Evolution for Bone Collimator-Detector Response Compensation in Iterative SPECT Imaging Reconstruction Algorithm

Transcription

1 GE Healthcare Evolution for Bone Collimator-Detector Response Compensation in Iterative SPECT Imaging Reconstruction Algorithm Version 1.0

2 Introduction The quality and quantitative accuracy of SPECT reconstructed images are affected by noise in the projection data, resolution degradation caused by the collimator-detector response (CDR) function of the imaging system, and photon attenuation and scatter in patient s body. Recently, significant progress has been achieved in the development of model-based corrective SPECT image reconstruction methods that include correction for these image quality-degrading factors. The corrective image reconstruction package (also referred to as Resolution Recovery) developed by JHU group provides improved SPECT images by including physical models of the imaging process into iterative image reconstruction algorithm. In particular, it provides accurate 3D models of the collimatordetector response functions for variety of clinical applications. This results in significant enhancement in image quality and potential improvement in diagnostic properties. Clinical application of these techniques are available and can be applied to reduced time SPECT acquisitions with equal or better image characteristics when compared to standard reconstructions (16). Collimator-detector response in SPECT Collimator-detector blur is the main factor affecting the resolution and noise properties of nuclear medicine images. In reconstructed SPECT images, these effects are strongly affected by the applied reconstruction algorithm and its parameters. In SPECT acquisition with parallel collimation, images of a point source located at different distances from the collimator fully describe (CDR) function for specific collimator design, photon energy and energy window parameters, and distance of the imaged object from collimator surface, as demonstrated in figure 1. Figure 1. SPECT acquisition geometry and collimator-detector response. CDR consists of four main components: intrinsic response (system without collimator) and the geometric, septal penetration and septal scatter components of the collimator parameters. The geometric component is the portion of the CDR resulting from photons by passing through the collimator holes without interaction with the collimator. The septal penetration component describes portion of the CDR due to photons reaching the detector after passing through one or more septa without scattering. The scatter component is due to photons that scatter in the septa and are then detected. For the majority of clinically used collimator designs, the last two components are generally significant for only mediumand high-energy imaging (see fig.2). The response functions are spatially invariant in planes parallel to the face of the collimator and that the intrinsic response is approximately spatially invariant in the detection plane.

3 Figure 3. Scheme of iterative reconstruction process. Compensation requires that a table of CDR functions at all relevant distances to collimator surface be pre-calculated and retrieved, or computed on the fly, during reconstruction. Onthe-fly calculations involve using the theoretical predicted collimator geometric response for an equivalent round hole shape [1, 2], and ignore penetration and septal scatter effects. These approximations work - well for conventional hexagonal shaped holes and low-energy isotopes, such as Tl and Tc- 99m. Figure 2. Examples of CDR function and its components. For each combination of acquisition system, radiopharmaceutical and particular acquisition protocol, the collimator detector response function provides the probability that a photon emitted from any point of the imaged object will contribute to a pixel of the resulting image. A CDR compensation technique was developed at the University of North Carolina Chapel Hill (UNC) and Johns Hopkins University (JHU) [3, 4, 5]. In fast rotation based iterative reconstruction implementations, CDR compensation is achieved by convolving the projected ray with corresponding Point Spread Function (PSF) in the course of projection and back-projection operations (Fig.4) Accurate predictions of the geometric response function for various collimator designs were derived [1, 2]. Penetration and scatter components of the collimator detector response function can be obtained and analyzed using Monte-Carlo simulation methods. Compensation for collimatordetector response in iterative reconstruction By including an accurate model of collimator-detector response function in an iterative SPECT reconstruction algorithm, the blurring effect may be included in the iterative reconstruction process, resulting in improved special resolution. Figure 4. Modeling of collimator-detector response in a rotation based projector. Adapted from: E.C. Frey and B.M.W. Tsui, Collimator-detector response compensation in SPECT In order to incorporate CDR into the reconstruction process, the following information is utilized during the course of image reconstruction:

4 Collimator design parameters : holes length and diameter, septa thickness Detector characteristics : intrinsic resolution, crystal thickness, collimator-detector gap Acquisition parameters: center-of-rotation to collimator face distances for every projection view acquired. This information for a variety of different collimators is stored in an additional look-up table which is part of the reconstruction package. Effect of collimator detector response compensation on reconstructed images data when no compensation is applied. On the other hand, image noise level tends to amplify with iteration number. Taking into account the differences in the iterative reconstruction process, with and without CDR, applicationspecific optimization of acquisition and processing parameters is essential to successful utilization of corrective reconstruction method [12, 13, 14, 15 ] (Fig.6) It was demonstrated (Fig.6) that application of corrective image reconstruction methods results in improved uniformity of myocardium wall in cardiac SPECT and specifically CDR compensation provides better recovery of lesion contrast in variety of clinical applications [8, 16]. The effect of CDR compensation on reconstructed images has been studied from qualitative [7], quantitative [8, 9, 10], and clinical task-based [11, 13, 15] prospective. It has been found that CDR corrected images demonstrate not only improved resolution and signal-to-noise ratio, but also lower noise variability in reconstructed images (Fig. 5). It has also been shown that the noise texture with CDR compensation changes, exhibiting wider correlations than non-corrected images, resulting in improved resolution without corresponding increase in noise. CDR is quite sensitive to incorrect modeling of the CDR function or the use of incorrect acquisition parameters. It is important that these parameters are accurately passed from the camera to the iterative reconstruction process. Figure 6. Reconstructed clinical Tc-99m myocardium SPECT images. Top line - attenuation correction only applied, bottom line attenuation and CDR compensation. Numbers mark number of updates (product of iterations number and subsets number) applied. From: E.C. Frey and B.M.W. Tsui, Collimator-detector response compensation in SPECT Mathematical and human observer studies demonstrated improvement in cold and hot lesion detectability with application of CDR compensation [15, 16]. Clinical Application CDR has been implemented on the Xeleris workstation for use in the Bone SPECT and Whole Body SPECT protocols. The application of CDR in shorter acquisition times has been shown to demonstrate equivalent or improved image quality over standard, full time acquisitions (16). Image improvement of half time CDR reconstructed SPECT over full time standard OSEM iterative reconstruction is demonstrated in (fig. 9). Figure 5. Hot sphere phantom images. Left: schematic presentation of insert locations; middle: slice of conventional reconstruction, right: slice of corrective reconstruction with collimator-detector response compensation. No post filtering was applied. Significant improvement of the resolution properties requires more iterations of the reconstruction method with compensation than are usually recommended for the same

5 Figure 8. Changes in Area under ROC curve (AUC) across acquisition/processing combinations for selected 10mm 2:1 contrast lesions results from Channelized Hotelling Observer study. CDR OSEM Figure 7. Examples of reconstructed image slices of Tc-99m bone SPECT containing 10 mm 6:1 contrast lesions; simulated data. Figures 7 and 8 show results from an investigation of image properties where application of CDR compensation gave rise to studies of potential usefulness of this compensation in shorter than usual clinical acquisition time to ensure nondegraded diagnostic quality [16, 17]. Figure 9. Demonstrated improvement in WB SPECT half time acquisition over standard OSEM iterative reconstruction process

6 Conclusion Detailed description of the effect of the collimator detector response on properties and diagnostic quality of SPECT images is a subject of extensive research. So far, significant evidence has been accumulated that diagnostic quality of NM images is substantially improved by incorporation of collimator-detector system modeling into iterative image reconstruction. References 1. Metz, C.E., The geometric transfer function component for scintillation camera collimators with straight parallel holes. Phys. Med. Biol., (6): p Tsui, B.M.W. and G.T. Gullberg, The Geometric Transfer- Function for Cone and Fan Beam Collimators. Physics in Medicine and Biology, (1): p Tsui, B.M.W., et al., Implementation of simultaneous attenuation and detector response correction in SPECT. IEEE Transactions on Nuclear Science, (1): p Tsui, B.M.W., et al., The importance and implementation of accurate three-dimensional compensation methods for quantitative SPECT. Phys Med Biol, (3): p Tsui, B.M.W., et al., Characteristics of reconstructed point response in three-dimensional spatially variant detector response compensation in SPECT, in Three-Dimensional Image Reconstruction in Radiology and Nuclear Medicine, P. Grangeat and J.-L. Amans, Editors. 1996, Kluwer Academic Publishers. p Wilson, D.W. and H.H. Barrett, The effects of incorrect modeling on noise and resolution properties of ML-EM images. Nuclear Science, IEEE Transactions on, (3): p Tsui BMW, Zhao XD*, Frey EC and McCartney WH. Quantitative SPECT: Basics and Clinical Considerations. Seminar in Nuclear Medicine, Vol. XXIV, No 1 (January), pp 38-65, Pretorius, P.H., et al., Reducing the influence of the partial volume effect on SPECT activity quantitation with 3D modelling of spatial resolution in iterative reconstruction. Physics in Medicine and Biology, (2): p Kohli, V., et al., Compensation for distance-dependent resolution in cardiac-perfusion SPECT: impact on uniformity of wall counts and wall thickness. Nuclear Science, IEEE Transactions on, (3): p Pretorius, P.H., et al., Comparison of detection accuracy of perfusion defects in SPECT for different reconstruction strategies using polar-map quantitation. Ieee Transactions on Nuclear Science, (5): p Narayanan, M.V., et al., Human-observer receiveroperating-characteristic evaluation of attenuation, scatter, and resolution compensation strategies for Tc-99m myocardial perfusion imaging. Journal of Nuclear Medicine, (11): p Sankaran, S., et al., Optimum compensation method and filter cutoff frequency in myocardial SPECT: A human observer study. Journal of Nuclear Medicine, (3): p Frey, E.C., K.L. Gilland, and B.M.W. Tsui, Application of taskbased measures of image quality to optimization and evaluation of three-dimensional reconstruction-based compensation methods in myocardial perfusion SPECT. IEEE Transactions on Medical Imaging, (9): p Gifford, H.C., et al., LROC analysis of detector-response compensation in SPECT. IEEE Transactions on Medical Imaging, (5): p He, X, et al., A mathematical observer study for the evaluation and optimization of compensation methods for Myocardial SPECT using a phantom population that realistically models patient variability. IEEE Transactions on Nuclear Science, (1): Volokh, L., et al., Efficacy of corrective reconstruction with collimator detector response compensation for short Tc- 99m bone SPECT acquisition in a bone lesion detection task./ Abstract presented at SNM Meeting, 2005/ 17. Keidar, Z. et al., Half-time bone SPECT acquisition - Assessment of a new Collimator Detector Response (CDR) reconstruction algorithm / Abstract submitted to RSNA Meeting, 2005 / 18. Frey, E.C., Tsui, B.M.W., Collimator-Detector Response Compensation in SPECT. in: Quantitative Analysis in Nuclear Medicine Imaging. Springer 2005 GE Healthcare Waukesha, WI U.S.A General Electric Company All rights reserved. GE Healthcare, a division of General Electric Company. General Electric Company reserves the right to make changes in specifications and features shown herein, or discontinue the product described at any time without notice or obligation. Contact your GE Representative for the most current information.