Non-Ionising Imaging

Question 1

State two physical properties of a materials which determine its acoustic impedance.

Answer 1

• speed of sound within a material

- density of the material

Question 2

Under what condition is ultrasound strongly reflected at a boundary between two types of material?

Answer 2

• where there is a large difference between the acoustic impedances of the two materials

Question 3

Explain two physical properties of the light produced by a laser which makes it different from the light produced by a filament lamp.

Answer 3

• Monochromatic (wave of single frequency/wavelength)
• Collimated (produces a parallel beam)
• Coherent (waves produced are in constant phase with each other)

Question 4

State the use of a filament lamp.

Answer 4

To allow the user to view the inside of a body.

Question 5

State the use of light from a laser.

Answer 5

To stop bleeding/To cut

Question 6

State where a coupling medium or gel is used and why it is necessary.

Answer 6

• A coupling medium is used between the skin and the transducer
• It is necessary to make the sound wave enter the body as the acoustic impedance of the gel is similar to that of the body.

Question 7

Why is it necessary for the pulse in an ultrasound to be short?

Answer 7

The probe is used as generator and receiver.

Question 8

How does a transducer produce short pulses?

Answer 8

• Electrodes are connected to a high EMF
• Crystal expands and contracts at a frequency of EMF
• Vibration of faces produces ultrasound waves
• Backing material damps oscillation of crystals
• to stop crystal oscillating between end of transmitted pulse and start of received pulse

Question 9

State the use of the coherent bundle.

Answer 9

To view an image

Question 10

State the use of the incoherent bundle.

Answer 10

To light area in the body

Question 11

Describe the structure a coherent bundle.

Answer 11

The fibres are aligned so that the position of the fibre is the same at both ends.

Question 12

Describe the structure of an incoherent bundle.

Answer 12

The fibres are randomly positioned.

Question 13

State advantages of using small diameter fibres in coherent bundles.

Answer 13

• Higher resolution image is produced.

- Endoscope is more flexible.

Question 14

Why is a glass cladding used around the central core of each fibre in a coherent bundle?

Answer 14

• so that scratches on the outer surface do not allow light to escape
• so that close contact between adjacent fibres, doesn’t allow light to pass from one fibre to another to ensure the image is not confused

Question 15

Explain why increasing the refractive index of the cladding increases the probability of light escaping from the fibre.

Answer 15

It increases the critical angle as there will now be a smaller difference between the indexes of the core and the cladding.

Question 16

Explain why bending the fibre into a tighter curve increases the probability of light escaping from the fibre.

Answer 16

It makes the internal angle of incidence at core-cladding interface more likely to be less than the critical angle.

Question 17

What would the acoustic impedance need to be for a coupling gel for ultrasound use?

Answer 17

1.7 x10^6

Question 18

What is the unit for acoustic impedance?

Answer 18

kg m^-2 s^-1

Question 19

Explain how a piezoelectric crystal is used to generate waves of ultrasound.

Answer 19

• Two faces of a thin slice of crystal act as electrodes
• Electrodes connected to high frequency source of EMF
• As the applied EMF alternates it applies an alternating electric field across the slice of crystal between the electrodes
• Crystal expands and contracts at the same frequency as the applied EMF
• The vibrations of the faces of the crystal slice produce ultrasound pressure waves

Question 20

State two applications of laser radiation.

Answer 20

• Treatments of defects of vision
• Welding of detached retina
• Removal of birthmarks
• Treatments of tumours
• Uses as cutting instruments