Final answer:
The interventricular septum is best visualized when the ultrasound beam is perpendicular to its long axis, allowing for optimal interaction and clear imaging.
Step-by-step explanation:
The thickness of the interventricular septum is best visualized when the US beam is perpendicular to the long axis of the septum.
In this orientation, the ultrasound waves can interact with the entire thickness of the septum, reflecting back to the transducer and providing a clear image on the ultrasound monitor.
This approach takes advantage of how ultrasound technology works, where a probe consisting of multiple ultrasound sources is phased to interfere constructively in a given direction, optimizing the visualization of structures.