What technique is best for demonstrating a cardiac shunt?

First-pass radionuclide angiography is the most effective technique for demonstrating a cardiac shunt. This method involves the rapid acquisition of images following the injection of a radiopharmaceutical. Because it captures the passage of the isotope through the heart and into the circulatory system in real-time, it provides immediate visual evidence of blood flow dynamics.

In the context of a cardiac shunt, this technique can effectively highlight abnormal connections between heart chambers or between the heart and major vessels, such as atrial septal defects or ventricular septal defects. The ability to visualize the arrival of the radiotracer in the right and left heart chambers or pulmonary circulation can confirm the presence and severity of a shunt.

While other imaging modalities, like single photon emission computed tomography (SPECT) or positron emission tomography (PET), can assess myocardial perfusion or viability, they are not as focused on the rapid transit of blood that characterizes a shunt. Computed tomography angiography (CTA) is great for anatomical evaluation but does not capture the physiological aspect of shunting as effectively as first-pass radionuclide angiography does. Therefore, first-pass radionuclide angiography stands out as the best option for identifying and demonstrating a cardiac shunt.