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EACVI Echocardiography Textbook - sample

Discover the EACVI Textbook of Echocardiography 2nd edition

8 Chapter 1 general

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. 10 5 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. V

ultrasound imaging principles 9 (a) (b) (c) (d) Fig. 1.9 Image quality optimization. (a) and (b) Harmonic imaging effect on image quality: see fundamental image (a) and harmonic image (b). Relying on the high-intensity central part of the beam, harmonic imaging improves lateral resolution and reduces lateral lobes artefacts. Relying on signal from higher depth, harmonic imaging reduces near-field artefact. Because attenuation of returning waves is higher for higher depth though, the mid field has the best image quality. Relying on longer ultrasound pulses, harmonic imaging reduces axial resolution. The pulse inversion harmonic imaging technique uses shorter pulses in pairs which cancel each other’s fundamental frequencies. This technique improves axial resolution but it reduces temporal resolution instead, having a lower frame rate. (c) Compress control effect: image (c) is image (b) with maximum compress. ‘Compression’ increase reduces heterogeneity of grey-scale image display, reducing differences in signal strength by amplifying more the weak signals. (d) Dynamic range control effect: image (d) is image (b) with maximum dynamic range. The ‘dynamic range’ is the range of signal strengths which can be processed—from the weaker detected signal to the stronger not inducing saturation signal. Increasing the dynamic range we increase the displayed number of shades of grey in the display; decreasing it we obtain a more black and white image. The major advantage of harmonic imaging resides in its capability to improve the signal-to-noise ratio. Image storage In older instruments, image storage was based on videorecording or paper printing and later on magneto-optical disk. Presently, image storage is based on digital acquisition of loops or freeze frames. Current instruments have large digital archiving memory, can write information on CD-ROMs and USB flash-drives, or send information to a computer workstation or an external database. Post-processing, analysis, and measurements can be performed on the machine or on the workstation after completing the scanning (offline). The studies are easy to retrieve for comparison at follow-up, for teaching, and for research. Digital memory stores numbers representing information. The image is divided into thousands of pixels forming a matrix. Each matrix gives a 2-bit memory—stores ‘black’ or ‘white’ for each pixel. More matrixes have to be used for detailed shading; for example, 8-bit memory stores 256 shades of grey. An even higher number of bits is used for current high-resolution displays. Images can be stored as raw data or Digital Imaging and Communication in Medicine (DICOM) data format, both suitable for subsequent measurements and analysis. Compressed files following standard protocols (e.g. AVI or MPEG for loops or JPEG for freeze frames) can be exported for use in presentations and publications, but are not suitable for subsequent measurements. Images can be communicated through a PACS (Picture Archiving and Communicating System). The image communication protocol for all manufacturers is standardized in DICOM format. Currently inter-vendor compatibility of DICOM ultrasound images is limited and advanced quantification options such as deformation imaging may require raw data for analysis. Further communication standardization within healthcare and with manufacturers is provided by HL7—Health Level Seven of the International Organization for Standardization (ISO). For example, HL7 provides echocardiography examination report coding systems. Image quality optimization Echocardiography instruments have controls which allow quality optimization before image storage in memory (pre-processing)

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