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Attenuation compensation of cardiac SPECT: A critical look at a confusing world
EDITORIAL Attenuation compensation of cardiac SPECT: A critical look at a confusing world Frans J. Th. Wackers, MD
See related article, p 361 In 1999 I wrote an editorial for the Journal of Nuclear Medicine entitled “Attenuation correction, or the emperor’s new clothes?”1 I noted the widespread desire to believe in single photon emission computed tomography (SPECT) attenuation correction, notwithstanding disappointing and confusing results in clinical studies using commercially available devices and software packages. Although many imaging facilities possessed nuclear imaging equipment with attenuation correction capability, it was noted that only a few laboratories used the devices in routine clinical practice. Moreover, when these devices were used, corrected images were often not interpreted independently but were reviewed side by side with uncorrected images. Thus information from attenuation correction appeared to be used selectively. Most vendors of nuclear imaging equipment presently market software packages and devices that intend to compensate for nonuniform tissue attenuation on SPECT images. However, the attenuation correction devices differ substantially from vendor to vendor with regard to design of hardware, type of transmission sources, and acquisition/processing algorithms. Two multicenter studies in relatively large numbers of patients, using commercially available equipment, showed improvement of specificity on corrected SPECT images compared with noncorrected SPECT images.2,3 However, in each of these studies more complex correction algorithms were applied than attenuation compensation alone. For example, both applied depth-dependent resolution compensation, but only one incorporated scatter correction. Although both clinical studies showed
From the Cardiovascular Nuclear Imaging and Stress Laboratories, Departments of Diagnostic Radiology and Internal Medicine, Yale University School of Medicine, New Haven, Conn. Reprint requests: Frans J. Th. Wackers, MD, Cardiovascular Nuclear Imaging and Stress Laboratories, Yale University School of Medicine, 333 Cedar St, Fitkin-3, PO Box 208042, New Haven, CT 06520-8042; [email protected]. J Nucl Cardiol 2002;9:438-40. Copyright © 2002 by the American Society of Nuclear Cardiology. 1071-3581/2002/$35.00 ⫹ 0 43/39/125813 doi:10.1067/mnc.2002.125813 438
modest improvement in specificity and normalcy rate, the values were not identical in both studies. Thus, despite these encouraging clinical results, the marked differences in technology and the lack of comparative testing criteria leave the potential buyer of equipment confused and uncertain about which choice to make and, most importantly, on which parameters to base a decision. Most experts agree that present state-of-the-art (commercially available) attenuation compensation devices are not sufficiently robust for routine use in clinical practice. Nevertheless, there is full agreement that the development of accurate and reliable methods for attenuation correction is of crucial importance for the continued advancement of SPECT myocardial perfusion imaging. The American Society of Nuclear Cardiology recently stated in a position statement on SPECT attenuation correction that “ . . . attenuation correction should be regarded as a rapidly evolving standard for SPECT myocardial imaging. Therefore it is our recommendation that the adjunctive technique of attenuation correction has become a method for which the weight of evidence and opinion is in favor of its usefulness.”4 In the aforementioned editorial,1 I suggested that in order to minimize the confusion of potential buyers about such complex systems, it is desirable that, before attenuation correction is implemented in routine clinical practice, each attenuation correction device is rigorously tested by a standardized testing protocol. Potential buyers of equipment must be able to compare the results of testing with various devices against predefined criteria in order to make an educated decision. I proposed that professional societies define the testing standards for SPECT attenuation correction systems. Because attenuation correction deals with a physics problem, it seems rational to first perform testing of attenuation correction devices in a well-designed series of physics experiments of cardiac phantoms that resemble as closely as feasible the human chest, including attenuating and fillable organs. Furthermore, I suggested that because improvements in reconstructed images might be rather subtle, quantitative analysis of the experimental results must be used to measure the effect of corrections on the heterogeneity/homogeneity of SPECT images. Only after extensive testing in phantoms has provided satisfactory results are clinical trials justified and necessary.
Journal of Nuclear Cardiology Volume 9, Number 4;438-40
In this issue of the Journal of Nuclear Cardiology, O’Connor and Kemp5 provide a unique consumers’ report on the comparative performance of 8 different attenuation correction devices: 7 SPECT systems and 1 positron emission tomography system. The investigators performed a series of carefully designed and standardized phantom studies with quantitative data analysis. The investigators traveled to the participating sites with the phantoms and acquired SPECT images with attenuation compensation using acquisition protocols as recommended by the manufacturers. The investigators imaged 8 different phantom configurations, varying from the isolated myocardial insert suspended in air without any surrounding attenuating material, to a large complex anthropomorphic chest phantom with attenuating “breast” attachments. Image data were reconstructed at the clinical sites and then exported in either DICOM or Interfile format to the investigators’ laboratory to ensure unbiased and uniform quantitative image analysis. Circumferential count profiles of reconstructed slices were used to quantify uniformity, or lack thereof, on reconstructed images. The investigators quantified uniformity as global phantom uniformity, slice uniformity, and apex-to-base uniformity. One important observation in the study is found in the very first line of the results section: “ . . . with the heart model in air, we found that for all systems, the application of AC [attenuation correction] resulted in degradation of image quality.” Thus, with the use of sophisticated attenuation compensation acquisition/reconstruction algorithms, the most simple phantom configuration—that is, a cardiac insert filled with a homogeneous technetium 99m solution, suspended in air, without external attenuating material— could not be reconstructed as the homogeneous image that it was supposed to be with any of the tested systems. In fact, in terms of uniformity, the results with attenuation correction were inferior to those with conventional filtered backprojection. Although in actuality the uniformity of reconstructed slices of this normal phantom should be near 0%, the best achievable uniformity was only approximately 11%. This observation underscores the complexity of SPECT imaging and reconstruction. Reconstructed SPECT images are influenced by many confounding variables. Attenuation is only one of those. The final reconstructed SPECT image is affected by a complex interplay between orbit, collimator, target organ location, and reconstruction algorithms. Furthermore, there are Compton scatter, partial volume effects, variable depthdependent resolution, significant uptake in adjacent structures, and (in patients) motion to consider. Thus it
Wackers A critical look at attenuation compensation of cardiac SPECT
439
may well be that perfect uniformity is an elusive goal and that one may have to settle for uniformity of approximately 11%. However, from this observation, it also follows that basic camera specifications and imaging parameters still can be improved upon. For example, it was recently demonstrated that, because of varying depth resolution, a 360° acquisition orbit is clearly superior to the widely used 180° acquisition orbit, the recommended standard.6,7 O’Connor and Kemp5 found considerable variation in the performance of the various commercial imaging systems for imaging the normal cardiac phantom in air. Slice uniformity varied from approximately 9% to 15%. In order to provide a frame of reference, the uniformity of typical inferior attenuation in a patient is approximately 20% to 25%. When the normal cardiac phantom was inserted in the anthropomorphic phantoms, the non–attenuation-corrected reconstructed images showed marked slice inhomogeneity, ranging from 21% to 35%. Attenuation correction resulted in modest, although statistically significant, improvement of uniformity to 18% to 22%. However, none of the systems improved uniformity comparable to that achieved with cardiac phantom in air and conventional backprojection. The presence of a “hot liver” had an additional and varying degrading effect on attenuation-corrected images obtained with various devices. The investigators also noted substantial differences in the ability to resolve “myocardial defects,” which appeared dependent on location and position of the defects in the phantom. The authors provide an excellent and useful overview of the marked technical differences between various compensation devices in terms of hardware and software. No two systems are alike. Although no system was perfect in correcting for attenuation, some performed considerably better than others. Surprisingly, the one positron emission tomography system that was tested did not perform noticeably better than the SPECT systems. The authors appropriately point out some of the limitations inherent to phantom studies. The cardiac phantoms may represent extreme conditions and may not exactly mimic the average patient body habitus. Thus it is conceivable that in some patients the devices may perform better than in the reported experiments. Nevertheless, I do not believe that one should accept such explanations or excuses too readily. Attenuation artifacts do create problems with interpretation, especially in markedly obese and large patients. In patients with average body habitus the experienced observer readily recognizes the presence of attenuation artifacts and adjusts the interpretation appropriately. Although considerable progress has been made, SPECT attenuation compensation technology in the year 2002 is still a work in progress. The plethora and
440
Wackers A critical look at attenuation compensation of cardiac SPECT
diversity of technical approaches remain confusing. It is unlikely that they are all correct. O’Connor and Kemp5 are to be commended for their unbiased and critical look at this confusing world. The series of phantom studies described in their study should serve as a model for the future testing of new iterations of attenuation compensation devices. On the basis of the results of O’Connor and Kemp,5 I propose the following standard testing of future attenuation compensation devices. A minimum of two phantom imaging experiments must be performed: (1) imaging of a normal cardiac phantom (ie, left ventricle only) with the “myocardium” filled with a homogeneous Tc99m solution suspended in air and (2) imaging of an anthropomorphic phantom with a normal left ventricular insert and a 2:1 liver–to–myocardial activity ratio. The uniformity of the normal phantom in air serves as a benchmark. Comparison of quantified uniformity of short-axis slices from the uncorrected and corrected phantom data of both phantom image acquisitions should give the user a good idea of the performance of the attenuation compensation device. Vendors should review the comparative results of O’Connor and Kemp’s study5 carefully and consider modifications of their algorithms. I trust that manufacturers and users will show restraint and that the new clothes of the emperor will remain in the closet.
Journal of Nuclear Cardiology July/August 2002
Acknowledgment Dr. Wackers discloses the following financial relationships: grants and research support from Bristol-Myers-Squibb Medical Imaging, Fujisawa Health, King Pharmaceuticals, CIS-US, and General Electric Medical Systems.
References 1. Wackers FJTh. Attenuation correction, or the emperor’s new clothes? [editorial]. J Nucl Med 1999;40:1310-2. 2. Hendel RC, Berman DS, Cullom SJ, et al. Multicenter clinical trial to evaluate the efficacy of correction for photon attenuation and scatter in SPECT myocardial perfusion imaging. Circulation 1999;99:2742-9. 3. Links JM, Becker LC, Rigo P, Taillefer R, Hanelin L, Anstett F, et al. Combined corrections for attenuation, depth-dependent blur, and motion in cardiac SPECT: a multicenter trial. J Nucl Cardiol 2000;7:414-25. 4. Hendel RC, Corbet JR, Cullom SJ, DePuey G, Garcia EV, Bateman TM. The value and practice of attenuation correction for myocardial perfusion SPECT imaging: a joint position statement from the American Society of Nuclear Cardiology and the Society of Nuclear Medicine. J Nucl Cardiol 2002;9:135-43. 5. O’Connor MK, Kemp B. A multicenter evaluation of commercial attenuation compensation techniques in cardiac SPECT using phantom models. J Nucl Cardiol 2002;9:361-76. 6. Liu YH, Lam PT, Sinusas AJ, Wackers FJTh. Differential effect of 180° and 360° acquisition orbits on the accuracy of SPECT imaging: quantitative evaluation in phantoms. J Nucl Med 2002. In press. 7. DePuey EG, Garcia EV. Updated imaging guidelines for nuclear cardiology procedures. Part 1. J Nucl Cardiol 2001;8:G1-G58.
See related article, p 361 In 1999 I wrote an editorial for the Journal of Nuclear Medicine entitled “Attenuation correction, or the emperor’s new clothes?”1 I noted the widespread desire to believe in single photon emission computed tomography (SPECT) attenuation correction, notwithstanding disappointing and confusing results in clinical studies using commercially available devices and software packages. Although many imaging facilities possessed nuclear imaging equipment with attenuation correction capability, it was noted that only a few laboratories used the devices in routine clinical practice. Moreover, when these devices were used, corrected images were often not interpreted independently but were reviewed side by side with uncorrected images. Thus information from attenuation correction appeared to be used selectively. Most vendors of nuclear imaging equipment presently market software packages and devices that intend to compensate for nonuniform tissue attenuation on SPECT images. However, the attenuation correction devices differ substantially from vendor to vendor with regard to design of hardware, type of transmission sources, and acquisition/processing algorithms. Two multicenter studies in relatively large numbers of patients, using commercially available equipment, showed improvement of specificity on corrected SPECT images compared with noncorrected SPECT images.2,3 However, in each of these studies more complex correction algorithms were applied than attenuation compensation alone. For example, both applied depth-dependent resolution compensation, but only one incorporated scatter correction. Although both clinical studies showed
From the Cardiovascular Nuclear Imaging and Stress Laboratories, Departments of Diagnostic Radiology and Internal Medicine, Yale University School of Medicine, New Haven, Conn. Reprint requests: Frans J. Th. Wackers, MD, Cardiovascular Nuclear Imaging and Stress Laboratories, Yale University School of Medicine, 333 Cedar St, Fitkin-3, PO Box 208042, New Haven, CT 06520-8042; [email protected]. J Nucl Cardiol 2002;9:438-40. Copyright © 2002 by the American Society of Nuclear Cardiology. 1071-3581/2002/$35.00 ⫹ 0 43/39/125813 doi:10.1067/mnc.2002.125813 438
modest improvement in specificity and normalcy rate, the values were not identical in both studies. Thus, despite these encouraging clinical results, the marked differences in technology and the lack of comparative testing criteria leave the potential buyer of equipment confused and uncertain about which choice to make and, most importantly, on which parameters to base a decision. Most experts agree that present state-of-the-art (commercially available) attenuation compensation devices are not sufficiently robust for routine use in clinical practice. Nevertheless, there is full agreement that the development of accurate and reliable methods for attenuation correction is of crucial importance for the continued advancement of SPECT myocardial perfusion imaging. The American Society of Nuclear Cardiology recently stated in a position statement on SPECT attenuation correction that “ . . . attenuation correction should be regarded as a rapidly evolving standard for SPECT myocardial imaging. Therefore it is our recommendation that the adjunctive technique of attenuation correction has become a method for which the weight of evidence and opinion is in favor of its usefulness.”4 In the aforementioned editorial,1 I suggested that in order to minimize the confusion of potential buyers about such complex systems, it is desirable that, before attenuation correction is implemented in routine clinical practice, each attenuation correction device is rigorously tested by a standardized testing protocol. Potential buyers of equipment must be able to compare the results of testing with various devices against predefined criteria in order to make an educated decision. I proposed that professional societies define the testing standards for SPECT attenuation correction systems. Because attenuation correction deals with a physics problem, it seems rational to first perform testing of attenuation correction devices in a well-designed series of physics experiments of cardiac phantoms that resemble as closely as feasible the human chest, including attenuating and fillable organs. Furthermore, I suggested that because improvements in reconstructed images might be rather subtle, quantitative analysis of the experimental results must be used to measure the effect of corrections on the heterogeneity/homogeneity of SPECT images. Only after extensive testing in phantoms has provided satisfactory results are clinical trials justified and necessary.
Journal of Nuclear Cardiology Volume 9, Number 4;438-40
In this issue of the Journal of Nuclear Cardiology, O’Connor and Kemp5 provide a unique consumers’ report on the comparative performance of 8 different attenuation correction devices: 7 SPECT systems and 1 positron emission tomography system. The investigators performed a series of carefully designed and standardized phantom studies with quantitative data analysis. The investigators traveled to the participating sites with the phantoms and acquired SPECT images with attenuation compensation using acquisition protocols as recommended by the manufacturers. The investigators imaged 8 different phantom configurations, varying from the isolated myocardial insert suspended in air without any surrounding attenuating material, to a large complex anthropomorphic chest phantom with attenuating “breast” attachments. Image data were reconstructed at the clinical sites and then exported in either DICOM or Interfile format to the investigators’ laboratory to ensure unbiased and uniform quantitative image analysis. Circumferential count profiles of reconstructed slices were used to quantify uniformity, or lack thereof, on reconstructed images. The investigators quantified uniformity as global phantom uniformity, slice uniformity, and apex-to-base uniformity. One important observation in the study is found in the very first line of the results section: “ . . . with the heart model in air, we found that for all systems, the application of AC [attenuation correction] resulted in degradation of image quality.” Thus, with the use of sophisticated attenuation compensation acquisition/reconstruction algorithms, the most simple phantom configuration—that is, a cardiac insert filled with a homogeneous technetium 99m solution, suspended in air, without external attenuating material— could not be reconstructed as the homogeneous image that it was supposed to be with any of the tested systems. In fact, in terms of uniformity, the results with attenuation correction were inferior to those with conventional filtered backprojection. Although in actuality the uniformity of reconstructed slices of this normal phantom should be near 0%, the best achievable uniformity was only approximately 11%. This observation underscores the complexity of SPECT imaging and reconstruction. Reconstructed SPECT images are influenced by many confounding variables. Attenuation is only one of those. The final reconstructed SPECT image is affected by a complex interplay between orbit, collimator, target organ location, and reconstruction algorithms. Furthermore, there are Compton scatter, partial volume effects, variable depthdependent resolution, significant uptake in adjacent structures, and (in patients) motion to consider. Thus it
Wackers A critical look at attenuation compensation of cardiac SPECT
439
may well be that perfect uniformity is an elusive goal and that one may have to settle for uniformity of approximately 11%. However, from this observation, it also follows that basic camera specifications and imaging parameters still can be improved upon. For example, it was recently demonstrated that, because of varying depth resolution, a 360° acquisition orbit is clearly superior to the widely used 180° acquisition orbit, the recommended standard.6,7 O’Connor and Kemp5 found considerable variation in the performance of the various commercial imaging systems for imaging the normal cardiac phantom in air. Slice uniformity varied from approximately 9% to 15%. In order to provide a frame of reference, the uniformity of typical inferior attenuation in a patient is approximately 20% to 25%. When the normal cardiac phantom was inserted in the anthropomorphic phantoms, the non–attenuation-corrected reconstructed images showed marked slice inhomogeneity, ranging from 21% to 35%. Attenuation correction resulted in modest, although statistically significant, improvement of uniformity to 18% to 22%. However, none of the systems improved uniformity comparable to that achieved with cardiac phantom in air and conventional backprojection. The presence of a “hot liver” had an additional and varying degrading effect on attenuation-corrected images obtained with various devices. The investigators also noted substantial differences in the ability to resolve “myocardial defects,” which appeared dependent on location and position of the defects in the phantom. The authors provide an excellent and useful overview of the marked technical differences between various compensation devices in terms of hardware and software. No two systems are alike. Although no system was perfect in correcting for attenuation, some performed considerably better than others. Surprisingly, the one positron emission tomography system that was tested did not perform noticeably better than the SPECT systems. The authors appropriately point out some of the limitations inherent to phantom studies. The cardiac phantoms may represent extreme conditions and may not exactly mimic the average patient body habitus. Thus it is conceivable that in some patients the devices may perform better than in the reported experiments. Nevertheless, I do not believe that one should accept such explanations or excuses too readily. Attenuation artifacts do create problems with interpretation, especially in markedly obese and large patients. In patients with average body habitus the experienced observer readily recognizes the presence of attenuation artifacts and adjusts the interpretation appropriately. Although considerable progress has been made, SPECT attenuation compensation technology in the year 2002 is still a work in progress. The plethora and
440
Wackers A critical look at attenuation compensation of cardiac SPECT
diversity of technical approaches remain confusing. It is unlikely that they are all correct. O’Connor and Kemp5 are to be commended for their unbiased and critical look at this confusing world. The series of phantom studies described in their study should serve as a model for the future testing of new iterations of attenuation compensation devices. On the basis of the results of O’Connor and Kemp,5 I propose the following standard testing of future attenuation compensation devices. A minimum of two phantom imaging experiments must be performed: (1) imaging of a normal cardiac phantom (ie, left ventricle only) with the “myocardium” filled with a homogeneous Tc99m solution suspended in air and (2) imaging of an anthropomorphic phantom with a normal left ventricular insert and a 2:1 liver–to–myocardial activity ratio. The uniformity of the normal phantom in air serves as a benchmark. Comparison of quantified uniformity of short-axis slices from the uncorrected and corrected phantom data of both phantom image acquisitions should give the user a good idea of the performance of the attenuation compensation device. Vendors should review the comparative results of O’Connor and Kemp’s study5 carefully and consider modifications of their algorithms. I trust that manufacturers and users will show restraint and that the new clothes of the emperor will remain in the closet.
Journal of Nuclear Cardiology July/August 2002
Acknowledgment Dr. Wackers discloses the following financial relationships: grants and research support from Bristol-Myers-Squibb Medical Imaging, Fujisawa Health, King Pharmaceuticals, CIS-US, and General Electric Medical Systems.
References 1. Wackers FJTh. Attenuation correction, or the emperor’s new clothes? [editorial]. J Nucl Med 1999;40:1310-2. 2. Hendel RC, Berman DS, Cullom SJ, et al. Multicenter clinical trial to evaluate the efficacy of correction for photon attenuation and scatter in SPECT myocardial perfusion imaging. Circulation 1999;99:2742-9. 3. Links JM, Becker LC, Rigo P, Taillefer R, Hanelin L, Anstett F, et al. Combined corrections for attenuation, depth-dependent blur, and motion in cardiac SPECT: a multicenter trial. J Nucl Cardiol 2000;7:414-25. 4. Hendel RC, Corbet JR, Cullom SJ, DePuey G, Garcia EV, Bateman TM. The value and practice of attenuation correction for myocardial perfusion SPECT imaging: a joint position statement from the American Society of Nuclear Cardiology and the Society of Nuclear Medicine. J Nucl Cardiol 2002;9:135-43. 5. O’Connor MK, Kemp B. A multicenter evaluation of commercial attenuation compensation techniques in cardiac SPECT using phantom models. J Nucl Cardiol 2002;9:361-76. 6. Liu YH, Lam PT, Sinusas AJ, Wackers FJTh. Differential effect of 180° and 360° acquisition orbits on the accuracy of SPECT imaging: quantitative evaluation in phantoms. J Nucl Med 2002. In press. 7. DePuey EG, Garcia EV. Updated imaging guidelines for nuclear cardiology procedures. Part 1. J Nucl Cardiol 2001;8:G1-G58.