Time-resolved velocity imaging using the magnetic resonance phase contrast technique can provide clinically important quantitative flow measurements in vivo but suffers from long scan times when based on conventional spin-warp sequences. This can be particularly problematic when imaging regions of the abdomen and thorax because of respiratory motion. We present a rapid phase contrast sequence based on an interleaved spiral k-space data acquisition that permits time-resolved, three-direction velocity imaging within a breath-hold. Results of steady and pulsatile flow phantom experiments are presented, which indicate excellent agreement between our technique and through plane flow measurements made with an in-line ultrasound probe. Also shown are results of normal volunteer studies of the carotids, renal arteries, and heart.

Purpose: To investigate a three‐pass reconstruction approach for metal artifact reduction in x‐ray CT. Method and Materials: The algorithm consists of: 1) Initial reconstruction from the original sinogram data; 2) Simple thresholding to identify high‐density regions (e.g. metal) that can cause artifacts; 3) Delineation of corresponding regions in the original sinogram that are replaced using linear interpolation; 4) Second reconstruction after the interpolation; 5) All pixels in the second image that lie between −500 and +500 HU are replaced with the mean of these pixels; 6) Rays in the sinogram through the metal are estimated a second time through forward projection of the segmented second image; 7) Third and final reconstruction. To avoid the need for forward projection across the entire native field‐of‐view (FOV) during step 6 above, a double‐wedge filter is applied in the 2DFT space of the sinogram so that objects outside of the reconstruction FOV are filtered out of the original sinogram. If k and p are the view‐angle and fan‐angle frequency variables, respectively, the double‐wedge filter consists of setting to zero all frequencies in the 2DFT of the sinogram for which |k/(k+p)|>R/L, where R is the reconstruction FOV and L is the source‐to‐isocenter distance. Results: The algorithm substantially reduces streak and blooming artifacts that are present in the original reconstruction for three scans with dental fillings, and performs better than linear interpolation across missing regions in the sinogram. The double‐wedge filter is effective in removing contributions to the sinogram from objects outside of the reconstruction FOV. Conclusion: The algorithm is effective for reducing metal artifacts as well as computationally practical. Conflicts of Interest: Funding was provided by GE Healthcare.

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Purpose: Dose efficiency of dual kVp imaging can be improved if the beams are filtered to remove photons in the common part of their spectra, thereby increasing spectral separation. While there are a number of advantages to rapid kVp‐switching for dual energy, it may not be feasible to have two different filters for the two spectra. Therefore, we are interested in whether a fixed added filter can improve the dose efficiency of kVp‐switching dual energy x‐ray systems.Methods: Simulations were done to decompose known phantoms into basis materials of aluminum and water, using 80 and 140 kVp x‐ray spectra. Precision of the decomposition was evaluated based on the propagation of the Poisson noise in the detected intensities through the decomposition function. We hypothesized that a K‐edge filter would provide the energy selectivity needed to remove overlap of the spectra and hence increase the precision at constant dose. When optimizing the filter design, we considered filtration material (Z), filter thickness, and the relative power allocated to the two beams. We imposed a constraint of <50% loss of x‐ray output due to filtration while maintaining constant entrance dose. The detector absorption efficiency of a CsI screen was also included in the simulation. Results: An improvement in precision of >30% at fixed dose (or lower dose for the same precision) was achieved with optimal filtration. The optimal filter material depends somewhat on the phantom composition and ranges across the lanthanide series. Increasing filtration thickness increases the average spectral separation. When ignoring tube power limitations, the precision improvement with increasing filtration continues increasing, although sublinearly. Conclusions: This study demonstrates the potential of fixed filtration to improve the dose efficiency and material decomposition precision for rapid kVp‐switching dual energy systems. GE Healthcare