Manoj Narayanan

LROC analysis of detector-response compensation in SPECT

Localization ROC (LROC) observer studies examined whether detector response compensation (DRC) in ordered-subset, expectation-maximization (OSEM) reconstructions helps in the detection and localization of hot tumors. Simulated gallium (Ga-67) images of the thoracic region were used in the study. The projection data modeled the acquisition of attenuated 93- and 185-keV photons with a medium-energy parallel-hole collimator, but scatter was not modeled. Images were reconstructed with five strategies: (1) OSEM with no DRC; (2) OSEM preceded by restoration filtering; (3) OSEM with iterative DRC; (4) OSEM with an ideal DRC; and (5) filtered backprojection (FBP) with no DRC. All strategies included attenuation correction. There were four LROC studies conducted. In a study using a single tumor activity, the ideal DRC offered the best performance, followed by iterative DRC, restoration filtering, OSEM with no DRC, and FBP. Statistical significance at the 5% level was found between all pairs of strategies except for restoration filtering and OSEM with no DRC. A similar ranking was found for a more realistic study using multiple tumor activities. Additional studies considered the effects of OSEM iteration number and tumor activity on the detection improvement that iterative DRC offered with respect to OSEM with no DRC.

Evaluation of a compartmental model for estimating tumor hypoxia via FMISO dynamic PET imaging.

This paper systematically evaluates a pharmacokinetic compartmental model for identifying tumor hypoxia using dynamic positron emission tomography (PET) imaging with 18F-fluoromisonidazole (FMISO). A generic irreversible one-plasma two-tissue compartmental model was used. A dynamic PET image dataset was simulated with three tumor regions-normoxic, hypoxic and necrotic-embedded in a normal-tissue background, and with an image-based arterial input function. Each voxelized tissues time activity curve (TAC) was simulated with typical values of kinetic parameters, as deduced from FMISO-PET data from nine head-and-neck cancer patients. The dynamic dataset was first produced without any statistical noise to ensure that correct kinetic parameters were reproducible. Next, to investigate the stability of kinetic parameter estimation in the presence of noise, 1000 noisy samples of the dynamic dataset were generated, from which 1000 noisy estimates of kinetic parameters were calculated and used to estimate the sample mean and covariance matrix. It is found that a more peaked input function gave less variation in various kinetic parameters, and the variation of kinetic parameters could also be reduced by two region-of-interest averaging techniques. To further investigate how bias in the arterial input function affected the kinetic parameter estimation, a shift error was introduced in the peak amplitude and peak location of the input TAC, and the bias of various kinetic parameters calculated. In summary, mathematical phantom studies have been used to determine the statistical accuracy and precision of model-based kinetic analysis, which helps to validate this analysis and provides guidance in planning clinical dynamic FMISO-PET studies.

Application of the Karhunen-Loeve transform to 4D reconstruction of cardiac gated SPECT images

Reconstruction of gated SPECT images is intrinsically a four-dimensional problem. Gated SPECT studies are normally reconstructed frame by frame; thus, the time frames are treated independently. This approach fails to exploit the strong signal correlations among the time frames which are critical for noise reduction and resolution recovery. The authors investigated two reconstruction approaches, developed for dynamic PET by Wernick et al. (1997, 1999), which utilize the compression and decorrelation properties of the Karhunen-Loeve (KL) transform. In Method I, the authors temporally filter the data by using only the first few KL components, then perform frame-by-frame reconstruction. In Method II, the authors KL transform the projection data, reconstruct only the significant KL component images, then perform an inverse KL transformation to obtain the full 4D image sequence. Results indicate that Methods I and II provide better noise performance than ordinary frame-by-frame reconstruction. Additionally, Method II requires substantially fewer computations than conventional reconstruction methods.

CHAPTER 22 – Attenuation, Scatter, and Spatial Resolution Compensation in SPECT

This chapter explains methods of correcting for complicating factors in the single-photon emission computed tomography (SPECT) image reconstruction process, namely, attenuation and scatter. SPECT imaging is not ideal. Inherent in SPECT imaging are degradations that distort the projection data. This chapter focuses on three such degradations and the compensation for them. The first is attenuation— in order for a photon to become part of a measured projection, it must escape the body, the second source of degradation is the inclusion of scatter in the projections, and the third source of degradation is the finite, distance-dependent spatial resolution of the imaging system. It is noted that, solely compensating for attenuation is not enough to improve SPECT image quality to its fullest extent. Instead, attenuation, scatter, resolution, correction of patient, physiological motion, and changes in localization during the course of acquisition, can impact image quality, and thus combined compensation is required.

An interior point iterative maximum-likelihood reconstruction algorithm incorporating upper and lower bounds with application to SPECT transmission imaging

The algorithm we consider here is a block-iterative (or ordered subset) version of the inferior point algorithm for transmission reconstruction. Our algorithm is an interior point method because each vector of the iterative sequence {x/sup k/}, k= 0, 1, 2,..., satisfies the constraints a/sub j/<x/sub j//sup k/<b/sub j/, j=1,..., J. Because it is a block-iterative algorithm that reconstructs the transmission attenuation map and places constraints above and below the pixel values of the reconstructed image, we call it the BITAB method. Computer simulations using the three-dimensional mathematical cardiac and torso phantom, reveal that the BITAB algorithm in conjunction with reasonably selected prior upper and lower bounds has the potential to improve the accuracy of the reconstructed attenuation coefficients from truncated fan beam transmission projections. By suitably selecting the bounds, it is possible to restrict the over estimation of coefficients outside the fully sampled region. that results from reconstructing truncated fan beam projections with iterative transmission algorithms such as the maximum-likelihood gradient type algorithm.

Effect of block-iterative acceleration on Ga-67 tumor detection in thoracic SPECT

A combination of human localization receiver operating characteristic (LROC) and channelized Hotelling observer (CHO) ROC psychophysical studies were used to investigate how accelerated ordered-subset expectation maximization (OSEM) and rescaled block-iterative (RBI) EM reconstruction affect tumor detection in simulated Ga-67 SPECT images, The tumors were 1-cm-diameter spheres within the chest region of the three-dimensional mathematical cardiac-torso phantom. Previous work with iterative detector resolution compensation showed that eight iterations of the OSEM algorithm with a subset size of eight (16 subsets) offered optimal observer performance. For the LROC study in this paper, the OSEM and RBI algorithms were implemented using subset sizes P and iterations K that satisfied the relation P=K for P=1, 2, 4, and 8. The CHO was applied to reconstruction strategies that deviated from this relation. Results show that using P/spl les/2 penalized observer performance compared to strategies with larger subset sizes. Other researchers have reported on the more stable convergence and noise properties of the RBI algorithm [(Byrne, 1996) and (Lalush and Tsui, 2000)]. In a similar vein, we found that an RBI strategy with a subset size of P produced the same performance as an OSEM strategy with subset size 2P. As neither algorithm displayed a decisive advantage in speed over the other, we conclude that the RBI algorithm is the better choice for accelerating the Ga-67 reconstructions.

Assessing a system to detect patient motion in SPECT imaging using stereo optical cameras

Patient motion, which causes artifacts in reconstructed images, can be a serious problem in SPECT imaging. If patient motion can be detected and quantified, the reconstruction algorithm can compensate for the motion. Most previous approaches to detecting patient motion have relied on only the acquired projection data, using, for example, consistency checks or motion-tracking to detect motion. Our approach is based on optical tracking of the patient using a pair of web cameras to acquire stereo images. The stereo images are analyzed by a visual tracking system (VTS) that detects changes in the stereo images over time to track locations on the patient surface. Patient surface motion can then be used to infer motion within the patient body, which will be used to correct for patient motion. The system consists of a three-headed SPECT system and two web cameras connected to a PC.

Comparison of scatter compensation strategies for myocardial perfusion imaging using Tc-99m labeled sestamibi

Scatter can be a confounding problem in the detection of perfusion defects in myocardial perfusion imaging. The goal of this investigation was to compare practical methods of scatter compensation representing different classes of compensation strategies. The methods investigated were: (1) modification of the energy window; (2) use of effective values of attenuation coefficients; (3) estimating the scatter distribution from energy spectrum information; and (4) estimating the scatter distribution in the spatial domain using the attenuation maps and scatter kernels. Monte Carlo simulated projections from two MCAT source distributions were used to investigate the impact of the various methods on residual scatter and myocardial uniformity. The investigation showed that each of the methods reduce the scatter significantly; however, the spatial domain method gave the least variation with the different source distributions and the best uniformity.

An iterative transmission algorithm incorporating cross-talk correction for SPECT.

Simultaneous emission/transmission acquisitions in cardiac SPECT with a Tc 99m / Gd 153 source combination offer the capability for nonuniform attenuation correction. However, cross-talk of Tc 99m photons downscattered into the Gd 153 energy window contaminates the reconstructed transmission map used for attenuation correction. The estimated cross-talk contribution can be subtracted prior to transmission reconstruction or incorporated in the reconstruction algorithm itself. In this work, we propose an iterative transmission algorithm (MLTG-S) based on the maximum-likelihood gradient algorithm (MLTG) that explicitly accounts for this cross-talk estimate. Clinical images were acquired on a three-headed SPECTcamera, acquiring Tc 99m emission and Gd 153 transmission images simultaneously. Subtracting the cross-talk estimate prior to transmission reconstruction can result in negative and zero values if the estimate is larger than or equal to the count in the transmission projection bin, especially with increased attenuator size or amount of cross-talk. This results in inaccurate attenuation coefficients for MLTG reconstructions with cross-talk subtraction. MLTG-S reconstructions on the other hand, yield better estimates of attenuation maps, by avoiding the subtraction of the cross-talk estimate. Comparison of emission slices corrected for nonuniform attenuation reveals that inaccuracies in the reconstructed attenuation map caused by cross-talk can artificially enhance the extra-cardiac activity, confounding the ability to visualize the left-ventricular walls.

An optimization of reconstruction parameters and investigation into the impact of photon scatter in /sup 67/Ga SPECT

During /sup 67/Ga citrate SPECT imaging, photon downscatter will occur from higher energy photons into lower energy acquisition windows thus possibly adversely affecting reconstructed image quality. With these additional scattered photons present in projection data, the effect of using more complex reconstruction strategies such as three-dimensional detector response compensation (3-D DRC) and attenuation correction (AC) is unclear. Using a combination of numerical channelized hotelling observers (CHO) and human localization receiver operating characteristics (LROC) studies it has been found that maximum lesion detectability occurs when projection data is reconstructed using two iterations of the rescaled block iterative (RBI) algorithm with both 3-D DRC and patient specific AC, followed by a postreconstruction low-pass 3-D Gaussian filtering with a FWHM of /spl ap/1 cm. These parameters deviate from optimal reconstruction parameters for primary photon-only projection data which finds that maximum lesion detection occurs after 4 RBI iterations with the same 3-D DRC, AC, and postfiltering. As expected, it is also observed that decreased tumor detectability results when scattered photons are present in /sup 67/Ga projection data compared to primary photon only reconstructions (reducing maximum A/sub L/ from 0.79 to 0.58). This decrease has been found to be statistically significant in both human LROC and numerical observer studies, suggesting the need for scatter compensation during /sup 67/Ga citrate imaging.