The Medical Physicist
About the medical physics professional
| Who are Medical Physicists?
Medical physicists are health care professionals with specialized training in the medical applications of physics. Their work often involves the use of x-rays, ultrasound, magnetic and electric fields, infra-red and ultraviolet light, heat and lasers in diagnosis and therapy. Most medical physicists work in hospital diagnostic imaging departments, cancer treatment facilities, or hospital-based research establishments. Others work in universities, government, and industry.
| What do Medical Physicists do?
Most medical physicists work in one or more of the following areas:
Clinical Service
The responsibilities of a clinical medical physicist lie predominantly in the areas of radiotherapy and diagnostic imaging. The roles of a medical physicist in radiotherapy include treatment planning and radiotherapy machine design, testing, calibration, and troubleshooting. The roles of a medical physicist in diagnostic imaging include machine purchasing and installation, testing, quality control, and operation.
- Radiation Safety
Medical physicists have expertise in radiation safety. Ghanaian regulations recognize medical physicists who are certified by the Allied Health Professions Council of Ghana (AHPC)
- Research and Development
Medical physicists are involved at the frontiers of research at all levels:
- basic, theoretical studies into new physical concepts that might be used for diagnosis and treatment
- development and testing of equipment
- the conduct of clinical trials of new imaging and treatment techniques.
Medical research work is almost always highly collaborative and multi-disciplinary. Collaborations typically involve basic scientists in universities, equipment manufactures and a range of different medical professionals, including radiographers, radiologists and radiation oncologists.
The recent rapid technical developments in equipment used in medical imaging and therapy mean that there is always a need for applied research and development work within hospitals. Finding the optimum way to use new equipment and designing practical and robust methods for implementing technology in a busy clinical workplace are challenges that face most medical physicists are some stage.
Teaching
Most medical physicists are affiliated with universities. Many medical physicists teach in graduate and undergraduate medical physics and physics programs. They also teach radiology and radiation oncology residents, medical students, and radiology, radiotherapy, and nuclear medicine technologists.
| Why is a medical physicist needed in X-ray imaging?
Radiation protection is afforded in X-ray imaging, as in any other activity using ionizing radiation, by the application of the International Commission on Radiological Protection’s (ICRP) principles of radiation protection, namely, justification of a given procedure, optimization of the radiation protection, and the limitation of dose for workers and members of the public. Responsibilities for radiation protection are shared among the different professionals in the imaging team, including the radiologists, other medical specialists, the medical radiation technologists and the medical physicists. However, it is the medical physicist who has the particular skills to either perform, or be responsible for, specific aspects in the implementation of these ICRP principles. Indeed, the specific skills of the medical physicist have prompted international standards and requirements to mandate their crucial role in radiation protection.
The roles and the responsibilities of the medical physicist in medical imaging fall into three main areas:
(a) imaging equipment,
(b) imaging procedures and
(c) administrative and regulatory aspects.
In addition, the medical physicist is usually involved in research and development in topics linked to the first two areas cited above. Research on equipment and procedures has allowed substantial advances in the management of radiation doses in medical imaging during recent years. Most commonly, it is the implementation of the optimization principle that dominates the medical physicist’s role; in contributing to processes and procedures that help ensure the use of the minimum patient exposure necessary to achieve the clinical purpose.
Role of the medical physicist—X-ray imaging equipment and X-ray facility
Appropriate selection of X-ray imaging equipment is a very important, yet complex process, which has ongoing implications for the imaging facility, including radiation protection. There are often many competing requirements for a given imaging device, such as imaging performance, ergonomics, dose features, cost, and servicing, and the medical physicist contributes to the selection process. For example, comparing the relative imaging performances of different devices for the same dose conditions is not a trivial task, and requires the specific knowledge and expertise of the medical physicist. Equipment is often offered with several additional options that can impact on radiation protection, such as radiation protection tools, software for patient dose reports and quality control (QC) tools in some specific imaging systems (e.g. rotational acquisitions). These options need to be evaluated and, if indicated, included in the purchase specification.
Once the purchase has been made, the medical physicist takes part in the acceptance tests and in conducting the commissioning procedures. Acceptance testing is primarily about ensuring you have received what you specified, including imaging and dose performance. The separate process of commissioning is to tailor the equipment to how it is going to be used within the facility, and to calibrate and establish the base line measurements for the quality control programme. Many modern imaging devices have a bewildering number of options for technical factors and other parameters, most of which affect the image quality–patient dose relationship. In performing base line measurements of image quality, doses and dose rates and other technical parameters, the medical physicist provides invaluable information to help establish optimized imaging protocols.
A system of quality assurance with quality control tests is required to ensure the ongoing acceptable imaging performance of the imaging equipment. The medical physicist develops, adapts and implements the physics aspects of such a quality assurance programme, by performing tests at set intervals and after major reparations or updates. Comparisons are made with the baseline measurements, and values outside control limits result in corrective actions to ensure continued acceptability of performance.
Data transfer of the relevant dosimetry and the procedure’s technical factors from the imaging equipment to the facility’s Radiological Information System (RIS), or other dedicated system for storage, and its subsequent analysis require the intervention of a medical physicist. The transmission and storage of patient dose data will normally include the cooperation of computer engineering staff as part of the clinical team. The results of the analyses should be periodically discussed and shared with the clinical team to promote improvement in the implementation of radiation protection. Similarly, the connectivity of the imaging systems with the RIS and PACS may also have some influence on radiation protection (e.g. repetition of examinations if some of the previous images have been lost) and should be tested by the medical physicist in cooperation with the computer engineer.
In addition to the imaging equipment, the room and the facility into which it is to be installed must be appropriately designed to ensure that appropriate radiation protection is afforded to medical personnel and to members of the public who may be inside or close to the facility. The design should specify the appropriate room layout and size, together with the necessary shielding requirements, and the medical physicist has the necessary expertise to perform this task. The radiation protection adequacy of the room and facility also must be verified. The final assessment is performed after the imaging equipment has been installed and commissioned, so that realistic scattered radiation measurements can be made.
Role of the medical physicist — X-ray imaging procedures
For a given imaging device, the number of technical factors that can influence the image quality, patient dose, or both is very large. The clinical needs such as the type of projections and the number of images must be appraised. High doses to patients are sometimes either the consequence of improper use of the X-ray system, the acquisition of images of higher quality than necessary, or gathering too many images during cine or DSA series. Implementation of the optimization principle of radiation protection requires that a systematic approach is taken to establish protocols for how a given imaging device will be used in performing a given procedure. The medical physicist is an important contributor to this process due to their expertise and understanding of the physics underpinning the way the particular imaging equipment operates.
Recent advances in high technology imaging systems have presented new challenges. For example, low dose modes in CT, rotational acquisitions in angiography, CT-like modes in angiography and cardiology, all require the medical physicist’s expertise to analyze their performance, and to propose corrective actions when the image quality has become degraded or patient doses have increased significantly.
Particular attention needs to be made with respect to the imaging of children. This has attracted considerable attention in recent years, and equipment manufacturers are now developing sophisticated tools for the optimization of radiation protection in imaging paediatrics. It is incumbent upon the medical physicist to ensure that these features are appropriately validated and employed.
Part of the implementation of optimization requires knowledge of the patient doses being used in the imaging facility. Medical physicists need to conduct or supervise the periodic assessment of patient doses for common imaging procedures, and this would typically be part of the quality assurance programme. Direct measurements are made on some systems, while for others it may be verification of the values being reported by the imaging device. Analysis of the facility’s doses with periodic comparisons to national diagnostic reference levels (DRLs) need to be made. If average values of patient doses are consistently higher than DRLs, or significantly below and with poor image quality, corrective optimization actions need to be discussed in cooperation with the other members of the imaging team.
In addition to the periodic surveys of patient dose, there is also an ad hoc need for specific dosimetry to be performed. For example, dose and risk estimates may need to be performed following the inadvertent irradiation of a pregnant woman. Assisting in the follow-up of high patient (or high staff) dose values is another important task the medical physicist must perform, as can and does occur in image-guided interventional procedures.
The principle of optimization of protection also applies to occupational exposure of medical personnel. The medical physicist has the expertise to provide specialist radiation protection training for staff, including equipment and room-specific practical radiation protection training. There is a strong inter-relationship between patient dose and personal dose in image-guided interventional procedures, and the medical physicist is best placed to provide training to ensure both parties are afforded optimized protection.
In some situations, a person, such as a mother, may willingly and knowingly act as a carer or comforter to a person undergoing an imaging procedure, which exposes them to a small amount of radiation. Estimating doses for such carers or comforters during medical exposures comes under the remit of the medical physicist, who also may propose dose constraints where appropriate to optimize the protection of the carers and comforters.
Role of the medical physicist—training and regulatory activities
As mentioned previously, training personnel in the X-ray imaging facility on imaging physics and radiation protection is an important role of the medical physicist. This includes training new staff and also the provision of refresher training as part of continuing professional development. The medical physicist also usually contributes to the training programme in imaging for radiology residents or registrars.
The medical physicist often plays a role in the implementation of the radiation protection principle of dose limitation as it applies to occupationally exposed personnel and to members of the public. This task includes managing the results of individual monitoring of occupationally exposed personnel, and the appropriate follow-up actions if an individual’s results are unusual for their particular procedures and workload. Area monitoring is also performed, and the medical physicist can advise on which staff require special personal dosimetry, such as the use of two dosimeters (one over and one under the apron) or finger dosimeters, and on precautions for pregnant workers.
X-ray imaging is frequently used as a tool to quantify changes in a given parameter (e.g. bone mineral density) resulting from a trial in biomedical research. The medical physicist‘s role in this case is to estimate the doses and risks for the volunteers in a proposed research programme and to present this information to the appropriate ethics committee to help their decision whether or not to approve a given proposal.
Unintended and accidental medical exposures do occur in X-ray imaging, including irradiating the wrong person, the wrong body part, using a much higher dose than specified, inadvertent exposure of the embryo or foetus, or some equipment or operator failure. As part of the investigation, the medical physicist’s job is to perform the necessary dose and risk estimates, and to provide input into any review of the optimization of protection that may be needed, such as when the dose was much higher than intended.
Finally, the medical physicist also liaises with the radiation protection regulatory body, providing records and documents attesting to, inter alia, equipment inventory, equipment acceptance tests, calibrations, results of the quality assurance programme and quality control tests, patient dosimetry results including comparisons with DRLs, personal monitoring results, and dose constraints for carers and comforters.
| What knowledge and skills of medical physicist working in X-ray imaging?
As outlined above, medical physicists are core members of the multi-disciplinary X-ray imaging team. The performance of the medical physicist has a significant impact on the quality and safety of the X-ray imaging services and they must be fully qualified with the academic knowledge, professional skills and competency to be able to perform their duties effectively and safely.