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Whole-Body ¹⁸F-FDG PET/CT in the Presence of Truncation Artifacts

¹ Department of Nuclear Medicine; University Hospital Essen, Essen, Germany; ² Institute of Diagnostic and Interventional Radiology and Neuroradiology; University Hospital Essen, Essen, Germany; and ³ Department of Nuclear Medicine; Technical University Munich, Munich. Germany

View larger version (45K): [in a new window]   FIGURE 1.  Phantom setups for object truncation (offset_trunc). Experimental outline is shown to the left and fused PET/CT before AC is shown to the right. (A) A 20-cm cylinder filled with 18F-FDG and water was placed at edge of CT FOV (double line). (B) The same cylinder simulating the trunk was centered and with 2 arms (8-cm diameter each) placed at edge of CT FOV. (C) National Electrical Manufacturers Association (NEMA)-quality phantom with 4 hot lesions (9:1 background ratio) was placed off-center. All phantom arrangements were contained within measured PET FOV (dashed line). ROIs were defined on corresponding fused uncorrected PET/CT images (right column). ROI_e = truncated/extended area; ROI_c = center; ROI_c1 and ROI_c2 = center of central cylinder; ROI_ac = central arm; ROI_at = truncated arm; ROI_b = background body; ROI_l1 = lesion 1; ROI_l2 = lesion 2; ROI_bt = background truncated. For each phantom study, regions of interest (ROIs) were the same size.

View larger version (10K): [in a new window]   FIGURE 2.  Outline of eFOV algorithm used in this study (9). Mirror truncated projections at maximum CT FOV to extrapolate the projections to limit of PET FOV (eFOV) (A), apply a cosine roll-off filter to extrapolated projections (B), apply a 25-channel smoothing kernel to filtered extended projections, and backproject extrapolated and filtered projections (not shown).

View larger version (92K): [in a new window]   FIGURE 3.  Phantom A. Axial CT images and PET images before (noAC) and after CT-based AC (CTAC) for phantom positioned centrally inside the FOV (A), phantom moved to edge of the FOV with 17% volume truncation (offset_trunc position) (B), and as (B) but after eFOV correction (offset_ext) (C). CT images are shown in soft-tissue window (top) and lung window (middle).

View larger version (40K): [in a new window]   FIGURE 4.  Phantom B. Axial CT images and PET images before (noAC) and after CT-based AC (CTAC) for phantom positioned centrally inside the FOV (A), phantom moved to edge of the FOV (offset_trunc position) (B), and as (B) but after eFOV correction (offset_ext) (C). CT images of after eFOV correction are shown in soft-tissue window and lung window for better appreciation of recovered CT object.

View larger version (78K): [in a new window]   FIGURE 5.  Phantom C. Axial CT images and PET images before (noAC) and after CT-based AC (CTAC) for phantom positioned centrally inside the FOV (A), phantom moved to the edge of the FOV such that the largest hot lesion is on edge of the CT FOV (offset_trunc position) (B), and as (B) but after eFOV correction (offset_ext) (C). CT images after eFOV correction are shown also in soft-tissue window and lung window.

View larger version (89K): [in a new window]   FIGURE 6.  Effect of truncation and eFOV correction on patient data from PET/CT using 2-slice (A) and 16-slice (B) CT. Coronal views of fused CT–CT images with truncation and with eFOV recovery (first row), nonattenuation-corrected PET (second row), attenuation-corrected PET with truncated CT (third row), and attenuation-corrected PET with eFOV CT (fourth row). Lateral profiles through CT and PET images of PET/CT study in (A) are shown in (C) and (D), respectively. Boxes on either side of profiles mark the area of truncation. In (D) a relative offset has been added to the profile through the uncorrected PET for improved visibility. noAC PET = PET before AC; xCT = truncated CT, eFOV CT = CT after eFOV correction; xCT AC = PET with AC based on xCT; eFOV AC = PET with AC based on eFOV CT.