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Evaluation of Image Registration in PET/CT of the Liver and Recommendations for Optimized Imaging

¹ Department of Nuclear Medicine, Radboud University Nijmegen Medical Center, Nijmegen, The Netherlands; ² Department of Surgery, Radboud University Nijmegen Medical Center, Nijmegen, The Netherlands; and ³ Department of Radiology, Radboud University Nijmegen Medical Center, Nijmegen, The Netherlands

View larger version (43K): [in this window] [in a new window]   FIGURE 1.  Localization of liver borders. Coronal slices of CT (A), attenuation-corrected PET (B), and uncorrected PET (C) of a single patient, acquired with hybrid PET/CT during expiration breath-hold. Circles represent slices through 3D ellipsoids that were mapped to diaphragmatic dome (green), right lateral border (blue), and caudal tip (red) to determine differences in their respective positions.

View larger version (20K): [in this window] [in a new window]   FIGURE 2.  Image registration errors of liver borders. Relative image registration errors at location of several liver borders for EB (hybrid PET/CT with breath-hold CT), FB (hybrid PET/CT with free-breathing CT), and SF (software fusion of dedicated PET and CT). Registration errors occur primarily in craniocaudal direction (diaphragm and caudal tip affected most) because of insufficient expiration during CT.

View larger version (57K): [in this window] [in a new window]   FIGURE 3.  Misregistration of a liver lesion on breath-hold PET/CT. Transverse (left) and coronal (right) images of large liver metastasis in hybrid PET/CT with breath-hold CT. Center of lesion is marked with red cross on CT and with blue cross on PET. Positioning differences of liver between PET and CT acquisition resulted in mismatch of 13 mm, measured as a 3D vector.

View larger version (16K): [in this window] [in a new window]   FIGURE 4.  Errors at diaphragmatic dome. Extent of image registration errors and attenuation-correction artifacts of >10 mm was comparable at location of diaphragmatic dome of liver for EB (hybrid PET/CT with breath-hold CT), FB (hybrid PET/CT with free-breathing CT), and SF (software fusion of dedicated PET and CT).

View larger version (28K): [in this window] [in a new window]   FIGURE 5.  PET/CT attenuation-correction artifacts. Hybrid PET/CT of large liver metastasis with central necrosis, with CT acquired during expiration breath-hold: coronal slices of CT (A), uncorrected PET (B), attenuation-corrected PET (C), and fused corrected PET with CT (D). Despite breathing instructions, comparison of A and B reveals a difference in diaphragm position between CT and PET acquisition. C and D demonstrate change in shape of liver on PET after attenuation correction, to falsely match CT. Liver metastasis appears partially in lung on corrected images and results from severe loss of signal intensity in region of mismatch.

View larger version (55K): [in this window] [in a new window]   FIGURE 6.  Breathing artifacts on CT: CT slices from different patients, acquired on hybrid PET/CT scanner. (A) Coronal slice of CT acquired with expiration breath-hold command. Arrows indicate artifact in middle of liver and spleen due to unsustained breath-hold. (B) Sagittal slice of CT acquired with expiration breath-hold command. Breathing commands were given relatively late and can be recognized by movement of abdominal wall (left arrow); resulting liver motion causes caudal tip of liver to appear twice (right arrow). (C) Coronal slice of CT acquired during free breathing. Breathing artifacts (arrows) are visible throughout image.