Page authurs

Dwayne Morris, MD

Matt Jones MD

 

SONOGRAPHY PRINCIPLES & INSTRUMENTATION

 

SPI outline and notes

  1. Clinical Safety, Patient Care, and Quality Assurance 10%

    1. Patient Care

    2. Quality Assurance

    3. New Technologies

  2. Physical Principles   15%

    1. frequency of hearing

      1. 20 - 20,000 Hz

    2. The typical range of frequency for diagnostic ultrasound imaging

      1. 1 to 20 MHz-often written as 1000kHz-20000 kHz

    3. Huygens Principle = each point of a wave acts as an individual point of propagation

    4. Sound is a form of energy. It is a pressure wave, created by a mechanical action, and is therefore called a mechanical wave

    5. All Sound is either transmitted or reflected

    6. Stiffness (elasticity) and the density (inertia) of the medium

      1. Stiffness is defined as the ability of an object to resist compression and relates to the hardness of a medium

    7. Only 2 factors increase with speed of propagation of sound when they are increased: Stiffness and temperature

    8. Speed of sound in medium = wavelength x frequency

      1. Wavelength (mm) = Propagation Speed / Frequency (MHz)

      2. Propagation Speed (m/s) = Frequency (Hz) x wavelength (m)

    9. Each cycle consists of two parts: a compression, where the molecules are pushed closer together, and a rarefaction

    10. Pressure is measured in pascals

    11. The three primary acoustic variables are pressure, density, and distance

    12. This resistance to the propagation of sound through a medium is called impedance (z).

    13. The deeper the wave travels, the more deformed, or nonsinusoidal, it becomes. This is called nonlinear propagation. Therefore harmonics are used.

    14. Tissue Interactions

      1. Specular reflections - occurs at flat, smooth interfaces where the transmitted wave is reflected in a single direction depending on the angle of incidence. Examples of specular reflectors are fascial sheaths, the diaphragm and walls of major vessels

      2. Diffuse Scattering

        1. Large Obstacle Scatter = posterior shadow, gb stone

        2. Small Obstacle Scatter = no shadow

      3. Rayleigh Scattering - micron size, red blood cell size

      4. Refraction - Snell’s Law

      5. Multiple Paths - will produce the mirroring artifact

    15. 13 microsec for every cm traveled and returned

    16. Attenuation in loss of sound energy, occurs from multiple things, largest contributor is mechanical conversion to heat

    17. Attenuation equation =  Half value layer, 3 dB of attenuation, the depth at which half the sound is gone

      1. Attenuation Coefficient (dB/cm) = Frequency (MHz)/2

      2. Attenuation Coefficient (dB/cm)  = 0.5dB / cm / MHz

      3. Total Attenuation (dB) = Attenuation Coefficient (dB/cm) X Distance

  3. Ultrasound Transducers  16%

    1. See the guide sheet for steering!

    2. piezoelectric materials-a material that generates electricity when pressure is applied to it

      1. PZT materials operate according to the principle of piezoelectricity, which states that pressure is created when voltage is applied to the material and electricity is created when a pressure is applied to the material

      2. Piezoelectric crystal. When a voltage is applied across the crystal, the highly polarized molecular dipoles rotate, causing the crystal to thicken and produce ultrasound. Conversely, when ultra- sound is received, the mechanical vibration of these structures produces an output voltage.

  1. damping-the process of reducing the numberof cycles of each pulse in order to improve axial resolution

      1. The backing or damping layer reduces the long "ring" of a vibrating crystal to two or three cycles per pulse

    1. grating lobes-an artifact caused by extraneous sound not along primary beam path; occurs with arrays; reduced or eliminated by apodization, subdicing, and tissue harmonics

      1. Or put another way apodization, which is used to decrease the risk of grating lobes. Apodization works by decreasing the strength of the voltage pulse sent to the outermost element

      2. Wave interference is important for the use of harmonics

        1. Tissue Harmonic Imaging

        2. Nonlinear propagation of sound Harmonic signals are produced by the patient, not the transducer

        3. Harmonic beam is weaker (lower amplitude than the fundamental) but travels only one way: from the patient to the transducer

        4. Second harmonic is twice the transmitted (fundamental) frequency

        5. Elimination of near-field artifacts (noise, reverberation)

        6. Elimination of grating lobes

    2. Speed of Sound is 1540 M/sec (average) in medium

      1. 1.54 mm/microsec

      2. Period x Frequency = 1

  1. Imaging Principles and Instrumentation  28%

    1. Types of Resolution

      1. Axial Resolution - improve with depth change

        1. Axial resolution (mm) = SPL (mm) / 2

        2. Axial resolution (mm) = (wavelength (mm) X # cycles in pulse) / 2

        3. Axial resolution (mm) = 0.77 x #cycles in pulse / Frequency (MHz)

      2. Lateral Resolution - improve with change in focal zone

        1. Near Zone Length (mm) = Diameter (squared) (mm) / 4 x wavelength (mm)

        2. Narrow beam- better lateral resolution

      3. Elevation Resolution - The resolution of the slice thickness, difficult to change or modify

    2. ring-down-a type of reverberation artifact caused by air

    3. rejection-function of the receiver that is used to reduce image noise; sets a threshold below which the signal will not be displayed

    4. scan converter-the part of the ultrasound machine that processes the signals from the receiver; consists of the A-to-D converter, computer memory, and D-to-A converter

    5. variations in impedance that help create reflections at the interface between adjacent tissues

    6. Pulse duration Only the active time, or "on" time, that a transducer is pulsing

    7. Spatial Pulse Length

      1. SPL is defined as the length of a pulse

        1. SPLs mean shorter PDs, which results in better axial resolution and improved overall image quality.

        2. If the number of cycles of the pulse increases, the SPL also increases. Either of these would result in a longer lasting pulse or a long PD. SPL, like PO, can be controlled with damping or backing material. Damping reduces the SPL by reducing the number of cycles of each pulse. This damping reduces the PO and SPL and subsequently improves axial resolution.

    8. pulse repetition frequency-the number of pulses of sound produced in 1 second

      1. The PRF changes whenever the sonographer adjusts the depth control on the ultrasound machine, the deeper the area of interest, the slower the PRF

    9. Key equations

      1. Pulse Duration (us) = # cycles / Frequency (MHz)

      2. Pulse Duration (us) = # cycles X period (us)

      3. Intensity (W/cm2) = Power (w) / Area (cm2)

      4. PRP = 1 / PRF

      5. PRF = 1 / PRP

      6. PRF x PRP = 1

      7. Duty Factor (%) = Pulse Duration / Pulse Repetition period X 10

      8. PRP = imaging depth (cm) X 13 microseconds / cm

      9. PRF (Hz) = 77,000 cm / s / imaging depth (cm)

  2. Doppler Imaging Concepts  31%

    1. Sound that is continuously transmitted is termed continuous wave (CW) sound. We can· not image using CW ultrasound, though it is often employed for Doppler studies

    2. oblique incidence-angle of incidence is lesser than or greater than 90° to the interface

    3. Important to know is the 13 microsecond rule: it takes 13 microseconds (microseconds) for sound to travel to a depth of 1 cm and return

    4. 1.54 mm/f.LS can be used to reveal an equation that assumes soft tissue: d = 0.77t

      1. This range equation-equation used to calculate the distance to the reflector; in soft tissue, d = 0.77t where "d" is the depth of the reflector and t represents round trip time.

    5. Aliasing

      1. correct aliasing

        1. lower baseline

        2. increase scale (increase PRF)

        3. Change to CW Doppler

    6. Nyquist Limit, when the returning wave frequency is over half the PRF. The high velocity flow can shift the returning frequency above the nyquist limit and thus information in lost

    7. Doppler Effect

      1. Cos of zero is one (ideal)

      2. Cos of 90 is zero

      3. Usually the best you get is about 40-60 degrees

    8.  

 

Reciprocal relationships – if one goes up the other goes down frequency – wavelength

      1. PRP – PRF

      2. duty factor – PRP

      3. frequency – penetration

      4. frequency – period

      5. beam area – intensity

      6. imaging depth-prf

Direct relationships – if one goes up the other goes up

      1. wavelength – propagation speed (wavelength depends on the propagation speed, propagation speed is determined by the medium

      2. duty factor – pulse duration

      3. duty factor – PRF

      4. path length – attenuation

      5. frequency – attenuation

      6. beam power – intensity

      7. density – propagation speed

 

 

 

Ultrasound waves

 

Artifacts 

 

 

The Doppler Effect.

This is what NASA has to say about it. 

This is what you should probably read for ultrasound. 

This is the best example available on the internet. 

Another great example.