Discover the EACVI Textbook of Echocardiography 2nd edition
4 Chapter 1 general principles of echocardiography Table 1.1 Currently used frequency range for cardiac imaging applications (including paediatric) Transthoracic Transoesophageal Intracardiac Epicardial Intracoronary 1–8 MHz 3–10 MHz 3–10 MHz 4–12 MHz 10–20 MHz Reflection and transmission of ultrasound at interfaces Travelling through tissues, the ultrasound encounters interfaces where acoustic properties change, influencing propagation. Propagation depends also on the angle of incidence (insonation angle) with the interface. Encountering an interface, the ultrasound partially returns towards the source and is partially transmitted (% Fig. 1.2). At a smooth and large interface the ultrasound obeys rules of specular reflection, returning towards the source with direction angle equal to the angle of incidence. Every medium has specific acoustic impedance (density × propagation speed, measured in Rayls). The impedance difference between media at an interface— acoustic impedance mismatch—influences the return signal ratio. Higher mismatch enhances reflection and lower mismatch enhances transmission. The high mismatch at air/soft tissue interfaces explains the need for using ultrasound gel as a coupling medium during examination. At a rough interface or when encountering small structures (with dimensions in the range of the wavelength) the ultrasound suffers scattering, returning towards the source and being transmitted in all directions (% Fig. 1.2). The proportion of ultrasound returning to the source (backscatter) is independent of the insonation angle. Scatter reflections allow the generation of an image of the examined structures instead of generating a mirror (specular) image of the transducer. Amplitude and intensity drop as ultrasound travels through tissues, a phenomenon named attenuation (measured in decibels, dB) and due predominantly to absorption but also to reflection and scattering (% Fig. 1.2). Attenuation increases with travelled distance and with ultrasound frequency. It depends on a specific attenuation coefficient—assumed as being constant in soft tissues (0.5 dB/cm/MHz). For image formation the ultrasound travels from and to the source, which doubles attenuation—1 dB/cm/MHz per centimetre of depth. Characteristic of an ultrasound wave Number of cycles per second = frequency Cycle Amplitude Pressure Amplitude Time Period (sec) High pressure Pressure Low pressure Distance Wavelength (mm) Baseline Compression Rarefaction Compression Baseline Direction of wave propagation Fig. 1.1 Characteristics of an ultrasound wave. Each cycle of complete pressure variation occurs over a certain length of time (period—measured in time units) and also occurs over a certain length of space (wavelength—measured in distance units, usually millimetres). The frequency is the number of complete cycles occurring in the unit of time, measured in hertz (one cycle per second). The maximal pressure variation above or below baseline represents the sound wave amplitude, measured in pressure units—megapascals (MPa) for ultrasound. As the wave travels, the encountered media particles are displaced, resulting in compression of the medium (increased particle density) corresponding to the high-pressure wave travelling past and rarefaction of the medium (decreased particles density) corresponding to low acoustic pressure.
transducers 5 Transmission 10 Reflection returns to transducer Destructive Interference 5 Point Scatterer Constructive Interference Specular Refraction Specular Refraction Back Scatterer Fig. 1.2 Specular reflection: the reflected ultrasound returns to the source in cases of perpendicular incidence, but does not return to the source in cases of an oblique incidence. Transmission of ultrasound continues in the same direction in cases of perpendicular incidence or occurs with a change in direction—refraction—in cases of oblique incidence. Scatter reflection: the backscatter is higher with higher ultrasound frequency and depends on scatterer size. A point scatterer sends ultrasound homogenously in all directions. The backscatter from the multitude of scatterers encountered by the ultrasound wave interfere enhancing (constructive interference) or neutralizing each other (destructive interference). This explains why the image of tissues contains speckles and apparent free spaces instead of having homogeneous appearance. bandwidth providing simultaneously high resolution and penetration (pure wave crystals, single crystal technology, etc.). The transducer has a housing case for the piezoelectric element. Elements backed with damping material stop backwards propagation of waves towards the case walls, to prevent interference with waves returning from examined structures (ring-down artefact). Damping restricts the number of oscillations per pulse (not necessary for continuous wave Doppler-only transducers) and their amplitude. Shorter pulse duration improves axial resolution and increases the range of generated frequencies. The generated ultrasound frequency is determined by the electric pulse frequency. The nominal frequency of the transducer depends on the resonance frequency of the piezoelectric element, generated when the element thickness is half the wavelength (only tenths of millimetres, explaining why elements break easily if we drop or bang the transducer). Damping and electrical current modulation help generate a broad band of frequencies around the nominal frequency. The piezoelectric element has much higher impedance than the tissues; direct contact would create a highly reflective interface. Materials with intermediate impedance are used to cover the element in one or more matching layers. The transducer generates a main ultrasound beam, which creates diagnostic images, and accessory beams (side lobes) which can also create images (artefact). The beam of a basic single discshaped element transducer has progressively increasing width due to diffraction (deviation from initial direction) after a natural focus point, separating the beam in two zones. The narrow beam of the near zone (Fresnel field) offers better lateral resolution. Its length is proportional with the ultrasound frequency and the transducer aperture dimensions. The wide beam of the far zone (Fraunhofer field) offers reduced image resolution. Transducers Transducer construction and characteristics The transducers are both generators and receivers of ultrasound waves used to create echocardiographic images, based on the piezoelectric effect (the conversion of electricity in ultrasound waves and the reverse, the conversion of ultrasound waves in electricity). The main component of the transducer is a piezoelectric element, which generates ultrasound waves as a result of electricity-induced deformation and generates electricity as a result of the returning ultrasound wave-induced deformation. The piezoelectric properties are natural or induced by heating the material up to a specific temperature—Curie temperature—at which the molecules align in a strong electric field. Modern transducers use manufactured piezoelectric materials. Crystals, ceramics, polymers, and composites with a range of electromechanical coupling efficiencies (ideally high) and acoustic impedances (ideally low) have been used over the years. The piezoelectric properties are lost with heating, a reason why heat-based transducer sterilization is inappropriate. Current transducers use new-generation crystals with high Transducer types The first transducers were single-element transducers, which could generate and receive ultrasound waves along a single line, resulting in A-mode and M-mode imaging (for explanation of A-mode and M-mode imaging see % ‘Ultrasound imaging principles’). The grey-scale M-mode had a high frame rate (2000–5000 frames/s), limited only by the time necessary for the waves to return from examined structures. Focusing was achieved with a concave lens or by giving curved shape to composite piezoelectric elements. Two dimensional (2D) imaging emerged from development of sector scanning necessitating beam steering—sweeping across the imaging plane. Steering prolonged imaging time, restricting frame rate, compared with single-line scanning. Mechanical steering was used first. The mechanical transducer had a small footprint though was inappropriate for Doppler imaging advances. Electronic steering was a breakthrough in the development of transducers (% Fig. 1.3a). It enabled parallel 2D and Doppler imaging advances, and in refined versions it enabled development of three dimensional (3D) imaging. Sequential firing of elements is also used for electronic focusing (% Fig. 1.3b).