# Ultrasound Physics

What is US? US is defined as a sound wave with a frequency above 20,000 cycles per second or hertz(Hz)One Hz means one event per second. Ultrasound frequency is inaudible to humans.
What is audible sound? Audible sound is defined as a sound wave that can be heard by man. The frequency range of audible sound is from 20Hz to 20,000 Hz.
What is infra sound? Infrasound is a sound wave with a frequency less then 20 Hz. So low it is in audible to human hearing.
Sound Travels through a medium & carries energy from one place to another & consists of orderly oscillations of particles within the medium. Oscillations produce compressions & rarefactions throughout the medium.
At what speed does sound travel through the body? speed of sound in soft tissue is 1,540 meters per second or 1.54mm a ms
Appx.one mile per second through the body.
Only the characteristics of the medium effect speed.
The speed of sound is NOT effected by the characteristics of the wave.
Rank these four media according to their propagation speed.(IE: how fast sound travels through them)
bone, fat, lung and soft tissue
1. lung
2. fat
3. soft tissue
4. bone
All sound waves travel at 1540 meters per second.
1. What is the frequency of a sound wave?
2. What units are used to report frequency?
3. What's the frequency range of US waves used in diagnostic imaging?
Frequency: # of times per second that particles in a medium oscillate back and forth as a sound wave propagates thru a medium.
2. Units: per second or Hz
3. Frequency Range 2-10Mhz (2-10 million cycles per second.
1. What is wavelength?
2. What are the units of wavelength?
3. How is wavelength determined?
1. Wavelength- distance occupied by a single cycle in a wave.
2. Measured in units of distance, most commonly mm's.
3. Wavelength is determined by the freq. of sound & the medium through which sound travels
Wavelength= prop.speed/frequency c/f
What is the importance of wavelength in diagnostic imaging? Helps to determine image quality. Shorter wavelength > Higher Quality image.
Wavelengths of US traveling thru soft tissue range from 0.2 – 0.8 mm.
Inverse relationship between frequency and wavelength. Higher freq. wavelengths have shorter wavelengths

What is the wavelength of 1 MHz sound in soft tissue? What is the equation for determining wavelength?

1.54 mm

wavelength (mm)= propagation speed/frequency

Three specific terms are used to describe the size or magnitude of a sound beam: pressure, power and intensity. What is the meaning of the term pressure? What units are used to measure pressure? Compression and rarefaction create a pressure wave. Pressure is the concentration of force within a particular area. Units are Pascals (Pa).
What is the meaning of the term power? What are the units to measure power? Power is the amount of energy per second that is delivered by an US beam. Power is measured in Watts.
What is the meaning of the term intensity? What units are used to measure intensity? Intensity is the concentration of power within an area of the soundbeam. It is measured in W/cm2.
What is a decibel? Decibel(dB)is used to measure relative change in the intensity or power of a sound beam.
What is the meaning of 3dB? -3dB?
What is the meaning of 10db? -10dB?

3dB means the beam's intensity has doubled.
-3dB means the beam's intensity has halved
10db means the beam's intensity is 10 x greater
-10dB means the beam's intensity is reduced by 1/10th

What is PRF? What units are used to measure PRF? What determines PRF?

PRF is the number of sound pulses emitted by a transducer in one second.

Units are Hertz (Hz). Hertz means per second.

PRF is determined by Depth of View. Shallow depth=high PRF. Deep depth= low PRF.

What is PRP? What units are used to measure PRP? What determines PRP? PRP is the TIME (period=time) from the start of one pulse to the start of the next pulse.
Units are any measure of time (seconds, ms)
Determined by the US system.
What is the duty factor? What are the units? Why is it important? What determines duty factor? Duty factor is the PERCENTAGE of time an US machine is transmitting sound. It has no units it is expressed as a percentage. Determined by sound source. Adjusted by changing depth of view.
Duty Factor equation duty factor(%)= pulse duration/ PRP, then multiple by 100 to get percent
What is reflection? What causes reflection? Reflection is the redirection of an ultrasound wave back towards the transducer. Reflection is caused by the difference in IMPEDANCE between two boundaries. High impedance=high reflection. Low impedance=low reflection.
What is specular reflection? What is scattering? What two factors affect scattering? specular reflection is when a wave strikes a smooth boundary; it is organized and regular. Scattering is reflection of a wave in many different directions (chaotic, disorganized). Scattering is determined by the boundary and frequency.
A sound wave travels through the body and strikes a boundary between to media (incidence).
What is normal incidence and what is oblique incidence?
Normal incidence is a sound wave striking the boundary at exactly 90 degrees (perpendicular, right angle, orthogonal). Oblique incidences is striking a boundary at and angle other than 90 degrees.
What is attenuation? What three processes contribute to attenuation? What is the relationship between attenuation and frequency? Attenuation is the weakening of the strength of a sound beam as it travels. The processes that contribute to attenuation are: reflection, scattering and absorption. High freq=high attenuation, hence why you use lower freq when imaging deeper
Attenuation of ultrasound in media, low to high water: extremely low
blood, urine: low
fat:low
soft tissue: medium
muscle: higher
bone and lung: even higher
air: highest

What is a transducer? What is the piezoelectric effect?

A transducer is ANY device that converts one form of energy into another.

Piezoelectric effect is the process by which pressure energy is converted into electrical energy.

What is the Curie temperature or Curie point? If a piezoelectric is heated to temperatures in excess of the Curie temp/ Curie point, it will permanently lose ifs piezoelectric properties. It is between 300 and 400 degrees C.
Name the components of an ultrasound transducer Active element (piezoelectric material), Matching layer, backing (damping) material, wire, case
What does the active element of the US transducer do? It transmits and receives acoustic pulses. Most active elements are made from lead zirconate titanate better known as PZT.
What does the matching layer of an US transducer do? The matching layer permits more sound energy to travel into the body during transmission and reflected out of the body during reception; it is in front of the active element (basically, it reduces the impedance b/t the active element and the skin)
What is impedance? What are the units used to measure impedance? How is it calculated? Impedance is the acoustic resistance to sound traveling in a medium; it is measured by rayls (Z). Determined by the medium only. Calculated by density of a medium X the prop. speed of the medium
For the maximum amount of sound energy to be transmitted from one medium to another… …the acoustic impedance's of adjacent media should be as similar as possible, i.e. the impedance of the matching layer should be as close as possible to the impedance of the gel
What is the function of the backing (damping) material of an US transducer? What material is it made out of? It prevents the a active material from "ringing". It is often made from tungsten fibers embedded into an epoxy mixture. In US transducer, the order is skin, gel, matching layer, active element, damping material
Name the "parts" of the ultrasound beam between the face of the transducer to the end of the sound beam Near zone (Fresnel zone)=to diameter of crystal
Focus=point where the beam is at its smallest diameter, it is surrounded by the focal zone
Far zone (Fraunhofer zone), beam progressively widens
What is the significance of the focal zone? It is the region of the US beam that provides the highest quality images. Lateral resolution is highest at this point.
What is diffraction? Sound waves produced by very small sources (i.e. tiny pieces of PZT) diverge in the shape of a V; these v shaped waves are known as diffraction patterns or Huygen's wavelets.
Describe Huygen's principle Huygen's principle states that a large active element may be thought of as millions of tiny, distinct sound sound sources. These numerous sources interfere constructively and an hourglass main sound beam is produced.
What is lateral resolution? What are the units? What are the synonyms? Lateral resolution is the ability to distinctly identify two structures that are very close together when the structures are PERPENDICULAR to the sound beam. Units of distance. LATA: Lateral, Angular, Transverse or Azimuthal.
What determines lateral resolution? Lateral resolution is determined by the width (diameter) of the sound beam (narrow beam: better lateral res.) Diameter is affected by focusing and frequency (and therefore depth)Improved by decreasing beam diameter.
What two factors have an effect on the length of the near zone? Hint: length of the near zone is also the depth of the focus. Two factors that affect the length of the near zone/depth of focus are: diameter of the transducer's active element and frequency of the US wave. The greater the diameter the longer the near zone.

How do you improve lateral resolution?

Increase the frequency

Decrease the beam diameter (focus)

What is axial resolution? What are the units and synonyms of axial resolution? Axial resolution describes the ability of the US system to identify two adjacent structures that lie PARALLEL to the sound beam. Units of distance. LAARD -Longitudinal, Axial, Radial, Range and Depth
What determines axial resolution? The spatial pulse length (spl)
SPL= # of cycles in a pulse x wavelength
The smaller the SPL, the better the axial resolution

How do you decrease spatial pulse length and therefore improve axial resolution?

You can increase the number of cycles in a pulse by damping OR

Decrease the wavelength by increasing frequency

REMEMBER: higher the freq, shorter the wavelength
wavelength= prop speed/freq

What is the bandwidth of an acoustic pulse? What effect does the backing material have on the bandwidth? Bandwidth represents the range of frequencies that are present within an acoustic pulse. In general, longer pulses had a narrow bandwidth and shorter pulses have a wide bandwidth; damping material keeps pulses short therefore keeps wide bandwidth.
What is the quality factor (q-factor)? What is the equation to determine q factor and what values are used in diagnostic imaging? Q factor is the degree to which a transducer dampens (shortens) an US pulse. q factor=freq/bandwidth. It is unitless, q factor for diagnostic imaging is b/t 2 and 4, and qfactor for imaging is less therapeutic US
What is the shape of the active elements and how are they arranged in a:
1. linear array transducer
2. annular array transducer
3. convex array transducer
1. Linear: rectangle shaped elements arranged in a straight line 2. Annular: ring shaped elements in a concentric pattern 3. Convex: rectangular shaped and arranged in an arc
Temporal Resolution (time) AKA: Frame rate Abilty to correctly display moving structures in real time. Frame rate =PRF/LPF.
Inc.Depth, dec. FR, dec PRF
Inc. focuses, dec. FR, dec PRF
Inc. LPF., dec.,FR, dec., PRf
Elevation Resolution Determined by beam Slice thickness. If slice is thick small cystic structures may not appear cystic because beam is wider than cystic structure.
Contrast Resolution Abilty to distinguish similiar structures with varying grey scales.
Reynolds Number The point @ which flow becomes turbulent. Above 2000.
Average flow speed x tube diameter x density
Poiseuelles law Amount of flow(Q)is dependant on.
-Pressure differential
-Length of the vessel
-Viscosity of the blood
inc. Friction- dec flow
Snells law Angle of sound transmission @ an interface between media having different propagation speeds.
Refraction of sound @ an interface obeys Snells law.
Bernoullis Law Energy cannot be created or destroyed, there must be equalibrium.
Increase velocity,Decrease pressure.
Rayeligh Scattering Occurs when reflector is smaller than wavelength of the beam. (RBC)
Increase freq.- Increase rayleigh scattering
Intensity Reflection Coefficient (IRC) %of sound reflected at an interface.
Intensity Transmission Coefficient (ITC) %of sound that continues thru the medium & is not reflected.
IRC + ITC = 100%
ITC = 1 – IRC
Sound Frequencies Infrasound < 20 Hz
Audible Sound 20-20,000 Hz
Ultrasound > 20,000 HZ
Specular Reflectors Wavelength smaller than interfaces.
Smooth surfaces
EX: boundries. EG: diaphragm, bladder wall
Non-Specular reflectors Wavelength larger than interface
heterogenous surfaces
EG: liver parenchyma, RBC
Pre & Post processing PRE-PROCESSING – Cannot be performed on a Frozen Image. POST PROCESSING – Can be performed on a Frozen Image.
Refraction Change of direction of sound with oblique incidence. Oi REFRACTION: -Oblique Incidence -Speed Mismatch -Angle of transmitted sound
Reflection -Perpendicular Incidence -Impedence Mismatch -%of intensities of reflected or transmitted sound
Continuity Rule Volumetric flow rate must be the same proximal, at and distal to a stenosis.
Order of generating a sonographic image Pulsar->beam former->receiver->memory->display
Normal Incidence aka: perpendicular or orthogonal occurs when angle of incidence is 90 deg. direction of travel is perpendicular to the interface between the media. Sound may be transmitted, reflected or both.
Roulex Formation Adhering of RBC that occurs @ lower velocities & low shear rates.
Produces a large echo.
Intima Layer of vessel wall that's a single layer of cells & backed by a thin layer of Elastin & collagen fibers.
Media Layer of vessel wall that contains smooth muscle fibers.
Doppler Frequency Shift Doppler beam encounters increased flow. A large difference occurs between transmitted & received frequencies.
SMPTE Test Pattern
(Society of Motion Picture & Television Engineers)
Aids in the setup & quality assurance of displays & cameras.
Hydrophone Measures acoustic pressure output.
Frank-Starlingart Mechanism When an arryhhmia is present, peak systolic velocity following a heart cycle with a longer interval is higher than one with a shorter time interval.
Nyquist Limit Doppler Limit @ which aliasing occurs.
=1/2 the prf
Aliasing is directly related to sample depth.
Math G-Giga 1,000,000,000 10 to the 9th
M-Mega 1,000,000 10 to the 6th
K-kilo 1,000 10 to the 3rd
Math (negative) d-Deci 0.1 10 -1
c-centi 0.01 10 -2
m-milli 0.001 10 -3
u-micro 0.000001 10 -6
n-nano 0.00000000 10 -9
Impedance Impedance = Density x Speed of Sound
z=pc

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