EACVI Echocardiography Textbook - sample

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.