Why does a cardiac MRI show little to no patent foramen ovale (PFO) but an echo bubble study shows a large PFO with many bubbles?

Discrepancy Between Cardiac MRI and Echo Bubble Study in PFO Detection

Echocardiography with bubble study is significantly more sensitive than cardiac MRI for detecting patent foramen ovale (PFO), with cardiac MRI detecting only 18.8% of mild shunts that are identified by transesophageal echocardiography (TEE). 1

Why the Discrepancy Exists

The difference in PFO detection between these two imaging modalities occurs for several technical and physiological reasons:

1. Sensitivity Differences:

   • Bubble studies during echocardiography provide real-time visualization of contrast bubbles crossing the interatrial septum
   • Cardiac MRI has inferior sensitivity for detecting contrast-enhanced right-to-left shunting 1
   • MRI particularly struggles with mild shunts, detecting only 18.8% of those identified by TEE 1

2. Temporal Resolution:

   • Echocardiography offers superior temporal resolution compared to MRI
   • Echo can capture the brief moments when bubbles cross through a PFO during specific phases of the cardiac cycle
   • The American Heart Association recognizes that bubble studies with Valsalva maneuver provide optimal detection of right-to-left shunting 2

3. Provocation Techniques:

   • Bubble studies typically incorporate Valsalva maneuver, which transiently increases right atrial pressure
   • This pressure increase enhances right-to-left shunting through the PFO, making it more detectable
   • MRI protocols may not effectively incorporate these provocation techniques

Clinical Implications

The superior sensitivity of echocardiography with bubble study for PFO detection has important clinical implications:

• Risk Stratification: The American Heart Association recommends screening for PFO with echocardiography with agitated saline bubble study for risk stratification in patients with pulmonary embolism 3

• Stroke Risk: The presence of a PFO in patients with pulmonary embolism increases the risk of death (relative risk 2.4), ischemic stroke (relative risk 5.9), and peripheral arterial embolism (relative risk 15) 3, 2

• Silent Brain Infarcts: In patients with PE, PFO presence is associated with a significantly increased risk of silent brain infarcts (33% vs. 2% in those without PFO) 3

Appropriate Testing Selection

When evaluating for PFO, the following approach is recommended:

1. Initial Screening:

   • Transthoracic echocardiography (TTE) with bubble study and Valsalva maneuver 2
   • Transcranial Doppler (TCD) with bubble study is also recommended by the American Heart Association 2

2. Definitive Assessment:

   • Transesophageal echocardiography (TEE) with bubble study is the gold standard 2
   • Essential when PFO closure is being considered due to its superior anatomical detail 2

3. Role of Cardiac MRI:

   • Better suited for planning transcatheter PFO closure procedures 2
   • Useful for detailed visualization of atrial septal anatomy before PFO closure 2
   • Not recommended as the primary diagnostic test for PFO detection

Pitfalls to Avoid

1. Relying solely on cardiac MRI for PFO detection, especially for mild shunts
2. Failing to incorporate Valsalva maneuver during bubble studies
3. Unnecessary PFO screening in patients with readily identifiable stroke causes (8.7% of patients referred for bubble studies had atrial fibrillation and 9.2% had significant carotid stenosis) 4
4. Assuming all detected PFOs are clinically significant - management was changed in only 31% of patients with detected PFOs in one study 4

In conclusion, the discrepancy between cardiac MRI and echo bubble study findings in PFO detection is well-documented, with echocardiography being significantly more sensitive. For accurate PFO assessment, especially when clinical management decisions depend on it, echocardiography with bubble study remains the preferred diagnostic approach.