Why would a cardiac MRI (Magnetic Resonance Imaging) indicate a small or no patent foramen ovale (PFO) but a transthoracic echocardiogram (TTE) with bubble study show a large PFO?

Discrepancy Between Cardiac MRI and TTE Bubble Study in PFO Detection

The discrepancy between cardiac MRI showing a small or no patent foramen ovale (PFO) and transthoracic echocardiogram (TTE) with bubble study showing a large PFO is primarily due to the superior dynamic functional assessment capabilities of bubble studies compared to the primarily anatomical assessment of cardiac MRI.

Why This Discrepancy Occurs

Differences in Detection Methods

• Cardiac MRI:

  • Provides excellent anatomical imaging but has limitations in detecting small intracardiac shunts
  • MRI is "probably not as accurate as color flow Doppler in visualizing small ventricular and atrial defects" 1
  • Primarily assesses anatomical structure rather than functional flow
  • May miss a PFO that appears small anatomically but functionally allows significant shunting

• TTE with Bubble Study:

  • Specifically designed to detect right-to-left shunts through dynamic assessment
  • "Saline contrast echocardiography ('bubble echo') may be useful in verifying the presence of a shunt and in differentiating intracardiac from extracardiac shunts" 1
  • Directly visualizes the passage of microbubbles across the septum during Valsalva maneuver
  • Assesses the functional significance of the PFO rather than just its anatomical size

Functional vs. Anatomical Assessment

• A PFO may appear small anatomically on MRI but function as a large shunt during certain physiological conditions:
  • The foramen ovale is a flap-like structure that may open widely during certain hemodynamic states
  • Valsalva maneuver during bubble study increases right atrial pressure, forcing the PFO open
  • The dynamic nature of the PFO means its functional size can be significantly larger than its anatomical appearance

Clinical Implications

Diagnostic Accuracy

• TEE with bubble study is considered more sensitive than MRI for PFO detection 1
• Studies have shown that patients with cryptogenic stroke have "larger patent foramen ovale with more extensive right-to-left interatrial shunting" than patients with stroke of determined cause 2
• The number of microbubbles crossing the septum correlates with the functional significance of the PFO 2

Clinical Decision Making

• For cryptogenic stroke evaluation, bubble studies are more clinically relevant than anatomical imaging alone
• In patients ≤60 years with cryptogenic stroke, PFO detection by bubble study may lead to consideration of closure 3
• A negative bubble test has a high negative predictive value (97.1%) for therapy-relevant findings on TEE 4

Pitfalls and Considerations

Potential for Overdiagnosis

• Bubble studies are frequently performed in patients who have readily identifiable causes of stroke, where any PFO detected is likely incidental 5
• Only 31% of patients with PFO detected on bubble study had a subsequent change in management 5
• The majority of bubble studies performed in non-cryptogenic stroke patients yield no change in management 3

Technical Considerations

• Proper technique during bubble study is essential:
  • Adequate Valsalva maneuver timing is critical
  • The number of microbubbles crossing the septum should be quantified
  • Timing of bubble appearance in left atrium helps differentiate PFO from pulmonary shunts

Conclusion

The discrepancy between cardiac MRI and TTE bubble study findings in PFO detection is expected due to their different assessment methods. While MRI provides excellent anatomical imaging, it may underestimate the functional significance of a PFO. TTE with bubble study directly visualizes the dynamic functional aspect of the PFO during conditions that promote right-to-left shunting, making it the preferred method for assessing the clinical significance of a PFO, particularly in the context of cryptogenic stroke evaluation.