Attenuation Correction Errors Research Articles

Improving C-band weather radar reflectivity attenuation based on two-dimensional video disdrometer data

A method was proposed for correcting the attenuation of C-band weather radar reflectivity data based on a 2-D video disdrometer. The relationship between the C-band weather radar reflectivity attenuation rate k and radar reflectivity factor Z was derived using radar meteorological equations. A video disdrometer was used to correct the deformation of large (1 to 9 mm) raindrops, and the coefficients a and b of the k − Z relationships for different precipitation types under Rayleigh scattering and Mie scattering conditions were inverted separately. Data from a weather detection instrument and rain gauge were combined with Newton’s iteration method for database-by-database accumulative corrections of the C-band radar reflectivity factor. Observed C-band weather radar data recorded in Xichang city and video disdrometer data from Xichang, Dechang, Mianning, and Xide counties during 2016 and 2017 were selected (all areas are in Liangshan Prefecture, Sichuan Province). The relationships between the raindrop size distribution and the radar reflectivity factor and attenuation rate in stratiform and convective cloud precipitation were analyzed, and the k − Z relationships under different precipitation types in Liangshan were presented. Additionally, radar reflectivity attenuation correction errors were analyzed using observed C-band weather radar data under strong convective storm precipitation. The proposed method achieves far smaller root mean square errors of the radar reflectivity factor than traditional attenuation correction methods in both the radial and the vertical directions. Overall, the proposed method has a better attenuation correction effect for C-band weather radar reflectivity data than traditional attenuation correction methods, which may enable the popularization and application of the proposed method elsewhere.

Effect of gradient field nonlinearity distortions in MRI-based attenuation maps for PET reconstruction

PurposeAttenuation correction is a requirement for quantification of the activity distribution in PET. The need to base attenuation correction on MRI instead of CT has arisen with the introduction of integrated PET/MRI systems. The aim was to describe the effect of residual gradient field nonlinearity distortions on PET attenuation correction. MethodsMRI distortions caused by gradient field nonlinearity were simulated in CT images used for attenuation correction in PET reconstructions. The simulations yielded radial distortion of up to ±2.3mm at 15cm from the scanner isocentre for distortion corrected images. The mean radial distortion of uncorrected images were 6.3mm at the same distance. Reconstructions of PET data were performed using the distortion corrected images as well as the images where no correction had been applied. ResultsThe mean relative difference in reconstructed PET uptake intensity due to incomplete distortion correction was less than ±5%. The magnitude of this difference varied between patients and the size of the distortions remaining after distortion correction. ConclusionsRadial distortions of 2mm at 15cm radius from the scanner isocentre lead to PET attenuation correction errors smaller than 5%. Keeping the gradient field nonlinearity distortions below this limit can be a reasonable goal for MRI systems used for attenuation correction in PET for quantification purposes. A higher geometrical accuracy may, however, be warranted for quantification of peripheral lesions. These distortions can, e.g., be controlled at acceptance testing and subsequent quality assurance intervals.

The need for quantitative PET in multicentre studies

Positron emission tomography (PET) was developed in the seventies of the last century with the first commercial scanner becoming available in 1978. Originally, this novel tomographic imaging technique was seen and used as a quantitative method for measuring human physiological and pathophysiological processes at regional level. The two most important characteristics of PET were its quantitative nature and its extremely high sensitivity, which allowed measurements down to picomolar level. Over the following 10–20 years the molecular processes that could be investigated using PET expanded rapidly, ranging from perfusion through metabolism, preand post-synaptic receptor density and affinity, neurotransmitter release, and enzyme activity, to drug delivery and uptake. Given that PET, as mentioned, was characterised by two unique features (quantification and sensitivity), it is of interest to note that by the turn of the century one of them, sensitivity, was starting to be applied in clinical medicine, in particular its sensitivity to detect (distant) metastases using F-FDG. This led to a significant improvement in staging, which had direct consequences with regard to the choice of appropriate therapy (e.g. avoidance of futile surgery). As a result PET became the fastest growing medical technology, which was in sharp contrast to the situation in the mid-nineties, when the field (and the manufacturing industry) had been struggling to survive. Clearly, to detect metastases (by finding hot spots), quantification was not necessary, as the purpose of a PET staging study is simply to establish whether they are there or not. In addition, it was convenient that scanning could be performed according to a protocol that is standard in clinical nuclear medicine practice, i.e. inject the tracer, wait for sufficient uptake in tissue and finally position and scan the patient. For F-FDG, this waiting period is 1 h and, from the perspective of patient throughput, it is very convenient that the scanner is not occupied during this uptake period. It is unfortunate and somewhat ironic, however, that one of the unique characteristics of PET (sensitivity) has indirectly been the reason why the other (quantification) is less recognised and indeed unknown to many clinicians who became familiar with PET through staging studies, in which quantification is not required. On the basis of its success in staging, however, PET is now seen as a very promising technique for monitoring and even predicting response to therapy. Indeed, F-FDG PET is already used as a surrogate endpoint in the development of new anticancer drugs. In addition, other more specific tracers are increasingly being used in drug development, both in oncology and in neurology. In this context, it is very tempting to implement the same methodology that is used in F-FDG staging studies, i.e. the application of a static or whole-body protocol at some time after injection of the tracer. It should be noted that there are many aspects to quantification that need to be taken into account, i.e. scanner, physics and patient-related factors [1]. A prerequisite for quantification is that the scanner be equipped with accurate randoms, scatter and attenuation correction procedures. This may seem trivial, but it is well-known that the most recent scanners (i.e. PET/MRI) are associated with substantial errors in attenuation correction, even in the Color figures online at http://link.springer.com/article/10.1007/ s40336-014-0074-y

Raindrop Size Distribution (DSD) Retrieval for X-Band Dual-Polarization Radar

Abstract A raindrop size distribution (DSD) retrieval method for a dual-polarization radar at attenuating frequency is proposed. The proposed method is developed such that the range profiles of the gamma DSD parameters, an intercept parameter Nw (mm−1 m−3), and a median volume diameter D0 (mm) can be estimated to match the dual-polarization measurements, measured equivalent reflectivity at horizontal polarization ZHm, measured differential reflectivity ZDRm, and measured differential propagation phase ΦDPm, where the forward scattering and backscattering are formulated simultaneously to avoid the two-step process of attenuation correction and DSD retrieval. Additionally, the proposed method does not have the attenuation-correction errors accumulated along range that traditional forward and backward processes have, since the range profiles of the DSD parameters are optimized in a radar beam simultaneously. In the simulation, the proposed algorithm showed fairly good accuracies for retrievals Nw and D0. Errors with the different axis ratio models or calibration biases in ZHm and ZDRm, which contaminate assumptions of the proposed method in real observational data, were also evaluated. Under a Gaussian fluctuation model, the estimation process, known as an iterative maximum-likelihood estimator, derives the best estimation in the statistical fluctuation conditions. This scheme could be extended to duplicative observation such as a radar network environment.

An investigation of the challenges in reconstructing PET images of a freely moving animal

Imaging the brain of a freely moving small animal using positron emission tomography (PET) while simultaneously observing its behaviour is an important goal for neuroscience. While we have successfully demonstrated the use of line-of-response (LOR) rebinning to correct the head motion of confined animals, a large proportion of events may need to be discarded because they either 'miss' the detector array after transformation or fall out of the acceptance range of a sinogram. The proportion of events that would have been measured had motion not occurred, so-called 'lost events', is expected to be even larger for freely moving animals. Moreover, the data acquisition in the case of a freely moving animal is further complicated by a complex attenuation field. The aims of this study were (a) to characterise the severity of the 'lostevents' problem for the freely moving animal scenario, and(b) to investigate the relative impact of attenuation correction errors on quantitative accuracy of reconstructed images. A phantom study was performed to simulate the uncorrelated motion of a target and non-target sourcevolume. A small animal PET scanner was used to acquirelist-mode data for different sets of phantom positions. The list-mode data were processed using the standard LOR rebinning approach, and multiple frame variants of this designed to reduce discarded events. We found that LOR rebinning caused up to 86 % 'lost events', and artifacts that we attribute to incomplete projections, when applied to a freely moving target. This fraction was reduced by up to 18 % using the variant approaches, resulting in slightly reduced image artifacts. The effect of the non-target compartment on attenuation correction of the target volume was surprisingly small. However, for certain poses where the target and non-target volumes are aligned transaxially in the field-of-view, the attenuation problem becomes more complex and sophisticated correction methods will be required. We conclude that there are limitations with the LOR rebinning approach and simplified attenuation correction for freely moving animals requiring the development and validation of more sophisticated approaches.

Motion artifacts occurring at the lung/diaphragm interface using 4D CT attenuation correction of 4D PET scans

For PET/CT, fast CT acquisition time can lead to errors in attenuation correction, particularly at the lung/diaphragm interface. Gated 4D PET can reduce motion artifacts, though residual artifacts may persist depending on the CT dataset used for attenuation correction. We performed phantom studies to evaluate 4D PET images of targets near a density interface using three different methods for attenuation correction: a single 3D CT (3D CTAC), an averaged 4D CT (CINE CTAC), and a fully phase matched 4D CT (4D CTAC). A phantom was designed with two density regions corresponding to diaphragm and lung. An 8 mL sphere phantom loaded with 18F‐FDG was used to represent a lung tumor and background FDG included at an 8:1 ratio. Motion patterns of sin(x) and sin4(x) were used for dynamic studies. Image data was acquired using a GE Discovery DVCT‐PET/CT scanner. Attenuation correction methods were compared based on normalized recovery coefficient (NRC), as well as a novel quantity “fixed activity volume” (FAV) introduced in our report. Image metrics were compared to those determined from a 3D PET scan with no motion present (3D STATIC). Values of FAV and NRC showed significant variation over the motion cycle when corrected by 3D CTAC images. 4D CTAC‐ and CINE CTAC–corrected PET images reduced these motion artifacts. The amount of artifact reduction is greater when the target is surrounded by lower density material and when motion was based on sin4(x). 4D CTAC reduced artifacts more than CINE CTAC for most scenarios. For a target surrounded by water equivalent material, there was no advantage to 4D CTAC over CINE CTAC when using the sin(x) motion pattern. Attenuation correction using both 4D CTAC or CINE CTAC can reduce motion artifacts in regions that include a tissue interface such as the lung/diaphragm border. 4D CTAC is more effective than CINE CTAC at reducing artifacts in some, but not all, scenarios.PACS numbers: 87.57.qp, 87.57.cp

SU‐GG‐J‐166: Reduction of Motion Artifacts Occurring at the Lung/diaphragm Interface Using 4D‐CT Attenuation Correction On 4D‐PET Scans: Implications for Radiation Therapy Planning

Purpose: For PET/CT the fast CT acquisition time can lead to errors in attenuation correction, particularly at the lung/diaphragm interface. Gated 4D‐PET can reduce motion artifacts, though it is not always attenuation corrected with a similarly acquired 4D‐CT. We performed phantom studies specifically designed to evaluate 4D‐PET images using three different methods for attenuation correction: a single 3D‐CT (3D‐CTAC), an averaged 4D‐CT (CINE‐CTAC), and a fully phase matched 4D‐CT (4D‐CTAC). Method and Materials: A phantom was designed with two density regions corresponding to diaphragm and lung. An 8ml vial loaded with FDG was used to represent a lung tumor and background FDG was also added with an 8:1 ratio. Imaging was performed with a GE Discovery DVCT‐PET/CT scanner. Periodic motion of 2 cm amplitude was used and the image data was reconstructed into 10 phase bins over the motion cycle. Image data was analyzed using a GE image processing workstation and in‐house developed software. Values of activity within the target for each relative phase were corrected for decay and compared to those derived from a 3D PET scan with no motion present (3D‐STATIC). Results: Activity values derived from 4D‐CTAC corrected PET images are generally closer to 3D‐STATIC images with no motion present. Mean activity over the known target volume over the motion cycle, normalized to the activity for 3D‐STATIC, was 93%, 101% and 98% for 3D‐CTAC, CINE‐CTAC and 4D‐CTAC images respectively with corresponding standard deviations of 6%, 10% and 3%. Conclusion: Compared to other attenuation correction methods, 4D‐CTAC corrected 4D‐PET images correspond more closely on average to similar 3D‐STATIC images. Using CINE‐CTAC for correction resulted in mean activity values slightly closer to 3D‐STATIC, but with much greater variation over the motion cycle. We believe these results have implications for the use of 4D‐PET imaging for radiation therapy target definition.

Cine CT for Attenuation Correction in Cardiac PET/CT

In dual-modality PET/CT systems, the CT scan provides the attenuation map for PET attenuation correction. The current clinical practice of obtaining a single helical CT scan provides only a snapshot of the respiratory cycle, whereas PET occurs over multiple respiratory cycles. Misalignment of the attenuation map and emission image because of respiratory motion causes errors in the attenuation correction factors and artifacts in the attenuation-corrected PET image. To rectify this problem, we evaluated the use of cine CT, which acquires multiple low-dose CT images during a respiratory cycle. We evaluated the average and the intensity-maximum image of cine CT for cardiac PET attenuation correction. Cine CT data and cardiac PET data were acquired from a cardiac phantom and from multiple patient studies. The conventional helical CT, cine CT, and PET data of an axially translating phantom were evaluated with and without respiratory motion. For the patient studies, we acquired 2 cine CT studies for each PET acquisition in a rest-stress (13)N-ammonia protocol. Three readers visually evaluated the alignment of 74 attenuation image sets versus the corresponding emission image and determined whether the alignment provided acceptable or unacceptable attenuation-corrected PET images. In the phantom study, the attenuation correction from helical CT caused a major artifactual defect in the lateral wall on the PET image. The attenuation correction from the average and from the intensity-maximum cine CT images reduced the defect by 20% and 60%, respectively. In the patient studies, 77% of the cases using the average of the cine CT images had acceptable alignment and 88% of the cases using the intensity maximum of the cine CT images had acceptable alignment. Cine CT offers an alternative to helical CT for compensating for respiratory motion in the attenuation correction of cardiac PET studies. Phantom studies suggest that the average and the intensity maximum of the cine CT images can reduce potential respiration-induced misalignment errors in attenuation correction. Patient studies reveal that cine CT provides acceptable alignment in most cases and suggest that the intensity-maximum cine image offers a more robust alternative to the average cine image.

Quality control for SPECT systems.

A comprehensive set of acceptance tests is the basis for an effective quality control program. Acceptance tests indicate whether the scintillation camera meets published specifications but should also include evaluation of certain parameters not specified by the vendor but that can seriously affect image quality. Once a camera is "accepted," initial quality control tests serve as a benchmark for future measurements. Tomographic systems must be subjected to the same basic quality control program as planar cameras. Flood field images must be acquired and evaluated daily; spatial resolution must be tested once each week. Less frequently, parameters such as multiple-window spatial registration, collimator uniformity, system sensitivity, and camera "dead time" should be evaluated. Single photon emission computed tomographic systems require additional tests to ensure optimum performance. Center-of-rotation calibration and verification of detector registration are needed to avoid losses of spatial resolution. Flood corrections based on high-count images eliminate residual detector nonuniformiaties and correct for subtle collimator defects. Pixel size must be calibrated to avoid errors in attenuation correction and distortion when cardiac and brain reorientation software is used. Completed clinical studies must be checked for unacceptable patient movement and incomplete views. A quality control program may be time-consuming, but, from the standpoint of maximum benefit to patients and confidence in clinical interpretations, there is no alternative.