Wissenschaftliche Highlights der GSF 2007

Transcription

1 H Forschungszentrum für Umwelt und Gesundheit GmbH in der Helmholtzgemeinschaft Wissenschaftlich-Technische Abteilung Wissenschaftliche Highlights der GSF 2007 Abfrage April 2007 Institut / Selbst. Abteilung / KKG / Nachwuchsgruppe: ISS () Medical Physics Group FE-Nr.: Kontaktperson für Rückfragen: Felix Schöfer, felix.schoefer@gsf.de, Titel des Highlights: Towards the reduction of diagnostic X-ray dose: How many photons do we actually need for optimum diagnosis? Keywords: Mathematical model, dose, optimization, radiography Kernaussage des Highlights in einem Satz: Optimum detection of a contrast is achieved with a radiation field leading to 20% transmission through any object. 1

2 Darstellung des Highlights: We wanted to answer the question, how many photons of what photon energy are needed to detect the presence of an added bit of material to an object with a certain confidence to optimize the radiation energy used for paediatric X-ray radiology. We developed a simple model of a general radiographic imaging task. The model is not depending on assumptions about the statistical properties of the imaging process. It therefore allows to check whether those assumptions made in other approaches are correct. When calculating the numbers needed for answering the question above we found that for the detection of a thickness contrast one most efficient radiation quality is determined: The optimal photon energy is found to be that one, where a fraction of one fifth is transmitted through the object. This is at a large scale independent from the difference in thickness to be detected as well as it is independent from the radiation s spectral composition. This has not been recognized before and is presently not used in clinical systems. Comparing the photon energy used to day with the found optimum means a possible dose reduction by 50% or more without loosing image quality. For the practical application of the results the model has to be validated by experiments and than to be gradually adapted to properties of realistic clinical diagnostic setups like the energy dependences of organ dose conversion factors and detector efficiency. The model offers the potential to calculate properties of the most efficient system possible for an individualized imaging task. Bezug zur GSF-Strategie: The method is aiming for the reduction of dose in medical imaging. This way it minimizes the potential risk to initiate cancer by repeated X-ray imaging procedures. After optimization the repetition rate of radiographic imaging and the accuracy of nearly every imaging technique are altered. This makes the early detection of various pathologies easier especially in populations of higher personal risk. GSF-interne Kooperationspartner, mit denen das Highlight ggf. erarbeitet wurde: (keine) 2

3 Towards the reduction of diagnostic X-ray dose: How many photons do we actually need for optimum diagnosis? Our work performed: Construction of a simple mathematical model of X-ray radiographic imaging Calculation of parameters for minimal exposure at fixed level of information content

4 Mathematical model of the imaging task Source Incident Photons Investigated Object Specimen No Added added material material present! Transmitted Photons Detector Question: Radiographic detection with certain confidence if material is present total thickness of object = k thickness of added material

5 Number of photons needed for a certain confidence number of photons (n) / number of photons (n) / number of photons (n) / Typical numbers of photons used today Minimum is always at ca.20% transmission % 0% 20% 10% 40% 10% 20% 30% 20% 60% 40% 30% 80% 50% 40% 60% 50% 100% 70% 60% 70% k=13.5 k=69 k=693transmission Transmission of Main Transmission Absorber of Main pabsorber of Main Absorber p p

6 Calculation Results Highlight: Optimal detection of a thickness contrast is always performed with radiation field resulting in 20% transmission through any object (this has not been recognized before and is presently not used in clinical systems). This transmission can be reached choosing the appropriate photon energy. The position of the minimum does hardly depend on the thickness ratio k.

7 Outlook The mathematical model needs further experimental validation. The model needs extension regarding detector properties (minimum around 30% transmission?). Adaptation on clinical geometries is required. The method offers the potential to pre-calculate the radiation field and detector parameters for the most efficient diagnostic system possible for an individualized imaging task. Felix Schöfer, Christoph Hoeschen