Filtration, Electron–Anode Interactions & Emission Spectrum

• INHERENT: built into tube (envelope, oil, beryllium window) — ~0.5–1.0 mm Al equiv.
• ADDED: aluminium sheets deliberately placed in beam — adds 1.0–2.5 mm Al equiv.
• TOTAL: inherent + added. Regulatory minimum above 70 kVp = 2.5 mm Al equiv.

• Removes LOW-ENERGY X-ray photons from the beam
• Low-energy photons: too weak to reach the detector → contribute nothing to image
• But DO dose the patient's superficial tissues
• Filtration removes them → reduces patient dose
• No meaningful loss of image quality

• OUTER SHELL interaction: most common (~99%) → produces HEAT only, no X-rays
• NUCLEAR interaction (BREMSSTRAHLUNG): variable energy → CONTINUOUS X-ray spectrum
• INNER SHELL interaction (CHARACTERISTIC): fixed energy → DISCRETE PEAKS on spectrum

• German for 'braking radiation'
• Electron decelerates near tungsten nucleus
• Lost kinetic energy released as X-ray photon
• Deceleration is random → photons of ALL energies produced
• Creates a CONTINUOUS SPECTRUM from near-zero to maximum (= kVp in keV)

• Photon energy = specific energy difference between electron shells in tungsten
• This energy is FIXED and CHARACTERISTIC of the tungsten atom
• Always the same specific energies — not random
• Appears as discrete PEAKS on the emission spectrum
• Different target materials produce different characteristic energies

• Step 1: High-energy filament electron ejects a K-shell electron from tungsten atom
• Step 2: Only occurs when kVp is high enough to overcome K-shell binding energy (~70 kV for tungsten)
• Step 3: Vacancy left in K-shell
• Step 4: Outer shell electron drops down to fill the vacancy
• Step 5: Energy difference between shells released as characteristic X-ray photon
• Step 6: Photon has a FIXED energy (e.g. 59 or 68 keV for tungsten K-transitions)
• Step 7: Appears as a sharp PEAK on the emission spectrum

• Graph of X-ray INTENSITY (y-axis) vs PHOTON ENERGY (x-axis)
• CONTINUOUS BREMSSTRAHLUNG CURVE: starts at minimum energy (set by filtration)
• Curve rises to a peak in mid-energy range
• Falls to zero at maximum energy (= kVp in keV)
• CHARACTERISTIC PEAKS: sharp spikes at specific energies superimposed on the curve
• Area under the curve = total X-ray output

• INCREASE kVp: entire spectrum shifts to HIGHER ENERGIES
• kVp: maximum energy increases (curve extends right)
• kVp: overall output increases
• kVp: characteristic peaks shift if new transitions become possible
• INCREASE mAs: HEIGHT of entire curve increases proportionally
• mAs: more photons at ALL energies — shape and maximum energy UNCHANGED
• mAs: curve scales up uniformly

• Filtration removes LOW-ENERGY photons preferentially
• Low-energy photons more easily absorbed (photoelectric absorption ∝ 1/E³)
• After filtration: fewer low-energy photons remain in the beam
• AVERAGE energy of remaining beam is HIGHER
• Higher average energy = more penetrating = 'harder' beam
• Maximum energy (set by kVp) is unchanged
• More dose deposited at depth, less at the skin surface