# Standardized uptake value

(Redirected from Standardized Uptake Value)
3-dimensional [18F]FDG-PET image with 3D-ROI generated in one mouse click by a threshold based algorithm. The blue dot in the MIP image bottom right marks the maximum SUV within the ROI.

The standardized uptake value (SUV) is often used in positron emission tomography (PET) imaging for a simple semiquantitative analysis.[1] Its use is particularly common in the analysis of [18F]fluorodeoxyglucose ([18F]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. If the arterial input function is available from blood sampling or advanced image analysis techniques, other measures like the fractional uptake rate (FUR), may be preferable, or more advanced pharmacokinetic modeling may be undertaken.

The SUV is calculated either pixel-wise yielding a parametric image, or over a region of interest (ROI). This may be done for any image acquired at time point t, or for all images of a dynamic series acquired at multiple time points. The SUV is commonly defined as the ratio of (1) the tissue radioactivity concentration c (e.g. in MBq/kg = kBq/g) at time point t, and (2) the injected activity (e.g. in MBq, extrapolated to the same time t) divided by the body weight (e.g. in kg):

$SUV(t) = \frac {c(t)}{injected\ activity(t) \quad / \quad body\ weight}$

In other words, the SUV represents the ratio of (1) the image derived radioactivity concentration found in a selected part of the body at a certain time point, and (2) as reference the radioactivity concentration in the hypothetical case of an even distribution of the injected radioactivity across the whole body. Here, the two radioactivity measures need to be from the same time point, e.g. the time of injection or the time of the image (or "image frame"). For example in the latter case, the injected activity is to be corrected for the physical decay between time of injection (t=0) and the time of the image frame (t). A (small) systematic error might be introduced since the image frame is acquired typically within minutes whereas the injected activity (more precisely, each of the two activity measurements of the syringe, before and after injection) is measured in a relatively short time interval, practically at a time point. Depending on the image frame duration compared to the radioisotope half life, e.g. the image frame midpoint may be considered a good approximation as time point of reference.

Moreover, the difference between (a) c(t) measured per volume of soft tissue via PET images and (b) the even distribution reference concentration per weight of the whole body is ignored here, assuming a mass density of 1 kg/L.

Instead of the body weight used in the equation above, some authors prefer the lean body weight[2] or the body surface area.[3]

Also for c(t) from a region of interest, different measures are found in the literature, e.g. the maximum intensity value within the ROI, the mean intensity value of the ROI,[4] or the mean intensity value of the ROI after applying an intensity threshold (thus excluding a number of pixels of the ROI).

The SUV can be significantly affected among other things by image noise, low image resolution and/or user biased ROI selection.[5] For the semi-quantitative analysis of 18F]FDG uptake in tissue or tumor, several corrections have been recommended (see [6] and references therein).

The ratio of the SUV from two different regions within the same PET image (i.e. from a target and a reference region) is commonly abbreviated as SUVR. For the SUVR, the injected activity and body weight that are part of the SUV, cancel.

The SUV concept has been only begun to be tested in connection with other radiotracers like [18F]fluorothymidine ([18F]FLT) and conclusions on its usefulness and robustness in these cases might be premature.[7]

In summary, SUVs are a convenient measure for the evaluation of [18F]FDG PET images within a subject to study therapy monitoring/therapy response, and also for comparison between subjects. However, care has to be taken with respect to its pitfalls and with respect to the interpretation of the results.