Page 272 - Biomedical Engineering and Design Handbook Volume 2, Applications

250 DIAGNOSTIC EQUIPMENT DESIGN

receive processing used to create this beam is referred to as beam formation. Due to its central
role in ultrasonic imaging, beam formation will be discussed in detail later on.
The strength of the received echoes is usually displayed as increased brightness on the screen (hence
the name for the basic ultrasonic imaging mode, B-mode, with B for brightness). A two-dimensional data
set is acquired as the transmitted beam is steered or its point of origin is moved to different locations on
the transducer face. The data set that is acquired in this manner will have some set of orientations of the
acoustic rays. The process of interpolating this data set to form a TV raster image is usually referred to as
scan conversion. With Doppler signal processing, mean Doppler shifts (which correspond to velocities of
the scatterers such as red blood cells) at each position in the image can be determined from as few as 4 to
12 repeated transmissions. The magnitudes of these mean frequencies can be displayed in color superim-
posed on the B-mode image and can be used to show areas with significant blood flow.

9.2.2 Physical Constants and Typical System Operating Parameters
It may be useful to consider typical system operating parameters. The following table lists some
physical constants that help define the operating parameters of today’s systems:
Typical attenuation in tissue 0.5 dB/cm ⋅ MHz for one-way travel
Speed of sound in tissue 1540 m/s (or roughly 13 μs/cm for round-trip travel)
One of the challenges of ultrasonic imaging relates to that very high attenuation. To put this in numerical
terms, a typical 5-MHz transducer is expected to penetrate approximately 10 cm. Thus, the signal leaving
the transmitting transducer will undergo attenuation in the order of 25 dB before it reaches a scattering
site. At that point, a small fraction of the energy will be reflected, let us say the reflected echo will be
another 30 dB down; and the return will bring about another 25 dB of attenuation. Thus, the entire atten-
uation has been about 80 dB. A typical ultrasound image will contain some 40 to 50 dB of dynamic range;
hence, needless to say, there is a strong need for careful low-noise designs for ultrasound front ends.
The following table gives some system design parameters commonly used for B-mode imaging:
Transducer frequencies 2–15 MHz
Transducer bandwidth 50–90% fractional bandwidth
Typical depths of penetration 18 cm (abdominal imaging)
16 cm (cardiac imaging)
5 cm (small parts and peripheral vascular imaging)
Time to acquire one 20-cm acoustic ray ~260 μs
Pulse repetition frequency (PRF) 4 kHz
Typical number of rays in an image 128–400
Data acquisition frame rates 10–80 frames/s
Due to frequency-dependent attenuation, applications with greater penetration requirements use the
lower transducer frequencies. The instrument parameters have been selected or have evolved to their cur-
rent values as manufacturers have optimized their instruments to the needs of the various clinical appli-
cations. Given compromises that have to be made, resolution of scanners is been limited to roughly 0.4
mm with 7- to 12-MHz transducers to approximately 2 to 4 mm with the 2.5- to 3.5-MHz transducers.
The penetration at which these resolutions can be achieved is approximately 4 cm for the higher fre-
quencies and 15 cm or more for the lower frequencies. Whether or not this performance can be achieved
on any given patient is dependent on factors such as uniformity of speed of sound, which is highly patient
dependent (O’Donnell, 1988). The degree of and correction for sound speed variations in ultrasound sys-
tems continues to receive much attention (Fink, 1992; Flax, 1988; Li, 1995; Nock, 1988; O’Donnell,
1988; Trahey, 1988; Zhu, 1993a, 1993b; Krishnan, 1996; Rigby, 2000; Silverstein, 2001).
9.2.3 Clinical Applications
B-mode imaging has found numerous uses in today’s clinic (Goldberg, 1990; Sarti, 1987). Some of
these are
Abdominal imaging Identification of tumors, cysts in liver, kidneys
Cardiology Valvular insufficiency (flail leaflet), myocardial dyskinesis,
septal defects, congenital malformations