Schär, Michael

ABSTRACT Purpose Standard spatial‐spectral excitation pulses apply a fly‐back gradient between sub‐pulses, limiting, for example, how thin the slices can be (> 4 mm). Without fly‐back gradients, slices can be as thin as 1.7 mm but require a phase calibration for the sub‐pulses with inverted gradients due to system imperfections. Here we propose and test a fast (< 1 s), localized pre‐scan enabling thin‐slice frequency‐selective excitation. Methods The calibration pre‐scan determines the correction phase between even and odd sub‐pulses by measuring the phase difference of two FIDs excited with either the last or second‐to‐last RF sub‐pulse of the spatial‐spectral pulse. Non‐localized and localized versions of the calibration pre‐scan are tested and validated for water‐only excitation in phantoms and the human heart. As an example, breath‐held coronary MR angiography is performed with a spiral multi‐slice sequence employing the proposed pulses with a slice thickness of 1.7 mm. Results The proposed calibration pre‐scan takes 11.1 ms per slice, whether it is localized or not. Localized calibration phases are validated with an image‐based calibration map. Slice profiles are visualized for pulses with different time‐bandwidth products. Fat is suppressed to below 20% and 25% of the surrounding water signal in both phantoms and the human heart, respectively. High‐quality coronary angiograms are demonstrated in all test subjects. Conclusion Spatial‐spectral pulses without fly‐back gradients can be calibrated quickly (up to 90 slices in 1 s), enabling thinner slices, higher excitation angles, or pulses with sharper slice profiles using waveforms with higher time‐bandwidth products.

ABSTRACT Purpose To simultaneously measure skeletal muscle energetics and blood flow (BF) before, during, and after dynamic plantar flexion exercise (PFE). Methods Non‐localized pulse‐acquire phosphorus‐31 magnetic resonance spectroscopy ( 31 P MRS) and phase contrast flow magnetic resonance imaging ( 1 H MRI) using golden‐angle rotated spiral readouts were acquired in an interleaved fashion. Data were collected at rest, during PFE to volitional fatigue, and post‐exercise recovery in 10 healthy adults. Interleaved popliteal artery BF was validated against conventional cine phase‐contrast measures at rest and post‐exercise recovery, while calf muscle energetics were validated by comparing to conventional 31 P MRS‐only measures at rest. To minimize motion artifacts during dynamic PFE, flow data were resolved in 3 dimensions (cardiac phase, pedal position, and exercise stage). Results Resting phosphocreatine (PCr) and inorganic phosphate concentrations were similar between both techniques (both p > 0.05). During PFE, PCr declined to 46% ± 10% of rest. PCr recovery time was 43 ± 15 s. Resting BF was 132 ± 43 mL/min and increased up to four‐fold during PFE depending on exercise stage and pedal position. Notably, there was excellent agreement between the interleaved and conventional BF measures ( r 2 = 0.95, slope = 0.977, p < 0.0001) with minimal bias (12.62 mL/min) over a wide range of physiologically relevant BF values (from ∼60 to 600 mL/min). Conclusion The feasibility of simultaneous assessment of skeletal muscle energetics and BF during dynamic exercise by interleaved 31 P MRS/ 1 H flow MRI was demonstrated along with good agreement between interleaved and conventional techniques during rest and recovery. One can now simultaneously quantify critical BF‐metabolism relationships and use this to probe disease‐related adaptations.