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Voxel-wise uncertainty in CT substitute derived from MRI
Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
SLU, Centre of Biostochastics.
Umeå University, Faculty of Medicine, Department of Radiation Sciences, Oncology.
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2012 (English)In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 39, no 6, 3283-3290 p.Article in journal (Refereed) Published
Abstract [en]

Purpose: In an earlier work, we demonstrated that substitutes for CT images can be derived from MR images using ultrashort echo time (UTE) sequences, conventional T2 weighted sequences, and Gaussian mixture regression (GMR). In this study, we extend this work by analyzing the uncertainties associated with the GMR model and the information contributions from the individual imaging sequences.

Methods: An analytical expression for the voxel-wise conditional expected absolute deviation (EAD) in substitute CT (s-CT) images was derived. The expression depends only on MR images and can thus be calculated along with each s-CT image. The uncertainty measure was evaluated by comparing the EAD to the true mean absolute prediction deviation (MAPD) between the s-CT and CT images for 14 patients. Further, the influence of the different MR images included in the GMR model on the generated s-CTs was investigated by removing one or more images and evaluating the MAPD for a spectrum of predicted radiological densities.

Results: The largest EAD was predicted at air-soft tissue and bone-soft tissue interfaces. The EAD agreed with the MAPD in both these regions and in regions with lower EADs, such as the brain. Two of the MR images included in the GMR model were found to be mutually redundant for the purpose of s-CT generation.

Conclusions: The presented uncertainty estimation method accurately predicts the voxel-wise MAPD in s-CT images. Also, the non-UTE sequence previously used in the model was found to be redundant.

Place, publisher, year, edition, pages
American Association of Physicists in Medicine , 2012. Vol. 39, no 6, 3283-3290 p.
Keyword [en]
magnetic resonance imaging, computed tomography substitute, ultrashort echo time, quality assurance, Gaussian mixture, dose calculation, attenuation correction
National Category
Physical Sciences
URN: urn:nbn:se:umu:diva-55694DOI: 10.1118/1.4711807OAI: diva2:528718
Available from: 2013-01-04 Created: 2012-05-28 Last updated: 2014-11-13Bibliographically approved
In thesis
1. Magnetic resonance imaging with ultrashort echo time as a substitute for X-ray computed tomography
Open this publication in new window or tab >>Magnetic resonance imaging with ultrashort echo time as a substitute for X-ray computed tomography
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Radiotherapy dose calculations have evolved from simple factor based methods performed with pen and paper, into computationally intensive simulations based on Monte Carlo theory and energy deposition kernel convolution.

Similarly, in the field of positron emission tomography (PET), attenuation correction, which was originally omitted entirely, is now a crucial component of any PET reconstruction algorithm.

Today, both of these applications – radiotherapy and PET – derive their needed in-tissue radiation attenuation coefficients from images acquired with X-ray computed tomography (CT). Since X-ray images are themselves acquired using ionizing radiation, the intensity at a point in an image will reflect the radiation interaction properties of the tissue located at that point.

Magnetic resonance imaging (MRI), on the other hand, does not use ionizing radiation. Instead MRI make use of the net transverse magnetization resulting from the spin polarization of hydrogen nuclei. MR image contrast can be varied to a greater extent than CT and the soft tissue contrast is, for most MR sequences, superior to that of CT. Therefore, for many cases, MR images provide a considerable advantage over CT when identifying or delineating tumors or other diseased tissues.

For this reason, there is an interest to replace CT with MRI for a great number of diagnostic and therapeutic workflows. Also, replacing CT with MRI would reduce the exposure to ionizing radiation experienced by patients and, by extension, reduce the associated risk to induce cancer.

In part MRI has already replaced CT, but for radiotherapy dose calculations and PET attenuation correction, CT examinations are still necessary in clinical practice. One of the reasons is that the net transverse magnetization imaged in MRI cannot be converted into attenuation coefficients for ionizing radiation in a straightforward way. More specifically, regions with similar appearance in magnetic resonance (MR) images, such as bone and air pockets, are found at different ends of the spectrum of attenuation coefficients present in the human body. In a CT image, bone will appear bright white and air as black corresponding to high and no attenuation, respectively. In an MR image, bone and air both appear dark due to the lack of net transverse magnetization.

The weak net transverse magnetization of bone is a result of low hydrogen density and rapid transverse relaxation. A particular category of MRI sequences with so-called ultrashort echo time (UTE) can sample the MRI signal from bone before it is lost due to transverse relaxation. Thus, UTE sequences permit bone to be imaged with MRI albeit with weak intensity and poor resolution.

Imaging with UTE in combination with careful image analysis can permit ionizing-radiation attenuation-maps to be derived from MR images. This dissertation and appended articles present a procedure for this very purpose. However, as attenuation coefficients are radiation-quality dependent the output of the method is a Hounsfield unit map, i.e. a substitute for a CT image. It can be converted into an attenuation map using conventional clinical procedure.

Obviating the use of CT would reduce the number of examinations that patients have to endure during preparation for radiotherapy. It would also permit PET attenuation correction to be performed on images from the new imaging modality that combines PET and MRI in one scanner – PET/MR.

Place, publisher, year, edition, pages
Umeå: Umeå universitet, 2014. 78 p.
Umeå University medical dissertations, ISSN 0346-6612 ; 1675
magnetic resonance imaging, computed tomography substitute, ultrashort echo time, parallel imaging, radial imaging
National Category
Physical Sciences
Research subject
urn:nbn:se:umu:diva-93053 (URN)978-91-7601-138-6 (ISBN)
Public defence
2014-12-05, Hörsal Betula, Unod L 0, Byggnad 6M, Norrlands universitetssjukhus, Umeå, 09:00 (English)
Available from: 2014-11-14 Created: 2014-09-11 Last updated: 2014-11-13Bibliographically approved

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