M.A. King

Optimization of iterative reconstructions of /sup 99m/Tc cardiac SPECT studies using numerical observers

In this paper, we investigate the use of a numerical observer to optimize ordered-subset expectation maximization (OSEM) reconstructions for the detection of coronary artery disease (CAD). The parameters optimized were the iteration number and the full-width at half-maximum of three-dimensional Gaussian postfiltering. The numerical observer employed in the optimization was the channelized Hotelling observer (CHO). The CHO had been used previously to rank tumor detection accuracy for different reconstruction strategies in Ga-67 images, showing good agreement with the rankings of human observers. The intent of this paper was to determine if this CHO could also be employed for the detection of CAD. Results indicate that when grayscale (quantized) images are used, the CHO optimization results correlate well with human observers. On the other hand, when the CHO was used with floating-point images, it provided very good detection performance even when the images were excessively filtered. This result was at odds with the human-observer performance which showed a decrease in detection accuracy with highly smoothed images. This reflects the need to better model the detection task of the human observers who usually view and rank grayscale images and by appropriately modeling the image noise that quantization introduces, we show that the CHO can better match human-observer detection performance.

Study of relative quantification of Tc-99 m with partial volume effect and spillover correction for SPECT oncology imaging

The apparent concentration of activity in structures in nuclear medicine images depends on their size relative to the system spatial resolution. This dependence is called the partial volume effect (PVE). Spillover (SO) or the blurring into a structure of counts originating in nearby structures also alters the apparent concentration of activity. In combination these effects impact the detection of lesions and quantification of activity within structures in the slices. The increased accessibility of dual-modality imaging systems makes available high-resolution anatomical information which is registered with the emission slices and can be used in correcting for the PVE and SO. In this study we investigated the use of the template projection-reconstruction method for correction of the PVE and SO. We examined the impact of correction on visual image quality and the quantification of activity in simulated spheres of varying contrast relative to a uniform background distribution of activity. Our enhancements to the template projection-reconstruction methodology included both an improvement in the matching of the blurring in the reconstructed templates of structures to the actual blurring in the reconstructed slices, and accounting for the fractional presence of structures in SPECT voxels. We determined that such corrections for the PVE and SO can dramatically improve both the visualization and quantification of activity within the source distributions we investigated.

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.

Optimization of regularization of attenuation and scatter corrected /sup 99 m/Tc cardiac SPECT studies for defect detection using hybrid images

Through means of an ROC study, the authors optimize the iteration number and 3-D Gaussian post-filtering of /sup 99 m/Tc cardiac emission OSEM reconstructions that implement corrections for both attenuation and scatter. Hybrid images were used for this optimization wherein perfusion defects were added artificially to clinical patient studies that were read as being normally perfused. The test conditions included 3 different iteration numbers of OSEM (1, 5 and 10), followed by 3-D Gaussian low-pass filtering at each iteration level. The level of Gaussian low-pass filtering was varied using standard deviations (/spl sigma/) of 0.6, 0.75, 1 and 1.25 pixels, in addition to a case where no post-filtering was applied. Four observers read 80 images for each of the 15 test conditions being investigated, providing confidence ratings as to the presence or absence of perfusion defects. Results indicate a slowly varying trend between very little filtering and quite heavy levels of smoothing with a gentle plateau for post-filters in the range of /spl sigma/=0.6 to 1 pixel. No significant improvement in detection accuracy was observed with increasing iteration number as long as the reconstructions are post-filtered with /spl sigma/ in the range of 0.6 to 1 pixel, suggesting that 1 complete iteration of OSEM should suffice.

Implementing visual search in human-model observers for emission tomography

Human observers have been described as statistical ideal observers with internal noise, but the reliance on prior statistical information for these model observers creates several difficulties. We present initial tests of a human-model framework for emission tomography that attempts to overcome these issues with prior information by considering specific features of the given test images. This framework is based on models of visual search (VS) for radiology, in which readings occur as a sequence involving fast scanning to identify candidate abnormalities followed by a lengthier focused analysis of these candidates. The identification of suspicious sites by the model observer is directed by the blob morphology of the given test image. Blob proximity to the boundaries of the region(s) of interest was an additional factor. A channelized NPW observer was used for the subsequent analysis of the suspicious locations. The VS model observer was compared against human observers in forced-choice studies with PET and SPECT images. Overall performances showed good model correlation, although comparisons of image-by-image responses show substantial differences between the human and VS observers.

FOUR-DIMENSIONAL RECONSTRUCTION OF GATED CARDIAC SPECT WITH ATTENUATION AND SCATTER COMPENSATION

In our recent work we proposed a motion-compensated approach for four-dimensional (4D) reconstruction of gated cardiac images in single photon emission tomography (SPECT), and demonstrated that motion-compensated temporal smoothing can be effective for noise reduction in the reconstructed images. In this paper we further develop our proposed 4D approach by considering attenuation and scattering which are two important degradation factors in SPECT imaging. Our simulation results demonstrate that with proper attenuation and scatter compensation, the 4D method can lead to improved reconstruction over independent reconstruction with spatial smoothing only. Our evaluation study was conducted by simulating Tc99m labeled sestamibi imaging based on the NURBS-based cardiac-torso (NCAT) phantom

Assessment of scatter compensation strategies for /sup 67/Ga tumor SPECT using numerical observers and human LROC studies

Ga-67 citrate SPECT imaging is often used for oncological studies in order to diagnose or stage patient lymphomas. Because the decay of Ga-67 involves multiple emission energies, it is possible that many down-scattered photons will be present in photopeak acquisition data. We have previously shown through human observer LROC studies, that the inclusion of these scattered photons significantly degrades lesion detectability in simulations. We have investigated the use of six different scatter compensation methods representing different strategies. These consist of i) perfect scatter rejection, ii) no scatter compensation, iii) ideal scatter compensation, iv) triple energy window estimation, v) effective scatter source estimation, and vi) post-reconstruction scatter subtraction. Each method has first been optimized using a channelized hotelling numerical observer, then ranked through the use of a human LROC study and by using a newly devised LROC numerical observer. Both human LROC and LROC numerical observer results indicate that both TEW and ESSE scatter compensation methods are able to improve lesion detectability over no compensation, but fail to achieve similar detectability to using perfect scatter rejection. Excellent agreement between the LROC numerical observer and human LROC studies indicate that the LROC observer may be good predictor of human performance in Ga-67 SPECT.

The impact of noisy attenuation maps and patient motion on human-observer performance at Ga-67 lesion detection in SPECT

The authors have demonstrated an improvement due to attenuation correction (AC) at the task of lesion detection in thoracic SPECT images. However, increased noise in the transmission data due to aging sources or very large patients, and misregistration of the emission and transmission maps, can reduce the benefits of AC and may result in a loss of lesion detectability. The authors investigated the impact of noise in and misregistration of transmission data, on the detection of Ga-67 thoracic lesions. Human-observer LROC methodology was used to assess performance. Both emission and transmission data were simulated using the MCAT computer phantom. Images were reconstructed using OSEM incorporating AC and detector resolution compensation. Clinical count levels were used in the emission data. The transmission-data noise levels ranged from zero (noise-free) to 32 times measured clinical levels. Transaxial misregistrations of 0.32, 0.63, and 1.27 cm between emission and transmission data were also examined. Results indicate that a 20-fold increase in the noise was required to eliminate the benefit afforded by AC but that smaller increases in noise could be detrimental, especially for low-contrast lesions. Misregistration errors are also a concern as even small errors here greatly reduce the performance gains of AC.

Accounting for anatomical noise in SPECT with a visual-search human-model observer

Reliable human-model observers for clinically realistic detection studies are of considerable interest in medical imaging research, but current model observers require frequent revalidation with human data. A visual-search (VS) observer framework may improve reliability by better simulating realistic etection-localization tasks. Under this framework, model observers execute a holistic search to identify tumor-like candidates and then perform careful analysis of these candidates. With emission tomography, anatomical noise in the form of elevated uptake in neighboring tissue often complicates the task. Some scanning model observers simulate the human ability to read around such noise by presubtracting the mean normal background from the test image, but this backgroundknown- exactly (BKE) assumption has several drawbacks. The extent to which the VS observer can overcome these drawbacks was investigated by comparing it against humans and a scanning observer for detection of solitary pulmonary nodules in a simulated SPECT lung study. Our results indicate that the VS observer offers a robust alternative to the scanning observer for modeling humans.

Estimation of the rigid-body motion from images using a generalized center-of-mass points approach

We present an analytical method for the estimation of rigid-body motion in three-dimensional SPECT and PET slices. This method utilizes mathematically defined generalized center-of-mass points in images, requiring neither segmentation nor an iterative process. It can be applied to compensation of the rigid-body motion in both SPECT and PET. We generalized the formula for the center-of-mass and obtained a family of points co-moving with the objects rigid-body motion. In calculation of the generalized center-of-mass points and estimation of the rigid-body motion, we optimized a Gaussian smoothing function and chose the best three points, which resulted in the minimum root-mean-square difference between images. The estimated motion was used to generate a summed image, or incorporated in the iterative reconstruction of the motion-present data. To evaluate this method for different noise levels, we performed simulations with the MCAT phantom. We observed that though noise degraded the motion-detection accuracy, this method helped in reducing the motion artifact both visually and quantitatively. We also acquired four sets of the emission and transmission data of the Data Spectrum Anthropomorphic Phantom positioned at four different locations and/or orientations. From these we generated a composite acquisition simulating phantom movements during acquisition. The simulated motion was calculated from the generalized center-of-mass points on the images reconstructed from individual acquisitions. We determined that motion-compensation greatly reduced the motion artifact. Finally, in a simulation with the gated MCAT phantom, an exaggerated rigid-body motion was applied to the end-systolic frame. The motion was estimated from the end-diastolic and end-systolic images, and used to sum them into a summed image without obvious artifact. As an image-driven approach this method assumes angularly complete data sets for each state of motion. We expect this method to be applied in correction of the respiratory motion in respiratory gated SPECT and respiratory or other rigid-body motion in PET.