & Ros, P. R. (2013). Abdominal Imaging. Springer.

Enteric Duplication Cysts

Most commonly found in the distal ileum, they may contain ectopic gastric or pancreatic mucosa, increasing the risk of bleeding, ulceration, or perforation. They can also act as a lead point for intussusception.

Fig 16: Characteristics of duplication cysts. Text adapted from: Hamm, B., & Ros, P. R. (2013). Abdominal Imaging. Springer. Illustration created by the author.

Fig 17: Ultrasonographic findings of enteric duplication cysts. Abdominal ultrasound image showing a cystic mass in the lower right quadrant with the “gut signature,” consistent with an enteric duplication cyst. Case courtesy of Eishah Mohammed Al-Shaibani, Radiopaedia.org, rID: 149210.

Abdominal Wall Defects

• Gastroschisis: right-sided abdominal wall defect with extruded bowel loops lacking a protective sac. Associated with short bowel, malrotation, atresia, and dysmotility.

Fig 18: Characteristics of Gastroschisis. Anteroposterior abdominal X-ray showing herniation of bowel loops through a defect in the abdominal wall. The loops are not contained by a membrane. Text based on and images taken from: Hamm, B., & Ros, P. R. (2013). Abdominal Imaging. Springer.

• Omphalocele: Herniation through the umbilicus, covered by peritoneum and amnion. Frequently linked to cardiac defects, chromosomal anomalies, and Beckwith-Wiedemann syndrome, with higher mortality than gastroschisis.

Fig 19: Characteristics of Omphalocele. A contrast-enhanced axial abdominal CT scan of a patient with omphalocele demonstrated a large abdominal wall defect with herniation of intra-abdominal organs, which are covered by a peritoneal sac. Text based on: Hamm, B., & Ros, P. R. (2013). Abdominal Imaging. Springer. Case courtesy of Fundación Cardio Infantil-LaCardio, Department of Radiology, 2016.

Omphalomesenteric Duct Remnants

The omphalomesenteric duct connects the yolk sac to the fetal midgut and should close before birth. Its persistence leads to anomalies such as:

Meckel’s Diverticulum: The most common small bowel anomaly, arising from the antimesenteric border of the distal ileum. It may contain ectopic gastric or pancreatic mucosa, increasing the risk of ulceration, bleeding, and obstruction.

Described by the Rule of 2s:

• Present in 2% of the population.
• Located <2 feet from the ileocecal valve.
• Measures ~2 inches in length.
• May contain 2 types of mucosa.
• Becomes symptomatic in the first 2 years of life.

Fig 20: Contrast-Enhanced Abdominal CT Scan in a 75-Year-Old Patient Under Evaluation for Abdominal Pain. The image shows a blind-ending bowel loop (green arrow) with an air-fluid level originating from the distal ileum. There is associated bowel wall thickening and inflammatory changes in the adjacent mesenteric fat, findings consistent with a complicated Meckel’s diverticulum. Case courtesy of Fundación Cardio Infantil-LaCardio, Department of Radiology, 2020.

Other anomalies: umbilical fistula, omphalomesenteric sinus, and duct remnants may also occur.

Hirschsprung Disease

• 15-20% of congenital obstructions.
• Lack of ganglia → Aperistalsis, obstruction, proximal dilation.
• 1 in 5,000 births, more common in males.
• Rectosigmoid (75-80%), Total colon (8%).
• Barium enema: inverted cone transition (50% cases).
• Cross-section: dilated proximal (normal), narrow distal (aganglionic).

Fig 21: Barium Enema Study in a 6-Year-Old Patient, Anteroposterior and Lateral Projections. The images show an “inverted cone” morphology of the colon, with sigmoid dilation and distal narrowing, findings characteristic of Hirschsprung disease. Case courtesy of Fundación Cardio Infantil-LaCardio, Department of Radiology, 2023.

Anorectal Anomalies

Clinical Presentation:

• Manifest as signs of low colonic obstruction in the neonatal period.
• Congenital Associations (up to 40%):
• Renal, vertebral, esophageal, and tracheal anomalies.

Related Syndromes:

• VACTERL association: vertebral, anorectal, cardiac, tracheal, esophageal, renal, and limb anomalies.
• Currarino triad: combination of anorectal malformation, sacral anomaly, and presacral mass.

Imaging Studies:

• Males: Voiding cystourethrogram to assess genitourinary involvement.
• Females: vaginogram or fistulogram to evaluate potential fistulous connections with the colon (4).

Fig 22: Imaging Studies in a 6-Month-Old Male Patient with Known VACTERL Association. (A) Anteroposterior abdominal radiograph showing distal sacral and coccygeal agenesis, along with butterfly-shaped lower thoracic vertebrae (red arrow). (B) Contrast study through a colostomy tube, revealing the absence of anal canal opacification and the presence of a thin rectoperineal fistulous tract (green arrow). Case courtesy of Fundación Cardio Infantil-LaCardio, Department of Radiology, 2024.