chapter 27 - medical physics Flashcards
1
Q
X-ray properties
A
- short wavelenght
- EM waves
- can be polarised and diffracted
- high intensity can be harmful to living cells
2
Q
production of X-rays in an X-ray tube
A
- use can evacuated tube containing 2 electrodes
- large pd between electrodes
- cathode is a heater which produces electrons by thermionic emissions
- electrons are accelerated towards the anode (target metal) eg tungsten - high mtp
- X-rays produced when electrons decelerate by hitting anode
- anode is shaped so rays are emitted in the desired direction
3
Q
how to prevent X-ray tubes heating up too much
A
- cooling fins help dissipate heat from the tube
- water and oil cooling systems
- target is rotated
4
Q
shortest wavlentg of X-rays produced in an X-ray tube
A
eV = hf
eV = hc/λ
λ = hc/eV
so λ prop to 1/V
increasing current just increases intensity (as more electrons emitted)
5
Q
X- ray spectra
A
- range of decelerations of electrons inside the x-ray tube result in Bremsstrahlung (overall curve)
- narrow intense lines (K-lines) are due to incident electrons removing electrons from the atom and these are filled by electrons dropping down energy levels and releasing photons of specific energies
6
Q
how x-ray spectra is effected by increasing pd
A
intensity increases - graph shifts up and lowest wavelength shifts left (decreases)
7
Q
attenuation
A
gradual decrease in intensity of the x-ray beam as it travels through a medium - bone attenuates more than soft tissue
8
Q
energy range of x-rays used to diagnose bones
A
100eV - 100keV
9
Q
4 attenuation mechanisms
A
- simple scatter
- photoelectric effect
- compton scattering
- pair production
10
Q
simple scatter
A
1-20keV
- photon changes direction
- energy unchanged
11
Q
photoelectric effect
A
<100keV
- X-ray photon absorbed
- electron emitted
- used in X-ray imaging
12
Q
compton scattering
A
0.5-5MeV
- in high energy x-rays (radiotherapy) - used to kill cancerous cells
- elastic scattering of X-rays by orbital electron
- X-ray incident - low energy photon + electron released
13
Q
pair production
A
1.02MeV
- X-ray photon -> electron-positron pair
14
Q
attenuation coefficient
A
- μ
- interaction of x-ray photons with matter reduces intensity of the beam
- transmitted intensity of x-rays depends on energy of photons + thickness/ type of substance
I = I0 e^-μx
15
Q
contrast medium
A
- soft tissue have low absorption coefficients but adding a contrast medium allows visibility (larger absorption coefficient)
- eg iodine and barium - have large atomic number which is related to absorption
16
Q
CAT scan
A
- patient lies on table that can slide in and out of large vertical ring
- x-ray tube on one side and an array of detectors on the other
- x-ray tube and detectors rotate
- x-ray tube produces a then beam of x-rays which are attenuated by different amounts by different tissues
- each time the tube rotates a 2D slice is formed - slices are turned into a 3D image by computer
16
Q
A
16
Q
A
17
Q
+ CAT
A
- 3D image
- more detailed
- can distinguish between soft tissues of similar attenuation coefficients
17
Q
A
17
Q
A
17
Q
A
18
Q
- CAT
A
- takes longer
- patients receive a higher dose of ionising radiation vs an x-ray
- have to lay still
18
Q
medical tracers used
A
- need to target desired tissures
- radioisotopes combined with elements to achieve this
19
gamma camera
- imaging tecnique to scan functions of organs
- images radiation from a tracer in the patients body
20
gamma knife
- can be used to kill tumour cells
- gamma emitted from multiple sources and concentrated to the target
- try to minimise damage to healthy cells while providing a high enough does to kill tumour cells
21
gamma diagnosis + therapy
- diagnosis - gamma camera
- therapy - gamma knife
22
chosing radioisotopes
imaging - isotopes have to be placed inside and detected outside
- gamma used as least ionising and most penetrating
- short half life- ensures patient doesnt remain radioactive for too long after and high enough activity from source so only small does required
23
gamma isotope used
Technetium - 99m
Tc - 99m
24
gamma camera parts
- collimator
- scintillator
- photomultiplier
- computer - outputs connected to a comp and use software to build image
25
collimator
lead tubes
- only gamma photons that travel along axes of lead tubes are detected
26
scinillator
- gamma ray photon produces many vissible light photons
27
photomultiplier
- electrical pulse produced from each photon of visible light entering a photomultiplier tube
- one visible light photon incident producing a photoelectron
- electron accelerated to first electrode
- high speed impact produces about 4 secondary electrons which are accelerated to the next higher V dynode
- repeats and number of electrons grows exponentially
28
positron emission tomography (PET) scans
- like CAT scans, PET scans produces slices through the body to build a 3D image but uses gamma not x-rays
- ring of gamma cameras around the patient
29
PET scan tracer
uses Fluorine-18
- positron emitter
- short half life
30
PET scan steps
- pair of gamma photons produced from electron positron annihilation, emitted in opposite directions (obey conservation of momentum)
- each photon detected at opposite detectors - arrival times recorded
- as know speed can find location of annihilation based on arrival times
- repeated until a 3D model can be produced
- tracer density determined from rate of photons emitted in each region
31
PET +
- non invasive
- diagnosis of cancer/ observes functionality of organs
- used to assess effects of new medcines on organs
32
PET -
- expensive due to facilities required for on site tracer production
- only found in main hospitals
- only used for patients with complex problems
33
ultrasound
- longitudinal
- >20kHz
- non ionising
- non invasive
- high freqs mean wavelength is small and smaller details can be detected (2-15MHz)
34
piezoelectric effect
a piezoelectric material expands/ contracts when a pd is applied across opposite faces
and vice versa
35
ultrasound transducer - emitting
- high freq alternating pd applied across the crystals
- the crystal resonates producing an intense ultrasound signal
36
ultrasound transducer - detecting
- incident ultrasound makes crystal vibrate (compress and expand)
- this generates an alternation emf across the ends of the crystal - detected by a ciruit
37
A scan
- simplest ultrasound scan
- single transducer
- used to determine thickness of bone / distance between lens and retina in eye
38
A scan - eye scan
- voltage pulse sent into the eye
- reflections at each boundary between tissues - partly reflected and partly transmitted
- the reflected pulse received by the transducer has less energy than original pulse
- pulse voltage seen on a screen
39
B scan
- provide a 2D image
- multiple A scans in different directions
- transducer moved over skin
- output connected to cop which produces a collection of dots on the screen
- each dot corresponds to the boundary between tissues
- brightness of dot prop to intensity of the reflected pulse
40
acoustic impedance
fraction of ultrasound intensity reflected at the boundary depends on acoustic impedance of both media
Z = pc
(density * speed of ultrasound in substance)
prop of signal reflected
Ir/I0 = (Z2-Z1)^2/(Z2+Z1)^2
more reflection when acoustic impedance is different - if the same not reflected
41
acoustic matching - coupling gel
- acoustic impedance of air is very different to body
- air packets get trapped between transducer and body
- gel stops air pockets and has a acoustic impedance similar to the body
- means more ultrasound waves are transmitted and less reflected
42
doppler imaging
freq of ultrasound changes when it is reflected off a moving object - Doppler effect
- can monitor flow of blood through vessel
43
colour doppler scans
- when ultrasound detected from moving blood cells - freq of detected pulse changes
- moving towards - freq ^
- moving away - decreases
- freq shift is prop to the speed of blood
- comp can colour code image to show blood moving away/towards
44
speed of blood flow
Δf = 2fv cosθ/c
