Fetal 3D cardiovascular cine image acquisition using radial sampling and compressed sensing.

PURPOSE: To explore a fetal 3D cardiovascular cine acquisition using a radial image acquisition and compressed-sensing reconstruction and compare image quality and scan time with conventional multislice 2D imaging. METHODS: Volumetric fetal cardiac data were acquired in 26 volunteers using a radial 3D balanced SSFP pulse sequence. Cardiac gating was performed using a Doppler ultrasound device. Images were reconstructed using a parallel-imaging and compressed-sensing algorithm. Multiplanar reformatting to standard cardiac views was performed before image analysis. Clinical 2D images were used for comparison. Qualitative and quantitative image evaluation were performed by two experienced observers (scale: 1-4). Volumes, mass, and function were assessed. RESULTS: Average scan time for the 3D imaging was 6 min, including one localizer. A 2D imaging stack covering the entire heart including localizer sequences took at least 6.5 min, depending on planning complexity. The 3D acquisition was successful in 7 of 26 subjects (27%). Overall image contrast and perceived resolution were lower in the 3D images. Nonetheless, the 3D images had, on average, a moderate cardiac diagnostic quality (median [range]: 3 [1-4]). Standard clinical 2D acquisitions had a high cardiac diagnostic quality (median [range]: 4 [3, 4]). Cardiac measurements were not different between 2D and 3D images (all p > 0.16). CONCLUSION: The presented free-breathing whole-heart fetal 3D radial cine MRI acquisition and reconstruction method enables retrospective visualization of all cardiac views while keeping examination times short. This proof-of-concept work produced images with diagnostic quality, while at the same time reducing the planning complexity to a single localizer.

Factors affecting performance of fetal blood T2 measurements for noninvasive estimation of oxygen saturation.

PURPOSE: To ultimately make accurate and precise fetal noninvasive oxygen saturation (sO2 ) measurements by T2 -prepared bSSFP more widely available by systematically assessing error sources in order to potentially reduce perinatal mortality in cardiovascular malformations and fetal growth restriction. METHODS: T2 -prepared bSSFP data were acquired in phantoms; in flowing blood in adults in the superior sagittal sinus, ascending and descending aorta, and main pulmonary artery; and in the fetal descending aorta and umbilical vein. T2 was assessed in relation to T2 two- or three-parameter curve-fitting techniques, SSFP readout, refocusing time delay (τ), constant and pulsatile blood flow, and impact of T1 recovery. Further, fetal T2 and sO2 variability were quantified in the descending aorta and umbilical vein in healthy fetuses and fetuses with cardiovascular malformation (gestational weeks 32-38). RESULTS: In phantoms, three-parameter fitting was accurate irrespective of phase FOV (<4 ms; i.e., <2%), and T2 was overestimated (up to 23 ms/10%; p = 0.001) beyond ±30 Hz off-resonance. In the adult aorta, T2 was underestimated during higher blood flow velocities and pulsatility for τ = 16 ms (-41 ms/-17%; p = 0.008). In fetuses, two-parameter fitting overestimated T2 compared with three-parameter fitting (+33 ms/+18%; p = 0.03). T2 variability was 18 ms/15% in the fetal descending aorta and 28 ms/14% in the umbilical vein. The resulting estimated sO2 variability was ∼10% (15% of sO2 value) in the fetal descending aorta. CONCLUSIONS: Errors due to T2 -fitting techniques, off-resonance, flow velocity, and insufficient T1 recovery between image acquisitions could be mitigated by using three-parameter fitting with included saturation-prepared images approximating infinite T2 -preparation time, adequate shimming covering the fetus and placenta, and by modifying acquisition parameters. Variability in fetal blood T2 and sO2 , however, indicate that it is currently not feasible to use these methods for prediction of disease.

Comparison of 2D and 4D Flow MRI in Neonates Without General Anesthesia.

BACKGROUND: Neonates with critical congenital heart disease require early intervention. Four-dimensional (4D) flow may facilitate surgical planning and improve outcome, but accuracy and precision in neonates are unknown. PURPOSE: To 1) validate two-dimensional (2D) and 4D flow MRI in a phantom and investigate the effect of spatial and temporal resolution; 2) investigate accuracy and precision of 4D flow and internal consistency of 2D and 4D flow in neonates; and 3) compare scan time of 4D flow to multiple 2D flows. STUDY TYPE: Phantom and prospective patients. POPULATION: A total of 17 neonates with surgically corrected aortic coarctation (age 18 days [IQR 11-20]) and a three-dimensional printed neonatal aorta phantom. FIELD STRENGTH/SEQUENCE: 1.5T, 2D flow and 4D flow. ASSESSMENT: In the phantom, 2D and 4D flow volumes (ascending and descending aorta, and aortic arch vessels) with different resolutions were compared to high-resolution reference 2D flow. In neonates, 4D flow was compared to 2D flow volumes at each vessel. Internal consistency was computed as the flow volume in the ascending aorta minus the sum of flow volumes in the aortic arch vessels and descending aorta, divided by ascending aortic flow. STATISTICAL TESTS: Bland-Altman plots, Pearson correlation coefficient (r), and Student's t-tests. RESULTS: In the phantom, 2D flow differed by 0.01 ± 0.02 liter/min with 1.5 mm spatial resolution and -0.01 ± 0.02 liter/min with 0.8 mm resolution; 4D flow differed by -0.05 ± 0.02 liter/min with 2.4 mm spatial and 42 msec temporal resolution, -0.01 ± 0.02 liter/min with 1.5 mm, 42 msec resolution and -0.01 ± 0.02 liter/min with 1.5 mm, 21 msec resolution. In patients, 4D flow and 2D flow differed by -0.06 ± 0.08 liter/min. Internal consistency in patients was -11% ± 17% for 2D flow and 5% ± 13% for 4D flow. Scan time was 17.1 minutes [IQR 15.5-18.5] for 2D flow and 6.2 minutes [IQR 5.3-6.9] for 4D flow, P < 0.0001. DATA CONCLUSION: Neonatal 4D flow MRI is time efficient and can be acquired with good internal consistency without contrast agents or general anesthesia, thus potentially expanding 4D flow use to the youngest and smallest patients. EVIDENCE LEVEL: 1 TECHNICAL EFFICACY: Stage 2.

4D Flow cardiovascular magnetic resonance consensus statement: 2023 update.

Hemodynamic assessment is an integral part of the diagnosis and management of cardiovascular disease. Four-dimensional cardiovascular magnetic resonance flow imaging (4D Flow CMR) allows comprehensive and accurate assessment of flow in a single acquisition. This consensus paper is an update from the 2015 '4D Flow CMR Consensus Statement'. We elaborate on 4D Flow CMR sequence options and imaging considerations. The document aims to assist centers starting out with 4D Flow CMR of the heart and great vessels with advice on acquisition parameters, post-processing workflows and integration into clinical practice. Furthermore, we define minimum quality assurance and validation standards for clinical centers. We also address the challenges faced in quality assurance and validation in the research setting. We also include a checklist for recommended publication standards, specifically for 4D Flow CMR. Finally, we discuss the current limitations and the future of 4D Flow CMR. This updated consensus paper will further facilitate widespread adoption of 4D Flow CMR in the clinical workflow across the globe and aid consistently high-quality publication standards.

Real-time imaging of respiratory effects on cerebrospinal fluid flow in small diameter passageways.

PURPOSE: Respiration-related CSF flow through the cerebral aqueduct may be useful for elucidating physiology and pathophysiology of the glymphatic system, which has been proposed as a mechanism of brain waste clearance. Therefore, we aimed to (1) develop a real-time (CSF) flow imaging method with high spatial and sufficient temporal resolution to capture respiratory effects, (2) validate the method in a phantom setup and numerical simulations, and (3) apply the method in vivo and quantify its repeatability and correlation with different respiratory conditions. METHODS: A golden-angle radial flow sequence (reconstructed temporal resolution 168 ms, spatial resolution 0.6 mm) was implemented on a 7T MRI scanner and reconstructed using compressed sensing. A phantom setup mimicked simultaneous cardiac and respiratory flow oscillations. The effect of temporal resolution and vessel diameter was investigated numerically. Healthy volunteers (n = 10) were scanned at four different respiratory conditions, including repeat scans. RESULTS: Phantom data show that the developed sequence accurately quantifies respiratory oscillations (ratio real-time/reference QR  = 0.96 ± 0.02), but underestimates the rapid cardiac oscillations (ratio QC  = 0.46 ± 0.14). Simulations suggest that QC can be improved by increasing temporal resolution. In vivo repeatability was moderate to very strong for cranial and caudal flow (intraclass correlation coefficient range: 0.55-0.99) and weak to strong for net flow (intraclass correlation coefficient range: 0.48-0.90). Net flow was influenced by respiratory condition (p < 0.01). CONCLUSIONS: The presented real-time flow MRI method can quantify respiratory-related variations of CSF flow in the cerebral aqueduct, but it underestimates rapid cardiac oscillations. In vivo, the method showed good repeatability and a relationship between flow and respiration.

Blood flow imaging by optimal matching of computational fluid dynamics to 4D-flow data.

PURPOSE: Three-dimensional, time-resolved blood flow measurement (4D-flow) is a powerful research and clinical tool, but improved resolution and scan times are needed. Therefore, this study aims to (1) present a postprocessing framework for optimization-driven simulation-based flow imaging, called 4D-flow High-resolution Imaging with a priori Knowledge Incorporating the Navier-Stokes equations and the discontinuous Galerkin method (4D-flow HIKING), (2) investigate the framework in synthetic tests, (3) perform phantom validation using laser particle imaging velocimetry, and (4) demonstrate the use of the framework in vivo. METHODS: An optimizing computational fluid dynamics solver including adjoint-based optimization was developed to fit computational fluid dynamics solutions to 4D-flow data. Synthetic tests were performed in 2D, and phantom validation was performed with pulsatile flow. Reference velocity data were acquired using particle imaging velocimetry, and 4D-flow data were acquired at 1.5 T. In vivo testing was performed on intracranial arteries in a healthy volunteer at 7 T, with 2D flow as the reference. RESULTS: Synthetic tests showed low error (0.4%-0.7%). Phantom validation showed improved agreement with laser particle imaging velocimetry compared with input 4D-flow in the horizontal (mean -0.05 vs -1.11 cm/s, P < .001; SD 1.86 vs 4.26 cm/s, P < .001) and vertical directions (mean 0.05 vs -0.04 cm/s, P = .29; SD 1.36 vs 3.95 cm/s, P < .001). In vivo data show a reduction in flow rate error from 14% to 3.5%. CONCLUSIONS: Phantom and in vivo results from 4D-flow HIKING show promise for future applications with higher resolution, shorter scan times, and accurate quantification of physiological parameters.

Doppler ultrasound cardiac gating of intracranial flow at 7T.

BACKGROUND: Ultra-high field magnetic resonance imaging (MR) may be used to improve intracranial blood flow measurements. However, standard cardiac synchronization methods tend to fail at ultra-high field MR. Therefore, this study aims to investigate an alternative synchronization technique using Doppler ultrasound. METHODS: Healthy subjects (n = 9) were examined with 7T MR. Flow was measured in the M1-branch of the middle cerebral artery (MCA) and in the cerebral aqueduct (CA) using through-plane phase contrast (2D flow). Flow in the circle of Willis was measured with three-dimensional, three-directional phase contrast (4D flow). Scans were gated with Doppler ultrasound (DUS) and electrocardiogram (ECG), and pulse oximetry data (POX) was collected simultaneously. False negative and false positive trigger events were counted for ECG, DUS and POX, and quantitative flow measures were compared. RESULTS: There were fewer false positive triggers for DUS compared to ECG (5.3 ± 11 vs. 25 ± 31, p = 0.031), while no other measured parameters differed significantly. Net blood flow in M1 was similar between DUS and ECG for 2D flow (1.5 ± 0.39 vs. 1.6 ± 0.41, bias ± 1.96SD: - 0.021 ± 0.36) and 4D flow (1.8 ± 0.48 vs. 9 ± 0.59, bias ± 1.96SD: - 0.086 ± 0.57 ml). Net CSF flow per heart beat in the CA was also similar for DUS and ECG (3.6 ± 2.1 vs. 3.0 ± 5.8, bias ± 1.96SD: 0.61 ± 13.6 µl). CONCLUSION: Gating with DUS produced fewer false trigger events than using ECG, with similar quantitative flow values. DUS gating is a promising technique for cardiac synchronization at 7T.

Independent validation of metric optimized gating for fetal cardiovascular phase-contrast flow imaging.

PURPOSE: To validate metric optimized gating phase-contrast MR (MOG PC-MR) flow measurements for a range of fetal flow velocities in phantom experiments. 2) To investigate intra- and interobserver variability for fetal flow measurements at an imaging center other than the original site. METHODS: MOG PC-MR was compared to timer/beaker measurements in a pulsatile flow phantom using a heart rate (∼145 bpm), nozzle diameter (∼6 mm), and flow range (∼130-700 mL/min) similar to fetal imaging. Fifteen healthy fetuses were included for intra- and interobserver variability in the fetal descending aorta and umbilical vein. RESULTS: Phantom MOG PC-MR flow bias and variability was 2% ± 23%. Accuracy of MOG PC-MR was degraded for flow profiles with low velocity-to-noise ratio. Intra- and interobserver coefficients of variation were 6% and 19%, respectively, for fetal descending aorta; and 10% and 17%, respectively, for the umbilical vein. CONCLUSION: Phantom validation showed good agreement between MOG and conventionally gated PC-MR, except for cases with low velocity-to-noise ratio, which resulted in MOG misgating and underestimated peak velocities and warranted optimization of sequence parameters to individual fetal vessels. Inter- and intraobserver variability for fetal MOG PC-MR imaging were comparable to previously reported values.

Subacute vessel wall imaging at 7-T MRI in post-thrombectomy stroke patients.

PURPOSE: Reports from 3-T vessel wall MRI imaging have shown contrast enhancement following thrombectomy for acute stroke, suggesting potential intimal damage. Comparisons have shown higher SNR and more lesions detected by vessel wall imaging when using 7 T compared with 3 T. The aim of this study was to investigate the vessel walls after stent retriever thrombectomy using high-resolution vessel wall imaging at 7 T. METHODS: Seven patients with acute stroke caused by occlusion of the distal internal carotid artery (T-occlusion), or proximal medial cerebral artery, and treated by stent retriever thrombectomy with complete recanalization were included and examined by 7-T MRI within 2 days. The MRI protocol included a high-resolution black blood sequence with prospective motion correction (iMOCO), acquired before and after contrast injection. Flow measurements were performed in the treated and untreated M1 segments. RESULTS: All subjects completed the MRI examination. Image quality was independently rated as excellent by two neuroradiologists for all cases, and the level of motion artifacts did not impair diagnostic quality, despite severe motion in some cases. Contrast enhancement correlated with the deployment location of the stent retrievers. Flow data showed complete restoration of flow after treatment. CONCLUSION: Vessel wall imaging with prospective motion correction can be performed in patients following thrombectomy with excellent imaging quality at 7 T. We show that vessel wall contrast enhancement is the normal post-operative state and corresponds to the deployment location of the stent retriever.

Validation and reproducibility of cardiovascular 4D-flow MRI from two vendors using 2 × 2 parallel imaging acceleration in pulsatile flow phantom and in vivo with and without respiratory gating.

BACKGROUND: 4D-flow magnetic resonance imaging (MRI) is increasingly used. PURPOSE: To validate 4D-flow sequences in phantom and in vivo, comparing volume flow and kinetic energy (KE) head-to-head, with and without respiratory gating. MATERIAL AND METHODS: Achieva dStream (Philips Healthcare) and MAGNETOM Aera (Siemens Healthcare) 1.5-T scanners were used. Phantom validation measured pulsatile, three-dimensional flow with 4D-flow MRI and laser particle imaging velocimetry (PIV) as reference standard. Ten healthy participants underwent three cardiac MRI examinations each, consisting of cine-imaging, 2D-flow (aorta, pulmonary artery), and 2 × 2 accelerated 4D-flow with (Resp+) and without (Resp-) respiratory gating. Examinations were acquired consecutively on both scanners and one examination repeated within two weeks. Volume flow in the great vessels was compared between 2D- and 4D-flow. KE were calculated for all time phases and voxels in the left ventricle. RESULTS: Phantom results showed high accuracy and precision for both scanners. In vivo, higher accuracy and precision ( P < 0.001) was found for volume flow for the Aera prototype with Resp+ (-3.7 ± 10.4 mL, r = 0.89) compared to the Achieva product sequence (-17.8 ± 18.6 mL, r = 0.56). 4D-flow Resp- on Aera had somewhat larger bias (-9.3 ± 9.6 mL, r = 0.90) compared to Resp+ ( P = 0.005). KE measurements showed larger differences between scanners on the same day compared to the same scanner at different days. CONCLUSION: Sequence-specific in vivo validation of 4D-flow is needed before clinical use. 4D-flow with the Aera prototype sequence with a clinically acceptable acquisition time (<10 min) showed acceptable bias in healthy controls to be considered for clinical use. Intra-individual KE comparisons should use the same sequence.

Hemodynamic forces using four-dimensional flow MRI: an independent biomarker of cardiac function in heart failure with left ventricular dyssynchrony?

Patients with heart failure with left ventricular (LV) dyssynchrony often do not respond to cardiac resynchronization therapy (CRT), indicating that the pathophysiology is insufficiently understood. Intracardiac hemodynamic forces computed from four-dimensional (4-D) flow MRI have been proposed as a new measure of cardiac function. We therefore aimed to investigate how hemodynamic forces are altered in LV dyssynchrony. Thirty-one patients with heart failure and LV dyssynchrony and 39 control subjects underwent cardiac MRI with the acquisition of 4-D flow. Hemodynamic forces were computed using Navier-Stokes equations and integrated over the manually delineated LV volume. The ratio between transverse (lateral-septal and inferior-anterior) and longitudinal (apical-basal) forces was calculated for systole and diastole separately and compared with QRS duration, aortic valve opening delay, global longitudinal strain, and ejection fraction (EF). Patients exhibited hemodynamic force patterns that were significantly altered compared with control subjects, including loss of longitudinal forces in diastole (force ratio, control subjects vs. patients: 0.32 vs. 0.90, P < 0.0001) and increased transverse force magnitudes. The systolic force ratio was correlated with global longitudinal strain and EF ( P < 0.01). The diastolic force ratio separated patients from control subjects (area under the curve: 0.98, P < 0.0001) but was not correlated to other dyssynchrony measures ( P > 0.05 for all). Hemodynamic forces by 4-D flow represent a new approach to the quantification of LV dyssynchrony. Diastolic force patterns separate healthy from diseased ventricles. Different force patterns in patients indicate the possible use of force analysis for risk stratification and CRT implantation guidance. NEW & NOTEWORTHY In this report, we demonstrate that patients with heart failure with left ventricular dyssynchrony exhibit significantly altered hemodynamic forces compared with normal. Force patterns in patients mechanistically reflect left ventricular dysfunction on the organ level, largely independent of traditional dyssynchrony measures. Force analysis may help clinical decision making and could potentially be used to improve therapy outcomes.

Altered biventricular hemodynamic forces in patients with repaired tetralogy of Fallot and right ventricular volume overload because of pulmonary regurgitation.

Intracardiac hemodynamic forces have been proposed to influence remodeling and be a marker of ventricular dysfunction. We aimed to quantify the hemodynamic forces in patients with repaired tetralogy of Fallot (rToF) to further understand the pathophysiological mechanisms as this could be a potential marker for pulmonary valve replacement (PVR) in these patients. Patients with rToF and pulmonary regurgitation (PR) > 20% ( n = 18) and healthy control subjects ( n = 15) underwent MRI, including four-dimensional flow. A subset of patients ( n = 8) underwent PVR and MRI after surgery. Time-resolved hemodynamic forces were quantified using 4D-flow data and indexed to ventricular volume. Patients had higher systolic and diastolic left ventricular (LV) hemodynamic forces compared with control subjects in the lateral-septal/LV outflow tract ( P = 0.011 and P = 0.0031) and inferior-anterior ( P < 0.0001 and P < 0.0001) directions, which are forces not aligned with blood flow. Forces did not change after PVR. Patients had higher RV diastolic forces compared with control subjects in the diaphragm-right ventricular (RV) outflow tract (RVOT; P < 0.001) and apical-basal ( P = 0.0017) directions. After PVR, RV systolic forces in the diaphragm-RVOT direction decreased ( P = 0.039) to lower levels than in control subjects ( P = 0.0064). RV diastolic forces decreased in all directions ( P = 0.0078, P = 0.0078, and P = 0.039) but were still higher than in control subjects in the diaphragm-RVOT direction ( P = 0.046). In conclusion, patients with rToF and PR had LV hemodynamic forces less aligned with intraventricular blood flow compared with control subjects and higher diastolic RV forces along the regurgitant flow direction in the RVOT and that of tricuspid inflow. Remaining force differences in the LV and RV after PVR suggest that biventricular pumping does not normalize after surgery. NEW & NOTEWORTHY Biventricular hemodynamic forces in patients with repaired tetralogy of Fallot and pulmonary regurgitation were quantified for the first time. Left ventricular hemodynamic forces were less aligned to the main blood flow direction in patients compared with control subjects. Higher right ventricular forces were seen along the pulmonary regurgitant and tricuspid inflow directions. Differences in forces versus control subjects remain after pulmonary valve replacement, suggesting that altered biventricular pumping does not normalize after surgery.

Investigation of cerebrospinal fluid flow in the cerebral aqueduct using high-resolution phase contrast measurements at 7T MRI.

Background The cerebral aqueduct is a central conduit for cerebrospinal fluid (CSF), and non-invasive quantification of CSF flow in the aqueduct may be an important tool for diagnosis and follow-up of treatment. Magnetic resonance (MR) methods at clinical field strengths are limited by low spatial resolution. Purpose To investigate the feasibility of high-resolution through-plane MR flow measurements (2D-PC) in the cerebral aqueduct at high field strength (7T). Material and Methods 2D-PC measurements in the aqueduct were performed in nine healthy individuals at 7T. Measurement accuracy was determined using a phantom. Aqueduct area, mean velocity, maximum velocity, minimum velocity, net flow, and mean flow were determined using in-plane resolutions 0.8 × 0.8, 0.5 × 0.5, 0.3 × 0.3, and 0.2 × 0.2 mm2. Feasibility criteria were defined based on scan time and spatial and temporal resolution. Results Phantom validation of 2D-PC MR showed good accuracy. In vivo, stroke volume was -8.2 ± 4.4, -4.7 ± 2.8, -6.0 ± 3.8, and -3.7 ± 2.1 µL for 0.8 × 0.8, 0.5 × 0.5, 0.3 × 0.3, and 0.2 × 0.2 mm2, respectively. The scan with 0.3 × 0.3 mm2 resolution fulfilled the feasibility criteria for a wide range of heart rates and aqueduct diameters. Conclusion 7T MR enables non-invasive quantification of CSF flow and velocity in the cerebral aqueduct with high spatial resolution.

Left and right ventricular hemodynamic forces in healthy volunteers and elite athletes assessed with 4D flow magnetic resonance imaging.

Intracardiac blood flow is driven by hemodynamic forces that are exchanged between the blood and myocardium. Previous studies have been limited to 2D measurements or investigated only left ventricular (LV) forces. Right ventricular (RV) forces and their mechanistic contribution to asymmetric redirection of flow in the RV have not been measured. We therefore aimed to quantify 3D hemodynamic forces in both ventricles in a cohort of healthy subjects, using magnetic resonance imaging 4D flow measurements. Twenty five controls, 14 elite endurance athletes, and 2 patients with LV dyssynchrony were included. 4D flow data were used as input for the Navier-Stokes equations to compute hemodynamic forces over the entire cardiac cycle. Hemodynamic forces were found in a qualitatively consistent pattern in all healthy subjects, with variations in amplitude. LV forces were mainly aligned along the apical-basal longitudinal axis, with an additional component aimed toward the aortic valve during systole. Conversely, RV forces were found in both longitudinal and short-axis planes, with a systolic force component driving a slingshot-like acceleration that explains the mechanism behind the redirection of blood flow toward the pulmonary valve. No differences were found between controls and athletes when indexing forces to ventricular volumes, indicating that cardiac force expenditures are tuned to accelerate blood similarly in small and large hearts. Patients' forces differed from controls in both timing and amplitude. Normal cardiac pumping is associated with specific force patterns for both ventricles, and deviation from these forces may be a sensitive marker of ventricular dysfunction. Reference values are provided for future studies.NEW & NOTEWORTHY Biventricular hemodynamic forces were quantified for the first time in healthy controls and elite athletes (n = 39). Hemodynamic forces constitute a slingshot-like mechanism in the right ventricle, redirecting blood flow toward the pulmonary circulation. Force patterns were similar between healthy subjects and athletes, indicating potential utility as a cardiac function biomarker.

Test-retest repeatability of human speech biomarkers from static and real-time dynamic magnetic resonance imaging.

Static anatomical and real-time dynamic magnetic resonance imaging (RT-MRI) of the upper airway is a valuable method for studying speech production in research and clinical settings. The test-retest repeatability of quantitative imaging biomarkers is an important parameter, since it limits the effect sizes and intragroup differences that can be studied. Therefore, this study aims to present a framework for determining the test-retest repeatability of quantitative speech biomarkers from static MRI and RT-MRI, and apply the framework to healthy volunteers. Subjects (n = 8, 4 females, 4 males) are imaged in two scans on the same day, including static images and dynamic RT-MRI of speech tasks. The inter-study agreement is quantified using intraclass correlation coefficient (ICC) and mean within-subject standard deviation (σe). Inter-study agreement is strong to very strong for static measures (ICC: min/median/max 0.71/0.89/0.98, σe: 0.90/2.20/6.72 mm), poor to strong for dynamic RT-MRI measures of articulator motion range (ICC: 0.26/0.75/0.90, σe: 1.6/2.5/3.6 mm), and poor to very strong for velocities (ICC: 0.21/0.56/0.93, σe: 2.2/4.4/16.7 cm/s). In conclusion, this study characterizes repeatability of static and dynamic MRI-derived speech biomarkers using state-of-the-art imaging. The introduced framework can be used to guide future development of speech biomarkers. Test-retest MRI data are provided free for research use.

Independent validation of four-dimensional flow MR velocities and vortex ring volume using particle imaging velocimetry and planar laser-Induced fluorescence.

PURPOSE: This study aimed to: (i) present and characterize a phantom setup for validation of four-dimensional (4D) flow using particle imaging velocimetry (PIV) and planar laser-induced fluorescence (PLIF); (ii) validate 4D flow velocity measurements using PIV; and (iii) validate 4D flow vortex ring volume (VV) using PLIF. METHODS: A pulsatile pump and a tank with a 25-mm nozzle were constructed. PIV measurements (1.5 × 1.5 mm pixels, temporal resolution 10 ms) were obtained on two occasions. The 4D flow (3 × 3 × 3 mm voxels, temporal resolution 50 ms) was acquired using SENSE = 2. VV was quantified using PLIF and 4D flow. RESULTS: PIV showed excellent day-to-day stability (R(2) = 0.99, bias -0.04 ± 0.72 cm/s). The 4D flow mean velocities agreed well with PIV (R(2) = 0.95, bias 0.16 ± 2.65 cm/s). Peak velocities in 4D flow were underestimated by 7-18% compared with PIV (y = 0.79x + 2.7, R(2) = 0.96, -12 ± 5%). VV showed excellent agreement between PLIF and 4D flow (R(2) = 0.99, 2.4 ± 1.5 mL). CONCLUSION: This study shows: (i) The proposed phantom enables reliable validation of 4D flow. (ii) 4D flow velocities show good agreement with PIV, but peak velocities were underestimated due to low spatial and temporal resolution. (iii) Vortex ring volume (VV) can be quantified using 4D flow.

Vortex-ring mixing as a measure of diastolic function of the human heart: Phantom validation and initial observations in healthy volunteers and patients with heart failure.

PURPOSE: To present and validate a new method for 4D flow quantification of vortex-ring mixing during early, rapid filling of the left ventricle (LV) as a potential index of diastolic dysfunction and heart failure. MATERIALS AND METHODS: 4D flow mixing measurements were validated using planar laser-induced fluorescence (PLIF) in a phantom setup. Controls (n = 23) and heart failure patients (n = 23) were studied using 4D flow at 1.5T (26 subjects) or 3T (20 subjects) to determine vortex volume (VV) and inflowing volume (VVinflow ). The volume mixed into the vortex-ring was quantified as VVmix-in = VV-VVinflow . The mixing ratio was defined as MXR = VVmix-in /VV. Furthermore, we quantified the fraction of the end-systolic volume (ESV) mixed into the vortex-ring (VVmix-in /ESV) and the fraction of the LV volume at diastasis (DV) occupied by the vortex-ring (VV/DV). RESULTS: PLIF validation of MXR showed fair agreement (R(2) = 0.45, mean ± SD 1 ± 6%). MXR was higher in patients compared to controls (28 ± 11% vs. 16 ± 10%, P < 0.001), while VVmix-in /ESV and VV/DV were lower in patients (10 ± 6% vs. 18 ± 12%, P < 0.01 and 25 ± 8% vs. 50 ± 6%, P < 0.0001). CONCLUSION: Vortex-ring mixing can be quantified using 4D flow. The differences in mixing parameters observed between controls and patients motivate further investigation as indices of diastolic dysfunction. J. Magn. Reson. Imaging 2016;43:1386-1397.

Left ventricular fluid kinetic energy time curves in heart failure from cardiovascular magnetic resonance 4D flow data.

BACKGROUND: Measurement of intracardiac kinetic energy (KE) provides new insights into cardiac hemodynamics and may improve assessment and understanding of heart failure. We therefore aimed to investigate left ventricular (LV) KE time curves in patients with heart failure and in controls. METHODS: Patients with heart failure (n = 29, NYHA class I-IV) and controls (n = 12) underwent cardiovascular magnetic resonance (CMR) including 4D flow. The vortex-ring boundary was computed using Lagrangian coherent structures. The LV endocardium and vortex-ring were manually delineated and KE was calculated as ½mv(2) of the blood within the whole LV and the vortex ring, respectively. RESULTS: The systolic average KE was higher in patients compared to controls (2.2 ± 1.4 mJ vs 1.6 ± 0.6 mJ, p = 0.048), but lower when indexing to EDV (6.3 ± 2.2 µJ/ml vs 8.0 ± 2.1 µJ/ml, p = 0.025). No difference was seen in diastolic average KE (3.2 ± 2.3 mJ vs 2.0 ± 0.8 mJ, p = 0.13) even when indexing to EDV (9.0 ± 4.4 µJ/ml vs 10.2 ± 3.3 µJ/ml, p = 0.41). In patients, a smaller fraction of diastolic average KE was observed inside the vortex ring compared to controls (72 ± 6% vs 54 ± 9%, p < 0.0001). Three distinctive KE time curves were seen in patients which were markedly different from findings in controls, and with a moderate agreement between KE time curve patterns and degree of diastolic dysfunction (Cohen's kappa = 0.49), but unrelated to NYHA classification (p = 0.12), or 6-minute walk test (p = 0.72). CONCLUSION: Patients with heart failure exhibit higher systolic average KE compared to controls, suggesting altered intracardiac blood flow. The different KE time curves seen in patients may represent a conceptually new approach for heart failure classification.

Whole-heart four-dimensional flow can be acquired with preserved quality without respiratory gating, facilitating clinical use: a head-to-head comparison.

BACKGROUND: Respiratory gating is often used in 4D-flow acquisition to reduce motion artifacts. However, gating increases scan time. The aim of this study was to investigate if respiratory gating can be excluded from 4D flow acquisitions without affecting quantitative intracardiac parameters. METHODS: Eight volunteers underwent CMR at 1.5 T with a 5-channel coil (5ch). Imaging included 2D flow measurements and whole-heart 4D flow with and without respiratory gating (Resp(+), Resp(-)). Stroke volume (SV), particle-trace volumes, kinetic energy, and vortex-ring volume were obtained from 4D flow-data. These parameters were compared between 5ch Resp(+) and 5ch Resp(-). In addition, 20 patients with heart failure were scanned using a 32-channel coil (32ch), and particle-trace volumes were compared to planimetric SV. Paired comparisons were performed using Wilcoxon's test and correlation analysis using Pearson r. Agreement was assessed as bias±SD. RESULTS: Stroke volume from 4D flow was lower compared to 2D flow both with and without respiratory gating (5ch Resp(+) 88±18 vs 97±24.0, p=0.001; 5ch Resp(-) 86±16 vs 97.1±22.7, p<0.01). There was a good correlation between Resp(+) and Resp(-) for particle-trace derived volumes (R2=0.82, 0.2±9.4 ml), mean kinetic energy (R2=0.86, 0.07±0.21 mJ), peak kinetic energy (R2=0.88, 0.14±0.77 mJ), and vortex-ring volume (R2=0.70, -2.5±9.4 ml). Furthermore, good correlation was found between particle-trace volume and planimetric SV in patients for 32ch Resp(-) (R2=0.62, -4.2±17.6 ml) and in healthy volunteers for 5ch Resp(+) (R2=0.89, -11±7 ml), and 5ch Resp(-) (R2=0.93, -7.5±5.4 ml), Average scan duration for Resp(-) was shorter compared to Resp(+) (27±9 min vs 61±19 min, p<0.05). CONCLUSIONS: Whole-heart 4D flow can be acquired with preserved quantitative results without respiratory gating, facilitating clinical use.

Random effects during training: Implications for deep learning-based medical image segmentation.

BACKGROUND: A single learning algorithm can produce deep learning-based image segmentation models that vary in performance purely due to random effects during training. This study assessed the effect of these random performance fluctuations on the reliability of standard methods of comparing segmentation models. METHODS: The influence of random effects during training was assessed by running a single learning algorithm (nnU-Net) with 50 different random seeds for three multiclass 3D medical image segmentation problems, including brain tumour, hippocampus, and cardiac segmentation. Recent literature was sampled to find the most common methods for estimating and comparing the performance of deep learning segmentation models. Based on this, segmentation performance was assessed using both hold-out validation and 5-fold cross-validation and the statistical significance of performance differences was measured using the Paired t-test and the Wilcoxon signed rank test on Dice scores. RESULTS: For the different segmentation problems, the seed producing the highest mean Dice score statistically significantly outperformed between 0 % and 76 % of the remaining seeds when estimating performance using hold-out validation, and between 10 % and 38 % when estimating performance using 5-fold cross-validation. CONCLUSION: Random effects during training can cause high rates of statistically-significant performance differences between segmentation models from the same learning algorithm. Whilst statistical testing is widely used in contemporary literature, our results indicate that a statistically-significant difference in segmentation performance is a weak and unreliable indicator of a true performance difference between two learning algorithms.