bmen 509 xrays and ct

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mcdonalds

p.3: Who produced and detected electromagnetic radiation in wavelength range known as X-rays or Röntgen rays? What date was this?

Wilhelm Röntgen. 8 November 1895.

p.3: When was the first "medical" x-ray print?

22 December 1895.

p.3: When was the first diagnostic radiographic image of a hand deformity published?

1896.

p.3: When did Röntgen receive a Nobel Prize for physics?

1901.

p.4: What are some advantages of x-rays?

Widely acceptable, low cost, and fast.

p.4: What are some disadvantages of x-rays?

Ionizing radiation, and only projection.

p.5: How are x-rays produced?

Substance (metal) bombarded by high speed electrons.

p.5: What is scattering? What is it also known as?

The arriving electron is deflected by the attraction of the nucleus and loses energy or "brakes".
Bremsstrahlung or braking radiation.

p.5: What is the first mechanism that takes place during substance bombardment of high-speed electrons in x-ray production?

Scattering.

p.5: Describe the energy of scattering and what it depends on.

This emission can have variable energy depending on the distance and thus, loss of kinetic energy during scattering.

p.5: What type of interaction is scattering?

Electron-nucleus interaction.

p.6: What is collision?

The arriving electron collides with an electron in the metal and knocks it out. An electron from a higher energy level fills the vacancy and loses energy in the process.

p.6: Describe the energy of collision and what type of radiation it is.

This emission has a very specific energy value and it is thus called characteristic radiation.

p.6: What type of interaction is collision?

Electron-electron interaction.

p.7: Where is the production of x-rays?

X-ray tube.

p.7: In an x-ray tube, what starts emission of electrons?

A thin metal filament (tungsten) is energized to be heated.

p.7: What happens after electrons are emitted from the cathode?

Electrons emitted from the tungsten filament (cathode) are accelerated towards a tungsten target (anode).

p.7: How are electrons emitted from the cathode?

By the application of a high voltage between the filament housing and the target (anode).

p.7: Where are electrons emitted from?

Tungsten filament (cathode).

p.7: What is the tungsten filament?

The cathode.

p.7: What is the tungsten target?

The anode.

p.7: How are x-rays produced?

Tungsten energized -> electrons emitted from cathode accelerates to anode -> electrons interact with electrons and the nuclei of the target atoms.

p.7: Why are x-rays collimated (parallel)?

To produce a useful x-ray beam of defined cross-sectional area.

p.8: Why are the components of an x-ray tube in a vacuum?

To prevent the electrons from interacting with gas molecules before they reach the target.

p.8: What part of the x-ray tube is rotating?

Filament target (anode).

p.8: Why is the anode continuously rotating?

To reduce heating.

p.9: What is a filament?

Small coil(s) of thin thoriated tungsten (thorium & tungsten alloy) wire.

p.9: What affects the beam width?

The choice of coil (of the thoriated tungsten).

p.9: What is the melting point of thoriated tungsten?

3370°C

p.9: What is a focusing cup?

A nickel structure designed to house the filament.

p.9: What is a focusing cup used for?

To condense the electron beam to a small area of the anode.

p.10: What is an anode?

An electrical and thermal conductor that receives the electrons emitted from the cathode.
Conductive target (tungsten, Z=74) positively charged (25-140 kV).

p.10: What is an anode's bevel used for?

To control the coverage.

p.11: How is the x-ray tube insulated?

The tube is embedded in oil.

p.11: In the x-ray tube, what is used for direct emission?

A lead shield.

p.11: What does the voltage on the anode control?

The maximum energy of emission.

p.11: What does the current on the cathode control?

The beam intensity (number of photos/time).

p.11: What determines the total number of photons?

Total exposure time.

p.11: In a graph of the number of photons vs. kVp, what do the small peaks correspond to?

The characteristic radiation of the target.

p.11: In a graph of the number of photons vs. kVp, what do the spikes show?

The electrons dropping in orbit.

p.11: In a graph of the number of photons vs. kVp, why don't we want any values to get too low?

Since it'll be absorbed.

p.12: X-rays travelling through matter can...

Transmit, attenuate, or scatter.

p.12: What does it mean when x-rays travelling through matter can transmit?

Pass through unaffected.

p.12: What does it mean when x-rays travelling through matter can attenuate? What is it also known as?

Transfer energy to the matter (photoelectric attenuation).

p.12: What does it mean when x-rays travelling through matter can scatter? What is it also known as?

Divert in a new direction (compton scattering).

p.12: True or false?
Scattering causes random background noise.

True.
Scattering are deflected photons that will be "wrongly" detected.

p.12: What does attenuation depend on?

The density of the matter and the energy of the x-ray.

p.14: How does photoelectric attenuation work?

The incident energy knocks out an inner-shell electron.
The outer-shell electron takes its place.
A characteristic x-ray is emitted (very low energy), but the x-ray does not reach the detector.

p.14: Which direction(s) does the characteristic x-ray emit?

It is shot (scattered) in all different directions.

p.15: How does compton scattering work?

The incident energy knocks out an outer-shell electron.
The x-ray deflects with a changed wavelength (energy loss).

p.15: True or false?
Attenuation accounts for all losses in x-ray energy.

True.

p.16: True or false?
Higher energy = higher attenuation.

False. Higher energy = less attenuation.

p.16: What is HVL?

Half-value layer: the thickness of tissue reducing beam by half.

p.16: What is the equation used for compton scattering and attenuation?

N = N₀ e^(-μ(E)x)
N = number of photons
N₀ = initial number of photons
μ(E) = attenuation as function of energy

p.16: How do you calculate μ(E)?

μ(E) = μ(E) photoelectric + μ(E) compton

p.16: look at diagram

p.17 What is beam-restrictor Collimator?

Absorbent sheets used to avoid radiating tissue not of interest. Note: Collimator is just a device that narrows a beam of light

p.17 What is a collimator grid?

Geometrical grid used to eliminate scattered X-rays and get rid of background noise

p.18 What do detectors do?

Convert X-rays into energy that can be visualized

p. 18 What do digital detectors that use optical-electrical use?

• Scintillator (material that emits light when exposed to ionizing radiation)

• Photo-diode (used for the exact measurement of the intensity of light)

p. 18 What do digital detectors that use direct electrical conversion use?

• Photoconductors (material that becomes more conductive when light shines on it)

• Capacitors

p.23 Who published mathematical analysis of tomographic image (without being aware of Radon's work)?

Allan Cormack (1963)

p.24 What configuration does tomography follow?

Geometrically ordered detector configuration

p. 24 How is CT achieved?

Mechanical movements and detector arrays

p.26 What are some features of the 1st generation CT?

• Single detector

• Translation + rotation

• 5 min per slice

p.26 What are some features of the 2nd generation CT?

• Detector array

• Translation + rotation

• 20 s per slice

p.24: What is required for a CT measurement?

A certain number of single projection measurements at different angles.

p.26 What are some features of the 4th generation CT?

• Detector ring

• << 1s slice

p. 27 How can other coverage be achieved?

Arranging the detectors and rotation (multi-slice or helical)

p.26 What are some features of the 3rd generation CT?

• Fan beam + detector array

• No translation

• 1 s slice

p.24 How does a CT scan create images? (Principle?)

• takes multiple X-ray images from different angles

• combines images to form a detailed 3D view of body

p.23 Who developed the first CT system with his own reconstruction method?

Godfrey Hounsfield (1917)

p.23: Who received the Nobel Prize for Physiology or Medicine?

Hounsfield and Cormack (1979)

p.23 Who established the mathematical framework for tomography?

Johann Radon (1917)

p.19 Where creates contrast in an X-ray image?

Contrast comes from differences in X-ray attenuation; how much different tissues absorb X-rays. Bones absorb a lot (appear white), soft tissues absorb some (appear gray), and air absorbs very little (appear black).

p.19 List some factors affecting SNR

1. Num of photons emitted

2. Photon energy

3. Patient size/body part

4. geometry of anti-scatter gird

5. Efficiency of the detector

p.19 The SNR is proportional to
a. Total num of photons
b. √Total num of photons
c. Total num of photons x 2
d. Total num of photons

b. √Total num of photons (√N)

p. 22 What factors affect resolution in X-ray imaging?

1. Pixel size (Smaller pixels improve detail, larger pixels cause blurring.)

2. LSF & MTF (how well system captures fine details)

3. Geometry & Penumbra (larger xray source or object distance increases blurring)

p.19 What causes noise in X-ray image?

Random variations in the num of X-ray photons reaching the detector. Randomness follows a Poisson distribution and makes the image look grainy.

p.20+21 How is contrast improved in X-ray images?

Changing the energy of the beam and using contrast agents (chemicals that enhances contrast like barium or iodine)

p.19 How does the Poisson distribution relate to X-ray noise?

Describes the random variation in the number of X-ray photons hitting each pixel of the detector; leads to image noise.

p.34: What type of radiation damages are there?

Deterministic and stochastic.

p.34: What is deterministic radiation damage?

Cellular damage leading to loss in function.

p.34: What is stochastic radiation damage?

Chromosomal damage on cells -- increasing risk of cancer or hereditary conditions.

p.35: What is radiation (absorbed) dose, D?

The radiation energy absorbed by a material (tissues).
[Gy = 1 J/kg = 100 rads]

p.28 What is a sinogram?

Graphic representation of projections as a function of the angle (see images on p.29 and p.30)

p.31 What is backprojection?

Process of combining the images from diff angles into a clear slice of the body

p.36: tissue weighting factor table

p.37: What is CTDI?

Computed tomography dose index.
Accounts for absorbed dose D₂ at specific positions, z, for a slice thickness, T.

there's an equation just look at the slide

p.37. What is CDTI₁₀₀?

It refines the concept for modern CT where large number of N slices are acquired.

there's an equation just look at the slide

p.37: What is CDTI(subscript w)?

Accounts for weighting depending on the position in the scan.

there's an equation just look at the slide

p.38: What does the effective dose (mSv) in the table represent?

shows how much radiation a patient receives from different medical imaging procedures. Higher numbers indicate more radiation exposure.

p.39: What are common uses of CT scans for the chest, abdomen, and pelvis?

Detects lung disease, organ issues, and tumors.

p.31 How does backprojection work

1. Each projection or image spreads the same or equal weight across all pixels

2. The end result is a slice

p.35: What is equivalent dose, H?

The dose accounting for radiation energy from all sources (i.e. x-rays, gamma rays, etc.)
[Sv = J/kg]

there's an equation just look at the slide

p.35: What is effective dose, E?

The dose accounting for tissue sensitivity to radiation.

there's an equation just look at the slide

p.33 What are Hounsfield Units (HU) in a CT scan, and how is the CT number calculated?

• Hounsfield Units (HU) represent the density of different tissues in a CT scan.

• CT scans show a map of CT numbers (HU), not the actual attenuation coefficients.

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p.31 Why is it better to have a lot of projections or images for backprojection?

The image becomes sharper (with only one or two, the image is blurry and unclear)

p.31 At 180 projections of a CT scan how will the final result look?

A clear cross-sectional slice of the body