Option - Chapter 4 Flashcards
What range of ultrasound frequencies are used in medicine?
1 and 10 MHz
What happens when an ultrasound scan is carried out?
1, Pulses of ultrasound waves are emitted by an ultrasound probe on the body’s surface
2, The ultrasound waves travel into the body and partically reflect at any boundaries which they encounter
3, The reflected pulses return to the surface and are detected by the probe and used to generate an image
How can a greater image be formed in a ultrasound?
Increasing the frequency and reducing the wavelength
Which organs low density or high density can be imaged in more detail and why?
Low density, because waves of a higher frequency can be used, in high density organs waves of a lower frequency have to be used otherwise they would be absorbed.
Describe the parts which make up an ultrasound probe and what are their uses?
An ultrasound probe is a piezoelectric disc, which vibrates when an alternating pd is put across it. There is an absorber block behind it, one which prevents ultrasound waves from cancelling themselves out. And a protective cover fixed to the piezoelectric disc.
Name a suitable material for the absorber block on a ultrasound probe?
Epoxy resin
Why are ultrasounds preferred over X-rays?
Because they use non ionising waves which won’t kill or damage living cells.
What would happen if a gel was not placed between the probe and the body?
The ultrasound waves would be almost completely reflect at the body’s surface.
Describe how an ultrasound works?
Pulses are passed into the body and each pulse is partially reflected at the surface of the organs and tissue boundaries. The reflected pulses are then detected by the same probe when it is not releasing another pulse.
What is the minimum time between each pulse and why?
No more than 1000 per second to allow the received pulses from each transmitted pulse to return to the probe before the next pulse is transmitted.
What is the difference between an A-scan and B-scan?
An A-scan are used to measure the distance between tissue boundaries or the distance from a tissue boundary to the body surface.
An B-scan is used to obtain an image.
Describe the set up of an A-scan?
The transducer probe is placed onto the patient. A pulse generator produces a pulse which is sent to the patient. The probe then receives a reflected pulse which is sent to an amplifier and onto an oscilloscope screen.
Describe what is shown on the oscilloscope screen from an A-scan and how to work out the distance from the transmitted pulse to the reflected pulse.
The transmitted pulse appears on the left-hand edge of the screen and the reflected pulse appears on the far right-hand side of the screen.
The distance can be calculated by measuring the distance on the screen and using the time base, where t (transmitted pulse) = d x the time base setting in time per distance.
How can you work out distance x ( distance from the probe to the edge of the boundary )?
x=(d/D)X
X is the distance from the probe to the far side of the body
D is the distance from the transmitted pulse to the far side reflected pulse from the body surface to the reflecting boundary.
What is the difference between a probe in an A and B scan?
In a B-scan, the probe contains serval transducers which transmit pulses simultaneously.
Does a B-scan give a 2D or 3D image?
2D
What is acoustic impedance?
Acoustic impedance is the opposition to the passage of ultrasound waves through a substance.
What is acoustic impedance defined as? And what formula is used to calculate it and its unit?
Defined as the product of its density and the speed of sound through the substance.
Z=Density x C (speed of sound through the substance)
The unit is kg m^-2 s^-1
What is attenuation?
It is the reduction in amplitude with distance as ultrasound waves pass through a substance.
What happens why ultrasound waves reach a boundary of different acoustic impedances?
Partial reflection occurs so the waves passing through it reduce in amplitude.
What is the reflection coefficient formula?
R=Ir/Io
What two factors are needed for total internal reflection to occur?
The incident substance has to have a larger refractive index than the other, the incident angle must exceed the critical angle.
What happens to the intensity when total internal reflection occurs?
No intensity is lost
What could cause a ray to leave an optical fibre?
If the angle of incidence at this boundary is less than or equal to the critical angle for this interface.
Or the outer surface is rough or greasy or in contact with another fibre at the point of incidence.
What elements make up an endoscope?
An air/water channel
An objective lens over the image channel
An instrument channel
An illumination channel contains an incoherent bundle of light
What is a coherent bundle made up of and why do the fibres need to be in the same relative position?
A coherent bundle is made up of thousands of fibres about 0.01mm wide. The fibres need to be in the same relative position to ensure the image formed by the lens is seen as a coherent image on the end outside the body by an observer. Without this, the image would be scrambled.
What is laser light used for in an endoscope and how does it do that?
Laser light is used to destroy diseased tissue or seal off leaking blood vessels. This is possible because the light is focused on a very small area and can, therefore, deliver more energy per second per unit area.
What is the formula used for calculating the viewing cone?
No sin imax = n1 sin(90-ic)
What does the amount the fibre can be bent at depend on, without loss of light?
Without loss of light, it depends on the diameter of the core.
How do you calculate the maximum angle at which an optical fibre can be bent at?
Sin ic= R/R+d
R is the radius of curvature
d is the diameter of the core
What are MR scanners used for?
To scan the hydrogen content in the body.
What is nuclear magnetic resonance?
It is the excitation of a hydrogen nucleus in a magnetic field using radio waves of the same frequency and as the frequency of the precession of the nucleus.
How can the frequency of precession be changed and why?
It can be changed by changing the magnetic flux density of the external field. This is because the frequency of precession is proportional to the magnetic flux density.
What happens why a short pulse of radio waves are applied into a sample containing hydrogen?
If it is the same precession frequency, some of the nuclei in the lower energy state each absorb a radio-wave photon and flip into the higher energy level.
When the pulse ends, some of the excited nuclei flip back into the lower energy state, each emitting a radio wave photon.
In an MR scanner, how is the emission of radio wave photons used?
The MR scanner detects the photons these are then used by the scanning software to determine the location of the nucleus that emitted it.
What are the key design features of an MR scanner?
That it applies a magnetic field of specific flux density to the patient at a precise well-defined location that changes systematically with time so that it can scan a cross-section of the patent.
What does the intensity of an MR scanner signal depend on?
The number of nuclei excited at that location.
How are the images displayed and where?
They are displayed on a TV screen, the location of the excited nuclei changes along a raster of successive straight lines in a single plane through the patient. These lines build up an image.
Why is tissue discrimination possible on an MR scanner?
It is possible because the rate of decay of the detected signal after each pulse (relaxation times) depends on the type of molecules surrounding the water molecules. And the magnetic fields of surrounding magnetic nuclei delay the de-excitation of excited nuclei to different extents. These relaxation times can then be compared to identify tissue types.
How is a large magnetic field generated in an MR scanner?
It is generated by using a large superconducting electromagnet. The coils provide magnetic fields with gradients that ripple through the patient with a magnetic field of the required flux density to make the hydrogen nuclei precess at a know location that changes systematically.
