Flashcards
A2 Physics Medical Imaging
Cards in this deck (61)
- 1X-raysA form of EM radiation
Produced when high-speed electrons hit metal targets - 2Production of X-raysHeated filament undergoes thermionic emission releasing high speed electrons
p.d. between cathode and anode causes electrons to accelerate
Electrons bombard metal target emitting X-rays which leave through a windown
Some kinetic energy of electrons transferred into metal target as thermal energy
Metal target is cooled by water or spun around to increase target area - 3Diagram of production of X-rays
- 4Tube currentRate of arrival of electrons at metal target
- 5Intensity of X-raysDepends on no. of electrons hitting anode per unit time
Proportional to the heater current
A more intense X-ray produces an image quicker - 6Hardness of X-raysDepends on acceleration of electrons
Proportional to p.d. between anode and cathode
A harder X-ray has more penetrating power - 7How to reduce the radiation doseSoft X-rays cannot fully pass through patient, becomes useless but still contributes to the total radiation dose of the patient
To reduce the radiation dose, cut off soft X-rays by placing an aluminum filer used to absorb them - 8X-ray spectrumEmerge from the tube with a range of energies as represented in the spectra below
- 9Graph of X-ray spectrum
- 10Braking radiationX-ray photons released when electrons decelerate as it strikes anode, attracted by the nucleus of an atom in the anode and loses energy
- 11Characteristic radiationRearrangement of electrons in anode when a high-speed electrons strikes, excites orbital electrons which then de-excite, emitting photons and giving rise to spectrum lines; specific to material of anode
- 12X-ray imagingX-ray radiation plates in the same way as visible light
Degree of blackening depends on total X-ray exposure
Mostly used to distinguish bones from tissue because bones have a higher density than surrounding tissue
Can be used to identify organs if the densities of surrounding tissue are sufficiently different - 13SharpnessThe ease with which the edge of a structure can be determined
- 14How to increase sharpness in X-raysReduce the area of target anode
Reduce aperture size: reduces beam width
Place lead grid in front of photographic film: absorbs scattered X-rays and reduces partial image - 15ContrastThe visual difference between the areas of blackening and light
- 16How to improve contrast of X-raysIncrease exposure time
Use harder X-rays: increases penetration power
Reduce scattering of X-ray beam
Use fluorescent 'contrast medium' - 17X-ray intensityIn medium where X-rays are absorbed, intensity of a parallel X-ray beam decreases by a constant fraction in passing through equal small thicknesses of the medium
I = I(0)e^ux
Where u is the linear absorption coefficient unique to medium
x is the thickness of the medium - 18Graph of X-ray intensity through a medium
- 19Half value thicknessThickness of the medium required to reduce the transmitted intensity to one half of its value
Related to linear absorption coefficient by
u = ln2 / xhalf - 20Computed Tomography (CT scan)X-ray imaging only produces a two dimensional image with no impression of depth, cannot tell if tissue is near to the surface or deep within the body
Tomography is a procedure which forms a 3 dimensional plane of the object
Putting together several planes produces a whole 3 dimensional image which can be rotated - 21Diagram of CT scan
- 22VoxelA small cube in a three dimensional image
- 23The number in each square (voxel)The density that the computer will register for that section of the object
- 24How voxel in CT scans workAs the scanner goes around each part has a different density which the computer can model
The computer then put these together to form a 3 dimensional shape - 25Diagram of how voxels work
- 26How to create a well defined CT scanX-ray beams must be well collimated so that it consists of parallel rays - rays must no spread
Detector must consist of regular array of tiny detecting elements - smaller the detector the better the image - 27Advantages of CT scanProduce images that show three-dimensional relationships between different tissues
Can distinguish tissues with quite similar densities - 28UltrasoundAny sound wave that has a frequency above the upper limit of human hearing, 20kHz
- 29Piezo electric transducerCan be used to record and produce ultrasonic waves
- 30Diagram of a piezo-electric transducer
- 31Generating ultrasonic wavesCharged atoms of a transducer in an electric field move close to oppositely charged plates and the overall crystal either compresses or extends
When an alternating voltage with frequency f is applied to the crystal, it causes it to contact and expand at the same frequency of f
This acts as the vibrating source of ultrasound waves - 32Receiving ultrasonic wavesUltrasonic waves change pressure in medium
Charged atoms in crystal shift position closer to plates
Opposite charges induced in the silver plates
Induced potential difference across the plates
Potential difference fluctuates which can be processed - 33Reflection in ultrasoundRequires ultrasonic waves to pass from one medium to another
When a beam of ultrasound wave reaches a boundary between two different media, the beam is partially refracted and partially reflected - 34Why I = Ir + ItConservation of energy
- 35Specific acoustic impedanceProduct of density of medium and speed of sound in medium
Z = pc
Between two media, the difference in acoustic impedance determines the fraction of incident intensity that is reflected - 36Intensity reflection coefficientRatio of intensiyt of reflected wave and intensity of incident wave
(Z2 - Z1)^2/(Z2 + Z1)^2 - 37Comparing acoustic impedanceVery large fraction reflected at air-tissue boundary
Large fraction reflected at tissue-bone boundary
Very little reflected at boundary between soft tissues - 38Purpose of gel in ultrasonic scansVery little transmission at an air-skin boundary
Almost complete transmission at a gel-skin boundary
Because acoustic impedance of gel and skin are very similar - 39Attenuation of ultrasonic wavesSimilar to X-rays, ultrasonic waves are also absorbed by the medium in which they are passing through
I = I(0)e^-kx
k is the absorption coefficient - 40Ultrasound Imaging ProcedureTransducer is placed in contact with skin and a gel acting as a coupling medium
Pulses of ultrasound are directed into the body
The wave is reflected at boundary between tissues
The reflected pulse is detected and processed
The time for return of echo gives information on depth
Amount of reflection gives information on structures - 41A-scanMeasures distance of of different boundaries from transducer held in one position graphically
- 42B-scanRepeats A-scan from multiple angles to form 2 dimensional image
- 43How do atomic nuclei with an odd number of protons behave in a magnetic fieldLike tiny magnets
- 44What atmoic nuclei is used most in MRIHydrogen because it is abundant in all organic tissue
- 45When there is no magnetic field, protons are aligned:Randomly
- 46When a magnetic field is introduced, most protons...Most protons align themselves with 'north' facing 'south' - stable low energy state
Some protons align themselves in the opposite way with 'south' facing 'south' - unstable high energy state - 47Aligned protons are not stationary; they...spin on their axis of rotation
- 48Atomic spinA fundamental property of a subatomic particle that defines how the particle rotates on its axis
- 49PrecessionThe movement of the axis of rotation of a spinning object (proton) around another external axis
- 50Larmor frequencyThe angular frequency of the circular path of precession of the object (proton)
- 51Process of nuclear magnetic resonanceThe applied external magnetic field causes nuclei to precess
Angular frequency of precession is called Larmor frequency
Frequency of radio pulses applied equals to the Larmor frequency
Causes hydrogen nuclei to resonate and 'flip' into higher energy state
Larmor freq = radio freq = resonance freq - 52Relaxtion of protonsThe protons in high energy state are unstable so must 'relax' and come back to lower energy state
The excess energy is transmitted back as radio waves which can be detected
The time taken for these radio waves to be detected determines the relaxation time - 53Relaxation timeTime taken for a nucleus to fall back to a lower energy state
Depends on environment of protons:
water and watery tissues - several seconds - high
Fatty tissues - hundreds of milliseconds - low
Cancerous tissue - intermediate - 54How MRI scanning worksA large uniform magnetic field causes all protons in body to have the same Larmor frequency
A non-uniform magnetic field is applied to locate a particular position of a proton within the person - 55Procedure of an MRIA strong constant magnetic field is applied along body
Hydrogen nuclei precess about the direction of the field
A radio frequency pulse is applied
The pulse is at the Larmor frequency which causes resonance in hydrogen nuclei
On relaxation, the nuclei de-excite and emit pulse of r.f.
r.f. pulses are detected, processed and displayed
A calibrated non-uniform magnetic field enables position of nuclei to be located and for location of detection to be changed - 56Advantages of X-raySharp image
Improvable contrast
Can form image where air is trapped - 57Drawbacks of X-rayEquipment heavy and not portable
May cause ionization and damage tissues - 58Benefits of ultrasoundPortable equipment
Less harmful than X-ray
Used for treatment of sprained joints - 59Drawbacks of ultrasoundCannot be used to form image where air is trapped
Image not sharp due to refraction
Rapid movements in tissue - may damage - 60Benefits of MRIClearer image than ultrasound
Image can be studied in any plane or direction
Lower health risk than X-ray/ultrasound - 61Drawbacks of MRIEquipment heavy and not portable
Requires patient to remain still
If patient body contains metal, heated/attracted
Pacemakers may be upset by magnetic field
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- AS level Physics Flashcards on A2 Physics Medical Imaging, created by Sean Byrne on 26/02/2018.
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- Sean Byrne
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