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Cause and Magnitude of the Error Induced by Oral CT Contrast Agent in CT-Based Attenuation Correction of PET Emission Studies

Division of Nuclear Medicine, University Hospital of Zurich, Zurich, Switzerland

View larger version (21K): [in a new window]   FIGURE 1. Linear attenuation coefficient extrapolated from narrow-beam measurements at different photon energies (4). Plots 1 and 2 represent content of iodine and barium in oral CT contrast agents Gastrografin and Micropaque Scanner, respectively, mixed with water, cortical bone (plot 3), and soft tissue (plot 4). High attenuation of bone at 511 keV is due to its high density.

View larger version (80K): [in a new window]   FIGURE 2. Attenuation images of NEMA phantom containing 2 cylindric acrylic inserts with oral CT contrast agent in clinical concentrations. Row A shows orthogonal CT sections (140 kV, 160 mA); row B shows matched PET transmission images acquired with 68Ge rods in 30 min. Slight attenuation increase originating from insert walls is notable in transmission images. 1 = Micropaque Scanner; 2 = Gastrografin; 3 = water.

View larger version (92K): [in a new window]   FIGURE 3. Example of the performed analysis. (A) CT image with thresholds [-160 HU, 240 HU] showing regions that define oral CT contrast, tumor, and reference tissue. (B) CT image after replacement of contrast-enhanced pixels by normal values. (C) PET emission image attenuation corrected with CT image A. (D) PET emission image attenuation corrected with CT image B, resulting in approximation of true 18F-FDG uptake. (E) Difference image C - D normalized to D, showing percentage error due to attenuation correction with CT contrast-enhanced pixels. Maximal difference is 9.6%.

View larger version (17K): [in a new window]   FIGURE 4. Average attenuation of 2 oral CT contrast agents at 3 different concentrations measured in CT and with PET 511-keV transmission scans. CT attenuation increases with increasing concentration of contrast agent. PET attenuation, however, depends minimally on concentration of contrast agent and is only slightly higher than attenuation of water. Transformation applied by scanner software to convert CT numbers into PET attenuation coefficients in Discovery LS scanner is indicated as dashed line (3). If it is applied to measured CT numbers, too high PET attenuation coefficients are calculated ().

View larger version (20K): [in a new window]   FIGURE 5. Average HU and SD measured in different sections of gastrointestinal tract for control group studied without oral contrast agent and for patient group.

View larger version (17K): [in a new window]   FIGURE 6. Overestimation of SUV in image regions containing oral CT contrast agent. Shown is percentage SUV error induced by CT-based PET attenuation correction with contrast enhancement present in CT images.

View larger version (14K): [in a new window]   FIGURE 7. Overestimation of SUV in image regions absent of oral CT contrast agent. Shown is percentage SUV error induced by CT-based PET attenuation correction with CT contrast enhancement present in other parts of image.