Ultrasound Beam Profile and Focusing

Electronic Focusing

• When an array transducer receives echoes, the electrical outputs of the elements can be timed to "listen" in a particular direction with a listening focus at a particular depth.
• Most modern scanners employ receive focus automatically in the background.

Dynamic Aperture

• Aperture refers to the surface area of the crystal(s) or element(s) used to generate and receive the sound beam.
• For arrays, the aperture relates to the group of elements pulsed collectively to generate the sound beam.
• A single-crystal, fixed-focus transducer aperture is equal to the transducer diameter.
• The aperture of an array can be changed electronically.

Ultrasound Beam Axis

• The sound beam axis is 3D, having two lateral and axial limits.
• The center of the main beam is known as the beam axis.
• In addition to the primary or main beam (lobe), all ultrasound transducers also generate secondary off-axis low-energy beams known as side lobes and/or grating lobes.

Focusing

• Focusing is the mechanical and/or electronic process of reducing beam width.
• There are two main types of focusing: mechanical and electronic.

Mechanical Focusing

• Focusing a transducer with an external lens or shaping the crystal is referred to as mechanical focusing.
• There are two types of mechanical focusing: external focusing and internal focusing.
• External focusing is inserting an acoustic lens in front of the crystal or by using a system of lens and mirrors.
• Internal focusing is shaping or curving the crystal.
• Internal focusing is generally used for frequencies 5 MHz and lower, while external focusing is usually applied for higher frequencies.

Electronic Focusing

• Electronic focusing is more complex than mechanical focusing.
• It is performed by applying small time delays in a specific way to the firing group of crystal elements in an array transducer.
• Only array transducers can be focused electronically.
• The advantage of electronic focusing is that the operator can change the depth at which the beam is focused.

Transmit Focusing

• Transmit focusing is applied during the transmission of the ultrasound pulse.
• Short timing delays are applied to the center elements group, causing the pulse to be focused at a certain depth.
• Increasing this delay will cause the pulse to be focused at a shallower depth, and conversely.

Ultrasound Beam Profile

• The ultrasound beam profile consists of the near field, transition point, and far field.
• The near field extends from the transducer face to the transition point and is characterized by beam convergence and a natural focus.
• The distance from the transducer face to the transition point is described as "one NF length".
• All unfocused transducers have a natural focus, which is the transition point.

Unfocused Ultrasound Beam

• Unfocused means no mechanical or electronic methods are used to change the natural beam profile and focus the beam to obtain a narrower beam width.
• The sound beam produced by an unfocused transducer has specific characteristics, divided into two distinct regions: near field and far field.
• The near field is characterized by beam convergence and a natural focus, while the far field is characterized by beam divergence.