Medical - Ear, eye and heart Flashcards
Explain what the pinna does
3 points
The pinna channels longitudinal waves into the auditory canal.
Conc in small A = increase in intensity.
Variations in air pressure causes eardrum to vibrate
What is the function of the:
Eardrum
Ossicle lever system
Oval window
Cochlea
Vibrates mechanically and transfers energy to the inner ear
Transmits vibrations from the eardrum to the oval window
Transmits vibrations into the fluid in the inner ear
Converts pressure waves in the fluid into electrical signals which are sent to the brain
Explain how components of the ear act to amplify pressure changes
(3 marks)
Ossicles lever system produce an increase in force
Area of the oval window is much smaller than the area of the eardrum
P = F/A so pressure greatly increases
How are vibrations of a sound wave transmitted to the inner ear from the outer ear?
(3 marks) - 4 possible marks
Longitudinal/pressure waves in the auditory canal
Forces eardrum into mechanical vibrations
Vibrations passed through the middle ear by lever system to the oval window
Sets up pressure waves in cochlea
Define intensity
What is the formula for intensity?
Unit?
The energy per second per unit area incident normally on a surface
I = P/A
Wm-2
What is the relation between intensity and amplitude?
Intensity is proportional to amplitude squared
Define threshold of hearing
The minimum intensity level that can be heard by a normal ear at a frequency of 1kHz
How can you obtain an equal loudness curve?
4 steps
- Using signal generator, generate a control frequency of 1000Hz at a particular intensity
- Using a second signal generator, set a different frequency and adjust the intensity level until the signal generators produce the same loudness. Make note of this f and I
- Repeat this step for several frequencies and plot curve
- Adjust the intensity of the control signal generator and repeat experiment
What causes hearing loss? What are the effects of these causes?
(2 marks)
Aging - hearing loss increases as frequency increases
Excessive noise - hearing loss increases up to 4kHz, then decreases after this frequency
Describe an electrocardiogram trace whilst staring each relevant time interval
- 2s - aorta contract
- 4s - ventricles contract and aorta relax
- 6 to 0.8s - ventricles relax
How is an ECG trace obtained
Electrode are placed on a patients chest or limbs and a change in potential difference is measured
What precautions are taken when taking an ECG trace?
Sandpaper skin - remove dead skin and hair cells to increase conductivity and decrease contact resistance
Securely attach electrodes - prevents voltage spikes by the electrostatic effect of motion
Conductive gel between skin and electrode - reduce contact resistance
What are the design features of the electrodes of an ECG
Large input impedance amplifier - reduces current between patient and machine and causes signal gain
High gain amplifier - amplifies signal, required because voltage is very small
Low noise amplifier - doesn’t pick up unwanted signals
What are the characteristics of the:
Cornea
Lens
Retina
Optic nerve
High refractive index and convex shape, focuses on objects
Muscles contract - lens fattens. Muscles relax - lens thins. These change the focal length
Contains light sensitive rods and cones and a very sensitive yellow spot
Carries signals from the rods to the brain
What are rods and cones?
How does it carry out its function?
Photoreceptors that detect light and convert to it an electrical signal
They bleach when light hits them due to a chemical pigment
This stimulates the cell, causing them to send signals to the brain via optic nerve
What do cones detect?
Colour
What is required for two objects to be distinguished between each other?
Where is spatial resolution the greatest? Why?
The light from the object must be hitting rods or cones that are attached to separate optic nerves
At the yellow spot. It contains the most densely packed cones and rods
Define principal focus
The point on the principal axis through which rays which were parallel to the principal axis pass after refraction by the lens
Define Focal length
What is it’s unit?
The distance between the lens and the focal plane
Diopters, D
Define power
The reciprocal of the focal length measured in meters
What does u and v stand for in the equation for in the lens equation?
u - the distance between the object and the lens
v - the distance between the lens and the image
What are the properties of the image produced from a diverging lens?
Virtual
Upright
Smaller than object
When does short sightedness occur?
3 points
When the:
Cornea and lens are too powerful
Eyeball too long
Image focus is in front of retina
How do you correct short sightedness?
Use a diverging lens with negative power with principal focus at the faulty far point and negative focal length to the far point
When does long sightedness occur?
3 points
When the:
Cornea and lens are too weak
Eyeball too short
Image focus behind retina
How do you correct long sightedness?
Use a converging lens with positive power with the virtual image produced at uncorrected near point of object in front of near point
What is the format of subscription of astigmatism?
Power and axis of cylindrical lens
Outline how the piezoelectric effect produces waves in an ultrasound transducer
(5 points)
- Two faces of a thin slice of crystal is used as electrodes
- Electrodes are connected to a high frequency emf source
- Alternating emf induces an alternating electric field across the piezoelectric crystals between the electrodes
- The crystals expand and contract at the same frequency as applied emf
- Vibrations of the crystals produce ultrasound pressure waves
How is a signal received by the transducer?
Waves are reflected at a boundary and are received by transducer, causing crystals to vibrate
Vibration of crystals induce an emf
Define acoustic impedance
The amount of reflection ultrasound waves experience at a boundary
What is attenuation?
What is done to reduce it? Why? What is the disadvantage of this method?
The scattering and absorption of ultra sound waves as they travel through a material
Low frequency impulses are used for deeper tissue. Intensity of reflected waves decrease as depth increases due to attenuation and low freq waves are less attenuated
Poorer resolution
How and why is a coupling media used in ultrasound?
It is used to displace air between the probe and tissue. Air and tissue have a very large difference in impedance which would cause all of the waves to be reflected before entering the tissue
Gell impedance = tissue impedance
How is an A scan produced?
Short pulses are transmitted and received by probe and signal sent to oscilloscope
What do the spacing of the pulses in an A scan show you? Wat can be found using this knowledge?
Echoes that are reflected back to transducer appear as voltage peaks on the graph
The spacing gives time delay between transmitted pulse and echo
Distance = 1/2 speed x time
How is a B scan produced?
Amplitude of pulses displayed as bright spots, forming an image
Advantages of ultrasound
3 points
Produces high quality imaging of soft tissue
Cheap, portable and quick
No ionizing radiation so no known side effects
Disadvantages of ultrasound
4 points
Doesn’t penetrate bone
Doesn’t give information on large masses
Cannot pass through air spaces in body
Poor resolution
Describe a coherent optical fiber bundle
The relative positions of the fibers remain the same at the start and end of the fiber bundle.
They transmit image and light
Image can be magnified by gradually increasing the diameter of the fibers
Describe a non-coherent optical fiber
The positions of the fibers change.
Cheaper to make but can only transmit light
What is an endoscope
What causes a loss of light and image? Why?
Bundles of coherent and incoherent optical fibers used for body examination
Large curvature of the fiber because the angle of incidence decreases and may decrease below the critical angle, resulting in no total internal reflection
What channels may an endoscope contain?
Water channel - used to clean objective lens
CO2 channel - used to make room in body
Why is keyhole surgery advantageous?
Reduces risk of infection
Reduces recovery time and hence price
Describe how the frequency and amplitude of the vibrations change as a wave is transmitted through the ear to the fluid in the inner ear
Frequency is the same
Amplitude is reduced
