Bioimaging Lecture 3: X-Ray Scattering and Applications

Lecturer: Dipanjan Roy
Position: Professor, School of AIDE, Indian Institute of Technology Jodhpur

Recap: Interactions of X-rays

  1. Coherent Scattering:

    • Definition: A process where the scatter photon retains all original energy but changes direction upon interaction with an atom.

    • Implication: This interaction does not change photon energy and can reduce image contrast, often resulting in film fog.

  2. Photoelectric Absorption:

    • Definition: The process where an x-ray photon is completely absorbed by an atom, resulting in the ejection of electrons.

    • Probability of absorption: Related to the atomic number (Z) and photon energy (E) according to the formula:
      extProbabilityextofextabsorptionhickapproxracZ3E3ext{Probability} ext{ of } ext{absorption} hickapprox rac{Z^3}{E^3}

    • Key Points:

      • For soft tissues, this phenomenon dominates up to 26 keV.

      • Higher atomic numbers have a higher limit of absorption.

      • Provides excellent imaging for bones due to high calcium content (Ca, Z = 20) and density.

Coherent Scattering

  1. Mechanism:

    • An incoming x-ray photon interacts with an atom in the body. The energy is absorbed temporarily by the entire atom.

    • The x-ray photon is then emitted as a scatter photon with unchanged energy but in a different direction.

  2. Diagrammatic Summary:

    • Incoming: X-ray

    • Outgoing:

      • Electron (stopped locally)

      • Characteristic x-ray

      • Secondary Radiation (Deposits Energy Locally)

    • Dependencies:

      • $Z^3$

      • $ rac{1}{E^3}$

    • Image Impact: Affects patient dose and staff dose significantly, primarily concerning primary contrast.

Compton Scattering

  1. Mechanism:

    • An incoming x-ray photon collides randomly with an electron cloud around a nucleus.

    • An electron is ejected, which deposits energy locally.

    • The scattered x-ray photon usually does not retain energy locally.

  2. Characteristics:

    • Dependencies are not strongly dependent on Z (number of protons) or E (energy).

    • Image Impact:

      • Causes a background haze in imaging.

    • Dose Impact:

      • The sequent electrons from Compton Scattering deposit significant doses and are a dominant source of stray radiation affecting staff.

  3. Additional Note:

    • Wavelength changes after scattering are subject to the Compton formula which states that wavelengths increase with the scattering angle.

Applications of X-rays

  1. Historical Images:

    • First image of a human hand with a ring (1895) versus a recent image highlighting advancements in imaging technologies.

    • Emphasizes the evolution of imaging with time.

  2. Computed Tomography (CT):

    • Involves creating summation images to visualize internal structures, such as transaxial sections of the human brain.

Properties of X-rays

  1. Nature of X-rays:

    • A form of electromagnetic radiation exhibiting properties of ionizing radiation.

    • First discovered by Wilhelm Conrad Röntgen in 1895, earning him the first Nobel Prize in Physics in 1901.

  2. Characteristics:

    • Frequency Range: 3imes1016extHz−3imes1019extHz3 imes 10^{16} ext{ Hz} - 3 imes 10^{19} ext{ Hz}

    • Wavelength Range: 0.01extnm−10extnm0.01 ext{ nm} - 10 ext{ nm}

    • Energy Range: 100exteV−100extkeV100 ext{ eV} - 100 ext{ keV}

    • X-rays are shorter than ultraviolet radiation but longer than gamma rays.

  3. Application Spectrum: Different applications utilize various parts of the x-ray spectrum depending on the requirement.

X-ray Radiography

  1. Data Types:

    • Produces 2-dimensional grayscale images.

    • Commonly used to image bone and soft tissue anatomy.

  2. Advantages:

    • Low radiation dose, inexpensive, and quick imaging; widespread availability.

  3. Disadvantages:

    • Limited tissue density range; images are represented as grayscale (2D matrices of intensity) commonly adhering to the Digital Imaging and Communications in Medicine (DICOM) standard.

    • Image size and resolution are device/scale-dependent.

Contrast of Imaging Techniques

  1. Photoelectric vs. Compton Effect:

    • Distinction in mechanisms leading to radiation absorption and scattering phenomena impacting imaging quality.

Other Medical Imaging Techniques

  1. Overview:

    • Non-invasive imaging techniques allow visualization of internal organs, tissues, and biological functions in two or three dimensions.

    • Commonly utilized in medical contexts for interventions and visual representations of organ function.

  2. Imaging Types:

    • Anatomical Imaging: Assesses damage or structural integrity of organs.

    • Functional Imaging: Evaluates conditions such as lesions or metabolic diseases, capturing physical parameters through intensity and color changes.

    • These changes allow for the detection, localization, and characterization of anatomy and function within the imaged region.