RT201 Topic

Review of Image Production and Evaluation

DENSITY

  • Definition: Amount of blackness on a given area of a radiograph.

  • Also known as the logarithm of opacity or optical density (OD).

  • Defined as the ratio of the amount of light incident on the film to the amount of light transmitted through the film.

  • Light Incident: Light striking the radiograph from the back (view box).

  • Light Transmitted: Light seen coming through the radiograph as viewed by the naked eye or a densitometer.

  • OD formula:

    • OD = log10 (Light Incident / Light Transmitted)

DENSITY (Continued)

  • Result of exit x-rays and light rays from intensifying screens striking the film’s emulsion.

  • Visible when the silver halide crystals in the film’s emulsion are converted to black metallic silver in the developer solution.

  • Factors controlling density:

    • The number of exit rays striking the film-screen combination.

    • Speed of the film-screen combination.

    • Processing conditions.

Factors Controlling and Influencing Density

  • mAs (milliamperes-seconds):

    • Controls the number of electrons passing from cathode to anode in the x-ray tube.

    • Governs the quantity of x-rays produced at the anode.

    • Affects the amount of radiation exiting the tube and the number of x-ray photons emerging from the patient.

    • Governed by the reciprocity law.

Factors Controlling and Influencing Density (Continued)

  • kVp (kilovolt peak):

    • Directly controls the energy or quality of the x-rays produced.

    • Affects density in a non-directly proportional manner.

    • Determines the penetrating ability of the x-ray beam affecting the number of x-rays exiting the patient.

Distance

  • Density is inversely proportional to the square of the distance (inverse square law):

    • Old mAs = (New Distance)² (mAso = Dn²)

    • New mAs = (Old Distance)² (mAsn = Do²)

  • Film-screen combination:

    • A direct relationship with density.

    • As speed (sensitivity) increases, density increases; as speed decreases, density decreases.

Additional Factors Influencing Density

  • Grids: Decrease the amount of scatter radiation striking the film.

  • Beam Restriction: Reduces density by limiting the size of the x-ray beam unless mAs is increased to compensate.

  • Anatomy & Pathology: Affects density through variation in atomic number, tissue thickness, and tissue density.

  • Anode Heel Effect: Affects density across the film.

  • Filtration: Has negligible effects on density but serves mainly as a radiation protection measure.

CONTRAST

  • Definition: Differences in adjacent densities on the radiograph.

  • Primary function: Make details visible.

  • High Contrast: Few gray tones; mainly black and white.

  • Low Contrast: Many gray tones; referred to as long-scale contrast.

Factors Controlling and Influencing Contrast

  • kVp:

    • Directly controls contrast through differential absorption of x-ray beam energy.

  • Grids:

    • Reduce scatter reaching the film, decreasing gray tones, which increases contrast.

  • Beam Restriction: Limits area being irradiated, affecting contrast.

Additional Factors Controlling Contrast

  • Filtration:

    • Increases filtration results in harder beam, leading to decreased contrast.

  • Anatomy and Pathology: Known as subject contrast, controlling contrast through variations in anatomy.

RECORDED DETAIL

  • Definition: Sharpness with which anatomic structures are displayed.

  • May be defined as the geometric representation of the part being radiographed.

  • Also referred to as detail sharpness, definition, or image resolution.

Factors Influencing Recorded Detail

  • Object-image distance (OID): Distance from the anatomic part being imaged to the image receptor (usually film).

  • Source-image distance (SID): Distance from the source of radiation (usually the anode) to the image receptor.

  • Focal Spot Size: Use of a smaller focal spot results in increased sharpness.

Additional Factors Influencing Recorded Detail

  • Film-screen combination: Slower speed film-screen systems result in increased sharpness.

  • Motion: Any motion results in image blur and loss of detail.

DISTORTION

  • Definition: Misrepresentation of anatomic structure on an image receptor altering size and/or shape.

  • Types:

    • Size Distortion: May be caused by excessive OID or insufficient SID, causing structures to appear larger.

    • Shape Distortion:

      • Elongation: Causes structures to appear longer than reality.

      • Foreshortening: Causes structures to appear shorter.

RADIOGRAPHIC FILM

  • Base: Made of polyester, approximately 0.008 inches thick, with added blue dye.

  • Emulsion Types:

    • Double emulsion film: Coated on both sides.

    • Single emulsion film: Coated on one side.

  • Emulsion consists of silver halide crystals suspended in gelatin.

Film Characteristics

  • Speed (Sensitivity): Determined by size/number of silver halide crystals and thickness of the emulsion.

  • Contrast: Determined by crystal size and emulsion thickness.

  • Latitude: Inherent contrast of the film, with wider exposure latitude at higher kVp levels.

Sensitometry: P & D Curves

  • Also known as sensitometric curves, characteristics curves, and D log E curves.

  • Curve typically shows a sigmoid shape, with the following sections:

    • Toe: Represents low exposure and density (base plus fog).

    • Body: Straight-line portion from 0.25 to 2.5 density (gamma).

    • Shoulder: Portion from 2.5 to maximum density (D-max); measures unusable densities.

Use of H & D Curves

  • Used to determine characteristics of specific films and compare several films.

  • Film Characteristics: Speed (sensitivity) indicated by curve closeness to y-axis (faster film).

FILM STORAGE

  • Storage Conditions:

    • Temperature kept between 68º to 70° F.

    • Humidity maintained between 40% to 60%.

  • Handling Issues:

    • Pressure marks, static, crinkle or half-moon marks, bending film over fingernail, and rough handling.

INTENSIFYING SCREENS

  • Base or Backing: Made of polyester, reflective layer increases system speed.

  • Active Layer: Phosphor layer adhering to base.

  • Protective Layer: Coating on top of the active layer for protection.

Screen Speed (Sensitivity)

  • Primarily controlled by the film-screen combination; speed mainly influenced by screens while contrast is influenced by kVp.

GRIDS

  • Use: Reduces scatter radiation reaching the film.

  • Construction: Lead strips separated by aluminum interspaces.

  • Grid Types: Linear, focused, and crossed grids.

  • Grid Characteristics:

    • Contrast Improvement Factor: Enhances contrast.

    • Grid Selectivity: Ratio of primary to secondary radiation transmitted.

Additional Information on Grids

  • Grid Conversion Factor (Bucky Factor): Exposure increase needed to compensate for absorption of image-forming rays and scatter.

  • Grid Motion: Stationary grids do not move during exposure; moving grids reciprocate.

Grid Errors

  • Common errors in focused grids: upside down, off-level, lateral decentering.

  • Air Gap Technique: Increases OID to allow scatter to miss the film (e.g., lateral cervical spine).

  • Grids enhance contrast by absorbing scatter rays, improving image quality.

TECHNIQUE CHARTS

  • Measurements: Part thickness should always be measured using calipers.

  • Types of Technique Charts:

    1. Fixed kVp-variable mAs: Assumes optimal kVp for the part being radiographed.

    2. Variable kVp: Varies kVp according to part thickness to ensure more penetrating rays.

    3. Variable Technique: Adjusts methods based on pathology, patient factors, and other variables.