medical physics

Question 1

how are x-rays prod

Answer 1

electrons are emitted from a metal filament (cathode) through thermionic emission. ( when it is heated)
the electrons are then accelerated by the high p.d between the cathode and the target metal which is usually tungsten (anode)
the x-ray tube is a vacuum so that electrons can pass through with out interacting with particles
when the electrons strike the target, the electrons decelerate and some of the kinetic energy is transformed to xray photons .

Question 2

wha is the range of x-ray wavelengths

Answer 2

10^-8 to 10^-13 m

Question 3

what is thermionic emission

Answer 3

heating a cathode to release electrons

Question 4

what are the 4 attenuation mechanisms and give a description of each

Answer 4

simple scatter- low energy photons bounce off an atom with no change in momentum
photoelectric effect- A photon is completely absorbed by a metal ejecting an electron from the surface
compton scatter- the xray photon interacts with an electron within an atom , and the electron is ejected and the photon is scattered with reduced energy
pair production- a photon interacts with a nucleus of an atom producing an electron positron pair.

Question 5

define attenuation

Answer 5

the decrease in intensity of electromagnetic radiation as it passes through matter.

Question 6

give 3 factors which effect attenuation of an xray

Answer 6

the specific material being xrayed
the photon energy
the thickness o the material

Question 7

define the attenuation coefficient

Answer 7

a measure of the amount of xray photons absorbed by a substance per metre

Question 8

how can we find the attenuation coefficient using count rate and the geiger muller tube experiment

Answer 8

C=C0 x e^-ux
carry out experiment and find BG radiation
C0 = the count rate of radiation source without a material present in between
plot graph of lnC against x
x= thickness of material
y intercept = lnC0
gradient = attenuation coefficient u

Question 9

describe how the components of a CAT scanner can produce a high quality image of the internal structure of a patient

Answer 9

the xray tube and the detectors rotate around the patient
a thin fan-shaped beam of xrays is emitted
the intensity of the transmitted xrays is recorded by the detectors
after each 360 degree rotation , an image of a thin cross-section of the body is produced showing the soft tissues
the tube continues to spiral down the body building up a 3d image
as the computer uses the signal from the detectors to produce a 3D image

Question 10

why would bone and muscle have a better xray image contrast than fat and muscle

Answer 10

because bone and muscle have a greater difference in attenuation coefficient contrast therefore better contrast

Question 11

an xray scan of the hearts and its blood vessels shows very poor contrast explain what can be done to improve this

Answer 11

a contrast medium / iodine can be injected into the blood
this has a higher attenuation contrast

Question 12

how can we calculate the max wavelength of the xrays for the pair production proces

Answer 12

max wavelength= min E
the min energy required for pair production is given by E=mc^2
m= 2me
then find wavelength after finding E

Question 14

Describe the shape of an intensity of x-rays against the wavelength graph

Answer 14

The spectrum is produced by electron striking the target of an x-ray tube. There are narrow peaks at specific wavelengths which are produced by the bombarding electrons which removes the electrons close to the nuclei. These gaps are then filled with electrons dropping from a higher energy state and photons are released at this wavelength.

Question 15

Explain what would happen to the intensity of x-rays against wavelength graph if the accelerating potential difference is increased in an x-ray tube

Answer 15

When the accelerating potential difference is increased the kinetic energy of the electrons increase therefore the maximum kinetic energy of the x-ray photos also increase however the shortest wavelength will decrease therefore the graph would shift to the left and have higher peaks for the electrons removing electrons close to the nuclei however these peaks will be in the same position the wavelength of the so-called K lines only depend on the material of the target

Question 16

Compare CAT scans and an x-ray tube

Answer 16

An x-ray scan is quicker and cheaper compared to a CAT scan
However CAT scans can produce a 3-D image
The x-rays can be harmful and CAT scan are prolonged therefore the patient is exposed to x-rays for longer

Question 17

What are medical tracers

Answer 17

A medical tracer is a radioactive substance which is injected into the patient or digested and is used for diagnosis or treatment of a patient
They are typically gamma source

Question 18

Describe the property of medical traces require

Answer 18

They are typically gamma sources so this way they can pass through tissue
The source is half-life must be long enough to be detected but short enough so that they do not remain in the patient’s body
it’s activity must be large enough so that it can be detected outside of the body
And it must be non-toxic

Question 19

What are the two medical traces/gamma sources used normally?

Answer 19

Technician 99m, and it is used to monitor the major organs such as the heart kidney brain
Fluorine 18, which is used in PET scan

Question 20

Describe how a gamma camera works

Answer 20

Gamma photons are passed due a collumbator which consists of thin lead tubes that only allows gamma photons that are travelling parallel to each other any travelling at an angle will be absorbed by this thin lead therefore the galleries are said to be columnated
They then passed through a scintillator which is a silicone iodide crystal every gammaray which strikes the Cator will produce multiple visible light photons on
These then passed through a photomultiplier tube where a single photo on a visible light is converted into an electrical signal such as a voltage
The electric electrical signals are then used to create an image of the concentration of the gum and hence the tracer

Question 21

Explain a PET scan

Answer 21

This is when a patient is surrounded by a ring of gamma detectors
And a tracer that emits a Poron as it decay through beta plus radiation is injected into the patient
The positron annihilate with an electron in the body
These produces two Gamaret photons which travel in the opposite direction due to the conservation of momentum
Then the delay time of the two photos is used to locate the location of the annihilation
Then a computer is connected cutie detector in order to form an image of the annihilation

Question 22

Explain the advantages and disadvantages of a PET scan

Answer 22

Advantage
it is a non-evasive technique and can produce real time images
Dis
It is very expensive and a radioactive source is used Therefore, the patient is exposed to a small amount of activity

Question 23

What does the photo multiplier in a gamma camera do?

Answer 23

It takes an input of a single photo of visible light from the creator and uses this to produce up to 1,000,000 electrons to produce an electrical signal

Question 24

How can the gamma camera produce a higher quality image?

Answer 24

This can be improved by using longer thinner culminated or by scanning for a longer time period

Question 25

Explain how the structure of a photomultiplier tube uses a single photo to produce up to 1 millions of electrons

Answer 25

A single photo on a visible light strikes the cathode the photon then releases an electron and the electron is accelerated towards the first dynode it strike dynode causing the release of several more electrons. These electrons are then accelerated towards the dynode and each of these electrons releases even more electrons. This will continue until the electron strike the anode at the end of the tube where a voltage is detected.

Question 26

PET scanners are expensive because they require nearby or on site particle accelerator that produce fluorine 18 discuss the ethical issues this raises in the treatment of patient

Answer 26

Doctors would have to make difficult decisions about who can and cannot have a PET scan Some patients will miss out on PET scans because of their location as not all patients will have access to these scans

Question 27

Explain how medical traces are used to diagnose the condition of organs

Answer 27

Teresa is injected into the body The tracer is absorbed by the organ And a beta detector/gamma camera is used to detect radiation from the body

Question 28

Explain how an ultrasound transducer can emit ultrasound

Answer 28

The piezoelectric effect where an alternating potential difference is applied across the crystal causing it to expand and contract/vibrate

Question 29

Explain how the reflection of ultrasound at a boundary between two tissues depend depends on the physical properties of the tissue

Answer 29

Each material has an acoustic impedance Z where? Z=pc If the two materials have the same value for Z, there will be no reflection if the two values are different the ultrasound will be reflected The fraction of the reflected intensity is given by the equation state the equation

Question 30

In ultrasound scanning explain what is meant by acoustic impedance matching and how it is achieved

Answer 30

This is when a gel with the same acoustic impedance of the skin is used therefore there is no reflection of ultrasound at the boundary

Question 31

Explain how an Ascan could be used to measure the thickness of a patient’s eye lense

Answer 31

A pulse is sent into the eye and reflections occur from the front and the back of the lens. These reflections are picked up on an a telescope where we can use the timebase to figure out the thickness. thickness= 1/2 speed x time

Question 32

Explain the advantages of using technetium-99m when imaging organs such as the brain

Answer 32

It emits gamma radiation and gamma is the least ionising radioactive substance It can diagnose the function of organs and detect tumours and has a short half life but can still be detected outside the body

Question 33

Explain how the transducer produces and receives ultrasound waves and also expressed the purpose of the gel

Answer 33

Producing ultrasound A high frequency alternating potential difference is applied across the piezoelectric crystal causing the crystal to stretch and compress/vibrate therefore the ultrasound wave omitted in the process Receiving ultrasound The reflected ultrasound causes the crystal to oscillate which induce is an alternating potential difference across the crystal Gel Without the gel the acoustic impedance of air is too different from the skin therefore the ultrasound is reflected before entering body however has a similar acoustic impedance as the skin therefore allowing a greater transmission of ultrasound into the body

Question 34

A B scan of an unborn baby is taken and there are parts of the scan which is pitch black explain why there is no signal received in these areas

Answer 34

Because there’s no change in acoustic impedance

Question 35

Explain a B scan and an A scan

Answer 35

Bscan A computer creates dots of different brightnesses corresponding on the strength of the echo This is done very rapidly allowing the transducer to move and produce images from different angles. B scans are effectively multiple overlay scans. Ascan A pulse is sent out and the reflections of ultrasounds are plotted as peaks on a telescope where we can then read off the time base and find the thickness which equals speed times time times 0.5

Question 36

What is an ultrasound?

Answer 36

It is a longitudinal wave that has a frequency of 20,000 Hz

Question 37

On a B scan, there is areas that are brighter than others. Explain why these areas are brighter.

Answer 37

Because there is a greater change of acoustic impede at these areas

Question 38

How do we calculate the wavelength of the gamma futon produced from annihilation?

Answer 38

Use the energy of one gamma photon therefore divide the energy required for the annihilation by 2 to get the energy of one gamma photo after the reaction and then use the equation for wavelength

Question 39

X-rays and gums are produced by different physical processes briefly describe these processes

Answer 39

X-rays are produced by the ionic emissions releasing electrons which are accelerated through a vacuum and hit a target entered where some of that kinetic energy is converted to x-rays Gamma rays are produced from the nuclei of unstable atoms

Question 40

Explain why high energy x-ray photo produce electrons and patrons as they passed through the body

Answer 40

The x-ray futon interacts with a nucleus and the energy of the photo is transformed into matter and antimatter per production occurs and a particle and an anti-particle are produced

Question 41

Explain how an ultrasound transducer operates a medical diagnosis explain how an ultrasound post is produced by the transducer and how the reflected ultrasound processes detected by the transducer and how the transducer can do both transmission and receiving

Answer 41

A high frequency alternating potential difference is applied across the PO electric crystal causing it to contract and expand and oscillate in the process sound waves/ultrasound waves admitted the frequency of the ultrasound waves are above 20 kHz Reflected the pressure , wave in crystal caused the crystal to expand and contract which produces a potential difference across the crystal The same transducer can act as a receiver and a transmitter as there is a short application of alternating current to produce short pulses. There is a use of backing in the material to damp and stop the vibrations of the crystal as the crystal must stop vibrating before the reflections reaches the crystal.

Question 42

How can we find the resolution of an ultrasound?

Answer 42

Calculate the wavelength of the ultrasound and this is equal to the resolution

Question 43

Discuss whether an ultrasound will be able to investigate a tumour inside the lung

Answer 43

No, because the ultrasound would be travelling from air inside the lung to the lung tissue therefore the difference in acoustic impedance is very large so most of the ultrasound will be reflected at the boundary

Question 44

Explain why a coupling gel is needed between an ultrasound probe and a patient skin

Answer 44

Without the gel there is air between the probe and the tissue therefore nearly all of the ultrasound waves are reflected at the boundary

Question 45

Describe the basic physical principles of an Ascan

Answer 45

A short pulse of ultrasound waves is transmitted into the organ and at internal boundaries. Some of the ultrasound waves are reflected and the rest transmit to the next boundary. The reflected pulse is received by the probe and a signal sent to the ossciliscope

Question 46

Outline the differences between an a scan and a b scans and describe the advantages and disadvantages of each type of scan and explain which type of scan would be suitable to insert a tube into a uterus to remove some cells and fluid around the fetus

Answer 46

Differences There are multiple sensors in bscans and there is only one sensor in a scans A scan intensity determines the amplitude whereas the b scan determines the brightness Adv/disadv An Ascan only produces a graph which is good for accurately determining distances however with complex structures it is difficult to identify which surface produces which echo therefore the structures may not lie along one line B scan produces a picture and a 2-D cross-section is obtained rather than a single line. This allows the structures to be identified more easily and allows features that are along one line to be identified. Due to this a B scan would be more suitable as it Qu33r shows the structure of the cells and the a scan would not be suitable for complex structures

Question 47

Discuss how x-rays are used to help provide the best and safest treatment for patients when treating a cancer tumour

Answer 47

The time of the x-rays used is minimised and detour is eradicated from different direction And the use of x-rays of the correct energy for the depth/size of tumour, ensures accuracy and safe

Question 48

Explain the advantages of a CT scanner over an x-ray image for head injuries

Answer 48

A patient lies in the sense of a ring and the tube rotates around the patient’s head after each rotation across section of the patient is produced and a 3-D image is obtained. A CT scanner is better for head injuries as there is a better defined image of the tissue boundaries inside the skull therefore we can identify any bleeding inside the skull whereas x-rays is just a simple 2-Dimage

Question 49

Explain the purpose of the lead shield around a gamma camera

Answer 49

It protects the crystal and the photomultipliers from random background signals

Question 50

Explain the differences of a gamma camera and x-rays

Answer 50

An x-ray image is a simple shadow photograph whereas the image produced by gamma cameras is an actual image of the actual emission points which can be used to monitor rapidly changing situations

Question 51

Explain why technetium-99m is suitable as a tracer

Answer 51

It has a half-life of six hours therefore it is long enough to allow the scan to take place however it is short enough to not expose the patient to excessive radiation however it is too short to keep stored therefore it must be produced on site

Question 52

Explain why gamma rays are suitable to be used as a tracer

Answer 52

Other radiation causes damage to the tissue, which is undetectable gammaray passed through the body and therefore can be detected outside of the body