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MR-guided attenuation map for prostate PET-MRI: an intensity and morphologic-based segmentation approach for
M Shirin Shandiz1,2, H Saligheh Rad1,2, P Ghafarian3,4
1Department of Medical Physics and Biomedical Engineering, Tehran University of Medical Sciences, Tehran, Iran.
Purpose:
Prostate imaging is one of the major application of hybrid PET/MRI systems. Inaccurate attenuation maps (µ-maps) derived by direct segmentation (SEG) in which the cortical bone is ignored and the volume of the air in cavities is underestimated is the main challenge of commercial PET/MRI systems for the quantitative analysis of the pelvic region. The present study considered the cortical bone and air cavity along with soft tissue, fat, and background air in the µ-map of the pelvic region using a method based on SEG. The proposed method uses a dedicated imaging technique that increases the contrast between regions and a hybrid segmentation method to classify MR images based on intensity and morphologic characteristics of tissues, such as symmetry and similarity of bony structures.
Procedures:
Ten healthy volunteers underwent MRI and ultra-low dose CT imaging. The dedicated MR imaging technique uses the short echo time (STE) based on the conventional sequencing implemented on a clinical 1.5T MRI scanner. The generation of a µ-map comprises the following steps: (1) bias field correction; (2) hybrid segmentation (HSEG), including segmenting images into clusters of cortical bone-air, soft tissue, and fat using spatial fuzzy c-means (SFCM), and separation of cortical bone and internal air cavities using morphologic characteristics; (3) the active contour approach for the separation of background air; and (4) the generation of a five-class μ-map for cortical bone, internal air cavity, soft tissue, fat tissue, and background air. Validation was done by comparison with segmented CT images.
Results:
The Dice and sensitivity metrics of cortical bone structures and internal air cavities were 72 ± 11 and 66 ± 13 and 73 ± 10 and 68 ± 20 %, respectively. High correlation was observed between CT and HSEG-based µ-maps (R 2 > 0.99) and the corresponding sinograms (R 2 > 0.98).
Conclusions:
Currently, pelvis µ-maps provided by the current PET/MRI systems and the ultra-short echo time and atlas-based methods tend to be inaccurate. The proposed method acceptably generated a five-class μ-map using only one image.
Insights
This study introduces an improved method for generating accurate attenuation maps (µ-maps) for pelvic PET/MRI scans. The new hybrid segmentation (HSEG) technique enhances quantitative analysis by incorporating cortical bone and air cavities.
Area of Science:
- Medical Imaging
- Quantitative Analysis
- Hybrid PET/MRI
Background:
- Prostate imaging is a key application for hybrid PET/MRI systems.
- Accurate attenuation maps (µ-maps) are crucial for quantitative analysis in pelvic PET/MRI.
- Current commercial systems struggle with µ-map accuracy due to issues with cortical bone and air cavities.
Purpose of the Study:
- To develop an improved µ-map generation method for pelvic PET/MRI.
- To accurately incorporate cortical bone, air cavities, soft tissue, fat, and background air into µ-maps.
- To enhance quantitative analysis in pelvic PET/MRI by addressing limitations of direct segmentation (SEG).
Main Methods:
- A novel hybrid segmentation (HSEG) method was developed using a dedicated short echo time (STE) MRI technique.
- The HSEG method incorporates spatial fuzzy c-means (SFCM) and morphologic characteristics for tissue classification.
- Validation was performed by comparing HSEG-derived µ-maps with ultra-low dose CT images.
Main Results:
- The HSEG method achieved Dice and sensitivity metrics of 72±11% and 66±13% for cortical bone, and 73±10% and 68±20% for internal air cavities.
- High correlation (R² > 0.99) was found between CT and HSEG-based µ-maps.
- Sinogram correlation also showed high agreement (R² > 0.98).
Conclusions:
- Existing pelvis µ-maps from PET/MRI, ultra-short echo time, and atlas-based methods are often inaccurate.
- The proposed HSEG method successfully generated an accurate five-class µ-map from a single MRI scan.
- This method offers a significant improvement for quantitative PET/MRI analysis in the pelvic region.
