New challenges in MDCT.

The performance of computed tomography has approximately doubled every two years since the mid 1980s. The introduction of multidetector CT (MDCT) has sustained this trend. While 4-slice scanners were able to scan faster or with higher resolution than single slice units, 16-slice scanners introduced fast isotropic imaging and 32-to 64-slice scanners made it possible to combine ECG-gated cardiac evaluation with other CT applications. New technologies such as z-flying focal spot improve image quality and artifact behaviour. At present, however, CT faces a number of challenges that make it difficult to extrapolate performance into the future. The most important challenge is the relationship between spatial resolution and image noise; for identical image noise at higher spatial resolution, radiation dose will have to be increased substantially. For example, if the spatial resolution is to be increased by a factor of 2 in all spatial directions, the dose will need to be increased by 2(4)=16. This x(4) relationship results from the lower number of quanta that penetrate a smaller voxel and from the fact that CT images have to be reconstructed from projectional data. Dose containment and noise suppression techniques will therefore become the defining factors for further improvement of spatial resolution in CT. Increasing detector width will become important for CT perfusion imaging but will face challenges from scattered radiation, cone beam artifacts and overscanning. Faster effective rotation speeds together with improved dose efficiency of image reconstruction will become the main challenges for cardiac CT imaging.