In the particular case of respiratory-related CAL-101 in vitro motion, the practical image resolution can be degraded by a factor of five over the intrinsic resolution of the system [54]. Furthermore, motion causes blurring of tumors within the patient, making them appear larger in size while having a lower mean radiotracer uptake which, in turn, creates errors in quantification. By having the total activity distributed over a larger region of interest (ROI), the mean and maximum SUVs of the tumor will be underestimated. Additionally,

such motion can entirely obscure the presence of smaller lesions. The problem is further exacerbated in dynamic imaging whereby any motion can potentially increase or decrease (in an unpredictable manner) the time activity course from a particular voxel resulting in decreased signal-to-noise and accuracy for estimating

kinetic parameters. Early selleckchem methods of motion correction in PET relied on realigning individual frames to a reference position and then summing the result to obtain a single volume [55]. Others have explored the use of external tracking devices and video cameras to record when movements take place during image acquisition and using these time stamps to start new frames that could then be retrospectively registered [55]. Building on this approach, investigators have employed optical tracking systems combined with motion sensors placed on the periphery of the body. While this can be of value in dedicated brain imaging where corrections based on rigid transformations are sufficient to realign head motion, tracking the motion of the chest, for instance, provides limited information about internal nonrigid motion, such as how the diaphragm and heart move during the respiratory cycle. Additionally, visual tracking methods are

often not applicable for PET–MR scanners as some RF coils preclude a clear view of the ROI being imaged. It is important to note that CT-based methods for motion correction are limited by the fact that the CT and PET acquisitions do not occur simultaneously; that is, any motion occurring Racecadotril between the transmission and emission acquisitions will cause a spatial mismatch between the two data sets, thereby compromising the integrity of the motion correction. As noted in Section 2, this misregistration of the attenuation map will also adversely affect the quantitative accuracy and could give rise to artifacts. Simultaneous PET–MRI potentially offers a practical solution to the problem of correcting motion occurring during a PET acquisition. A natural way to make use of MR images to correct for PET motion is to simultaneously acquire high-spatial-resolution MR images while the PET data are being acquired. The MR images can then be retrospectively registered at the conclusion of data collection, and the appropriate transformations can then be applied to the PET data.