Multithreaded cardiac CT.

Phase-correlated CT, as it is used for cardiac imaging, is the most popular and the most important but also the most demanding special CT application in the clinical routine, today. Basically, it fulfills the four-dimensional imaging task of depicting a quasiperiodically moving object at any desired motion phase with significantly reduced motion artifacts. Although image quality with phase-correlated reconstruction is far better than with standard reconstruction, there are motion artifacts remaining and improvements of temporal resolution are required. As a well-known alternative to simply decreasing rotation time, we consider a spiral cone-beam CT scanner that has G x-ray guns and detectors mounted. We call this a multisource or a multithreaded CT scanner. Aiming for improved temporal resolution the relative temporal resolution tau, which measures the fraction of a motion period that enters the image, is studied as a function of the motion rate (heart rate) and the degree of scan overlap (pitch value) for various configurations. The parameters to optimize are the number of threads G and the interthread parameters delta alpha and delta z, which are the angular and the longitudinal separation between adjacent threads, respectively. To demonstrate the improvements approximate image reconstruction of multithreaded raw data is performed by using a generalization of the extended parallel back projection cone-beam reconstruction algorithm [Med. Phys. 31(6), 1623-1641 (2004)] to the case of multithreaded CT. Reconstructions of a simulated cardiac motion phantom and of simulated semi-antropomorphic phantoms are presented for two and three threads and compared to the single-threaded case to demonstrate the potential of multithreaded cardiac CT. Patient data were acquired using a clinical double-threaded CT scanner to validate the theoretical results. The optimum angle delta alpha between the tubes is 90 degrees for a double-threaded system, and for triple-threaded scanners it is 60 degrees or 120 degrees. In all cases, delta z = 0 results as an optimum, which means that the threads should be mounted in the same transversal plane. However, the dependency of the temporal resolution on delta z is very weak and a longitudinal separation delta z not = 0 would not deteriorate image quality. The mean temporal resolution achievable with an optimized multithreaded CT scanner is a factor of G better than the mean temporal resolution obtained with a single-threaded scanner. The standard reconstructions showed decreased cone-beam artifacts with multithreaded CT compared to the single-threaded case. Our phase-correlated reconstructions demonstrate that temporal resolution is significantly improved with multithreaded CT. The clinical patient data confirm our results.