Medical Imaging: X-Ray Radiography

Medical Imaging: X-Ray Radiography

• X-rays are used in medical imaging.
• A graphic shows an X-ray tube that produces X-rays to penetrate a body part.

Half Value Layer (HVL)

• HVL is the material thickness needed to halve X-ray/gamma-ray beam intensity.
• The formula for HVL calculation is: HVL = 0.693/μ (where μ is the linear attenuation coefficient)
• A 2000 monoenergetic photon beam reduced to 1000 photons by a 0.01m thick copper slab example is given, in which the linear attenuation coefficient (μ) of the copper for these photons is calculated as 69.3 m⁻¹

Planar Radiography/Projection Imaging

• It produces a 2D image of a patient's 3D anatomy.
• The diagram shows an X-ray source, patient, and detector, illustrating the principles.

Instrumentation of Planar Radiography

• Basic components of a planar X-ray radiography system include:
  • X-ray tube: emits X-rays.
  • Collimator: restricts the beam to the area of interest.
  • Anti-scatter grid: reduces scattered X-rays.
  • Detector: converts X-ray energy into light.

Space Charge

• Negative electrons stay close to the filament.
• Electron cloud surrounds filament, making it more difficult to release additional electrons.

Kilovoltage and Space Charge

• Gradually increasing kilovoltage overcomes space charge.
• At high enough kilovoltage, saturation occurs, releasing all possible electrons.

Saturation Voltage

• The kilovoltage at which increasing it further doesn't increase tube current.
• Tube current is emission limited at this point.

Focal Spot

• The area of the anode struck by the electron stream.
• Focal spot size affects image resolution.

Focusing Cup

• It is negatively charged to focus the electron stream to the target.
• It prevents electron spread due to mutual repulsion.

Focal Spots (Further Details)

• Most X-ray tubes have multiple filaments and focal spots.
• Typically, only one is active at a time.
• Small focal spots provide better resolution.
• Large focal spots enhance heat capacity.

Cross-Section of X-Ray Tube

• Illustration displays the internal anatomy of an X-ray tube, including the horn, filaments, and focal spots.
• Also illustrates the stinger, the target, and highlights of GTS products.

Line Focus Principle

• Focal spots are angled / slanted.
• When viewed from the target, the spot appears much smaller compared to the filament.
• The larger apparent size at the filament allows for a better heat capacity.

Target Angle

• The angle between the target and the perpendicular to the X-ray tube axis.
• Typically ranges from 7 to 15 degrees.
• Its affect on heat capacity is highlighted, and smaller angles have greater field coverage, and larger target angles have a poorer heat distribution.

Line Focus (continued)

• The apparent focal spot is calculated from the actual focal spot's value, according to the angle.

Target Angle (continued)

• Larger target angles have greater heat capacities.
• Smaller target angles decrease heat capacity, but offer greater field coverage.

Heel Effect

• The intensity of the X-ray beam is significantly reduced on the anode side.
• The beam passes through more material when exiting the anode.

X-Ray Detectors

• Traditional X-ray film uses screen-film radiography.
• Digital detectors use computed and/or digital radiography.

Screen-Film Cassette

• Composed of a cassette, intensifying screens, and a film sheet.
• The film is a thin plastic sheet with a photosensitive emulsion on one or both sides.

X-ray Film

• Darker films indicate higher X-ray exposure.
• Optical density (OD) quantifies the film's darkness (quantifying the film's light transmittance).
• The densitometer is the tool calculating OD values, and the formula given illustrates this calculation.

Example: Contrast Calculation

• Contrast between regions on a radiograph can be calculated using optical densities. For example a contrast of 0.176 equates to regions with 1.0 and 1.5 corresponding optical densities, respectively.