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(D) 11 Bioeffects
MIT232
27
Medical
Undergraduate 2
06/04/2014

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Term
What is the spatial average?
Definition
The AVERAGE intensity ACROSS the beam.
Term
What is spatial peak?
Definition
The MAX intensity ALONG the beam.
Term
What is TP?
Definition
- The temporal peak is the maximum intensity during a pulse.
Term
What is PA?
Definition
- The pulse average is the average intensity during a pulse.
Term
What is TA?
Definition
The average intensity over PRP.
Term
What are the different Intensities?
Definition
- SPTP
- SPPA
- SPTA

- SATP
- SAPA
- SATA
Term
Which intensity has the highest value?
Definition
- SPTP
Term
Which intensity has the lowest value?
Definition
- SATA
Term
Explain Duty Factor and its units.
Definition
Recall DF = PD/PRP
so in terms of intensity: DF = TA/PA
units: none
- Since the TA < PA, DF < 1
Term
Explain BUR
Definition
- Beam Uniformity Ratio
BUR = SP/SA
- Units: none
Since SP > SA, BUR > 1
Term
What are bioeffects and what are the different kinds?
Definition
- Bioeffects are the effects that ultrasound causes on biological tissues.
There are two kinds:
> Thermal
> Mechanical
Term
Explain Fetal tissue thermal bioeffect
Definition
- When the mechanical wave of the ultrasound is converted into heat.
- The normal absorbtion of soft tissue is 0.5 db/cm*MHZ
- For bone it is 15db/cm*MHZ
- FETAL TISSUE ABSORBS THE MOST ULTRASOUND (SPECIFICALLY FETAL BONE)
Term
Explain how much energy is required to do damage with thermal bioeffects.
Definition
- If heating is significant, >2 degrees Celsius, protein coagulation may occur causing cellular damage.
- 2 degrees Celsius for <50 hrs doesn't't have any bioeffects on adult tissue.
- The more energy delivered over TIME (temporal average) the greater the thermal bioeffects.
- * It depends on:
> Intensity of the SPTA and Thermal Index (TI)
> PD
> PRF
Term
Explain thermal bioeffects in terms of ISPTA and Thermal Index
Definition
- ISPTA and Thermal Index BEST represent the amount of energy absorbed by the body over time.
- AIUM states that Thermal bioeffects are not reported for the following:
- ISPTA < 100 mW/cm^2 for an UNFOCUSED BEAM
- ISPTA < 1,000 mW/cm^2, and TI <2 for a FOCUSED BEAM.
- A common value for ISPTA = 720 mW/cm^2.
Term
Explain Thermal Index (TI)
Definition
- The thermal index is a ratio of the output power over the power required to cause heating by 1 degree Celsius.
TI = Output Power/P to cause heating by 1 degree C.
- There are three sub-indices
> Thermal Index for soft tissue (TIS)
> Thermal Index for bones (TIB)
> Thermal Index for Cranium (TIC)
Term
Explain Thermal Bioeffect for the different modes of US
Definition
- In B-mode and color mode multiple lines are obtained, meaning heat is DISTRIBUTED over a larger area.
- * But in non-imaging modes, a single line is rescanned multiple times, increasing the likelihood of thermal bioeffects:
> A-mode
> M-mode
> PW doppler
> CW doppler
Term
Explain Mechanical Bioeffects
Definition
- In ultrasound, sound is a mechanical wave with periods of rarefactions and compressions.
- Cavitation may result when small bubbles burst or implode.
- Bursting is more likely to occur however because because stretching is limited to surface tension, so bursting is most likely to occur with PEAK RAREFACTIONAL pressure.
Term
What are the types of Cavitation?
Definition
- Stable
- Transient
Term
Explain Stable Cavitation
Definition
- Micro gas bubbles expand and contract but do NOT BURST.
- It puts stress on surrounding tissues and if liquid is surrounding the bubbles than it undergoes MICROSTREAMING.
Term
Explain Transient Cavitation
Definition
- Gas bubbles expand and contract so violently that they explode.
- The result is shock waves with tremendous mechanical stress on surrounding tissues.
- High temperature MAY also result.
Term
What does mechanical bioeffect depend on?
Definition
- The peak intensity, ISPPA
- Mechanical Index
> MI = Peak Rarefactional pressure/Square root of the frequency
- Mechanical Bioeffects are not reported for a MI < 0.3
Term
* What are the AIUM recommendations to reduce bioeffects?
Definition
- Do not perform studies without reason
- Minimize scanning time
- Use low output power and maximum amplification to optimize the image
- Follow ALARA principle:
> As Low As Reasonably Achievable
Term
Explain Hydrophone
Definition
- A device (transducer) that measures acoustic pressure (amplitude) in a water tank.
- It is positioned along the three dimensions (axial, lateral, and elevational) at different angles.
- Data is plotted and analyzed.
Term
Explain the Schlieren System
Definition
- A high intensity focused light is used on a clear medium insonated by the ultrasound beam.
- Light is strobed in phase with ultrasound pulses.
- Medium disturbances caused by ultrasound are recorded.
Term
Explain the medical use of thermal bioeffects.
Definition
- Low frequency ultrasound (1-3MHZ) is the best form of heat treatment for soft tissue injuries. It is best to treat joint and muscle sprains, bursitis, and tendonitis.
- High Intensity Focused Ultrasound is used to thermally necrose tumors or cauterize bleeeds.
Term
Explain the medical use with mechanical bioeffects.
Definition
- Cavitation is used for:
> lithiotripsy (breaking down kidney stones)
> in dental (scaling of plaque)
> as a sterilization technique
Term
What are the other potential dangers with ultrasound?
Definition
- Electrical hazards:
- Transducer with a damaged casing
- Any other electrical component without proper grounding
* REMEMBER THAT HIGH CURRENT, NOT HIGH VOLTAGE POSES A GREAT DANGER *
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