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Beam focusing refers to creating a narrow point in the cross-section of the ultrasound beam called the focal point. It is at the focal point where the lateral resolution of the beam is the greatest also. Before the focal point is the near field or Fresnel zone, where beams converge. Distal to this focal point is the far field or Fraunhofer zone where beams diverge.
The two broad types of focusing are fixed and adjustable, which are discussed below.
Single-element ultrasound transducers have a fixed focal depth determined by the design of the transducer either with a lens in front of the element or a concave piezoelectric element with the transmitted ultrasound beam focusing to a fixed focal point. These types of transducers were present on the earliest ultrasound machines but are rare today and have the disadvantage of only have one focal distance.
Modern ultrasound machines use two main techniques to focus the image:
- this occurs by adding a time delay to the firing of each of the piezoelectric elements
- the outer most elements are fired first with the center-most element fired last
- the ultrasound pulses constructively interact to create a composite pulse which converges at the focal point
- the focal depth is determined by the time delay between these pulses
- greater focal depths are achieved by reducing the difference in the time delay between the elements resulting in more beam divergence and greater depths
- shallower focal depths increase the difference in the time delay between the elements - both linear and array probes may use this
dynamic receiving focusing:
- echoes received at the outer most elements of the array travel a longer distance than those at the center of the array hence re-phasing is needed to prevent a loss of resolution
- dynamic receiving focusing re-phases the signal by introducing electronic delays as a function of depth
- a smaller time delay is needed for echoes returning from a greater depth and a larger time delay is needed for echoes returning from a shallower depth
- this technique is used in phased arrays but many linear arrays also allow this
So-called 1.5D transducers (with an additional 5-7 rows of elements in the vertical direction) allows for transmitting and receiving focusing in this direction leading to improvement of elevational resolution also.
Some systems allow for multiple focal zones, however with the present technologies for this, there are trade offs in terms of temporal resolution 3.