PET imaging of rodents is increasingly used in preclinical research, but its utility is limited by spatial resolution and signal-to-noise ratio of the images. A recently developed preclinical PET system uses a clustered-pinhole collimator, enabling high-resolution, simultaneous imaging of PET and SPECT tracers. Pinhole collimation strongly departs from traditional electronic collimation achieved via coincidence detection in PET. We investigated the potential of such a design by direct comparison to a traditional PET scanner. Methods: Two small-animal PET scanners, 1 with electronic collimation and 1 with physical collimation using clustered pinholes, were used to acquire data from Jaszczak (hot rod) and uniform phantoms. Mouse brain imaging using 18 F-FDG PET was performed on each system and compared with quantitative ex vivo autoradiography as a gold standard. Bone imaging using 18 F-NaF allowed comparison of imaging in the mouse body. Images were visually and quantitatively compared using measures of contrast and noise. Results: Pinhole PET resolved the smallest rods (diameter, 0.85 mm) in the Jaszczak phantom, whereas the coincidence system resolved 1.1-mm-diameter rods. Contrast-to-noise ratios were better for pinhole PET when imaging small rods (,1.1 mm) for a wide range of activity levels, but this reversed for larger rods. Image uniformity on the coincidence system (,3%) was superior to that on the pinhole system (5%). The high 18 F-FDG uptake in the striatum of the mouse brain was fully resolved using the pinhole system, with contrast to nearby regions equaling that from autoradiography; a lower contrast was found using the coincidence PET system. For shortduration images (low-count), the coincidence system was superior. Conclusion: In the cases for which small regions need to be resolved in scans with reasonably high activity or reasonably long scan times, a first-generation clustered-pinhole system can provide image quality in terms of resolution, contrast, and the contrast-to-noise ratio superior to a traditional PET system. Smal l-animal PET is an increasingly important tool in biomedical research. A recent development in scanner technology has been the introduction of a focused, clustered-pinhole collimator that enables simultaneous high-spatial-resolution PET and SPECT imaging. Physical collimation of the 511-keV photons produced by positron-electron annihilation, using clustered pinholes, is a substantial change in scanner design as compared with electronic collimation via coincidence detection. Clustered pinholes can offer improved spatial resolution (1) in rodent images, despite a low fraction of single 511-keV photons being detected as compared with the fraction of detected photon pairs in traditional PET. Such scanners are likely suited to different applications. The commonly encountered sensitivity-resolution trade-off has been investigated previously: it has been known for several decades that an improvement in image quality can be achieved via a gain in spatial resolution, even if this gain...
Introduction: Reliable quantification in positron emission tomography (PET) requires accurate attenuation correction of emission data, which in turn entails accurate determination of the attenuation map (µ-map) of the object under study. One of the main steps involved in CTbased attenuation correction (CTAC) is energy-mapping, or the conversion of linear attenuation coefficients (µ) calculated at the effective CT energy to those corresponding to 511 keV. Materials and methods: The aim of this study is to compare different energy-mapping techniques including scaling, segmentation, the hybrid method, the bilinear calibration curve technique and the dual-energy approach to generate the µ-maps required for attenuation correction. In addition, our newly proposed method involving a quadratic polynomial calibration curve was also assessed. The µ-maps generated for both phantom and clinical studies were assessed qualitatively and quantitatively. A cylindrical polyethylene phantom containing different concentrations of K 2 HPO 4 in water was scanned and the µ-maps calculated from the corresponding CT images using the above-referenced energy-mapping methods. The CT images of five whole-body data sets acquired on a GE Discovery LS PET/ CT scanner were employed to generate µ-maps using different energy-mapping approaches that were compared with the µ-maps generated at 511 keV using 68 Ge/ 68 Ga rod sources. In another experiment, the evaluation was performed on PET images of a clinical study corrected for attenuation using µ-maps generated using the above described methods. The evaluation was performed for three different tissue types, namely, soft tissue, lung, and bone. Results and Discussion: All energy-mapping methods yielded almost similar results for soft tissues. The mean relative differences between scaling, segmentation, hybrid, bilinear, and quadratic polynomial calibration curve methods and the transmission scan serving as reference were 6.60%, 6.56%, 6.60%, 5.96%, and 7.36%, respectively. However, the scaling method produced the largest difference (16%) for bone tissues. For lung tissues, the segmentation method produced the largest difference (14.9%). The results for reconstructed PET imagesCorrespondence to: Mohammad R. Ay; e-mail: mohammadreza_ay@tums.ac.ir followed a similar trend. For soft tissues, all energy-mapping methods yield results in nearly the same range. However, in bone tissues, the scaling method resulted in considerable bias in the µ-maps and the reconstructed PET images. The segmentation method also produced noticeable bias especially in regions with variable densities such as the lung, since a single µ is assigned to the lungs. Apart from the aforementioned case, despite small differences in the generated µ-maps, the use of different energy-mapping methods does not affect, to a visible or measurable extent, the reconstructed PET images.
The use of X-ray CT images for CT-based attenuation correction (CTAC) of PET data results in the decrease of overall scanning time and creates a noise-free attenuation map (μmap). The linear attenuation coefficient (LAC) measured with CT is calculated at the x-ray energy rather than at the 511 keV. It is therefore necessary to convert the linear attenuation coefficients obtained from the CT scan to those corresponding to the 511 keV. Several conversion strategies have been developed including scaling, segmentation, hybrid, bilinear and dual-energy decomposition methods. The aim of this study is to compare the accuracy of different energy mapping methods for generation of attenuation map form CT images. An in-house made polyethylene phantom with different concentrations of K2HPO4 was used in order to quantitatively measure the accuracy of the nominated methods, using quantitative analysis of created μmaps. The generated μmaps using different methods compared with theoretical values calculated using XCOM cross section library. Accurate quantitative analysis showed that for low concentrations of K2HPO4 all these methods produce acceptable attenuation maps at 511 keV, but for high concentration of K2HPO4 the last three methods produced the lowest errors (10.1% in hybrid, 9.8% in bilinear, and 4.7% in dual energy method). The results also showed that in dual energy method, combination of 80 and 140 kVps produces the least error (4.2%) compared to other combinations of kVps.
-Attenuation correction of PET emission data using spatially correlated CT images is fast and precise yielding a noise-free attenuation map (µmap) in comparison with radionuclide transmission scanning (TX). However, it is essential to convert the linear attenuation coefficients obtained from CT scans to those corresponding to 511 keV. Several conversion strategies have been developed including scaling, segmentation, hybrid and bilinear methods. The aim of this study is to compare the accuracy of different energy mapping methods for generation of 511 keV µmap using clinical studies. The procedure for generation of attenuation map from CT images using different energy mapping methods was assessed using clinical studies and the results compared to the TX image derived using Ga-68 rod sources acquired on the Discovery LS PET/CT scanner, were used as gold standard in this study. A region of interest analysis was performed at different locations of the µmaps. It was shown that for soft tissues, the relative difference of scaling, segmentation, hybrid and bilinear methods compared to TX technique were 11.3%, 9.2%, 11.3% and 10.8% respectively (no major difference). For bony structures, the quantitative analysis showed that the scaling method produces a substantial relative difference (31%). The relative difference of segmentation, hybrid and bilinear methods compared to TX were 29%, 14% and 18% respectively. However these results for lung tissue were 4%, 13%, 4% and 4% respectively for scaling, segmentation, hybrid and bilinear methods with a great difference for segmentation method. It can be concluded that for soft tissues all energy mapping methods give satisfactory results. For bone, the scaling and segmentation methods yield substantial relative differences but the other 2 methods give acceptable results. For lung tissue the results are approximately close to each other except for segmentation method.
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