Ultrasound is the first-line investigation for suspected intussusception, with sensitivity and specificity both approaching 100% in experienced hands. It is one of the studies where the sonographer's findings directly and immediately determine management — a positive scan sends the child to reduction, often within the hour.

It is also a study where a confident negative is genuinely valuable, because it spares a child an enema.

Intussusception

Who and When

Ileocolic intussusception is most common between roughly 3 months and 3 years, peaking in infants around 6 to 9 months. The classic triad of intermittent colicky pain, vomiting, and redcurrant-jelly stool is present in a minority of cases — most children present with some subset, often just intermittent distress and vomiting.

Technique

Probe: High-frequency linear, 7–15 MHz for most infants. A curvilinear probe helps in larger or more distended children.

Approach: Systematic survey of the colon. Start in the right lower quadrant, follow the ascending colon up to the hepatic flexure, across the transverse colon, down the descending colon. Ileocolic intussusception most often sits in the right upper quadrant or along the transverse colon, because the intussusception migrates as peristalsis drives it forward.

Graded compression displaces bowel gas and brings the target closer to the probe. Be gentle and patient — a distressed, crying child tenses the abdominal wall and makes the study much harder. Give the child time, let a parent hold them, and warm the gel.

The Findings

Target sign (donut sign) — the diagnostic appearance in transverse section. Concentric alternating hypoechoic and hyperechoic rings created by the layers of bowel telescoped inside bowel: outer hypoechoic bowel wall, inner echogenic mucosa and mesenteric fat, and the central intussuspiens.

Pseudokidney sign — the same structure in longitudinal section, where the layered mass resembles a kidney with echogenic central mesenteric fat mimicking a renal sinus.

Size criteria:

Outer diameter typically greater than 2.5 cm for ileocolic intussusception
A smaller target, generally under about 2 cm with a thin wall and short segment, more likely represents a small bowel–small bowel intussusception, which is frequently transient and self-resolving. Distinguishing these matters — transient small bowel intussusception usually needs observation, not an enema.

Additional features to document:

Trapped mesenteric fluid between the layers
Mesenteric lymph nodes dragged into the intussusception
Length of the intussuscepted segment
Colour Doppler flow within the bowel wall
Free intraperitoneal fluid

Features Suggesting Reduction May Fail

These change the urgency and the conversation with the surgical team, so report them explicitly:

Absent colour Doppler flow in the intussusceptum, suggesting ischaemia
Trapped fluid between the intussusceptum and intussuscipiens
Long-standing symptoms with marked bowel wall thickening
Free fluid in significant volume
A pathological lead point

Lead Points

Most intussusception in the typical age range is idiopathic, attributed to hypertrophied lymphoid tissue. A pathological lead point becomes more likely outside that age range — in children under 3 months, over 3 years, and particularly in older children.

Look for: Meckel's diverticulum, duplication cyst, polyp, lymphoma, or a Henoch–Schönlein purpura–related intramural haematoma.

An older child with intussusception deserves a deliberate search for a lead point, and the report should say whether one was seen or not.

Reporting

The report should give the surgical and radiology team what they need to act:

"Target lesion in the right upper quadrant measuring 3.1 cm in outer diameter with a 4.5 cm intussuscepted segment, demonstrating the pseudokidney appearance in longitudinal section. Mesenteric lymph nodes are drawn into the intussusceptum. Preserved colour Doppler flow within the bowel wall. Small volume free fluid in the pelvis. No pathological lead point identified. Findings consistent with ileocolic intussusception."

Malrotation and Midgut Volvulus

A different and more urgent problem, and one where ultrasound has an increasingly recognized role.

Why it matters: Midgut volvulus is a surgical emergency. Bowel infarction can occur within hours. Classic presentation is bilious vomiting in a neonate, and that clinical picture should trigger immediate imaging.

The Normal Relationship

In normal anatomy, the superior mesenteric vein (SMV) lies to the right of the superior mesenteric artery (SMA) in transverse section at the level of the pancreas.

Technique: Transverse epigastric view. Identify the aorta, then the SMA arising anteriorly, then the SMV. Use colour Doppler to confirm which vessel is which — the relationship is the whole point, so getting the identification right matters.

Abnormal Findings

SMA/SMV inversion — the SMV lying anterior or to the left of the SMA. This suggests malrotation. Important caveat: a normal relationship does not exclude malrotation, and an abnormal one is not entirely specific. It is a flag, not a verdict.

Whirlpool sign — the specific and alarming finding. The SMV and mesentery wrap clockwise around the SMA, producing a visible spiral on colour Doppler as you sweep through the mesenteric root. This indicates midgut volvulus and is a surgical emergency. Communicate it immediately and directly — do not leave it in a report queue.

Supporting findings: duodenal dilatation with distal tapering, thickened bowel wall, free fluid.

Upper GI contrast study has traditionally been the reference standard for malrotation, and ultrasound findings usually prompt it or, where volvulus is demonstrated, direct surgical management. Know your institution's pathway.

Other Pediatric Bowel Findings

Mesenteric adenitis: Clustered enlarged mesenteric nodes, typically right lower quadrant, with a normal appendix. A diagnosis of exclusion — the appendix must be visualized and normal before calling it.

Bowel wall thickening: Non-specific. Consider infectious enteritis, inflammatory bowel disease in older children, and Henoch–Schönlein purpura. Note the distribution, degree of thickening, wall stratification, and mural Doppler signal.

Duplication cyst: Cystic structure with the characteristic gut signature — an inner echogenic mucosal layer and outer hypoechoic muscular layer. Can act as an intussusception lead point.

Practical Notes for Scanning Children

Warm the gel. It matters more than you would think.
Let a parent hold the child. A calm child is a diagnostic study; a screaming child is not.
Use the highest frequency that reaches. Children are small — take the resolution.
Distraction works. A phone playing something, a toy, a sibling.
Take your time on the appendix. If you are scanning for intussusception and it is negative, the appendix is usually the next question, and it is worth answering in the same study.

SonoBuddy is a reference tool, not a diagnostic authority. Suspected midgut volvulus requires immediate direct communication with the clinical team.

Intussusception and Pediatric Bowel Ultrasound