T1/T2*-weighted MRI provides clinically relevant pseudo-CT density data for the pelvic bones in MRI-only based radiotherapy treatment planning.

Department of Oncology, Helsinki University Central Hospital , HUS , Finland.
Acta oncologica (Stockholm, Sweden) (Impact Factor: 2.27). 06/2012; DOI: 10.3109/0284186X.2012.692883
Source: PubMed

ABSTRACT Background and purpose. In radiotherapy (RT), target soft tissues are best defined on MR images. In several cases, CT imaging is needed only for dose calculation and generation of digitally reconstructed radiographs (DRRs). Image co-registration errors between MRI and CT can be avoided by using MRI-only based treatment planning, especially in the pelvis. Since electron density information can not be directly derived from the MRI, a method is needed to convert MRI data into CT like data. We investigated whether there is a relationship between MRI intensity and Hounsfield unit (HU) values for the pelvic bones. The aim was to generate a method to convert bone MRI intensity into HU data surrogate for RT treatment planning. Material and methods. The HU conversion model was generated for 10 randomly chosen prostate cancer patients and independent validation was performed in another 10 patients. Data consisted of 800 image voxels chosen within the pelvic bones in both T1/T2*-weighted gradient echo and CT images. Relation between MRI intensity and electron density was derived from calibrated HU-values. The proposed method was tested by constructing five "pseudo"-CT series. Results. We found that the MRI intensity is related to the HU value within a HU range from 0 to 1400 within the pelvic bones. The mean prediction error of the conversion model was 135 HU. Dose calculation based on the pseudo-CT images was accurate and the generated DRRs were of good quality. Conclusions. The proposed method enables generation of clinically relevant pseudo-CT data for the pelvic bones from one MRI series. It is simpler than previously reported approaches which require either acquisition of several MRI series or T2* maps with special imaging sequences. The method can be applied with commercial clinical image processing software. The application requires segmentation of the bones in the MR images.

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    ASTRO 56th annual meeting; 09/2014
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    ABSTRACT: Purpose: To determine whether bones could be localized accurately by using MR images only in radiotherapy treatment planning. Furthermore, to measure absorbed dose in a material behind different parts of the bone, and to evaluate dose calculation error in a pseudo-CT image by assuming a single electron density for the bones. Methods: A dedicated phantom was constructed using fresh deer bones and gelatine. The accuracy of the bone edge location and the bone diameter in MR images were evaluated by comparing those in the images with the actual measures. The absorbed dose behind the bones was measured by a matrix detector at 6 and 15 MV. Thedose calculation error in the bulk density pseudo-CT image was quantified by comparing the calculation results with those obtained in a standard CT image by superposition and Monte Carlo algorithms (TPSs: Xio 4.60 and Monaco 3.00, Elekta CMS Software). Results: The examination of bone position revealed that the bones can be localized within a 1-mm-pixel-size in the MR images. The measured dose behind less than 2.5-cm-thick femur indicated that the absorbed dose behind the middle part of the bone is approximately one percentage unit (6 MV: 1.3%, 15 MV: 0.9%) smallerthan that of the physically narrower bone edge. The calculations illustrated that the bulk density pseudo-CT image used causes errors up to nearly 2% to the dose behind the middle part, but also, the edge of the femur. Conclusions: This research ascertains that the bone localization is not a restrictive issue for radiotherapy treatment planning by using MR imageonly. The work indicates also that the decrease in absorbed dose is not necessarily dependent on the diameter of the bone. Future research shouldinvestigate the generation of more complex pseudo-CT images and the dosecalculations by using these. Supported by Elekta.
    Medical Physics 06/2012; 39(6):3664. · 2.91 Impact Factor
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    ABSTRACT: Purpose: The lack of electron density information in magnetic resonance images (MRI) poses a major challenge for MRI-based radiotherapy treatment planning (RTP). In this study the authors convert MRI intensity values into Hounsfield units (HUs) in the male pelvis and thus enable accurate MRI-based RTP for prostate cancer patients with varying tissue anatomy and body fat contents.Methods: T1∕T2∗-weighted MRI intensity values and standard computed tomography (CT) image HUs in the male pelvis were analyzed using image data of 10 prostate cancer patients. The collected data were utilized to generate a dual model HU conversion technique from MRI intensity values of the single image set separately within and outside of contoured pelvic bones. Within the bone segment local MRI intensity values were converted to HUs by applying a second-order polynomial model. This model was tuned for each patient by two patient-specific adjustments: MR signal normalization to correct shifts in absolute intensity level and application of a cutoff value to accurately represent low density bony tissue HUs. For soft tissues, such as fat and muscle, located outside of the bone contours, a threshold-based segmentation method without requirements for any patient-specific adjustments was introduced to convert MRI intensity values into HUs. The dual model HU conversion technique was implemented by constructing pseudo-CT images for 10 other prostate cancer patients. The feasibility of these images for RTP was evaluated by comparing HUs in the generated pseudo-CT images with those in standard CT images, and by determining deviations in MRI-based dose distributions compared to those in CT images with 7-field intensity modulated radiation therapy (IMRT) with the anisotropic analytical algorithm and 360° volumetric-modulated arc therapy (VMAT) with the Voxel Monte Carlo algorithm.Results: The average HU differences between the constructed pseudo-CT images and standard CT images of each test patient ranged from -2 to 5 HUs and from 22 to 78 HUs in soft and bony tissues, respectively. The average local absolute value differences were 11 HUs in soft tissues and 99 HUs in bones. The planning target volume doses (volumes 95%, 50%, 5%) in the pseudo-CT images were within 0.8% compared to those in CT images in all of the 20 treatment plans. The average deviation was 0.3%. With all the test patients over 94% (IMRT) and 92% (VMAT) of dose points within body (lower than 10% of maximum dose suppressed) passed the 1 mm and 1% 2D gamma index criterion. The statistical tests (t- and F-tests) showed significantly improved (p ≤ 0.05) HU and dose calculation accuracies with the soft tissue conversion method instead of homogeneous representation of these tissues in MRI-based RTP images.Conclusions: This study indicates that it is possible to construct high quality pseudo-CT images by converting the intensity values of a single MRI series into HUs in the male pelvis, and to use these images for accurate MRI-based prostate RTP dose calculations.
    Medical Physics 01/2014; 41(1):011704. · 2.91 Impact Factor

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