Andy Welch

Toward accurate attenuation correction in SPECT without transmission measurements

The current trend in attenuation correction for single photon emission computed tomography (SPECT) is to measure and reconstruct the attenuation coefficient map using a transmission scan, performed either sequentially or simultaneously with the emission scan. This approach requires dedicated hardware and increases the cost (and in some cases the scanning time) required to produce a clinical SPECT image. Furthermore, if short focal-length fan-beam collimators are used for transmission imaging, the projection data may be truncated, leading to errors in the attenuation coefficient map. Our goal is to obtain information about the attenuation distribution from only the measured emission data by exploiting the fact that only certain attenuation distributions are consistent with a given emission dataset. Ultimately this consistency information will either be used directly to compensate for attenuation or combined with the incomplete information from fan-beam transmission measurements to produce a more accurate attenuation coefficient map. In this manuscript the consistency conditions (which relate the measured SPECT data to the sinogram of the attenuation distribution) are used to find the uniform elliptical attenuation object which is most consistent with the measured emission data. This object is then used for attenuation correction during the reconstruction of the emission data. The method is tested using both simulated and experimentally acquired data from uniformly and nonuniformly attenuating objects. The results show that, for uniform elliptical attenuators, the consistency conditions of the SPECT data can be used to produce an accurate estimate of the attenuation map without performing any transmission measurements. The results also show that, in certain circumstances, the consistency conditions can prove useful for attenuation compensation with nonuniform attenuators.

Image reconstruction for a novel SPECT system with rotating slant-hole collimators

We are investigating the use of rotating slant-hole (RSH) collimators on conventional SPECT machines. The main interest in this configuration is the potential for increased photon sensitivity over standard parallel-hole collimator systems. Projection data from an RSH-SPECT system presents a novel but tractable image reconstruction problem. Special features of RSH-SPECT, such as the dimensions of the field-of-view and the rotation requirements of the detector head, are discussed, and image reconstruction is presented in some detail. Data from a Jaszczak cardiac phantom were acquired on an experimental RSH system and reconstructed. Limited-angle artifacts were clearly seen on images reconstructed from a single set of data (classical RSH). The reconstructions from multi-view datasets verify the full tomographic capability of the RSH-SPECT system.<<ETX>>

An investigation of the effect of finite system resolution and photon noise on the bias and precision of dynamic cardiac SPECT parameters

The exchange of a radionuclide (and similarly of oxygen) between blood and the myocardium is a dynamic process. Dynamic SPECT offers the possibility of directly quantifying the kinetic parameters which describe this process. In this paper we investigate and quantify the effect of typical SPECT system resolution and photon counting statistics on the bias and precision of dynamic cardiac SPECT parameters. System resolution and photon noise are only two of the image degrading processes which occur in SPECT. Therefore, the bias and precision quoted should be viewed as a lower limit on those which could be expected with an experimental study. Dynamic SPECT projection data are simulated using a realistic human torso phantom. Data are simulated assuming both perfect system resolution and a system resolution typical of a clinical SPECT system. A triple detector SPECT system which acquires a full set of projection data in 10 s using continuous detector motion is modelled. Kinetic parameters are estimated using a number of myocardial regions of interest. The results show that the rate constant characterizing the washing of activity into the myocardium is more sensitive to region of interest position than is the washout rate constant. The bias and precision of the dynamic parameters are estimated from multiple realizations of projection data exhibiting various noise levels. The main effect of increased photon noise in the projection data is to decrease the precision of the estimated parameters. However, there is also some evidence that for high noise levels the bias of the parameters may also be affected.

Comparison of three applications of ConTraSPECT

Today, the trend in SPECT attenuation correction is to acquire a transmission data set and reconstruct the corresponding attenuation map, for incorporating into the emission reconstruction. Recently, some techniques have been developed to determine the attenuation map directly from the emission data, by exploiting the fact that in the absence of noise only some specific transmission data can be consistent with given emission data. The consistency conditions relating the measured emission data to the sinogram of the attenuation distribution could theoretically allow recovery of the exact attenuation map. However it is necessary to include some a priori information so that the consistency conditions can be used stably. In this paper, three types of a priori information are used: a uniform ellipse, a uniform spline curve, and a uniform spline curve knowing the untruncated part of the transmission sinogram. Once the corresponding attenuation map has been computed, it is used to correct for attenuation in the emission image reconstruction process. The three methods have been tested on both simulations and experimentally acquired data. The results show that the two first methods give equivalent acceptable results, while the third method provides reconstructions which better fit baseline reconstructions obtained when the attenuation map is computed from acquired transmission data.

Implementation of a model-based nonuniform scatter correction scheme for SPECT

Four scatter-compensation schemes are considered. The 4 schemes are all based on a previously developed two-dimensional (2-D) scatter model. Reconstruction is achieved using the iterative expectation-maximization maximum-likelihood (EM-ML) algorithm. The schemes consist of: (1) including the model in both the forward and back projector; (2) just including the model in the forward projector; (3) and (4) implementing the model in a subtraction and addition scheme, respectively. Monte Carlo simulated projection data are used to test the accuracy, convergence properties, and noise properties of the 4 scatter-compensation schemes. Data are simulated for both uniformly and nonuniformly attenuating objects. The results show that all 4 correction schemes yield images which are similar in terms of accuracy to that obtained from reconstructing scatter-free data. The subtraction scheme is shown to converge faster than the other compensation schemes, both in terms of iterations and actual time required for reconstruction. The scheme in which the model is only used in the forward-projector and the scatter-addition scheme both performs slightly better, in terms of signal-to-noise ratio (SNR), than the subtraction scheme. However, the forward projector scheme requires significantly more CPU time for reconstruction. The correction scheme in which the scatter model was included in both the forward and backprojectors is shown to produce accurate images with SNRs higher than even a perfect scatter rejection scheme. While the scatter correction scheme with the model in both the forward projector and backprojector has superior noise properties to the other algorithms, the results suggest that the faster subtraction/addition schemes will probably prove most useful for routine clinical scatter compensation.

Accurate attenuation correction in SPECT without transmission measurements

The current trend in attenuation correction for SPECT is to measure and reconstruct the attenuation coefficient map using a transmission scan, performed either sequentially or simultaneously with the emission scan. This approach requires dedicated hardware and increases the cost (and in some cases the scanning time) required to produce a clinical SPECT image. Furthermore, if short focal length fan-beam collimators are used for transmission imaging, the projection data may be truncated, leading to errors in the attenuation coefficient map. Our goal is to obtain information about the attenuation distribution from only the measured emission data by exploiting the fact that only certain attenuation distributions are consistent with a given emission dataset. Ultimately this consistency information will either be used directly to compensate for attenuation or combined with the incomplete information from fan-beam transmission measurements to produce a more accurate attenuation coefficient map. The simulations and phantom studies performed in this investigation show that, in certain circumstances, the consistency conditions of the SPECT data can be used to produce an accurate estimate of the attenuation map without performing any transmission measurements.

A comparison of Gd/Tc versus Tc/Tl simultaneous transmission and emission imaging using both single and triple detector fan-beam SPECT systems

In this study, the authors compare the accuracy of the transmission and emission maps, produced using both a Gd/Tc and a Tc/Tl transmission-emission source combination. In both cases, the transmission and emission data are acquired simultaneously. This results in contamination of the data acquired in the lower energy window by events from the higher energy isotope. The authors use standard methods to correct for this contamination when using a Gd/Tc combination and propose a new method for use with the Tc/Tl combination. The Tc/Tl combination yields more accurate transmission images than the Gd/Tc combination. However, when a single factor is used to scale the attenuation coefficients measured using the transmission source to those appropriate for the emission isotope/energy, the resulting error in the attenuation map used for attenuation correction is larger for the Tc/Tl combination than for the Gd/Tc combination. For this study, neither combination yielded emission maps which were significantly superior (in terms of reconstructed emission ratios) to the other. >

Energy window optimization in simultaneous technetium-99m TCT and thallium-201 SPECT data acquisition

In simultaneous Tc-99m TCT and Tl-201 SPECT data acquisition, the emission images are degraded by not only scatter from the emission source, but also cross-contamination from the transmission photons. Also, the transmission images are degraded by scatter from the transmission source and cross-contamination from the emission photons. In this work, a criterion based on maximizing a figure-of-merit (FOM) was used to reduce image degradations by optimizing the position and width of the energy windows used in the data acquisition in a triple-camera fan-beam SPECT system. The FOM determines a compromise between increased detection efficiency of primary photons and reduction of scatter and cross-contamination photons. Scatter in the projection data was modeled using Monte Carlo (MC) simulation methods. Experimental studies were performed to verify the simulations and to estimate the amount of Pb X-rays and scatter in the collimator which were not modeled in the MC program. Results suggest that cross-contamination degrades the emission and transmission data. However, the contamination does not significantly alter the optimal energy window settings, which should be centered at /spl sim/77 keV with a window width of /spl sim/34% for detecting the Ti emission photons, and centered at 140 keV with a window width of /spl sim/20% for detecting the Tc transmission photons. >

An investigation of dual energy transmission measurements in simultaneous transmission emission imaging

In this study we investigate the feasibility of acquiring dual energy transmission projection data using a conventional gamma camera. Transmission projection data are acquired, simultaneously with /sup 99m/Tc emission data, using the two major photopeaks of /sup 153/Gd, at 44 keV and 99 keV. The degree of contamination of the data in the 44 keV window, by scattered 99 keV transmission photons, is investigated using Monte Carlo simulations. A modified scaling method is proposed to convert the two attenuation maps, obtained from transmission data acquired at 44 keV and 99 keV, to one appropriate for /sup 99m/Tc. This attenuation map is compared to one produced by scaling the 99 keV attenuation map, for the attenuation correction of /sup 99m/Tc SPECT images. The use of dual energy transmission imaging is shown to improve the quantitative accuracy of the reconstructed SPECT images compared to a single scale factor method. However, for situations where a dual scaling factor method is possible, this is shown to perform as well as the dual energy method. Dual energy transmission imaging is shown to be more effective at visualizing areas of bone, particularly in regions near the edge of the untruncated field of view.

Performance evaluation of a transmission reconstruction algorithm with simultaneous transmission-emission SPECT system in a presence of data truncation

A simultaneous transmission-emission SPECT system (STEP) was developed on a three-detector gamma camera (Picker Prism 3000) equipped with fan-beam collimators (65 cm focal length) and a transmission line source. With this system, fan-beam geometry can cause transmission projection data to be truncated. An iterative transmission reconstruction algorithm was formulated to determine the distribution of attenuation coefficients from the system of linear equations for only measured projections. In this paper we evaluated this algorithm using phantom data with varying degree of data truncation. The results showed that with up to 30% truncation, differences in partial attenuation integrals in the non-truncated region were statistically not significant (p<0.05). Also, a study was performed to determine the minimal number of iterations necessary to obtain quantitatively accurate results. It was shown that partial attenuation integrals were not significantly different (p<0.05) when 9 to 100 iterations were performed. We conclude that the described transmission reconstruction algorithm using nine iterations is quantitatively accurate and is able to correct for the truncation of the data.<<ETX>>