 MRI is non-invasive, but it is expensive.  We can use different non-invasive techniques instead.  Non-invasive techniques do not involve ionising.

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Presentation transcript:

 MRI is non-invasive, but it is expensive.  We can use different non-invasive techniques instead.  Non-invasive techniques do not involve ionising radiation.  Methods we will look at:  Endoscopy  Ultrasound

 These use optical fibres  These can be inserted into body openings.  Light is passed down one set of optic fibres.  These are arranged so that they have the exact same arrangement in the bundle at the bottom as at the top.  This allows the doctor to see what is being examined through an eye piece.

 These are waves that are above the audible sound frequency range (20 Hz to 20,000 Hz)  In medical ultrasounds the frequencies are in the megahertz range.  These cause no ionisation.  Ultrasound can show both muscle and blood.  A low frequency is used as often as possible as high frequency ultrasounds can be destructive to tissues.  Will be examined in greater detail later on

 If you move towards a stationary source of sound you will not hear the same frequency as when you were stationary.  Extra waves will have passed into your ears.  This change in frequency due to the persons movement is called the doppler effect.

 Stationary source  200 Hz  c = 340 m/s  Wavefronts = 1.7 m apart  If person is stationary, wavefronts go past at 200/s  Imagine you are travelling towards the source at 30 m/s.  You will have passed an extra (30/1.7) = (17.6 wavefronts.  Altogether, wavefronts will have passed.  This means the frequency will be Hz. You will hear a higher pitch.

 When the source of the sound is moving, it creates a different effect.  Wavelengths behind the source are larger.  Wavelengths in from are shorter.  This is the principle of ultrasound.

 Can you think of 2 examples where the wave source is travelling faster than the wave itself?

 Calculate the frequency heard when a train, travelling towards you with a speed of 60 m/s, sounds its whistle of frequency 400 Hz.  Velocity of sound is 340 m/s.