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Ionising radiation dosimetry and principles of measurement

Introduction

The measurement of ionizing radiation requires a thorough understanding of the interaction of radiation and matter, and an understanding of the mechanisms of the various measurement systems available, e.g., ionization chambers, thermoluminescent dosimeters (TLDs), optically stimulated luminescent (OSL) dosimeters, diodes, etc. The spectral content of the x-ray beam is important as well as the dose rate. Maintenance and calibration of dosimetry systems are other critical areas in the dosimetry of ionizing radiation.

Important Principles

First, and foremost, it is essential to understand radiation quantities and units, formalism and uncertainty estimation, and the various types of dosimeters available for use in diagnostic radiology. Dosimetry for the five diagnostic modalities (radiography, fluoroscopy, mammography, CT, and dentistry) is relatively straightforward, with or without phantoms. However, the complexity increases in situations where components of scattered radiation may be the primary source, e.g., dosimetry for an interventional radiologist or cardiologist where the exposure is from scattered radiation.
Radiation dosimetry must consider whether the measurements are made free in air (no backscatter), with phantoms, or on patients. An understanding of the amount and spectral characteristics of backscatter provides insight into the anticipated differences in such measurements. Energy and flux rate dependence can result in significant errors in measurements and must be taken into consideration for accurate dosimetry measurements.

In addition to patient dose, other units of measurements are used today, e.g., KAP (DAP), DLP, CTDI, etc. The kerma area product, the kerma times the area of the patient exposed, is a better representation of the risk to the patient than incident kerma alone. The CT dose-length product (DLP) is analogous to the KAP in that this is a measure of the dose for a single slice times the z-axis length of the patient exposed. The computed tomography dose index (CTDI) is often misunderstood as representing patient dose. In actuality, it is a dose index which provides the ability to compare radiation characteristics from one CT scanner to another, or for different techniques, e.g., kilovoltage (kVp).

Introduction to References

There are many references available on the measurement of ionizing radiation. The ones provided in “Essential References” should be on the bookshelf of every diagnostic medical physicist.
The book by Attix, although over 20 years old, is considered the standard reference on the measurement of ionizing radiation. (Likewise, Johns and Cunningham, in the Supplemental References, is considered a standard reference.) The IAEA’s International Code of Practice serves as an excellent textbook, in addition to its purpose as a code of practice. The publication by Nagel provides a thorough discussion of the principles of CT dosimetry with examples.