EACVI Echocardiography Textbook - sample

10
Chapter 1 general principles of echocardiography
or after image retrieval from memory in order to be displayed
(post-processing). In other words, pre-processing determines the
quality of image formation, operating on the transmitted and
received ultrasound. Post-processing determines the quality of
image display on the screen, operating before display or even
offline, on stored images. The names of the controls are manufacturer
specific and so is their level of interference in image quality
and the ability to post-process offline-acquired images. Changes
and improvements are rapidly occurring. Understanding the
principles, we can use the specific controls of each instrument
accordingly.
The controls of the instrument can change the following:
1. Ultrasound frequency: we can select the appropriate frequency
(see % ‘Transducer selection’) for the examination performed
without changing transducer. The higher the frequency, the
higher the resolution but the lower the penetration. The lower
the frequency, the higher the penetration but the lower the
resolution.
2. Depth: we can increase depth to encompass all the structures
of interest; the higher the depth, the lower the PRF so the
lower the frame rate. We can also reduce depth in case the
structures of interest are near shallow to maximize their display
on the screen; the lower the depth, the higher the PRF so
the higher the frame rate (see % ‘Signal processing’).
3. Output power: we can change the amount of energy emitted by
the transducer. The output power is measured in percentages
of the maximum power or in decibels. Output power reduction
results in lower amplitude of the returning waves and
therefore weaker signal. Output power increase enhances the
amplitude of the returning signal, but excessive increase raises
concerns regarding biological effects (see % ‘Biological effects
of ultrasound and safety’).
4. Focus level: we can change the ultrasound beam focus level to
optimize resolution at a specific distance from the transducer.
Structures proximal to the focus level are better visualized.
5. Angle or sector width: we can change the area swept by the
ultrasound beam. Reducing the width, we reduce the beam
steering time and consequently the imaging time, achieving a
higher frame rate.
6. Tilt: we can orientate the image sector laterally, to facilitate
exploration of peripheral structures with better resolution
(using the axial resolution).
7. Gain: we can change the overall amplification of the electrical
signal induced by the returning ultrasound waves, in a similar
way as we can change the volume control in an audio system.
8. Time gain compensation (TGC): we can differentially adjust
the gain along the length of the ultrasound beam, to compensate
for the longer time taken by waves returning from
higher depth to reach the transducer. Owing to attenuation,
signals returning from progressively higher depth (later)
are weaker. The TGC provides a series of controls allowing
progressively higher amplification of signal returning from
progressively higher depth (later arriving) to compensate for
attenuation.
9. Lateral gain compensation (LGC): this control is used to allow
higher amplification of the weaker lateral signal on older
instruments. It is no longer needed due to improvements in
image quality.
10. Reject: we can set an appropriate strength threshold for a signal
to be detected, excluding weaker signals (noise).
11. Freeze: we can stop the moving heart display, during real-time
scanning or offline, to select a single frame of interest in order
to perform measurements or print.
12. Dynamic range and compress: see % ‘Imaging modes’ and
% Fig. 1.9c,d.
13. Edge enhancement: we can improve border delineation enabling
more accurate measurements and better visualization
of the endocardium for systolic function and regional wall
motion assessment.
A range of ready-made grey scale or colourized scale (B colour)
settings are also available for post-processing image optimization,
some with a better contrast resolution and some with a
more smooth appearance. The smooth appearance is obtained
with pixel interpolation and persistence. With pixel interpolation
smoothening is achieved by filling in the gaps with grey-scale pixels,
progressively more with higher depth because the scan lines
progressively diverge. Persistence makes moving images smooth
by adding frames which are an average of previous and next.
Colourization improves contrast resolution. Shades of orange are
widely popular for both 2D and 3D imaging.
Post-processing abilities are refined in 3D imaging, allowing
colourization, shading, smoothening, contrast resolution optimization,
and 3D adjustment of gain to improve the perception of
perspective. Post-processing includes rendering and cropping of
the full volume pyramid and changes of the angle of display.
Artefacts and pitfalls of imaging
Echocardiography can create images of structures in the wrong
place, distorted images (in size, shape, and brightness), images
of false structures, or it can miss structures in the shadow of
other structures. These artefacts are due to ultrasound physics or
operator interference. Some artefacts can be avoided by changing
transducer position/angulation or imaging settings/technique.
Artefacts are less frequent with current technology.
The near-field clutter is an artefact due to the high amplitude
of oscillations obscuring structures present in the near field. It is
reduced by harmonic imaging.
Artefact recognition is crucial for image interpretations. To
facilitate recognition, the artefacts have been illustrated with
examples and described in the figure captions (see % Figs 1.10–
1.14).