Download Chapter 23 Technological methods of medical diagnosis

CHAPTER 23 TECHNOLOGICAL METHODS OF MEDICAL
DIAGNOSIS
Name:
QUESTIONS
23.1 Ultrasound
1.
Calculate the wavelength of the note middle C, which has a frequency of 256 Hz, if it
travels through the air at 330 m s–1.
2.
Calculate the wavelength of an ultrasound signal travelling through air at 330 m s–1 if the
frequency of the signal is 25 000 Hz.
3.
An ultrasound signal of frequency 30 000 Hz has a wavelength of 3 mm when it is
travelling through some body tissue. How fast is it travelling?
4.
An ultrasound wave of 2.5 MHz is travelling through a liver. Calculate:
(a)
the wavelength of the ultrasound
(b)
the acoustic impedance of the liver.
5.
Calculate the acoustic impedance of the lens of an eye if it was found that ultrasound of
frequency 2.0 MHz travelled at 1620 m s–1 through the lens. Assume the density of the
lens is 1140 kg m–3.
6.
Using information from table 23.2, calculate the percentage of ultrasound that is reflected
at the junction between air and fat.
7.
Calculate the percentage of ultrasound that is transmitted through bone of density
1400 kg m–3 when the ultrasound is travelling from muscle to bone.
23.2 X-rays
8.
Outline why soft X-rays are not preferred for imaging.
9.
Outline why soft X-rays are removed from the X-ray beam.
© John Wiley & Sons Australia, Ltd
1
QUEENSLAND PHYSICS
10. ‘When we see an X-ray, we are really seeing a shadow.’ Explain what this statement
means.
11. Outline why bones are very good for producing X-ray images.
23.3 CT scans
12. Outline two reasons why X-rays or ultrasound are often used in preference to CT scans.
13. Explain why conventional X-rays won’t provide fine detail in images of the brain.
Review questions
Understanding
1.
Contrast ultrasound with the sound detected by people with normal hearing.
2.
High frequency ultrasound allows the detection of more detail than low frequency
ultrasound. Why is high frequency ultrasound not used to scan internal organs?
3.
Outline the difference between an ultrasonic A-scan and an ultrasonic B-scan.
4.
(a)
With the aid of a labelled diagram, give a description of the way in which X-rays
are produced.
(b)
Explain why the X-rays usually pass through a thin filter before they are used to
image the patient.
(a)
Describe how optical fibres are positioned in a coherent bundle.
(b)
Explain why a coherent bundle is necessary in an endoscope.
(c)
Explain why the endoscope has to be used in conjunction with a powerful light
source.
(d)
Which properties of the fibre bundle affect the ability of the observer to see small
details when using the instrument?
5.
6.
What properties of lasers make them suitable for the treatment of diseased organs?
© John Wiley & Sons Australia, Ltd
2
QUEENSLAND PHYSICS
Application
7.
If the acoustic impedance of blood is 1.59 × 106 kg m–2 s–1 and the velocity of sound
through the blood is 1570 m s–1, calculate the density of blood.
8.
Use the data from table 23.2 (page 593) to calculate:
9.
(a)
the acoustic impedance of soft tissue
(b)
the acoustic impedance of bone of density 1600 kg m–3
(c)
the fraction of incident ultrasound intensity reflected from a liver–muscle interface.
I 
The value of the ratio of reflected intensity to incident intensity  r  for ultrasound at
 Io 
various interfaces is found in the table that follows. Use it to answer the following
questions.
(a)
Identify the tissue interface at which there is the most reflection.
(b)
Identify the tissue interface at which there is the least reflection.
(c)
Identify the tissue interface at which the greatest amount of absorption occurs.
(d)
If an ultrasound signal of intensity 60 mW cm–2 meets a fat–bone interface,
calculate the intensity of the reflected signal.
(e)
If the ultrasound signal striking a fat–muscle interface is 80 mW cm–2, calculate
how much energy travels into the muscle.
(f)
Describe what happens to the energy in (e).
10. Using the information from table 23.2, compare the percentage of ultrasound reflected at
the junction between fat and liver with the percentage of ultrasound reflected at the
junction between liver and fat.
11. A pregnant woman needs to have a bladder full of urine if she wishes to have a successful
ultrasound of her baby. Explain why an empty bladder would make an ultrasound
unsuccessful.
12. A low intensity ultrasonic beam of 15 mW cm–2 is used to study the lens of the eye. Use
the data in table 23.2 to calculate the intensity of the reflected beam if we assume the
fluid in front of the lens is aqueous humour.
© John Wiley & Sons Australia, Ltd
3
QUEENSLAND PHYSICS
13. Outline how Doppler ultrasound is used to monitor foetal heart rate.
14. Describe how ultrasound is used to measure bone density.
15. (a)
(b)
Outline how the attenuation of X-rays changes for different materials in the body.
Describe and account for the appearance of an X-ray image of part of the body
containing bone, muscle and air spaces.
16. X-rays can be classified as hard or soft.
(a)
How are hard X-rays different from soft X-rays?
(b)
Why are hard X-rays preferred for imaging the human body?
17. Use a table to summarise situations in which CT scans are a superior diagnostic tool to Xrays and ultrasound.
18. An endoscope is used to take a biopsy of a small tumour in the oesophagus, which leads
from the mouth to the stomach. Explain how an endoscope can be used to do this.
19. List some advantages of keyhole surgery over other methods of operating on diseased
organs.
Challenges
20. For the following question, assume the density of skin is 1010 kg m–3 and the velocity of
sound through skin is 1540 m s–1. A 1 MHz transducer requires the use of a gel on the
skin to avoid acoustic mismatch at the skin–transducer interface.
(a)
Describe what would happen if air was between the transducer and the skin.
(b)
Calculate the optimum acoustic impedance of the gel and justify your answer.
(c)
What is the speed of the ultrasound in the gel if the gel is made of material of
density 1200 kg m–3?
21. A body structure at a depth of 350 mm is to be imaged using an ultrasound B-scan. In
order to obtain a clear image the reflected signal must be received before the next pulse is
sent.
© John Wiley & Sons Australia, Ltd
4
QUEENSLAND PHYSICS
22. (a)
(b)
Assuming the sound speed is 1540 m s–1 in the body, calculate the minimum time
between pulses that may be used to provide an unambiguous image.
Explain why a faster rate of pulse would produce an image that was not clear.
Notes:
© John Wiley & Sons Australia, Ltd
5