Nonuniform Attenuation Compensation Research Articles

Wavelet‐based denoising and its impact on analytical SPECT reconstruction with nonuniform attenuation compensation

AbstractIn single photon emission computed tomography (SPECT), the nonstationary Poisson noise in projection data (sinogram) is a major cause of compromising the quality of reconstructed images. To improve the quality, we must suppress the Poisson noise in the sinogram before or during image reconstruction. However, the conventional space or frequency domain denoising methods will likely remove some information that is very important for accurate image reconstruction, especially for analytical SPECT reconstruction with compensation for nonuniform attenuation. As a time‐frequency analysis tool, wavelet transform has been widely used in the signal and image processing fields and demonstrated its powerful functions in the application of denoising. In this article, we studied the denoising abilities of wavelet‐based denoising method and the impact of the denoising on analytical SPECT reconstruction with nonuniform attenuation. Six popular wavelet‐based denoising methods were tested. The reconstruction results showed that the Revised BivaShrink method with complex wavelet is better than others in analytical SPECT reconstruction with nonuniform attenuation compensation. Meanwhile, we found that the effect of the Anscombe transform for denoising is not significant on the wavelet‐based denoising methods, and the wavelet‐based de‐noise methods can obtain good denoising result even if we do not use Anscombe transform. The wavelet‐based denoising methods are the good choice for analytical SPECT reconstruction with compensation for nonuniform attenuation. © 2013 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 23, 36–43, 2013

Super resolution SPECT reconstruction with non-uniform attenuation

In this paper, based on Novikov's explicit inversion formula for the attenuated Radon transform, we present a super resolution SPECT reconstruction algorithm with compensation for non-uniform attenuation. Unlike the former methods improving the medical image resolution via super resolution (SR) in the reconstructed image, the proposed method apply the SR algorithm in the low resolution (LR) sinogram, which needs only 1-D shift of the detector, and the PSF is easy to obtain. Simulation results show that our reconstruction algorithm is effective.

A wavelet‐based SPECT reconstruction algorithm for nonuniformly attenuated Radon transform

Single photon emission computed tomography (SPECT) is a tomography technique that can greatly show information about the metabolic activity in the body and improve the clinical diagnosis. In SPECT, because of photoelectric absorption and Compton scattering, the emitted gamma photons are attenuated inside the body before arriving at the detector. The goal of quantitative SPECT reconstruction is to obtain an accurate image of the radioactivity distribution in the interested area of a human body, so the compensation for nonuniform attenuation and the treatment of Poisson noise are necessary in the quantitative SPECT reconstruction. The authors know that the wavelet transform has characteristics of multiresolution and localized nature, and these characteristics can be applied for denoising and localized reconstruction. Based on the explicit inversion formula for the attenuated Radon transform, the authors present a wavelet-based SPECT reconstruction algorithm with compensation for nonuniform attenuation. The wavelet-based SPECT reconstruction algorithm offers the ability for denoising in the reconstruction processing. In simulation, 128 projections were simulated evenly spaced over 360 degrees by a circular orbit, each with 128 bins. Simulation results show that the wavelet-based denoising is effective in SPECT reconstruction. The authors present a wavelet-based SPECT reconstruction algorithm with compensation for nonuniform attenuation. The reconstruction results from computer simulations show that the wavelet-based SPECT reconstruction algorithm is accurate.

Quantitative Rotating Multisegment Slant-Hole SPECT Mammography With Attenuation and Collimator-Detector Response Compensation

Rotating multisegment slant-hole (RMSSH) single photon emission computed tomography (SPECT) is suitable for detecting small and low-contrast breast lesions since it has much higher detection efficiency than conventional SPECT with a parallel-hole collimator and can image the breast at a closer distance. Our RMSSH SPECT reconstruction extends a previous rotation-shear transformation-based method to include nonuniform attenuation and collimator-detector response (CDR) compensation. To evaluate our reconstruction method, we performed two phantom simulation studies with 1) an isolated breast and 2) a breast phantom attached to the body torso. The reconstructed RMSSH SPECT images with attenuation and CDR compensation showed improved quantitative accuracy and less image artifacts than without. To evaluate the clinical efficacy of RMSSH SPECT mammography, we used a simulation study to compare with planar scintimammography in terms of the signal-to-noise ratio (SNR) value of a breast lesion. The RMSSH SPECT reconstruction images showed higher SNR value than the planar scintimammography images and even more so as we applied compensation for attenuation and collimator detector response. We conclude that attenuation and CDR compensation provide RMSSH SPECT mammography images with improved quality and quantitative accuracy.

FAST ANALYTICAL RECONSTRUCTION OF GATED CARDIAC SPECT WITH NON-UNIFORM ATTENUATION COMPENSATION

Conventionally, the inverse problem of gated cardiac SPECT is solved by reconstructing the images frame-by-frame, ignoring the inter-frame correlation along the time dimension. To compensate for the non-uniform attenuation for quantitative cardiac imaging, iterative image reconstruction has been a choice which could utilize an a priori constraint on the inter-frame correlation for a penalized maximum likelihood (ML) solution. However, iterative image reconstruction in the 4D space involves intensive computations. In this paper, an efficient method for 4D gated SPECT reconstruction is developed based on Karhune-Loève (KL) transform and Novikov's inverse formula. The temporal KL transform is first applied on the data sequence to de-correlate the inter-frame correlation and then the 3D principal components in the KL domain are reconstructed frame-by-frame using Novikov's inverse formula with non-uniform attenuation compensation. Finally an inverse KL transform is performed to obtain quantitatively-reconstructed 4D images in the original space. With the proposed method, 4D reconstruction can be achieved at a reasonable computational cost. The results from computer simulations are very encouraging as compared to conventional frame-by-frame filtered back-projection and iterative ordered-subsets ML reconstructions. By discarding high-order KL components for further noise reduction, the computation time could be further reduced.

Analytical reconstruction of 4-D dynamic cardiac SPECT with noise-reduction and non-uniform attenuation compensation

Background This study focuses on quantitative reconstruction of dynamic cardiac SPECT. Material and methods The Karhunen–Loeve (KL) transform is first applied to gated frames or sinogram data to de-correlate the dynamic information along the time direction. Then the Poisson noise is treated by a penalized weighted least-squares minimization, followed by an analytical inversion of the treated sinograms with attenuation compensation via the Novikov formula. The reconstruction is completed by inverse KL transform. Results Dynamic or gated cardiac sinograms were simulated from the NCAT phantom mimicking the human torso, and their reconstructions by the presented method showed significant improvement over the conventional methods of frame-by-frame reconstruction. Conclusion This analytical reconstruction of dynamic SPECT is consistent and efficient with a good potential for practical use.

An analytical algorithm for skew‐slit imaging geometry with nonuniform attenuation correction

The pinhole collimator is currently the collimator of choice in small animal single photon emission computed tomography (SPECT) imaging because it can provide high spatial resolution and reasonable sensitivity when the animal is placed very close to the pinhole. It is well known that if the collimator rotates around the object (e.g., a small animal) in a circular orbit to form a cone-beam imaging geometry with a planar trajectory, the acquired data are not sufficient for an exact artifact-free image reconstruction. In this paper a novel skew-slit collimator is mounted instead of the pinhole collimator in order to significantly reduce the image artifacts caused by the geometry. The skew-slit imaging geometry is a more generalized version of the pinhole imaging geometry. The multiple pinhole geometry can also be extended to the multiple-skew-slit geometry. An analytical algorithm for image reconstruction based on the tilted fan-beam inversion is developed with nonuniform attenuation compensation. Numerical simulation shows that the axial artifacts are evidently suppressed in the skew-slit images compared to the pinhole images and the attenuation correction is effective.

Attenuation correction for rotating multi-segment slant-hole SPECT in breast imaging

Rotating multi-segment slant-hole (RMSSH) SPECT is suitable for detecting small and low-contrast breast lesions since it has much higher detection efficiency compared with parallel-hole SPECT for the same resolution and can image the breast at a closer distance. Our RMSSH SPECT image reconstruction extends a previous rotation-shear transformation based 3D iterative reconstruction method to include non-uniform attenuation compensation. Reconstructed RMSSH SPECT images with attenuation compensation show much improved quantitative accuracy and less image artifacts than without. We conclude that attenuation compensation provides RMSSH SPECT images with improved quality and quantitative accuracy.

An efficient reconstruction method for nonuniform attenuation compensation in nonparallel beam geometries based on Novikov's explicit inversion formula

This paper investigates an accurate reconstruction method to invert the attenuated Radon transform in nonparallel beam (NPB) geometries. The reconstruction method contains three major steps: 1) performing one-dimensional phase-shift rebinning; 2) implementing nonuniform Hilbert transform; and 3) applying Novikov's explicit inversion formula. The method seems to be adaptive to different settings of fan-beam geometry from very long to very short focal lengths without sacrificing reconstruction accuracy. Compared to the conventional bilinear rebinning technique, the presented method showed a better spatial resolution, as measured by modulation transfer function. Numerical experiments demonstrated its computational efficiency and stability to different levels of Poisson noise. Even with complicated geometries such as varying focal-length and asymmetrical fan-beam collimation, the presented method achieved nearly the same reconstruction quality of parallel-beam geometry. This effort can facilitate quantitative reconstruction of single photon emission computed tomography for cardiac imaging, which may need NPB collimation geometries and require high computational efficiency.

Investigation of large field-of-view transmission imaging for SPECT attenuation compensation with Gd-153, Tc-99m and Ce-139 sources

A recently developed, large field-of-view (FOV), dual head SPECT system was investigated for fast sequential fan-beam (FB) transmission computed tomography (TCT) with various transmission sources. For TCT, the system utilizes a fixed line source at the 77 cm focal line of asymmetric FB collimator. The TCT maps are used for non-uniform attenuation compensation (NUAC) of Tc-99m myocardial SPECT. The heads each have three degrees of freedom (transaxial tilt, radial linear and transaxial linear travel). When the heads are tilted and height adjusted to accommodate different size FOVs (34-53 cm diameter), the symmetric FB geometry only samples part of the FOV. Thus, with full 360 degree acquisitions, the pseudo-asymmetric FB TCT detector utilizes measured conjugate views to avoid truncation artifacts without sacrificing spatial resolution, and minimizes emission contamination compared with other TCT geometries. Transmission line source energies of 100 to 166 keV (from Gd-153, Tc-99m, and Ce-139 line sources) were utilized with rod and sphere, and anthropsmorphic phantoms. The TCT images were reconstructed with iterative ordered subsets estimation maximization (OSEM). Comparisons were made between the emission reconstructions utilizing filtered backprojection (without and with iterative and multiplicative Chang NUAC) and OSEM (without and with NUAC). Based on a derived noise equivalent count rate determination of the three sources, the highest energy yet weakest intensity TCT source outperforms the other sources in the primary myocardial region of interest by approximately a factor of two. The emission contamination for the highest energy transmission source was an order of magnitude smaller than for the other sources with this acquisition geometry. These line sources each demonstrate the potential for suitably compensated images and quantitative myocardial activity profiles with this geometry. Higher energy transmission sources in particular result in more physically accurate attenuation maps useful for SPECT quantitation.

Energy-based compensation for nonuniform attenuation in Ga-67 SPECT imaging

Describes a method to estimate unattenuated projection images directly from Ga-67 SPECT data acquired over 360/spl deg/ at 3 photopeak energies. The algorithm compensates to third order for the effects of nonuniform attenuation, thereby yielding unattenuated projections with <3% accuracy, provided that scattered photons have been successfully subtracted from each energy window. Previously, the authors demonstrated an approach for using prior information to control noise in the estimated projections. They describe here an accelerated, robust algorithm for implementing these constraints. They have also evaluated the accuracy of the method for a region-of-interest (ROI) activity estimation task as a function of the residual error following scatter subtraction, and under conditions of nonstationary spatial resolution. When the maximum residual scatter subtraction errors at 93, 185, and 300 keV, respectively, were assumed to be 8%, 4%, and 2%, all ROI estimates were biased less than /spl plusmn/7% after attenuation compensation. Projection inconsistencies arising from distance-dependent geometric response and energy-dependent collimator penetration increased the standard deviation of ROI estimates (over location) to /spl sim/10%; however, methods of correcting for distance-dependent collimator response can probably be adapted well to Ga-67 imaging. If scatter can be adequately removed from the 3 Ga-67 photopeak windows, this technique is expected to provide reliable compensation for nonuniform attenuation with no need for an independently measured attenuation map.

Comparison of Hotelling observer models and human observers in defect detection from myocardial SPECT imaging

The objective of this study was to compare defect detection performance using a channelized Hotelling observer with radially symmetric channel (RSC) and oriented channels (OC) to that found in a previously reported human observer ROC study (see K.J. LaCroix and B.M.W. Tsui, "An evaluation ofthe effect of nonuniform attenuation compensation on defect detection for Tc-99m myocardial SPECT images," J. Nucl. Med., vol. 38, p. 19P, 1997). The observer's task involved the detection of left-ventricular myocardial perfusion defects for Tc-99m sestamibi myocardial perfusion SPECT images. The images were reconstructed using the filtered-backprojection (FBP) algorithm without attenuation compensation or the maximum-likelihood expectation-maximization (ML-EM) algorithm with non-uniform attenuation compensation (AC). This was performed for cases with and without significant attenuation artifacts. The areas under the ROC curves for the Hotelling observers were calculated and compared to those for the human observers. The ML-EM reconstructed images (with AC) had high defect detectability across all anatomy types, while with FBP, a lower detectability was found for cases where the reconstructed images contained attenuation artifacts in the myocardium. Similar trends between the channelized Hotelling observer and those from the human observer study were found with both radially symmetric channel (RSC) and oriented-channel (OC) models. Further investigation of the channel models is needed to determine the number and alignment of orientations and the number and cutoff of frequency bands to improve agreement between human and Hotelling observer results.

A spline-regularized minimal residual algorithm for iterative attenuation correction in SPECT

In SPECT, regularization is necessary to avoid divergence of the iterative algorithms used for non-uniform attenuation compensation. In this paper, we propose a spline-based regularization method for the minimal residual algorithm. First, the acquisition noise is filtered using a statistical model involving spline smoothing so that the filtered projections belong to a Sobolev space with specific continuity and derivability properties. Then, during the iterative reconstruction procedure, the continuity of the inverse Radon transform between Sobolev spaces is used to design a spline-regularized filtered backprojection method, by which the known regularity properties of the projections determine those of the corresponding reconstructed slices. This ensures that the activity distributions estimated at each iteration present regularity properties, which avoids computational noise amplification, thus stabilizing the iterative process. Analytical and Monte Carlo simulations are used to show that the proposed spline-regularized minimal residual algorithm converges to a satisfactory stable solution in terms of restored activity and homogeneity, using at most 25 iterations, whereas the non regularized version of the algorithm diverges. Choosing the number of iterations is therefore no longer a critical issue for this reconstruction procedure.

Bayesian SPECT lung imaging for visualization and quantification of pulmonary perfusion

In this paper, the authors quantitatively and qualitatively examine the use of a Gibbs prior in maximum a posteriori (MAP) reconstruction of SPECT images of pulmonary perfusion using the expectation-maximization (EM) algorithm. This Bayesian approach is applied to SPECT projection data acquired from a realistic torso phantom with spherical defects in the lungs simulating perfusion deficits. Both the scatter subtraction constant (k) and the smoothing parameter beta (/spl beta/) characterizing the prior are varied to study their effect on image quality and quantification. Region of interest (ROI) analysis is used to compare MAP-EM radionuclide concentration estimates with those derived from a implementation of filtered backprojection (CFBP), and a quantitative implementation of FBP (QFBP) utilizing nonuniform attenuation and scatter compensation. Qualitatively, the MAP-EM images contain reduced artifacts near the lung boundaries relative to the FBP implementations. Generally, the MAP-EM image's visual quality and the ability to discern the areas of reduced radionuclide concentration in the lungs depend on the value of /spl beta/ and the total number of iterations. For certain choices of /spl beta/ and total iterations, MAP-EM lung images are visually comparable to FBP. Based on profile and ROI analysis, SPECT QFBP and MAP-EM images have the potential to provide quantitatively accurate reconstructions when compared to CFBP. The computational burden, however, is greater for the MAP-EM approach. To demonstrate the clinical efficacy of the methods, the authors present pulmonary images of a patient with lung cancer.

ROC evaluation of SPECT myocardial lesion detectability with and without single iteration non-uniform Chang attenuation compensation using an anthropomorphic female phantom

The purpose of this work was to evaluate lesion detectability with and without nonuniform attenuation compensation (AC) in myocardial perfusion SPECT imaging in women using an anthropomorphic phantom and receiver operating characteristics (ROC) methodology. Breast attenuation causes artifacts in reconstructed images and may increase the difficulty of diagnosis of myocardial perfusion imaging in women. The null hypothesis tested using the ROC study was that nonuniform AC does not change the lesion detectability in myocardial perfusion SPECT imaging in women. The authors used a filtered backprojection (FBP) reconstruction algorithm and Chang's (1978) single iteration method for AC. In conclusion, with the authors' proposed myocardial defect model nuclear medicine physicians demonstrated no significant difference for the detection of the anterior wall defect; however, a greater accuracy for the detection of the inferior wall defect was observed without nonuniform AC than with it (P-value=0.0034). Medical physicists did not demonstrate any statistically significant difference in defect detection accuracy with or without nonuniform AC in the female phantom.

Comparison of transmission acquisition approaches for SPECT nonuniform attenuation compensation

This study compared two approaches for acquiring transmission computed tomography (TCT) data for SPECT nonuniform attenuation compensation. One approach, which has been implemented in commercial SPECT systems, acquires the TCT and SPECT data simultaneously using a scanning transmission line source, dual head SPECT system and parallel beam collimation (PB-sim). The other approach acquires the TCT and SPECT data sequentially using long focus, off-set fan beam collimation with a non-scanning line source and a triple head system (FB-seq). The two systems were compared based on: (a) the noise level of the TCT projection data, (b) the spatial resolution of the TCT projection data, and (c) the quality of reconstructed TCT and SPECT images of a thorax phantom. For the thorax phantom data a fast TCT scan (2 min.) was used and total scan time (TCT and SPECT) was the same for the two systems. The results from the TCT noise measurements showed that for the source activities used here (400 mCi for PB-sim, 56 mCi for FB-seq), PB-sim had higher count density in the lung region whereas FB-seq had higher in the heart and liver regions. The measured TCT spatial resolution for the two systems was comparable in the axial direction but was superior with FB-seq in the transaxial direction. The resolution difference was apparent in the reconstructed TCT images. These results suggest that the FB-seq system offers a viable approach for TCT acquisition and one that compares favorably with current commercial approaches based on TCT noise, resolution and reconstructed image quality.

Compensation for nonuniform attenuation in SPECT brain imaging

Accurate compensation for photon attenuation is needed to perform quantitative brain single-photon-emission computed tomographic (SPECT) imaging. Bellini's attenuation-compensation method (1979) has been used with nonuniform attenuation map to account for the nonuniform attenuation properties of the head. Simulation studies using a three-dimensional (3-D) digitized anthropomorphic brain phantom were conducted to compare quantitative accuracy of reconstructions obtained with the nonuniform Bellini method to that obtained with the Chang method (1978) and to iterative reconstruction using maximum-likelihood expectation maximization (ML-EM). Using the Chang method and assuming the head to be a uniform attenuator gave reconstructions with an average bias of approximately 6-8%, whereas using the Bellini or the iterative ML-EM method with a nonuniform attenuation map gave an average bias of approximately 1%. The computation time required to implement nonuniform attenuation compensation with the Bellini algorithm is approximately equivalent to the time required to perform one iteration of ML-EM. Thus, using the Bellini method with a nonuniform attenuation map provides accurate compensation for photon attenuation within the head, and the method can be implemented in computation times suitable for routine clinical use.

S-H5-03 Spect nonuniform attenuation compensation using transmission CT attenuation maps: Jaszczak, RJ. Departments of Radiology and Biomedical Engineering, Duke University (USA)

S-H5-03 Spect nonuniform attenuation compensation using transmission CT attenuation maps: Jaszczak, RJ. Departments of Radiology and Biomedical Engineering, Duke University (USA)

An analytical approach for compensation of non-uniform attenuation in cardiac SPECT imaging

Photon attenuation can reduce the diagnostic accuracy of cardiac SPECT imaging. Bellini et al. have previously derived a mathematically exact method to compensate for attenuation in a uniform attenuator. Since the human thorax contains structures with differing attenuation properties, non-uniform attenuation compensation is required in cardiac SPECT. Given an estimate of the patient attenuation map, we show that the Bellini attenuation compensation method can be used in cardiac SPECT to provide a quantitatively accurate reconstruction of a central region in the image which includes the heart and surrounding soft tissue. Simulations using a mathematical cardiac-torso phantom were conducted to evaluate the Bellini method and to compare its performance to the ML-EM iterative algorithm, and to 180 degrees and 360 degrees filtered backprojection (FBP) with no attenuation compensation. 'Bulls-eye' polar maps and circumferential profiles showed that both the Bellini method and the ML-EM algorithm provided quantitatively accurate reconstructions of the myocardium, with a substantial reduction in attenuation-induced artifacts that were observed in the FBP images. The computational load required to implement the Bellini method is approximately equivalent to that required for one iteration of the ML-EM algorithm, thus it is suitable for routine clinical use.

Comparison of 180/spl deg/ and 360/spl deg/ iterative reconstruction with non-uniform attenuation compensation for Tl-201 SPECT

This study compares Tl-201 SPECT images reconstructed from 180/spl deg/ and 360/spl deg/ projection data, using iterative reconstruction with non-uniform attenuation compensation. Using a mathematical phantom of the human thorax, Monte Carlo simulation is used to produce both 180/spl deg/ (45/spl deg/ RAO to 45/spl deg/ LPO) and 360/spl deg/ Tl-201 SPECT projection data. Included in the simulation are the effects of non-uniform attenuation, detector response, scatter and statistical noise. The 180/spl deg/ and 360/spl deg/ data were taken over 64 and 128 angles, respectively, assuming the same total acquisition time for both. The simulated projection data were reconstructed with nonuniform attenuation compensation using the ML-EM algorithm for 50 iterations and, for comparison, were also reconstructed without attenuation compensation using the filtered backprojection method with a ramp filter. Transaxial slices of the reconstructed images, image profiles and Bull's-eye plots all show that images reconstructed with attenuation compensation, unlike those reconstructed without attenuation compensation, are essentially the same within the region of the myocardium for the 180/spl deg/ and 360/spl deg/ data, with few artifacts and similar image contrast. Furthermore, for the same total imaging time, the images reconstructed from the 180/spl deg/ data have slightly less noise in the myocardial region.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>