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Quantitative MRI

Research group 8.13

Simultaneous PET-MR

Simultaneous Positron Emission Tomography (PET) – Magnetic Resonance (MR) combines the excellent sensitivity of PET with the versatility of MRI. PET provides metabolic information, and the range of different PET tracers offers the possibility to image different molecular targets. MRI yields high-resolution anatomical images with different contrasts as well as functional information about blood flow and microscopic tissue structure. This makes simultaneous PET-MR a highly promising imaging technique, especially for cancer diagnosis and cancer management, but also for cardiology.

In contrast to PET-CT, simultaneous PET-MR yields a true simultaneous acquisition of PET and MR data allowing for an optimal combination of the two data streams. (MR: 3D cine acquisition, PET: standard FDG scan, data was acquired as part of a canine study)

Attenuation correction maps

Attenuation correction (AC) maps provide information about the attenuation of the PET signal due to different tissue types with different densities (e.g. bone mass has a high density and strong attenuation, lung tissue has a low density and weak attenuation). AC maps are required to compensate for this effect and allow for quantitative PET reconstructions.

For simultaneous PET-MR acquisitions in the thorax or abdomen these AC maps are obtained in a single breathhold. This can lead to misregistration errors between breathhold AC and free-breathing PET data. We are working on methods to obtain accurate AC information during free-breathing and to yield additional respiratory motion fields which can be utilised in motion-compensated MR and PET reconstructions to improve image quality and quantification accuracy (Link to motion compensation project).

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Multi-echo MR sequences are used to determine attenuation correction (AC) maps for PET-MR applications. These scans allow for the separation and classification of image data into fat, soft tissue, air inside and air outside the body. The attenuation correction coefficients are then assigned based on this classification.

For PET-MR exam of the abdomen, the multi-echo scan is acquired during a breathhold. This can lead to errors during the PET reconstruction, because the PET data are obtained during free-breathing. To overcome this problem, we are developing novel techniques which obtain AC information during free-breathing. The MR data also provides respiratory motion information which can be used in motion-compensated MR and PET image reconstructions to improve image quality and quantification accuracy.

Motion-compensated image reconstruction

The combination of PET and MR allows for advanced motion-compensated image reconstruction approaches. Physiological motion information obtained with one of the two modalities can also be utilised for the image reconstruction of the other modality. Furthermore, the two data streams can be combined to obtain motion information with improved accuracy.

In cardiac applications PET-MR allows for example for the correction of both respiratory and cardiac motion information leading to an improvement of more than 30 % in PET tracer quantification.

Further information on motion compensation

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Simultaneous FDG-PET-MR acquisition of the heart during free-breathing without cardiac triggering in three patients. MR data was acquired with a 3D Golden Radial Phase Encoding (GRPE) scheme which allowed for the reconstruction of cardiac and respiratory resolved 3D images. Cardiac and respiratory resolved PET images were also reconstructed and motion resolved MR and PET images were utilised in a joint image registration approach to estimate accurate motion information. Motion information was then used in a motion-compensated image reconstruction to improve MR and PET image quality.

MR and PET images without any motion correction (uncorr) show blurring of the heart and the surrounding anatomy due to cardiac and/or respiratory motion. Cardiac and respiratory motion correction (corr) strongly improves the image quality of MR and PET and ensures accurate alignment between the two modalities.

Clinical applications

A wide range of complementary diagnostic information can be obtained with PET-MR within one scan. PET-MR holds the promise to become an important tool especially for cancer diagnostics and modern cancer management, but also for cardiac applications.

The PTB is part of a consortium led by Charité-Universitätsmedizin Berlin, which has purchased and installed the first simultaneous whole-body PET-MR scanner in Berlin. We are collaborating closely with Dr. Winfried Brenner from the Department of Nuclear Medicine and Dr. Marcus Makowski from the Department of Radiology at Charité Berlin to develop novel PET-MR techniques which will have a direct impact on patient care.

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Current projects

Respiratory motion correction for simultaneous PET-MR of the liver

Novel approaches to estimate respiratory motion from MR are developed as part of the DFG research training group BIOQIC (BIOphysical Quantitative Imaging Towards Clinical Diagnosis) and in collaboration with Charité Berlin. This motion information can then be utilized to improve the accuracy and precision of quantitative MR and PET images.

Cooperation with:
Prof. Dr. Marcus Makowski, Department of Radiology, Charité Berlin, Germany
Prof. Dr. Winfried Brenner, Department of Nulcear Medicine, Charité Berlin, Germany

Respiratory-resolved attenuation correction maps for motion-compensated PET reconstructions

Attenuation correction information required for quantitative PET imaging is commonly obtained with a multi-echo MR acquisition. We have developed an MR scan which yields both accurate attenuation correction information and respiratory motion information in one efficient MR acquisition. This scan allows for respiratory motion-compensated PET reconstructions strongly improving PET image quality and tracer uptake quantification in lesions.

Further information on motion compensation

Cooperation with:
Division of Imaging Sciences and Biomedical Engineering, King’s College London, UK
Siemens Healthcare, Research Collaborations, Frimley, UK

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Selected references

C. Kolbitsch, R. Neji, M. Fenchel, A. Mallia, P. Marsden, T. Schaeffter,
Respiratory-resolved MR-based attenuation correction for motion-compensated cardiac PET-MR
Opens external link in new window Physics in Medicine & Biology 63, 135008 (2018).

C. Kolbitsch, R. Neji, M. Fenchel, A. Mallia, P. Marsden, T. Schaeffter,
Fully integrated 3D high-resolution multicontrast abdominal PET-MR with high scan efficiency
Opens external link in new window Magnetic Resonance in Medicine 79, 900-911 (2018).

C. Kolbitsch, M. Ahlman, C. Davies-Venn, R. Evers, M. Hansen, D. Peressutti, P. Marsden, P. Kellman, D. Bluemke, T. Schaeffter
Cardiac and Respiratory Motion Correction for Simultaneous Cardiac PET/MR
Opens external link in new window Journal of Nuclear Medicine 58, 846-852 (2017).

C. Munoz, C. Kolbitsch, A. Reader, P. Marsden, T. Schaeffter, C. Prieto
MR-Based Cardiac and Respiratory Motion-Compensation Techniques for PET-MR Imaging
Opens external link in new window PET Clinics 11, 179-191 (2016).

D. Balfour, P. Marsden, I. Polycarpou, C. Kolbitsch, A. King
Respiratory motion correction of PET using MR-constrained PET-PET registration
Opens external link in new window BioMedical Engineering OnLine 14, 85 (2015). 

C. Kolbitsch, C. Prieto, C. Tsoumpas, T. Schaeffter
A 3D MR-acquisition scheme for nonrigid bulk motion correction in simultaneous PET-MR
Opens external link in new window Medical Physics 41, 082304 (2014).

C. Baumgartner, C. Kolbitsch, D. Balfour, P. Marsden, J. McClelland, D. Rueckert, A. King
High-resolution dynamic MR imaging of the thorax for respiratory motion correction of PET using groupwise manifold alignment
Opens external link in new window Med Image Anal 18, 939-52 (2014).

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