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Pseudo-CT – when radiation dose is calculated by MRI

Currently, the radiation dose in radiotherapy is often calculated based on CT imaging. Such images provide a good understanding of how well the radiation dose is absorbed by the body. But what happens when only MRI images are available, which do not provide that particular information? One approach may be to find a substitute for CT.

– There are a number of reasons why the radiation dose should be calculated based solely on MRI images. One reason may simply be that CT images are unavailable. It may also be the case that you have drawn in the organs at risk on your MR image, the organs that you wish to avoid irradiating, and you want to use that particular image as a basis for dose calculations, says Jens Sjölund, industrial PhD student at Elekta and a participant in the Swedish Testbed for Innovative Radiotherapy.

He is a member of a research team associated with Linköping University that has been investigating a new method to create substitute CT images, also referred to as pseudo CT or synthetic CT. The results were recently published in Physics in Medicine and Biology.

Common problem – only MRI images available

– This may be a concern for us at Elekta when a patient is scheduled for treatment with the gamma knife. We commonly encounter this situation where only MRI images are available – we simply have no CT images on which to base our calculations. In such situations, we have to guess how the CT would have looked. This is a way to make a better guess, says Jens Sjölund and adds:

– The ultimate goal is to calculate what radiation dose the patient should receive based on the pseudo CT.

New method easy to implement

The method that the research team has explored uses images from prior patients, for whom both CTs and MRIs are available. Then these images are matched to a new patient and the result shows how the CT image should look.

One advantage of the new method is that users do not need to deviate from their normal routine.

– Implementation of new imaging techniques is unnecessary, and the new method can be used with the standard MRI protocol – along with the information from prior patients – to estimate how a CT image should look” says Jens Sjölund.

More input needed from clinics

– We still don’t know exactly how accurate an estimate you actually need for the CT image in order to arrive at an acceptable estimate of the radiation dose. What is good enough? asks Jens Sjölund.

– We want to investigate this, but it will require more input from those who routinely administer radiotherapy. We need to compare calculations based on real CTs with pseudo CTs using various tests. And this is what we hope to achieve through the collaboration between industry and hospitals that we’ve established within the Swedish Testbed for Innovative Radiotherapy and Gentle Radiotherapy.

 

Jens Sjölund, the testbed project MRI-based radiotherapy
Jens Sjölund, the testbed project MRI-based radiotherapy

 

MRI in radiotherapy – under development in two national projects

By integrating modern imaging techniques with radiotherapy, we will better be able to locate tumors, irradiate with greater precision – and see early on how the tumor is responding to treatment. It is common for certain parts of a tumor to be more resistant and require extra high dosing. With proper functional imaging technology, we can precisely deliver extra radiation to those specific areas, while avoiding irradiation of other organs.

MRI technology has shown great promise in this area, but completely integrating the concept into clinical practice will require new methods, new technology and new software. Sweden’s University Hospitals have joined forces to solve the task in collaboration with Swedish industry through two national projects, the Swedish Testbed for Innovative Radiotherapy and Gentle Radiotherapy.