Sound is a physical phenomenon that transfers energy from one point to another. In this respect, it is similar to radiation. It differs from radiation, however, in that sound can pass only through matter and not through a vacuum as radiation can. This is because sound waves are actually vibrations passing through a material. If there is no material, nothing can vibrate and sound cannot exist.
One of the most
significant characteristics of sound is its frequency, which is the rate
at which the sound source and the material vibrate. The basic unit for
specifying frequency is the hertz, which is one vibration, or cycle, per
second. Pitch is a term commonly used as a synonym for frequency of sound.
The transducer is the component of the ultrasound imaging equipment that is placed in direct contact with the patient's body. It performs several functions as will be described in detail later. It's first function is to produce the ultrasound pulses when electrical pulses are applied to it. A short time later, when echo pulses return to the body surface they are picked up by the transducer and converted back into electrical pulses that are then processed by the system and formed into an image.
When a beam of ultrasound pulses is passed into a body, several things happen. Most of the ultrasound energy is absorbed and the beam is attenuated. This is undesirable and does not contribute to the formation of an image like in x-ray imaging. Some of the pulses will be reflected by internal body structures and send echoes back to the surface where they are collected by the transducer and used to form the image. Therefore, the general ultrasound image is a display of structures or reflecting surfaces in the body that produce echoes as illustrated below.
Echoes, which show up as bright or white spots in the image are produced by surfaces or boundaries between two different types of tissues. Most anatomical areas are composed of a "mixture" of different tissue types and many surfaces that produce the general gray and white background that we see in the image. Since there are no reflecting surfaces within a fluid, such as a cyst, it is dark in the image. Therefore, the general ultrasound image, sometimes called a "B mode" image, is a display of echo producing sites within the anatomical area.
The ultrasound image is a display showing the location of reflecting structures or echo sites within the body. The location of a reflecting structure (interface) in the horizontal direction is determined by the position of the beam. In the depth direction, it is determined by the time required for the pulse to travel to the reflecting site and for the echo pulse to return.
Another physical characteristic that can be imaged with ultrasound is motion, specifically the motion of flowing blood. This uses the Doppler principle and the images are usually displayed with different colors representing the different flow velocities and directions. This will be covered in a later chapter.
The basic functional components of an ultrasound imaging system are shown below.
Modern ultrasound systems use digital computer electronics to control most of the functions in the imaging process. Therefore, the boxes in the illustration above represent functions performed by the computer and other electronic circuits and not individual physical components.
We will now
consider some of these functions in more detail and how they contribute to
The transducer is the component of the ultrasound system that is placed in direct contact with the patient's body. It alternates between two major functions: (1) producing ultrasound pulses and (2) receiving or detecting the returning echoes. Within the transducer there are one or more piezoelectric elements. When an electrical pulse is applied to the piezoelectric element it vibrates and produces the ultrasound. Also, when the piezoelectric element is vibrated by the returning echo pulse it produces a pulse of electricity.
The transducer also focuses the beam of pulses to give it a specific size and shape at various depths within the body and also scans the beam over the anatomical area that is being imaged.
The pulse generator produces the electrical pulses that are applied to the transducer. For conventional ultrasound imaging the pulses are produced at a rate of approximately 1,000 pulses per second. NOTE: This is the pulse rate (pulses per second) and not the frequency which is the number of cycles or vibrations per second within each pulse. The principal control associated with the pulse generator is the size of the electrical pulses that can be used to change the intensity and energy of the ultrasound beam.
Amplification is used to increase the size of the electrical pulses coming from the transducer after an echo is received.. The amount of amplification is determined by the gain setting. The principal control associated with the amplifier is the time gain compensation (TGC), which allows the user to adjust the gain in relationship to the depth of echo sites within the body. This function will be considered in much more detail in the next section.
The scan generator controls the scanning of the ultrasound beam over the body section being imaged. This is usually done by controlling the sequence in which the electrical pulses are applied to the piezoelectric elements within the transducer. This is also considered in more detail later.
Scan conversion is the function that converts from the format of the scanning ultrasound beam into a digital image matrix format for processing and display.
The digital image is processed to produce the desired characteristics for display. This includes giving it specific contrast characteristics and reformatting the image if necessary.
The digital ultrasound images are viewed on the equipment display (monitor) and usually transferred to the physician display or work station.
One component of the ultrasound imaging system that is not shown is the digital storage device that is used to store images for later viewing if that process is used.
The basic principles of ultrasound pulse production
and transmission are illustrated below.
The Production of an Ultrasound Pulse
The source of sound is a vibrating object, the piezoelectric transducer element. Since the vibrating source is in contact with the tissue, it is caused to vibrate. The vibrations in the region of tissue next to the transducer are passed on to the adjacent tissue. This process continues, and the vibrations, or sound, is passed along from one region of tissue to another. The rate at which the tissue structures vibrate back and forth is the frequency of the sound. The rate at which the vibrations move through the tissue is the velocity of the sound.
The sound in
most diagnostic ultrasound systems is emitted in pulses rather than a
continuous stream of vibrations. At any instant, the vibrations are
contained within a relatively small volume of the material. It is this
volume of vibrating material that is referred to as the ultrasound pulse.
As the vibrations are passed from one region of material to another, the
ultrasound pulse, but not the material, moves away from the source.
Ultrasound pulses have several physical characteristics that should be considered by the user in order to adjust the imaging procedure for specific diagnostic applications. The most significant characteristics are illustrated here.
Characteristics of Ultrasound Pulses That Have an Effect on the Imaging
Ultrasound Pulse Frequency
The frequency of ultrasound pulses must
be carefully selected to provide a proper balance between image detail and
depth of penetration. In general, high frequency pulses produce higher
quality images but cannot penetrate very far into the body. These issues
will be discussed in greater detail later.
Factors Related to Ultrasound Pulse Velocity
The significance of ultrasound velocity is that it is used to determine the depth location of structures in the body. The velocity with which sound travels through a medium is determined by the characteristics of the material and not characteristics of the sound. The velocity of longitudinal sound waves in a liquid type medium like tissue is given by
where r is the density of the material, and
E is a
factor related to the elastic properties or "stiffness" of the material. The velocities
of sound through several materials of interest are given in the following
Most ultrasound systems are set up to determine distances using an assumed velocity of 1540 m/sec. This means that displayed depths will not be completely accurate in materials that produce other ultrasound velocities such as fat and fluid.
The distance sound travels during the period of one vibration is
known as the wavelength,
l. Although wavelength is not a unique property
of a given ultrasound pulse, it is of some significance because it
determines the size (length) of the ultrasound pulse. This has an effect
on image quality, as we will see later.
Wavelength (l) = v/f.
The Temporal and Length Characteristics of an Ultrasound Pulse
In ultrasound imaging the significance of wavelength is that short wavelengths are required to produce short pulses for good anatomical detail (in the depth direction) and this requires higher frequencies as illustrated below.
Dependence of Pulse Length on Wavelength and Frequency
The amplitude of an ultrasound pulse is the range of pressure excursions as below.
Ultrasound Pulse Amplitude, Intensity, and Energy
. The pressure is related to the degree of tissue
displacement caused by the vibration. The amplitude is related to the
energy content, or "loudness," of the ultrasound pulse. The
amplitude of the pulse as it leaves the transducer is generally determined by how
hard the crystal is "struck" by the electrical pulse.
Most systems have a control on the pulse generator that changes the size
of the electrical pulse and the ultrasound pulse amplitude. We designate
this as the intensity control, although different names are used by
various equipment manufacturers.
In diagnostic applications, it is usually necessary to know only the
relative amplitude of ultrasound pulses. For example, it is necessary to
know how much the amplitude, A, of a pulse decreases as it passes through
a given thickness of tissue. The relative amplitude of two ultrasound
pulses, or of one pulse after it has undergone an amplitude change, can be
expressed by means of a ratio as follows:
Relative amplitude (ratio) = A2/A1.
There are advantages in expressing relative pulse amplitude in terms of
the logarithm of the amplitude ratio. When this is done the relative
amplitude is specified in units of decibels (dB). The relative pulse
amplitude, in decibels, is related to the actual amplitude ratio by
Relative amplitude (dB) = 20 log A2/A1
When the amplitude ratio is greater than 1 (comparing a large pulse to a
smaller one), the relative pulse amplitude has a positive decibel value;
when the ratio is less than 1, the decibel value is negative. In other
words, if the amplitude of a pulse is increased by some means, it will
gain decibels, and if it is reduced, it will lose decibels.
illustration compares decibel values to pulse amplitude ratios and percent
values. The first two pulses differ in amplitude by 1 dB. In comparing the
second pulse to the first, this corresponds to an amplitude ratio of 0.89,
or a reduction of approximately 11%. If the pulse is reduced in amplitude
by another 11%, it will be 2 dB smaller than the original pulse. If the
pulse is once again reduced in amplitude by 11 % (of 79%), it will have an
amplitude ratio (with respect to the first pulse) of 0.71:1, or will be 3
Pulse Amplitudes Expressed in Decibels and Percentages
During its lifetime, an ultrasound pulse undergoes many reductions in amplitude as it passes through tissue because of absorption. If the amount of each reduction is known in decibels, the total reduction can be found by simply adding all of the decibel losses. This is much easier than multiplying the various amplitude ratios.
Power is the rate of energy transfer and is expressed in the units of
watts. Intensity is the rate at which power passes through a specified
area. It is the amount of power per unit area and is expressed in the
units of watts per square centimeter. Intensity is the rate at which
ultrasound energy is applied to a specific tissue location within the
patient's body. It is the quantity that must be considered with respect to
producing biological effects and safety. The intensity of most diagnostic
ultrasound beams at the transducer surface is on the order of a few
milliwatts per square centimeter.
Intensity is related to the pressure amplitude of the individual pulses
and the pulse rate. Since the pulse rate is fixed in most systems, the
intensity is determined by the pulse amplitude.
The relative intensity of two pulses (I1 and I2) can be expressed in the
units of decibels by:
Relative Intensity = 10 log I2/I1.
Note that when intensities are being considered, a factor of 10 appears in
the equation rather than a factor of 20, which is used for relative
amplitudes. This is because intensity is proportional to the square of the
pressure amplitude, which introduces a factor of 2 in the logarithmic
relationship. The intensity of an ultrasound beam is not constant with
respect to time nor uniform with respect to spatial area, as shown in the
following figure. This must be taken into consideration when describing
intensity. It must be determined if it is the peak intensity or the
average intensity that is being considered.
The Temporal and Spatial Characteristics of Ultrasound Pulses That Affect Intensity Values
above shows two sequential pulses. Two important time intervals are
the pulse duration and the pulse repetition period. The ratio of the pulse
duration to the pulse repetition period is the duty factor. The duty
factor is the fraction of time that an ultrasound pulse is actually being
produced. If the ultrasound is produced as a continuous wave (CW), the
duty factor will have a value of 1. Intensity and power are proportional
to the duty factor. Duty factors are relatively small, less than 0.01, for
most pulsed imaging applications.
With respect to time there are three possible power (intensity) values. One is the peak power, which is associated with the time of maximum pressure. Another is the average power within a pulse. The lowest value is the average power over the pulse repetition period for an extended time. This is related to the duty factor.
The energy or intensity is generally not distributed uniformly over the area of an ultrasound pulse. It can be expressed either as the peak intensity, which is often in the center of the pulse, for as the average intensity over a designated area.
There is some significance associated with each of the intensity
expressions. However, they are not all used to express the intensity with
respect to potential biological effects.
Three Types of Ultrasound Pulse Interactions Within a Body
As an ultrasound pulse passes through matter, such as human tissue, it interacts in several different ways. Some of these interactions are necessary to form an ultrasound image, whereas others absorb much of the ultrasound energy or produce artifacts and are generally undesirable in diagnostic examinations. The ability to conduct and interpret the results of an ultrasound examination depends on a thorough understanding of these ultrasound interactions.
The Reduction of Pulse Amplitude by Absorption of It's Energy
As the ultrasound pulse moves through matter, it continuously loses
energy. This is generally referred to as attenuation. Several factors
contribute to this reduction in energy. One of the most significant is the
absorption of the ultrasound energy by the material and its conversion
into heat. Ultrasound pulses lose energy continuously as they move through
matter. This is unlike x-ray photons, which lose energy in
"one-shot" photoelectric or Compton interactions. Scattering and
refraction interactions also remove some of the energy from the pulse and
contribute to its overall attenuation, but absorption is the most
The rate at which an ultrasound pulse is absorbed generally depends on two
factors: (1) the material through which it is passing, and (2) the
frequency of the ultrasound. The attenuation (absorption) rate is
specified in terms of an attenuation coefficient in the units of decibels
per centimeter. Since the attenuation in tissue increases with frequency,
it is necessary to specify the frequency when an attenuation rate is
given. The attenuation through a thickness of material, x, is given by:
Attenuation (dB) = (a) (f) (x)
where a is the attenuation coefficient (in decibels per centimeter at 1 MHz), and f is the ultrasound frequency, in megahertz.
Approximate values of the attenuation coefficient for various materials of interest are given in the following table.
From the attenuation coefficient values given in
the above table, it is apparent
that there is a considerable variation in attenuation rate from material
to material. The significance of these values is now considered. Of all
the materials listed, water produces by far the least attenuation. This
means that water is a very good conductor of ultrasound. Water within the
body, such as in cysts and the bladder, forms "windows" through
which underlying structures can be easily imaged. Most of the soft tissues
of the body have attenuation coefficient values of approximately 1 dB per
cm per MHz, with the exception of fat and muscle. Muscle has a range of
values that depends on the direction of the ultrasound with respect to the
muscle fibers. Lung has a much higher attenuation rate than either air or
soft tissue. This is because the small pockets of air in the alveoli are
very effective in scattering ultrasound energy. Because of this, the
normal lung structure is extremely difficult to penetrate with ultrasound.
Compared to the soft tissues of the body, bone has a relatively high
attenuation rate. Bone, in effect, shields some parts of the body against
easy access by ultrasound.
The following illustration shows the decrease in pulse amplitude as ultrasound passes through various materials found in the human body.
The Effect of Absorption on Ultrasound Pulse Amplitude in Relation to Distance or Depth in the Body
The reflection of ultrasound pulses by structures within the body is the
interaction that creates the ultrasound image. The reflection of an
ultrasound pulse occurs at the interface, or boundary, between two
dissimilar materials, as shown in the following figure. In order to form a
reflection interface, the two materials must differ in terms of a physical
characteristic known as acoustic impedance Z. Although the traditional
symbol for impedance, Z, is the same symbol used for atomic number, the
two quantities are in no way related. Acoustic impedance is a
characteristic of a material related to its density and elastic
properties. Since the velocity is related to the same material
characteristics, a relationship exists between tissue impedance and
ultrasound velocity. The relationship is such that the impedance, Z, is
the product of the velocity, v, and the material density, Y, which can be
Impedance = (Y) (v).
The Production of an Echo and Penetrating Pulse at a Tissue Interface
Reflection loss (dB) = 20 log (Z2 -
Z1)/(Z2 + Z1).
At most soft tissue interfaces, only a small fraction of the pulse is reflected. Therefore, the reflection process produces relatively weak echoes. At interfaces between soft tissue and materials such as bone, stones, and gas, strong reflections are produced. The reduction in pulse amplitude during reflection at several different interfaces is given in the following table.
The amplitude of a pulse is attenuated both by absorption and reflection losses. Because of this, an echo returning to the transducer is much smaller than the original pulse produced by the transducer.
When an ultrasound pulse passes through an interface at a relatively small angle (between the beam direction and interface surface), the penetrating pulse direction will be shifted by the refraction process. This can produce certain artifacts as we will see later..
An important characteristic of an ultrasound pulse is its diameter, which
is also the width of the ultrasound beam. The diameter of a pulse changes
as it moves along the beam path. The effect of pulse size on image detail
will be considered in the next chapter.. At this point we will observe the change
in pulse diameter as it moves along the beam and show how it can be
The diameter of the pulse is determined by the characteristics of the transducer. At the transducer surface, the diameter of the pulse is the same as the diameter of the vibrating crystal. As the pulse moves through the body, the diameter generally changes. This is determined by the focusing characteristics of the transducer.
Transducers can be designed to produce either a focused or non-focused beam, as shown in the following figure. A focused beam is desirable for most imaging applications because it produces pulses with a small diameter which in turn gives better visibility of detail in the image. The best detail will be obtained for structures within the focal zone. The distance between the transducer and the focal zone is the focal depth.
Beam Width and Pulse Diameter Characteristics of Both Unfocused and Focused Transducers
An unfocused transducer produces a beam with two distinct regions, as
shown in the previous figure. One is the so-called near field or Fresnel zone and
the other is the far field or Fraunhofer zone.
In the near field, the beam has a constant diameter that is determined by
the diameter of the transducer. The length of the near field is related to
the diameter, D, of the transducer and the wavelength, l, of the
Near field length = D2/4l.
Recall that the wavelength is inversely related to frequency. Therefore,
for a given transducer size, the length of the near field is proportional
to frequency. Another characteristic of the near field is that the
intensity along the beam axis is not constant; it oscillates between
maximum and zero several times between the transducer surface and the
boundary between the near and far field. This is because of the
interference patterns created by the sound waves from the transducer
surface. An intensity of zero at a point along the axis simply means that
the sound vibrations are concentrated around the periphery of the beam. A
picture of the ultrasound pulse in that region would look more like
concentric rings or "donuts" than the disk that has been shown
in various illustrations.
The major characteristic of the far field is that the beam diverges. This
causes the ultrasound pulses to be larger in diameter but to have less
intensity along the central axis. The approximate angle of divergence is
related to the diameter of the transducer, D, and the wavelength, l, by
Divergence angle (degrees) = 70l/D.
Because of the inverse relationship between wavelength and frequency,
divergence is decreased by increasing frequency. The major advantage of
using the higher ultrasound frequencies (shorter wavelengths) is that the
beams are less divergent and generally produce less blur and better
The previous figure is a representation of the ideal ultrasound beam. However, some transducers produce beams with side lobes. These secondary beams fan out around the primary beam. The principal concern is that under some conditions echoes will be produced by the side lobes and produce artifacts in the image.
A transducer can be designed to produce a focused ultrasound
beam by using a concaved piezoelectric element or an acoustic lens in
front of the element. Transducers are designed with different degrees of
focusing. Relatively weak focusing produces a longer focal zone and
greater focal depth. A strongly focused transducer will have a shorter
focal zone and a shorter focal depth.
Fixed focus transducers have the obvious disadvantages of not being able to produce the same image detail at all depths within the body.
The focusing of some transducers can be adjusted to a specific depth for each transmitted pulse. This concept is illustrated in the following figure. The transducer is made up of an array of several piezoelectric elements rather than a single element as in the fixed focus transducer. There are two basic array configurations: linear and annular. In the linear array the elements are arranged in either a straight or curved line. The annular array transducer consists of concentric transducer elements as shown. Although these two designs have different clinical applications, the focusing principles are similar.
The Principle of Electronic Focusing with an Array Transducer
Focusing is achieved by not applying the electrical pulses to all of the
transducer elements simultaneously. The pulse to each element is passed
through an electronic delay. Now let's observe the sequence in which the
transducer elements are pulsed in the figure above. The outermost element
(annular) or elements (linear) will be pulsed first. This produces
ultrasound that begins to move away from the transducer. The other
elements are then pulsed in sequence, working toward the center of the
array. The centermost element will receive the last pulse. The pulses from
the individual elements combine in a constructive manner to create a
curved composite pulse, which will converge on a focal point at some
specific distance (depth) from the transducer.
The focal depth is determined by the time delay between the electrical
pulses. This can be changed electronically to focus pulses to give good
image detail at various depths within the body rather than just one depth
as with the fixed focus transducer. One approach is to create an image by
using a sequence of pulses, each one focused to a different depth or zone
within the body.
One distinction between the two transducer designs illustrated here is that the annular array focuses the pulse in two dimensions whereas the linear array can only focus in the one dimension; that is, in the plane of the transducer.
The focusing of an array transducer can also be
changed electronically when it is in the echo receiving mode. This is
achieved by processing the electrical pulses from the individual
transducer elements through different time delays before they are combined
to form a composite electrical pulse. The effect of this is to give the
transducer a high sensitivity for echoes coming from a specific depth
along the central axis of the beam. This produces a focusing effect for
the returning echoes.
An important factor is that the receiving focal depth can be changed rapidly. Since echoes at different depths do not arrive at the transducer at the same time, the focusing can be swept down through the depth range to pick up the echoes as they occur. This is the major distinction between dynamic or sweep focusing during the receive mode and adjustable transmit focus. Any one transmitted pulse can only be focused to one specific depth. However, during the receive mode, the focus can be swept through a range of depths to pick up the multiple echoes produced by one transmit pulse.
The ultrasound image is produced by interactions of ultrasound pulses with the anatomical structures within the human body. The basic B mode image is a display of echoes or reflections from the structures and objects. The absorption of the ultrasound as it passes into and back out of the body is generally undesirable because it limits the depth of imaging, adversely affects the amplitude of echoes that form the image, and can be the source of artifacts.
The size of the ultrasound pulse at different depths within the imaged area determines the amount of blurring and image detail.
An understanding of the physical characteristics of ultrasound and how it interacts with the body enhances the ability to analyze images and make accurate diagnostic decisions.