Chapter 27 Medica l Flashcards
How is an x ray produced
1) structure of the x ray tube
Inside an x ray tube
- the x ray tube consists of an EVACUATED CHAMBER
- there are TWO circuits
1) the first circuit produces electrons ( with 0 asssumed ke ) through therminonic emission
2) the second circuit accelerated these electrons with a use of an electric field , to collide with a SHEET OF TUNGSTEN = anode
3) the energy transferred when the electrons decelerate when colliding is used to produce x rays
4) the tungsten sheet is angled in the direction of where the X-rays should exit
How x rays are produced
1) thermionic emission occurs
- this is where a filament is heated by a string current high enough that electrons gain enough ke to escape the atom
- they emit from the atom by thermionic emission.
- we ASSUME THEY EMIT AT 0KE
2) Acceleration
- another circuit runs between the filament and then tungsten
- the filament acts as the cathode, and tungsten as an anode
- by electric field, the electrons feel a constant force and are accelerated towards the anode ( which is positive)
3) the collision
- electron collided and decelerated , and energy of electron is transfered to produce an x ray
Explain energy changes and how to find min frewuency max wavelength of x ray
Assuming the electrons left the cathode emitted with 0 ke, by the time they reach the anode they have work done on them = QE
Assuming all the energy is transferred to the x ray being produced, then equate EQ = hf and rearrange
In reality what are the energy changes
In reality almost 99% of ke of electrons are lost as thermal energy , only 1% actually goes in and excites and de excites the electrons in tungsten to produce x rays
As so much energy is lost upon collidjg the electron with tungsten what is advised in the d ray tube?
2 WAYS
1) the tungsten is rotated so the heat is spread out
2) use oil to cool the anode
And remember how could we find the speed of the electrons collidjg the tungsten (easy)
So why must be collided
Assume all QE is transfered to KE
This is why it must be evacuated, so electrins don’t collide with any air molecules and lose energy
Why is there A continous spectrum of x rays produced at different intensities of course
2) what are k lines?
1) this is because the electrons don’t always collide with ground state electrons, some higher etc, thus different frequencies of x rays are produced .
2) the k lines are because when an electron from a high state, an electron from ground state goes to fill the gap, which produces a UNIQUE wvalenght of photon, that is gonna stay there no matter what
What happens if we increased the current of the filament circuit, and the pd of the accelerating plates circuit to the intensity graoh?
IMPORTANT
1)Increasing the current will only increase the amount of electrons being thermionically emitted .
- as these electrons produce x rays in 1:1 ratio, this increases the number and hence INTENSITY OF X RAYS wavelengths produced
= shifts the graph higher
2) increasign accelerating pd increases the work done and max ke and hence assuming all ke is transfered to the x ray, the max frequency and hence DECREASES min wavelength
- thus the graph STRETCHES and parts on the left are filled in
How does x ray imaging work
What is attenuation
On the idea that the media you are imaging has different materials of different strength Ustinov coefficients, such that it is SELECTIVELY attenuating
Attenuation is used to describe the DECREASE in intensity on the other end as a material absorbs something
So we want to image the bone, as the bones have higher attenuation coefficients than the soft tissue, they absorb the x rays more, leaving less intensity of x ray on the other side when being detected by the digital plate
- based on the intensities detected by the digital plate, an image can be produced
How does the digital plate make an image
For example for bone, where higher intensities of x ray is received, ( soft tissue), it colours it black
Where lower , colours it white
And this produces an image
Attenuated DEFINITON
Selectively attenuated
Decrease of intensity in the direction of travel as it passes through something
This means some parts of the media are more attenuating than others, which will owe to contrast
What are the 4 ways x rays can become attenuated we need to know
In order of energy
1) simple scattering
2) photo electric effect
3) Compton scattering
4) pair production
Simple scatter
The eneegy of the x ray is not enough to remove the electron by photoelectric effect
So it just bounces off ( scatters) with NO change to its original energy
= as it’s moved out the way= reduced intensity = attenuated
2) photoelectric effect
The entirety of the energy of the x ray is absorbed by an electron and used to escape the atom, by the photoelectric effect, as it met the work function
X ray COMPLETLEY disappears
= as the x ray gone= reduced intensity = attenuated
3) Compton scattering
The s ray interacts with an electron in the atom, and energy from the x ray is used to cause the electron to escape the atom
HOWEVER, the x ray doesn’t disappears, instead is now scattered with REDUCED ENERGY
= hence attenuated
Why can the Compton effect even happen, I thought for a photon it had to give up all its energy or nothing! But electron could give how much it wants
Technically the electron it collided with is on the outside of the shell which is “already free”
Hence as its free already it can give it as much energy as it wants, and hence ejected the atom , and is scattered with reduced energy
Stipulation , don’t need to know
4) pair production
The x ray goes close enough and interacts with the nucleus, and via the string nuclear force , it interacts and dissapeaes to form a position and electron
For this to happen the x ray must have enough energy for 2ME! Else it can’t happen
As x ray COMPLETLEY disappear, attenuated
All 4 attenuation mechanisms
1) simple scatter - the x ray does not have enough energy to cause the electron to leave, hence is scattered with same energy
2) photoelectric effect
- the x ray is completely absorbed by an electron as it meets its work function, and escapes the atom. X ray disappears
3) Compton scattering
- energy from x ray is used to eject and electron, but x ray does not dissoear, instead is scattered with reduced energy
4) pair production
- x ray interacts with the nucleus via string nuclear force and dis spears to produce a positron electron, with energy of x ray atleast = 2me
What is the most common attenuation mechanism with x rays in hospital imaging
Is photoelectric effect bevause this happens for x rays below 100keV , and hospitals have x rays of 30 to 100
Simple scatter doesn’t occur bevause less than 30, and hospitals don’t run that low
Why are hospitals against low frequency x rays
- what do they do
Why hospitals don’t prefer high frequency x rays
- but why ntd
Because they too low enough to create contrast in image, and just contribute to dosage
- therefore they use a metal net to absorb these
High ones don’t contribute to picture, I’m fact REDUCE CONTRAST because makes it harder to differniate between black extreme, , but not to dosage either as so strign COMPLETLEY pass through, but ntd
Attenuation amount transmitted formula
Intensity transmitted
= I0 E^-UX, where U is the attenuation coeffeint of the object , and x is the distance of the object
What if you want to investigate between things of low similar attenuation coeffeicjt d that are low, like blood flow in organs, how can we increase the contrast?
We need a CONTRAST MEDIUM to be injected which has high attenuation coefficient that will thus show higher contrast between what we want to image.
This is because when we try to image an organ, as the soft tissue and blood have similar attenuation coefficients, there won’t be much contrast in the resulting image, leaving it quite hard to differniate
How does contrast medium and why does it work
It has MUCH higher attenuation coeffeicjt than others, such that contrast will show
For interest the attenuation coefficient is proprtinsl to the molar mass cubed. Hence as iodine and barium is used, there’s 100s time more attenuation coefficient
Now when x ray, there’s difference in mediums which mean that different intensities of s ray will meet the digital plate and hence a picture can firm
What are the two types of contrast medium we need to know !
Iodine for blood
Barium as part of barium meal for digestive system
Two things that affect the contrast of a picture
1) if the attenuation coefficients are similar, then contrast between picture will be low
2) if the x ray is left for long, then eventually the whole image will recieve high intensity, and thus the contrast becomes worse
How to fix soft x ray and hard x ray problem?
Soft x ray, place a metal filter between patient and emitter, which will absorb these low frequency s rays
Hard x rays, turn down the voltage so these aren’t produced. These reduce the contrast as makes things harder to see
How to keep picture clear with x ray
3 CRUCIAL POINTS WE NEED TO KNOW
1) hold a filter between patient and digital plate
- this will stop any attenuated x rays from random postions coming
2) ensure detector plate is held close to the person
- so all of the transmitted x rays reach it
3) domt hold the x ray tube too far
- else the x rays might not reach the patient parallel fashion, and the postion of the x rays becomes more uncertain making the image blurry
How to use x rays in therapeutic rather than imaging?
As high frequency higj energy, used to treat tumours in cancer too , to kill the cells
