Tissue Classification as a Potential Approach for Attenuation Correction in Whole-Body PET/MRI: Evaluation with PET/CT Data

Abstract

Attenuation correction (AC) of whole-body PET data in combined PET/MRI tomographs is expected to be a technical challenge. In this study, a potential solution based on a segmented attenuation map is proposed and evaluated in clinical PET/CT cases. Methods: Segmentation of the attenuation map into 4 classes (background, lungs, fat, and soft tissue) was hypothesized to be sufficient for AC purposes. The segmentation was applied to CT-based attenuation maps from 18 F-FDG PET/CT oncologic examinations of 35 patients with 52 18 F-FDG-avid lesions in the lungs (n 5 15), bones (n 5 21), and neck (n 5 16). The standardized uptake values (SUVs) of the lesions were determined from PET images reconstructed with nonsegmented and segmented attenuation maps, and an experienced observer interpreted both PET images with no knowledge of the attenuation map status. The feasibility of the method was also evaluated with 2 patients who underwent both PET/CT and MRI. Results: The use of a segmented attenuation map resulted in average SUV changes of 8% 6 3% (mean 6 SD) for bone lesions, 4% 6 2% for neck lesions, and 2% 6 3% for lung lesions. The largest SUV change was 13.1%, for a lesion in the pelvic bone. There were no differences in the clinical interpretations made by the experienced observer with both types of attenuation maps. Conclusion: A segmented attenuation map with 4 classes derived from CT data had only a small effect on the SUVs of 18 F-FDGavid lesions and did not change the interpretation for any patient. This approach appears to be practical and valid for MRI-based AC.Key Words: instrumentation; PET/CT; PET/MRI; attenuation correction J Nucl Med 2009; 50:520-526 DOI: 10.2967/jnumed.108.054726 Int he same way in which PET/CT has been shown to be a powerful multimodality imaging tool, there are compelling reasons for combining PET and MRI. PET/MRI would have the following advantages: improved soft-tissue contrast; the possibility of performing truly simultaneous instead of sequential acquisitions; and the availability of sophisticated MRI sequences, such as diffusion and perfusion imaging, functional MRI, and MR spectroscopy, which can add important information. Moreover, the use of PET/MRI would result in a significant decrease in radiation exposure, which is of foremost importance for serial follow-up and pediatric imaging.Thus, a combined PET/MRI scanner would provide an alternative to a combined PET/CT scanner for whole-body oncologic imaging (1,2); improved accuracy could be achieved in the detection, staging, and characterization of several cancers (3-10). Moreover, the combination of PETand MRI is perfectly suited to neurologic imaging and offers new possibilities for cardiovascular imaging (11,12). Consequently, much research effort is being directed toward the development of combined imaging devices, and the initial results are promising (13-18).However, a still-unsolved technical challenge for combined whole-body PET/MRI is the correction of attenuation and scatter in the PET data (19). For this purpose, an...

Feasibility of using respiration‐averaged MR images for attenuation correction of cardiac PET/MR imaging

“…Martinez‐Moller et al (14) proposed using a four‐class segmentation scheme with the Dixon technique, in which fat is separated from nonfat soft tissue and assigned a different attenuation coefficient. Although evaluating different segmentation‐based attenuation correction methods was beyond the scope of our study, it should be noted that the Dixon technique can be integrated into our proposed AMR protocol with a modification of the MR sequence, to separate fat and nonfat soft tissue while maintaining similar temporal resolution.…”

“…This mismatch, which reflects the different breathing states captured by the respective imaging modalities, is largely due to the disparity in modalities' imaging speeds (13) . Unlike PET data, which are averaged over several minutes, each CT slice is captured in less than 1 s. Similarly, in whole‐body PET/MR imaging, MR images for attenuation correction, unlike PET data, are usually acquired using a breath‐hold Dixon sequence, which takes about 18 s for each 21 cm bed position (14) . Examples of respiration associated attenuation artifacts in clinical whole‐body PET/MR have been reported by Keller et al (15) The difference in image acquisition time suggests that artifacts caused by spatial mismatch can also occur in cardiac PET/MR imaging.…”

The Use of Anatomical Information for Molecular Image Reconstruction Algorithms: Attenuation/Scatter Correction, Motion Compensation, and Noise Reduction

“…and to assign a single value to each class. Using discretized attenuation values for μ-maps instead of a continuous attenuation image was evaluated in [28,29] and the accuracy of reconstructed images is dependent on locations. It was claimed that it is feasible to use discretized μ-maps clinically.…”

“…Using anatomical information, these effects can be compensated during image reconstruction. Attenuation correction methods have been investigated using CT [19,[23][24][25][26] or MR [27][28][29]. Scatter correction methods have also been studied in [30][31][32][33][34].…”

PET/MR attenuation correction: where have we come from and where are we going?

“…This sequence, which involves a few seconds of acquisition time, allows the separation of water and fat tissue by using the chemical shift of fat relative to that of water. This information facilitates in turn the segmentation of MR images into four to five different classes (lung, fat tissue, nonfat tissue, mixture of fat/nonfat tissue, air) [6]. It is important to highlight that once segmented, fixed 511 keV linear attenuation coefficients (LACs) are assigned to each of the considered tissue types, largely ignoring tissue heterogeneities.…”