The reflected echo scatters in many different directions resulting in echoes of similar weak amplitudes. An ultrasound wave hitting a smooth mirror-like interface at a 90° angle will result in a perpendicular reflection. The next pulse-echo cycle can only be started after receiving the echo from the selected maximum depth. If the ultrasound wave hits a small object, a circular reflection wave is created. At the interface of tissues with different density, there is a partial or complete reflection of the ultrasound wave. This means that as the amplitude of a wave decreases (such as with deeper penetration), the energy carried by the wave reduces drastically, Defines as the energy (E) delivered per unit time (t). The degree of reflection is high for air because air has an extremely low acoustic impedance (0.0004) relative to other body tissues. It is responsible for both the production of ultrasound waves and, after a set period of time, the reception of waves reflected from the tissues. The near-field resolution is poor, sector array transducers have the smallest footprint and are used to for imaging deep structures. You can change your ad preferences anytime. (Reprinted with permission from Philip Peng Educational Series). If you continue browsing the site, you agree to the use of cookies on this website. To compensate for attenuation, it is possible to amplify the signal intensity of the returning echo. Reflection in biological tissues is not always specular. The bone also produces a strong reflection because its acoustic impedance is extremely high (7.8) relative to other body tissues. Medical imaging uses high-frequency waves (1–20 MHz), The speed of propagation of a sound wave through a medium (m/s). We use your LinkedIn profile and activity data to personalize ads and to show you more relevant ads. The returning wave is the crucial component in diagnostic ultrasound, which provides the information about the tissue structure. essentials of ultrasound physics 1e 9780815198529. output measurenients for medical ultrasound. For diagnostic ultrasound frequencies between 2–10 MHz (Megahertz) are used (Singer et al, 2006). kidney stone]. (Reprinted with permission from Philip Peng Educational Series), A longitudinal wave showing alternating compression and rarefaction. The transducer transforms the echo (mechanical energy) into an electrical signal which is processed and displayed as an image on the screen. The ultrasound waves are generated using the piezoelectric effect: a voltage change on a quartz crystal leads to the deformation of the crystal. Ultrasound: Medical Definition • Diagnostic Medical Ultrasound is the use of high frequency sound to aid in the diagnosis and treatment of patients. Its current importance can be judged by the fact that, of all the various kinds of diagnostic images produced in the world, 1 in 4 is an ultrasound scan. The lateral imaging resolution depends on the geometry of the transducer and is highest in the focal zone. The summation of all waves generated by the piezoelectric crystals forms the ultrasound beam. As the ultrasound wave travels through the tissues, it loses amplitude, and hence energy (attenuation), which is the summative effect of absorption, reflection, and refraction of ultrasound waves. This explains why it is difficult to visualize a needle inserted at a steep angle (>45° to the skin surface). Pulse repetition frequency (PRF) is the rate of pulses emitted by the transducer (number of pulses per unit time) (Fig. PRINCIPLES OF ULTRASONOGRAPHY Jerome A 2. The PRF for medical imaging devices ranges from 1 to 10 kHz, Anatomy of a transducer. Narrow shadow dorsal to the lateral boundary of a circular structure. Pulse repetition frequency. For this reason, it is clinically important to apply sufficient conducting gel (an acoustic coupling medium) on the transducer surface to eliminate any air pockets between the transducer and skin surface. Increasing the gain will amplify only the returning signal and not the transmit signal. Ultrasound waves can be reflected several times between two parallel interfaces, these additional reflection waves are computed according to the additional time to a greater depth. It is the product of its frequency (f) and wavelength (λ), The energy of a sound wave is proportional to the square of its amplitude (A). The interference of the scattered sound waves produces a grainy echo of actually homogeneous tissues (speckle-noise). The interaction of ulrasound waves with tissues can lead to disturbances of the image structure, which do not correspond to the tissue anatomy. Ultrasound pulses must be spaced with enough time between pulses to permit the sound to reach the target of interest and return to the transducer before the next pulse is generated. Testicular cyst with acoustic enhancement, with kind permission by Prof. Dr. R. Harzmann, Augsburg. 1.3). If the ultrasound wave hits nonperpendicular an interface of tissues with different density, the advancing non-reflected wave is deflected in its direction of propagation. The angle of the incidence is also a major determinant of reflection. The ultrasonic waves are attenuated by various mechanisms (reflection, scattering, interference and absorption). In soft tissue, 80% of the attenuation of the sound wave is caused by absorption resulting in heat production. An ultrasound wave is generated when an electric field is applied to an array of piezoelectric crystals located on the transducer surface. This is called pulsed ultrasound. Thus, it is the inverse of the period (T) of a wave. Strong reflecting interfaces may cause a specular reflection of the ultrasound wave leading to mirrow image. Dorsal echo free area behind strongly reflecting or absorbing interfaces (stone, bone, air) [fig.