Double track sign | Radiology Signs

Answer:

The double track sign is two parallel linear streaks of barium or fluid separated by an intervening lucent band, seen traversing the pyloric canal. The streaks are contrast or gastric fluid squeezed along the crevices of crowded, redundant pyloric mucosa, and the lucent band between them is the mucosa itself. On an upper GI series it is barium tracking through the narrowed elongated channel; on ultrasound it is fluid compressed into two tracks by the circumferentially thickened circular muscle. It is a well-known feature of hypertrophic pyloric stenosis but is explicitly not pathognomonic: it occurs in pylorospasm, in which the muscle measurements are normal and the appearance is transient. Diagnosis therefore rests on the quantitative sonographic criteria of muscle thickness (3 mm or more) and channel length (15-17 mm or more), with real-time observation of channel opening. It differs from the string sign, a single thin barium column, and the beak sign, barium entering only the canal entrance.

Why is it called so?

The two parallel streaks of contrast running along the pyloric canal look like a pair of railway tracks.

Pathophysiology

Hypertrophy of the pyloric circular muscle narrows and elongates the canal. The mucosa lining it is not similarly reduced and so becomes redundant, thrown into longitudinal folds that fill most of the residual lumen. Contrast or fluid forced through under gastric peristalsis can only pass along the narrow crevices between these folds, and typically follows two of them, producing paired linear streaks with the intervening mucosal fold as a lucent band. Because transient spasm can crowd the mucosa in the same way without true muscular hypertrophy, pylorospasm reproduces the appearance, which is why the sign alone does not establish the diagnosis.

Alternative names: Double-track sign, twin track sign

Other associated named signs: Target sign, cervix sign, antral nipple sign, shoulder sign, string sign and beak sign

References

1. Cohen HL, Blumer SL, Zucconi WB. The sonographic double-track sign: not pathognomonic for hypertrophic pyloric stenosis; can be seen in pylorospasm. J Ultrasound Med. 2004;23(5):641-6. PMID: 15154530.