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Experimental determination of the weighting factor for the energy window subtraction-based downscatter correction for I-123 in brain SPECT studies.

de Nijs R, Holm S, Thomsen G, Ziebell M, Svarer C - J Med Phys (2010)

Bottom Line: After correction, the measured specific binding ratio (image contrast) increased significantly for healthy subjects, typically by more than 20%, while the background counts outside of all subjects were effectively removed.A weighting factor of 1.1-1.2 can be applied in clinical practice.This correction effectively removes downscatter and significantly improves image contrast inside the brain.

View Article: PubMed Central - PubMed

Affiliation: Department of Clinical Physiology, Nuclear Medicine, PET and Cyclotron Unit, Copenhagen University Hospital, Copenhagen, Denmark.

ABSTRACT
Correction for downscatter in I-123 SPECT can be performed by the subtraction of a secondary energy window from the main window, as in the triple-energy window method. This is potentially noise sensitive. For studies with limited amount of counts (e.g. dynamic studies), a broad subtraction window with identical width is preferred. This secondary window needs to be weighted with a factor higher than one, due to a broad backscatter peak from high-energy photons appearing at 172 keV. Spatial dependency and the numerical value of this weighting factor and the image contrast improvement of this correction were investigated in this study. Energy windows with a width of 32 keV were centered at 159 keV and 200 keV. The weighting factor was measured both with an I-123 point source and in a dopamine transporter brain SPECT study in 10 human subjects (5 healthy subjects and 5 patients) by minimizing the background outside the head. Weighting factors ranged from 1.11 to 1.13 for the point source and from 1.16 to 1.18 for human subjects. Point source measurements revealed no position dependence. After correction, the measured specific binding ratio (image contrast) increased significantly for healthy subjects, typically by more than 20%, while the background counts outside of all subjects were effectively removed. A weighting factor of 1.1-1.2 can be applied in clinical practice. This correction effectively removes downscatter and significantly improves image contrast inside the brain.

No MeSH data available.


Related in: MedlinePlus

A slice of a reconstructed SPECT image for subject V (patient) in Table 1. The images shown correspond to those in Figure 3.
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Figure 0004: A slice of a reconstructed SPECT image for subject V (patient) in Table 1. The images shown correspond to those in Figure 3.

Mentions: where subscript S refers to the striatum, VOI to the volume of interest around the striatum, and the subscript ‘ref’ to the reference region. Identical VOIs were used for both reconstructions, with and without downscatter correction. In order to ensure that all the striatal counts were contained in the large VOI, an extra top and bottom slice with VOIs were added. Examples of the VOIs are shown in Figures 3a and 4a. A standard striatal volume of 11.2 mL was assumed.[19] However, due to individual variations, this might be significantly different from the volume measured by a structural MRI scan.[20] For this reason, some caution is needed before interpreting the SBR values determined in this way. In this study, however, the improvement in SBR by downscatter correction is important, and since the striatal volume is a constant scaling factor in Equation 4, it cancels out in the calculation of the relative improvement in SBR.


Experimental determination of the weighting factor for the energy window subtraction-based downscatter correction for I-123 in brain SPECT studies.

de Nijs R, Holm S, Thomsen G, Ziebell M, Svarer C - J Med Phys (2010)

A slice of a reconstructed SPECT image for subject V (patient) in Table 1. The images shown correspond to those in Figure 3.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2990116&req=5

Figure 0004: A slice of a reconstructed SPECT image for subject V (patient) in Table 1. The images shown correspond to those in Figure 3.
Mentions: where subscript S refers to the striatum, VOI to the volume of interest around the striatum, and the subscript ‘ref’ to the reference region. Identical VOIs were used for both reconstructions, with and without downscatter correction. In order to ensure that all the striatal counts were contained in the large VOI, an extra top and bottom slice with VOIs were added. Examples of the VOIs are shown in Figures 3a and 4a. A standard striatal volume of 11.2 mL was assumed.[19] However, due to individual variations, this might be significantly different from the volume measured by a structural MRI scan.[20] For this reason, some caution is needed before interpreting the SBR values determined in this way. In this study, however, the improvement in SBR by downscatter correction is important, and since the striatal volume is a constant scaling factor in Equation 4, it cancels out in the calculation of the relative improvement in SBR.

Bottom Line: After correction, the measured specific binding ratio (image contrast) increased significantly for healthy subjects, typically by more than 20%, while the background counts outside of all subjects were effectively removed.A weighting factor of 1.1-1.2 can be applied in clinical practice.This correction effectively removes downscatter and significantly improves image contrast inside the brain.

View Article: PubMed Central - PubMed

Affiliation: Department of Clinical Physiology, Nuclear Medicine, PET and Cyclotron Unit, Copenhagen University Hospital, Copenhagen, Denmark.

ABSTRACT
Correction for downscatter in I-123 SPECT can be performed by the subtraction of a secondary energy window from the main window, as in the triple-energy window method. This is potentially noise sensitive. For studies with limited amount of counts (e.g. dynamic studies), a broad subtraction window with identical width is preferred. This secondary window needs to be weighted with a factor higher than one, due to a broad backscatter peak from high-energy photons appearing at 172 keV. Spatial dependency and the numerical value of this weighting factor and the image contrast improvement of this correction were investigated in this study. Energy windows with a width of 32 keV were centered at 159 keV and 200 keV. The weighting factor was measured both with an I-123 point source and in a dopamine transporter brain SPECT study in 10 human subjects (5 healthy subjects and 5 patients) by minimizing the background outside the head. Weighting factors ranged from 1.11 to 1.13 for the point source and from 1.16 to 1.18 for human subjects. Point source measurements revealed no position dependence. After correction, the measured specific binding ratio (image contrast) increased significantly for healthy subjects, typically by more than 20%, while the background counts outside of all subjects were effectively removed. A weighting factor of 1.1-1.2 can be applied in clinical practice. This correction effectively removes downscatter and significantly improves image contrast inside the brain.

No MeSH data available.


Related in: MedlinePlus