beam restriction notes

Beam Restriction

Introduction to Beam Restriction

  • Chapter 15: Beam Restriction, Course: RT106 - Spring 2025, Instructor: Joy MacMahan

Objectives

  • Identify factors affecting scatter radiation production.
  • Discuss primary methods to control scatter radiation reaching the image receptor (IR).
  • Explain the purpose and construction of beam-restricting devices.
  • Compare advantages and disadvantages of various beam-restricting devices.
  • Describe the effect of beam restriction on image quality and patient dose.

X-ray Interactions

Interaction Mechanisms

  1. Transmission: An X-ray may penetrate the patient without interacting and reach the IR.
  2. Absorption: An X-ray may completely interact with patient matter and deposit all energy (Photoelectric interaction).
  3. Scatter: An X-ray may interact with the patient and deflect, depositing part of its energy.

Scatter Radiation

Definition

  • Scatter: Radiation that is deflected and travels in different directions after interacting with matter.
  • Creates secondary radiation via Compton interactions.
  • Impacts image quality by causing radiographic fog on the IR.
  • Contributes to dose received by the radiographer.

Production of Scatter

  • Origin:
    • Comes from large body parts, expansive field sizes, and high kilovolt peak (kVp) levels.
    • Generated when primary X-ray photons interact with tissue and objects when the light field is improperly collimated (should be 0.5 to 1 inch from skin).
  • Effects of Scatter:
    • Scattered photons create an evenly distributed exposure (fog).
    • Decreases visibility of subject contrast, leading to loss of detail in images.

Effects of Scatter

  • Primary Contributor: Personnel exposure is primarily due to scatter.
  • Image Formation: X-rays that scatter in a forward direction may help in image formation, while those scattered at angles can degrade image quality and increase receptor exposure.

Factors Affecting Scatter Production

Kilovoltage (kVp)

  • Selection: Based on the size of the body part.
  • Role:
    • Affects beam penetrability.
    • Higher kVp reduces interactions, allowing more photons to pass through to the IR.
  • Implications:
    • Increased kVp results in higher Compton scatter but lower photoelectric absorption, hence reducing patient dose.

Irradiated Material

  • Volume & Atomic Number: Affects the amount of scatter generated during interactions.
    • Volume controlled by field size and patient thickness.
  • Effects:
    • Higher atomic number materials (like bone) absorb more radiation via photoelectric interactions, resulting in reduced scatter compared to soft tissues.

Patient Thickness

  • Larger patient thickness necessitates higher kVp for adequate penetration.
    • Increasing mAs can lead to increased radiation dose.
    • A minimal increase in kVp is often sufficient, yielding lower patient doses, but is associated with increased scatter and reduced image contrast.

Controlling Scatter

  • Purpose: Minimize scatter production during Compton interactions to enhance image quality.
  • Methods:
    • Beam Restriction: Reduces the number of primary and scattered photons.
    • Use of Grids: Absorbs scatter radiation and improves image quality.
    • Patient Dose Management: Number of primary photons directly affects patient dose.

Beam Restrictors/Cataloging Devices

Types of Beam Restrictors

  • Collimators:
    • Restrict primary beam to various field sizes.
    • Contains lead shutters at right angles, moving in opposing pairs.
    • Bottom Shutters: Reduce penumbra (geometric unsharpness) at beam periphery through greater distance from focal spot.
    • Upper Shutters: Absorb off-focus radiation to prevent it from reaching the IR.

Collimator Structure

  • Mirror Assembly: Mounted at a 45-degree angle within the x-ray beam path.
    • Ensures alignment of the light field and x-ray field due to equal distance from the source.
    • Includes a light source that projects through the collimator to define the field size.

Positive Beam Limitation (PBL)

  • Device Function: Automatically adjusts collimation to match the IR size.
  • Rules:
    1. The field size should never exceed the IR size.
    2. Must include added filtration of 1mm aluminum.

Grids

  • Devices positioned between patient and IR to minimize scatter radiation.
  • Function: Only effective means to enhance image quality post scatter photon creation.
  • Results in a decrease in photons interacting with the IR, enhancing overall image quality.

Alternative Beam Restrictors

Aperture Diaphragm & Cones/Cylinders

  • Aperture Diaphragm: Simple shape to restrict beam size.
  • Cones/Cylinders: Shaped devices that limit beam field to specific geometries.

Ancillary Devices

  • Lead Blockers/Lead Masks: Restricted beam shapes tailored for specific examinations.
    • Lead Blocker: A flexible sheet of lead-impregnated material that can be molded.
    • Lead Mask: Custom-shaped masks secured to collimators for precise beam restriction.

Overview of Beam Restriction Impact

  • Decoupling Scatter Production: Smaller field sizes produce fewer scatter photons.
  • Principal Factors of Scatter Production: kVp and type of irradiated material.
  • Atomic Number's Effect: Increasing atomic number correlates with reduced scatter production.
  • Collimator Functionality: Manages primary beam field size effectively.
  • Positive Beam Limitation: Standardizes field size to the IR, reducing excess exposure.
  • Patient Dose Relationship: Effective beam restriction lowers patient dose while improving image quality.

Introduction to Attenuation

Attenuation Defined

  • Attenuation: Reduction of intensity and total number of X-ray photons remaining in the beam after passing through material.
  • A result of X-rays interacting with matter (absorbed or scattered).
  • Determining Factors:
    • Type of irradiated material and its thickness.
    • Density and atomic number of tissues involved.

Human Body as an Attenuator

  • Major substances affecting x-ray absorption:
    1. Air: Effective atomic number 7.78
    2. Fat: Effective atomic number 6.46
    3. Muscle: Effective atomic number 7.64
    4. Bone: Effective atomic number ranging from 12.31 to 13.8

Interaction of Different Tissues with X-rays

  1. Air (Effective atomic number 7.78):
    • Lower density; thus absorbs fewer photons and allows greater receptor exposure (lungs, sinuses, GI tract).
  2. Fat (Effective atomic number 6.46):
    • Softer tissue, slightly less dense than muscle; visible structures like kidneys on abdomen X-ray.
  3. Muscle (Effective atomic number 7.64):
    • Higher density; acts as a major attenuator (psoas muscles visible due to radiation absorption).
  4. Bone (Effective atomic number 12.31-13.8):
    • High density and atomic number, immense absorption, contributes to low receptor exposure.

Patient Influence on Image Quality

Relationship Components

  1. Subject Density: Variability of tissue influences IR exposures.
  2. Subject Contrast: Differences in tissue absorption levels creating variances in image contrast.
  3. Subject Detail: Sharpness influenced by subject proximity to IR.
  4. Subject Distortion: Misrepresentation of structure's size or shape (e.g., lumbar spine view differences).

Pathology Impact on Radiation Absorption

  • Additive Conditions: Increase tissue thickness/density resulting in greater x-ray beam attenuation (e.g., pneumonia).
  • Destructive Conditions: Decrease thickness or density, leading to lesser attenuation (e.g., emphysema).
Knowledge Check
  • High atomic number materials attenuate more than low atomic number materials (e.g., Lead (82) vs Hydrogen (1)).
  • Tissue density impacts attenuation; tightly packed solids absorb more radiation than liquids or gases.
Clinical Considerations
  • Conditions affecting multiple systems, such as Edema, require adjustments in technical factors (5-15% increase in kVp).

Student Projects

  • Exploration of additive vs. destructive pathologies and their presentations on imaging, with a rubric available on the course platform.