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Image distortions in magnetic resonance imaging (MRI)

Due to its high soft-tissue contrast, MRI is routinely used to define the planning target volume (PTV) for radiotherapy. For this the MR images are registered with images from computed tomography (CT). Geometrical distortions in the MR images due to inhomogeneities in magnetic fields of the MRI device can lead to geometrical uncertainties in the PTV definition.
A phantom together with an evaluation software is developed to quantify the size of the image distortions over a volume relevant for whole-body irradiations. The images may be corrected if necessary. Although there are attempts by the manufacturers of MRI devices to provide a correction for image distortion, the resulting images have still to be checked for their geometrical accuracy, if they are to be used for radiotherapy. The dependence of the image distortions on the imaging sequences has also to be checked.

Geometrical accuracy of multimodal image registration

The accuracy of stereotactic registration of multimodal images is mainly dependent on the mechanical accuracy of the localizer system, which may be well quantified. Other image registration methods (e.g. mutual information), however, use the content of the images and it is rather of the difficult to specify the achieved accuracy quantitatively. Furthermore, the achieved accuracy may be dependent on the selected anatomical region and the individual motility of related organs.
In the case of PET and SPECT additional problems may occur, if tumor-specific markers (e.g. Gd-DOTATOC) are used. In this situation, almost no anatomical information is available for the image registration procedure.
It is the aim to analyze the clinically achieved registration accuracy for different combination of image modalities and for different anatomical regions.

Cooperations

  • Dr. R. Bendl, Dept. of Medical Physics in Radiation Oncology, DKFZ, Heidelberg, Germany
  • Prof. Dr. L. Schad, Dept. of Medical Physics in Radiology, DKFZ, Heidelberg, Germany
  • Dr. M. Henze, Dept. for Nuclear Medicine, University Clinic Heidelberg, Germany

Selected References

  • Karger C.P., Hipp P., Henze M., Echner G., Höss A., Schad L., Hartmann G.H.: Stereotactic imaging for radiotherapy: accuracy of CT, MRI, PET and SPECT. Physics in Medicine and Biology 48, 211-221, 2003
  • Schad L.R., Ehricke H.H., Wowra B., Layer G., Engenhart R., Kauczor H.U., Zabel H.J., Brix G., Lorenz W.J.: Correction of spatial distortion in magnetic resonance angiography for radiosurgical treatment planning of cerebral arteriovenous malformations. Magn. Reson. Imaging 10, 609-21, 1992
  • Schad L. Lott S. Schmitt F. Sturm V. Lorenz W.J.: Correction of spatial distortion in MR imaging: a prerequisite for accurate stereotaxy. J. Comput. Assist. Tomogr. 11, 499-505, 1987

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