Real-time estimation of excess atmospheric attenuation using an artificial neural network with a two-frequency input
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A simple artificial neural network is considered for real-time estimation of excess atmospheric attenuation on a satellite communication link with known attenuation at two frequencies. All atmospheric contributors to attenuation are considered except for gases. The network has a two-layer feed-forward structure with 32 neurons in the hidden layer. Its performance is evaluated by computer simulation using 447 hours of measured attenuation data at 20, 40, and 50 GHz. Estimated attenuation tracks well the measured attenuation at 50 GHz. Estimation error standard deviation is 0.36 dB. RMS error is a function of attenuation: it increases slowly with attenuation, but the ratio of error to attenuation decreases with increasing attenuation. This approach accurately estimates excess attenuation without requiring assumptions, but required training data. (4 pages)Keywords:
Correction for attenuation
This paper describes and compares procedures to obtain attenuation maps used for the absorption correction (AC) of PET brain scans if a transmission scan is not available as in the case of future MR-PET scanners. A previously reported approach called MBA (MRT-based attenuation correction) used T1- weighted MR images which were segmented into four tissue types representing brain tissue, bone, other tissue and sinus to which appropriate attenuation coefficients were assigned. In this work a template-based attenuation correction (TBA) is presented which applies an attenuation template to single subjects. A common attenuation template was created from transmission scans of 10 normal volunteers and spatially normalized to the SPM2 standard brain shape. For each subject the T1-MR template of SPM2 was warped onto the subject's individual MR image. The resulting warping matrix was applied to the common attenuation template so that an attenuation map matching the subject's brain shape was obtained. The attenuation maps of MBA and TBA were forward projected into attenuation factors which were alternatively used for AC. FDG scans of four subjects were reconstructed after AC with MBA and TBA and compared to images whose ACs were based on conventional attenuation maps (PBA=PET-based attenuation correction). Using PBA as refer- ence in a region of interest analysis, MBA and TBA showed similar under- and overestimation of the reconstructed radioac- tivity up to −10% and 9%, respectively. The procedure to obtain the attenuation template needs still some improvements. Never- theless, the TBA method of attenuation correction is a promising alternative to MBA with its still complex and not yet resolved accurate segmentation of MR images.
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Attenuation correction is necessary for the reconstruction of SPECT images. One report has mentioned that attenuation correction by X-ray computed tomography (CT) is effective for a non-uniform attenuation body. We examined the effect of attenuation correction on SPECT images by changing the scanning conditions of CT, and evaluated the possibility of attenuation correction by low-dose CT. The phantom was scanned under several X-ray tube conditions varying from 80 kV to 135 kV and from 7.5 mAs to 200 mAs. We obtained equations of attenuation correction based on the Hounsfield Unit (HU) units of each pixel and compared the effects of attenuation correction. The results showed that the equation for attenuation correction under each condition did not vary significantly, and the effects of attenuation correction by the equations did not vary significantly between CT of low dose and that of clinical dose. This result suggest that the attenuation correction obtained by low-dose CT was equal to that obtained by the clinical dose. In conclusion, it seemed that the equation and map of attenuation correction matched with each radionuclide yielded more adequate attenuation correction than conventional methods.
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Hounsfield scale
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There has been considerable debate about the desirability of attenuation correction in whole-body PET oncology imaging. The advantages of attenuation correction are quantitative accuracy, whereas the perceived disadvantages are loss of contrast, noise amplification, and increased scanning time. In this work, we explain contrast changes between images reconstructed with and without attenuation correction.To analytically explain both well-known and surprising phenomena in images reconstructed without attenuation correction, we performed a series of simulation studies, a phantom experiment, and a patient experiment.We showed that it is possible to calculate a priori the appearance of images reconstructed without attenuation correction. Compared with attenuation-corrected images, images without attenuation correction may have locally enhanced contrast in the abdomen or other regions of uniform attenuation, although the amount of enhancement varies with position in a complex manner. In regions of nonuniform attenuation, such as the thorax, it is possible that foci of increased tracer uptake disappear in images reconstructed without attenuation correction. The critical tracer concentration for this zero-contrast effect depends on the size, location, and density of the foci. Above the critical value, foci are visible in images with and without attenuation correction, whereas below the critical value, foci are visible in attenuation-corrected images but appear as photopenic regions in images without attenuation correction.Even though images without attenuation correction may be desired, these results suggest that all studies should at least be reconstructed with attenuation correction to avoid missing regions of elevated tracer uptake.
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Correction for attenuation
Cardiac PET
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Attenuation correction in SPECT has been used for uniformly absorptive objects like the head. On the other hand, it has seldom been applied to nonuniform absorptive objects like the heart and surrounding lungs because of the difficulty and inaccuracy of data processing. However, since attenuation correction using a transmission source recently became practical, we were able to apply this method to a nonuniform absorptive object. Therefore, we evaluated the usefulness of this attenuation correction system with a transmission source in myocardial SPECT. The dose linearity, defect/normal ratio using a myocardial phantom, and myocardial count distribution in clinical cases was examined with and without the attenuation correction system. We found that all data processed with attenuation correction were better than those without attenuation correction. For example, in myocardial count distribution, while there was a difference between men and women without attenuation correction, which was considered to be caused by differences in body shape, after processing with attenuation correction, myocardial count distribution was almost the same in all cases. In conclusion, these results suggested that attenuation correction with a transmission source was useful in myocardial SPECT.
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In PET imaging, attenuation and scatter corrections are an essential requirement to accurately quantify the radionuclide uptake. In the context of PET/MR scanners, obtaining the attenuation information can be challenging. Various authors have quantified the effect of an imprecise attenuation map on the reconstructed PET image but its influence on scatter correction has usually been ignored.
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In this study, we assessed the importance of attenuation correction by quantitative evaluation of errors associated with attenuation in myocardial SPECT in a phantom study. To do attenuation correction we use an attenuation map derived from X-ray CT data. The succession of attenuation correction highly depends on high quality of attenuation maps. CT derived attenuation map in related to non-uniform attenuation correction is used to do transmission dependent scatter correction. The OSEM algorithm with attenuation model was developed and used for attenuation correction during image reconstruction. Finally a comparison was done between reconstructed images using our OSEM code and analytical FBP method. The results of measurements show that: Our programs are capable to reconstruct SPECT images and correct the attenuation effects. Moreover to evaluate reconstructed image quality before and after attenuation correction we applied a famous approach using Image Quality Index. Attenuation correction increases the quality and quantity factors in both methods. This increasing is independent of activity in quantity factor and decrease with activity in quality factor. Both quantitative and qualitative of SPECT images were improved by attenuation correction. In both OSEM and FBP the activity ratio of heart phantom in comparison with the markers was very increased. So the attenuation correction in fat patients and low activity is recommended. Attenuation correction with CT images and OSEM reconstruction in the condition of complete registration yields superior results.
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Correction for attenuation
Tomographic reconstruction
Emission computed tomography
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Diagnosis , staging and treatment of disease depends on the morphological and functional information obtained from multimodality molecular imaging systems. The combination of functional and morphological information is now routinely performed to overcome the limitations of each individual modality. Attenuation of photons in the object under study is one of the main limitations of quantitative PET imaging. Attenuation correction plays a pivotal role in PET imaging. However, the availability of CT data on hybrid PET/CT scanners made it possible to build an accurate attenuation map. One of the well-known methods for generation of the attenuation map on PE/MRI systems is MR-based attenuation correction (MRAC) where image segmentation is used to classify MRI into several classes corresponding to different attenuation factors. In this study we investigate the effect of using different numbers of classes for the generation of attenuation maps on the accuracy of attenuation correction of PET data. The study was carried out using simulations of the XCAT phantom and 10 clinical studies. For the later, CT and PET images of 10 patients were used with CT-based attenuation correction assumed as reference. MRI was classified into different classes to produce two, three and four-class attenuation maps using the ITK library. The relative error showed that the lower number of classes will increase the global error over 8%. The elimination of bony structures from the attenuation map will cause a local error over 3%. In clinical studies, SUV mean and SUV max were calculated for each AC method. The results seem to indicate an underestimation of 11% because of neglecting bone.
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PET quantification requires accurate correction for the attenuation of 511 keV photons in tissue. In PET/CT imaging, the attenuation map is derived from a CT image. Contrary to CT, a direct conversation from MRI intensity values to attenuation coefficients is not applicable. Hence, attenuation correction remains one of the major challenges in the development of PET-MR scanners. Many studies have shown the feasibility of MR-based attenuation correction in clinical practice. However, some drawbacks remain. Alternatively, methods have been suggested to derive the attenuation map from the emission data or by using an external transmission source inside the FOV of the PET scanner. In this work three approaches based on transmission and/or emission data were evaluated and compared with a simulation study using GATE. In the first approach an annulus shaped transmission source was inserted inside the FOV of the PET scanner. An iterative MLTR-MLEM algorithm was used to reconstruct the attenuation and the PET image sequentially. In the second approach only the emission data is used and the attenuation coefficients and the PET image are reconstructed simultaneously with the MLAA algorithm. Finally, an MLAA+ method is proposed in which both the emission and transmission data are used for determining the attenuation map and the PET image simultaneously. Results show that the use of TOF information in the emission-based approach is mandatory and the algorithm can be improved significantly by including additional transmission data coming from an external source, especially in cases where the attenuation medium is not fully supported by the activity distribution. Additionally, the presence of the transmission data allows compensation for the low-frequency cross-talk between the reconstructed attenuation coefficients and the PET image. The absolute error of reconstructed attenuation coefficients in the lungs, soft tissue and spine was respectively more then 40%, 15% and 9% higher when only emission data was used. Contrary, in the MLTR approach, where the transmission-data is extracted from the emission data using TOF information, misclassifications cause inaccuracies in the reconstructed attenuation maps and higher inter-tissue variance in the reconstructed PET image compared to emission based methods. These effects are also reduced when the attenuation map and activity distribution are reconstructed simultaneously.
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