Chapter 27 Medical Physics Flashcards
Recap on anode vs cathode
Anodes attract anions thus must be positive, cathodes attract cations thus must be NEGATIVE
What was themrionic emission again and how did it work
It is the process by which a current is used to heat up the filament, giving electrons here Ke
When they had enough kE, they could escape the filament all together
We make the assumption that they escape the filament with 0KE
Once thermionic emission happened, how are electrons accelerated? What’s the energy they gain by the time these finished accelerating and what speed travel at once done?
In an electron gun
After they escape, there is ANOTHER CIRCUIT.
Here a voltsge supply is connected to two parallel metal plates to create a uniform electric field
- the Parallel plates involve the CATHODE which is the filament and an ANODE which is something else
- the cathode needs to be the filament, as this is negative, so it will repel the electrons (electric field needs to run in this way for electrons to be acclersted correct way)
2) the electrons will accelerate due to constant F being applied towards the Anode. If by they reach the whole anode, that means they have experienced the whole voltage and thus have gained energy = QVas V= WD / Q.
- this converted entirely to Ke so Ke = VQ and thus know the velocity
3) if there is a hole in the anode, then they all escape with a certain velocity !
Why must there be a vacuum
Must be a vacuum so electrons don’t collide with gas particles and transfer energy to the,
What’s the aim with the electrons in an X ray tube to produce s rays (hows it done)?
We want to make X day photons
- this is dine by the accelerated electrons to COLLIDE with tungsten , transfer there KE to excite electrons in tungsten and thus decelerate colliding electrons , and when the excited electrons unexcited, they produce X RAYS!
So now describe each component of an x day tube used to produce an x ray
- two circuits, with an EVACUATED TUBE
1) first circuit connects to a filament in the tube - a current is run across the filament which caused electrons to be emitted by thermionic emission
2) then there is a second circuit connecting the filament to a piece of TUNGSTEN METAL
- this makes the filament a cathode and the tungsten an anode
- a HIGH voltage is run across this circuit
- this acceleatyed the emitted electrons towards the tungsten anode
3) with the energy of the electrons, electrons in tungsten excite and then de excite later. This produces X RAYS
Finally how is the x ray tube allow for x rays to be emitted in certain direction?
The tungsten is aligned such that they are emitted through a window
How to find the MINIMUM wavelength that a colliding electron can produce?
Assumign that electron had NO ke when thermionically emitted
- that means energy gained = vQ = 1/2mv2
- assuming all this energy was transferred to electron and de excited back
Then eV = energy of x ray photon = h f (where f is max frequency )
Thus csn find min wavelength ntd
How to increase the frewuencies or decrease min wavelengths of the x rays?
Increasing the ACCELERATING VOLTAGE of 2nd circuit means more energy = higher f = lower wavelengths of x rays!
How to increase the INTENSITY ( number of x rays)
Important
As each electron produces 1 x ray, to incresse number of x rays, have to increase number of electrons
Thus to increase number of electrons, need to increase emitted
- do this by INCREASING THE CURRENT of the FIRST CIRCUIT, increased the amount of electrons emitted thus amount of x rays emitted too!
In reality , is all the kE from colliding electron transferred to electron in tungsten shell?
Hell nah!
99% is transferred to HEAT upon colliding, only 1 % goes snd acc transferred to excite the electron snd so x ray acc gonna have way less energy and thus way higher min wavelength
How to ensure heat felt by tungsten isn’t a lot as 99% of electron energy converted to heat here?
1) rotate the tungsten so heat spread out
2) use o Icl to cool the anode
Why does the x rays produced actually a CONTINOUS spectrum of different wavelengths ?
What happens when increase accelerating v / initsl current ?
The electrons colliding don’t necessarily collide with the OUTER SHELLS of tungsten
= some could go deeper in , and thus excite electron less ( as it was at higher energy level) and thus lower res length of x ray produced
- as this happens at all levels, a CONTINOUS spectrum of x rays are produced , like a normal distribution
2) increase current = intensity up so all shift up extend
- increase pd = increase max frewuency so reduced min wavelentgh so graph increases to the LEFT and it stretches !
DONT NEED TO KNOW
What are the k lines Prdouced on the CONTINOUS spec and why?
These are random lines on the curve
- this is due to the Fingerprint of the TARGET METAL
- when electron excites, electron of higher energy shell moves down to fill Valentine gap, releasing an x ray as a result
- this causes the spike
- it’s unique to each target metal because each target metal had its own divisions between energy levels so different x rays produced when this happens etc
Causing different spikes due to different wavelengths being produced at MUCH HIGHER INTENSITIES
How does x ray imagine bsdicslly work
It works on the fact that human body is SELECTIVLY ATTENUATING
- as a result, x rays will be able to pass through soft tissue (blood muscle fat ) but not hard tissue (bone)
- x rays travel to another screen behind which is digital. Where it picks up an x ray it can colour whatever, typically it covers it in BLACK, so thst is the absence of the x ray (bone) it becomes WHITE
What does attenuated mean?
Body being selectively attenuated means what
If it attenuated something, it has REDUCED THE INTENSITY OF THE X RAY in the ORIGNAL DIRECTION OF TRAVEL, this could mean absorbed or deflected out of the way
So selectively attenuating means some parts are MORE attenuating than others, such that whole parts are isolated whereas rest are through
NEW CHAPTER
Attenuation mechanisms
What are all 4 attenuation mechanisms In order of how they happen with increasing eV of x ray?
1) simple scatter
2) photoelectric effect
3) compote scatter
4) pair production
1) simple scatter
- happens at 1 to 20 kEV energy ( so not really used in hospitals as they always sue above this )
X ray photo interacts with an electron in atom it meets, but because it simply doesn’t have ENOUGH energy to remove it,
- it just BOUNCES OFF ELASTICALLY , keeping all its energy , but moved out tje way
As moved out the way= scatter = reduced intensity
2)photo electric effect
Happens less than 100kev
- the x ray is compleltey absorbed by the electron as it met the work function needed for it to escape the atom
- so x ray COMPLETLEY DISAPPEARS
Why is photo electric effect the MOST FREQUENT METHOD OF ATTENUATION IN HOSPITALS
Because hospitals tend to use 20 to 100kev for x rays, which means simple scatter cannot happen , but photovelevtkc effect can and is only one thst really happens here!
So due to its range as photo goes to 100
3) Compton scattering?
For energy to 5 MEV
- x ray photon interacts with electron , transfers energy, caused it to escape atom, similar to photoelectric but…
The X RAY DOESNT COMPLETELY DISSAPEAR
- it retains some energy and is just SCATTERED But with REDUCED ENERGY to what it came in
(For understanding ) how is Compton scattering even possible, I thought photons had to give up all their energy or nine?
It’s considered to hit an electron on the OUTSIDE OF THE SHELL, which is FREE.
As it’s free stipulation is it can give SOME OF ITS energy up not all and still cause electron to be ejected
Whereas photoelectric is a LOW energy electron but not outer shell low, so has to be discrete there!
Finally 4) pair production
What MUST ENERGY OF X RAY MUST ATLEAST BE
Energy must be greater than 1 MeV
Here the x ray interacts with the NUCLEUS INSTEAD
- via the string nuclear force, it interacts and dis spears to become an electron and a positron
Thus energy of x ray must atleast be 2me c 2!
All 4 attenuation mechanisms summary
1) simple scatter
- from 0 to 20 ev , as it has low energy, it interscts with electron but csnf remove it, so just bounces off elastically with exact same energy. As bounce off = attenuated
2) photoelectric effect to 100ev
- x ray interacts with electron and hsd enough energy to surpass its work function causing to go be ejected from atom .
- as x ray disspear = attenuated
3) Compton scattering
- for energy to 5 MEV
- interacts with electron and causes it to eject but this time RETAINS some energy too
- gets scattered with REDUCED energy
- can do so because only interacts with outer shell electrons considered free already , which dint require rule to give away all energy!
4) pair production
Energy to 1 MeV
- interacts with the nucleus and via string nuclear force created positron electron
- as x ray dispear= attenuated
Most common attenuation = hospitals because range of x rays are 20 to 100 which it fills under
Atrnustion coeffeint formula
I = I0 e ^- x K
Just make sure x and k same units and you’re good
This shows you how much intensity it lets through sbsed on coeffeicmt which is obvs different for each material
What if you want to investigate specially the BLOOD VESSELS or blood flow in a tissue like heart, what’s problem and what’s in torched
As BOTH BLOOD VESSELS AND HEART TISSUE have similar attenuation coefficient , the contrast between them will be low! (Bith will alprear same colour )
- thus a CONTRAST MEDIUM is introduced to single one component of soft tissue out .
How does a contrast medium work?
It is a substance with TREMEDNOUSLY More attenuation coeffeicmt than other structured you want to differnstie from like tissue, vessels etc
It is ingested and then x ray is run
Now where it is in acts like BONES, quite attenuated leaving back white, whereas everything else not attenjated so x ray goes though snd makes it black
Importsnt
WHAT TWO TYPES OF CONTRAST MEDIUM IS USED
Iodine and barium !
Iodine for blood, barium for organs
What really affects the contrast of a picture ?
- if the attenuation coefficient same then cotrsdt very low
- time left is for long time then eventually it will all be black and can’t tell anything
Why DONT WE WANT SOFT X RAYS and not HARD X RAYS either?
Soft x rays
- too weak , so nothign is transmitted barely, and case of white vs less white low contrast
- also they are ABSORBED BY HUMAN SO INCREASE DOSAGE so dangerous
Hard x rays
- too strong they just go through and it’s a case of really black vs less black Cant fell
- however don’t corntibute to dosage as so powerful just goes through
How to fix hard x ray and soft x ray problem
Hard x ray = want to reduced this to medium x ray
- so reduce accelerating pd to reduced max frewuency kf x ray produced
Soft x ray
- place a filter between pateint and emitter because there always will be soft x rays in any emission
How to keep picture clear and not blurry with x ray?
1) hold a filter between patient and detector plate
- this will stop any attunated x rays coming in that direction
2’ ensure detector plate is close to person
- so all of the, reach
3) don’t hold x ray tube too far back
- so we know for a fact they enter parallel
