Accuracy and Precision of Radioactivity Quantification in Nuclear Medicine Images
Section snippets
Reliability of Quantitative Images
The goal of activity quantification is to enable diagnostic or therapeutic decisions based on estimates of activity in objects or regions in the body. Two criteria characterize the reliability of such estimates: accuracy and precision. Accuracy describes the deviation of an estimate of some quantity from the true value, whereas precision describes the variability of an estimate about its mean. Accuracy is often expressed in terms of the bias or relative error, whereas precision is described by
Factors Affecting Reliability of Image-Based Activity Estimates
The process of estimating activity often involves implicit or explicit modeling of the image formation process. The factors in imaging systems that result in the failure to satisfy this model are often referred to as image degrading factors. Degrading factors can be divided into the following 3 categories: effects resulting from the physics of the image formation, factors resulting from the choice of image protocol parameters, and factors resulting from biology and physiology of the patient.
Quantitative Planar Imaging
Imaging with a scintillation camera is measurement of 2D projections of a 3D activity distribution. Consequently, the source depth and extent in the direction parallel to the projection is not resolved. This complicates activity quantification because more than one organ can contribute to a particular pixel value in the projection image.
Quantitative SPECT
There have been several published reviews on quantitative SPECT. Tsui et al3 and the Society of Nuclear Medicine Computer and Instrumentation Council4 provide a taxonomy of the image degrading effects, as well as a description of compensation methods, and include some profiles that demonstrate the quantitative accuracy achievable with SPECT. Due to the theoretical advantages of statistical iterative reconstruction (IR) algorithms, in the following we confine our discussion to IR-based
Quantitative PET
PET is construed as a highly quantitative imaging modality due to the ability to calibrate PET scanners to provide image readout data directly in units of Bq/cc (μCi/cc). Calibration is performed by acquiring images on the PET scanner of a known 18F activity after uniform mixing in a cylindrical phantom of known volume. This known activity concentration in the phantom is used as the basis to convert the count density information into an activity concentration in the reconstructed, attenuation
Acknowledgments
Portions of this work have been supported by the Swedish Cancer Society. Eric Frey would like to thank Bin He, Ph.D., for comments on the manuscript and acknowledge support for portions of this work by Public Health Service grant R01-CA109234.
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