Dosimetry

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1. Introduction
2. Ionization chambers and other detectors
3. Patient dose assessment
4. Patient dose management
5. Dose management systems (software)

Introduction

In diagnostic radiology, measurements of absorbed dose (or air kerma) are necessary to test performance of imaging equipment, for optimization of practice, or for risk assessment. As various imaging modalities are employed in X ray diagnostic radiology (e.g., radiography, fluoroscopy, interventional radiology, mammography, CT, and dental radiography), these measurements are performed in a variety of conditions and in radiation fields that differ in size and shape.

The dosimetric quantities used in diagnostic radiology can be divided into:

(i) Practical dosimetric quantities (application specific), which may be directly measured and tailored to specific situations or modalities. Examples include incident air kerma (IAK), air kerma–area product (KAP) and computed tomography (CT) air kerma indices;

(ii) Risk related quantities, used to estimate radiation detriment or risk and are thus measures of absorbed dose. Examples include organ dose and mean glandular dose (MGD) (for mammography).

The doses delivered in diagnostic radiological procedures should be accurately determined to maintain a reasonable balance between image quality and patient exposure. Dosimetry methods used shall ensure the appropriate levels of accuracy and long-term stability. ?The design and performance of the dosimetry equipment must be matched to the needs of the clinical situation, e.g. the dosimeters must have a suitable energy response in the low photon energies used in diagnostic radiology. ?The appropriate selection and use of dosimeters, as well as the interpretation of the dosimetry data, requires specific skills and knowledge. The standards for dosimeter accuracy are generally less strict than in radiation therapy, however the measurements of dose and dose rate span a wide range. It is therefore essential to understand radiation quantities and units, formalism and uncertainty estimation, and the performance of various types of dosimeters available for use in diagnostic radiology. Considerations must be made as to whether the measurements are performed in free air (no backscatter), using phantoms, or on patients. There must be an understanding of the amount of backscatter radiation and its spectral characteristics as this provides insight into the anticipated differences in such measurements.

Medical physicists are responsible for dosimetry in diagnostic radiology and for assessment of radiation doses for specific diagnostic radiology procedures, which requires the use of specialized instrumentation. The duties of the medical physicists include dosimetry measurements, development of methods to analyse the results of the measurements, verifying the accuracy of doses delivered to patients, and in special cases individual patient dose calculations.

Ionization chambers and other detectors

The measurement of ionizing radiation, using a dosimeter, requires a thorough understanding of the interaction of radiation and matter as well as an understanding of the mechanisms of the various measurement systems. It usually comprises a measuring assembly, referred to as an electrometer, and one or more detectors, which may or may not be an integral part of the measuring assembly. In diagnostic radiology, dosimetric instruments can be classified as either active (e.g., ionization chambers and semiconductor detectors) or passive (e.g., thermoluminescent dosimeters (TLDs), optically stimulated luminescent (OSL) dosimeters, diodes, etc).

In addition to radiation beams of varying size and shape, it is important to note the use of a wide range of X ray spectra, generated by X ray tube voltages from about 20 kV to 150 kV and various combinations of anode and filtration materials. An accurate measurement of dose requires a dosimeter calibration in radiation fields of known properties, referred to as radiation quality. Radiation qualities are usually specified in terms of the X ray tube voltage first and, sometimes, second half value layer (HVL).

As the dosimeters are used for diverse type of X ray imaging modalities and exposure conditions, selecting the suitable instrument is critical for reliable dosimetry. Radiation dosimeters must have various desired properties, including sensitivity, linearity, energy and angular dependence and leakage current.

As noted above, several types of dosimeters can be used for the measurement of air kerma (and its derivatives). Most commercial dosimeters may be used for both radiographic and fluoroscopic applications, and they can employ either the cumulative air kerma over time in integrate mode or the air kerma rate mode. The ionization chambers of a few cubic centimetres in volume or semiconductor-based detectors particularly built for such measurements are generally utilized. Dosimeters must not interfere with the patient examination if they are used to take measurements during it. These devices are also used for determination of the HVL. For computed tomography (CT), mammography, and interventional radiology dosimetry, special types of ionization chambers are used. In addition to ionization chambers, diagnostic radiology dosimeters based on semiconductor technology (diodes and MOSFETs) have found widespread use. Owing to their small size and rigidity, they are convenient for use in many applications. In general, the semiconductors have pronounced energy dependence compared to ionization chambers. Although most of the semiconductor-based dosimeters use compensation to correct for the energy dependence at specified beam qualities, the energy dependence for non-specified beam must be investigated and considered in the uncertainty assessment.

To make accurate and repeatable measurements, the dosimetry systems must be properly maintained and calibrated. Maintenance and calibration of measurement equipment is a critical responsibility of the medical physicist. Calibrations are usually carried out on an annual or biennial (every two years) basis and apply to all types of dosimeters or measuring devices. However, cross checks of equipment accuracy between calibrations are considered a good practice, along with proper documentation of these checks.

Patient dose assessment

There are two fundamental reasons for patient exposure monitoring and dose assessment in diagnostic radiology.? The first is related to establishing and maintaining standards of good practice, whereas the second is focused on assessing detriment for the purposes of justification and risk assessment. Monitoring of patient radiation exposure provides relevant information to the professionals to pursue these tasks. It is a complex endeavour, which requires careful planning and selection of appropriate dose metrics and mechanisms and processes for data recording, collection, and analysis, including implementation considerations of the collected data.

International Basic Safety Standards ( http://www.dgdingfa.net/publications/search/type/safety-standards-series) requires establishment of Diagnostic Reference Levels (DRL) for medical exposures incurred in medical imaging, in the attempt to optimize clinical practice. Such diagnostic reference levels shall be based on, whenever possible, wide scale surveys.?

The dosimetric techniques form the basis of the dose surveys. For example, a survey could be designed to collect patient data for common X ray examination, classified based on anatomical region and clinical indication, every 3–5 years. The sample can be selected to best represent the population being studied and large enough to ensure statistically meaningful results. Otherwise, constrains in terms of certain anatomical parameters (e.g., patient weight or breast thickness) must be applied. The typical doses may then be found from the median of this distribution. For paediatric patients, it is necessary to use several size groupings. Typical doses may be compared to DRL values as a part of an optimization exercise.

A medical physicist is a leading team member responsible for designing dose surveys, deciding on the dose metrics to be used for patient or phantom measurements, as appropriate, and choice of a patient cohort to assure statistical validity. The medical physicist also provides training and guidance to the staff involved in the data collection to ensure quality, accuracy and correctness of the collected data. Finally, the medical physicist analyses the data for further use, e.g., in optimization process.

Patient dose management

Incorporating a dose management strategy into the medical imaging practices assures the appropriate amount of radiation is used for the intended clinical task. The dose management is therefore implicit in the optimization task. ?It is important for each patient, especially patients whose health condition requires repeated radiological examinations, to combine a dose management plan with medical imaging procedures. Patient doses can only be successfully managed if information is available on the magnitude and range of doses encountered in clinical practice, and diagnostic reference levels (DRLs) are set using this data. ?Medical physicists? have responsibilities in reviewing procedures and equipment when DRLs are consistently exceeded in standard procedures and subsequently in the optimization of the physical and technical aspects of the different processes used to produce medical images and the necessary imaging equipment.

Dose management systems (software)

Collecting, analysing and evaluating patient dose information are core activities of patient dose management, particularly in the context of Diagnostic Reference Levels (DRL) process. While simple resources such as templates and spreadsheets may still be used for manual data acquisition, electronic data recording and automated systems are preferred for these tasks whenever available. In many countries, radiation exposure data is facilitated by use of electronic tools such as Hospital Information Systems (HIS), Radiology Information Systems (RIS), Picture Archiving and Communication Systems (PACS) and software tools for dose monitoring.

Dose Management Systems (DMS) are designed for health professionals, e.g., radiologist, radiographers and medical physicists, involved in diagnostic radiology process, to support their tasks and duties. Including some specific activities such as: collecting dosimetry data to establish ?and implement DRLs (e.g., optimization of patient exposure), preventing, detecting and reporting of unintended exposures, structured consolidation of dose documentation, reporting and tracking, or for local, regional or national benchmarking of patient exposure for modalities and procedures. For example, with the help of the DMS it is possible to perform real-time, multiparametric evaluations and to compare the dose data from all X ray examinations from the patient's medical history. Other useful DMS features are related to: calculating organ doses using computational dosimetry methods such as Monte Carlo simulations and virtual phantoms in CT, skin dose mapping in interventional procedures or deriving Size Specific Dose Estimates (SSDEs) in computed tomography (CT). In spite of the differences in DMS provided by different developers and vendors, a common feature of all systems is ability to provide immediate results in critical clinical cases thereby aiding medical physicists in performing time consuming tasks.

On another note, DMS can contribute towards improving overall quality and safety in radiology department. By recording the downtime of X ray machines, the system allows for the planning of quality assurance activities. It can facilitate automated image quality assessment, identification of errors and implementation of corrective measures to improve the quality of the daily operations of the radiology department and other routine tasks in hospitals.

Successful utilization of DMS requires careful planning and addressing several implementation considerations related to its integration into clinical environment, connection to relevant imaging modalities, transmission of dosimetry data or procedure and protocols nomenclature used.

The medical physicist is a key health professional who should be involved in selection, procurement, installation, and subsequent operation of a DMS, including relevant quality assurance activities.