MEDICAL PHYSICS d.p.
- The properties of ultrasound waves can
be used as diagnostic tools.
Identify the
differences between ultrasound and sound in normal hearing range.
The frequencies that can normally be heard by humans range from about 20 Hz to around 20 000 Hz. This varies between individuals with older people hearing less range of frequencies than younger people. Ultrasound begins where normal hearing ends with frequencies greater than 20 000 Hz being classified as ultrasound. Ultrasound used for medical applications usually ranges from 1 to 20 MHz.
Describe the
piezoelectric effect and the effect of using an alternating potential difference with a
piezoelectric crystal.
Some crystals such as quartz or tourmaline produce a potential difference between opposite faces when pressure is applied to them. Similarly if a potential difference is applied to opposite sides of the crystal there will be distortion of the crystal. If the applied voltage is alternating then the crystal will pulsate at the same frequency as the applied voltage. If the crystal is in contact with air then it will produce sound waves with the same frequency as the potential difference.
Define acoustic
impedance: Z = rv and identify that different materials have
different acoustic impedances.
Acoustic impedance, Z, is the resistance to the passage of sound waves through a medium. It is equal to the product of the density of the medium, r, and the speed v, of the ultrasound through the medium.
The body is made up of a variety of materials such as blood, bone, fat and muscle and these different materials hinder the passage of ultrasound through them at different rates, i.e. they have different acoustic impedance.
Solve problems and
analyse information to calculate the acoustic impedance of a range of
materials, including bone, muscle, soft tissue, fat, blood and air and explain
the types of tissues that ultrasound can be used to examine.
Complete the following table:
|
Material |
Acoustic Impedance (x106 kgm-2s-1) |
Velocity (ms-1) |
Density (kgm-3) |
|
Air |
|
340 |
1.265 |
|
Fat |
1.38 |
|
952 |
|
Water |
|
1540 |
1000 |
|
Brain |
1.68 |
1541 |
|
|
Blood |
1.61 |
|
1025 |
|
Muscle |
1.70 |
1585 |
|
|
Boneaverage |
6.5 |
|
1850 |
|
Skull Bone |
|
4080 |
1912 |
|
Liver |
1.65 |
1549 |
|
|
Kidney |
|
1561 |
1038 |
Completed table:
|
Material |
Acoustic Impedance (x106 kgm-2s-1) |
Velocity (ms-1) |
Density (kgm-3) |
|
Air |
0.00043 |
340 |
1.265 |
|
Fat |
1.38 |
1450 |
952 |
|
Water |
1.54 |
1540 |
1000 |
|
Brain |
1.68 |
1541 |
1090 |
|
Blood |
1.61 |
1570 |
1025 |
|
Muscle |
1.70 |
1585 |
1073 |
|
Boneaverage |
6.5 |
3314 |
1850 |
|
Skull Bone |
7.8 |
4080 |
1912 |
|
Liver |
1.65 |
1549 |
1065 |
|
Kidney |
1.62 |
1561 |
1038 |
Ultrasound will pass through the body until it reaches a boundary between one type of tissue and another. Some of the ultrasound will be reflected from the boundary and some will be transmitted. The ratio of reflected to transmitted ultrasound depends on the difference in acoustic impedance between the two types of substance. If there is a large difference then very little of the signal will cross the boundary and there will be a strong echo.
Bone has the highest acoustic impedance so very little ultrasound can penetrate the bone. Consequently ultrasound is not suitable for examining tissue surrounded by bone such as the brain which is surrounded by the skull.
Air has the lowest acoustic impedance so almost all of the ultrasound is reflected. Consequently ultrasound is not suitable for examining tissue with enclosed air such as the lungs.
Tissues with similar acoustic impedances allow some of the ultrasound to be transmitted and some to be reflected. Each layer of tissue will give an echo with a time delay between the echoes of different layers. Tissues such as muscle and fat can be examined by analysing the ultrasound that has just passed through them.
Describe how the
principles of acoustic impedance and reflection and refraction are applied to ultrasound.
The laws of reflection and refraction for ultrasound are the same as those for sound that we dealt with in “The World Communicates” in the preliminary course. At each boundary, some of the ultrasound wave will be reflected and some will be refracted. The time difference between the echoes from two successive echoes can be used to determine the thickness of layers of various tissues
Solve problems and
analyse information using Z = rv
and 
= 
Ir and Ii refer to the reflected and
initial intensities of the signals and the ratio is called the
intensity reflection coefficient. Note that because of the squares the
intensity reflection coefficient is always positive and because the reflected
intensity is always less than the initial intensity, the intensity reflection
coefficient is always less than 1.
Exercises:
Calculate the intensity reflection coefficient of an ultrasound wave passing from
(i) muscle to bone
(ii) muscle to fat
(iii) fat to muscle
Answers: (i) 0.343 (ii) 0.011 (iii) 0.011
Identify that the
greater the difference in acoustic impedance between two materials, the greater
is the reflected proportion of the incident pulse.
The greater the value of Z2 – Z1, the
greater will be the ratio of
This follows mathematically from the equation; =
Describe the
situations in which A scans, B scans and phase and sector scans would be used
and the reasons for the use of each.
Identify data sources
and gather information to observe the flow of blood through the heart from a
Doppler ultrasound video image.
Describe the Doppler effect in sound waves and how it is used in ultrasonics to
obtain flow characteristics of blood moving through the heart.
Outline some cardiac
problems that can be detected through the use of the Doppler effect.
The speed of blood flow can be determined by the amount of Doppler shift. If deposits in a vein or artery cause it to be narrowed then the speed of the blood flow and the corresponding Doppler shift will be greater.
Several different Doppler shifts indicate a turbulent blood flow, symptomatic of deposits in the blood vessel.
The direction of the Doppler shift indicates the direction of blood flow and can detect valves that are not functioning properly.
Identify data
sources, gather, process and analyse information to
describe how ultrasound is used to measure bone density.
Define the ratio of
reflected to initial intensity as: =
Like the syllabus says; it is defined.