Ultrasound Physics

Ultrasound Array Transducer
  • Linear array: In a linear array transducer, all the elements form a line. The element usually as a rectangle shape with width in the array direction or lateral direction, and height in elevation direction. The center to center distance of the two adjacent elements is defined as pitch size and gap between them is called kerf. Normally the pitch size is required to be smaller than 1 wavelength, but some cases it can be 1.25 wavelength.  The kerf is normally around 50 microns depends on the blade thickness to cut the elements. Linear array is widely used when acoustic window is big enough, such as abdominal or limb vessel scan. In a linear array, each time only a group of elements work together to transmit or receive, i.e. the aperture size is smaller than he transducer active surface. The ultrasound beam is perpendicular to the transducer surface, and scan a rectangle area.
  • Phased Linear Array: it is exactly the same as linear array in term of element arrangement. For phased linear array, the element pitch size is required to be smaller than half of the wavelength. In a phased array, all the elements work together, i. e. the aperture are all the elements, and the aperture size is the whole transducer active surface. Phase array steer the beam by apply different delay on each element, and it requires small acoustic window. It is widely used in cardiovascular scan where the rib gap is the small acoustic window.
  • Curved array: Curved array is very similar to the linear array. All the elements form a line, but it is curved and most likely is convex. Compared to the linear array, it gives a bigger scan area when acoustic window is limited.
  • Annular array: An annular array consists concentric rings elements with the center one having a round shape. Annular array elements work together and normally have time delay to achieve focused beam. Due to its special geometry shape, annular give best focused beam with focal depth adjustable electronically. Most annular array have equal area elements to keep impedance of each the same.
  • Circular array: a circular array has all the elements form a circle, facing one side.
  • 1.25D 1.5D 1.75D array: in linear array, either linear scan or phased scan, there is only one element in elevation direction, i.e. the direction perpendicular to the scan direction and depth. The focal depth cannot be changed in this direction also it can be dynamically changed in lateral direction. To improve this capability, the element is divided into several pieces in elevation direction, with the first one, or the primary one is bigger in size. Depends on the number of pieces and the size, it is called 1.25D. 1.5D, or 1.75D. It partially improves the elevation focus property.
  • 2D array: in 2D array, the element forms a M X N matrix. Beam can be steered in all the directions and thus the transducer is capable of scanning a volume, generating echo information for a 3D image. A 64 x 64 element array will require more than 3600 channels for beamforming, and it increase the cost of the imaging system greatly.