What major issue arises due to varying blood velocities in vessels?

The major issue that arises due to varying blood velocities in vessels is complete signal cancellation. This phenomenon, often referred to as "phase cancellation," occurs when the different velocities of flowing blood can lead to varying phase shifts between the moving protons. In magnetic resonance imaging (MRI), signals from protons moving at different speeds can destructively interfere with each other when the timing of their echo signals is misaligned. This can cause the signals to cancel each other out, leading to reduced or absent signals in certain areas of the image.

This situation is particularly pronounced in regions with rapidly changing velocities, such as in large blood vessels or areas of turbulence. The cancellation effect can result in poor visualization of blood flow and structures, making it a significant challenge in MRI imaging of cardiovascular conditions. Understanding this issue is crucial for medical imaging professionals in order to improve image quality and accurately assess vascular anatomy and pathology.