The ultrasound beam is not uniform and its nature changes with distance from the

• A. Transducer
• B. Tissue
• C. Gel
• D. Patient

Answer: (A)

The key idea is that the ultrasound beam is created by a finite-sized transducer, so its geometry is set at the source and changes as it propagates due to diffraction. Each point of the transducer acts as a tiny source, and the waves from all points interfere. This creates a curved wavefront and a cross-sectional pattern that evolves with distance: in the near field, the pattern can be complex and the beam may not be uniform; in the focal region the beam concentrates and becomes narrower and more intense; beyond the focus the beam spreads out again and its intensity falls. This distance-dependent variation is inherent to the transducer’s aperture and diffraction effects, not to the surrounding media. Gel mainly ensures good coupling and doesn’t define the beam’s geometry; tissue and the patient affect attenuation, speed, and reflections once the wave is in tissue, but they don’t establish the basic nonuniform propagation pattern that originates at the transducer.