Lecture 1 Flashcards

To learn for the first coursework exam.

You may prefer our related Brainscape-certified flashcards:
1
Q

BASICS –ELECTROMAGNETIC SPECTRUM

A

Range of frequencies in X-ray spectrum 10^17 – 10^20Hz
Range of wavelengths? Wavelength 10^-8 to 10^-13 meters (short) (c = fʎ), f = frequency, ʎ = wavelength, c = speed of light 3 x 10^8 m/s
Range of photon energies in eV? Photon energies in the range 100 eV to 100 keV (electron volts) (E = hf= hc/ʎ), h = planks constant (6.626 ×10-34 joule/second) , f = frequency

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

X-RAY 1901

A

Wilhelm Röntgen

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

X-RAY TUBE

A

An early Crookes x-ray tube from a museum dedicated to Wilhelm Conrad Roentgen in Würzburg, Germany-First Nobel prize Physics December 10th 1901, Philipp Lenard-Nobel prize Physics December 10th 1905

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

X-RAY

A

What do you know?
• How are x-rays produced?
• What do you need to produce x-rays?
• What are the 3 main parts of the imaging system?

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

The three main components of an x-ray imaging system

A

The three main components of an x-ray imaging system

  1. The x-ray tube
  2. The operating console
  3. The high-voltage generator
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

X-RAY RADIOGRAPHIC TABLE

A

What do you know about a radiographic table?

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

X-RAY Tilting Table

A

Tilting tables are designed for both diagnostic and fluoroscopic work • Tilting models usually tilt to 90 degrees in one direction and 15-30 degrees in the other direction • Tilting models include ancillary equipment; footboard, shoulder support, handgrips, compression bands.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

X-RAY FLUROSCOPY TABLES

A

The tube is under the table, image capture is above the patient. Monitor>Video camera>Optical coupling>Image Intensifier>Grid>Patient>Table>Filtration>Collimator>X-ray tube>X-ray generator

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

BASICS –THERMIONIC EMISSION

A

Thermionic emission is a process of emission of charged particles (known as thermions)from the surface of a heated metal. • The charged particles normally are electrons.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

BASICS -X-RAY TUBE TUNGSTEN ANODE

A

• Energy of X-rays are produced at the anode when the beam of electrons strike the anode in the X-ray tube.
• Two types of X-rays are produced: • Braking X-rays (Bremsstrahlung radiation) • Characteristic X-rays (from K-shell transitions)
Characteristic X-rays (Spikes)
Braking X-rays (Curve)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

ELECTRON ANODE INTERACTION -BRAKING X-RAYS

A

• Imagine the energy needed to propel electron from 0 to half the speed of light in one to three centimeters. • The electrons that travel from the cathode to the anode are called projectile electrons. • When they strike the heavy metal atoms of the anode they interact with the atoms and transfer their kinetic energy to the target. • These interactions happen at a very small depth of penetration into the target. • If the incident electron passes close to the nucleus of an atom in the target anode, it will decelerate as it passes the nucleus, and its kinetic energy will decrease. The excess kinetic energy is given off as an X-ray photon. • More than 99% of the kinetic energy of the projectile electron is converted to thermal energy.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

ELECTRON ANODE INTERACTION – CHARACTERISTIC X-RAYS

A

If an incident electron ejects an electron in the innermost K-shell of an atom, other electrons will transition downwards into the K shell. The energy lost by these electrons in transition is released as X-ray photons.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Characteristic vs. Bremsstrahlung X-rays

A
  • Characteristic X-ray require 70 kVp or higher. Based upon the energy of the k-shell electron. • Characteristic x-rays have a precisely fixed or discrete energies. • These energies are characteristic of the differences between electron binding energies of a particular element. • For tungsten you can have one of 15 energies.
  • Bremsstrahlung X-rays can be produced at any projectile electron energy. In diagnostic radiography most of the x-rays are bremsstrahlung x-rays.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Four Factors Influencing the X-ray Emission Spectrum

A
  1. The electrons accelerated from the cathode do not all have the peak kinetic energy. Depending upon the type of rectification and high voltage circuits, many electrons will have very low energy that produces low energy x-rays. 2. The target is relatively thick. Many of the bremsstrahlung x-ray emitted result from multiple interactions of the projectile electrons. Each successive interaction results in less energy. 3. Low energy x-rays are more likely absorbed by the target. 4. External filtration is always added to the tube assembly. This added filtration serves to selectively remove the lower energy photon.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Factors affecting the size and relative position of the x-ray emission spectra.

A
  1. Tube Current (mA)affects the amplitude 2. Tube Voltage affects the amplitude and position. 3. Added Filtration affects Amplitude most effective at low energies. 4. Target material affects spectrum and position of the line spectrum. 5. Voltage waveform affects the amplitude, most effective at high energies
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Influence of Tube Current

A

A change in mA or mAs results in a proportional change in the amplitude of the x-ray emission spectrum at all energies and the intensity of the output.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

Influence of Tube Potential

A

•Unlike tube current, a change in kVp affects both the amplitude and the position of the x-ray emission spectrum. •When kVp increases the relative distribution of emitted photons shifts to the right or to higher energies. •15% increase in kVp is equivalent to doubling the mAs.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

Influence of Added Filtration

A

•Adding filtration to an x-ray machine has an effect on the relative shape of the spectrum similar to that of increasing the kVp. •Added filtration effectively absorbs more low energy x-ray than high energy x-rays, therefore the spectrum is reduced more to the left.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

X-ray Filtration

A
  • Filtration of the x-ray beam has two components: •Inherent Filtration •Added Filtration
  • Filtration is required by law.
  • Aluminum is most common material
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

Filtration Affects the Beam Spectrum

A

•Filtration removes the lower energy photons that do not contribute to image production. •Added filtration results in an increased half value layer or higher quality beam. •The overall result is an increase in the effective energy of the beam •The discrete and maximum energy of the x-ray spectrum is not effected.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

Influence of Target Material

A

•As the atomic number of the target material increases, the efficiency of the continuous spectrum x-rays increase. •The discrete spectrum also shifts to the right representing higher energy characteristic radiation. •Tungsten is used for general radiography. •Some specialty tube use gold. •Molybdenum is used for mammography. It has a lower atomic number so the discrete spectrum is of a lower energy. This is ideal for soft tissue studies such as mammography.

22
Q

X-RAY AEC

A

Automatic Exposure Control •Uses an ionization chamber •Technologist sets kVp, mA, back-up time & sensors •Exposure terminates the IR has proper OD •Patient positioning must be absolutely accurate

23
Q

X-RAY CONTROLS

A

•Line Compensation, kVp, mA and time •Quantity = # of x-rays •Milliroentges(mR) or (mR/mAs) •Quality = the pentrability •Kilovolts peak (kVp)

24
Q

X-RAY CIRCUIT

A

3 Divisions of Circuit Board • PRIMARY (CONTROL PANEL) • SECONDARY (HIGH VOLTAGE) • FILAMENT (LOW CURRENT)

25
Q

X-RAY LINE COMPENSATOR

A

Most imaging systems are designed to operate on 220 V. (some 110 V or 440 V). However power from the wall is not always accurate continuously. Line compensators are: •Wired to the autotransformer is the line compensator •Designed to maintain the accurate voltage required for consistent production of high-quality images •Today’s line compensators are automatic and are not displayed on the control panel

26
Q

X-RAY AUTOTRANSFORMER

A
  • The power for the x-ray imaging system is delivered first to the autotransformer •The autotransformer works on the principle of electromagnetic induction •It has one winding and one core •There are a number of connections along its length
  • A’s = primary connections & power into the transformer •Other connections allow for variations of voltages •Is designed to step up voltage to about twice the input voltage value •The increase in voltage is directly related to the number of turns
27
Q

X-RAY TUBE CURRENT OR FILAMENT CIRCUIT

A

• A separate circuit crossing from cathode to anode • Measured in milliampers(mA) • What determines how many x-rays are created? • # of e-is determined by the temperature of the filament. The hotter the filament the more e• Are their any limiting factors to thermionic emission?

28
Q

X-RAY: FOCAL SPOT

A

• The effective focal spot is the beam projected onto the patient. • As the anode angle decreases, the effective focal spot decreases. • Diagnostic angels range from 5 to 20ᵒ

29
Q

X-RAY: SPATIAL RESOLUTION

A

Spatial resolution of the image is determined by the spatial distribution of the electrons. A focusing cup is used to produce a tight uniform beam of electrons

30
Q

X-RAY: OFF FOCUS RADIATION

A

Off focus radiation: photons that were not produced at the focal spot or extra focal radiation.

31
Q

X-RAY: FILAMENT

A

• Most commonly used is tungsten-rhenium alloy, 2-10% rhenium. Pure tungsten cracks. • Tungsten:highmeltingpoint-3370ᵒC high atomic number–74 low vapor pressure–10-7 bar

32
Q

X-RAY: FOCAL SPOT

A

• The relationship between the actual focal spot size F the effective focal spot size, f is given by the equation below where θ is the bevel angle.
f = F sin θ Effective focal spot = 0.3 mm for mammography, 0.6-1.2 mm for planar radiography.

33
Q

X-RAY: TUBE CURRENT

A
  • Radiography:50–400mA • Fluoroscopy:1–5mA • CT Scan: up to 1000 mA
  • Also note: the higher the vacuum the higher the velocity of the electrons, therefore a shorter exposure time is required. Motion-induced artifacts are decreased in moving structures(heart).
34
Q

X-RAY: ANODE HEATING

A

• Anode heating problems limits the tube output. • Anode rotates at ~3000rpm to dissipate heat. • Maximum tube output is proportional to the square root of the rotation speed. • The rotor assembly is made of molybdenum, high melting point and low thermal conductivity. Heat loss is via radiation through the vacuum to the vessel walls.

35
Q

X-RAY: BEAM INTENSITY

A

• The intensity, I of the X-ray beam is defined to be the power incident per unit time. Units: joules/square meter. • Power is dependent on: 1)the total number of X-rays and 2)the energy of the X-rays. • The number of X-ray beams produced by the source is proportional to the tube current. • The energy of the X-ray beam is proportional to the square of the accelerating voltage

36
Q

X-RAY-BEAM INTENSITY

A

• Therefore, Iα(kVp)2 (mA) • Beam intensity is not uniform due to the heel effect. • The heel effect is due to a portion of the X-ray beam being absorbed by the anode. • The resultant X-ray beam is less intense on the anode side and more intense on the cathode side. • The heel effect is more pronounced with steeper anode angles.

37
Q

X-RAY: FILTERS

A

• Low energy X-rays cannot penetrate the patient sufficiently to reach the detector. • 30μm thick molybdenum filters X-rays less than 30kV(mammograms) • 0.5mm thick Aluminum filters X-rays up to 50kV • 1.5mm thick Aluminum filters X-rays between 50-70kV • 2.5mm thick Aluminum filters X-rays above70kV • Filters can reduce skin dose by a factor of 80

38
Q

X-RAY: INTERACTIONS WITH TISSUE

A

• Contrast between tissues is due to differential attenuation as X-rays pass through the body. • Primary radiation: pass through the body without attenuation • Secondary radiation: scattered radiation–Coherent scatter – Compton scatter • Absorbed radiation: completely absorbed by the body – Photoelectric interactions

39
Q

X-RAY: COHERENT SCATTERING

A

• Nonionizing interaction between X-rays and tissue. • The X-ray energy is converted into harmonic motions of the electrons in the atoms of the tissue. • The probability of a coherent scattering event occurring is given by Pcoherent α Zeff^8/3/E2, Zeff: the effective number of the tissue E2: energy of the incident ray

40
Q

X-RAY- COHERENT SCATTERING

A

There are three main steps in coherent scatter. 1.An incoming xray photon with less than 10keV (so a very low energy x ray photon) interacts with an outer orbital electron. 2. The incoming x ray photon transfers ALL of its energy to the outer orbital electron. The incoming xray photon no longer exists after transferring its energy. This makes the outer orbital electron excited. 3. The outer orbital electron gives off the excess energy(in the form of an xray photon) in a different direction than the original incoming x ray photon. The new xray photon has the same energy as the incoming xray photon.

41
Q

X-RAY: COMPTON SCATTERING

A

There are three main steps in Compton scatter. 1. An incoming x ray photon interacts with an outer orbital electron. 2. The incoming x ray photon knocks the orbital electron out of it orbit (becoming a recoil electron). The recoil electron loses energy as heat or creates bremsstrahlung radiation within the object being imaged. 3. The x ray photon is deflected in a different direction with less energy (determined by subtracting the binding energy of the orbital electron). The higher the energy of the incoming x ray photon, the smaller angle of deflection (meaning the x ray photon will continue closer to it’s original path).

42
Q

X-RAY: THE PHOTOELECTRIC EFFECT

A

• The energy of the incident ray is absorbed by an atom in the tissue. • A tightly bonded electron is emitted from the K or L shell as a “photoelectron”. • The kinetic energy of the photoelectron is the difference between the energy of the incident X-ray and the binding energy of the electron. An electron from a higher energy level then fills the void. • The X-ray produced has low energy and is absorbed in a short distance.

43
Q

X-RAY: LINEAR AND MASS ATTENUATION COEFFICIENTS OF X-RAYS IN TISSUES

A

• Mathematical expression for attenuation in tissue Ix = Io * e-μx, Io: intensity of the incident ray Ix : is the X-ray intensity at a distance x from the source μ: linear attenuation coefficient of tissue (cm-1) A high value of μ corresponds to efficient absorption by the tissue. μ= μ photoelectric+ μ Compton + μ Coherent (μ Coherent is usually small and ignored)

44
Q

INSTRUMENTATION FOR PLANAR X-RAY IMAGING

A

Collimators: The collimator restricts the beam of X-rays so as to irradiate only the region of interest. The anti scatter grid increases tissue contrast by reducing the number of detected X-rays that have been scattered by tissue.
ANTISCATTER GRIDS: The grid is placed between the patient’s body and the receptor, as shown below. It is constructed of alternate strips of an x-ray absorbing material, such as lead, and a relatively non-absorbing interspace material, such as fiber, carbon, or aluminum. Under normal operating conditions, the grid strips are aligned with the direction of the primary x-ray beam. In most grids, the interspaces are angled so as to align with a specific point in space. These are designated focused grids. The focal point of the grid should coincide with the focal spot of the x-ray tube, which is the source of the primary radiation.

45
Q

INTENSIFYING SCREENS

A
  • Film is relatively insensitive to X-rays directly • Only about 2% of the X-rays would interact with the emulsion • Requires unacceptably high doses to give a diagnostic image • An intensifying screen is a phosphor sheet the same size as the film, which converts the X-rays to a pattern of light photons • The intensity of the light is proportional to the intensity of X-rays • The pattern of light is then captured by the film • One exception is intraoral dental radiography, where screens are not practical
  • Modern intensifying screens use rare earth materials, which emit light that is matched to the sensitivity of the film being used • Spectral match between the emission of the screen and the absorption in the film e.g. blue or green • K-edges clinically relevant(39-61keV) • Rare earth screens used as they very efficient at converted absorbed Xray energy into light • Results in a ‘faster’ (more sensitive)system •The sensitive emulsion of the film must be in close contact with the screen
46
Q

INTENSIFYING-SCREENS

A

Image on the left shows a schematic of two intensifying screens enclosing a film (yellow). An intensifying screen is composed of a supporting base (purple), a layer containing the phosphors (light blue), and a protective coat (orange). The detailed view on the right shows x-ray photons entering at the top, traveling through the base, and striking phosphors in the base. The phosphors emit visible light, exposing the film. Some visible light photons may reflect off the reflecting layer of the base

47
Q

X-RAY FILM CASSETTE

A

• Flat, light tight box with pressure pads to ensure film in good contact with the screen(s) mounted on the front(and back) • The tube side of the cassette is low atomic number material (Z~6) to minimise attenuation • Rear of cassette often lead backed to minimise back scatter (not in mammo)

48
Q

THE CHARACTERISTIC CURVE

A
  • Plotting Optical Density against log exposure(log Ii/It) gives the Characteristic Curve of the X-ray film • Different types of film – subtle differences but all basically the same
  • Depends on type of film, processing and storage • Fog: Background blackening due to manufacture and storage (undesirable) • Linear portion: useful part of the curve in which optical density (blackening) is proportional to the log of X-ray exposure • The gradient of the linear portion determines contrast in an image and patient exposures must lie within this region • Need to match this to the clinical task! • Hence, film suffers from a limited and fixed dynamic range
49
Q

FILM SPEED

A

• Definition: 1 / ExposureB+F+1 • Reciprocal of Exposure to cause an OD of 1 above base plus fog • Speed of film = sensitivity = amount of radiation required to produce a radiograph of standard density • Fast film requires less radiation (lower patient dose) • Speed is generally used as a relative term defined at a certain OD; one film may be faster than another at a certain point on the curve

50
Q

FACTORS AFFECTING SPEED

A

• Size of grains –larger means faster • This is the main factor and conflicts with the need for small crystals to give good image sharpness. • Fast films are grainier but reduce patient dose • Thickness of emulsion • Double layers of emulsion give faster films • Radiosensitisers added • X-ray energy

51
Q

EFFECT OF DEVELOPING CONDITIONS

A

• Increasing developer temperature, concentration or time increases speed at the expense of fog • Developer conditions should be optimised for maximum gamma, and minimum fog • Automatic processor has temperature controls and time maintained by roller speed • Concentration is controlled by automatic replenishment of the chemicals

52
Q

IMAGE QUALITY: SCREEN-UNSHARPNESS

A
  • Speed class should be chosen carefully to match the application • e.g.400-speed(thick phosphor)for thick sections of the body (abdo/pelvis), • e.g.100-150-speed(thin phosphor) for extremeties(require detail) • Also may have reflective layer on top of phosphor to increase sensitivity (reflect light photons back to the film)at the expense of resolution • Colour dyes to absorb light photons at wider angles(longer path lengths) –at the expense of sensitivity
  • Crossover –light photons from the front screen may be absorbed by the rear emulsion(and vice-versa) • Crossover is a significant contributor to overall unsharpness • Reason for only using one screen in mammography where resolution is critical • Minimise screen-unsharpness by ensuring good contact between the screen and film • Poor contact may result from damage to the film cassette