5. Inside The Body

Question 1

I) Who discovered x-rays in 1895?

Answer 1

Wilhelm Röntgen

Question 2

I) What are some of the applications of x-rays?

Answer 2

Unobtrusive investigations of internal problems

Security e.g. Monitoring bags at airports

Question 3

I) Why are lower energy x-rays removed from the x-ray beam in medical applications?

Answer 3

Because lower energy x-rays are readily absorbed by the body and are therefore harmful.

Question 4

I) What does the intensity of the x-ray beam after passing through a patient’s body depend on?

Answer 4

The thickness and density of the tissue and bones through which the beam has passed.

Question 5

I) How does the intensity of the x-ray beam change with thickness?

Answer 5

Exponentially
I = Ioe^-mx
Where: I is the intensity, Io is the initial intensity, m is the attenuation coefficient, x is the distance travelled by the beam.

Question 6

I) What 3 things does the value of the ‘attenuation coefficient/m’ depend on when considering an x-ray beam?

Answer 6

The energy of the x-rays
The proton number of the absorbing material
The density of the absorbing material

Question 7

I) What effect does a higher proton number of the absorbing material have on the ‘attenuation coefficient/m’ when considering an x-ray beam?

Answer 7

Elements with high proton numbers have higher values of the ‘attenuation coefficient/m’

Question 8

I) For a given proton number and photon energy, what is the relationship between the ‘attenuation coefficient/m’ and density?

Answer 8

The ‘attenuation coefficient/m’ is proportional to the density

Question 9

I) What is a material’s half-value thickness?

Answer 9

The thickness of an absorber that reduces the intensity of EM radiation to half its original value.

Question 10

I) How would you calculate a material’s half-value thickness/X1/2?

Answer 10

X1/2 = ln2/mu

Question 11

I) How are the darker and the lighter parts of an x-ray formed?

Answer 11

The dark parts of the image are formed where there has been little adsorption of the radiation. The lighter images are formed of bones which have a high density, and a high value of ‘mu’ and so baron more radiation.

Question 12

I) How can doctors spot diseased organs?

Answer 12

Normally the images of organs are consistent and dark.

Diseased organs produce images of varying intensity that can be interpreted by doctors.

Question 13

I) How does the short wavelength of x-rays affect the way that they travel through the body of a patient.

Answer 13

It means that x-rays are not diffracted or scattered significantly by the atoms in the body so travel through the body in almost straight lines.

Question 14

I) Describe how the scattering of x-rays is dealt with in order to improve image quality.

Answer 14

The scattering of the beam that does occur would blur the image so the contrast of the image is improved by using a lead grid.
The lead grid absorbs scattered x-rays so only radiation that travels directly through the grid channels reaches the photographic film.

Question 15

I) Describe how a ‘contrast medium’ is used to improve the image quality of an x-ray. Give examples of contrast mediums and situations where they are used.

Answer 15

Due to the small difference in density between organs such as the stomach and surrounding tissue, a contrast medium is used.
To produce a good contrast a patient drinks a barium meal which consists of barium sulphate. The barium absorbs x-rays which improves the image.
Similarly, blood vessels can be shown more clearly if a solution of iodine is introduced into the bloodstream of the patient.

Question 16

I) Describe how an image intensifier tube works and why it is useful to doctors.

Answer 16

The x-ray image can be enhanced electronically and allows doctors to study real-time movement inside the body.

X-rays form an image on a fluorescent screen which emits light photons that cause electron emission from the photocathode.
These electrons are focused to produce an image on the fluorescent viewing screen.
The image is enhanced by a factor of a 1000 so the radiation dose received by the patient is reduced significantly.

Question 17

I) What does a CT scanner stand for?

Answer 17

Computerised tomography scanner

Question 18

I) Briefly describe how a CT scanner works?

Answer 18

An x-ray source rotates around a patient whilst measuring the intensity of x-rays transmitted through the body.

At the same time the patient travels through the electron beam.

The beam fans out from the x-ray source and sensors on the other side of the body record the intensity of the transmitted beam.

Each rotation of the beam provides data for a thin cross-section of the body.

Analysis of all the data by powerful computer software produces a 3D image of the body which can be manipulated.

Question 19

I) What precautions must a radiologist take when performing an x-ray?

Answer 19

They leave the room when an x-ray is being taken and are protected by screens that are designed to reduce radiation to negligible levels.

Checks are made by using radiation badges to check the levels of radiation to which the operator has been exposed.

Question 20

I) What are x-rays?

Answer 20

X-rays are electromagnetic radiation with wavelengths from about 0.03 to 3 nm.

Question 21

I) What is the difference between gamma radiation and x-ray radiation?

Answer 21

Gamma rays come from energy changes in the nucleus of an atom, whereas x-rays are produced either by rapid deceleration of electrons when they strike a metal target or by electron transitions between electron energy levels in atoms.

THEIR FREQUENCIES CAN OVERLAP!

Question 22

LOOOOOOOOOOOOOOOOOOOOVE

Answer 22

MEEEEEEEEEEEE

Question 23

I) Describe the 2 main parts and their purpose in a rotating anode X-ray tube.

Answer 23

The filament - electrons are produced at the heated filament by the process of thermionic emission.

The anode - A high potential difference is set up between the filament and the anode which produces an electric field which accelerates the electron towards the anode. The anode is made of tungsten and is shaped like a disc.

Question 24

I) Describe the 2 main parts and their purpose in a rotating anode X-ray tube.

Answer 24

The filament - electrons are produced at the heated filament by the process of thermionic emission.

The anode - A high potential difference is set up between the filament and the anode which produces an electric field which accelerates the electron towards the anode. The anode is made of tungsten and is shaped like a disc.

Question 25

I) What happens to the electrons as they hit the tungsten anode in a rotating anode X-ray tube?

Answer 25

They decelerate rapidly and the energy lost from each electron becomes a photon of X radiation.
An electron may loose its energy in one collision or in many collisions in which different amounts of energy are lost by the electron.
Therefore, the resulting X-ray beam has photons with a range of energies giving rise to a continuous spectrum of x-rays.

Question 26

I) Why does the anode rotate in a rotating anode X-ray tube?

Answer 26

When the electron beam strikes the anode, only about 1% of the energy becomes X radiation. The remainder becomes internal energy of the target, raising the temperature. If the anode rotates, the point at which the electrons hit the anode changes continuously which prevents the anode from overheating.

Question 27

I) Define the term ‘characteristic x-rays’.

Answer 27

Shortwave radiation of specific frequencies that depend on the element forming the target for the electrons in an x-ray tube.

Question 28

I) State and describe the reasons for why the intensity of an X-ray beam is reduced as it passes through matter.

Answer 28

The energy may be scattered if the energy of the photon is too low to be absorbed.

The photons use their energy to ionise an atom, knocking an electron out of one of the energy levels.

The photons may excite an electron in an atom.

A high energy photon may collide with an electron in a ‘billiard ball’ like collision, giving the electron kinetic energy. This is called Compton Scattering.

At high enough energies, the photon may collide with the nucleus of an atom leaden to the production of an electron-positron pair.

Question 29

I) What is ultrasound?

Answer 29

High recency longitudinal waves that are above the range of human hearing (above 20kHz).

Question 30

I) How is ultrasound produced?

Answer 30

Practical ultrasound transducers use a ceramic crystal called Lead Zirconate Titanate.

The crystal exhibits a piezoelectric effect so when it is compressed, a potential difference is produced across is and conversely, when a potential difference is applied to the crystal, it changes shape.

An alternating potential difference oscillating at the natural frequency of the crystal is applied of that resonance occurs.

This produces a large amplitude of vibration and waves are produced in the medium that is in contact with the crystal.

The amplitude of the ultrasound is controlled by the size of the voltage applied across the crystal.

Question 31

I) What are some of the uses of ultrasound in medicine?

Answer 31

Used for imaging and diagnostic purposes.
Cleaning surgical instruments.
Cleaning teeth in dentistry.
The generation of localised heating in the treatment of benign and malignant tumours.

Question 32

I) How many pulses of ultrasound are produced per second in medical imaging?

Answer 32

1000 pulses per second

Question 33

I) The level of detail that can be observed by ultrasound imaging depends on what 2 things?

Answer 33

The wavelength of the waves

The diameter of the beam

Question 34

I) How does the wavelength of ultrasound affect the detail in the imaging?

Answer 34

A shorter wavelength would provide a better image quality and suggests that using a higher frequency would be the best solution.

HOWEVER, higher frequencies are more readily absorbed so these are less useful for investigations deep inside the body.

Question 35

I) What is the main idea when trying to obtain a good quality image using ultrasound?

Answer 35

The energy needs to penetrate deep inside the body and needs to loose as little energy as possible due to absorption and refraction.

Question 36

I) What does the amount on energy reflected back form a change in medium depend on in ultrasound imaging?

Answer 36

The difference in acoustic impedances between the two materials.

Question 37

I) Why is gel used between the ultrasound transducer and the skin in ultrasound imaging?

Answer 37

It reduces the difference in acoustic impedance between the ultrasound transducer and the skin and ensure more efficient energy transfer into the body.

Question 38

I) How is an image produced in ultrasound imaging?

How is the depth of different tissues calculated?

Answer 38

The image is produced by waves reflected from parts of the body made of different tissues.

The ultrasound image is built up by sending out pulses of ultrasound and changing the beam direction so that is scans the body.

Some energy is reflected from any interface that the beam meets as it travels through the body.

The darker parts of an image indicate where there is little or no reflection and the lighter parts a where a higher proportion of the incident ray is reflected.

The depth of these surfaces is calculated from the time it takes the echo to return to the transducer after being emitted.

Each pulse is sent in a slightly different direction to the last so that over a short period the ultrasound scans a cross-section through the body and is displayed on the screen..

Question 39

I) When ultrasound passes from one type of tissue to another (1 – > 2), How would you calculate the reflected intensity / Ir

Answer 39

Ir = Ii (Z2-Z1/Z2+Z1)^2 where Ii is the incident intensity.

Question 40

I) How might you use the doppler effect to measure the rate of blood flow when using an ultrasound transmitter?
How would you process blood flow in different directions?
What formula would you use?
How would you calculate the volume flow rate?

Answer 40

Using the doppler effect, the velocities of blood in a blood vessel can be found from the change in frequency of an ultrasound wave when it is reflected by a moving blood cell.

The reflections from a scanning beam can be processed so that flows in different directions show up as different colours on the screen.

For blood flowing at an angle θ,
The velocity is given by: 2vcosθ/c = Δλ/λ = Δf/f
(The factor 2 occurs because the frequency shift is caused by the echo from an moving reflector)

The volume flow rate can be calculated fro Av where A is the cross-sectional area or the artery.

Question 41

Compare Ultrasound with x-rays base on the following:

• The production of the image
• The risk of ionising the patient’s cells
• The number/duration of examinations possible
• Detail of the image
• Risk to the operator

Answer 41

X-rays are highly penetrating and produce images by transmission through the body. Ultrasound produces images by reflection.

X-rays are highly ionising. Ultrasound is not

Because of its ionising effects, there are limitations on how often X-rays can be used to examine patients. In addition, careful precautions are needed to protect the operator. On the other hand, ultrasound is not harmful so can be used frequently and continuously if required. No safety precautions have to be made for the operator.

X-rays are more detailed the ultrasound due to their short wavelengths.

Question 42

I) In MRI, what creates the contrast in image.

Answer 42

The differences in density of hydrogen nuclei in different areas of the body.

Question 43

I) Does MRI have an ionising effect? Explain why.

Answer 43

No because only radio waves are used so there is no ionising radiation to damage the patient.

Question 44

I) Describe the process of producing an image with an MRI scan.

Answer 44

Protons are aligned in a strong magnetic field.

A gradient magnetic field is produced across the body.

Radio waves of a suitable frequency are transmitted through the body. If there are protons that precess at this frequency (the lamor frequency) they absorb energy from the radio waves, changing their spin state.

The radio waves are switched off and the radiation from an excited protons is detected as the protons return to their original spin state.

The data is stored and further data is collected for different radio frequencies and for different directions of the beam.

The data is analysed by powerful computers to produce a final image.

Question 45

I) What is the lamor frequency?

Answer 45

(0.04258B)Hz Where B is the magnetic flux density in nanotesla….we probs don’t need to know this…

Question 46

I) What is the magnetic flux density inside a coil of radius ‘r’ with ‘N’ turns and carrying a current ‘I’?

Answer 46

B = μ0NI/2r

where μ0 is the permeability of free space

Question 47

I) Why are super conductors useful in MRI?

Answer 47

Resistive coils producing the required fields wound generate a lot of energy which would require cooling and be costly and wasteful of energy.

Although superconductors need to be cooled to very low temperatures, the currents and therefore the fields can be larger without generating thermal energy. Larger fields lead to a greater proportion of protons being aligned in the field and a better quality image.

Question 48

I) What are some of the disadvantages of MRI?

Answer 48

MRI scanners are expensive to buy, run and rep are.

Because of their cost, hospitals may only have one MRI scanner meaning that patients may have to wait for a long time to be scanned.

When being scanned, the patient has to remain still for a long period of time which could make them feel claustrophobic.

The limited size of the core means that the technique can’t be used for large patient.

Patients with metal replacement, pacemakers and metal implants can’t be scanned.

Question 49

I) What is an endoscope?

Answer 49

An instrument used to examine and observe the interior of a hollow organ or cavity of the body using light.

Question 50

I) Where would flexible and rigid endoscopes be used?

Answer 50

Flexible: used to examine the gastrointestinal tract and the colon.
Rigid: use in arthroscopy for examining and carrying out operations on damaged knee cartilage.

Question 51

I) In a standard endoscope, what are the 4 main tubes?

Answer 51

Air/water channel - keeps face of endoscope clear
Image channel - allows light to go up the endoscope
Instrument channel - carries controls for tools
Illumination channel - provides the lighting for observation

Question 52

I) In an optic fibre, which has the highest refractive index;
the core or the cladding?

Answer 52

The core has a higher refractive index than the cladding surrounding it.

Question 53

I) What is total internal reflection?

Answer 53

A phenomenon which occurs when a propagating wave strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface.

Question 54

I) What is the critical angle?

Answer 54

The angle between the incident ray and the normal at an interface that produces an angle or refraction of 90 degrees.

Question 55

I) What is the field of view and how would you calculate it?

Answer 55

The angle within which an image is obtained. This may be an image visible to the eye or another image forming device such as a camera or endoscope.

No sin(imax) = root (N1 - N2)
Where in an endoscope:
No is the refractive index of the air
N1 is the refractive index of the core
N2 is the refractive index of the cladding 
And imax is the maximum half angle

Question 56

I) What is an incoherent bundle?

What is a coherent bundle?

Answer 56

An incoherent bundle is a bundle of fibres which are used to illuminate objects.

A coherent bundle is a bundle of optical fibres that retain the same spatial configuration so that they transmit a true image.

Question 57

I) What is multi path dispersion in an optic fibre?

Answer 57

The dispersion that occurs in optical fibres due to the light taking paths of different total length.

Question 58

I) How would you reduce light from leaking from one fibre to the other?

Answer 58

By coating the fibre with dark glass.

Question 59

I) What does CCD stand for?

Answer 59

Charged Coupled Device

Question 60

I) Where was the potential for CCD’s first recognised?

Answer 60

For use in astronomy where light levels are often very low.

Question 61

I) What are some of the advantages of CCD’s compared to photographic film?

Answer 61

CCD’s are much more sensitive to light than photographic film and the image can be built up over a long time.

Because the image consists of electrical signals it can be enhanced electronically and can be used to produce moving images without the need for chemical processing.

Question 62

I) Give some of the current uses for CCD’s.

Answer 62

In astronomy where light levels are often very low.
Fitted to endoscopes in place of the magnifying lens.
Used in digital cameras.

Question 63

I) Describe how an image is formed on a CCD.

Answer 63

The image produced by the camera lens falls on the silicon integrated circuit chip which is an array of metal-oxide semiconductor capacitors. These are called photosites.

Each photosite produces the image on one pixel element (pixel) in the final image.

Each photo site is a photodiode which has the ability to store the charge that is generated. When light falls on a photosite, photons liberate electrons from atoms.

The photosites are insulated from one another and charge builds up of each site.

Where the light is brighter there is a greater amount of charge on the photosites as more photons liberate more electrons.

The detail obtained in the final image depends on the number of these photo sites in the area where the image is formed (more photo sites = clearer image).

Question 64

I) What is quantum efficiency?

Answer 64

The percentage of photons that falls on a photosensitive area that are used to generate an image.

Question 65

I) Why must the quantum efficiency of all the photosites be constant?

Answer 65

So that the relative intensity of each part of the image is the same as in the scene being recorded.

Question 66

I) Compare the quantum efficiencies of a retina, a photographic plate and a CCD.

Answer 66

Retina = 1%
Photographic plate = 4%
CCD = 70%

Question 67

I) Why does a CCD’s higher quantum efficiency give it an advantage over a photographic plate?

Answer 67

Better at producing images in low light conditions.

Enables much short exposure times under normal lighting conditions so more images can be taken in a given time.

Question 68

Describe how image information would be transferred onto a screen from a CCD. How would the pixels be matched to the output stream of information?

Answer 68

A clock pulse shifts the charge in each row downwards so that the charge on each photosite is transferred to the photosite below it.

The bottom row is fed into a shift register where the charge on each pixel is measured and another clock pulse empties the row by moving the charge horizontally.

The output of the shift register is a series of pulses that represent the charge level in each photosite.

This procedure is repeated until all the sites have been processed.

The system can identify which charge level in the serial stream of information goes with which pixel on the screen and the image can be built up.
For example the first one in the complete stream goes with the bottom right hand side of the image, and the last one with the top left hand side.

Question 69

I) What does the final the final colour of an image formed on an a CCD depend on (in basic terms)?

Answer 69

The relative intensity of each of the primary colours.

Question 70

I) Describe the arrangement of CCD’s in cameras used in astronomy and expensive video cameras. How do they produced coloured images?

Answer 70

3 CCD’s produce 3 different images that depend on the colour of the light.

Mirrors and lenses direct the light to each CCD.

Red, Blue and Green filters are placed in front of the CCDs so that each image depends on the intensity of these colours in the incident light.

These 3 images are then processed to produce the final image.

Question 71

I) Describe the 2 possible arrangements of CCD’s in a digital camera. How do they produced coloured images?

Answer 71

Digital cameras only use one CCD.

In the simplest array, a mask is placed over the CCD array so that 3 adjacent pixels are covered by red, blue and green filters. This means that in a 12 megapixel camera, there would be 4 million photosites registering the intensities of each colour which can then be processed to produce the final image.

However in practice, the photosites are in pairs of alternate colours (e.g. red and blue or blue and green). The intensity of say, red and green in a particular regain is measured directly. The intensity of blue is computed electronically from the averages of the signals from the adjacent green and blue covered photosites or by subtracting the red and green intensities from the intensity of white light.