Discover the EACVI Textbook of Echocardiography 2nd edition
6 Chapter 1 general principles of echocardiography t 1 t 1 t 2 >t 1 t 2 >t 1 t 3 >t 2 t 3 >t 2 >t 1 t 4>t 3 t 2 >t 1 t 5 >t 4 t 1 (a) (b) Fig. 1.3 Electronic steering and focusing. (a) Electronic steering: the phased-array transducers consist of a series of rectangular piezoelectric elements sequentially fired (see activation time delays t 1 < t 2 < t 3 < t 4 < t 5 ) in order to obtain ultrasound beam steering. (b) Electronic focusing: see sequence of elements activation (t 1 < t 2 < t 3 ) used for electronic focusing. Transmitted ultrasound beam focusing changes the length of the near zone by adjusting the depth of focus, to include the examined structure and improve resolution. The beam width is reduced to minimum in the focal area and diverges immediately after. The focal zone has maximum intensity and provides maximal lateral resolution. Electronic aperture variation, allowing preservation of focal beam width for a range of focus depths, is obtained by changing the fired elements number. The conventional phased-array transducer is the one-dimensional (1D) transducer (% Fig. 1.4). Matrix array transducers are currently used (% Fig. 1.5). The evolution of the matrix array transducers established 3D imaging (% Figs. 1.6 and 1.7). The received beam has electronic steering and focusing as well. Current transducers have multiple transmission focusing and dynamic reception focusing, improving resolution throughout the image depth. Dynamic reception focusing needs dynamic aperture to preserve focal beam width. Fig. 1.4 Conventional phased-array transducer. The 1D transducer consists of one row of piezoelectric elements aligned in the imaging plane. This allows electronic focusing only in the imaging plane, adjusting the beam width responsible for lateral resolution. See the accessory beams (side lobes) illustrated in red. Fig. 1.5 Matrix array transducer. In transducer development, the 1.5D and the 2D transducers were created by adding rows of elements in the elevation plane. This allows electronic focusing in the elevation plane as well, adjusting the beam thickness responsible for the tomographic slice thickness. The 1.5D transducer has fewer elements rows in the elevation plane and has shared electric wiring for pairs of added elements. The 2D arrays have more elements rows with separate electric wiring. Transducer advances were based on the ability to cut the elements very small, to isolate them, to fit their electric wiring in the case, and to command complex firing algorithms. The first transducers for 2D and then 3D imaging were sparse-arrays, concomitantly using only part of their elements. Full matrix array transducers are currently used. Side lobes of constituent elements add up in grating lobes of the array (illustrated in red in the image). They can be reduced by firing peripheral elements at lower amplitude—apodization—with continuous variation—dynamic apodization. They can also be reduced by cutting subelements within elements.
ultrasound imaging principles 7 Complex transmission and reception focusing and the use of short intermittent pulses with multiple frequencies and damping make the current actual ultrasound beam difficult to illustrate. Presented images (% Figs. 1.4–1.7) represent the beam pathway, with the transmitted beam fading away due to attenuation. Transducer selection Transducer selection is less of an issue with current broad-band transducers and equipment features allowing fine tuning in image optimization. A wide range of patients is scanned without changing transducer. Technology evolution resulted in a decrease in transducer size and footprint (aperture). Smaller transducers need a smaller acoustic window and are easier to handle. Aperture size reduction is limited by concomitant drop in near-field length. Higher frequency is needed for better resolution imaging of shallow structures (e.g. heart in children and cardiac apex or coronaries in adults). Lower frequency is needed for better penetration to deeper structures (e.g. large size adults). There is a penetration– spatial resolution trade-off (for details of transducer selection, see % ‘Principles of imaging’ in Chapter 2). Fig. 1.6 Real-time 3D imaging transducer beam. Currently, full matrix array transducers with more than 2000 elements enable second-generation realtime 3D imaging, concomitantly collecting information in sagittal, coronal, and transverse planes with a pyramidal ultrasound beam. The first 3D images were reconstructed from a series of 2D images acquired with free hand scanning or mechanically driven transducers. Later, sparse-array matrix transducers with 256 elements made possible the first-generation real-time 3D transthoracic imaging with poor spatial resolution. Ultrasound imaging principles Echocardiography images represent the display of returning ultrasound waves from examined structures, with location determined by their travelling time. Each returning ultrasound wave generates an electrical radiofrequency signal. Fig. 1.7 Use of 3D imaging transducer for multiplane imaging. The 3D imaging transducer can acquire and display two or three concomitant 2D tomographic images from the same cardiac cycle, using rows of transducers generating beams in two or three imaging planes. Imaging modes (advantages and limitations) The A-mode represents a display of received signal amplitude from each depth along a scan line. The amplitude of specular reflections is much higher than that of scatter reflections, giving the signal a large dynamic range. Amplitude transformation in grey-scale display creates B-mode imaging—based on signal brightness. The electrical signal amplitude is analogue—proportional with the received wave pressure. Analogue to digital (A/D) conversion designates amplitude a number (from 0 to 255). The grey-scale display results from representing low amplitudes in black (0), high amplitudes in white (255), and intermediate amplitudes in shades of grey. The weak scatter reflections would be almost lost at conversion without logarithmic compression of amplitude numeric values which restricts the dynamic range. The M-mode—motion imaging mode—is a grey-scale display of amplitude from each depth along a scan line over time, with high temporal resolution. The 2D image is a grey-scale display of amplitude over depth information from several scan lines in an imaging plane. The result is a real-time tomographic section providing spatial resolution at the expense of temporal resolution. The 3D image is a colourized-scale display of amplitude over depth from several imaging planes in a pyramidal volume. The