# 79. Theoretical and Technical Basis of Ultrasound Diagnostics

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:

λWavelength
cPropagation Speed
fFrequency

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):

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

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:

RFraction of incident energy reflected
Z2 & Z1Acoustic impedances of 2 media

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

Bone6.5mm
Brain/liver/Lungs15-30mm

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