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Small field-of-view dedicated cardiac SPECT systems: impact of projection truncation.

Xiao J, Verzijlbergen FJ, Viergever MA, Beekman FJ - Eur. J. Nucl. Med. Mol. Imaging (2009)

Bottom Line: The maximum deviation in defected segments was found to be 49% in the worst-case scenario.However, artificially extending projections reduced deviations in defected segments to a few percent.For simultaneous (99m)Tc/(201)Tl studies, artificial projection extension almost fully eliminates the adverse effects of projection truncation.

View Article: PubMed Central - PubMed

Affiliation: Image Sciences Institute, University Medical Centre Utrecht, Universiteitsweg 100, STR 5.203, 3584 CG, Utrecht, The Netherlands. j.xiao@robeco.nl

ABSTRACT

Purpose: Small field-of-view (FOV) dedicated cardiac SPECT systems suffer from truncated projection data. This results in (1) neglect of liver activity that otherwise could be used to estimate (and subsequently correct) the amount of scatter in the myocardium by model-based scatter correction, and (2) distorted attenuation maps. In this study, we investigated to what extent truncation impacts attenuation correction and model-based scatter correction in the cases of (99m)Tc, (201)Tl, and simultaneous (99m)Tc/(201)Tl studies. In addition, we evaluated a simple correction method to mitigate the effects of truncation.

Methods: Digital thorax phantoms of different sizes were used to simulate the full FOV SPECT projections for (99m)Tc, (201)Tl, and simultaneous (99m)Tc/(201)Tl studies. Small FOV projections were obtained by artificially truncating the full FOV projections. Deviations from ideal heart positioning were simulated by axially shifting projections resulting in more severe liver truncation. Effects of truncation on SPECT images were tested for ordered subset (OS) expectation maximization reconstruction with (1) attenuation correction and detector response modelling (OS-AD), and (2) with additional Monte-Carlo-based scatter correction (OS-ADS). To correct truncation-induced artefacts, we axially extended truncated projections on both sides by duplicating pixel values on the projection edge.

Results: For both (99m)Tc and (201)Tl, differences in the reconstructed myocardium between full FOV and small FOV projections were negligible. In the nine myocardial segments, the maximum deviations of the average pixel values were 1.3% for OS-AD and 3.5% for OS-ADS. For the simultaneous (99m)Tc/(201)Tl studies, reconstructed (201)Tl SPECT images from full FOV and small FOV projections showed clearly different image profiles due to truncation. The maximum deviation in defected segments was found to be 49% in the worst-case scenario. However, artificially extending projections reduced deviations in defected segments to a few percent.

Conclusion: Our results indicate that, for single isotope studies, using small FOV systems has little impact on attenuation correction and model-based scatter correction. For simultaneous (99m)Tc/(201)Tl studies, artificial projection extension almost fully eliminates the adverse effects of projection truncation.

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201Tl images in a simultaneous 99mTc/201Tl study, reconstructed with the OS-ADS method. Top: Short-axis (SAX) views based on full FOV, small FOV, and extended small FOV projections for the worst-case scenario. Bottom: Vertical profiles through the inferior defect
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Fig4: 201Tl images in a simultaneous 99mTc/201Tl study, reconstructed with the OS-ADS method. Top: Short-axis (SAX) views based on full FOV, small FOV, and extended small FOV projections for the worst-case scenario. Bottom: Vertical profiles through the inferior defect

Mentions: For simultaneous 99mTc/201Tl imaging, vertical profiles in the short-axis views in Fig. 4, reconstructed with the OS-ADS method, clearly show that truncation of the liver tissue with the small FOV systems led to deterioration of the lesion contrast in the worst-case scenario. Axial extension of the small FOV by half FWHM led to partial recovery of lesion contrast and extension by one FWHM resulted in almost the same lesion contrast as provided by the full FOV data. A quantitative comparison for each myocardial segment is shown in Table 3. Extending the small FOV by one FWHM decreased the deviations in the basal part of the inferior and septal walls containing the defect from 49% to 1.8% and from 23% to 1.4%, respectively. The deviations were also calculated for the normal sized NCAT phantom with a normal sized heart and nonshifted projection. Extension by one FWHM was a good correction in this case too. Table 4 shows activity deviations in the myocardial segments containing a perfusion defect and illustrates the effect of truncation and the effectiveness of axial projection extension in correcting the deviations in all the scenarios. As shown in Table 4, artificial projection extension almost fully eliminated the adverse effects of projection truncation.Fig. 4


Small field-of-view dedicated cardiac SPECT systems: impact of projection truncation.

Xiao J, Verzijlbergen FJ, Viergever MA, Beekman FJ - Eur. J. Nucl. Med. Mol. Imaging (2009)

201Tl images in a simultaneous 99mTc/201Tl study, reconstructed with the OS-ADS method. Top: Short-axis (SAX) views based on full FOV, small FOV, and extended small FOV projections for the worst-case scenario. Bottom: Vertical profiles through the inferior defect
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Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2822234&req=5

Fig4: 201Tl images in a simultaneous 99mTc/201Tl study, reconstructed with the OS-ADS method. Top: Short-axis (SAX) views based on full FOV, small FOV, and extended small FOV projections for the worst-case scenario. Bottom: Vertical profiles through the inferior defect
Mentions: For simultaneous 99mTc/201Tl imaging, vertical profiles in the short-axis views in Fig. 4, reconstructed with the OS-ADS method, clearly show that truncation of the liver tissue with the small FOV systems led to deterioration of the lesion contrast in the worst-case scenario. Axial extension of the small FOV by half FWHM led to partial recovery of lesion contrast and extension by one FWHM resulted in almost the same lesion contrast as provided by the full FOV data. A quantitative comparison for each myocardial segment is shown in Table 3. Extending the small FOV by one FWHM decreased the deviations in the basal part of the inferior and septal walls containing the defect from 49% to 1.8% and from 23% to 1.4%, respectively. The deviations were also calculated for the normal sized NCAT phantom with a normal sized heart and nonshifted projection. Extension by one FWHM was a good correction in this case too. Table 4 shows activity deviations in the myocardial segments containing a perfusion defect and illustrates the effect of truncation and the effectiveness of axial projection extension in correcting the deviations in all the scenarios. As shown in Table 4, artificial projection extension almost fully eliminated the adverse effects of projection truncation.Fig. 4

Bottom Line: The maximum deviation in defected segments was found to be 49% in the worst-case scenario.However, artificially extending projections reduced deviations in defected segments to a few percent.For simultaneous (99m)Tc/(201)Tl studies, artificial projection extension almost fully eliminates the adverse effects of projection truncation.

View Article: PubMed Central - PubMed

Affiliation: Image Sciences Institute, University Medical Centre Utrecht, Universiteitsweg 100, STR 5.203, 3584 CG, Utrecht, The Netherlands. j.xiao@robeco.nl

ABSTRACT

Purpose: Small field-of-view (FOV) dedicated cardiac SPECT systems suffer from truncated projection data. This results in (1) neglect of liver activity that otherwise could be used to estimate (and subsequently correct) the amount of scatter in the myocardium by model-based scatter correction, and (2) distorted attenuation maps. In this study, we investigated to what extent truncation impacts attenuation correction and model-based scatter correction in the cases of (99m)Tc, (201)Tl, and simultaneous (99m)Tc/(201)Tl studies. In addition, we evaluated a simple correction method to mitigate the effects of truncation.

Methods: Digital thorax phantoms of different sizes were used to simulate the full FOV SPECT projections for (99m)Tc, (201)Tl, and simultaneous (99m)Tc/(201)Tl studies. Small FOV projections were obtained by artificially truncating the full FOV projections. Deviations from ideal heart positioning were simulated by axially shifting projections resulting in more severe liver truncation. Effects of truncation on SPECT images were tested for ordered subset (OS) expectation maximization reconstruction with (1) attenuation correction and detector response modelling (OS-AD), and (2) with additional Monte-Carlo-based scatter correction (OS-ADS). To correct truncation-induced artefacts, we axially extended truncated projections on both sides by duplicating pixel values on the projection edge.

Results: For both (99m)Tc and (201)Tl, differences in the reconstructed myocardium between full FOV and small FOV projections were negligible. In the nine myocardial segments, the maximum deviations of the average pixel values were 1.3% for OS-AD and 3.5% for OS-ADS. For the simultaneous (99m)Tc/(201)Tl studies, reconstructed (201)Tl SPECT images from full FOV and small FOV projections showed clearly different image profiles due to truncation. The maximum deviation in defected segments was found to be 49% in the worst-case scenario. However, artificially extending projections reduced deviations in defected segments to a few percent.

Conclusion: Our results indicate that, for single isotope studies, using small FOV systems has little impact on attenuation correction and model-based scatter correction. For simultaneous (99m)Tc/(201)Tl studies, artificial projection extension almost fully eliminates the adverse effects of projection truncation.

Show MeSH