X-ray Detection and Interaction Mechanisms

X-ray Detection

• X-ray photons are detected when they impart all or part of their energy to the detector material.
• Radiation energy is totally or partially absorbed by the detector material.
• For ionizing electromagnetic radiation (x-rays and gamma-rays), photon energy (E) is calculated as E = hc/λ = hf.

Interaction Mechanisms

• Photon energy absorption occurs through two mechanisms: excitation and ionization.
• Excitation: Electrons are excited to higher energy states within atoms, molecules, or crystals.
• Ionization: Electrons are completely removed from atoms or molecules, creating ions.
• Both processes can cause chemical changes or induce visible light or electrical charge in the circuit.

Amplification

• Due to the small photon energy, signal amplification is usually necessary.
• Amplification methods vary depending on the detection mechanism (electrical or chemical).
• Examples include the photomultiplier tube and the image intensifier.

Flux and Intensity

• Flux (F) is the number of photons per unit time per unit cross-sectional area (photons/second/cm²).
• Intensity (I) is power per unit area (Watts/cm²) or joules/second/cm².
• Intensity is an integrated value across the energy spectrum.
• Intensity is proportional to count rate multiplied by the average photon energy.
• Intensity and flux are directly proportional if the mean beam energy remains constant.

Main Detector Types

• Fluorescence: Incoming photons are absorbed, producing lower-energy radiation (e.g., film screens, scintillator-type detectors).
• Energy trapping: Occurs in computed radiography (CR) image plates and thermoluminescent dosimeters (TLDs).
• Ionization: Ionization chambers, Geiger-Müller tubes, and proportional counters convert radiation into ion pairs.
• Digital radiography systems: Electrical charges are induced in the circuit.

Some Useful Definitions

• Luminescence: Emission of light by a material, including fluorescence, phosphorescence, thermoluminescence, optically stimulated luminescence, and chemiluminescence.
• Phosphors and scintillators are often used interchangeably to represent luminescent materials.
• In radiography, phosphors and scintillators are used to immediately convert x-ray energy into light energy.

Two Forms of Detector Systems

• Detect each photon (pulse-mode): Each photon is detected individually.
• Detect the beam energy (current-mode): The output measures the energy in the incident beam; the output is integrated or smoothed over time.

Detector Types

• Fluorescence detectors use a phosphor scintillator to convert x-ray photons to optical photons.
• A common scintillator material for gamma spectroscopy is NaI.
• Scintillator physical forms include deposition onto substrates.

Detector Characteristics

• All detectors have a characteristic curve relating detector output to input dose or exposure.
• A certain input dose (offset) is needed before detector output is measurable.
• Above the offset, a curve (potentially non-linear) describes the relationship between input and output.
• The H&D curve is nearly linear in the diagnostic range.

Characteristics

• If linear, detector characteristics have a constant gradient (sensitivity).
• Sensitivity = (Δoutput)/(Δinput).
• Film is not linear.
• Modern computer-based and digital systems are linear over a range.
• For zero input, a minimum output level (offset) is detected.
• Above a certain maximum input, output levels do not increase (saturation).
• Noise is always present in detectors.

Dynamic Range

• Latitude or dynamic range: The range of intensities to which the device responds without saturation, usually 3 or more orders of magnitude.
• Dynamic range is crucial for capturing details in both bright and dark image areas without saturation.
• Devices with wide dynamic ranges produce better contrast and detail for diverse tissue densities.

Digital Receptor Dynamic Range

• A wide dynamic range of exposure is characteristic of many digital radiography systems.
• This means that the receptor responds to x-ray exposure and produces digital data over a wide range of exposures.

Film Latitude or Dynamic Range

• Most film systems have a very limited dynamic range of exposure.
• Latitude is the range of exposure that forms an image and is correlated with the slope portion of the H&D curve.

1-Digital Detectors

• To understand signal processing, learn about detectors including photo-stimulable phosphor plates and photoconductive materials.
• Detectors consist of receptor material (e.g., BaF(H)(Eu)) and signal readout/conversion electronics.

Best Described by three Elements

• Capture element: Where the x-ray is absorbed.
• Coupling element: Transfers the radiation signal to the collection element.
• Collection element: Photodiode, CCD, or TFT.

Direct and Indirect Systems

• Direct systems convert x-rays into an electric charge via a photoconductor (e.g., amorphous selenium).
• Indirect systems convert x-rays into visible light using a scintillator layer (e.g., cesium iodide). The light is then converted to an electric charge by a photodiode.

Flat Panel Technology

• Direct conversion uses a semiconductor material.
• Indirect conversion uses scintillator + a photoconductor.

Evolution of Digital Radiography Detectors

• Shows the historical advancement of detector systems from screen-film to digital systems.

Diode Detectors

• Photon energy is absorbed in the semiconductor material, producing electron-hole pairs.
• The circuit collects this charge directly or as current.
• Suitable for current mode usage and for conversion of x-ray energy to light energy by use with phosphors.

lonization Chambers

• Gas-filled detectors where the incoming photon energy ionizes gas atoms.
• Heavy gases (like Xe) have higher stopping power.
• Older CT scanners use xenon detector arrays.
• An electric field collects electrons and cations within the chamber.

Air lonization Chambers

• Air-filled chambers used to measure incident radiation intensity (exposure, E).
• Converts exposure values to dose.
• Dose-area product can be measured—independent of distance from focal spot (assumed a point source).
• Operates in current mode.

Proportional Counters

• Proportional region following ionization region.
• Charge collected is proportional to applied voltage.
• Amplification depends on electric field.
• Has a potential dead time.

Geiger-Müller (G-M) Counters

• Significant higher voltage than ionization or proportional counters (special gas used).
• Useful for detecting contamination after radioactive material spillage.
• Need thin, low-Z windows for detecting alpha particles.
• Can operate in pulse mode, but not a spectrometer or dose-rate meter.
• G-M tubes are inexpensive to build.
• Output pulse is fed to a speaker or ammeter.
• High bias voltage needed.
• "Dead time" or "paralysis" in high radiation fields.

Scintillation Detectors

• Use scintillating crystals as detection medium instead of gas.
• Light photons produced are proportional to incident x-ray energy.

Scintillation Detectors (continued)

• Sodium iodide (NaI) mixed with thallium (Tl) is a common scintillator material.
• The crystal must be coupled optically to a photomultiplier tube (PMT).
• PMT converts light photons into electrical pulses.

Scintillation Detectors (continued)

• Shows a diagram of a scintillator crystal and PMT combination.

Scintillation Detectors (continued)

• Incident radiation excites atoms in the scintillator.
• Excited atoms decay, emitting visible light.
• Light strikes the PMT photocathode, releasing electrons.
• Electrons are multiplied within the PMT to produce output pulses (proportional to energy).

Scintillation Detectors (continued)

• PMT operational range: 800-1200 V.
• PMT gain (amplification): up to 10⁸.
• Increased voltage between dynodes increases amplification, but beyond an optimum, noise amplification becomes unacceptably high.
• PMT voltage must be stabilized.

Solid State (diode) Detectors

• Semiconductor devices (e.g., germanium (Ge), silicon (Si)).
• Ge detectors require lithium (Li) doping.
• Used in applications needing high energy resolution.
• Superior energy resolution compared to proportional counters and NaI(Tl) detectors.

Solid State (diode) Detectors (continued)

• Radiation ionizes semiconductor atoms, producing charge pulses.
• Charge pulses are converted to voltage pulses using resistors.
• Semiconductor materials are denser than gases, resulting in better stopping power and detector efficiency.

Solid State (diode) Detectors (continued)

• Ionization requires lower energy (about 3 eV) compared with gas detectors (35 eV)
• Consequently, about 10 times more ions are produced in a solid state detector for a given y-ray energy.

Thermoluminescence Detectors (TLDs)

• Certain crystalline materials absorb x-ray energy and store it for a time.
• Radiation dose is determined by measuring light output after heating the material through a specific temperature cycle.
• Used for personal dose monitoring, specifically in nuclear medicine studies.

Thermoluminescence Detectors (continued)

• Common TLD compounds include LiF:Mg, CaF2:Dy, and CaF2:Mn.
• Doping elements enhance sensitivity.
• The height of the highest temperature glow curve peak and the total area under the curve are directly proportional to the energy deposited by ionizing radiation.
• LiF response is relatively independent of energies encountered in medical applications of radiation, similar to soft tissue.