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Contents Main topic subtopic page 1a. The structure of the ear 1b. Operation of the ear 1c. Range of human hearing 1d. Sound intensity 1e. The response of the ear 1f. Intensity and intensity level 1g. Discomfort level 1h. Problem solving 1i. Effects of exposure to noise 1j. Intensity level graphs 3 3 4 4 4 4 5 5 6 7 8 2a. Attenuation and half thickness 2b. Deriving the relationship 2c. Problem solving 2d. X-ray detection 2e. X-ray imaging 2f. Computer tomography 2g. Generation and detection of ultrasound 2h. Acoustic impedance 2i. Problem solving 2j. A and B scans 2k. Choice of diagnostic frequency 2l. NMR imaging 2m. Use of lasers 9 9 10 11 12 12 13 14 15 15 16 17 18 20 3a. Radiation dosimetry 3b. Protection 3c. Balanced risk 3d. Half-life 3e. Problem solving 3f. Radiation therapy 3g. Choice of isotope 3h. Problem solving 23 23 25 25 26 27 27 28 29 1. The Ear and hearing 2. Medical imaging 3. Radiation and medicine 4. Common exam mistakes 5. Questions 6. Answers 30 31 32 2 Option I Medical Physics I1 The Ear and hearing 1a The structure of the ear -the ear is split into three sections, the outer, middle and inner ear. outer ear semicircular canals inner ear auditory nerve pinna malleus auditory canal cochlea oval window eardrum incus middle ear stapes Eustachian tube outer -the outer ear consists of the pinna, the external sound collecting cone. -this is the external part we can see. -next is the auditory canal down which the sound waves are channelled. -the sound reaches the eardrum, a thin membrane that vibrates with the sound. middle -the middle ear contains a small air cavity about 1.5-2.5cm3 in volume. -connected to the eardrum is a series of three bones, the ossicles. -the three bones are the hammer (malleus), the anvil (incus) and the stirrup (stapes). -the ossicles act as a lever system increasing the small movement of the eardrum. -the middle ear is connected to the throat via the Eustachian tube. -this tube allows the pressure each side of the eardrum to balance out. -e.g. when in a plane, ears get blocked as the pressure behind the eardrum stays normal while the outside pressure drops in the plane. Due to the extra pressure inside, air is forced down the Eustachian tube leading to the ear ‘popping’ sensation, which is helped by jaw motion through sucking boiled sweets. inner -the inner ear is stars with the oval window, to which the other end of ossicles lever system is attached. -the entrance cavity is called the vestibule and is filled with a fluid, surrounded by bone. -the semicircular canals in the inner ear are for balance not hearing. -the entrance cavity leads to the cochlea, a coiled tube of 2-3 turns. Key points -the ear has three parts, outer, middle and inner ear. -the pinna and auditory canal make the outer ear. -the eardrum and ossicles make the middle ear. -the oval window cochlea and basilar membrane make the inner ear. 3 1b Operation of the ear -the pinna collects the sound which is channelled down the auditory canal. -this causes the eardrum to vibrate: the movement is magnified by the ossicles: a bone lever system, the malleus, incus and stapes. -the ossicles are attached to the oval membrane and the cochlea into which the vibrations are sent. -the cochlea distinguishes between the frequencies present. -inside the cochlea is the basilar membrane which extends its whole length. -the stiffness of the membrane decreases along its length. -from wave study the stiffer a material, the faster the wave speed. -consider sending a wave down the length of a stretched slinky and. -the greater the tension in the slinky, the faster the wave. -the basilar membrane relies on resonance. -resonance occurs when a driving frequency is close to the natural frequency of an object and it starts to vibrate. -different parts of the membrane resonate with different frequencies. -the brain identifies the frequency by determining which part of the membrane resonates. -the further along the membrane that resonates, the lower the frequency. Key points -the pinna collects the sound and sends it down the auditory canal to the eardrum. -the ossicles magnify the eardrum vibrations sending them to the cochlea. -the basilar membrane inside the cochlea separates frequencies. -the auditory nerve sends the signal to the brain. 1c Range of human hearing -the range of human hearing is from 20Hz to 20kHz. -the higher frequency end reduces with age: 25kHz in the teens to 15kHz in old age. Task 1: the frequency doubles for each octave so this covers how many octaves? Middle C is 256Hz. How many octaves above middle C can a normal human hear? How many octaves below? Key points: the range of human hearing is 20 - 20kHz. 1d Sound intensity -sound intensity (I) is the sound energy per second per unit area. -it is measured in joules/sec/metre² (Js-1m-2) or Wm-2. -the larger the amplitude of the sound waves the greater the intensity. -intensity I, increases with the square of the amplitude A² (I ∝ A²). -so if the amplitude is doubled, the intensity increases by four. -if the amplitude trebles, the intensity is nine times greater. Key points: sound intensity is energy per second per unit area. 1e The response of the ear -the human ear does not have a linear response to loudness. -it follows a logarithmic relationship, by sensing equal fractional increases. -the eye is the same in sensing fractional increases in brightness. 4