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Imaging equipment: X-ray, CT, MRI, PET, PET/CT


Imaging has become the primary source of information in the design of radiation therapy. It is required for delineation of target volumes and organs at risk, as well as for computerized treatment planning. As such, it is of critical importance that the signal contained in these images is well understood, and the spatial distribution is precise and accurate. Failure in this aspect can result in serious deleterious effects including failure to control the disease and/or induction of unforeseen toxicities. In addition to planar x-ray imaging and X-ray computed tomography (CT), magnetic resonance imaging (MRI) plays an important role in defining the location and local extent of disease. Also positron emission tomography (PET) is playing an increasing role in the radiation therapy treatment planning process since the introduction of dedicated hybrid PET-CT simulators.

Important Principles

Before the equipment starts to be used to image patients, safety tests should be performed at commissioning. Some of these tests are necessary to set up the baseline values needed for the periodic QA programme. In addition to its role in structure delineation, CT is the primary modality for treatment simulation and treatment planning, including dose calculation. The role of CT in simulation requires that the geometry of the image is faithfully representing the object, both at the CT scanner and after transfer to the treatment planning system (TPS). The role of CT in dose calculation means that an appropriate CT number-to-electron-density conversion is developed and tested in the TPS, both for validation of the density of the object being scanned, as well as for the effect of heterogeneities on the dose calculation algorithm.

Introduction to References

Extensive information on imaging equipment is provided in the IAEA Handbook on the Physics of Diagnostic Radiology. The report of the Consultant's Meeting provides an overview of current use and future developments of imaging in radiotherapy. The report of AAPM Task Group 66 discusses image acquisition, image transfer, treatment planning and DRR generation for CT simulators.