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PET/CT: Comparison of Quantitative Tracer Uptake Between Germanium and CT Transmission Attenuation-Corrected Images

¹ Division of Nuclear Medicine, Johns Hopkins School of Medicine, Baltimore, Maryland
² Department of Environmental Health Sciences, Johns Hopkins School of Public Health, Baltimore, Maryland
³ General Electric Medical Systems, Milwaukee, Wisconsin

View larger version (17K): [in a new window]   FIGURE 1. Conversion scale used to transform CT Hounsfield units (HU) into attenuation coefficients in Discovery LS (General Electric Medical Systems, Waukesha, WI). Bilinear function is defined by 3 coordinates (-1,000 HU, 0 cm-1), (0 HU, 0.093 cm-1), and (3,071 HU, 0.262 cm-1). In this system, CT values > 3,071 HU are regarded as 3,071 HU.

View larger version (31K): [in a new window]   FIGURE 2. Representative images for analysis. After we displayed PET emission images reconstructed using 2 different attenuation maps of CT (A) and 68Ge (B), and corresponding CT images (C), we evaluated quantitative tracer uptake values for same location. This figure shows example of assessment of S8 segment in liver.

View larger version (21K): [in a new window]   FIGURE 3. %Diff between CT-corrected images and germanium-corrected images for mean values (A) and maximum values (B). When CT-corrected images show larger uptake values, %Diff is calculated as positive by definition of formula. In lung, plots show comparable scatter in both positive and negative areas, but in all other organs, positive values are predominant, that is, CT attenuation correction values have higher apparent radioactivity concentration than do images reconstructed using 68Ge attenuation correction. TE = temporal lobe; CE = cerebellum; LU = lung; S8 = liver segment S8; S6 = liver segment S6; RC = renal cortex; RP = renal pelvis; PE = pelvis. • = mean ± SD.

View larger version (16K): [in a new window]   FIGURE 4. %Diff between CT-corrected images and germanium-corrected images for tumor and tumor-to-background ratios (A) and their location-based %Diff for mean values (B) and maximum values (C). As is shown in normal organs, CT-corrected images tended to show higher values than did germanium-corrected images. According to location-based data, %Diff was higher in bone lesions than in nonbone lesions for mean (P < 0.05) and maximum (P < 0.01) values. Lesions, including colon tumor or lymph node metastasis, that were located in pelvis were classified as pelvis. Lymphadenopathy in pelvic region was excluded from this category and included in pelvis. LN = lymph node; max = maximum; • = mean ± SD.

View larger version (14K): [in a new window]   FIGURE 5. Relationship between %Diff and CT value for mean values (A) and maximum values (B). Weak positive correlation was observed between CT values and mean values (r = 0.38; P = 0.021) and between CT values and maximum values (r = 0.45; P = 0.007).