Newsletter 29
- Details
Steve Ebdon-Jackson (HPA, UK)
Background
Nuclear Medicine imaging relies on the tracer principle first established in 1913 by Georg de Hevesy. It is used to demonstrate physiological processes and involves the administration of a small amount of a radioactive material to the patient which then distributes within the body and accumulates in particular areas or organs. The distribution depends upon the particular material administered, which is chosen depending on the organ of interest. The radiation (usually gamma rays) emitted by the radionuclide are detected and an image of the distribution of the radioactivity within the body is constructed.
A key challenge for nuclear medicine is to detect an adequate number of gamma rays in order to acquire an image that contains enough information for accurate diagnosis while keeping the radiation dose as low as reasonably practicable. This might be achieved by reducing the administered activity but imaging for a longer time. For procedures where the distribution is fixed, this is possible within the limits of the patients’ ability to lie completely still. Typically imaging times which exceed 30 minutes are prone to patient motion. This is not possible for some procedures where the distribution within the organ may be changing while we are imaging
The primary tool for nuclear medicine imaging is the gamma camera. The gamma rays emitted by the radionuclide are detected by a crystal and an image of the distribution of the radioactivity is built up. Because the gamma rays are emitted from the patient in all directions a collimator is used to acquire an accurate image of their distribution. Gamma rays which are not coming orthogonally from the patient are absorbed by the collimator and eliminated from the final image.
A collimator with small holes will provide better resolution but has lower sensitivity as it absorbs more of the emitted gamma rays. A collimator with larger holes is more sensitive (as it allows more gamma rays to be detected) but has poorer resolution. In nuclear medicine there is always a trade off between resolution and sensitivity. In practice the collimator choice depends upon the organ being imaged and the type of imaging.
Single Photon Emission Computed Tomography Imaging
As with other imaging modalities, it is possible to produce 2 dimensional planar images or, to use planar images acquired from a range of angles to reconstruct a full 3 dimensional distribution. In conventional nuclear medicine this is known as Single Photon Emission Computed Tomography (SPECT) imaging.
All SPECT reconstruction techniques have limitations. There are issues with:
- Attenuation (gamma rays lost due to absorption in the patient)
- Scatter (gamma rays are scattered within the patient before detection)
- Resolution (becomes poorer with increasing distance from patient to camera)
- Noise (becomes higher with reduced counts)
- Computation time (significant for accurate methodology)
In practice, three reconstruction methods have been used:
- Filtered back projection has been the standard approach for many years. It is fast but amplifies noise and attenuation and scatter corrections produce errors.
- 2-D iterative reconstruction techniques are now available with greater computer power and attenuation and scatter correction have become possible. These techniques are slower than filtered back projection, reconstructing each slice separately, but they deal with noise effectively.
- 3-D iterative reconstruction is now available where all slices are reconstructed together. This is even slower than the 2-D approach but has the advantage that an additional correction can be made for the variation of resolution with depth within the patient - resolution recovery.
Resolution Recovery – Implications for ALARA
All manufacturers now offer resolution recovery software packages. These are gamma camera, collimator and procedure specific. Generic products are also available. Each would need to be validated against conventional techniques. If expected performance is verified, resolution recovery software should be able to change the current balance between image quality, administered activity and scan time.
In most cases these products were developed and marketed with the intention that image quality would be maintained or improved, administered activities would remain unchanged and scan times would be reduced thus improving the efficiency and cost effectiveness of the nuclear medicine service. These products may however offer the potential to maintain image quality and scan times while reducing the administered activity to the patient. This has a positive impact on patient dose but coincidently may also help nuclear medicine services use available 99mTc more effectively, ensuring that costs are reduced and procedures undertaken as required.
A number of concerns and unknowns exist about the routine use of resolution recovery software. Published patient studies have concentrated on the “unchanged activity/ decreased imaging time” approach and moving towards the “decreased activity/routine imaging time” paradigm will require national and local validation.
Pilot Study – Use of Resolution Recovery in Myocardial Perfusion Imaging
To go some way towards addressing these issues, the Administration of Radioactive Substances Advisory Committee (ARSAC), a statutory advisory committee, set up a sub-group in collaboration with the Institute of Physics and Engineering in Medicine (IPEM) Nuclear Medicine Special Interest Group (NMSIG) and Software Validation Working Party. The aims of this group were:
- To establish what products are available, how they work, what is required in order to use them and what costs are associated with each product
- To evaluate at least one of the available products, as a pilot study, to establish whether RR can maintain or improve image quality, and hence image interpretation, and compensate for a reduction in administered activity, when compared to conventional imaging protocols
- To develop a wider study protocol for use in validation of a full range of products.
The pilot study considered myocardial perfusion imaging (MPI), the second most common nuclear medicine procedure in the UK. It is a high dose procedure which offers the potential for significant dose reduction. The diagnostic reference level DRL) for MPI with 99mTc is 1600 MBq (for patients who have both stress and rest components of the study). The principal objectives of the pilot study were:
- To determine whether the interpretation of images obtained with half the normal administered activity and processed with resolution recovery software can be the same as the interpretation from that obtained with normal activity and processed in the standard way
- To determine whether objective quantitative parameters calculated from gated images obtained with half the normal administered activity and processed with resolution recovery software are the same as those obtained with normal activity and processed in the standard way.
The pilot study was carried out using GE Evolution for Cardiac resolution recovery software in the Central Manchester Nuclear Medicine Centre.
Results
The study involved rest and stress data from 44 patients. Each patient was administered the routine activity (1600MBq in total) and gated images acquired but data from the studies were collected, stored and processed to enable full count data to be compared to half count data with resolution recovery applied ie a study using half the administered activity was simulated.
Double reporting resulted in only 2 of the 44 cases having a clinically significant report and of these only one resulted in different patient management. Quantitative left ventricular function analysis showed no significant difference in the LVEF values calculated from the full-count and half-count data at both stress and rest. Further details of the pilot study are included in a report by the ARSAC on the impact of 99mMo shortages on nuclear medicine services, published in November 2010 (www.arsac.org.uk).
Summary and Conclusions
Resolution recovery software was developed to reduce imaging time in busy nuclear medicine departments. Taking an ALARA perspective, this software may be used instead to reduce administered activity and hence patient dose while keeping scan times the same.
Initial results are promising and in MPI show that this approach produces images of accepted image quality from half the administered activity.. Further work will be required to validate this at a local level, for a range of procedures, equipment and software combinations.
- Details
Anne Catrine Traegde Martinsen, Hilde Kjernlie Saether (The Interventional Centre, Oslo University Hospital, Norway)
Introduction
Over the last 30 years, the technological developments in radiology and nuclear medicine have been tremendous, and therefore technological competence in the hospitals is demanded more than ever. In future, the need for technologists in hospitals will further increase, since the technological development continues and the use of high tech advanced equipment is increasing rapidly, including the need for advanced hybrid surgical theatres where advanced radiological equipment is used during operations.
Physicists are necessary to ensure the quality of equipment, optimize examinations with respect to radiation dose and image quality, and to develop new methods and implement new techniques. Diagnostic physicists must collaborate closely with radiologists and radiographers and other users of the equipment to ensure good diagnostic quality of the examinations. This multi-disciplinary collaboration, combined with the implementation of advanced technology in clinical practice, is making the work as a medical physicist especially challenging.
Regional physicist service in the South Eastern part of Norway
Oslo University Hospital (OUH) established a group of physicists specialized in diagnostic radiology, nuclear medicine and intervention, serving most of the hospitals in the southeastern part of Norway in 2005. Today we provide a service to 35 radiological and nuclear medicine departments outside the OUH. This is a non-profit service; the salary for physicists and traveling costs related to the work done in a hospital are paid for by the receiving hospital. As far as possible, each hospital has one contact physicist working together with the radiologist and technicians in the radiology department, and multidisciplinary teamwork is one important factor of success. The services offered are:
- System acceptance tests
- Image quality and dose
- Quality assurance tests annually
- Multidisciplinary dose- and image quality optimizing projects
- CT
- Trauma
- Neuroradiology
- Intervention
- Pediatrics
- Lectures for surgical personnel using X-ray equipment
- Lectures at the radiological and nuclear medicine departments
- Dose measurements and dose estimates
- Consultancy in purchases of new radiology modalities
The economical benefits of a Regional Physicist Centre are that less personnel are needed because of recirculation of lectures, reports and knowledge between the physicists in the department. Also less measuring equipments, phantoms, etc. is needed in the region due to a centralised pool of equipment.
Other benefits in the region are the enhanced competence in CT, X-ray, MR, and Nuclear medicine due to the exchange of experience and knowledge from different laboratories and hospitals. Technological problems are solved by experience from previous corresponding problems on other sites, and development of QA methods and procedures are consolidated in the group of physicists.
Quality Assurance
Physicists have the responsiblity for quality control of radiological and nuclear medicine equipment, monitoring of radiation protection in the hospital, and teaching and radiation protection training for surgical personnel who use the c-arms during the operation.
Annually we are performing QA on more than 400 X-ray and nuclear medicine machines. Annual inspections of the equipment are often carried out in cooperation with the radiographers responsible for the modality, as well as in dialogue with the technical department regarding issues that need follow-up or service. Small errors are often identified during these inspections, which makes it possible to correct them and avoid potentially larger problems with the equipment. Sometimes even serious errors that require adjustments can be repaired immediately.
The results from these inspections give us knowledge about baselines and reference values for different types of equipment for all vendors in the Norwegian market. This knowledge has been useful in several follow-up cases between hospitals and the vendors, and has even led to development of new reconstruction algorithms and improved automatic dose modulation for one CT vendor.
In deciding on the replacement of radiological and nuclear medicine equipment, the diagnostic physicist is a resource in evaluating performance and diagnostic quality compared to the corresponding modern equipment. Physicists have an overview of performance over time through annual inspections, and evaluate quality in relation to the international radiation protection guidelines and other international recommendations.
When purchasing new radiological equipment, physicists are involved together with radiologists, radiographers, technical personnel and procurement personnel. This is expensive and sophisticated equipment, and it therefore demands an orderly and well thought out process in which all options are carefully considered and discussed. This type of work demands for multidisciplinary collaboration and the ability to discuss across professions.
Optimisation of examinations with respect to radiation dose and image quality
In 2008 CT examinations accounted for 80% of the total population radiation exposure from medicine in Norway [1]. Therefore, optimisation of the CT examinations with respect to radiation dose and image quality is necessary. Further development of new imaging techniques to improve image quality while reducing the radiation dose to patients is required. To succeed in such processes, multidisciplinary collaboration between radiologists, radiographers and physicists is essential.
Through the regional services we have achieved high competence in CT, X-ray, MR, nuclear medicine and ultrasound. Now, we have experience of optimisation from all vendors on the Norwegian market, for example from single detector to 256 multi detector CT scanners. Also, we have a large multi disciplinary network in the region, and experience from optimisation work in several hospitals. The physicists are participating with measuring equipment, image quality and dosimetry phantoms and advanced image analysis.
At the same time, the skills in the radiology department and nuclear medicine department inside Oslo University Hospital and in the hospitals we are working for are increasing because of multi-disciplinary image optimizing and dose optimizing projects.
We are now working on standardizing CT exams for the most common clinical problems and oncology follow-up exams in the hospitals in the region. Today repeated CT exams are often performed as the patients are sent from one hospital to another. In the future, unnecessary CT scans could be avoided, with implementation of optimized, standardized CT exams made by common consensus of all hospitals in this region. This is a multidisciplinary task, where radiologists, technicians and physicists need to work together.
In the work of optimisation, it is necessary that the physicists have knowledge of clinically relevant issues, and the clinical and practical limitations of the examinations. This kind of knowledge, in combination with the technological knowledge, is necessary to give advice to the radiologist related to the implementation and further development of advanced imaging technology. In Oslo University Hospital, a multi disciplinary CT task group was established three years ago. The group meets every week to discuss optimal and sub optimal CT examinations, radiation protection, and optimisation of the examinations with respect to image quality and radiation exposure and the optimisation of iodine contrast about where to buy viagra. Our experience from this kind of collaboration is entirely positive, even though it might be challenging sometimes. The cooperation in this group has resulted in several interesting follow-up projects which all have contributed to further technological and clinical improvement, and also have resulted in new reconstruction algorithms for one CT vendor and improved technology for the automatic tube current modulation for another vendor.
In 2008, CT colonoscopy was introduced at the hospital and, related to introduction of this new technology, a task group consisting of radiologists, radiographers, physicists, and gastro surgeons and gastrologists were established. CT colonoscopy is now performed routinely in our hospital. In addition to the implementation of a new diagnostic method for the colon, this multi disciplinary collaboration has resulted in a national course in CT colonoscopy for radiologists, gastro surgeons, gastrologists, radiographers and physicists. Also, the Nordic CT-colonoscopy school that has been arranged three times is a result of this multi disciplinary collaboration.
Science and research
In addition to quality assurance and operational radiation protection, our department is involved in science and research. One professor of physics at the University of Oslo is employed in the department. 6 PhDs and 2 post-docs in MR-physics and one PhD in CT-physics are related to our department. We are also co-responsible for the daily follow-up and management of the PET-CT core facility in the region. In addition, comparison studies of different modalities, optimisation of radiation protection in paediatrics, interventional radiology and internal dosimetry are also fields of research.
In 2010, the department published more than 20 peer-reviewed scientific publications. Also, several abstracts were accepted for presentations at international congresses in 2010.
The department is also co-responsible for post-graduate courses for radiographers at the Oslo university college and for a MSc course in X-ray physics at the University of Oslo.
Conclusion
Over the last 30 years the technological development in radiology and nuclear medicine has been tremendous. The implementation and optimisation of the clinical use of this equipment in the hospitals requires multi disciplinary collaboration. Therefore, a further increase in the multi-disciplinary work of diagnostic physicists in hospitals is necessary to ensure that the ALARA principle is met in the future.
References
[1] Almén A, Friberg EG, Widmark A, Olerud HM. Radiology in Norway anno 2008. Trends in examination frequency and collective effective dose to the population. StrålevernRapport 2010:12. Østerås: Norwegian Radiation Protection Authority, 2010. (In Norwegian)