Ultrasound Production and Interactions
Perry Sprawls, Ph.D.

Online Textbook

Table of Contents

 

INTRODUCTION AND OVERVIEW
THE ULTRASOUND IMAGING SYSTEM
     Transducer
     Pulse Generator
      Amplification
      Scan Generator
    Scan Converter
    Image Processor
    Display
THE ULTRASOUND PULSE
ULTRASOUND CHARACTERISTICS
      Frequency
      Velocity
      Wavelength
      Amplitude
INTENSITY AND POWER
     Temporal Characteristics
     Spatial Characteristics
     Temporal/Spatial Combinations
INTERACTIONS OF ULTRASOUND WITH MATTER
     Absorption and Attenuation
     Reflection
     Refraction
PULSE DIAMETER AND BEAM WIDTH
     Transducer Focusing
     Unfocused Transducers
     Fixed Focus
     Adjustable Transmit Focus
     Dynamic Receive Focus
SUMMARY

 

INTRODUCTION AND OVERVIEW

CONTENTS

 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 human ear cannot hear or respond to all sound frequencies. The range of frequencies that can be heard by a normal young adult is from approximately 20 Hz to 20,000 Hz (20 kHz). Ultrasound has a frequency above this range. Frequencies in the range of 2 MHz (million cycles per second) to 20 MHz are used in diagnostic ultrasound. Ultrasound is used as a diagnostic tool because it can be focused into small, well-defined beams that can probe the human body and interact with the tissue structures to form images.

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.

 


The Basic Ultrasound Imaging Process

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 ULTRASOUND IMAGING SYSTEM

CONTENTS

      The basic functional components of an ultrasound imaging system are shown below.

The Principal Components of an Ultrasound Imaging System

The Principal Functional Components of an Ultrasound Imaging System

  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 image formation.

 

   Transducer

CONTENTS

  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.

 

   Pulse Generator

CONTENTS

    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

CONTENTS

    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.

 

   Scan Generator

CONTENTS

    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 Converter

CONTENTS

    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.

 

   Image Processor

CONTENTS

    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.

 

   Display

CONTENTS

   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 ULTRASOUND PULSE

CONTENTS

  The basic principles of ultrasound pulse production and transmission are illustrated below.

The Production of an Ultrasound Pulse

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.

   In soft tissue and fluid materials the direction of vibration is the same as the direction of pulse movement away from the transducer. This is characterized as longitudinal vibration as opposed to the transverse vibrations that occur in solid materials. As the longitudinal vibrations pass through a region of tissue, alternating changes in pressure are produced. During one half of the vibration cycle the tissue will be compressed with an increased pressure. During the other half of the cycle there is a reduction in pressure and a condition of rarefaction. Therefore, as an ultrasound pulse moves through tissue, each location is subjected to alternating compression and rarefaction pressures.

 As shown above, the space through which the ultrasound pulse moves is the beam. In a diagnostic system, pulses are emitted at a rate of approximately 1,000 per second. The pulse rate (pulses per second) should not be confused with the frequency, which is the rate of vibration of the tissue within the pulse and is in the range of 2-20 MHz.

 

ULTRASOUND CHARACTERISTICS

CONTENTS

   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.

The Characteristics of Ultrasound Pulses That Have an Effect on the Imaging Process
 

   Frequency

CONTENTS

 

   

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.
 
    The frequency of sound is determined by the source. For example, in a piano, the source of sound is a string that is caused to vibrate by striking it. Each string within the piano is adjusted, or tuned, to vibrate with a specific resonant frequency. In diagnostic ultrasound equipment, the sound is generated by the transducer. The major element within the transducer is a crystal designed to vibrate with the desired frequency. A special property of the crystal material is that it is piezoelectric. This means that the crystal will deform if electricity is applied to it. Therefore, if an electrical pulse is applied to the crystal it will have essentially the same effect as the striking of a piano string: the crystal will vibrate. If the transducer is activated by a single electrical pulse, the transducer will vibrate, or "ring," for a short period of time. This creates an ultrasound pulse as opposed to a continuous ultrasound wave. The ultrasound pulse travels into the tissue in contact with the transducer and moves away from the transducer surface, as shown in the above figure. A given transducer is often designed to vibrate with only one frequency, called its resonant frequency. Therefore, the only way to change ultrasound frequency is to change transducers. This is a factor that must be considered when selecting a transducer for a specific clinical procedure. Certain frequencies are more appropriate for certain types of examinations than others. Some transducers are capable of producing different frequencies. For these the ultrasound frequency is determined by the electrical pulses applied to the transducer.

 

    Velocity

CONTENTS

   

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

                  ___
Velocity = E/r

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 table.

Approximate Velocity of Sound in Various Materials

Material

Velocity (m/sec)

Fat

1450

Water

1480

Soft tissue (average)

1540

Bone

4100

   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.

 

   Wavelength

CONTENTS

   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.

   The illustration below shows both temporal and spatial (length) characteristics related to the wavelength. A typical ultrasound pulse consists of several wavelengths or vibration cycles. The number of cycles within a pulse is determined by the damping characteristics of the transducer. Damping is what keeps the transducer element from continuing to vibrate and produce a long pulse. The wavelength is determined by the velocity, v, and frequency, f, in this relationship:

Wavelength (l) = v/f.

 

The Temporal and Length Characteristics of an Ultrasound Pulse

The Temporal and Length Characteristics of an Ultrasound Pulse


   The period is the time required for one vibration cycle. It is the reciprocal of the frequency. Increasing the frequency decreases the period. In other words, wavelength is simply the ratio of velocity to frequency or the product of velocity and the period. This means that the wavelength of ultrasound is determined by the characteristics of both the transducer (frequency) and the material through which the sound is passing (velocity).

   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

 

Amplitude

CONTENTS

   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.

   The following 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 dB smaller.


Pulse Amplitudes Expressed in Decibels and Percentages

Pulse Amplitudes Expressed in Decibels and Percentages



   Perhaps the best way to establish a "feel" for the relationship between pulse amplitude expressed in decibels and in percentage is to notice that amplitudes that differ by a factor of 2 differ by 6 dB. A reduction in amplitude of -6 dB divides the amplitude by a factor of 2, or 50%. The doubling of a pulse amplitude increases it by +6 dB.

   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.

 

INTENSITY AND POWER

CONTENTS

    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

The Temporal and Spatial Characteristics of Ultrasound Pulses That Affect Intensity Values

 

   Temporal Characteristics

CONTENTS

   The figure 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.  

 

   Spatial Characteristics

CONTENTS

   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.

 

   Temporal/Spatial Combinations

CONTENTS

   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.
 Thermal effects are most closely related to the spatial-peak and temporal-average intensity (ISPTA). This expresses the maximum intensity delivered to any tissue averaged over the duration of the exposure. Thermal effects (increase in temperature) also depend on the duration of the exposure to the ultrasound.
Mechanical effects such as cavitation are more closely related to the spatial-peak, pulse-average intensity (ISPPA).

 

INTERACTIONS OF ULTRASOUND WITH MATTER

CONTENTS

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.

 

   Absorption and Attenuation

CONTENTS

  

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 significant.

   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.

Approximate Attenuation Coefficient Values for Various Materials

Material

Coefficient
 (dB/cm MHz)

Water

0.002

Fat

0.66

Soft tissue (average)

0.9

Muscle (average) 2.0
Air 12.0
Bone 20.0

Lung

40.0

 

   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 Effect of Absorption on Ultrasound Pulse Amplitude in Relation to Distance or Depth in the Body

 

   Reflection

CONTENTS

    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 written as

Impedance = (Y) (v).


The Production of an Echo and Penetrating Pulse at a Tissue Interface

The Production of an Echo and Penetrating Pulse at a Tissue Interface

   At most interfaces within the body, only a portion of the ultrasound pulse is reflected. The pulse is divided into two pulses, and one pulse, the echo, is reflected back toward the transducer and the other penetrates into the other material, as shown in the above figure. The brightness of a structure in an ultrasound image depends on the strength of the reflection, or echo. This in turn depends on how much the two materials differ in terms of acoustic impedance. The amplitude ratio of the reflected to the incident pulse is related to the tissue impedance values by

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.


Pulse Amplitude Loss Produced by a Reflection

Interface

Amplitude Loss (dB)

Ideal reflector

0.0

Tissue-air

-0.01

Bone-soft tissue

-3.8

Fat-Muscle -20.0
Tissue-water -26.0
Muscle-blood -30.0

 

   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.

 

   Refraction

CONTENTS

    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..

 

PULSE DIAMETER AND BEAM WIDTH

CONTENTS

    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 controlled.

   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.

 

  Transducer Focusing

CONTENTS

    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

Beam Width and Pulse Diameter Characteristics of Both Unfocused and Focused Transducers

 

  Unfocused Transducers

CONTENTS

    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 ultrasound pulse maintains a relatively constant diameter that can be used for imaging.

   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 ultrasound by

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 detail.

   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.

 

   Fixed Focus

CONTENTS

     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.

 

   Adjustable Transmit Focus

CONTENTS

     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

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.

 

  Dynamic Receive Focus

CONTENTS

    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.

 

SUMMARY

CONTENTS

    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.