Deep Learning Automated Background Phase Error Correction for Abdominopelvic 4D Flow MRI

Background Four-dimensional (4D) flow MRI has the potential to provide hemodynamic insights for a variety of abdominopelvic vascular diseases, but its clinical utility is currently impaired by background phase error, which can be challenging to correct. Purpose To assess the feasibility of using deep learning to automatically perform image-based background phase error correction in 4D flow MRI and to compare its effectiveness relative to manual image-based correction. Materials and Methods A convenience sample of 139 abdominopelvic 4D flow MRI acquisitions performed between January 2016 and July 2020 was retrospectively collected. Manual phase error correction was performed using dedicated imaging software and served as the reference standard. After reserving 40 examinations for testing, the remaining examinations were randomly divided into training (86% [85 of 99]) and validation (14% [14 of 99]) data sets to train a multichannel three-dimensional U-Net convolutional neural network. Flow measurements were obtained for the infrarenal aorta, common iliac arteries, common iliac veins, and inferior vena cava. Statistical analyses included Pearson correlation, Bland-Altman analysis, and F tests with Bonferroni correction. Results A total of 139 patients (mean age, 47 years ± 14 [standard deviation]; 108 women) were included. Inflow-outflow correlation improved after manual correction (ρ = 0.94, P < .001) compared with that before correction (ρ = 0.50, P < .001). Automated correction showed similar results (ρ = 0.91, P < .001) and demonstrated very strong correlation with manual correction (ρ = 0.98, P < .001). Both correction methods reduced inflow-outflow variance, improving mean difference from -0.14 L/min (95% limits of agreement: -1.61, 1.32) (uncorrected) to 0.05 L/min (95% limits of agreement: -0.32, 0.42) (manually corrected) and 0.05 L/min (95% limits of agreement: -0.38, 0.49) (automatically corrected). There was no significant difference in inflow-outflow variance between manual and automated correction methods (P = .10). Conclusion Deep learning automated phase error correction reduced inflow-outflow bias and variance of volumetric flow measurements in four-dimensional flow MRI, achieving results comparable with manual image-based phase error correction. © RSNA, 2021 See also the editorial by Roldán-Alzate and Grist in this issue.

Quantification of the Hemodynamic Changes of Cirrhosis with Free-Breathing Self-Navigated MRI

BACKGROUND

Non-invasive assessment of the hemodynamic changes of cirrhosis might help guide management of patients with liver disease but are currently limited.

PURPOSE

To determine whether free-breathing 4D flow MRI can be used to quantify the hemodynamic effects of cirrhosis and introduce hydraulic circuit indexes of severity.

STUDY TYPE

Retrospective.

POPULATION

Forty-seven patients including 26 with cirrhosis.

FIELD STRENGTH/SEQUENCE

3 T/free-breathing 4D flow MRI with soft gating and golden-angle view ordering.

ASSESSMENT

Measurements of the supra-celiac abdominal aorta, supra-renal abdominal aorta (SRA), celiac trunk (CeT), superior mesenteric artery (SMA), splenic artery (SpA), common hepatic artery (CHA), portal vein (PV), and supra-renal inferior vena cava (IVC) were made by two radiologists. Measures of hepatic vascular resistance (hepatic arterial relative resistance [HARR]; portal resistive index [PRI]) were proposed and calculated.

STATISTICAL ANALYSIS

Bland-Altman, Pearson's correlation, Tukey's multiple comparison, and Cohen's kappa. P < 0.05 was considered significant.

RESULTS

Forty-four of 47 studies yielded adequate image quality for flow quantification (94%). Arterial structures showed high inter-reader concordance (range; ρ = 0.948-0.987) and the IVC (ρ = 0.972), with moderate concordance in the PV (ρ = 0.866). Conservation of mass analysis showed concordance between large vessels (SRA vs. IVC; ρ = 0.806), small vessels (celiac vs. CHA + SpA; ρ = 0.939), and across capillary beds (CeT + SMA vs. PV; ρ = 0.862). Splanchnic flow was increased in patients with portosystemic shunting (PSS) relative to control patients and patients with cirrhosis without PSS (P < 0.05, difference range 0.11-0.68 liter/m). HARR was elevated and PRI was decreased in patients with PSS (3.55 and 1.49, respectively) compared to both the control (2.11/3.18) and non-PSS (2.11/2.35) cohorts.

DATA CONCLUSION

4D flow MRI with self-navigation was technically feasible, showing promise in quantifying the hemodynamic effects of cirrhosis. Proposed quantitative metrics of hepatic vascular resistance correlated with PSS.

LEVEL OF EVIDENCE

3 TECHNICAL EFFICACY STAGE: 2.

Diffusion-weighted double-echo steady-state with a three-dimensional cones trajectory for non-contrast-enhanced breast MRI

The image quality limitations of echo-planar diffusion-weighted imaging (DWI) are an obstacle to its widespread adoption in the breast. Steady-state DWI is an alternative DWI method with more robust image quality but its contrast for imaging breast cancer is not well-understood. The aim of this study was to develop and evaluate diffusion-weighted double-echo steady-state imaging with a three-dimensional cones trajectory (DW-DESS-Cones) as an alternative to conventional DWI for non-contrast-enhanced MRI in the breast. This prospective study included 28 women undergoing clinically indicated breast MRI and six asymptomatic volunteers. In vivo studies were performed at 3 T and included DW-DESS-Cones, DW-DESS-Cartesian, DWI, and CE-MRI acquisitions. Phantom experiments (diffusion phantom, High Precision Devices) and simulations were performed to establish framework for contrast of DW-DESS-Cones in comparison to DWI in the breast. Motion artifacts of DW-DESS-Cones were measured with artifact-to-noise ratio in volunteers and patients. Lesion-to-fibroglandular tissue signal ratios were measured, lesions were categorized as hyperintense or hypointense, and an image quality observer study was performed in DW-DESS-Cones and DWI in patients. Effect of DW-DESS-Cones method on motion artifacts was tested by mixed-effects generalized linear model. Effect of DW-DESS-Cones on signal in phantom was tested by quadratic regression. Correlation was calculated between DW-DESS-Cones and DWI lesion-to-fibroglandular tissue signal ratios. Inter-observer agreement was assessed with Gwet's AC. Simulations predicted hyperintensity of lesions with DW-DESS-Cones but at a 3% to 67% lower degree than with DWI. Motion artifacts were reduced with DW-DESS-Cones versus DW-DESS-Cartesian (p < 0.05). Lesion-to-fibroglandular tissue signal ratios were not correlated between DW-DESS-Cones and DWI (r = 0.25, p = 0.38). Concordant hyperintensity/hypointensity was observed between DW-DESS-Cones and DWI in 11/14 lesions. DW-DESS-Cones improved sharpness, distortion, and overall image quality versus DWI. DW-DESS-Cones may be able to eliminate motion artifacts in the breast allowing for investigation of higher degrees of steady-state diffusion weighting. Malignant breast lesions in DW-DESS-Cones demonstrated hyperintensity with respect to surrounding tissue without an injection of contrast. LEVEL OF EVIDENCE: 2. TECHNICAL EFFICACY STAGE: 1.

Rosette Trajectories Enable Ungated, Motion-Robust, Simultaneous Cardiac and Liver T2 * Iron Assessment

BACKGROUND

Quantitative T₂ * MRI is the standard of care for the assessment of iron overload. However, patient motion corrupts T₂ * estimates.

PURPOSE

To develop and evaluate a motion-robust, simultaneous cardiac and liver T₂ * imaging approach using non-Cartesian, rosette sampling and a model-based reconstruction as compared to clinical-standard Cartesian MRI.

STUDY TYPE

Prospective.

PHANTOM/POPULATION

Six ferumoxytol-containing phantoms (26-288 μg/mL). Eight healthy subjects and 18 patients referred for clinically indicated iron overload assessment.

FIELD STRENGTH/SEQUENCE

1.5T, 2D Cartesian and rosette gradient echo (GRE) ASSESSMENT: GRE T₂ * values were validated in ferumoxytol phantoms. In healthy subjects, test-retest and spatial coefficient of variation (CoV) analysis was performed during three breathing conditions. Cartesian and rosette T₂ * were compared using correlation and Bland-Altman analysis. Images were rated by three experienced radiologists on a 5-point scale.

STATISTICAL TESTS

Linear regression, analysis of variance (ANOVA), and paired Student's t-testing were used to compare reproducibility and variability metrics in Cartesian and rosette scans. The Wilcoxon rank test was used to assess reader score comparisons and reader reliability was measured using intraclass correlation analysis.

RESULTS

Rosette R2* (1/T₂ *) was linearly correlated with ferumoxytol concentration (r² = 1.00) and not significantly different than Cartesian values (P = 0.16). During breath-holding, ungated rosette liver and heart T₂ * had lower spatial CoV (liver: 18.4 ± 9.3% Cartesian, 8.8% ± 3.4% rosette, P = 0.02, heart: 37.7% ± 14.3% Cartesian, 13.4% ± 1.7% rosette, P = 0.001) and higher-quality scores (liver: 3.3 [3.0-3.6] Cartesian, 4.7 [4.1-4.9] rosette, P = 0.005, heart: 3.0 [2.3-3] Cartesian, 4.5 [3.8-5.0] rosette, P = 0.005) compared to Cartesian values. During free-breathing and failed breath-holding, Cartesian images had very poor to average image quality with significant artifacts, whereas rosette remained very good, with minimal artifacts (P = 0.001).

DATA CONCLUSION

Rosette k-sampling with a model-based reconstruction offers a clinically useful motion-robust T₂ * mapping approach for iron quantification. J. MAGN. RESON. IMAGING 2020;52:1688-1698.

Direct measurement of atrioventricular valve regurgitant jets using 4D flow cardiovascular magnetic resonance is accurate and reliable for children with congenital heart disease: a retrospective cohort study

BACKGROUND

3D-time resolved flow (4DF) cardiovascular magnetic resonance (CMR) with retrospective analysis of atrioventricular valve regurgitation (AVVR) allows for internal validation by multiple direct and indirect methods. Limited data exist on direct measurement of AVVR by 4DF CMR in pediatric congenital heart disease (CHD). We aimed to validate direct measurement of the AVVR jet as accurate and reliable compared to the volumetric method (clinical standard by 2D CMR) and as a superior method of internal validation than the annular inflow method.

METHODS

We identified 44 consecutive patients with diverse CHD referred for evaluation of AVVR by CMR. 1.5 T or 3 T scanners, intravenous contrast, and a combination of parallel imaging and compressed sensing were used. Four methods of measuring AVVR volume (RVol) were used: volumetric method (VOL; the clinical standard) = stroke volume by 2D balanced steady-state free precession - semilunar valve forward flow (SLFF); annular inflow method (AIM) = atrioventricular valve forward flow [AVFF] - semilunar valve net flow (SLNF); and direct measurement (JET). AVFF was measured using static and retrospective valve tracking planes. SLFF, SLNF, AVFF, and JET were measured by 4DF phase contrast. Regurgitant fraction was calculated as [RVol/(RVol+SLNF)]× 100. Statistical methods included Spearman, Wilcoxon rank sum test/Student paired t-test, Bland Altman analysis, and intra-class coefficient (ICC), where appropriate.

RESULTS

Regurgitant fraction by JET strongly correlated with the indirect methods (VOL and AIM) (ρ = 0.73-0.80, p < 0.001) and was similar to VOL with a median difference (interquartile range) of - 1.5% (- 8.3-7.2%; p = 0.624). VOL had weaker correlations with AIM and JET (ρ = 0.69-0.73, p < 0.001). AIM underestimated RF by 3.6-6.9% compared to VOL and JET, p < 0.03. Intra- and inter- observer reliability were excellent for all methods (ICC 0.94-0.99). The mean (±standard deviation) inter-observer difference for VOL was 2.4% (±5.1%), p < 0.05.

CONCLUSIONS

In a diverse cohort of pediatric CHD, measurement of AVVR using JET is accurate and reliable to VOL and is a superior method of internal validation compared to AIM. This study supports use of 4DF CMR for measurement of AVVR, obviating need for expert prospective prescription during image acquisition by 2D CMR.

Combined 5-minute double-echo in steady-state with separated echoes and 2-minute proton-density-weighted 2D FSE sequence for comprehensive whole-joint knee MRI assessment

BACKGROUND

Clinical knee MRI protocols require upwards of 15 minutes of scan time.

PURPOSE/HYPOTHESIS

To compare the imaging appearance of knee abnormalities depicted with a 5-minute 3D double-echo in steady-state (DESS) sequence with separate echo images, with that of a routine clinical knee MRI protocol. A secondary goal was to compare the imaging appearance of knee abnormalities depicted with 5-minute DESS paired with a 2-minute coronal proton-density fat-saturated (PDFS) sequence.

STUDY TYPE

Prospective.

SUBJECTS

Thirty-six consecutive patients (19 male) referred for a routine knee MRI.

FIELD STRENGTH/SEQUENCES

DESS and PDFS at 3T.

ASSESSMENT

Five musculoskeletal radiologists evaluated all images for the presence of internal knee derangement using DESS, DESS+PDFS, and the conventional imaging protocol, and their associated diagnostic confidence of the reading.

STATISTICAL TESTS

Differences in positive and negative percent agreement (PPA and NPA, respectively) and 95% confidence intervals (CIs) for DESS and DESS+PDFS compared with the conventional protocol were calculated and tested using exact McNemar tests. The percentage of observations where DESS or DESS+PDFS had equivalent confidence ratings to DESS+Conv were tested with exact symmetry tests. Interreader agreement was calculated using Krippendorff's alpha.

RESULTS

DESS had a PPA of 90% (88-92% CI) and NPA of 99% (99-99% CI). DESS+PDFS had increased PPA of 99% (95-99% CI) and NPA of 100% (99-100% CI) compared with DESS (both P < 0.001). DESS had equivalent diagnostic confidence to DESS+Conv in 94% of findings, whereas DESS+PDFS had equivalent diagnostic confidence in 99% of findings (both P < 0.001). All readers had moderate concordance for all three protocols (Krippendorff's alpha 47-48%).

DATA CONCLUSION

Both 1) 5-minute 3D-DESS with separated echoes and 2) 5-minute 3D-DESS paired with a 2-minute coronal PDFS sequence depicted knee abnormalities similarly to a routine clinical knee MRI protocol, which may be a promising technique for abbreviated knee MRI.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2018.

Targeted rapid knee MRI exam using T2 shuffling

BACKGROUND

MRI is commonly used to evaluate pediatric musculoskeletal pathologies, but same-day/near-term scheduling and short exams remain challenges.

PURPOSE

To investigate the feasibility of a targeted rapid pediatric knee MRI exam, with the goal of reducing cost and enabling same-day MRI access.

STUDY TYPE

A cost effectiveness study done prospectively.

SUBJECTS

Forty-seven pediatric patients.

FIELD STRENGTH/SEQUENCE

3T. The 10-minute protocol was based on T2 Shuffling, a four-dimensional acquisition and reconstruction of images with variable T2 contrast, and a T1 2D fast spin-echo (FSE) sequence. A distributed, compressed sensing-based reconstruction was implemented on a four-node high-performance compute cluster and integrated into the clinical workflow.

ASSESSMENT

In an Institutional Review Board-approved study with informed consent/assent, we implemented a targeted pediatric knee MRI exam for assessing pediatric knee pain. Pediatric patients were subselected for the exam based on insurance plan and clinical indication. Over a 2-year period, 47 subjects were recruited for the study and 49 MRIs were ordered. Date and time information was recorded for MRI referral, registration, and completion. Image quality was assessed from 0 (nondiagnostic) to 5 (outstanding) by two readers, and consensus was subsequently reached.

STATISTICAL TESTS

A Wilcoxon rank-sum test assessed the null hypothesis that the targeted exam times compared with conventional knee exam times were unchanged.

RESULTS

Of the 49 cases, 20 were completed on the same day as exam referral. Median time from registration to exam completion was 18.7 minutes. Median reconstruction time for T2 Shuffling was reduced from 18.9 minutes to 95 seconds using the distributed implementation. Technical fees charged for the targeted exam were one-third that of the routine clinical knee exam. No subject had to return for additional imaging.

DATA CONCLUSION

The targeted knee MRI exam is feasible and reduces the imaging time, cost, and barrier to same-day MRI access for pediatric patients.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 6 J. Magn. Reson. Imaging 2019.

Flow dynamics of type II endoleaks can determine sac expansion after endovascular aneurysm repair using four-dimensional flow-sensitive magnetic resonance imaging analysis

OBJECTIVE

The objective of this study was to investigate the hemodynamic parameters of type II endoleaks (T2ELs) to predict sac expansion using four-dimensional flow-sensitive magnetic resonance imaging (4D-flow MRI) analysis.

METHODS

Patients who underwent endovascular aneurysm repair (EVAR) and were diagnosed with a T2EL were included in the study. Using 4D-flow MRI at 7 days, the peak flow velocity and amplitude of dynamics of blood flow per minute were measured in each T2EL vessel. The peak flow velocity was defined as the maximum of the absolute value of the blood flow velocity. The amplitude of dynamics of blood flow in the tributary arteries was defined as the sum of the absolute values of the inflow and outflow volume in each vessel. The amplitude of dynamics of blood flow in the tributary arteries per sac was calculated in each sac. The aneurysm sac diameter was measured by computed tomography (CT) at 1 year. The patients were divided into two groups according to the presence or absence of sac expansion.

RESULTS

Of 155 patients who underwent EVAR, both CT angiography and 4D-flow MRI were performed in 107 patients at 7 days after EVAR. Among them, 39 (36.4%) were found to have a T2EL, of whom 28 were re-evaluated with CT angiography and 4D-flow at 1 year; 7 patients had expanding sacs (expanding group), whereas 21 had nonexpanding sacs (not-expanding group). At 7 days, 28 patients had 80 T2EL vessels detected by 4D-flow MRI, of which 39 vessels (48.8%) had stopped flowing at 1 year (transient vessels); 41 vessels (51.3%) had sustained flow (persistent vessels). The persistent vessels had significantly larger peak flow velocity and amplitude of dynamics of blood flow. The comprehensive analysis of T2EL vessels per sac identified that the amplitude of dynamics of blood flow in the tributary arteries per sac was significantly higher in the expanding group than in the not-expanding group. A receiver operating characteristic curve analysis revealed that the sensitivity and specificity of sac enlargement at a cutoff value of 3750 mm³/min were 85.7% and 76.2%, respectively.

CONCLUSIONS

The fate of aneurysm sacs with T2ELs after EVAR has remained difficult to predict. A comprehensive analysis of concurrent multiple T2EL vessels using 4D-flow MRI analysis may enable prediction of the sac expansion after EVAR.

4D flow MRI quantification of mitral and tricuspid regurgitation: Reproducibility and consistency relative to conventional MRI

BACKGROUND

In patients with mitral or tricuspid valve regurgitation, evaluation of regurgitant severity is essential for determining the need for surgery. While transthoracic echocardiography is widely accessible, it has limited reproducibility for grading inlet valve regurgitation. Multiplanar cardiac MRI is the quantitative standard but requires specialized local expertise, and is thus not widely available. Volumetric 4D flow MRI has potential for quantitatively grading the severity of inlet valve regurgitation in adult patients.

PURPOSE

To evaluate the accuracy and reproducibility of volumetric 4D flow MRI for quantification of inlet valvular regurgitation compared to conventional multiplanar MRI, which may simplify and improve accessibility of cardiac MRI.

STUDY TYPE

This retrospective, HIPAA-compliant imaging-based comparison study was conducted at a single institution.

SUBJECTS

Twenty-one patients who underwent concurrent multiplanar and 4D flow cardiac MRI between April 2015 and January 2017.

FIELD STRENGTH/SEQUENCES

3T; steady-state free-precession (SSFP), 2D phase contrast (2D-PC), and postcontrast 4D flow.

ASSESSMENT

We evaluated the intertechnique (4D flow vs. 2D-PC), intermethod (direct vs. indirect measurement), interobserver and intraobserver reproducibility of measurements of regurgitant flow volume (RFV), fraction (RF), and volume (RVol).

STATISTICAL TESTS

Statistical analysis included Pearson correlation, Bland-Altman statistics, and intraclass correlation coefficients.

RESULTS

There was high concordance between 4D flow and multiplanar MRI, whether using direct or indirect methods of quantifying regurgitation (r = 0.813-0.985). Direct interrogation of the regurgitant jet with 4D flow showed high intraobserver consistency (r = 0.976-0.999) and interobserver consistency (r = 0.861-0.992), and correlated well with traditional indirect measurements obtained as the difference between stroke volume and forward outlet valve flow.

DATA CONCLUSION

4D flow MRI provides highly reproducible measurements of mitral and tricuspid regurgitant volume, and may be used in place of conventional multiplanar MRI.

LEVEL OF EVIDENCE

4 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2018;48:1147-1158.

High-resolution 3D volumetric contrast-enhanced MR angiography with a blood pool agent (ferumoxytol) for diagnostic evaluation of pediatric brain arteriovenous malformations

OBJECTIVE Patients with brain arteriovenous malformations (AVMs) often require repeat imaging with MRI or MR angiography (MRA), CT angiography (CTA), and digital subtraction angiography (DSA). The ideal imaging modality provides excellent vascular visualization without incurring added risks, such as radiation exposure. The purpose of this study is to evaluate the performance of ferumoxytol-enhanced MRA using a high-resolution 3D volumetric sequence (fe-SPGR) for visualizing and grading pediatric brain AVMs in comparison with CTA and DSA, which is the current imaging gold standard. METHODS In this retrospective cohort study, 21 patients with AVMs evaluated by fe-SPGR, CTA, and DSA between April 2014 and August 2017 were included. Two experienced raters graded AVMs using Spetzler-Martin criteria on all imaging studies. Lesion conspicuity (LC) and diagnostic confidence (DC) were assessed using a 5-point Likert scale, and interrater agreement was determined. The Kruskal-Wallis test was performed to assess the raters' grades and scores of LC and DC, with subsequent post hoc pairwise comparisons to assess for statistically significant differences between pairs of groups at p < 0.05. RESULTS Assigned Spetzler-Martin grades for AVMs on DSA, fe-SPGR, and CTA were not significantly different (p = 0.991). LC and DC scores were higher with fe-SPGR than with CTA (p < 0.05). A significant difference in LC scores was found between CTA and fe-SPGR (p < 0.001) and CTA and DSA (p < 0.001) but not between fe-SPGR and DSA (p = 0.146). A significant difference in DC scores was found among DSA, fe-SPGR, and CTA (p < 0.001) and between all pairs of the groups (p < 0.05). Interrater agreement was good to very good for all image groups (κ = 0.77-1.0, p < 0.001). CONCLUSIONS Fe-SPGR performed robustly in the diagnostic evaluation of brain AVMs, with improved visual depiction of AVMs compared with CTA and comparable Spetzler-Martin grading relative to CTA and DSA.

Volumetric segmentation-free method for rapid visualization of vascular wall shear stress using 4D flow MRI

PURPOSE

To develop a rapid segmentation-free method to visualize and compute wall shear stress (WSS) throughout the aorta using 4D Flow MRI data. WSS is the drag force-per-area the vessel endothelium exerts on luminal blood; abnormal levels of WSS are associated with cardiovascular pathologies. Previous methods for computing WSS are bottlenecked by labor-intensive manual segmentation of vessel boundaries. A rapid automated segmentation-free method for computing WSS is presented.

THEORY AND METHODS

Shear stress is the dot-product of the viscous stress tensor and the inward normal vector. The inward normal vectors are approximated as the gradient of fluid speed at every voxel. Subsequently, a 4D map of shear stress is computed as the partial derivatives of velocity with respect to the inward normal vectors. We highlight the shear stress near the wall by fusing visualization with edge-emphasized anatomical data.

RESULTS

As a proof-of-concept, four cases with aortic pathologies are presented. Visualization allows for rapid localization of pathologic WSS. Subsequent analysis of these pathological regions enables quantification of WSS. Average WSS during peak systole measures approximately 50-60 cPa in nonpathological regions of the aorta and is elevated in regions of stenosis, coarctation, and dissection. WSS is reduced in regions of aneurysm.

CONCLUSION

A volumetric technique for calculation and visualization of WSS from 4D Flow MRI data is presented. Traditional labor-intensive methods for WSS rely on explicit manual segmentation of vessel boundaries before visualization. This automated volumetric strategy for visualization and quantification of WSS may facilitate its clinical translation.

T2 shuffling: Sharp, multicontrast, volumetric fast spin-echo imaging

PURPOSE

A new acquisition and reconstruction method called T2 Shuffling is presented for volumetric fast spin-echo (three-dimensional [3D] FSE) imaging. T2 Shuffling reduces blurring and recovers many images at multiple T2 contrasts from a single acquisition at clinically feasible scan times (6-7 min).

THEORY AND METHODS

The parallel imaging forward model is modified to account for temporal signal relaxation during the echo train. Scan efficiency is improved by acquiring data during the transient signal decay and by increasing echo train lengths without loss in signal-to-noise ratio (SNR). By (1) randomly shuffling the phase encode view ordering, (2) constraining the temporal signal evolution to a low-dimensional subspace, and (3) promoting spatio-temporal correlations through locally low rank regularization, a time series of virtual echo time images is recovered from a single scan. A convex formulation is presented that is robust to partial voluming and radiofrequency field inhomogeneity.

RESULTS

Retrospective undersampling and in vivo scans confirm the increase in sharpness afforded by T2 Shuffling. Multiple image contrasts are recovered and used to highlight pathology in pediatric patients. A proof-of-principle method is integrated into a clinical musculoskeletal imaging workflow.

CONCLUSION

The proposed T2 Shuffling method improves the diagnostic utility of 3D FSE by reducing blurring and producing multiple image contrasts from a single scan. Magn Reson Med 77:180-195, 2017. © 2016 Wiley Periodicals, Inc.

Feasibility of ferumoxytol-enhanced neonatal and young infant cardiac MRI without general anesthesia

PURPOSE

To assess the feasibility of ferumoxytol-enhanced anesthesia-free cardiac MRI in neonates and young infants for complex congenital heart disease (CHD).

MATERIALS AND METHODS

With Institutional Review Board approval, 21 consecutive neonates and young infants (1 day to 11 weeks old; median age of 3 days) who underwent a rapid two-sequence (MR angiography [MRA] and four-dimensional [4D] flow) MRI protocol with intravenous ferumoxytol without sedation (n = 17) or light sedation (n = 4) at 3 Tesla (T) (except one case at 1.5T) between June 2014 and February 2016 were retrospectively identified. Medical records were reviewed for indication, any complications, if further diagnostic imaging was performed after MRI, and surgical findings. Two radiologists scored the images in two sessions on a 5-point scale for overall image quality and delineation of various anatomical structures. Confidence interval of proportions for likelihood of requiring additional diagnostic imaging after MRI was determined. For the possibility of reducing the protocol to a single rapid sequence, Wilcoxon-rank sum test was used to assess whether 4D flow and MRA significantly differed in anatomical delineation.

RESULTS

One of 21 patients (4.8%, 80% confidence interval 0-11%) required additional imaging, a computed tomography angiography to assess lung parenchyma and peripheral pulmonary arteries. Only 1 of 13 patients (7.7%) with operative confirmation had a minor discrepancy between radiology and operative reports (80% confidence interval 0-17%). 4D flow was significantly superior to MRA (P < 0.05) for the evaluation of systemic arteries, valves, ventricular trabeculae, and overall quality. Using Cohen's kappa coefficient, there was good interobserver agreement for the evaluation of systemic arteries by 4D flow (κ = 0.782), and systemic veins and pulmonary arteries by MRA (κ > 0.6). Overall 4D flow measurements (mean κ = 0.64-0.74) had better internal agreement compared with MRA (mean κ = 0.30-0.64).

CONCLUSION

Ferumoxytol-enhanced cardiac MRI, without anesthesia, is feasible for the evaluation of complex CHD in neonates and young infants, with a low likelihood of need for additional diagnostic studies. The decreased risk by avoiding anesthesia must be balanced against the potential for adverse reactions with ferumoxytol.

LEVEL OF EVIDENCE

2 J. MAGN. RESON. IMAGING 2017;45:1407-1418.

Non-invasive evaluation of pulmonary arterial blood flow and wall shear stress in pulmonary arterial hypertension with 3D phase contrast magnetic resonance imaging

Background

Recently, time-resolved 3D phase contrast magnetic resonance imaging (4D-flow) allows flow dynamics in patients with pulmonary arterial hypertension to be measured. Abnormal flow dynamics, such as vortex blood flow pattern in the pulmonary artery (PA), may reflect progression of pulmonary arterial hypertension (PAH). Some reports suggested that abnormal blood flow parameters including wall shear stress (WSS) could be markers of PAH. However, it was not fully assessed clinical usefulness of these variables. We aimed to assess whether these flow dynamic parameters, such as vortex formation time (VFT) and WSS, were associated with right ventricular (RV) function.

Results

Fifteen subjects, nine with PAH and six healthy volunteers, underwent 4D-flow. Differences of Blood flow patterns, blood flow velocities and WSS between PAH patients and healthy volunteers were evaluated. We also assessed the association between VFT, WSS and RV function in PAH patients. Both vortex blood flow patterns and early systolic retrograde flow in the main PA were observed in all patients with PAH. The PA flow velocities and WSS in patients with PAH were lower than those in healthy volunteers, but that blood flow volumes in the MPA, RPA and LPA and SV in the MPA were broadly comparable between the groups. The mean VFT was 35.0 ± 16.6 % of the cardiac cycle. The VFT significantly correlated with RV ejection fraction, RV end systolic volume, and RV end systolic volume index (RVEF = 75.1 + (−85.7)·VFT, p = 0.003, RVESV = 12.4 + 181.8·VFT, p = 0.037 and RVESVI = 10.6 + 114.8·VFT, p = 0.038, respectively) in PAH patients, whereas WSS did not correlate with RV function.

Conclusions

We confirmed that abnormal blood flow dynamics, including the vortex formation and the early onset of retrograde flow, low WSS in the PA were characteristics of PAH. The VFT may be associated with right ventricular dysfunction, whereas WSS was not. Our results suggest that 4D-flow is an effective means of detecting right heart failure as well as diagnosing PAH.

Clinical trial registration URL: https://upload.umin.ac.jp/cgi-open-bin/ctr/ctr.cgi. Unique identifier: UMIN000011128

Electronic supplementary material

The online version of this article (doi:10.1186/s40064-016-2755-7) contains supplementary material, which is available to authorized users.

Resolving phase ambiguity in dual-echo dixon imaging using a projected power method

PURPOSE

To develop a fast and robust method to resolve phase ambiguity in dual-echo Dixon imaging.

METHODS

A major challenge in dual-echo Dixon imaging is to estimate the phase error resulting from field inhomogeneity. In this work, a binary quadratic optimization program was formulated to resolve the phase ambiguity. A projected power method was developed to efficiently solve the optimization problem. Both the 1-peak fat model and 6-peak fat model were applied to three-dimensional (3D) datasets. Additionally, the proposed method was extended to dynamic magnetic resonance imaging (MRI) applications using the 6-peak fat model. With institutional review board (IRB) approval and patient consent/assent, the proposed method was evaluated and compared with region growing on 29 consecutive 3D high-resolution patient datasets.

RESULTS

Fast and robust water/fat separation was achieved by the proposed method in different representative 3D datasets and dynamic 3D datasets. Superior water/fat separation was achieved using the 6-peak fat model compared with the 1-peak fat model. Compared to region growing, the proposed method reduced water/fat swaps from 76 to 7% of the patient cohort.

CONCLUSION

The proposed method can achieve fast and robust phase error estimation in dual-echo Dixon imaging. Magn Reson Med 77:2066-2076, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Assessment of the precision and reproducibility of ventricular volume, function, and mass measurements with ferumoxytol-enhanced 4D flow MRI

PURPOSE

To compare the precision and interobserver agreement of ventricular volume, function, and mass quantification by 3D time-resolved (4D) flow MRI relative to cine steady-state free precession (SSFP).

MATERIALS AND METHODS

With Institutional Research Board approval, informed consent, and HIPAA compliance, 22 consecutive patients with congenital heart disease (CHD) (10 males, 6.4 ± 4.8 years) referred for 3T ferumoxytol-enhanced cardiac MRI were prospectively recruited. Complete ventricular coverage with standard 2D short-axis cine SSFP and whole chest coverage with axial 4D flow were obtained. Two blinded radiologists independently segmented images for left ventricular (LV) and right ventricular (RV) myocardium at end systole (ES) and end diastole (ED). Statistical analysis included linear regression, analysis of variance (ANOVA), Bland-Altman (BA) analysis, and intraclass correlation (ICC).

RESULTS

Significant positive correlations were found between 4D flow and SSFP for ventricular volumes (r = 0.808-0.972, P < 0.001), ejection fraction (EF) (r = 0.900-928, P < 0.001), and mass (r = 0.884-0.934, P < 0.001). BA relative limits of agreement for both ventricles were between -52% to 34% for volumes, -29% to 27% for EF, and -41% to 48% for mass, with wider limits of agreement for the RV compared to the LV. There was no significant difference between techniques with respect to mean square difference of ED-ES mass for either LV (F = 2.05, P = 0.159) or RV (F = 0.625, P = 0.434). Interobserver agreement was moderate to good with both 4D flow (ICC 0.523-0.993) and SSFP (ICC 0.619-0.982), with overlapping confidence intervals.

CONCLUSION

Quantification of ventricular volume, function, and mass can be accomplished with 4D flow MRI with precision and interobserver agreement comparable to that of cine SSFP. J. Magn. Reson. Imaging 2016;44:383-392.

High-resolution diffusion-weighted imaging of the breast with multiband 2D radiofrequency pulses and a generalized parallel imaging reconstruction

PURPOSE

To develop a technique for high-resolution diffusion-weighted imaging (DWI) and to compare it with standard DWI methods.

METHODS

Multiple in-plane bands of magnetization were simultaneously excited by identically phase modulating each subpulse of a two-dimensional (2D) RF pulse. Several excitations with the same multiband pattern progressively shifted in the phase-encode direction were used to cover the prescribed field of view (FOV). The phase-encoded FOV was limited to the width of a single band to reduce off-resonance-induced distortion and blurring. Parallel imaging (PI) techniques were used to resolve aliasing from the other bands and to combine the different excitations. Following validation in phantoms and healthy volunteers, a preliminary study in breast cancer patients (N=14) was performed to compare the proposed method to conventional DWI with PI and to reduced-FOV DWI.

RESULTS

The proposed method gave high-resolution diffusion-weighted images with minimal artifacts at the band intersections. Compared to PI alone, higher phase-encoded FOV-reduction factors and reduced noise amplification were obtained, which translated to higher resolution images than conventional (non-multiband) DWI. The same resolution and image quality achievable over targeted regions using existing reduced-FOV methods was obtained, but the proposed method also enables complete bilateral coverage.

CONCLUSION

We developed an in-plane multiband technique for high-resolution DWI and compared its performance with other standard DWI methods. Magn Reson Med 77:209-220, 2017. © 2016 Wiley Periodicals, Inc.

Comprehensive motion-compensated highly accelerated 4D flow MRI with ferumoxytol enhancement for pediatric congenital heart disease

PURPOSE

To develop and evaluate motion-compensation and compressed-sensing techniques in 4D flow MRI for anatomical assessment in a comprehensive ferumoxytol-enhanced congenital heart disease (CHD) exam.

MATERIALS AND METHODS

A Cartesian 4D flow sequence was developed to enable intrinsic navigation and two variable-density sampling schemes: VDPoisson and VDRad. Four compressed-sensing methods were developed: A) VDPoisson scan reconstructed using spatial wavelets; B) added temporal total variation to A; C) VDRad scan using the same reconstruction as in B; and D) added motion compensation to C. With Institutional Review Board (IRB) approval and Health Insurance Portability and Accountability Act (HIPAA) compliance, 23 consecutive patients (eight females, mean 6.3 years) referred for ferumoxytol-enhanced CHD 3T MRI were recruited. Images were acquired and reconstructed using methods A-D. Two cardiovascular radiologists independently scored the images on a 5-point scale. These readers performed a paired wall motion and functional assessment between method D and 2D balanced steady-state free precession (bSSFP) CINE for 16 cases.

RESULTS

Method D had higher diagnostic image quality for most anatomical features (mean 3.8-4.8) compared to A (2.0-3.6), B (2.2-3.7), and C (2.9-3.9) with P < 0.05 with good interobserver agreement (κ ≥ 0.49). Method D had similar or better assessment of myocardial borders and cardiac motion compared to 2D bSSFP (P < 0.05, κ ≥ 0.77). All methods had good internal agreement in comparing aortic with pulmonic flow (BA mean < 0.02%, r > 0.85) and compared to method A (BA mean < 0.13%, r > 0.84) with P < 0.01.

CONCLUSION

Flow, functional, and anatomical assessment in CHD with ferumoxytol-enhanced 4D flow is feasible and can be significantly improved using motion compensation and compressed sensing. J. Magn. Reson. Imaging 2016;43:1355-1368.

Improved quantification and mapping of anomalous pulmonary venous flow with four-dimensional phase-contrast MRI and interactive streamline rendering

BACKGROUND

Cardiac MRI is routinely performed for quantification of shunt flow in patients with anomalous pulmonary veins, but can be technically-challenging to perform. Four-dimensional phase-contrast (4D-PC) MRI has potential to simplify this exam. We sought to determine whether 4D-PC may be a viable clinical alternative to conventional 2D phase-contrast MR imaging.

METHODS

With institutional review board approval and HIPAA-compliance, we retrospectively identified all patients with anomalous pulmonary veins who underwent cardiac MRI at either 1.5 Tesla (T) or 3T with parallel-imaging compressed-sensing (PI-CS) 4D-PC between April, 2011 and October, 2013. A total of 15 exams were included (10 male, 5 female). Algorithms for interactive streamline visualization were developed and integrated into in-house software. Blood flow was measured at the valves, pulmonary arteries and veins, cavae, and any associated shunts. Pulmonary veins were mapped to their receiving atrial chamber with streamlines. The intraobserver, interobserver, internal consistency of flow measurements, and consistency with conventional MRI were then evaluated with Pearson correlation and Bland-Altman analysis.

RESULTS

Triplicate measurements of blood flow from 4D-PC were highly consistent, particularly at the aortic and pulmonary valves (cv 2-3%). Flow measurements were reproducible by a second observer (ρ = 0.986-0.999). Direct measurements of shunt volume from anomalous veins and intracardiac shunts matched indirect estimates from the outflow valves (ρ = 0.966). Measurements of shunt fraction using 4D-PC using any approach were more consistent with ventricular volumetric displacements than conventional 2D-PC (ρ = 0.972-0.991 versus 0.929).

CONCLUSION

Shunt flow may be reliably quantified with 4D-PC MRI, either indirectly or with detailed delineation of flow from multiple shunts. The 4D-PC may be a more accurate alternative to conventional MRI.

Congenital heart disease assessment with 4D flow MRI

With improvements in surgical and medical management, patients with congenital heart disease (CHD) are often living well into adulthood. MRI provides critical data for diagnosis and monitoring of these patients, yielding information on cardiac anatomy, blood flow, and cardiac function. Though historically these exams have been complex and lengthy, four-dimensional (4D) flow is emerging as a single fast technique for comprehensive assessment of CHD. The 4D flow consists of a volumetric time-resolved acquisition that is gated to the cardiac cycle, providing a time-varying vector field of blood flow as well as registered anatomic images. In this article, we provide an overview of MRI evaluation of congenital heart disease by means of example of three relatively common representative conditions: tetralogy of Fallot, aortic coarctation, and anomalous pulmonary venous drainage. Then 4D flow data acquisition, data correction, and postprocessing techniques are reviewed. We conclude with several examples that highlight the comprehensive nature of the evaluation of congenital heart disease with 4D flow.

Fast pediatric 3D free-breathing abdominal dynamic contrast enhanced MRI with high spatiotemporal resolution

PURPOSE

To develop a method for fast pediatric 3D free-breathing abdominal dynamic contrast enhanced (DCE) magnetic resonance imaging (MRI) and investigate its clinical feasibility.

MATERIALS AND METHODS

A combined locally low rank parallel imaging method with soft gating is proposed for free-breathing DCE MRI acquisition. With Institutional Review Board (IRB) approval and informed consent/assent, 23 consecutive pediatric patients were recruited for this study. Free-breathing DCE MRI with ∼1 mm(3) spatial resolution and a 6.5-sec frame rate was acquired on a 3T scanner. Undersampled data were reconstructed with a compressed sensing method without motion correction (FB-CS) and the proposed method (FB-LR). A follow-up respiratory-triggered acquisition (RT-CS) was performed as a reference standard. The reconstructed images were evaluated independently by two radiologists. Wilcoxon tests were performed to test the hypothesis that there was no significant difference between different reconstructions. Quantitative evaluation of contrast dynamics was also performed.

RESULTS

The mean score of overall image quality of FB-LR was 4.0 on a 5-point scale, significantly better (P < 0.05) than FB-CS reconstruction (mean score 2.9), and similar to RT-CS (mean score 4.1). FB-LR also matched the temporal fidelity of contrast dynamics with a root mean square error less than 5%.

CONCLUSION

Fast 3D free-breathing DCE MRI with high scan efficiency and image quality similar to respiratory-triggered acquisition is feasible in a pediatric clinical setting.

Free-breathing pediatric MRI with nonrigid motion correction and acceleration

PURPOSE

To develop and assess motion correction techniques for high-resolution pediatric abdominal volumetric magnetic resonance images acquired free-breathing with high scan efficiency.

MATERIALS AND METHODS

First, variable-density sampling and radial-like phase-encode ordering were incorporated into the 3D Cartesian acquisition. Second, intrinsic multichannel butterfly navigators were used to measure respiratory motion. Lastly, these estimates are applied for both motion-weighted data-consistency in a compressed sensing and parallel imaging reconstruction, and for nonrigid motion correction using a localized autofocusing framework. With Institutional Review Board approval and informed consent/assent, studies were performed on 22 consecutive pediatric patients. Two radiologists independently scored the images for overall image quality, degree of motion artifacts, and sharpness of hepatic vessels and the diaphragm. The results were assessed using paired Wilcoxon test and weighted kappa coefficient for interobserver agreements.

RESULTS

The complete procedure yielded significantly better overall image quality (mean score of 4.7 out of 5) when compared to using no correction (mean score of 3.4, P < 0.05) and to using motion-weighted accelerated imaging (mean score of 3.9, P < 0.05). With an average scan time of 28 seconds, the proposed method resulted in comparable image quality to conventional prospective respiratory-triggered acquisitions with an average scan time of 91 seconds (mean score of 4.5).

CONCLUSION

With the proposed methods, diagnosable high-resolution abdominal volumetric scans can be obtained from free-breathing data acquisitions.