X-ray imaging

• What are X-rays?


  high frequency waves that make up part of the EM spectrum, formed when high energy electrons collide with their atoms and release some of their energy as a consequence
• What is thermionic emission?

  When electrons gain enough thermal energy to overcome the attractive forces that hold them to the positive ions in the metal material
• How are gaps created in tungsten atom's energy levels?

  Electrons collide with sufficient energy, knocking electrons out of the target atoms at low energy levels, leaving gaps which electrons of higher energy levels can demote themselves into/fill up
• How do identify a material from the intensity x wavelength graph?

  There are characteristic peaks unique to different materials which depend on the spacing/energy gap in the target atoms of the specific material
• What is the Bremsstrahlung/braking radiation?

  -feature on intensity x wavelength graph

  -it's a curve consistent in all X-ray spectra

  -caused by constant emissions of X-rays from the originally accelerated free electrons as they gradually slow down
• How an X-ray tube works

  1. Cathode is heated by the heat resulting from the resistance of a thin filament

  2. Causes cathode to release electrons through a process of thermionic emission

  3. Electrons are accelerated using a high voltage of around 150kV towards a target (made of tungsten metal sheet), which is attached to an anode

  4. Electrons strike target, collide with atoms in it and where there's sufficient energy, they knock electrons out of their target atoms, creating positive ions

  5. This creates gaps in electron configuration of Tungsten sheet's atoms, often in lower energy levels

  6. Higher energy electrons within the atom drop down into lower energy levels to fill the void/stabilise the ion

  7. When the atomic electrons undergo this demotion they release X-rays
• What do we see on a characteristic X-ray spectra?


  -The material's characteristic wavelength peaks

  -the bremsstrahlung curve
• Why do we see multiple characteristic wavelength peaks?

  Electrons in the target atoms can drop down by a variety of different energy levels each resulting in a different peak
• What is the energy of electrons as they're accelerated?

  E = eV

  E=energy

  e=charge of electron

  V=Voltage applied across tube to accelerate
• What is the energy of a photon produced?

  E=hf

  E=energy

  h=Plancks constant

  f=frequency of photon
• What is the minimum wavelength of emitted photons from X-ray Tubes?

  λmin = hc/E = hc/eV

  h=plancks

  E=energy

  c=speed of light

  e=charge of electron

  V=voltage applied

  • Any of these equations work
• Why is Tungsten metal commonly used as the target for X-ray tubes?

  -High specific heat capacity

  -High melting point

  -Large nuclei with lots of positive charge
• Why is it important for the target in X-ray tubes to have a high melting point/ SHC?

  -less than 1% of the energy from accelerated electrons ends up as X-ray radiation

  -The rest raises the internal energy of target

  -so it must be able to conduct thermal energy well and not melt
• Why are the tungsten target and anode spinning in X-ray tubes?

  -heat dissipation

  -all areas of target are hit - there's no build up of thermal energy in one spot only
• What is the intensity of an X-ray beam?

  Total energy emitted at all wavelengths per second through a given unit area
• How do we manipulate X-ray intensity at a given voltage?

  -manipulating rate at which electrons strike the target

  -more electrons = more collisions = more X-rays

  -therefore X-ray intensity is dependant on current in X-ray tube which we can control by varying the initial current supplied to the cathode filament
• What are the 2 ways of generating images using X-rays?

  -first involves passing X-ray through the area of body we want to be imaged and onto a detector, where a shadowy image is generated

  -second is computer tomography (CT scan); using a series of narrow X-ray beams scan across the patient's body, a computer measures the intensity levels, interprets the attenuation of different parts of body and compiles the data into a detailed image
• Why does blurring/unsharpness occur in X-ray imaging?

  -geometric unsharpness

  -motion unsharpness

  -detector unsharpness
• What causes geometric unsharpness?

  -clarity and sharpness of X-ray image relies on X-ray being as narrow as possible

  -This is an issue when the X-ray source has a broad area instead of being a point source.
• How do we combat geometric unsharpness?

  -target in X-ray tube angled towards the beam and the detector is kept as close to the patient as possible to limit spread/widening of the beam before it's detected - limiting blur

  -moving X-ray source away from the patient
• What is the issue with moving the source of X-rays away from the patient?

  Exposure times would need to increase due to inverse square law

  -distance increase = less intensity
• What is motion unsharpness?

  caused by movement of patient, source of X-rays or detectors

  -patients often asked to remain still and even hold breath for the few seconds the X-ray image takes
• What is detector unsharpness?

  Every detector has its own resolving power which affects the resolution of an image

  for X-ray film, the size of the photographic grains within the film material depicts the level of resolution available

  • This can't be fixed - only solution is to use a different detector with increased resolving power
• What colour is air, tissue and bone and why?

  Air - black

  soft tissues - greyish

  bone - white

  -depends on how they absorb X-rays. Bones can absorb X-rays as they're dense and very little passes through and is detected by the X-ray film, leaving it blank/white
• Why is maximising contrast important?

  Allows doctors to identify different components easier and means a more accurate interpretation

  -easier to discern abnormalities
• Why are X-rays harmful?

  They're ionising radiation
• What affects the patient's X-ray dosage?

  -exposure time

  -X-ray intensity
• How can a more sensitive detector limit dosage of X-rays?

  -More sensitive detector - less intensity required

  -intensity of the beam depends on voltage across tube and current across cathode

  -we need high voltage to produce shorter wavelengths/higher frequency X-rays to provide a better image

  -so our only option is to reduce the cathode current
• How else can we limit dosage?

  Beam hardening
• What is beam hardening?

  -filtering the beam to remove lower energy photons as they're unlikely to reach the detector anyways so therefore don't contribute to image
• How do we do beam hardening in practice?

  Use a thin filter of metal like copper or aluminium, which is designed to readily filter out photons with a lower energy allowing only the high-energy photons to pass
• How does beam hardening affect mean energy of beams and number of photons emitted?

  -Less total number of photons

  -The mean photon energy increases
• How were X- rays done when they were first discovered?

  -photographic film impregnated with silver halides that turned black when exposed to X-rays

  -more X-ray exposure = more grains blacken and darker image
• What is the issue with photographic film?

  -not sensitive to X-ray radiation

  -only absorbs about 0.1% of X-ray energy exposed to it

  -increases exposure of patient/doctor

  -more chance of motion unsharpness
• How can we capture X-ray images digitally nowadays?

  -Digital detection instead of photographic film

  -electronic scanning
• How does digital detection work using a flat panel detector (FPD)?

  -FPD's screen is lined with thin film transistors (TFTs) that comprise a series of photodiodes

  -photodiodes are semiconductors that convert light into electrical energy

  -Photodiodes are attached to a scintillator material like Caesium Iodide (CsI) that emits flashes of light in response to high energy photons, such as X-rays.

  -photodiode picks up these flashes of light from scintillator

  -X-ray energy converted to electrical energy via light energy
• How is the electrical energy used to produce a final X-ray image using Digital detection?

  -light energy from scintillator is proportional to energy of incident X-rays, each photodiode in the TFT acts as an individual pixel or picture element

  -electrical energy produced by photodiode in response to light is proportional, and the signal from each diode is read through the TFT switch.

  -this converts the electrical signal to a digital output which can then be interpreted and built into a final image on a computer
• What is the benefit of the Digital detection method?

  -produces an almost instant X-ray image

  -photodiode/TFT mechanism is very sensitive so less exposure time and lower doses required

  -digital images produced are easily stored/communicated around the hospital

  -images can be manipulated and enhanced using software
• Benefits of Computed radiography?

  -image is digital, easily stored and communicated

  -easy to process and enhance image quickly
• What issue do we face with identifying cancerous tissue?

  -contrast between healthy and cancerous tissue is sometimes unclear

  -both soft, similar compositions, including proton numbers

  -absorb/transmit X-rays to a similar degree