Dosimetry Principles

DOSIMETRY PRINCIPLES

Objectives

• Describe dosimetry and dosimeter.
• Describe properties of dosimetry.

Describe dosimeters and their functions.

Dosimetry Principles

Dosimetry is the science of measuring and monitoring the dose of ionizing radiation absorbed by matter, particularly human tissue, to ensure safety and accuracy in diagnostic radiology.

• Dosimetry is essential for:
• Radiation safety of patients and personnel.
• Quality control in imaging systems.

Compliance with regulatory standards.

Dosimetry Principles

A dosimeter can be defined as any device that is capable of providing a reading that is a measure of the absorbed dose deposited in a sensitive volume by ionizing radiation.

In diagnostic radiology, dosimetric instruments can be classified as either ;

Active dosimeters

Active dosimeters display the dose value directly.

These include ionization chambers and/or semiconductor detectors (sometimes loosely referred to as solid state detectors) used to measure absorbed dose in the primary beam condition Applicable in measurement of patients exit dose and CT phantom dose that use ionization chambers

Passive dosimeters

Passive dosimeters cannot display the dose value directly, but record a dose signal when exposed to radiation, which must be subsequently retrieved and converted to dose by a reading device.

These include solid state devices such as thermoluminescent dosimeters (TLDs), optically stimulated luminescent (OSL) dosimeters and dosimeters (including radiochromic film) that may be placed on a patient’s skin or inside cavities to measure the skin or organ doses.

Properties of radiation dosimeters:

Dosimeters are used for various types of X ray unit and exposure conditions, the choice of the appropriate instrument is important, in order for the radiation measurement to be sufficiently accurate. Properties of radiation dosimeters;

• Accuracy and precision
• Linearity
• Dose and Dose Rate Dependence
• Energy response
• Directional dependence

Spatial resolution

a) Accuracy and Precision

• Accuracy: The closeness of the dosimeter reading to the true radiation dose.
• Precision: Consistency of repeated measurements under the same condition
• Linearity

The dosimeter's response should be proportional to the radiation dose across its operational range.

Ensures reliable readings at varying exposure levels.

c) Dose and Dose Rate Dependence

Dosimeters must function accurately across a range of doses and dose rates, from low (e.g., fluoroscopy) to high (e.g., CT scans).

• Energy Response
• Dosimeters should respond consistently across different X-ray energies.

Energy correction factors may be applied to ensure accuracy.

e) Directional Dependence

The response of some dosimeters (e.g., ionization chambers) may vary with the angle of incident radiation.

• Proper positioning minimizes errors.
• Spatial Resolution

The ability to measure radiation dose in small, localized areas, important for precise patient dosimetry.

Ionization chambers

Ionization chambers consist of a gas-filled cavity between two electrodes with an applied electric field.

When ionizing radiation passes through the chamber, it ionizes the gas molecules, creating positive ions and free electrons.

The electric field directs the ions to the electrodes, producing a current proportional to the radiation dose.

This current is measured and converted to display the radiation dose in terms of air kerma.

Clinical Applications

Radiography: Used to measure radiation dose during routine X-ray procedures to ensure proper exposure.

Fluoroscopy: Monitors dose rates during real-time imaging to prevent excessive exposure.

Mammography: Measures low doses accurately, ensuring optimal image quality with minimal radiation.

Computed Tomography (CT): Used in pencil-type ionization chambers to measure dose-length products (DLP) for CT dose assessments.

Quality Assurance: Employed in equipment calibration and routine quality control to verify radiation output consistency and compliance with safety standards.

Dose length product (DLP) is a measure of CT tube radiation output/exposure (measured in mGy.cm).

Semiconductor Dosimeter

Semiconductor dosimeters are made of materials like silicon or gallium arsenide. When exposed to ionizing radiation, electron-hole pairs are generated within the semiconductor material.

The resulting electrical current or voltage change is proportional to the absorbed radiation dose.

Unlike ionization chambers, semiconductor dosimeters do not require high voltage and provide a highly sensitive and immediate response. They are compact, rugged, and capable of real-time dose measurement.

Clinical Applications

Interventional Radiology: Real-time dose monitoring during procedures to ensure patient and staff safety, especially in high-dose scenarios.

Dose Measurement in Complex Procedures: Useful for precise dose assessment in procedures requiring rapid dose adjustments (e.g., fluoroscopy).

Quality Control: Employed in the calibration of imaging equipment and verification of dose settings.

Skin Dose Measurement: Ideal for measuring localized radiation doses on the skin during interventional or therapeutic procedures.

Radiation Safety Monitoring: Used for occupational exposure monitoring, providing accurate and sensitive measurements of accumulated doses.

Other dosimeters

Film Dosimeter

Records radiation exposure through changes in the optical density of radiation-sensitive film. The degree of darkening correlates with the radiation dose.

Clinical Application:

• Commonly used for personal radiation monitoring (film badges).

Suitable for cumulative dose assessment over time.

Thermoluminescent Dosimeter (TLD)

Stores radiation energy in crystal form, which is released as light upon heating. The emitted light intensity is proportional to the absorbed radiation dose.

Clinical Application:

Used for patient and personnel dosimetry.

Particularly useful in measuring skin and organ doses in phantoms and direct patient applications.

Optically Stimulated Luminescence Dosimeter (OSL)

Emits light when exposed to a specific wavelength after radiation exposure. The emitted light intensity corresponds to the radiation dose received.

Clinical Application:

• Used for both patient and occupational dosimetry.

Ideal for long-term dose monitoring due to high sensitivity and reusability.