This document includes basic physics principle of ultrasound and instrumentation knowledge for college students, sonographer, and engineers. It focuses on using animations and simulations to help you understand how ultrasound wave is propagating, how the beam is formed and eventually how is image obtained.

### Ultrasound Definition

Ultrasound is cyclic mechanical vibration with a frequency greater than the upper limit of human hearing, which is approximately 25 kilohertz (25,000 hertz) in healthy, young adults

### Vibration and Wave in physics

• A vibration source in a mechanical medium will cause wave propagation.
• Ultrasound is a mechanical wave, cannot exist in vacuum.
• Ultrasound transfer energy within the medium, does not transfer mass. See a particle vibration example
• Longitudinal wave: particle movement in the same direction to the wave propagation. See a one-dimension example
• compression: Ultrasound propagation causes local medium density and pressure varying with time. The density or pressure will increase when it is compressed.
• rarefaction: When low pressure pass through, the local density will drease.
• Transverse wave: particle movement in the direction perpendicular to the wave propagation. See a one-dimension example

### Ultrasound Parameters

• Frequency: The number of times a vibrating particle goes through its original position within one second. The unit is “Hertz” or simplified as “Hz”. For ultrasound, the frequency is around mega hertz, or “MHz”.
• Propagation speed: The distance that the wave peak pass within one second. It is determined by elasticity and density property of the acoustic medium.
• Wave length: The distance between the two adjacent wave peaks
• Amplitude: For vibration, the amplitude is maximal distance a particle moves away from its original position.
• Pressure: Wave propagation can be  understood as the particles vibration abound their balance position, also can be think as pressure alternatively variation in the acoustic medium. Pressure, especially from the surface, cause particle to move. Take a small volume inside the acoustic medium, when more neighbor particles move in, a high pressure is introduced, and on the contrary, when the particle move out, a low pressure in generated.
• Power: Since the particles are vibrating, it possesses kinetic velocity and energy. An ultrasound source will generate energy, and this energy divided by time is power. When frequency is fixed, the power is proportional to the density and second power of particle vibration amplitude. When particles have the same vibration amplitude, the power will be proportional to the fourth power of the frequency.
• Intensity: it is the power on unit area.

### Medium Acoustic Property

• Air: Only low frequency ultrasound can propagate in air at a speed of 300m/s with a very high attenuation. The acoustic impedance of air near zeros.
• Water: Sound velocity in water is around 1500m/s at room temperature. Water has very little attenuation to ultrasound. The acoustic impedance of water is about 1.5MRayl.
• Soft tissue: Sound velocity in soft tissue is around 1540m/s. The attenuation of soft tissue is around 0.3 dB/cm/MHz. The acoustic impedance of soft tissue is around 1.5MRayl.
• Bone: Sound velocity in bone is much fast that in soft tissue. Bone also has a higher attenuation. Normally, ultrasound beam cannot penetrate bone.

### Ultrasound Reflection

• Big surface: The surface must be big compared to the ultrasound wavelength when reflection happens.
• Impedance mismatch: The acoustic impedance on both side of the surface must be different.
• Wavefront pattern: The reflected wave from a spherical wave emitted from a point has the same shape as that emitted from its mirrored counterpart.
• Dependence on angle: For plane wave, the reflected wave has the same angle to the surface as that of incident wave.

### Ultrasound Scattering

• Small particle size: Ultrasound will be scattered when encounters a small particle compared to its wavelength.
• Impedance mismatch: The particle must have a different acoustic impedance from its background.
• Independence of angle: The scattered wave will spherical when the particle size is very small.

### Ultrasound Refraction

• Big surface: Ultrasound refraction only happen at big surface compared to its wavelength.
• Velocity mismatch: The acoustic medium at both sides of the surface must have different sound velocity.
• Dependence on angle: The refracted wave obey Snell's Law.

### Ultrasound Attenuation

• Causes of ultrasound attenuation:
• Scattering
• Absorption
• Reflection
• Ultrasound attenuation values in type of tissue: Water has minimal attenuation to ultrasound. Blood has an attenuation of 0.03dB/cm/MHz. Soft tissue has a value of 0.3dB/cm/MHz. Air, bone, stone, and metal will normally attenuate all the imaging ultrasound beam energy they encountered.
• Ultrasound attenuation values increase with frequency.
• Effects on image: Far field image will look darker without compensation. Resolution in far field is lower than near field due to the downshift of the center frequency of the echo.

### Ultrasound Frequency Range and Application

• HIFU: Depends on application, low than half of the diagnostic frequency.
• Abdominal imaging: 3.5 ~ 5 MHz.
• Cardiovascular imaging: 2.5 ~ 3.5MHz.
• Ophthalmology, eye application: 15 ~ 50MHz.
• Dermatology, skin application: 15 ~ 50 MHz.
• Small organ: 5 ~ 12MHz.
• Peripheral vascular: 5 ~ 10 MHz.
• Intravascular (IVUS): 10 ~ 50 MHz.
• Frequency, Penetration, and spatial resolution: Higher frequency gives better resolution, and less penetration. The transmitted power of ultrasound pulse for imaging is regulated by FDA. The highest energy within FDA regulation is always used for the best signal to noise ratio. As long as the signal from the farthest depth has enough SNR, the highest frequency is always the first choice.

### Image Characteristics Related Terminology

• Echogenic: Scatter or reflect strong echo in general, like bone, stone, or air.
• Anechoic: No echo area, like water or blood pool.
• Hyperechoic: Generate stronger or increased echo than surrounding area, like a solid mass in soft tissue.
• Hypoechoic: Opposite to hperechoic, generate weaker or decreased echo than surrounding area, like a lipid pool or cyst.
• Isoechoic: Generate echo with normal amplitude, like normal soft tissue.
• Homogeneity: Parameters like acoustic impedance, geometry texture structure uniform in the area, such as healthy liver tissue.
• Heterogeneity: Contrary to homogeneity, contains dissimilar elements, like kidney or breast.

### Piezoelectric Effect

• Piezoelectricity is the ability of some materials (notably crystals and certain ceramics) to generate an electric potential in response to applied mechanical stress. The material that shows piezoelectricity is called piezoelectric material. Applied electrical charge on both sides of a piece of piezoelectric material, it will cause stress inside and thus generate deform. If the electrical charge is alternative, the piece of material will oscillate and generate mechanical wave. The piezoelectric material has a special structure that will cause positive and negative charge center mismatch when an external stress is introduced from certain direction. Piezoelectric ceramic have many small regions inside it, called “domain”, and each domain has its own piezoelectric direction. When an external stress is introduced, some domains give positive charge if they are lined up according to the stress direction; some domains may give very minimal charge if its own direction is perpendicular to the stress direction; and some domains will give negative charge if it is against the stress direction. The domains are very small at level of a few microns to hundreds microns, and normally they are randomly distributed, without special processing to line up all the domains, the material will not show piezoelectric as a whole piece. The processing is called poling, use a high DC voltage applied on both sides of the piece of material for a short duration of time, such as 1 to 10 seconds. Different material needs different voltage to reverse the domains, and this voltage is called coercive voltage. Pure piezoelectric crystal may be a single domain and doesn’t need poling.
• Curie temperature: When temperature is high enough, the piezoelectric domains inside ceramic will have such a high kinetic energy and it will break away from the poling direction and resume to its original random direction. This temperature is called Curie temperature. Piezoelectric ceramic will lose its piezoelectricity when its temperature is above its Curie temperature.
• Kt: It is thickness mode mechanical-electrical coupling efficient, the key indicator of piezoelectricity performance of the material in thickness mode. By definition, it is the ratio of energy send out to the energy stored by the material. Without piezoelectricity, a ceramic plate with two sides coated with electrodes will behave as a capacitor, the impedance will only have imaginary part, no real part. The current go through it and the voltage applied on it will be always 90 degree to each other and thus no energy is emit out but all stored and released. With piezoelectricity effect, at resonant frequency, the impedance will have real part and imaginary part, the real part will consume electrical energy, convert it into acoustic.
• Common piezoelectric materials: commonly used piezoelectric materials are ceramics, crystals and polymers. Crystal usually has lower Kt, and it not good for thickness mode, but good in bar mode. Ceramic has a better Kt, good in thickness mode. Both of crystal and ceramic have high acoustic impedance, usually above 30Mryls. Matching layers are required to transmit acoustic

### Transducer Construction and Characteristics

• Thickness resonance mode: The positive and negative charge center will mismatch and form a dipole when external force is applied. The dipole direction maybe parallel or perpendicular to the external force direction. For most ultrasound application, the transducer is a plate of piezoelectric material with two sides coated with electrodes. With this structure, the dipole direction will be parallel to the external force, called 3-3 mode or thickness mode. There are also transducers in the other way, the stress and electrical field perpendicular, called 1-3 mode, are common in low frequency application range.
• Bandwidth and Q: When it says the transducer has a center frequency of 5MHz, it doesn’t mean the transducer only works at exactly 5.0MHz, and it won’t work at 5.1MHz or 4.9MHz. It always has a range, and it is called spectrum if it drawn with vertical axis as magnitude and horizontal axis as frequency. Most good transducer will have bell or Gaussian shape spectrum curve. It has best response to the input excitation at center frequency, and the response will become weak as the excitation frequency moves away from the center frequency. On the spectrum, with the maximum point marked as 0dB, two points can be found at both sides with magnitude of -3dB, -6dB, or any other number you can name. The frequency range between these two data points is called Bandwidth. It is obvious that bandwidth is always linked with a dB level, such -3dB bandwidth or -6dB bandwidth. On the voltage spectrum, -6dB is often used, and on the power spectrum, -3dB is more commonly used. Q is a simple name of “Quality factor”, is the ratio of center frequency over the bandwidth. The lower the Q, the wider the bandwidth, and the pulse will be short. For ultrasound imaging, the transducer need transmit a very short pulse to achieve sharp resolution, and thus a low Q is required for the whole system, or we can say, the imaging system is a wide band width system.  High Q system is for resonant, for example, a crystal watch has a very high Q.
• Damping: Damping is to decrease the system Q. For ultrasound transducer, it normally means the backing layer. Heavy damping results in wide bandwidth, short pulse length, but lower sensitivity. Doppler transducer usually has lower damping, and thus a higher sensitivity can be achieved since the Doppler signal is normally weak because it is generated from blood scattering.
• Matching layer: Most medical ultrasound transducer is based on piezoelectric ceramic or crystal, having a very high acoustic impedance (about 30MRyl), and human acoustic impedance is only about 1.5MRyls. Without matching layer, the vibration of the ceramics will be bounce back and forth inside itself and gradually die out, only a small port of energy can be released to the tissue with each time of bouncing. The final pulse enter the tissue will be long with a lower amplitude. With a proper matching layer, the pulse will enter the tissue with minimal lengthened.

### Ultrasound Beamformation

• Interference phenomena: Sound is an acoustic wave, following the superposition principle. Acoustic wave cause pressure at each location of the medium vary with time, and thus the particles to vibrate with time. If the pressure or the displacement of the particle is recorded, in most cases, it is a sinusoidal function: y = A sin (2π f t + Ф ), A is the amplitude, f is frequency and Ф  is the initial phase.  Inside the acoustic medium, or the acoustic field, Ф varied with location, and amplitude and frequency are the same if we don’t consider the wave spread and attenuation. If two pressure waves propagate to a single point, the final pressure at the point will be arithmetic summation of these two: y =  A1 sin (2π f1 t + Ф1 ) + A2 sin (2π f2 t + Ф2 ).  Depends on frequency and initial phase, the combination can be stronger or weak compared to the incident waves, and can even become zero if the two have the same amplitude and frequency, but opposite initial phase. If the wave sources and acoustic field are fixed, the initial phase for each location will be fixed too, and thus vibration at some points will be enhanced while other points may be weakened consistently.
• Huygen's principle: This principle of wave analysis, proposed by the physicist Christiaan Huygens (1629-1695), basically states that: Every point of a wave front may be considered the source of secondary wavelets that spread out in all directions with a speed equal to the speed of propagation of the waves.
• Aperture size and wavelength: The aperture is the active area that transmits or receives acoustic wave at certain moment. For a single-element transducer, the aperture size is the transducer element size. For array transducer, the aperture are all the elements that works together simultaneously. To achieve a confined beam, the aperture size need to be much larger compared to the sound wave length. At 5MHz, the ultrasound wavelength is about 0.3mm in water, and a 5mm diameter transducer will give a decent beam. However, at normal sound frequency such as 1kHz, the wavelength is about 0.3m, it need a 5m diameter speaker to give a sound beam that propagate forward. Since most speakers are small compared to the sound wavelength, and they behave like a point source, with sound spread all the directions.
• Beam field from a piston aperture: The most simple transducer shape is a piston transducer. The beam from a piston transducer is similar to a flash light beam.
• Acoustic pressure along central axis have many maximums and minimums and from the last peak, it goes down monotonously.
• Cross section view of the beam at different depth vary with depth.
• Longitudinal section view view
• Main lobe and side lobes
• Near field and farfield:
•  At each sound field point location, the acoustic pressure is the summation of contributions from each point at transducer surface.   When aperture size is much bigger than the wavelength, the points locations within the transducer area and close to the center see an unlimited aperture,  at same depth, will receive the same amount of acoustic contribution from the nearly unlimited transducer surface, and thus ultrasound wave behaves like plane wave. However, the locations close to the edge still see the limited aperture, and thus the plane wave area is smaller than the aperture area. Moving away from transducer, this plane wave zone decreases quickly.
• For a point at the central axis of the aperture surface, the biggest time difference for sound to travel from different points on aperture surface to it is from aperture center point compared from aperture edge point. This time difference vary lot at distance close to aperture surface, and acoustic pressure will become maximum minimum alternatively. At certain depth it became one wavelength, and from there it slowly decrease  to infinitesimal when depth goes to infinity, and accordingly the acoustic pressure will decrease monotoneuosly.
• The acoustic field before this depth is called near field, and beyond this depth is called farfield. Since acoustic intensity is unpredictable in near field, and strictly speaking, it should be avoid to use it for echo information. However, for imaging ultrasound, since it is wide bandwidth, the acoustic intensity is also uniform in near field, and thus near field is not so serious.
• Beamwidth: Beam width is usually calculated from the cross-sectional or longitudinal section acoustic field view, and it is a parameter related with dB level. On cross-section view, draw a line through the center, or on longitudinal section view, draw a line at certain depth perpendicular to the central axis, a 1-D acoustic profile is obtained. On this profile, -xdB level bean width is the distance between the two points that have this dB level intensity.  Beam width can also be represented in angle. At certain distance, normally the focal or natural focal depth, draw a half circle center at the aperture center, and along this half circle, a 1D acoustic profile can be obtained. One this 1-D profile, the horizontal axis is angle from -90 to +90 degree, the beam width will be the angle difference between the two dB level points.
• Natural focus: ultrasound beam from a flat aperture will get narrow and then spread out within and angle range. The depth where beam is most narrow is the natural focus of the aperture.

### Ultrasound Imaging Resolution

• Axial resolution: Axial resolution is the minimal distance in depth, or ultrasound propagation direction that the imaging system can distinguish. Because ultrasound imaging using pulse-echo method, the pulse length determines the axial resolution. In ideal situation, the pulse is a Gaussian shape sinusoidal wave.  The echoes from two point targets on the beam path will be totally separated when their distance is larger enough, for example, larger than half the -40dB pulse length. The echoes will be get closer when the targets distance decrease, and will merge together when they are close enough, such as the distance is smaller than half of the -3dB pulse length. Since the pulse length is related with bandwidth, the shorter the pulse, the wider the bandwidth, and thus the wide bandwidth system is required to achieve higher axial resolution. For Gaussian shape sinusoidal pulse, it need contain minimal one cycle of sine wave, and thus the higher is the frequency, the shorter is the pulse length when bandwidth is fixed. For this reason, high frequency system will give better axial resolution.
• Lateral: In ultrasound imaging, ultrasound pulse travel in depth direction, and perpendicular to the depth direction, the beam scan direction is called lateral direction. Lateral direction is also parallel to the transducer surface.
• Point spread function: In a perfection imaging system, a point target will have a point correspond to it on the image. However, for ultrasound imaging, the ultrasound pulse has irregular 3D spatial shape, and thus, the image correspond to  a point target will be spread out, also called Point Spread Function (PSF). A typical ultrasound PSF looks like a flying bird.
• Lateral resolution from a focused aperture: The lateral resolution is determined by the beam width, and the higher the frequency, the thinner the focused beam width. To achieve higher lateral resolution, high frequency and strong focus is required.
• Slice Thickness Resolution (Elevational Resolution): Perpendicular to the depth and beam scan direction, is called elevation direction. And the resolution in this direction is called elevation resolution. For a round or square transducer, the beam is symmetry in lateral and elevation. However, if the aperture is rectangle, or other asymmetry shape, elevation resolution and lateral resolution is different. Normally, elevation or slice thickness resolution is worse than lateral.

### Ultrasound Array Transducer

• Linear array: In a linear array transducer, all the elements form a line. The element usually as a rectangle shape with width in the array direction or lateral direction, and height in elevation direction. The center to center distance of the two adjacent elements is defined as pitch size and gap between them is called kerf. Normally the pitch size is required to be smaller than 1 wavelength, but some cases it can be 1.25 wavelength.  The kerf is normally around 50 microns depends on the blade thickness to cut the elements. Linear array is widely used when acoustic window is big enough, such as abdominal or limb vessel scan. In a linear array, each time only a group of elements work together to transmit or receive, i.e. the aperture size is smaller than he transducer active surface. The ultrasound beam is perpendicular to the transducer surface, and scan a rectangle area.

• Phased Linear Array: it is exactly the same as linear array in term of element arrangement. For phased linear array, the element pitch size is required to be smaller than half of the wavelength. In a phased array, all the elements work together, i. e. the aperture are all the elements, and the aperture size is the whole transducer active surface. Phase array steer the beam by apply different delay on each element, and it requires small acoustic window. It is widely used in cardiovascular scan where the rib gap is the small acoustic window.
• Curved array: Curved array is very similar to the linear array. All the elements form a line, but it is curved and most likely is convex. Compared to the linear array, it gives a bigger scan area when acoustic window is limited.

• Annular array: An annular array consists concentric rings elements with the center one having a round shape. Annular array elements work together and normally have time delay to achieve focused beam. Due to its special geometry shape, annular give best focused beam with focal depth adjustable electronically. Most annular array have equal area elements to keep impedance of each the same.

• Circular array: a circular array has all the elements form a circle, facing one side.
• 1.25D 1.5D 1.75D array: in linear array, either linear scan or phased scan, there is only one element in elevation direction, i.e. the direction perpendicular to the scan direction and depth. The focal depth cannot be changed in this direction also it can be dynamically changed in lateral direction. To improve this capability, the element is divided into several pieces in elevation direction, with the first one, or the primary one is bigger in size. Depends on the number of pieces and the size, it is called 1.25D. 1.5D, or 1.75D. It partially improves the elevation focus property.
• 2D array: in 2D array, the element forms a M X N matrix. Beam can be steered in all the directions and thus the transducer is capable of scanning a volume, generating echo information for a 3D image. A 64 x 64 element array will require more than 3600 channels for beamforming, and it increase the cost of the imaging system greatly.

### Ultrasound Beam Focus

• Method: When aperture is concave, the uniform excitation on the aperture will generate a concave wave front, and it will converge to a point when propagating. As shown in the following figure: The concave wave front that is necessary to achieve the focused beam can be obtained by mechanically shape the aperture into concave, or the using an array transducer and control the transmit delay of each element. Theoretically, the round concave surface gives the best focus effect. The focal effect from of transducer can be improved with more element, smaller element size, and high resolution delay control.
• F number: is the ratio of focal depth over aperture equivalent diameter, it is the diameter for round aperture, and the total aperture element length for  array transducer. Focus effect is greatly related with frequency, focal depth and aperture size.  When focal depth and aperture size are combined into f number, and so the focus effect will be determined by f number and frequency.
• Focal zone characteristics
• Beam width: The beam width is narrowest at focal point, and it is governed by the f number and center frequency:
• Focal distance (length): For a focused beam, it is narrowest at focal depth, and spread on both sides away from the focal depth.
• Maximal Intensity: Beam also get maximal intensity at focal point.

### Ultrasound Beam Steering

• Mechanical: The transducer can be swing from side to side or rotation driven by a motor and in this way the beam can be steering like a flashlight beam scanning an area. Because more mechanical scanning transducer is a big single element, and most likely is a round shape, it has best beam focus quality for fixed focal depth. However, since the transducer is in motion, there must be an enclosure to hold the whole motor-transducer assembly, and the whole chamber has to be filled with special liquid for acoustic coupling. For this reason. The mechanical scan transducer will have a shorter life time. The common failures includes the bubbles  in the chamber, swing or rotation non-uniform, and motor failure. Most mechanical scan transducer use serve motor instead of step motor, the transducer is in constant motion while transmitting and receiving, and thus it is difficult for some application that requires multiple transmits at the same locations such as Doppler imaging.
• Phase array:
• Apodization:
• Dynamic Aperture:
• Dynamic focus:

### Ultrasound Pulse Echo Method

Ultrasound imaging is based on pulse-echo method. The ultrasound transducer transmits an ultrasound pulse and the switch to listen mode, recording the echo reaches the transducer surface. The echoes from targets close to the transducer will return firstly, and later for the echoes from further targets. Since the ultrasound travels inside homogeneous medium at a constant velocity and along a straight line, and thus the distance can be easily calculated. Most targets are moving and thus the pulse has to be repeated in certain frequency to track the target.

• Range equation

In human tissue or room temperature, the sound velocity is around 1540m/s or 1.54mm/ms. Assuming the pulse is transmitted at time 0, and at time T, the echo from target arrives, then the distance from transducer surface to target D is calculated:

For example, if it takes 10ms to receive the echo, then the distance from the target to transducer surface is about 7.7mm in water.

• Pulse duration

When transmits an ultrasound pulse, the pulse has a time duration.  If the pulse center frequency is 1MHz, then a single cycle of the carrier wave is 1ms. The pulse transmitted has to include at least one cycle since it is an alternative signal or energy. At certain frequency and amplitude, the longer the time duration of the pulse, the more energy of it, and thus it can transmit further before it die out due to the attenuation. If the pulse is too long, the echoes from targets that are close to each other may merged together. In this way, a short pulse is preferred to distinguish close target. It is obvious that with the same cycle number, higher frequency pulse will result in shorter pulse, and can detected finer target.

• Pulse repetition frequency, period

As mentioned above, pulse need to be repeatedly transmitted to track targets if it is moving. However, the next pulse can be sent out only when the echo from the furthest target has returned. Otherwise, the echo from the far target of the previous pulse and the echo from the near target of the recent pulse will come the same time and true target location cannot be determined. So if the maximal detection depth is D, and sound velocity is c, then the minimal time interval between two pulse is :

This time interval is also called pulse repetition period, and according f = 1/T is called pulse repetition frequency (PRF). The higher the PRF, the lower the maximal detection depth, and the faster the detectable moving target.

• Pulse spatial shape

As soon as the ultrasound pulse energy leaves a piston transducer surface and moves forward, it starts to spreads in all the directions, but the main energy will confine in a disk shape when it is very close to the transducer surface. The diameter of the disk is the same as the transducer surface and the thickness of the disk is sound velocity multiply pulse duration. This disk spreads as it moving forward, and eventually will become a dome shape.

• Duty factor

The definition of duty factor is the pulse duration over the pulse repetition period, or the pulse duration multiply PRF. For imaging ultrasound, energy duty factor is very low, and thus have very less averaged power.

### Ultrasound Transmitter

• Pulser
• Linear

• Protection circuit, TR switch
• Impedance control
• Gain

### Time Gain Compensation (TGC)

• Effect on Image
• Method

### Envelope Detection

• Demodulation
• Rectification
• Rejection

### Dynamic range compression

• Dynamic range from ultrasound signal
• Dynamic range of display equipment
• Method

• A-Mode
• B-Mode
• M-Mode
• C-Mode
• BD-Mode

### Ultrasound Image Frame Rate

• Image depth and PRF
• Frame rate, Number of lines per frame, and depth

### Preprocessing and Postprocessing

• Preprocessing
• Postprocessing
• Freeze frame
• Black/white inversion
• Contrast variation
• Frame averaging
• Edge enhancement

### Ultrasound Spectrum Doppler

• Doppler effect
• Continue wave ( CW )Doppler
• Pulsed wave ( PW )Doppler
• Aliasing
• Range ambiguity
• Frequency and flow speed
• Narrow band width transducer
• Demodulater
• Wall filter
• Demodulater

### Ultrasound Color Doppler Flow Imaging

• Color Flow Map
• Transmit method
• Autocorrelation
• Flow direction
• Average velocity
• Velocity variance
• Time domain process
• Color maps
• Hue
• Saturation
• Luminace

• Method
• Application

### Ultrasound Ultrasound Image Artifacts

• Definition
• Resolution related
• Speckle
• Propagation related
• Mirror image
• reverberation
• Comet-tail
• Ring-down
• Side lobe
• Attenuation related
• Enhancement
• Focal Enhancement or Focal Bandin
• Doppler related
• Aliasing
• Incident Beam Angle
• Clutter
• Ghosting or Flash

### Acoustic Output

• Pressure
• Unit
• Peak pressure
• hydrophone
• Power
• Intensity
• SATA
• SPTA
• SPPA
• SPTP
• Cavitation
• Mechanical index
• Thermal index
• TIS
• TIB
• TIC

### Guidelines and Regulations

• American Institute of Ultrasound in Medicine (AIUM) Statements
• National Electrical Manufacturers Association (NEMA)
• Food and Drug Administration (FDA)