Imaging with X-rays Flashcards by Andy Rad

Compton Scatter

Increases with:

increasing denisty
increasing electron density
lower x-ray beam energy

Increasing beam energy results in lower scatter (more energy goes to recoil electron which gets absorbed)

At 50keV Compton & Photoelectric interactions occur equally

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What is Elastic Scatter (also known as Coherent or Rayleigh Scatter)

Occurs where photon energy is less than binding energy

Photon bounces off electron

No contribution to dose

More likely to occur with high atomic number elements (as electrons more tightly bound) and low photon energy

No ionisation takes place
No energy lost
No absorption takes place

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Bremmstrahlung Radiation

What is bremmstrahlung proportional to ?

Probability of bremmstrahlung is proportional to:

Atomic number²

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Curve Shape

What effect on the curve does changing target do?

Changing target:

changes average beam energy
changes the characteristic peaks
does NOT affect peak beam energy

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X-ray Quantity (Number of photons)

What is x ray quantity proportional to?

Directly proportional to the tube current

Proportional to the peak kv²

Dependent on the choice of target material (higher atomic number = more photons)

(Target material does not affect energyO

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Computed Radiography

What colour light stimulates and what colour is released?

Red light laser stimulates electrons

Blue light released

Barium fluorohalide doped with europium is schintillator layer

The log of the light output from the screen is directly proportional to the log of the x-ray dose

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Electromagnetic vs Sound waves

How do both waves move?

Electromagmentic waves move as TRANSVERSE WAVES

Sound waves are longitudinal waves that go through periods of rarefaction and compression

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Geometric Unsharpness

What effect does exposure time have on geometric unsharpness?

Shortening of exposure time causes INCREASED geometric unsharpness

(shorter exposure means higher mA and blooming in size of focal spot)

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Grids

What is usual line density?

How thick are lead strips?

What is grid ratio?

Line density of a grid usually 30-80cm (number of lead strips per cm)

Lead strips are 0.05 - 0.07mm

Grid ratio is depth divided by width

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What is tube output proportional to?

The square of the peak kV

(in lower dose systems like mammography, it is equal to the peak kV cubed)

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Tube kV tested with?

Digital kV meter

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Cathode Filament voltage?

10v

(Power dissipation of 40 watts)

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What are the Focal Spot sizes?

Mammography

Radiography

Fluoro

1mm - magnification mammography
3mm - normal mammography and magnification radiography
6mm - conventional radiography (small focal spot) and fluoro
2mm - conventional radiography (large focal spot)

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DSA

What matrix size used in DSA?

Image intensifiers are used to reconstruct images

DSA matrix size is 1024 x 1024

Spatial resolution is 2-3lp/mm

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Target angle

What is target angle?

Do large or small target angles improve fine detail?

Usually 7-20 degrees

Smaller target angles improve fine detail imaging (reduces geometric unsharpness)

Smaller target angle used in mammography

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Heat Dissipation in the tube

How is heat dispersed from tube?

Radiation is the primary method

Convection currents flow in the oil

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Wave Rectification

How many diodes needed?

AC or DC current?

Converts AC to DC

Uses diodes

Full wave rectification needs at least 4 diodes

How to calculate Beam Intensity?

What is it proportional to ?

Does filtration affect beam intensity?

Number of photons per unit area x mean photon energy

Unit time

Beam intensity is directly proportional to the tube current (number of electrons flowing from cathode to anode)

Approximately proportional to kV squared

Intensity will DECREASE after filtration as some of the photons are filtered out

Unwanted Radiation

Made up of

Leakage radiation - is what is transmitted throught tube housing
Scattered radiation - is what has changed direction after leaving tube

Leakage radiation should not exceed 1mGy/hr at 1m from the focus

Heat Loading - what are units of measurement?

How is it calculated?

Measured in joules or heat units (old term)

For constant three phase potential it is equal to:

kV x mA

For single phase its is equal to:

0.7 x kV x mA

X-ray Spectrum

What effect on curve will increasing kV and mA have?

Increasing the tube current (mA) will NOT affect the shape of the x-ray spectrum

will increase amount of characteristic and bremmstrahlung radiation

Increasing the kvP will shift curve up and to the right

Are the max energy of x-ray photons affected by the anode or three phase supply?

NOT affected by:

anode material
voltage waveform (this affects mean energy)

How to calculate HVL?

HVL = 0.693/LAC

It is the thickness of material to attenuate a POLYENERGETIC beam by 50%

LAC and MAC are used for monoenergetic beams

What is Mass Attenuation Coefficient ?

What are units?

The fraction of photons removed from a monochromatic energy beam by a homogenous absorber per unit mass

Is LAC/Density

Units are cm squared/kg or cm squared/g

Attenuation of the beam = MAC x mass thickness

Depends on the radiation interaction

Focal Spot Size Quality Control How to measure focal spot size and resolution?

Focal Spot size measured using **Pinhole imaging** Resolution measured using a **Star Test Pattern**

Tomography What does narrow angle tomography produce? Angle? Blur?

*Tomography most useful when imaging structures with High inherent contrast* Contrast depends on slice thickness * thicker slices produce better contrast Structures to the right angle of the film appear blurred * those in line of sight between tube and detector are sharp **In Narrow Angle (thicker slices)** * **reduces blur of structures outside focal plane** * **preferred for imaging tissues with low contrast** * **increased tendance for phantom image formation**

How is heat lost from anode??

**RADIATION MAINLY**

What is Inherent Filtration equal to?

Mainly occurs at glass envelope but includes oil, tube housing, target material Equivilent to approx 0.5 - 1mm of aluminium **Inherent filtration reduces tissue contrast**

What is an Auger Electron ?

Results after the photoelectric effect where an additional electron is released * Are an important source of dose in radiology * Occurs as an alternative process to characteristic radiation release The kinetic energy of auger electron is the binding energy of the K shell minus twice the energy of the L shell

What is normal Bit Depth? Compression techniques

For DR: * usually 12 - 16bit depth * wide the dynamic range the wider the bit depth needed Compression: 1. Lossless * reversible * can be restored into an identical version of the original * can reduce file size by a factor of 2 to 3 1. Lossy * reduces file size by up to 40 * irreversible

Detector Dose indicators (in CR)

Provide a measure of the signal from the image plate AVERAGED over a broad region of the plate * equivilent to the optical density of the film * doubling dose does NOT double DDI

What is Noise calculation?

Quantum Noise: proportional to the square root of number of photons **Noise is inversley proportional to the square root of number of photons** **SNR = M/ square root of M (M = number of photons)** Signal to noise ratio is PROPORTIONAL to square root of number of photons

What is an electron volt?

When an electron is accelerated through 1 volt the unit used is an electron volt The rest mass of an electron is 511keV

Three phase supply effect on photon quality and quantity

Increases both quality and quantity Shifts curve up and to right

What is Isotropic Scatter ?

Where at low energies in compton scatter: * photons are given off in all directions EQUALLY

What increases contrast in radiography?

* Increasing thickness of structure **Contast is proportional to the difference in LAC of tissues x tissue thickness** Contrast decreases as kV increases

How to reduce scatter and improve contrast?

1. Reduce **field size** 2. Decrease **patient thickness** 3. **Reduce kV** 4. Use of **antiscatter grid** 5. Increased **Focus to detector distance**

What is grid ratio?

Depth of interspace / width Typical ratio is 8:1 Higher the ratio the better the contrast

What 3 factors are needed for focus grids?

1. Must be at specified distance from anode 2. Should be tilted progressively to point towards tube focus 3. Tube must be accurately angle over the grid Can result in grid cutoff if above measures arent adhered to

What is standard number of bits used in radiography?

Usually 12 to 14 bits

What is Nyquist frequency?

The max frequency that can be sampled Equal to half the sampling frequency

What size of anode disc is used in radiography?

Usually 20 - 30**cm in diameter**