Simultaneous brain and neck time-of-flight MRA using spiral multiband with localized quadratic encoding.

PURPOSE: To develop a method that achieves simultaneous brain and neck time-of-flight (ToF) magnetic resonance angiography (MRA) within feasible scan timeframes. METHODS: Localized quadratic (LQ) encoding is efficient for both signal-to-noise ratio (SNR) and in-flow enhancement. We proposed a spiral multiband LQ method to enable simultaneous intracranial and carotid ToF-MRA within a single scan. To address the venous signal contamination that becomes a challenge with multiband (MB) ToF, tilt-optimized non-saturated excitation (TONE) and partial-Fourier slice selection (PFSS) were further introduced in the LQ framework to mitigate the venous signal and improve artery contrast. A sequential spiral MB and LQ reconstruction pipeline was employed to obtain the brain-and-neck image volumes. RESULTS: The proposed MB method was able to achieve simultaneous brain and neck ToF-MRA within a 2:50-min scan. The complementarily boosted SNR-efficiency by MB and LQ acquisitions allows for the increased spatial coverage without increase in scan time or noticeable compromise in SNR. The incorporation of both TONE and PFSS effectively alleviated the venous contamination with improved small vessel sensitivity. Selection of scan parameters such as the LQ factor and flip angle reflected the trade-off among SNR, blood contrast, and venous suppression. CONCLUSIONS: A novel MB spiral LQ approach was proposed to enable fast intracranial and carotid ToF-MRA with minimized venous corruption. The method has shown promise in MRA applications where large spatial coverage is necessary.

Generating spiral gradient waveforms with a compact frequency spectrum.

PURPOSE: To generate efficient gradient waveforms for spiral MRI which mitigate the high-frequency attenuation inherent in gradient systems. THEORY AND METHODS: Spiral MRI has many clinical advantages, including high temporal and SNR efficiency. One of the challenges for robust spiral MRI is a high sensitivity to imperfections in the gradient system, which requires some form of correction in order to map data correctly in k-space. A previous numerical algorithm for generating spiral gradient waveforms was modified to reduce its high-frequency content with minimal increase in waveform duration. RESULTS: Examples are shown of compact frequency gradient waveforms. Software implementing the algorithm is made available. CONCLUSION: An algorithm to produce gradient waveforms with a compact frequency spectrum is described. This algorithm results in greatly reduced overall error and better compatibility with gradient systems than the original algorithm from which it was derived.

Effects of motion on MRI signal decay from micron-scale particles.

Transverse decay rate (R2∗) mapping is an established method for measuring iron overload in various biological tissues. Recently, R2∗ mapping was used to measure the mean 3D concentration distribution of micron-size particles dispersed in turbulent flows. However, some discrepancy was observed between the measured R2∗ and the expected decay based on existing theory. The present paper examines three flow-related mechanisms that could be responsible for this discrepancy. Computational simulations were used to study the effects of relative particle-fluid motion and preferential concentration by turbulence, while the effect of enhanced proton dispersion due to turbulence was examined via the existing MRI relaxation theory. Each flow phenomenon was shown to produce a different effect on the signal-time curve, as well as the extracted R2∗. Comparison to experimental data in a square channel flow showed that relative motion between the particles and fluid was the most likely cause of the discrepancy in the previous experiments; however, all three effects may be present in both medical and non-medical flows, and their differing effects on the MRI signal may eventually allow for their identification from MRI data.