MEDICAL PHYSICS d.p.

 

  1. 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.

 

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