3.10.4 Non-Ionising Imaging Flashcards
What longitudinal pressure waves (sound) is the human ear sensitive to?
20 Hz - 20 000 Hz
What is sound with a frequency higher than 20 000 Hz called?
Ultrasound
What are the frequency used for pre-natal scans?
1 MHz - 5 MHz
Why are prenatal scans grainy images?
The frequencies used have wavelengths of around 1mm, which limits the resolution
Why aren’t frequencies higher than 5 MHz used for prenatal scans?
Even though they would give better resolutions;
The increase in kinetic energy (vibrations) causes an increase in thermal energy, which can damage muscles and tissues.
What is the resistance that sound waves moving through a medium encounters called?
Acoustic impendance, Z
What is the equation for acoustic impedance?
acoustic impedance = density of medium * speed of sound in the medium
Z = p * v
Why is an ultrasound gel (coupling agent) used during a scan?
The gel has an acoustic impedance similar to that of soft tissue so that less reflection occurs because it removes the air gap.
What is the equation for calculating refraction?
R = Ir/Ii
= ((Z2-Z1)/(Z2+Z1))^2
What is a transducer?
Provides and detects ultrasound
What are the components of a transducer?
Metal outer casting,
Power cable,
Acoustic insulator surrounding the Damping material,
Piezoelectric crystal surrounded by 2 electrodes that supply an alternating p.d.,
Plastic ‘nose’
What happens when an alternating p.d. is applied to the piezoelectric crystal?
The crystal switches between being compressed and expanding, so ‘vibrates’.
What happens when pressure is applied to the plastic ‘nose’ of the transducer?
Compression causes a voltage and therefore induces a current.
(echo is detected - the ultrasound pulse returns, applying pressure)
What is caused by the ‘vibration’ of the crystal?
Pressure is applied to the air.
(The resonant frequency of the crystal is in the ultrasound range)
(Max. displacement when applied p.d. = resonant freq.)
Why does there need to be damping material in a transducer?
The pulses need to be short to detect clear echoes, so the ultrasound vibrations must stop quickly.
(The damping material has a high acoustic impendence)
What does the insulating material in a transducer do?
Ensures the sound doesn’t leave the transducer.
What is attenuation?
The losing of energy, by the wave, at each boundary, due to absorption, scattering refraction.
What is resolution often referring to?
Wavelength
What happens to an ultrasound wave of smaller wavelength, in terms of resolution?
The ultrasound wave can reflect off of smaller structures, allowing finer details to be resolved.
This is why higher freq. ultrasound waves (shorter wavelength) are used for imaging small or shallow structures)
What happens to an ultrasound wave of longer wavelength, in terms of resolution?
Longer wavelengths cannot resolve small structures, as they miss finer features, so have lower imaging resolution.
(Lower frequency ultrasound waves are used for deeper imaging, but sacrifices resolution)
What are the two types of scan?
A (Amplitude) Scan
B (Brightness) Scan
What are the five main components of an oscilloscope?
Cathode,
Anode,
Horizontal deflection plates,
Vertical deflection plates,
fluorescent screen
What is the basic purpose of an oscilloscope?
To visualize electrical signals as they vary over time.
What makes up the electron gun in an oscilloscope?
The cathode and anode
What does the ‘electron gun’ in an oscilloscope do?
Sends a beam of electrons through the tube towards the fluorescent screen.
What do the horizontal deflection plates do?
Affects the horizontal motion of the beam of electrons.
They move the beam left and right across the screen continuously, using an alternating p.d.
What do the vertical deflection plates do?
Vertically deflect the electron beam.
(A source of p.d. is connected to the vertical deflection plates - they will have the same p.d. across them as the source)
What are A Scans mostly used for?
When looking at structures at the same depth, along a line.
What is the general process of an A Scan?
Transducer emits short pulses of ultrasound and cathode ray oscilloscope (CRO) is triggered, starting a moving spot on the screen.
Pulses travel through body and reflects back when it hits an interface.
Reflected pulses detected by transducer, generating a p.d and electric signal, which is processed (amplified) and sent to oscilloscope Y-plates, causing a vertical deflection on screen.
(Spikes represent returning echoes, showing position of internal structures)
Echo’s displacement on screen represents time taken for pulse to travel to surface and back.
How to calculate distance from an A Scan?
If the speed of sound (in tissue) is known, the depth of the reflecting surface can be calculated:
S = (vt) / 2
What needs to be done for deeper surfaces during an A Scan?
Additional echoes from deeper surfaces need to be amplified more due to attenuation.
What do B Scans produce?
2D images
What is the general B scan process?
Transducer sends ultrasound pulses into the body.
The echoes reflect from internal structures and return to transducer.
Electron beam sweeps down the screen, rather than across.
The amplitude of relflected pulses is displayed as the brightness of the spot:
- strong echoes = brighter areas
- weak echoes = darker areas
Transducer moves, collecting multiple echoes; these signals form a 2D cross-sectional image of the body.
(Linear array of transducers to produce a 2D image)
How does an optical fibre work?
Light/EM wave enters glass rod at incident angle (> critical angle), TIR occurs.
This continues along whole length and wave is transmitted at other end (with little intensity lost)
What does an optical fibre consist of?
Made from high purity silica
Surrounded by cladding (lower refractive index than core so TIR can occur)
What does cladding do in an optic fibre?
Protects core from damage
Prevents cross-talk
What is cross-talk?
When EM waves cross into another fibre in the bundles
What are the 2 types of bundles?
Coherent
Incorehernt
What are coherent bundles?
Relative position of fibres remain constant
Good for imaging - smaller diameter fibres give higher resolution
What are incoherent bundles?
Relative positions of fibres don’t remain constant
Images would be ‘jumbled’, but good for transferring light.
(also cheaper and easier to lay)
What does MRI stand for?
Magnetic Resonance Imaging
What does the human body mostly consist of?
Water and Fat
= high hydrogen content
What is the hydrogen nucleus also?
Proton
What property does hydrogen nucleus (proton) have?
Spin
- means proton can exist in 2 possible spin states, and can be affected by magnetic fields.
How does the spin work (in hydrogen nucleus) ?
Creates a small mag. field around nucleus
= mag. dipole moment
(under normal circumstances, these are randomly aligned and cancel each other out)
What happens to hydrogen nuclei when an external mag. field is applied?
The hydrogen nuclei behave like little magnets and line up parallel (or antiparallel) to the field lines.
What does a pulse of radiofrequency EM radiation applied in MRI cause?
Causes the hydrogen nuclei to align in a diff. direction.
(when pulse is removed, nuclei returns to the equilibrium positions and releases radiofrequency radiation)
What is relaxation time?
The time period over which the application and release of radiofrequency radiation occurs.
[diff. types of tissue have diff. relaxation times]
How does an MRI scanner generally work?
The patient lays inside a large, very strong cylindrical magnet.
A pulse of radiofrequency radiation is applied.
A ring of detectors pick up the emitted radiation pulses
Info is fed to a computer which translates the info into a sectional image.
What are advantages to MRI?
Non-ionising = no harmful effects.
Ability to distinguish diff. types of soft tissue.
Resolution is better, for soft tissue.
Creates 3D and cross-sectional images.
Non-invasive.
What are disadvantages of MRI?
Can’t be used if patient has any metallic implants.
Patient needs to be still for up to 1h.
Bone and calcium do not show up in images.
Cost is greater than any other scan.
