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79. Theoretical and Technical Basis of Ultrasound Diagnostics

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yaboiemil's version from 2016-02-16 11:21

Section

1. Ultrasound is above the level of human hearing (20,000Hz)
2. Diagnostic Applications of Ultrasound use 2-20MHz
3. Longitudinal waves are used for diagnostic applications
4. The velocity of ultrasound as it passes through a medium of soft biological tissue is
1500-1600m/s

Equations for wavelength of ultrasound:

 

 

Question Answer
λWavelength
cPropagation Speed
fFrequency
memorize

 

5. Ultrasound is similar to light in that is can be FOCUSED as a beam using an acoustic lens,
and may also be REFLECTED, REFRACTED, ABSORBED and SCATTERED.

Characteristic impedances of some biological media in RAYL (kg/m^2.s):

 

Question Answer
Air0.0004 x10^6
Water1.52 x10^6
Lung0.26 x10^6
Fat1.35 x10^6
Brain1.55-1.66 x10^6
Blood1.62 x10^6
Soft Tissues1.6 - 1.74 x10^6
Bone3.75-7.38 x10^6
memorize

 

6. The acoustic impedance will tell if a beam of ultrasound will be reflected or refracted at
the boundary of 2 media.

Equation for amount of incident ultrasound energy reflected:

 

 

Question Answer
RFraction of incident energy reflected
Z2 & Z1Acoustic impedances of 2 media
memorize

 

7. Ultrasound cannot travel through a medium indefinitely, and will eventually be attenuated by
ABSORPTION and SCATTERING.
8. The attenuated depends on the frequency, the higher the frequency the higher the attenuation.
9. Attenuation is measured in db/cm OR half-value thickness.

Common Half-value thickness at 2.5MHz

 

Question Answer
Bone6.5mm
Brain/liver/Lungs15-30mm
memorize

 

10. A piezoelectric crystal is used to generate and detect Ultrasound. AC voltage is applied to the
crystal at the desired frequency - Ultrasound of this frequency is created.
11. If the crystal is compressed by reflected ultrasound, electrical potential arises at that
magnitude /direction (i.e. a receiver role)
12. THE MAIN MECHANISM OF ULTRASOUND IS ITS REFLECTION AT ACOUSTIC BOUNDARIES.
13. Gas totally reflects ultrasound, meaning you cannot examine gas-containing structures.
Also, you must exclude any air between the probe and the structure using a gel with similar impedance
14. Low frequency > Examination of deep structures
High frequency > Examination of superficial structures

 

 

Question Answer
Level of Human Hearing20,000Hz
Diagnostic Applications of Ultrasound2-20MHz, Longitudinal Waves
Speed of Ultrasound as it passes through tissue1500-1600m/s
Ultrasound Acoustic Impedance measured inRayl - Kg/m^2.s
Attenuation is higher whenfrequency is higher
Units for Ultrasound AttenuationdB/cm OR Half-value thickness
Half-value thicknessDistance in which initial intensity 1/2
The main mechanism of ultrasoundReflection at acoustic boundaries
Low frequency UltrasoundExamination of deep structures
High frequency UltrasoundExamination of superficial structures
memorize