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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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OS-ADS method. Top: Short-axis (SAX) views based on full FOV and small FOV projections for the worst-case scenario. Bottom: Vertical profiles through the inferior defect
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Fig3: OS-ADS method. Top: Short-axis (SAX) views based on full FOV and small FOV projections for the worst-case scenario. Bottom: Vertical profiles through the inferior defect

Mentions: For OS-AD, Fig. 2 shows short-axis views of a large myocardium in the worst-case scenario. Vertical profiles through the perfusion defect in the inferior wall show little difference in the reconstructed myocardium between the full FOV and small FOV data, both for 99mTc and 201Tl. As shown in Table 2, within the nine myocardial segments, the greatest deviations were 1.3% for 201Tl and 0.95% for 99mTc. Less-severe deviations were found for the smaller phantom and with a centrally positioned heart. The deviations caused by projection truncation can be regarded as marginal and should not affect diagnosis. For OS-ADS, little difference was also shown between the full FOV and small FOV data (Fig. 3). The greatest deviations (Table 2) were 3.5% for 201Tl and 2.9% for 99mTc. Only very small deviations were found for smaller thorax phantoms. In all cases, the deviations seemed to be too limited to influence the outcome of clinical diagnosis. Therefore, we can conclude that the missing liver tissue has little impact on the accuracy of both OS-AD and OS-ADS, both for 99mTc and 201Tl. As shown in Table 2, deviations in the myocardium were marginally larger with OS-ADS than with OS-AD. In this study, the truncation was only present on the reverse side of the thorax and it was far from the heart even for the worst-case scenario. The cupping artefact due to truncation in the emission data therefore has minimal impact on the myocardium.Fig. 2


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)

OS-ADS method. Top: Short-axis (SAX) views based on full FOV and small FOV projections for the worst-case scenario. Bottom: Vertical profiles through the inferior defect
© Copyright Policy
Related In: Results  -  Collection

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

Fig3: OS-ADS method. Top: Short-axis (SAX) views based on full FOV and small FOV projections for the worst-case scenario. Bottom: Vertical profiles through the inferior defect
Mentions: For OS-AD, Fig. 2 shows short-axis views of a large myocardium in the worst-case scenario. Vertical profiles through the perfusion defect in the inferior wall show little difference in the reconstructed myocardium between the full FOV and small FOV data, both for 99mTc and 201Tl. As shown in Table 2, within the nine myocardial segments, the greatest deviations were 1.3% for 201Tl and 0.95% for 99mTc. Less-severe deviations were found for the smaller phantom and with a centrally positioned heart. The deviations caused by projection truncation can be regarded as marginal and should not affect diagnosis. For OS-ADS, little difference was also shown between the full FOV and small FOV data (Fig. 3). The greatest deviations (Table 2) were 3.5% for 201Tl and 2.9% for 99mTc. Only very small deviations were found for smaller thorax phantoms. In all cases, the deviations seemed to be too limited to influence the outcome of clinical diagnosis. Therefore, we can conclude that the missing liver tissue has little impact on the accuracy of both OS-AD and OS-ADS, both for 99mTc and 201Tl. As shown in Table 2, deviations in the myocardium were marginally larger with OS-ADS than with OS-AD. In this study, the truncation was only present on the reverse side of the thorax and it was far from the heart even for the worst-case scenario. The cupping artefact due to truncation in the emission data therefore has minimal impact on the myocardium.Fig. 2

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