Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Ultrasound Physics for the Clinician
Introduction
Since its introduction to the medical community in the 1950s, ultrasound (US) has become a widespread imaging modality because of its portability, nonionizing energy, and relatively low cost. This chapter reviews basic physics concepts behind the formation of an US image, emphasizing conventional B-mode imaging. Other approaches, including US-based elasticity assessment, Doppler, and three-dimensional imaging, as well as magnetic resonance imaging, are discussed in other chapters.
Components of an Imaging System
In an imaging system, a source produces a stimulus that interacts with the object to be imaged. That interaction produces a response within the object, which is sensed by a detector, and transformed into a signal that an image generator uses to produce the image.
The Stimulus
In a clinical US system, the stimulus is a US pulse. This is a short disturbance (about one-millionth of a second) that progressively moves from the source (the US transducer, described subsequently) through the object (the human body). This disturbance consists of a sound wave, which is a series of pressure increments and reductions that cause small volumes (approx. 0.1 mm 3 ) of tissue to sequentially shrink (compression) and expand (decompression, also called rarefaction). One compression followed by one rarefaction is called a cycle, and a typical US pulse might contain between one and four cycles.
The amplitude of the wave can be described by the maximum compression achieved by the pulse. The frequency is the number of compression and decompression cycles that a small volume experiences per second. The unit “one cycle per second” is called a Hertz (Hz). Humans are able to hear sound waves with frequencies between about 20 Hz and 20 kHz. The prefix ultra - refers to sound waves with frequency beyond the audible range, i.e., higher than 20 kHz. Frequencies in diagnostic obstetric US span roughly 1 to 10 MHz.
The volume occupied by the US pulse (pulse size) is about 0.25 mm 3 and is defined in three dimensions: the axial length, the lateral width, and the elevational width (crosshairs in Fig. 169.1 ). The axial dimension is parallel to the propagation direction of the pulse and is called the pulse length, i.e., the extent occupied by adjacent compression/decompression zones within the pulse (about 0.5 mm). The lateral and elevational dimensions are perpendicular to the propagation direction—one within and one outside of the image plane, respectively. These dimensions are on the order of 1 mm and vary with the distance of the US pulse from the source, with the US beam geometry having an hourglass shape (see dashed blue lines in Fig. 169.1 ) determined by the size of the source (see red area in the transducer in Fig. 169.1 ). The depth at which the lateral and elevational dimensions are the smallest is the focus (see green mark in Fig. 169.1 ).
You're Reading a Preview
Become a Clinical Tree membership for Full access and enjoy Unlimited articles
If you are a member. Log in here