Standardized uptake value

The standardized uptake value (SUV) is a nuclear medicine term, used in positron emission tomography (PET) as well as in modern calibrated Single Photon Emission Tomography (SPECT) imaging for a (semi)quantitative analysis. Its use is particularly common in the analysis of fluorodeoxyglucose (FDG) images of cancer patients. It can also be used with other PET agents especially when no arterial input function is available for more detailed pharmacokinetic modeling. Otherwise measures like the fractional uptake rate (FUR) or parameters from more advanced pharmacokinetic modeling may be preferable. The standardized uptake value (SUV) is a nuclear medicine term, used in positron emission tomography (PET) as well as in modern calibrated Single Photon Emission Tomography (SPECT) imaging for a (semi)quantitative analysis. Its use is particularly common in the analysis of fluorodeoxyglucose (FDG) images of cancer patients. It can also be used with other PET agents especially when no arterial input function is available for more detailed pharmacokinetic modeling. Otherwise measures like the fractional uptake rate (FUR) or parameters from more advanced pharmacokinetic modeling may be preferable. The SUV is the ratio of the image derived radioactivity concentration cimg and the whole body concentration of the injected radioactivity cinj, S U V = c i m g c i n j {displaystyle SUV={frac {mathit {c_{img}}}{mathit {c_{inj}}}}} While this equation looks simple, there are a number of details that need to be discussed, such as (1) the origin of cimg data, (2) the origin of cinj data, (3) time, and (4) units. The cimg data may be the pixel intensities of a calibrated PET image. Calculated SUV data can then be visualized as parametric SUV image. Alternatively, groups of such pixels may be selected e.g. by manually drawing or otherwise segmenting a region of interest (ROI) on the PET image. Then e.g. the average intensity of that ROI may be used as cimg input to calculate SUVs. The cinj value is calculated as ratio of two independent measurements: the injected radioactivity (injected dose, ID) and the body weight (BW) of the subject. The ID can be estimated e.g. as difference in the radioactivity of the syringe before and after injection, if deemed necessary with correction for physical decay between each of those measurements and the time of injection. Conventionally the time of injection is t=0. This reference concentration represents the hypothetical case of an even distribution of the injected radioactivity across the whole body. SUV values thus quantify the measured deviation from this even radioactivity distribution. The injection of radioactivity is often followed by a waiting time interval and then a time span during which the PET image data are acquired. After image reconstruction, the image cimg (t) data need to be decay corrected to the injection time point t=0. The time point t may be the image acquisition start time, or in case of a long acquisition duration e.g. the midpoint of the PET image acquisition may be more appropriate. This decay correction needs to be done for each image in case of a series of images acquired after a single injection ('dynamic imaging'). The unit of cimg is MBq/mL or equivalent, based on (a) the pixel intensity calibrated with a radioactive source ('phantom') itself of known radioactivity and volume, and (b) the pixel volume or ROI volume. The unit of cinj is MBq/g or equivalent, based on the measured radioactivity and the subject's body weight. This would give SUV in units of g/mL or equivalent. However, SUV is typically presented as a unitless parameter. The reason is that the ROI is usually defined over soft tissue which has a mass density of approximately 1 g/mL. Thus the image derived intensities are implicitly converted by dividing by 1 g/mL to yield cimg in the same units as cinj. This results in a unitless SUV parameter. In summary this gives the following equation to calculate SUV at time t post injection,