Semi-Quantitative Metrics in Positron Emission Tomography
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The Standardized Uptake Value (SUV) is a method for semiquantitative evaluation of radiotracer accumulation on PET scans. Changes in SUV can be used to determine treatment response. However, SUV measurements are influenced by a variety of biological and technological factors, including image reconstruction parameters.
There are other semiquantitative metrics used in PET that relate to the total metabolic activity of a tumor. Current metrics of this type (e.g., Total Lesion Glycolysis) use a combination of SUV and an object volume. Such concentration-based metrics may not capture all radioactivity of an object. We propose a more direct method to assess total radiotracer uptake (TRU): the total radioactivity in a large VOI is measured and background is subtracted.
Phantom studies were performed to assess the effect of image reconstruction parameters on SUV, and to compare the TRU with concentration-based metrics. Patient images were evaluated to estimate the percent error of the TRU metric in imaging of humans.
A whole body phantom with 1 cm hot spheres was scanned with a GE Discovery 690 PET/CT scanner, with time of flight (TOF) capability. Data were reconstructed several different ways to examine the effect of image matrix size, amount of smoothing, field of view (FOV) size, TOF vs. non-TOF reconstruction, iterations of reconstruction algorithm, and image matrix shift on SUV.
An additional whole body phantom was scanned on the same system to compare the accuracy and variability of the new TRU metric with existing measures.
Reconstruction parameters had substantial effects on SUV for 1 cm spheres. Varying the FOV from 35 to 70 cm produced an 11% change in average normalized SUV. Changing the image matrix size from 128x128 pixels to 256x256 pixels produced an 5.3% difference. Shifting the image matrix produced up to a 12% change in SUV. TOF vs. non-TOF reconstruction resulted in up to a 29% difference in SUV for two iterations.
The TRU method was more accurate than TLG and SUV for all sphere types in images with 0 mm to 10 mm of smoothing. Mean errors of TRU were between 1-12%. The TRU method was less variable than TLG in unsmoothed images with acquisition lengths of 1, 2, and 4 minutes. Coefficients of variation were between from 2-17% for TRU measurements, compared to 5-19% for TLG measurements. Simulation of TRU applied to human images shows potential error from 10-18% for 10:1 lesions 1-4 cm in diameter.
Changes in image reconstruction parameters could significantly influence the SUV for small, 1 cm lesions. These effects are reduced for larger, 2.5 cm lesions.
TRU can accurately quantify small lesions in a phantom study. In some cases, TRU is less variable than TLG and SUV. Computer simulations of error in TRU when applied to human studies show low percentage errors for realistic tumor contrasts and volumes.
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