Discover the EACVI Textbook of Echocardiography 2nd edition
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).
iological effects of ultrasound and safety 11 (a) (b) Fig. 1.10 Mirror image and refraction artefacts. (a) Mirror image artefact: a second image (reflection) of the same structure is formed behind a strong reflector. The second image is weaker than the first. In the example, effort was made to enhance the mirror image for illustration purposes. A whole second parasternal long axis view of the heart is formed behind a highly reflective pericardium. (b) Refraction artefact: a second image of the same object positioned lateral to the real image is formed by the refracted waves. In the example (left image), a whole second interventricular septum appears to be present. By changing settings (increasing frequency and changing technique—harmonic imaging instead of pulse inversion) and slightly angulating the transducer, the artefact was resolved (right image). Biological effects of ultrasound and safety Ultrasound interacts with tissues, being a compression wave (MI-dependent mechanical effects) and having its energy absorbed (thermal index (TI)-dependent heating effects). The exposure is proportional with pressure amplitude, power, and intensity. We increase it by increasing power, MI, and with focusing. The spatial peak-temporal average intensity (I SPTA ) is used to describe exposure to intermittent, damped, and attenuated ultrasound pulses. I SPTA is the average temporal intensity at the point of maximum intensity in space. I SPTA is higher for Doppler imaging. Biological effects are investigated with cell cultures and plant and animal experiments. There is no evidence of risk; nevertheless rationalization is advised. Manufacturers have to comply with safety requirements for instruments output, by restricting and displaying the MI and TI (index value 1). Final responsibility remains with the operator. (a) (b) (c) Fig. 1.11 Reverberation artefacts. (a) Reverberation artefact: reverberations are multiple reflections produced by a strong reflector, giving a series of gradually weaker parallel images of the same structure behind the first. When they consist of a series of short lines near each other, they can take the form of a comet tail. In the example, in 2D, the prosthetic mitral valve creates reverberations which almost take the form of two comet tails. (b) Reverberation artefact: reverberation artefact is generated in 3D imaging as well. They can be bigger and more obscuring of normal structures than in 2D. In the example, the 3D image belongs to the same patient, during the same study as (a). It is possible to observe a multiple parallel disc-like appearance of the reverberation with tornado shape. (c) Reverberation artefact: this is another example of 3D reverberation artefact from mechanical mitral valve prosthesis, giving multiple parallel plate-like appearance when cropped and displayed to be seen from two angles.