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Doppler echocardiography as applied in clinical practice, as well as more advanced applications, such as tissue Doppler techniques, myocardial strain imaging, and three-dimensional echocardiography Physical Principles of Ultrasonic Imaging This chapter includes a limited introduction to the physical principles of ultrasound because some background knowledge is relevant for optimization of images in clinical practice Those interested in more details on the physics of imaging should consult more comprehensive reviews or specialized textbooks of echocardiography.1 Physical Properties of Ultrasound An acoustic wave is a mechanical wave causing local compression and refraction while traveling through a medium Ultrasonic waves are longitudinal sound waves with a frequency greater than 20 kHz, the highest frequency that can be detected by the human ear The waves are generated and detected by a piezoelectric crystal, which deforms under the influence of an electrical field The velocity of a sound wave is dependent upon the density and stiffness of the medium Stiffness is the hardness or resistance of the material to compression Density is the concentration of matter Increase in stiffness also increases speed, whereas an increase in density decreases speed Transmission of ultrasonic waves is slow in air or gasses but fast in solids In the body tissues, the velocity of sound is relatively constant, at 1540 m/s The frequency represents the numbers of cycles occurring in each second of time and is expressed in hertz (Hz), with 1 Hz corresponding to one cycle per second The wavelength corresponds to the length over space over which one cycle occurs The amplitude reflects the strength divided by the intensity of a soundwave and is expressed in decibels The velocity of transmission, the frequency, and the wavelength are related by the formula c = f × λ, where c is the speed of sound through the medium, f is the frequency of the wave, and λ is the wavelength The range of frequencies used for medical imaging is from 2 to 12 megahertz (mHz) Corresponding wavelengths are in the range 0.8 to 0.13 mm Wavelength is directly related to spatial resolution because two structures need to be separated by more than one wavelength to be resolved and imaged as two separate structures This relationship explains why a higher-frequency probe has a better spatial resolution compared with a lower-frequency probe When sound travels through a heterogeneous medium, different interactions occur, such as reflection, attenuation, refraction, and scattering Reflection occurs at a boundary or interface between two mediums having a different acoustic density The difference in acoustic impedance between the two tissues causes reflection of the sound wave in the direction of the transducer Reflection from a smooth or specular interface between tissues causes the sound wave to return to the transducer Irregular interfaces will cause scatter in different directions The ultrasonic image is created based on the reflected waves as received by the transducer Refraction is the phenomenon that the ultrasound wave, which passes through the tissue, is refracted based on the incidence of the beam The incidence is oblique when the direction of the sound beam is not at a right angle to the boundary of the two mediums This same phenomenon explains why a straight pencil that sits in a glass of water appears to have a bend in it Attenuation is the loss of sonic energy as sound propagates through a medium It is produced by the absorption of the ultrasonic energy by conversion to heat, as well as by reflection and scattering The deeper the wave travels in the body, the weaker it becomes The amplitude and strength of the wave decrease with increasing depth Overall attenuation is dependent upon the frequency, such that ultrasound with lower frequencies penetrates more deeply into the body than ultrasound with higher frequencies The shorter the wavelength (i.e., the higher the frequency), the faster the particle motion and the larger the viscous effects Higher-frequency waves will thus attenuate more and penetrate less deep into the tissue Probes with lower frequencies produce better penetration but, as mentioned previously, have lower spatial resolution Probes with higher frequencies have lower penetration but higher spatial resolution For pediatric echocardiography, high spatial resolution is generally required to visualize small cardiac structures; therefore imaging with the highest-possible frequency probe is recommended Attenuation also depends on acoustic impedance and mismatch in impedance between adjacent structures Because air has a very high acoustic impedance, any air between the transducer and the cardiac structures of interest results in substantial attenuation of the signal produced This is avoided during transthoracic examinations by use of water-soluble gel to form an airless contact between the transducer and the skin The air-filled lungs are avoided by careful positioning of the patient and by the use of acoustic windows that allow access of the ultrasonic beam to the cardiac structures without the need to pass through intervening lung tissue Attenuation causes decrease in amplitude as the wave passes through the tissue In most ultrasonic systems, this is corrected for by automatic compensation Further manual compensation in terms of gain in depth ... mentioned previously, have lower spatial resolution Probes with higher frequencies have lower penetration but higher spatial resolution For pediatric echocardiography, high spatial resolution is generally required to visualize small cardiac structures; therefore imaging with the highest-possible frequency probe