Patent foramen ovale and left atrial septal pouch

Imaging description A patent foramen ovale (PFO) occurs when the two components of the atrial septum, the septum primum and septum secundum, fail to fuse after birth. The result is a persistent valve-like potential pathway between the left and right atria. In fetal life, this pathway allows blood from the inferior vena cava to cross into the left atrium, bypassing the lungs. In adults, there is potential for blood to cross from right to left if there is reversal of the normal interatrial pressure gradient, which can occur during straining or a Valsalva maneuver. On imaging, a patent foramen ovale can occasionally be identified by cardiac CT, which demonstrates a slit-like, vertically oriented channel in the atrial septum between the left and right atria. A jet of contrast will often be seen exiting the channel into the right atrium inferiorly (Figure 20.1). Occasionally, a jet of unopacified blood will result in a blush of negative contrast in the left atrium if there is rightto-left flow across the PFO during the exam. Similar findings may be seen on dynamic contrast-enhanced MRI. The atrial septal pouch is a related anatomic structure created by partial fusion of the septum primum and septum secundum. This can be recognized at cardiac CT by a small channel in the left atrial side of the atrial septum that fills with contrast but does not communicate with the right side (Figure 20.2). Importance Some studies have found an increased incidence of PFO in patients with cryptogenic stroke, suggesting a causative role in thromboembolism. However, results from other studies have been contradictory and currently the role of PFO in management of patients with stroke is uncertain. A potential for blood stasis and thrombus within atrial septal pouches (ASP) has been suggested, and case reports have found thrombus within ASP in patients with embolic stroke. However, a larger case-control study of 187 patients with first-ever stroke and 157 controls found no increased incidence of atrial septal pouch in patients with stroke.