X-Ray Radiography Basics

X-Ray Radiography

• X-rays are used in medical imaging.
• Half-value layer (HVL) is the thickness of material required to reduce the intensity of an x-or gamma-ray beam to one-half its initial value.
• HVL = 0.693/μ
• A narrow beam of 2000 monoenergetic photons reduced to 1000 photons by a 0.01 m thick copper slab has a linear attenuation coefficient (μ) of approximately 69.3 m⁻¹.

Planar Radiography/Projection Imaging

• Planar radiography acquires a 2D image of a patient's 3D anatomy.
• X-rays travel from a source through the patient to a detector.

Instrumentation of Planar Radiography

• Key components of an X-ray radiography system include and X-ray tube, a collimator, anti-scatter grid, and a detector.
• A collimator reduces patient dose and Compton scattered X-rays.
• An anti-scatter grid further reduces scattered X-rays.
• A detector converts the energy of transmitted X-rays into light.

Space Charge

• Electrons leaving the filament create a positive charge.
• A cloud of negative electrons surrounds the filament.
• This cloud repels new electrons, limiting electron flow from cathode to anode.

Kilovoltage & Space Charge

• Increasing kilovoltage gradually overcomes space charge.
• Higher kilovoltage allows more electrons to reach the anode.
• At a high enough kilovoltage, saturation occurs, meaning all electrons liberated from the filament reach the target.
• Further increasing kilovoltage doesn't increase the number of electrons reaching the anode.

Saturation Voltage

• Saturation voltage is the kilovoltage at which increasing it further does not increase tube current.
• At this point, all electrons that can be generated are already traveling to the target.
• Tube current is emission-limited; increasing it only happens by increasing filament temperature.

Focal Spot

• Focal spot is the area on the anode where electrons strike, affecting resolution and heat capacity.
• Smaller focal spots improve resolution.
• Larger focal spots allow for better heat capacity, but reduce resolution.

Focusing Cup

• A focusing cup uses a negative charge to control and focus the electron stream, overcoming the natural repulsion between electrons.

Focal Spots (cont.)

• Most X-ray tubes have two filaments and thus two focal spots.
• Only one filament is used at a time.
• Small focal spot - improved resolution.
• Large focal spot - improved heat capacity, but lower resolution.

Cross-Section of X-Ray Tube

• An X-ray tube has various components, including filaments, focal spots, a target, a cathode and anode, and other hardware.

Line Focus Principle

• Focal spot is angled to create a smaller apparent focal spot, improving resolution from the patient's perspective.
• The actual focal spot appears larger from the filament than it appears from imaging areas.

Target Angle

• The target angle is the angle between the target and the perpendicular to the tube axis, typically 7-15 degrees.
• A smaller target angle leads to a smaller apparent focal spot, better heat ratings, but reduced field coverage.
• A larger target angle leads to a larger apparent focal spot, better heat capacity, and improved field coverage.

Heel Effect

• The intensity of the X-ray beam is reduced on the anode side of the beam, because the beam travels through more material exiting from that side.

X-Ray Detectors

• Traditional X-ray film (screen-film radiography)
• Digital detectors (Computed and digital radiography)

Screen-Film Cassette

• Screen-film detectors consist of a cassette, intensifying screens, and film.
• Film is a thin plastic sheet with photosensitive emulsion on one or both sides.

X-ray Film

• X-ray film is a negative recorder.
• Increased X-ray exposure results in darker film.
• Optical density (OD) quantifies the darkness of the film.

Example: Contrast

• Contrast is calculated by comparing the log of ratios of the optical densities of two regions, as compared. An example uses optical densities of 1.0 and 1.5