Term
| What is the spatial average? |
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Definition
| The AVERAGE intensity ACROSS the beam. |
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Term
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Definition
| The MAX intensity ALONG the beam. |
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Term
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Definition
| - The temporal peak is the maximum intensity during a pulse. |
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Term
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Definition
| - The pulse average is the average intensity during a pulse. |
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Term
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Definition
| The average intensity over PRP. |
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Term
| What are the different Intensities? |
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Definition
- SPTP
- SPPA
- SPTA
- SATP
- SAPA
- SATA |
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Term
| Which intensity has the highest value? |
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Definition
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Term
| Which intensity has the lowest value? |
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Definition
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Term
| Explain Duty Factor and its units. |
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Definition
Recall DF = PD/PRP
so in terms of intensity: DF = TA/PA
units: none
- Since the TA < PA, DF < 1 |
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Term
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Definition
- Beam Uniformity Ratio
BUR = SP/SA
- Units: none
Since SP > SA, BUR > 1 |
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Term
| What are bioeffects and what are the different kinds? |
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Definition
- Bioeffects are the effects that ultrasound causes on biological tissues.
There are two kinds:
> Thermal
> Mechanical |
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Term
| Explain Fetal tissue thermal bioeffect |
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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) |
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Term
| Explain how much energy is required to do damage with thermal bioeffects. |
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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 |
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Term
| Explain thermal bioeffects in terms of ISPTA and Thermal Index |
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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. |
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Term
| Explain Thermal Index (TI) |
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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) |
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Term
| Explain Thermal Bioeffect for the different modes of US |
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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 |
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Term
| Explain Mechanical Bioeffects |
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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. |
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Term
| What are the types of Cavitation? |
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Definition
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Term
| Explain Stable Cavitation |
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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. |
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Term
| Explain Transient Cavitation |
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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. |
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Term
| What does mechanical bioeffect depend on? |
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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 |
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Term
| * What are the AIUM recommendations to reduce bioeffects? |
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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 |
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Term
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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. |
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Term
| Explain the Schlieren System |
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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. |
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Term
| Explain the medical use of thermal bioeffects. |
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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. |
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Term
| Explain the medical use with mechanical bioeffects. |
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Definition
- Cavitation is used for:
> lithiotripsy (breaking down kidney stones)
> in dental (scaling of plaque)
> as a sterilization technique |
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Term
| What are the other potential dangers with ultrasound? |
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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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