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The Essential Physics of Medical Imaging > Radiography > Flashcards

Radiography Flashcards

1
Q

This is the production of a two-dimensional image from a three-dimensonal object, the patient.

A

Radiography

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2
Q

The procedure projects the x-ray shadows of the patient’s anatomy onto the detector and is often called what?

A

Projection radiograph

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3
Q

For most radiographic examinations, the source to imager distance (SID) is fixed at how many cm?

A

100 cm

And there are usually detents in the radiographic equipment that help the technologists to set this distance.

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4
Q

An exemption to the 100-cm (40 inches) SID is for upright chest radiography, where the SID is typically set to how many cm?

A

183 (72 inches)

To reduce the differential magnification in lung parenchyma

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5
Q

This occurs due to beam divergence

A

Magnification

The collimated xray beam becomes larger in area and less intense with increasing distance from the source

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6
Q

This is determined by the ratio of the length of the image to the length of the object.

A

Magnification

Largest when the object is close to the focal spot
Decreases with distance to the focal spot
Approaches a value of 1 as the object approaches image plane

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7
Q

How can focal spot blur can be minimized?

A

By keeping the object close to the image plane

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8
Q

In _______ radiography, a sheet of film with a light-sensitive emulsion on both sides is sandwiched between two intensifying screens.

A

Screen-film radiography

A light tight cassette encapsulates the screens and film

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9
Q

This is composed of scintillator (Gd 2 O2 S crystals) held together by a binder material.

A

Intensifying screen

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10
Q

What is the purpose of a scintillator?

A

It converts incident xray photons to visible light, which then exposes the silver halid emulsion on film

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11
Q

Why is close physical contact between screen and film important?

A

Gaps promote excessive lateral light propagation which blur the image

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12
Q

Why are intensifying screens composed of high atomic compounds? (Gd2 O2 S, LaOBr, YTaO4)

A

They have high absorption efficiency for x-rays

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13
Q

Thicker intensifying screens absorbs more x-ray photons than thinner screens, but the scintillation light that is emitted in a thicker screen diffuses greater distances to reach and expose film emulsion.

What is the result?

A

This results in more light spread, which reduces the spatial resolution of the screen.

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14
Q

This is composed of a thin plastic base coated on one or both sides with a layer of light sensitive emulsion consisting of silver halide crystals held in water soluble gelatin.

A

Film

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15
Q

Why are the silver halide crystal engineered in a tabular “T” grain shape?

A

To increase surface

For improving light capture efficiency and reducing silver content to save cost

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16
Q

Silver halide grains have small of amount of AgS, that does what?

A

That introduces defects in the ionic crystal structures, where negative charge builds up on the surface and a net positive charge is more central in the crystals

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17
Q

This is a protrusion of the positive charge that reaches the surface of the crystal.

A

A sensitivity speck

It is induced by the lattice defects caused by AgS

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18
Q

When silver halide grain is exposed to visible light, what will happen?

A

A small number of Ag ions are reduced and converted to metallic Ag

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19
Q

If there is greater than or equal to 5 Ag ions are reduced , what will be formed?

A

A stable latent image center

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20
Q

An exposed film not yet chemically develop contains what?

A

Contains a latent image consisting of a pattern of invisible silver halide grains with latent image centers

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21
Q

There is also a slight intensity rate effect, giving rise to slight differences in film density depending on the exposure rate, called what?

A

Reciprocity law failure

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22
Q

The cassette is usually made of what?

A

Usually made of carbon fiber or a material of low atomic number

The inside is light tight to keep ambient light from exposing the film

Designed to maximize transmission of xrays

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23
Q

Why is the gelatin layer permeable to aqueous solution?

A

It is necessary for the aqueous developing chemicals to come into contact with the silver crystals.

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24
Q

When the film is developed in an aqueous chemical bath containing a reducing agent, called the developer, the metallic Ag atoms at the latent image centers acts as a catalyst causing what?

A

Causing the remaining silver ions in the grain to be reduced.

A grain reduced metallic silver atoms appears as a black speck on the film.

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25
Q

After the film has passed through the developer, it passes through a bath of an aqueous oxidizing solution called the fixer, that does what?

A

That dissolves the remaining (inactivated) silver halide from the emulsion layer areas that were not exposed (or were underexposed) to light.

The film is then rinsed with water to remove residual developer and fixer and fixer, and is dried.

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26
Q

It is worth noting that the catalytic reaction that is promoted by the sensitivity speck in the presence of a strong reducing agent results in what?

A

In an approximately 10(9) gain in optical densityon the film with very little added noise - this is a chemical amplification process that few electronic detector systems can match.

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27
Q

This illustrates the optical density of film changes nonlinearly with the xray exposure level.

A

Hurter and Driffield Curve

Higher contrast in linear region
If unexposed film is developeed, its OD will be the base + fog level

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28
Q

Systems cans be more sensitive (and have hight speed), which means what?

A

Less x-ray radiation is required to produce a give OD

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29
Q

Slower screen-film systems required more radiation to produce a given level of film darkening, which results in what?

A

Results in more radiation dose, but because more x-ray photons are used to make the image, the statistical integrity of the image is better (lower quantum noise)

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30
Q

A change in screen-film speed is seen as what?

A

It is seen as a lateral shift of the H and D curve

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31
Q

How is contrast in screen film being adjusted?

A

By altering the grain size and other parameters in the film emulsion.

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32
Q

Is high contrast suitable for every application?

A

No.

For example, a large difference in x-ray fluence to the detector occurs in chest radiography, because of the highly attenuating mediastinum and the hightly transmissive lung fields - a low contrast, wide-latitude film is required for this setting, otherwise the heart silhoutte would be too white and the lung fields be to dark on the processed film.

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33
Q

In mammography, what is more applicable, a high or low contrast setting?

A

The compressed breast is almost uniformly thick and this the x-ray fluence striking the detector is relatively uniform , and thus a very high contrast (narrow latitude) film can be used.

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34
Q

This refers to a photostimulable phosphor detector (PSP) systems, which are historically housed in a cassette similar to a screen-film cassette.

A

Computed radiography

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35
Q

When x-rays are absorbed by photostimulable phosphors, what will happen?

A

Some light is promptly emitted, but a fraction of the absorbed x-ray energy is trapped in the PSP screen and can be red out later using laser light.

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36
Q

This are composed of mixture of BaFBr and other halide-based phosphors, often referred to as barium fluorohalid.

A

CR imaging plates

It is a flexible screen that is enclosed in a light-tight cassette.

The CR cassette is exposed to x-rays during the radiographic examination and is subsequently placed in a CR reader.

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37
Q

What are steps once the plate is placed on the CR reader?

A
  1. Cassette is moved into reader unit and the imaging plate is mechanically removed
  2. Imaging plate is translated vertically and scanned horizontally by a laser beam of approximately 700 nm wavelength.
  3. Red laser light stimulated emission of trapped energy. Blue-green visible light is emitted from storage phosphor
  4. Emitted light is collected by a fiber optic light guide and strikes a multiplier tube - produces an electronic signal
  5. Plate is exposed to bright white light to erase residual trapped energy
  6. Imaging plate is returned to cassette.
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38
Q

This form images from visible light.

It is also used in commercial-grade television cameras and in scientific applications such as astronomy.

A

Charge-coupled (CCD) detectors

39
Q

This refers to the stage where the number of quanta is the lowest and statistical integrity of the signal is the worst.

A

Quantum sink

40
Q

Ideally, in a radiographic image, the quantum sink should be at the stage where the x-ray photons are absorbed in the converter (scintillator or solid state detector). This is refered to as what?

A

X-ray quantum limited detector

41
Q

If there is a subsequent stage where the number of quanta are less than the number of absorbed x-rays, the image statistics and consequently the image noise will be dominated by this stage. What will occur next?

A

A secondary quantum sink

Although an image can be produced, an x-ray detector system with a secondary quantum sink will have image quality that is not commensurate with the x-ray dose used to make the image.

42
Q

What arrays are used in the slot-scan x-ray system?

A

Linear CCD arrays optically coupled to an x-ray scintillator by fiberoptic channel plates

43
Q

How does the slot-scan x-ray system operate?

A

Operate using a narrow-fan beam with pre-and post-patient collimators, acquiring an image by scanning the beam over the anatomy for several seconds.

44
Q

What is the advantage of the slot-scan x-ray system geometry?

A

Excellent scatter rejection achieved, allowing the elimination of the anti-scatter grid and the associated dose penalty.

Slot-scan system imaging have show excellent clinical usefulness in x-ray examinations of the chest and in full-body trauma imaging.

45
Q

What are the disadvantages of slot-scan systems?

A

Longer exposure time required for scan
Increase the potential for motion artifacts
Substantially increased x-ray tube loading

46
Q

This arrays are an alternative to the CCD arrays.

It is based upon a crystalline silicon matrix, these arrays are essentially random access memory chips with built-in photo-sensitive detectors, storage capacitors, and active readout electronics, operating at low voltage (3 to 5 volts).

A

Complementary Metal-Oxide Semiconductor (CMOS) light sensitive arrays

47
Q

These array detectors make use of technology similar to that used in flat panel displays, and much of this has to do with the wiring requirements of a huge number of individual display elements.

A

Flat panel Thin-Film-Transistor (TFT) array detectors

A horizontal and vertical electrical lines is used which, when combined with appropriate readout logic, can address each individual display elements.

48
Q

What are flat-panel TFT arrays are made of?

A

Made of amorphous silicon.

A large area of TFT array is divided into individual detector elements (dexels), arranged in a row and column matrix.

49
Q

What is a thin-film transistor (TFT)?

A

It is an electronic switch that is comprised of three connections: gate, source, and drain

50
Q

These arrays use a scintillator to convert x-rays to light with optical coupling of the scintillator to the active matrix.

A

Indirect x-ray conversion TFT arrays

Photons propagate laterally, compromising resolution, there is lateral spread of the detected signal from one xray photon interaction

51
Q

This refers to the percent of the area of each dexel that is photosensitive.

A

Fill factor

Typically is 80% for 200 um x 200 um dexel dimensions

52
Q

These arrays use a semiconductor material that produces electron-hole pairs in proportion to the incident x-ray intensity.

A

Direct x-ray conversion TFT arrays

Absorbed x-ray energy is directly converted into charge in the detector- there is n intermediate step involving the production of visible light photons.

53
Q

This is the semiconductor most widely used, and is layered between two surface-area electrodes connected to the bias voltage and dielectric layer.

A

Amorphous selenium

Electric field in the converter almost completely eliminates lateral spreading of the charges during transit through the semiconductor, resulting in high spatial resolution.

54
Q

For bone imaging and barium contrast use, what kV is used?

High or low?

A

Low kV

However, the kV is also adjusted to accommodated the thickness of the body part.

55
Q

Why is high kV (typically 120kV) used for chest radiographh?

A

In order to decrease the conspicuity of the ribs and reducing photoelectric iteractions

56
Q

What is the product of mA and time?

A

mAs

57
Q

Modern x-ray generators and tubes designed for radiography can operate at relatively high mA (such as 500 to 800 mA). Why?

A

High mA settings allow the exposure time to be shorter, which “freezes” patient motion.

58
Q

Most intensifying screens are comprised of what?

A

Comprised of fine-grain crystalline scintillating powders (phosphors) formed into a uniformly thick intensifying screen that is held together by a binder.

59
Q

What are the two factors that are important in x-ray detectors?

A

Absorption efficiency and conversion efficiency

60
Q

Absorption efficiency in the phosphor layer is determined by what?

A

Determined by the phosphor composition and screen thickness

The absorption efficiency is also dependent on the x-ray beam energy.

61
Q

This term is used to described how well x-ray detectors capture the incident x-ray photon beam.

A

Quantum detection efficiency

62
Q

Thicker, denser phosphor layers absorb more of the incident x-ray beam, and increased absorption efficiency is always desirable. However, increasing screen thickness has to be balance. Why?

A

There is concomitant increase in blurring and loss of spatial resolution that results in thicker screens.

63
Q

This relates to how well the optical signal is conveyed from the scintillator to the silicon photodetector and how it is then amplified and converted to signal in the image.

A

Conversion efficiency

64
Q

Increasing the speed of detector (and lowering the radiation dose) by increasing the conversion efficiency also increase what?

A

Increases image noise

65
Q

Increasing absorption efficiency can reduce dose. What will happen to image noise?

A

No detrimental effect on quantum noise on image

66
Q

Scattered radiation that does not strike the detector has no effect on the image; however, scattered radiation emanating from the patient is of concern for surrounding personnel. Why?

A

Due to associated radiation dose

67
Q

What happens when scattered radiation is detected by the image receptor?

A

It does have an effect on the image and can be a significant cause of image degradation

Scattered radiation generates image gray scale where it does not belong, and this can significantly reduce contrast in screen-film radiography.

68
Q

What does scatter do in digital radiograhic images?

A

it acts chiefly as a source of noise, degrading the signal-to-noise ration.

69
Q

The amount of scatter detected in an image is characterized by what?

A

The scatter-to-primary ratio (SPR) or scatter fracton

SPR is defined as the amount of energy deposited in a specific location in the detector by scattered photons, divided by the amount of energy deposited by primary (non-scattered) photons in that same location.

70
Q

For an SPR of 1, what happens?

A

Half of the energy is deposited on the detector (at that location) is from scatter - that is, 50% of the information in the image is largely useless.

71
Q

What are the instances that SPR increases?

A

SPR increases typically as the volume of the tissue that is irradiated bu the x-ray beam increases.

The SPR increases as the field size increases and as the thickness of the patient increases.

72
Q

This is the most widely used technology fir reducing scatter in radiography, fluoroscopy, and mammography.

A

Anti-scatter grid or scatter grid

73
Q

Where is the anti-scatter grid placed?

A

Placed between the detector and the patient.

74
Q

What happens when there is errors in the alignment of the grid and the focal spot?

A

The alignment of the grid with the focal spot is crucial to its efficient operation, and errors in this alignment can cause reduced grid performance or artifacts in the image .

75
Q

What are the several parameters that characterize the anti-scatter grid?

A 
Grid ratio
Interspace material
Grid frequency
Grid type
Focal length 
Moving grids
Bucky factor
Primary transition factor
Scatter transmission factor
Selectivity
Contrast degradation
76
Q

This is the ratio of the height of the interspace material to its width.

A

Grid ratio -septa dimensions do not affect grid ratio metric

The grid septa in a grid are typically manufactured from lead

77
Q

What are the grid ratios in general diagnostic radiology?

A

6, 8, 10, 12, or 14, with central three values most commonly used.

Grid ratios are lower (5) in mammography

78
Q

Ideally, air would be the interspace material, but it is not used. Why?

A

Lead septa are very malleable and require support for structural integrity.

Therefore, a solid material is placed in the interspace in the typical linear grid used in general radiography to keep the septa aligned.

79
Q

Low-cost grids in diagnostic radiology use material for interspace material?

A

Aluminum. But aluminum can absorb an appreciable number of the primary photons (especially lower x-ray energies)

Carbon fiber interspaced grids are more common on state-of-the-art imaging systems.

80
Q

This is the number of grid septa per centimeter.

A

Grid frequency

81
Q

What grid type is often used in diagnostic radiology?

A

Linear grid

Crossed grids are widely used in mammography.

82
Q

What is the focal length of a grid for most radiographic suites?

A

100 cm and 183 cm for most upright chest imaging units.

If the x-ray tube is accidentally located at a different distance from the grid, then grid cutoff will occur.

83
Q

This is a grid that moves with a reciprocating motion during the x-ray exposure, causing the grid bars to be blurred bu this motion and not visible in the image.

A

Bucky grid

Stationary grids - more common in upright screen-film chest radiography systems

84
Q

This describes the relative increase in x-ray intesity or equivalent, mAs, neede when a grid is used, compared to when a grid is not used.

A

Bucky factor

This essentially describes the radiatio dose penalty of using the grid

85
Q

The Bucky factor is relevant in screen-film radiography, but less so with digital imaging systems. Why?

A

In screen-film radiography, the use of the grid slight reduces the amount of detected primary radiation and substantially reduces the amount of scattered radiation detected, and both of theses effects reduce OD of the resulting film. Thus, the x-ray technique has to be increased by the Bucky factor to replaced this lost of radiation, in order to deliver films of the same OD.

Digital systems does not have to.

86
Q

Is a grid necessary for thin anatomical structures?

A

No.

It should be noted that thin anatomical structures, there is very little scattered radiation, and the use of a grid is not necessary.

87
Q

This is the fraction of primary photons transmitted through the grid; ideally 1.0.

A

Primary transmission factor

88
Q

This is the fraction of scattered radiation that penetrates the grid.

A

Scattered transmission factor

Ideally, it would be 0

89
Q

This refers to the reduction of contrast due to scattered radiation.

A

Contrast degradation factor

90
Q

What is the principle of air gaps?

A

By moving patient away from the detectors, less of the scatter is emitted from the patient will strike the detector.

91
Q

What practical factors limit the utility of the air gape method for scatter reduction?

A

As magnification of the patient anatomy increases, the coverage of a given detector dimension is reduced, and there is loss in spatial resolution due to the increased blurring of the finite focal spot with magnification.

92
Q

One of the most effective ways of reducing the detection of scattered x-ray.

A

Scan-slot system for scatter reduction

93
Q

Scan-slot radiography can be regarded as the gold standard in scatter reduction methods. Why?

A

The method words because a very small field is being imaged at any point in time, most scatter is prevented from reaching the image receptor, and the image is produce by scanning the small slot across the entire field of view.

In addition to excellent scatter rejection, it does not require a grid and therefore has the potential to be more dose efficient.

94
Q

What are the limitations of scan-slot system?

A

Scanning approaches requires significantly longer acquisition times, and the potential for patient motion during the acquisition increases.

The narrow aperture for scanning requires a high x-ray tube current and the longer scan time requires significant heat loading of the x-ray tube anode.

Require more attenuation from service personnel.

The Essential Physics of Medical Imaging (11 decks)

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