ACTIVITY 1 - Jacaranda



CHAPTER 23 TECHNOLOGICAL METHODS OF MEDICAL DIAGNOSIS | |

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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.

     

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.

     

14. High frequency ultrasound allows the detection of more detail than low frequency ultrasound. Why is high frequency ultrasound not used to scan internal organs?

     

15. Outline the difference between an ultrasonic A-scan and an ultrasonic B-scan.

     

16. (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.

     

17. (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?

     

18. What properties of lasers make them suitable for the treatment of diseased organs?

     

Application

19. 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.

     

20. Use the data from table 23.2 (page 593) to calculate:

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.

     

21. The value of the ratio of reflected intensity to incident intensity [pic] for ultrasound at 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).

     

22. 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.

     

23. 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.

     

24. 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.

     

25. Outline how Doppler ultrasound is used to monitor foetal heart rate.

     

26. Describe how ultrasound is used to measure bone density.

     

27. (a) Outline how the attenuation of X-rays changes for different materials in the body.

     

(b) Describe and account for the appearance of an X-ray image of part of the body containing bone, muscle and air spaces.

     

28. 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?

     

29. Use a table to summarise situations in which CT scans are a superior diagnostic tool to X-rays and ultrasound.

     

30. 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.

     

31. List some advantages of keyhole surgery over other methods of operating on diseased organs.

     

Challenges

32. 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?

     

33. 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.

     

34. (a) 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.

     

(b) Explain why a faster rate of pulse would produce an image that was not clear.

     

Notes:

     

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