Ultrasound Physics Volume 2

An ultrasound artifact is a reflection error. Artifacts can be classified or grouped in a number of different ways; for example, they can be described as reflections that:

• Are from a reflector out of the field of view or not from a reflector at all
• Are reflected but do not return to the transducer and therefore do not appear on the image
• Create an image of a differently shaped or sized object
• Create an object in an incorrect position
• Create an object of incorrect brightness

Artifacts can also be defined according to the cause of the artifact. The causes of artifacts can be grouped into:

• Artifacts caused by violation of any of the six assumptions of ultrasound systems
• Artifacts caused by equipment design or malfunction
• Artifacts caused by physical principles
• Artifacts caused by operator error

By combining both the classification systems above, it is possible to group artifacts into four classes:

• Resolution artifacts are related to: 
  • The ability of the ultrasound system to resolve or distinguish objects in the three imaging dimensions
  • The assumption that:
    • Reflectors are always positioned in the central axis of the main beam
    • The imaging plane (elevation plane) is infinitely thin

• Location artifacts are related to the errors that:
  • Are from a reflector out of the field of view or not from a reflector at all
  • Are reflected but do not return to the transducer and therefore do not appear on the image
  • Create an image of a differently shaped or sized object

• Attenuation artifacts are the group of artifacts that relate to:
  • Creating an object in an incorrect position
  • Creating an object of incorrect brightness
  • The assumption that the strength or intensity of the reflection is related only to the characteristics of the object causing the reflection

• Doppler artifacts are the group of artifacts that relate to:
  • Spectral Doppler - velocity measurement errors
  • Color Doppler - presence or absence of a signal error

• Limited lateral and axial resolution
  'Lateral resolution' describes the ability to resolve or separate two structures in the lateral dimension, or side by side.

  
  Phantom images of
  detail resolution

  
  Lateral resolution

  Lateral resolution is limited by beam width. If the two objects are closer than the width of the beam, they appear as a single object in the ultrasound image.
  Lateral resolution changes with depth and is best at the depth where the beam is narrowest.

  Lateral resolution = Lateral beam width

  Range (or axial) resolution refers specifically to the ability of an ultrasound system to distinguish and display two objects that are close together when the objects are lying parallel to the main axis of the sound beam. Axial resolution is determined by the spatial pulse length (SPL).

  
  Axial resolution

  

  Axial resolution is improved when:
  • Spatial pulse length (SPL) is minimized
  • Pulse duration (PD) is shortened
  • Frequency is maximized
  • Number of cycles per pulse is minimized

  Lateral and axial resolution artifacts occur when two or more structures or objects in the lateral or axial plane are seen as one object image on the ultrasound image.

• Limited elevation resolution
  The elevation plane is often forgotten. The two-dimensional image on the ultrasound machine gives little indication that there is a third dimension (slice thickness). One of the assumptions of an ultrasound system is that the slice thickness in the elevation plane is very narrow. However, it is determined by the dimensions of the crystal and increases with distance away from the focal zone. This means that structures located in the elevation plane may produce an echo that results in the object being incorrectly located in the image.

  
  Elevation resolution

• Refraction
  Refraction describes the bent path traveled by a sound wave when it moves from one medium through another. The change in the wave course is facilitated by differences in the propagation speeds of the two tissues concerned and obliqueness in the angle of the sound beam. When a sound wave strikes the edge of a structure lying above the structure under investigation, the sound wave is refracted or bent and ceases to travel in the original straight line. If the tissues on each side of this sound-refracting structure offer different propagation speeds, two reflections of the tissue under investigation appear on the screen. The ultrasound system always assumes that echoes are received from straight ahead; so the structure is artificially drawn in the image, laterally displaced.

  
  Refraction artifacts

  
  Refraction artifact
  image

• Reverberation
  Although many sound waves travel directly to the reflecting tissue and back to the transducer, some do not. If there are two tissue interfaces, both perpendicular to the axis, strong reflectors, and lying relatively superficially beneath the transducer, sound waves bounce back and forth between the two hyperechoic interfaces before returning to the transducer. The image therefore contains a series of parallel lines that look like the steps of a ladder, descending to deeper tissues. They create the impression that there are multiple structures in the beam path. In fact, the top two lines indicate the real tissue interfaces such as subcutaneous fat and muscle, and the others simply indicate sound wave reverberations.

  
  Reverberation
  artifact

  
  Ultrasound image
  of reverberation
  

  In addition to reverberation associated with parallel tissue planes or interfaces, there are two further types of reverberation artifacts:
  • Comet tail artifact is when there is reverberation of high amplitude echoes within a small object, typically a surgical clip or small metallic foreign body. Comet tails are distinguished as closely spaced, discrete lines, decreasing in width and brightness posterior to the object

    
    Comet tail

  • Ring down artifacts occur when an ultrasound beam passes through a group of small air bubbles. Reverberation occurs between the reflective interfaces between the bubbles. Ring down is distinguished by the appearance of a posterior tail that does not taper in width or decrease in brightness

    
    Ring down

• Multipath artifact
  One of the assumptions of ultrasound is that the sound pulse always travels directly to the structure and back to the transducer. Sometimes transmit and return paths differ because a pulse strikes a second structure on the way out or on the way back. This means the travel time is longer than expected, resulting in the misplacement of objects in the image - they are placed deeper because of the increased travel time.

  
  Multipath artifacts

• Side lobes and grating lobes
  Side lobes are created by single element transducers. Grating lobes are created by array transducers. These lobes are low intensity off-axis sound beams. They act like small weak sound beams that are capable of generating a weak reflected signal. Objects that reflect echoes arising from these lobes are misplaced as though they are lying in the main axis. This has the effect of decreasing lateral resolution and contradicts the assumption that all reflectors lie in the central axis of the main beam.

  
  Side lobes and
  grating lobes

• Speed error or range ambiguity
  Speed errors occur from the assumption that sound always travels 1540 m/s in soft tissue. A sound beam may take more or less time to make the return journey because of slight variations in propagation speed in the different types of soft tissue. The object (or part of an object) will be located in the image closer to or further from the transducer than it really is. In the following example there appears to be a fracture through the right kidney due to a difference in propagation speed for the sound traveling through the liver.

  
  Speed error

• Mirror images
  Sometimes the axis of the sound beam does not directly strike the structure of interest (true reflector), but instead strikes a very highly reflective structure (which is of no interest) situated to one side. A confusing double reflection then appears on the screen. If the mirroring structure lies in a straight line and at a 45 degree angle to the structure of interest, sound waves bounce off the mirror (strong reflector) towards the structure of interest and back to the transducer via two pathways: directly to the transducer, or back to the mirror, and then to the transducer. The structure of interest is therefore reproduced twice on the ultrasound screen, once in its proper position, and a second time at a greater depth in the body than the structure in fact has. This effect is created because the mirror surface reflects depth as well as the structure of interest, so the reflection will appear beyond the mirror, at the same distance from the mirror as the true reflector.

  
  Mirror image artifact

  
  Ultrasound image of
  mirror artifact

• Acoustic shadowing
  When sound waves strike a medium that characteristically attenuates sound very rapidly, the waves are attenuated due to a combination of absorption, reflection, and refraction. The sound beam fails to travel through this structure to the structures lying beneath it. This means the structures beneath are 'lost' due to the sound shadow from above. The eclipsed or darkened area is known as hypoechoic or anechoic.

  
  Acoustic shadow

  
  Ultrasound image of
  acoustic shadow

• Enhancement
  When sound waves strike a medium that characteristically has extremely low attenuation, the waves travel very easily through the structure to those beneath it. This means the structure beneath is strongly 'illuminated' as sound waves spill from above and cause it to be seen more brightly. The lighter area is known as enhancement  (or posterior acoustic enhancement) with the segment of tissue appearing to be hyperechoic.

  
  Enhancement

  
  Ultrasound image of
  enhancement
• Speckle
  Speckle is a form of noise that appears as tissue texture in the near field of an image close to the face of an ultrasound transducer. It is a form of constructive and destructive interference that creates bright and dark spots not related to a true anatomic structure.

• Aliasing (spectral and color)
  

  Spectral aliasing appears as a wraparound of spectral Doppler information from one channel to another. It is almost like information overload because the PRF has been set too low for the magnitude of the signal.

  
  Spectral aliasing

  Color aliasing appears for a similar reason. Too much information is to be displayed in color and so the extra information is wrapped around into the channel assigned to display information about flow in the opposite direction. The artifact gives the appearance of flow reversal when in fact there has been none.

  
  Color aliasing

• Range ambiguity
  Doppler range ambiguity concerns the receiving of Doppler information from vessels that lie deeper than the structure or vessel being interrogated and that is displayed as though the information was arising from the structure.
• Spectral mirror
  Mirror images of a spectral Doppler waveform may be displayed on the opposite side of the baseline. When the Doppler gain is set too high, the mirroring is due to information overload with noise reflected as an inverted signal in the second channel. Reducing the output power or gain will clean up the noise from the second channel. When Doppler angles greater than 60 degrees (and close to 90 degrees) are used, small changes in the angle of insonation allow for ambiguous signals to be received due to the fluctuating spiral flow, or mild turbulence creating the appearance of bidirectional flow.

  
  Spectral mirror
  artifact

• Spectral spread (broadening)
  Spectral broadening is a useful way of assessing the range of velocities occurring at a given point in a vessel. Spectral broadening is a diagnostic feature of post-stenotic turbulence and helps confirm the presence and severity of the stenosis. However, a spectral broadening artifact that is an artificial broadening of the spectral display is a confusing error that may be caused by:
  • Using a Doppler angle of greater than 60 degrees. As the Doppler angle approaches 90 degrees the effect of small changes in the Doppler angle can result in ambiguous changes in the calculated spectral information, resulting in signals being displayed over an artificially wide velocity range
  • Incorrect placement of the Doppler sample volume, so that it is too close to the wall of the vessel, may result in slower velocities due to the viscosity of the blood and corresponding frictional drag exerted by the wall being displayed in the spectral window

  Normal waveform

  Pre-stenosis waveform

  Stenosis waveform

  Post-stenosis waveform

• Blossoming (spectral and color)
  Blossoming is the term used to describe the appearance of overgain in the spectral Doppler trace. It may give the appearance of spectral broadening but is usually distinguishable by the appearance of noise in the usually black background of the trace. It may be caused by setting either the transmit power, or the receiver gain, too high. As well as causing artificial spectral broadening, the overgain may display an artificially increased peak velocity.

  
  Spectral blossoming

• Wall filter saturation (spectral and color)
  Wall filters are used to clean up the appearance of a spectral waveform by filtering strong reflections from vessel wall motion or from the effect of certain prosthetic heart valves. Overutilizing or underutilizing the wall filter setting may result in a confusing or erroneous spectral waveform.

  
  Wall filter

The acoustic power of an ultrasound machine, or the total amount of energy produced per second, is measured in milliwatts (mW).
1 Watt = 1 joule/second
1 joule = 0.24 calories
1 calorie is the amount of energy required to raise the temperature of 1 gram of water by 1 degree Celsius.

Acoustic power, or sound energy, can be measured in a number of ways and by assessing a number of the effects produced by a sound beam:

• Acoustic pressure can be measured using a hydrophone
  A hydrophone is a small probe with a small piece of piezoelectric material at the end. It is used to measure acoustic pressure at specific locations and is usually calibrated to provide a relationship between acoustic pressure and PZT voltage.
• Absorption by the production of heat can be measured by:
  • Calorimetry - a calorimeter measures total power by absorption. An ultrasound beam is directed into a liquid-filled calorimeter. The acoustic power is measured by recording the temperature increase and the time taken
  • Thermocouple - a small electronic thermometer
  • Liquid crystals - crystals that change color with temperature
• Radiation force
  An ultrasound beam exerts a force on objects in the path of the beam. The force is called 'radiation force'. Radiation force is directly related to power output. It can be measured by directing the beam onto a highly sensitive disc, measuring the displacement and converting it to a measure of acoustic power.
• Acousto-optics
  The principle of acousto-optics involves an interaction between sound and light; Schlieren imaging is used to view the shape of a sound beam in a medium.

The study of the parameters of ultrasound beams that may cause bioeffects is part of the field known as dosimetry. The study of potential ultrasound bioeffects is an ongoing area of research. The research may be organized as:

• In vivo research, which means research undertaken within a living plant or animal, this may involve laboratory experiments on animals
• In vitro research, which means research undertaken outside a living plant or animal, this may include computer modeling

Epidemiology is the study of diseases in populations. Epidemiological studies are used to collect data about the ultrasound exposure of the fetus during obstetric ultrasound. Epidemiological studies are limited when:

• Studies are retrospective
• Data is ambiguous
• Other factors are not taken into account

The most reliable epidemiology studies are prospective and randomized.

The American Institute of Ultrasound in Medicine (AIUM) has developed the policies that govern the use of medical diagnostic ultrasound.
A simple summary of the policies regarding bioeffects states:

No confirmed biological effects on patients or instrument operators by exposure at intensities typical of present diagnostic ultrasound machines have been reported.

The bioeffect and safety statements are found on the AIUM website http://www.aium.org/publications/statements.aspx.

The following statements from the website summarize current policies.

• Statement on heat
• Prudent use in obstetrics
• Prudent use and clinical safety
• Bioeffects of diagnostic ultrasound with gas body contrast agents

ALARA is an acronym for as low as reasonably achievable. The acronym is used in the AIUM Statements.

The AIUM statement can be summarized as stating the sonographer's responsibility to use the lowest possible power output and the lowest exposure time to yield appropriate clinical results.

Although the remote possibility of bioeffects does exist, the benefits of diagnostic medical ultrasound outweigh the risk, provided the lowest possible power output and the lowest exposure times are used at all times.

As ultrasound is a form of energy, the impact of this energy may affect the normal function of cells, causing structural damage and possible cell death. Bioeffects are the consequences of ultrasound interacting with living tissues. The severity of the effect will depend upon:

• The mechanism of the interaction
• The ultrasound exposure factors

Bioeffects can be categorized as either thermal or mechanical, according to the manner in which they exert their effect.

Bioeffects that are not related to heating are described as mechanical bioeffects. These mechanical effects include the effects of cavitation, the effects of radiation force, and the effects of microstreaming.

Cavitation is the most significant mechanical bioeffect. It can be further classified as stable or transient cavitation. Cavitation is caused during the rarefaction or decompression phase of a sound pulse. The reduced pressure that accompanies the separation of the wave fronts can cause small bubbles to arise or existing bubbles to vibrate, oscillate, and increase in size. In addition to the effect of a rarefaction pressure drop, resonance may cause the bubbles to increase up to 100 times their normal size.

Cavitation

• Stable cavitation
  Stable cavitation occurs at lower intensities. Pre-existing submicroscopic bubbles increase in size, but vibrate in a regular, organized, and expected manner. The bubble movement agitates the surrounding body fluid, which streams away. The shifting of fluid is called microstreaming. Microstreaming causes pressure that can stretch and break cell walls.
• Transient (inertial) cavitation
  At higher intensities the microbubbles may implode, or collapse in on themselves, or break up into even tinier bubbles. This is known as transient or inertial cavitation, and is potentially more severe than stable cavitation. Bubble collapse occurs because of the pressure changes around the bubble caused by the particles of the surrounding medium being alternately rarefied and compressed during sound wave propagation. During rarefaction, pressure outside the bubble decreases, so pressure within the bubble stretches the bubble bigger. Then compression around the bubble squeezes it smaller. As rarefaction occurs again, the bubble is stretched even more, and its boundary thus becomes weaker and more prone to collapse when compression is continually repeated. The process leading to bubble collapse can generate high-amplitude shock waves and sudden increases in temperature.

The factors causing cavitation may include:

• Pressure amplitude (peak rarefaction pressure) - the most significant factor in cavitation
• Transducer frequency
• Beam parameters (focusing, continuous or pulsed wave, spatial pulse length)
• PRF and exposure time
• Nature and state of the medium (gas bubbles and temperature)

Intensity is the expression of power per unit of area. Intensity can be expressed as a measure of energy per unit time per unit area.

The unit of measurement is milliwatts per square centimeter (mW/cm²).

Intensity can be calculated as:

The concept of intensity can be more difficult to comprehend than the concepts of energy or power due to three variables:

• Intensity varies throughout an ultrasound beam spatially due to interference, diffraction, and beam focusing
• Pulsed wave ultrasound intensity varies with time and is zero between pulses
• Intensity varies during an ultrasound pulse

Continuous wave intensity
Continuous wave ultrasound does not vary in time. Intensity is measured in terms of:

• Spatial peak intensity (I_SP)
• Spatial average intensity (I_SA)

Intensity and safety
There is a range of ways of expressing intensity of an ultrasound beam because of the lack of uniformity in time and space. The American Institute of Ultrasound in Medicine (AIUM) statements about safety refer to a maximum safe intensity (I_SPTA) of 720 mW/cm².
The relative values of intensity are summarized in the following diagram and table:

Relative values of
intensity

The intensity at any time for an ultrasound system depends on the output power setting as well as the following machine settings, which all cause changes in intensity:

• Pulse length - increased pulse length means more on time and increased intensity; pulse length increases for deeper fields of view
• Depth of focus - when the depth of focus is increased the beam is narrower at depth and therefore more concentrated; intensity increases
• Use of multiple focal zones increases intensity - as the number of focal zones increases the number of sound pulses increases, with one pulse per focal zone per line of sight
• Frame rate - increased frame rate causes increased number of pulses and increased intensity
• Line density - scan lines closer together means more sound pulses and increased intensity
• Pulse repetition frequency (PRF) - relates to an increase in duty factor; more on time

Beam uniformity can be expressed as a coefficient (or ratio). The intensity of an ultrasound beam varies in space. The cross section area of the beam varies as the sound pulse travels further from the transducer. The beam uniformity coefficient (BUC) is a measure of how uniform in space the beam is.

Beam uniformity coefficient (BUC)

As an example, consider an ultrasound beam with:

I_SATA = 2 mW/cm²

DF = 0.004

BUC= 10

The various intensities can be described as:

I_SPTA = 20 mW/cm²

I_SAPA = 500 mW/cm²

I_SPPA = 5000 mW/cm²

The duty factor is expressed as a percentage and describes the very small amount of time the transducer takes to transmit a single pulse.

It is calculated by dividing the length of time that the pulse sounds, by the combined length of time of the pulse and the pause (listening time) that follows it. In diagnostic ultrasound, the transducer should spend more time receiving ultrasound data (between 99.5% and 99.8% of the time) than in transmitting sound pulses (between 0.5% and 0.2% of the time).

Thus:
A duty factor of 0.2% in pulsed ultrasound is typical and acceptable. The higher the duty factor, the more time the transducer is spending transmitting sound waves, and the more likely that heat will build up and contribute to thermal bioeffects.

The ratio of pulse duration to pulse repetition period is identical to that of temporal average intensity to pulse average intensity:

Therefore, to find the temporal average, when the pulse average is known:

PA x DF = TA

Conversely, to find the pulse average, when the temporal average is known:

The use of both harmonic imaging and contrast agents in diagnostic medical ultrasound are recent developments.

Harmonic imaging refers to the use of harmonic frequencies (reflected sound waves at double the transmit frequency) to create an ultrasound image. Two types of harmonics are used in diagnostic ultrasound: contrast harmonics and tissue harmonics.

Harmonic imaging

When an ultrasound pulse insonates and is reflected off contrast agent microbubbles, some of the reflected signal is in the harmonic frequency. The reflected harmonic frequency is in response to the nonlinear effects of resonance.

Contrast harmonics

Tissue harmonics occur quite separately from the nonlinear interaction of sound waves with microbubbles. Sound waves are a series of pressure waves or compressions and rarefactions. Sound waves travel slightly faster during compression and slower during rarefaction. The difference in speed is nonlinear and gives rise to the generation of small reflected tissue harmonic frequencies in the main beam.

Tissue harmonics
from the main
beam

The generation of tissue harmonics can be summarized as occurring:

• Deeper in the tissue
• During transmission
• Due to the nonlinear speed difference between compression and rarefaction
• Along the axis of the main beam 

The extent or amount of harmonic reflection that is produced can be estimated by the mechanical index (MI).

MI is calculated from the transmit frequency and the pressure of the sound wave.

Increased MI values, and therefore increased harmonics, are related to lower frequency sound waves and higher sound wave pressures.

The generation of contrast harmonics can be summarized as occurring:

• During reflection
• Due to nonlinear resonance within microbubbles
• In proportion to the MI
• Due to both the shell and the gas content of the contrast microbubble

Contrast agents and contrast harmonics remain at the front of ongoing research. The following areas are currently evolving:

• Non-vascular contrast and harmonics - contrast agents can be used in sonosalpingography to visualize fallopian tube patency; and to detect vesicoureteric reflux by introducing contrast into the urinary bladder
• Vascular contrast and harmonics - agents can be used in echocardiography to assess systolic and ventricular function
• Quantification and functional studies - bolus injections of contrast can be used to assess transit times through a specific organ or lesion. An important application is in the area of functional assessment of liver cirrhosis and malignancy. Contrast will arrive in the liver much sooner due to increased vascularity in malignancy and arteriovenous shunting in cirrhosis
• Liver specific microbubbles - initial research and experience with contrast agents revealed an early arterial phase; that is, the early arrival of contrast at the site of interest as a direct result of the injection of contrast into the blood stream. More recently a late parenchymal phase has been identified. In this late phase, the contrast does not appear to be flowing or within the veins or arteries, but in the reticuloendothelial system or pooling in the sinusoids. Assessment of this late phase appearance appears to be proving useful in the detection of hepatocellular carcinoma
• High intensity focused ultrasound (HIFU) - is being developed as a possible clinical tool for inducing a rapid focal increase in temperature for the treatment of liver, prostate, and breast tumors
• Drug and gene delivery - by coating the surface of microbubbles with ligands that recognize certain receptors on cell surfaces it may be possible to target the delivery of agents to specific cells and regions. Insonation will cause the bubbles to burst at a specific location

  
  Drug and gene
  delivery

  
  Microbubble
  insonation and
  bursting

Quality assurance (QA) is the routine and periodic validation of consistency and accuracy of ultrasound images and measurements. It is performed on every ultrasound machine with every transducer and is both a medical and a legal requirement.

The requirements of a QA program include:

• The assessment of all system components including the machine, transducers, and image recording devices
• The repair and replacement of faulty or worn parts including transducers and transducer cables
• Preventative maintenance by replacing parts before they become faulty
• Accurate record keeping of maintenance schedules, faults, and damages

The goals of a program include:

• Ensuring the correct operation of all ultrasound equipment
• Detection of gradual changes in the performance or accuracy of any piece of equipment
• Minimizing the amount of downtime due to unexpected faults or breakdowns
• Minimizing the number of nondiagnostic examinations performed due to faulty or damaged equipment
• Reducing the number of repeat scans due to faulty or damaged equipment

There are four test devices or phantoms that can be used as part of a QA program to assist the sonographer to validate the accuracy and consistency of the ultrasound systems: test object; tissue equivalent phantom; Doppler phantom; slice thickness phantom. In some cases a multipurpose phantom can be used to combine the function of the test object with the tissue equivalent phantom.

A test object is a fluid-filled chamber with various arrangements of metal or plastic pins positioned vertically, horizontally, and obliquely. The test object is used to assess:

• Axial resolution - when the array of pins is parallel to the main axis of the sound beam
• Lateral resolution - when the array of pins is perpendicular
• Caliper accuracy - by measuring the known distance between pins
• Dead zone - by scanning pins close to the face of the transducer

The propagation speed of the test object is set at 1.54 km/s but it does not have the attenuation properties of soft tissue.

Test object

Ultrasound image of test object

Tissue equivalent phantoms are used to evaluate grayscale. They have similar attenuation, scattering, and echogenicity properties to soft tissue.

Tissue phantom

Ultrasound image of tissue phantom

Doppler phantoms can be made from vibrating strings, moving belts, or calibrated flow pumps. They are used to test all the features of Doppler, including continuous and pulsed wave, color, spectral, and power Doppler.

The slice thickness phantom is used in addition to the soft tissue phantom to add testing of the slice thickness in the elevation plane.

The test objects are used to perform a range of measurements that build a profile of the performance quality of the ultrasound machines and the various transducers. The following measurements are routinely performed as part of a QA program:

• Sensitivity
  Sensitivity is the term that describes the ability of an ultrasound system to detect and display low-level echoes. Sensitivity is measured using a tissue phantom.
  • Normal sensitivity is determined by adjusting output, TGC, and gain in order to display all the pins and mass objects in the phantom
  • A range from minimum to maximum sensitivity can also be established by setting the TGC to minimum and adjusting power and gain for minimum sensitivity
  • Setting the power and gain to maximum and adjusting TGC determines maximum sensitivity

• Dead zone
  Dead zone refers to the region immediately in front of the face of the transducer, where images are inaccurate. The dead zone is due to transducer ringing and the built-in delay between transmitting and receiving. The dead zone extends from the face of the transducer to the depth that objects first appear in the image. It is assessed using the very superficial pins in a tissue phantom.
• Registration accuracy
  Registration is the ability of a system to accurately locate an object in an image when the object is scanned from different approaches. Registration accuracy can be described in terms of vertical and horizontal calibration.
• Focal zone
  Focal zones are evaluated by altering the depth and number of zones while imaging a tissue phantom. The focal zone is the point where the beam is narrowest and the intensity is greatest.
• Axial resolution
  Axial resolution is assessed with pins that are parallel to the long axis of the ultrasound beam.
• Lateral resolution
  Lateral resolution is determined with an array of pins that are side by side and perpendicular to the long axis of the beam.
• Uniformity
  Uniformity is the ability to display similar reflectors from different depths with the same degree of brightness. It is an assessment of the TGC function.
• Display
  Once an ultrasound system has been optimized, all monitors and remote displays and recording devices can then be adjusted to give the same image appearance as the original system image.

The American Institute of Ultrasound in Medicine (AIUM) has developed standards for the accreditation of ultrasound practices. The essential motivation for seeking accreditation would be to:

• Demonstrate clinical excellence
• Demonstrate commitment to high quality patient care
• Provide credibility to peers
• Meet the QA requirement for insurance companies

The Standards and Guidelines for the Accreditation of Ultrasound Practices can be viewed at the AIUM website

Quality assurance is a part of the requirements for accreditation, but accreditation goes further and also includes specifications for:

• Licensing and educational requirements for staff who perform and interpret ultrasound
• Staff performance
• Scientific interpretation
• QA of equipment
• Record keeping
• Space management