Dosimetry in Diagnostic Radiology

Dosimetric Quantities & Units

  • Fluence: \phi = \frac{dN}{dA}, Unit: m^-2

  • Energy Fluence: \Psi = \frac{dR}{dA}, Unit: J/m^2

  • Kerma: K = \frac{dE_{tr}}{dm}, Unit: J/Kg = Gy

  • Kerma Rate: Kerma per second, Unit: J/Kg/sec = Gy/sec

  • Incident Air Kerma (Ki): Kerma to air from incident X-ray beam on central axis at patient/phantom surface, excluding backscatter. Unit: J/kg = Gy

  • Entrance Surface Air Kerma (Ke): Kerma to air on central axis at patient/phantom surface, including backscatter. Unit: J/kg = Gy, Ke = K_iB

  • Absorbed Dose: D = \frac{dE}{dm}, Unit: Gy, 1 Gy = 1 J/kg

  • Equivalent Dose: HT = wRDT = w_R \times \frac{dE}{dm}, Unit: Sv

  • Effective Dose: E = \sum wT HT = \sum wT \times D \times wR, Unit: Sv

Importance of Radiation Dosimetry

  • Medical exposures are the largest contributor to population radiation dose.

  • Accurate dosimetry monitors technological impacts, ensures optimized techniques, and provides health risk information.

Aspects of Dosimetry in Radiology

  • Assessment of doses to patients

  • Measurement of X-ray equipment performance.

  • Doses assessed for patient groups are compared with DRLs.

Assessment of Doses to Patients

  • Patient dose assessment programs reduce exposures and are required by legislation.

  • Exposure factors or related quantities are recorded to estimate individual patient dose if needed.

Measurement of Performance of X-Ray Equipment

  • Assessed by measuring doses, dose-rates, and other variables.

  • Ensures equipment performs to specification and identifies changes over time.

  • Information helps select exposure factors and establish clinical protocols.

Patient Dose Assessment in Radiography & Fluoroscopy

  • ESD and DAP are used for dose assessment.

  • ESD is skin dose where X-ray beam enters.

Entrance Surface Dose (ESD)

  • Includes backscatter, increasing dose by ~30%.

Dose Area Product (DAP)

  • DAP is the product of mean air kerma and beam area, measured using an ionization chamber.

  • Indicates dose level during examination.

Diagnostic Reference Level (DRL)

  • Provides a reference dose for comparisons, aiding in dose standardization.

  • Defined in terms of ESD and DAP.

  • DAP values are more variable for fluoroscopy.

Australian National DRL

  • ARPANSA national DRL is the 75th percentile of median doses from a national survey.

  • Local practice reference level (PRL) is the median dose at a local practice.

Objectives of DRL

  • Avoid excessive radiation dose without additional clinical information.

  • Used as investigation levels, advisory, not a dose limit.

  • PRL establishes a reference dose for local practices.

  • Used for international comparative dosimetry.

Applications of DRL

  • Reduce unnecessary patient doses and risks.

  • Improve dose distributions, promote narrower dose ranges, and provide a common dose metric.

  • Assess dose impact of new protocols and ensure regulatory compliance.

Regulatory Requirements

  • State and territory bodies require ARPANSA Code of Practice implementation, including DRL application.

  • Responsible persons must ensure periodic comparison with DRLs and review if consistently exceeded.

Measurement Quantities in DRL

  • MDCT: volume computed tomography dose index

  • Fluoroscopy: dose area product (DAP), screening time, number of acquired frames

  • Radiography: entrance skin dose (ESD) or dose area product (DAP)

  • Mammography: mean glandular dose (MGD)

  • Nuclear Medicine: adult reference activity

Mammography Dose Assessment

  • MGD is assessed due to glandular tissue cancer risk.

  • Calculated from incident air kerma for compressed breast thickness.

Mammography Dose Assessment

  • Conversion factors derived from Monte Carlo simulations to derive MGD from air kerma measurements.

Recommended Diagnostic Reference Doses

  • Includes specific values for radiography, fluoroscopy/interventional examinations, and mammography.

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Dosimetric Quantities & Units

Fluence: \phi = \frac{dN}{dA}, Unit: m^-2

  • Represents the number of photons (or particles) striking a unit area. Important in understanding the radiation interaction with matter, particularly in radiation therapy and radiology.

Energy Fluence: \Psi = \frac{dR}{dA}, Unit: J/m^2

  • Indicates the amount of energy delivered per unit area. Essential in evaluating the effectiveness of radiation treatments and assessing dose distributions.

Kerma: K = \frac{dE_{tr}}{dm}, Unit: J/Kg = Gy

  • Stands for Kinetic Energy Released per Unit Mass, which quantifies the energy transfer from uncharged radiation to charged particles in a material. This is vital for assessing radiation doses in tissues.

Kerma Rate: Kerma per second, Unit: J/Kg/sec = Gy/sec

  • This metric provides insight into the rate at which kerma occurs, important for determining dose rates in real-time scenarios during radiation therapy.

Incident Air Kerma (Ki): Kerma to air from incident X-ray beam on central axis at patient/phantom surface, excluding backscatter. Unit: J/kg = Gy

  • Ki is crucial for calculating patient exposure and ensuring safety protocols during imaging procedures, where ensuring minimal radiation exposure is a priority.

Entrance Surface Air Kerma (Ke): Kerma to air on central axis at patient/phantom surface, including backscatter. Unit: J/kg = Gy, Ke = K_iB

  • Includes contributions from backscattered radiation, providing a comprehensive understanding of the dose received at the patient surface level, essential for the safety and effectiveness of radiological examinations.

Absorbed Dose: D = \frac{dE}{dm}, Unit: Gy, 1 Gy = 1 J/kg

  • Measures the energy absorbed per unit mass of tissue. Integral for understanding the biological effects of radiation exposure on human tissues.

Equivalent Dose: HT = wR DT = wR \times \frac{dE}{dm}, Unit: Sv

  • Takes into account the type of radiation and its biological effects, providing a way to compare the impact of different types of radiation on human health.

Effective Dose: E = \sum wT HT = \sum wT \times D \times wR, Unit: Sv

  • Integrates equivalent doses across various organs based on their radiosensitivity, allowing for a risk assessment for exposure to ionizing radiation.

Importance of Radiation Dosimetry

Medical exposures are the largest contributor to population radiation dose.

  • Understanding dosimetry is critical as it helps in evaluating and optimizing the practices in medical imaging and therapy, which can impact large populations.

Accurate dosimetry monitors technological impacts, ensures optimized techniques, and provides health risk information.

  • This involves the continuous assessment of radiation doses to ensure safety and efficacy, ultimately guiding improvements in equipment and techniques.

Aspects of Dosimetry in Radiology

Assessment of doses to patients

  • Involves regularly monitoring and documenting patient exposure levels to ensure they remain within safe limits.

Measurement of X-ray equipment performance.

  • Regular checks are necessary to verify that devices operate according to standards, reducing the risk of excessive patient exposure.

Doses assessed for patient groups are compared with Diagnostic Reference Levels (DRLs).

  • This comparison helps identify outlier practices that may indicate unsafe levels of radiation exposure.

Assessment of Doses to Patients

Patient dose assessment programs reduce exposures and are required by legislation.

  • Regulatory compliance is critical in maintaining safety and promoting public trust in medical imaging practices.

Exposure factors or related quantities are recorded to estimate individual patient dose if needed.

  • This data can be instrumental in tailoring radiological exams to individuals, reducing unnecessary exposures.

Measurement of Performance of X-Ray Equipment

Assessed by measuring doses, dose rates, and other variables.

  • Evaluations should consider the quality and consistency of radiation output, which can affect patient dosing.

Ensures equipment performs to specification and identifies changes over time.

  • Understanding how equipment performance evolves can help in timely repairs and adjustments to maintain safety protocols.

Information helps select exposure factors and establish clinical protocols.

  • Maintaining up-to-date protocols guides staff on appropriate settings based on current technology and regulatory guidelines.

Patient Dose Assessment in Radiography & Fluoroscopy

Entrance Surface Dose (ESD) and Dose Area Product (DAP) are used for dose assessment.

  • These metrics are valuable for evaluating potential risks to patients during procedures and ensuring adherence to safety standards.

Entrance Surface Dose (ESD)

Includes backscatter, increasing dose by ~30%.

  • This consideration is introduced into calculations to better approximate the true exposure a patient faces during procedures.

Dose Area Product (DAP)

DAP is the product of mean air kerma and beam area, measured using an ionization chamber.

  • A highly useful metric in routine assessments as it correlates with the overall dose delivered to the patient.

Indicates dose level during examination.

  • This information is critical for evaluating the safety and effectiveness of various examination procedures.

Diagnostic Reference Level (DRL)

Provides a reference dose for comparisons, aiding in dose standardization.

  • Utilizing DRLs facilitates the setting of safe practice standards across different institutions and geographical locations.

Defined in terms of ESD and DAP.

  • Standardizing DRLs around these two metrics supports clarity and accuracy in dose assessments across practices.

DAP values are more variable for fluoroscopy.

  • Recognizing this variability emphasizes the need for regular updates to DRLs in the context of changing technology and practice standards.

Australian National DRL

ARPANSA national DRL is the 75th percentile of median doses from a national survey.

  • This percentile provides a benchmark that is realistic and achievable for health practitioners while ensuring patient safety.

Local practice reference level (PRL) is the median dose at a local practice.

  • Establishing a PRL accommodates the variations in practices and patient demographics across different settings.

Objectives of DRL

Avoid excessive radiation dose without additional clinical information.

  • Striking a balance between necessary diagnostics and minimizing risks is paramount in radiological practice.

Used as investigation levels, advisory, not a dose limit.

  • This advisory nature encourages continuous improvement in practice without imposing unnecessary restrictions that might hinder diagnostics.

PRL establishes a reference dose for local practices.

  • Ensures that each facility has benchmark data tailored to their specific populations and equipment.

Used for international comparative dosimetry.

  • Facilitating worldwide sharing of information and practices helps elevate safety standards on a global scale.

Applications of DRL

Reduce unnecessary patient doses and risks.

  • Active application of DRLs supports patient safety while maintaining diagnostic efficacy.

Improve dose distributions, promote narrower dose ranges, and provide a common dose metric.

  • Advancements in technology can be leveraged to achieve optimal dosing protocols.

Assess dose impact of new protocols and ensure regulatory compliance.

  • Continuous monitoring and assessment ensure ongoing compliance with safety standards.

Regulatory Requirements

State and territory bodies require ARPANSA Code of Practice implementation, including DRL application.

  • Compliance with ARPANSA ensures that all imaging modalities are held to consistent high standards for safety and effectiveness.

Responsible persons must ensure periodic comparison with DRLs and review if consistently exceeded.

  • Regular audits of practice against DRLs support the identification of areas for improvement and potential training needs for staff.

Measurement Quantities in DRL

MDCT: volume computed tomography dose index.

  • Provides a comprehensive measure of radiation dose across the volume of tissue being imaged.

Fluoroscopy: dose area product (DAP), screening time, number of acquired frames.

  • These measurements ensure practitioners can evaluate both the radiation dose and the procedural efficiency.

Radiography: entrance skin dose (ESD) or dose area product (DAP).

  • Essential for ongoing assessments of equipment performance and overall patient safety during imaging.

Mammography: mean glandular dose (MGD).

  • MGD specifically addresses the risks associated with breast tissue exposures, emphasizing the importance of targeted dose assessments.

Nuclear Medicine: adult reference activity.

  • Ensures consistent dosing based on up-to-date practices and safety benchmarks.

Mammography Dose Assessment

MGD is assessed due to glandular tissue cancer risk.

  • Tailored protocols based on patient demographic and risk factors are fundamental to optimizing patient care.

Calculated from incident air kerma for compressed breast thickness.

  • Measurements should adjust for physical parameters to maintain precision in assessments.

Mammography Dose Assessment

Conversion factors derived from Monte Carlo simulations to derive MGD from air kerma measurements.

  • These simulations provide a reliable method to estimate doses effectively, incorporating detailed modeling of radiation interactions.

Recommended Diagnostic Reference Doses

Includes specific values for radiography, fluoroscopy/interventional examinations, and mammography.

  • Ongoing updates to reference doses based on clinical outcomes ensure practices remain current and patient-centered.