Traveling Volunteers: A Multi‐Vendor, Multi‐Center Study on Reproducibility and Comparability of 4D Flow Derived Aortic Hemodynamics in Cardiovascular Magnetic Resonance

Background: Implementation of four-dimensional flow magnetic resonance (4D Flow MR) in clinical routine requires awareness of confounders. Purpose: To investigate inter-vendor comparability of 4D Flow MR derived aortic hemodynamic parameters, assess scan-rescan repeatability, and intra- and interobserver reproducibility. Study type: Prospective multicenter study. Population: Fifteen healthy volunteers (age 24.5 ± 5.3 years, 8 females). Field strength/sequence: 3 T, vendor-provided and clinically used 4D Flow MR sequences of each site. Assessment: Forward flow volume, peak velocity, average, and maximum wall shear stress (WSS) were assessed via nine planes (P1-P9) throughout the thoracic aorta by a single observer (AD, 2 years of experience). Inter-vendor comparability as well as scan-rescan, intra- and interobserver reproducibility were examined. Statistical tests: Equivalence was tested setting the 95% confidence interval of intraobserver and scan-rescan difference as the limit of clinical acceptable disagreement. Intraclass correlation coefficient (ICC) and Bland-Altman plots were used for scan-rescan reproducibility and intra- and interobserver agreement. A P-value 0.9: excellent, 0.75-0.9: good). Results: Ten volunteers finished the complete study successfully. 4D flow derived hemodynamic parameters between scanners of three different vendors are not equivalent exceeding the equivalence range. P3-P9 differed significantly between all three scanners for forward flow (59.1 ± 13.1 mL vs. 68.1 ± 12.0 mL vs. 55.4 ± 13.1 mL), maximum WSS (1842.0 ± 190.5 mPa vs. 1969.5 ± 398.7 mPa vs. 1500.6 ± 247.2 mPa), average WSS (1400.0 ± 149.3 mPa vs. 1322.6 ± 211.8 mPa vs. 1142.0 ± 198.5 mPa), and peak velocity between scanners I vs. III (114.7 ± 12.6 cm/s vs. 101.3 ± 15.6 cm/s). Overall, the plane location at the sinotubular junction (P1) presented most inter-vendor stability (forward: 78.5 ± 15.1 mL vs. 80.3 ± 15.4 mL vs. 79.5 ± 19.9 mL [P = 0.368]; peak: 126.4 ± 16.7 cm/s vs. 119.7 ± 13.6 cm/s vs. 111.2 ± 22.6 cm/s [P = 0.097]). Scan-rescan reproducibility and intra- and interobserver variability were good to excellent (ICC ≥ 0.8) with best agreement for forward flow (ICC ≥ 0.98). Data conclusion: The clinical protocol used at three different sites led to differences in hemodynamic parameters assessed by 4D flow. Level of evidence: 2 TECHNICAL EFFICACY STAGE: 2

Autocalibrated cardiac tissue phase mapping with multiband imaging and k-t acceleration

PURPOSE: To develop an autocalibrated multiband (MB) CAIPIRINHA acquisition scheme with in-plane k-t acceleration enabling multislice three-directional tissue phase mapping in one breath-hold. METHODS: A k-t undersampling scheme was integrated into a time-resolved electrocardiographic-triggered autocalibrated MB gradient-echo sequence. The sequence was used to acquire data on 4 healthy volunteers with MB factors of two (MB2) and three (MB3), which were reconstructed using a joint reconstruction algorithm that tackles both k-t and MB acceleration. Forward simulations of the imaging process were used to tune the reconstruction model hyperparameters. Direct comparisons between MB and single-band tissue phase-mapping measurements were performed. RESULTS: Simulations showed that the velocities could be accurately reproduced with MB2 k-t (average ± twice the SD of the RMS error of 0.08 ± 0.22 cm/s and velocity peak reduction of 1.03% ± 6.47% compared with fully sampled velocities), whereas acceptable results were obtained with MB3 k-t (RMS error of 0.13 ± 0.58 cm/s and peak reduction of 2.21% ± 13.45%). When applied to tissue phase-mapping data, the proposed technique allowed three-directional velocity encoding to be simultaneously acquired at two/three slices in a single breath-hold of 18 heartbeats. No statistically significant differences were detected between MB2/MB3 k-t and single-band k-t motion traces averaged over the myocardium. Regional differences were found, however, when using the American Heart Association model for segmentation. CONCLUSION: An autocalibrated MB k-t acquisition / reconstruction framework is presented that allows three-directional velocity encoding of the myocardial velocities at multiple slices in one breath-hold

Evaluating an acoustically quiet EPI sequence for use in fMRI studies of speech and auditory processing.

Echoplanar MRI is associated with significant acoustic noise, which can interfere with the presentation of auditory stimuli, create a more challenging listening environment, and increase discomfort felt by participants. Here we investigate a scanning sequence that significantly reduces the amplitude of acoustic noise associated with echoplanar imaging (EPI). This is accomplished using a constant phase encoding gradient and a sinusoidal readout echo train to produce a narrow-band acoustic frequency spectrum, which is adapted to the scanner's frequency response function by choosing an optimum gradient switching frequency. To evaluate the effect of these nonstandard parameters we conducted a speech experiment comparing four different EPI sequences: Quiet, Sparse, Standard, and Matched Standard (using the same readout duration as Quiet). For each sequence participants listened to sentences and signal-correlated noise (SCN), which provides an unintelligible amplitudematched control condition. We used BOLD sensitivity maps to quantify sensitivity loss caused by the longer EPI readout duration used in the Quiet and Matched Standard EPI sequences. We found that the Quiet sequence provided more robust activation for SCN in primary auditory areas and comparable activation in frontal and temporal regions for Sentences N SCN, but less sentence-related activity in inferotemporal cortex. The increased listening effort associated with the louder Standard sequence relative to the Quiet sequence resulted in increased activation in the left temporal and inferior parietal cortices. Together, these results suggest that the Quiet sequence is suitable, and perhaps preferable, for many auditory studies. However, its applicability depends on the specific brain regions of interest. Crow

Wideband Self‐Grounded Bow‐Tie Antenna for Thermal MR

The objective of this study was the design, implementation, evaluation and application of a compact wideband self‐grounded bow‐tie (SGBT) radiofrequency (RF) antenna building block that supports anatomical proton (1H) MRI, fluorine (19F) MRI, MR thermometry and broadband thermal intervention integrated in a whole‐body 7.0 T system. Design considerations and optimizations were conducted with numerical electromagnetic field (EMF) simulations to facilitate a broadband thermal intervention frequency of the RF antenna building block. RF transmission (B1+) field efficiency and specific absorption rate (SAR) were obtained in a phantom, and the thigh of human voxel models (Ella, Duke) for 1H and 19F MRI at 7.0 T. B1+ efficiency simulations were validated with actual flip‐angle imaging measurements. The feasibility of thermal intervention was examined by temperature simulations (f = 300, 400 and 500 MHz) in a phantom. The RF heating intervention (Pin = 100 W, t = 120 seconds) was validated experimentally using the proton resonance shift method and fiberoptic probes for temperature monitoring. The applicability of the SGBT RF antenna building block for in vivo 1H and 19F MRI was demonstrated for the thigh and forearm of a healthy volunteer. The SGBT RF antenna building block facilitated 19F and 1H MRI at 7.0 T as well as broadband thermal intervention (234‐561 MHz). For the thigh of the human voxel models, a B1+ efficiency ≥11.8 μT/√kW was achieved at a depth of 50 mm. Temperature simulations and heating experiments in a phantom demonstrated a temperature increase ΔT >7 K at a depth of 10 mm. The compact SGBT antenna building block provides technology for the design of integrated high‐density RF applicators and for the study of the role of temperature in (patho‐) physiological processes by adding a thermal intervention dimension to an MRI device (Thermal MR).BMBF, 13GW0102A, KMU-Innovativ - Verbundprojekt: Forschung für Tumortherapie mit lokalisierter Hochfrequenz-Hyperthermie: Diagnostik, Therapiesteuerung und -kontrolle mit ultrahochfeld MRT (3-IN-1:THERAHEAT) - Teilvorhaben: Erforschung von Hochfrequenzantennen für Tumortherapie mittels kontrollierter Hochfrequenz-HyperthermieEC/H2020/EU/743077/Thermal Magnetic Resonance: A New Instrument to Define the Role of Temperature in Biological Systems and Disease for Diagnosis and Therapy/ThermalM

Entwicklung von geräuscharmen Bildgebungstechniken für die funktionelle Magnetresonanztomographie

Ein bekanntes Problem in der funktionellen MRI (fMRI) ist, dass der Schalldruckpegel des MR-Scanners, der durch die Schaltung der Gradientenspulen erzeugt wird, ein BOLD-Signal im auditorischen Cortex hervorruft. In dieser Arbeit wird eine neue, geräuscharme echoplanare (EPI) Bildgebungssequenz vorgestellt und für auditorische Messungen optimiert. Durch die Verwendung eines sinusförmigen Auslesegradienten und eines konstanten Phasenkodiergradienten erzeugt die Sequenz ein schmalbandiges akustisches Spektrum. Dessen Frequenz ist an die Frequenzantwortfunktion des Auslesegradienten angepasst. Verglichen mit einer konventionellen EPI-Sequenz des Herstellers ist der Schalldruckpegel um bis zu 29 dBA reduziert. Die Sequenz wird in einem Block-Design-Experiment mit einem sinusförmigen Stimuluston evaluiert und mit der konventionellen EPI-Sequenz verglichen. In der statistischen Auswertung zeigt sich bei Verwendung der geräuscharmen Sequenz ein höheres Signifikanzlevel der auditorischen Aktivierung. Dies deutet stark darauf hin, dass die geräuscharme Sequenz einen höheren auditorischen BOLD-Kontrast erzeugt, als die in der fMRI konventionell eingesetzten EPI-Sequenzen

4D flow imaging with UNFOLD in a reduced FOV

PURPOSE: Two-dimensional selective excitation (2DRF) allows shortening 4D flow scan times by reducing the FOV, but the longer 2DRF pulse duration decreases the temporal resolution, yielding underestimated peak flow values. Multiple k-space lines per cardiac phase, n(l) ≥ 2, are commonly applied in 4D flow MRI to shorten the inherent long scan times. We demonstrate that 2DRF 4D flow with n(l) ≥ 2 can be easily combined with UNFOLD (UNaliasing by Fourier-encoding the Overlaps using the temporaL Dimension), a technique that allows regaining nominally the temporal resolution of the respective acquisition with n(l) = 1, to assure peak flow quantification. METHODS: Two different 2DRF pulses with spiral k-space trajectories were designed and integrated into a 4D flow sequence. Flow phantom experiments and 7 healthy control 4D flow in vivo measurements, with and without UNFOLD reconstructions, were compared with conventional reconstruction and 1D slab-selective excitation (1DRF) by evaluating time-resolved flow curves, peak flow, peak velocity, blood flow volume per cardiac cycle, and spatial aliasing. RESULTS: Applying UNFOLD to 4D flow imaging with 2DRF and reduced FOV increased the quantified in vivo peak flow values significantly by 3.7% ± 2.3% to 5.2% ± 2.4% (P < .05). Accordingly, the peak flow underestimation of 2DRF scans compared with conventional 1DRF scans decreased with UNFOLD. Finally, 2DRF combined with UNFOLD accelerated the 4D flow acquisition 3.5 ± 1.4 fold by reducing the FOV and increasing the effective temporal resolution by 6.7% compared with conventional 1D selective excitation, with 2 k-space lines per cardiac phase. CONCLUSION: Two-dimensional selective excitation combined with UNFOLD allows limiting the FOV to shorten 4D flow scan times and compensates for the loss in temporal resolution with 2DRF (Δt = 64.8 ms) compared with 1DRF (Δt = 43.2 ms), yielding an effective resolution of Δt(eff) = 40.5 ms to enhance peak flow quantification