Physics, Techniques and ProceduresUltrasonography
an imaging
modality using
ultrasound.
Short historical review
Man's practical use of
ultrasound had to await the discovery of the
piezoelectric crystal. In 1880, the brothers Jacques and Pierre Curie demonstrated the piezoelectric effect which makes possible the generation and detection of high-frequency pressure waves. The ability of a piezoelectric crystal to emit
ultrasound as a beam in a predetermined direction and to detect the echoes reflected from objects struck by the beam, was first exploited in World War I in the detection of enemy submarines. This technique was later developed into the well-known SONAR (sound navigation and ranging) system. The first published attempt to use
ultrasound for medical diagnosis did not appear until 1942, when K.T. Dussik tried to use transmitted
ultrasound through the intact skull to diagnose brain tumours. The attempt was, however, unsuccessful. In 1949, G.D. Ludwig and F.W. Struthers authored the first publication on the use of the pulse - echo technique for medical diagnostic imaging. By emitting the
ultrasound beam as short pulses into the human body, the same piezoelectric crystal could
act as both transmitter and receiver of
ultrasound, the duration and repetition rate of the emitted pulses being such that relevant echoes of one pulse were received before emission of the next one.
Physical principle
All ultrasonography is based on the
pulse echo method where an
ultrasound transducer transmits brief pulses of
ultrasound that propagate into the tissues. Each pulse travels in a narrow
ultrasound beam, the shape of which is determined by the dimensions of the transducer, the
ultrasound wavelength and the degree of mechanical or electronic
focusing. The propagational speed (
speed of sound) of the
ultrasound pulses is determined by the elasticity and density of the medium, and is nearly constant in the soft tissues of the body (approximately 1 540 m/s). Whenever there is a change in
acoustic impedance, some of the
ultrasound is reflected or backscattered to the transducer as echoes. The duration of each pulse is in the order of 1-2
ms, and the
pulse repetition frequency PRF is typically 1-5 kHz (1 000-5 000 pulses per second). Between pulse transmissions, i.e. approximately 99.7-99.9 % of the time, the transducer serves as a detector of the echoes. The time interval (t) from pulse transmission to reception of an echo is used to determine the transducer-to-reflector distance or range (
r):
r =
c t/2, where
c is the speed of sound (1 540 m/s). The factor 2 is included to account for the round trip distance, 2r.
The detected echoes may be displayed in one-dimensional formats such as A mode or M mode, but in radiology, the two-dimensional B mode format is used almost exclusively. The basic components of a B-mode ultrasound imaging system are shown in Fig.1 (left). The transducer transmits the ultrasound beam, which is swept through the region of interest by mechanic or electronic means. In electronic array scanning the transmitted ultrasound beam is electronically steered. The echoes are detected by the piezoelectric crystal of the transducer, where mechanical deformation of the crystal is converted into radiofrequency (RF) electronic signals (top, right). The electronic signals go through several steps of signal processing then stored in the scan converter memory, where an image is built up and retained during the scan. The vertical location of the signals in the image memory are determined by the echo return times, and the horizontal locations by the position of the beam axis (scan line) when the echoes were detected. The output from the image memory is fed through a digital-to-analogue converter (DAC) and finally to a monitor where the B-mode image is displayed. (Note: the electronic signals are shown with amplitude along the ordinate and time along the abscissa.)
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