EACVI Echocardiography Textbook - sample

8
Chapter 1 general principles of echocardiography
result is a real-time image with higher spatial resolution due to a
larger field of view, but even lower temporal resolution because of
prolonged image formation time. To overcome this issue, a ‘full
volume’ 3D data set is formed from stitching together subvolumes
acquired over two to seven sequential cardiac cycles (usually
four). This is a ‘near-real-time’ rather than a ‘real-time’ mode, but
it allows higher line density with relatively high frame rate (still
lower than the current 2D frame rate). ‘Full volume’ acquisition in
one cardiac cycle is also available.
Signal processing
The transducer should receive all returning waves from an
ultrasound pulse before generating a new pulse, to avoid range
ambiguity artefact (confining waves returning late from one pulse
in the same location with waves returning early from the next
pulse). Consequently, the number of pulses per second—pulse
repetition frequency (PRF)—depends on the maximum travelling
time which depends on the imaging depth. For every scan line, the
PRF determines the frame rate—the number of images generated
per second and stored in memory. The higher the depth, the lower
the PRF so the lower the frame rate. The higher the number of
scan lines, the lower the frame rate. Higher imaging sector width
needs more scan lines resulting in lower frame rate. The frame rate
determines the temporal resolution.
In current transducers, able to transmit and receive a range of
frequencies, a different PRF corresponds to each frequency:
◆ 5 MHz = >71.8 PRF
◆ 4 MHz = >60.9 PRF
◆ 3.3 MHz = >55.8 PRF.
From the image stored in the memory an image display is created
and presented. The number of images displayed per second
defines the refresh rate which is smaller or equal to the frame rate.
Both high frame rate and high refresh rate are needed for real-time
imaging (rapid display of images during scanning, presenting the
examined structures in motion).
In older echocardiography machines, image display was performed
in a cathode-ray tube and presented on a television screen
or, later, on a computer monitor. The stored digital information
was converted back to analogue voltages which induced a
proportional strength electron beam generating a proportional
brightness spot of light in the fluorescent tube. The spot moved
across the tube creating the image display.
In current echocardiography machines, both image display
and presentation can be performed on a flat-panel consisting of a
matrix of thousands of liquid crystal display (LCD) elements, acting
as electrically activated light valves which can create grey-scale
or colour images.
Harmonic imaging
Harmonic imaging substantially improves the image quality
in 2D and M-mode. In non-harmonic imaging (fundamental
imaging), the transducer receives only ultrasound signals from
cardiac structures of the same frequency as that of the emitted
ultrasound wave (fundamental frequency). In harmonic imaging,
the transducer receives ultrasound signals from cardiac structures
not only of fundamental frequency but also of higher frequency.
The ultrasound wave produces compression and rarefaction of
tissue particles (see % Fig. 1.1) and also oscillation of these particles
both at fundamental frequency and at higher frequency and
lower amplitude. These higher oscillation frequencies (harmonic
frequencies) are exact multiples of the fundamental frequency. The
harmonic amplitude is proportional with the square of the fundamental
amplitude. Harmonic imaging exploits this non-linear
response of tissue particles to the ultrasound beam.
The generation of harmonic frequencies is a cumulative
phenomenon, progressively augmenting with depth of tissue penetration.
However, it results in a reduction of axial resolution of
the ultrasound image.
Harmonics generation increases with administration of contrast
agents consisting of microbubbles with non-linear properties
which suffer volume resonant oscillations with pressure variations
(see % Chapter 8).
Harmonic imaging places great demands on the transducer. It
must be able both to generate low-frequency but high-amplitude
ultrasound waves and to receive high-frequency but low-amplitude
ultrasound waves. Broad bandwidth transducers have enabled the
use of harmonic frequencies (% Fig. 1.8) resulting in significant
image quality improvement (% Fig. 1.9a,b). The use of the second
harmonic (twice the fundamental frequency) minimizes artefact.
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Fig. 1.8 Principle of harmonic imaging. The propagation of ultrasound in
tissues is non-linear, faster during higher pressure because of higher media
particles compression. This results in progressive wave-shape change because
of added frequencies (harmonics) to the transducer-generated frequency
(fundamental). The phenomenon is more accentuated with higher ultrasound
intensity and at higher depth. Harmonic imaging is obtained by filtering
out the returning fundamental frequencies and receiving only the second
harmonic frequency.
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