The milestone in the history of three-dimensional echocardiography (3DE) has been the development of fully-sampled matrix array transthoracic transducers based on advanced digital processing and improved image formation algorithms which allowed the operators to obtain on cart, with short acquisition time, higher spatial and temporal resolution for transthoracic (TTE) real-time volumetric imaging. Further technological developments (i.e. advances in miaturization of the electronics and in element interconnection technology) made possible to insert a full matrix array into the tip of a transesophageal (TOE) probe and provide TOE real-time volumetric imaging.
2D phased array beamforming
A conventional 2D phased array transducer (link) is composed by multiple piezoelectric elements, electrically isolated from each other, arranged in a single row. Individual ultrasound wave fronts are generated by firing individual elements in a specific sequence with a delay in phase with respect to the transmit initiation time. Each element adds and subtracts pulses to generate a single ultrasound wave with a specific direction that constitutes a radially propagating scan line (Figure 1). The linear array can be steered in two dimensions (vertical (axial) and lateral (azimuthal), while resolution in the z axis (elevation) is fixed by the thickness of the tomographic slice, which, in turn, is related to the vertical dimension of piezoelectric elements.
Figure 1. Schematic drawing of beamforming using a conventional 2D phased array transducer. During transmission (left panel), focused beams of ultrasound are produced by pulsing each piezoelectric element with pre-calculated time delays (i.e. phasing).
During reception (right panel), focusing is achieved by applying selective delays at echo signals received by the different piezoelectric elements in order to create isophase signals that will be summed in a coherent way.
3DE matrix-array transducers
Currently, 3DE matrix-array transducers are composed of about 3000 independent piezoelectric elements with operating frequencies ranging from 2 to 4 MHz and 5 to 7 MHz for TTE and TOE, respectively. These piezoelectric elements are arranged in a matrix configuration within the transducer (Figure 2) in order to steer the ultrasound beam electronically. The electronically controlled phasic firing of the elements in that matrix generates a scan line that propagates radially and can be steered both laterally (azimuth) and in the elevation in order to acquire a volumetric pyramid of data. The main technological breakthrough which allowed manufacturers to develop the matrix transducers has been the miniaturization of electronics that, on one end, allowed the development of individual electrical interconnections for every piezoelectric require element which could be independently controlled, both in transmission and in reception, and, on the other end, the microbeamforming that allows the same size of the 2D cable to be used with 3D probes despite the large number of digital channels for these fully sampled elements to be connected.
Figure 2. Schematic drawing of a full matrix array transducer were about 3000 acoustically indepepndent piezoelectric elements are arranged in row and columns and used to steer the beam electronically. This matrix arrangement of piezoelectric elements allows their phasic firing to produce and ultrasound beam that can be steered in vertical (axial), lateral (azimuthal) and antero-posterior (elevation) directions in order to acquire a volumetric (pyramidal) data set.
Beamforming (link) is a general processing technique used to control the directionality of the reception or transmission of a signal on a transducer array. In 2D echocardiography, all the electronic components for the beamforming (high-voltage transmitters, low-noise receivers, analog-to-digital converter, digital ontrollers, digital delay lines) are in the system and consume a loto of power (around 100W, and 1,500 cm2 of personal computer electronics board area). IF the same beamforming approach would have been used for matrix array transducers used in 3D echocardiography, it would require around 4kW power consumption and a huge PC board area to accommodate all the needed electronics. To reduce both power consumption and the size of the connecting cable, several miniaturized circuit boards are incorporated into the transducer, allowing partial beamforming to be performed in the probe (Figure 3). The 3000 channel circuit boards within the transducer control the fine steering by delaying and summing signals within subsections of the matrix, known as patches. This allows to reduce the number of the digital channels to be put into the cable that connects the probe to the ultrasound system from 3000 to the conventional 125-256. Coarse steering is controlled by the ultrasound system where the analog-to-digital conversion occurs using digital delay lines (Figure 3)
Figure 3. Beamforming with 3D matrix array transducers. To save power, electronic circuitry needs (costs) and reduce the connection cable size the beamforming and steering processes have been splitted in two: the transducer and the ultrasound machine levels. The transducer contains about 3000 piezoelectric elements arranged in a matrix array, interconnection technology and integrated analog circuits (DELAY) to control transmit and receive signals using different subsection of the matrix (patches) to control analog pre-beamforming and fine steering. Signals from each patch are summed in order to reduce the number of digital lines in the coaxial cable that connects the transducer to the ultrasound system from 3000 to the conventional size of 128-256 channels. At the ultrasound machine level, analog-to-digital (A/D) convertors amplify, filter and digitize the elements signals. The resulting digital signals are focused (coarse steering) using digital delay (DELAY) circuitry and summed together (Ξ) to form the received signal from a desired object.
Latest developments in transducer technology
Additionally, developments in transducer technology have resulted in a reduced transducer footprint, improved side-lobe suppression, increased sensitivity and penetration, and the implementation of harmonic capabilities that can be used for both gray-scale and contrast imaging. The most recent generation of matrix transducers are significantly smaller than the previous ones and the quality of two-dimensional (2D) and 3D imaging has improved significantly, allowing a single transducer to acquire both 2D and 3DE studies, as well as of acquiring the whole left ventricular cavity in a single beat.
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