Purpose
The aim of this work is to shed light on the issue of reproducibility in MR image reconstruction in the context of a challenge. Participants had to recreate the results of “Advances in ...sensitivity encoding with arbitrary k‐space trajectories" by Pruessmann et al.
Methods
The task of the challenge was to reconstruct radially acquired multicoil k‐space data (brain/heart) following the method in the original paper, reproducing its key figures. Results were compared to consolidated reference implementations created after the challenge, accounting for the two most common programming languages used in the submissions (Matlab/Python).
Results
Visually, differences between submissions were small. Pixel‐wise differences originated from image orientation, assumed field‐of‐view, or resolution. The reference implementations were in good agreement, both visually and in terms of image similarity metrics.
Discussion and Conclusion
While the description level of the published algorithm enabled participants to reproduce CG‐SENSE in general, details of the implementation varied, for example, density compensation or Tikhonov regularization. Implicit assumptions about the data lead to further differences, emphasizing the importance of sufficient metadata accompanying open datasets. Defining reproducibility quantitatively turned out to be nontrivial for this image reconstruction challenge, in the absence of ground‐truth results. Typical similarity measures like NMSE of SSIM were misled by image intensity scaling and outlier pixels. Thus, to facilitate reproducibility, researchers are encouraged to publish code and data alongside the original paper. Future methodological papers on MR image reconstruction might benefit from the consolidated reference implementations of CG‐SENSE presented here, as a benchmark for methods comparison.
Pulmonary hypertension is definitively diagnosed by the measurement of mean pulmonary artery (PA) pressure (mPAP) using right heart catheterization. Cardiovascular magnetic resonance (CMR) ...four-dimensional (4D) flow analysis can estimate mPAP from blood flow vortex duration in the PA, with excellent results. Moreover, the peak systolic tricuspid regurgitation (TR) pressure gradient (TRPG) measured by Doppler echocardiography is commonly used in clinical routine to estimate systolic PA pressure. This study aimed to compare CMR and echocardiography with regards to quantitative and categorical agreement, and diagnostic yield for detecting increased PA pressure.
Consecutive clinically referred patients (n = 60, median interquartile range age 60 48-68 years, 33% female) underwent echocardiography and CMR at 1.5 T (n = 43) or 3 T (n = 17). PA vortex duration was used to estimate mPAP using a commercially available time-resolved multiple 2D slice phase contrast three-directional velocity encoded sequence covering the main PA. Transthoracic Doppler echocardiography was performed to measure TR and derive TRPG. Diagnostic yield was defined as the fraction of cases in which CMR or echocardiography detected an increased PA pressure, defined as vortex duration ≥15% of the cardiac cycle (mPAP ≥25 mmHg) or TR velocity > 2.8 m/s (TRPG > 31 mmHg).
Both CMR and echocardiography showed normal PA pressure in 39/60 (65%) patients and increased PA pressure in 9/60 (15%) patients, overall agreement in 48/60 (80%) patients, kappa 0.49 (95% confidence interval 0.27-0.71). CMR had a higher diagnostic yield for detecting increased PA pressure compared to echocardiography (21/60 (35%) vs 9/60 (15%), p < 0.001). In cases with both an observable PA vortex and measurable TR velocity (34/60, 56%), TRPG was correlated with mPAP (R
= 0.65, p < 0.001).
There is good quantitative and fair categorical agreement between estimated mPAP from CMR and TRPG from echocardiography. CMR has higher diagnostic yield for detecting increased PA pressure compared to echocardiography, potentially due to a lower sensitivity of echocardiography in detecting increased PA pressure compared to CMR, related to limitations in the ability to adequately visualize and measure the TR jet by echocardiography. Future comparison between echocardiography, CMR and invasive measurements are justified to definitively confirm these findings.
Computed tomography pulmonary angiography (CTPA) is the gold standard diagnostic method for patients with suspected pulmonary embolism (PE), but it has its drawbacks, including exposure to ionizing ...radiation and iodinated contrast agent. The present study aims to evaluate the diagnostic performance of our in-house developed non-contrast MRI protocol for PE diagnosis in reference to CTPA.
107 patients were included, all of whom underwent MRI immediately before or within 36 hours after CTPA. Additional cases examined only with MRI and a negative result were added to reach a PE prevalence of approximately 20%. The protocol was a non-contrast 2D steady-state free precession (SSFP) sequence under free-breathing, without respiratory or cardiac gating, and repeated five times to capture the vessels at different breathing/cardiac phases. The MRIs were blinded and read by two radiologists and the results were compared to CTPA.
Of the 243 patients included, 47 were positive for PE. Readers 1 and 2 demonstrated 89% and 87% sensitivity, 100% specificity, 98% accuracy and Cohen’s kappa of 0.88 on patient level. In the per embolus comparison, readers 1 and 2 detected, 60 and 59/61 (98, 97%) proximal, 101 and 94/113 (89, 83%) segmental, and 5 and 2/32 (16, 6%) subsegmental emboli, resulting in 81 and 75% sensitivity respectively.
The repeated 2D SSFP can reliably be used for the diagnosis of acute PE at the proximal and segmental artery levels.
•The protocol was a 2D non-contrast steady-state free precession (SSFP) sequence under free-breathing, without respiratory or cardiac gating, and repeated five times to capture the vessels at different breathing and cardiac phases.•The examination time was less than 10 minutes.•The readers demonstrated a sensitivity of 87 and 89%, specificity of 100%, accuracy of 97 and 98% with a kappa value of 0.88 at patient level.
Purpose
Through‐time spiral GRAPPA is a real‐time imaging technique that enables ungated, free‐breathing evaluation of the left ventricle. However, it requires a separate fully‐sampled calibration ...scan to calculate GRAPPA weights. A self‐calibrated through‐time spiral GRAPPA method is proposed that uses a specially designed spiral trajectory with interleaved arm ordering such that consecutive undersampled frames can be merged to form calibration data, eliminating the separate fully‐sampled acquisition.
Theory and Methods
The proposed method considers the time needed to acquire data at all points in a GRAPPA calibration kernel when using interleaved arm ordering. Using this metric, simulations were performed to design a spiral trajectory for self‐calibrated GRAPPA. Data were acquired in healthy volunteers using the proposed method and a comparison electrocardiogram‐gated and breath‐held cine scan. Left ventricular functional values and image quality are compared.
Results
A 12‐arm spiral trajectory was designed with a temporal resolution of 32.72 ms/cardiac phase with an acceleration factor of 3. Functional values calculated using the proposed method and the gold‐standard method were not statistically significantly different (paired t‐test, p < 0.05). Image quality ratings were lower for the proposed method, with statistically significantly different ratings (Wilcoxon signed rank test, p < 0.05) for two of five image quality aspects rated (level of artifact, blood‐myocardium contrast).
Conclusions
A self‐calibrated through‐time spiral GRAPPA reconstruction can enable ungated, free‐breathing evaluation of the left ventricle in 71 s. Functional values are equivalent to a gold‐standard cine technique, although some aspects of image quality may be inferior due to the real‐time nature of the data collection.
Purpose
To develop and evaluate a 3D sector‐wise golden‐angle (3D‐SWIG) profile ordering scheme for cardiovascular MR cine imaging that maintains high k‐space uniformity after electrocardiogram (ECG) ...binning.
Method
Cardiovascular MR (CMR) was performed at 1.5 T. A balanced SSFP pulse sequence was implemented with a novel 3D‐SWIG radial ordering, where k‐space was divided into wedges, and each wedge was acquired in a separate heartbeat. The high uniformity of k‐space coverage after physiological binning can be used to perform functional imaging using a very short acquisition. The 3D‐SWIG was compared with two commonly used 3D radial trajectories for CMR (i.e., double golden angle and spiral phyllotaxis) in numerical simulations. Free‐breathing 3D‐SWIG and conventional breath‐held 2D cine were compared in patients (n = 17) referred clinically for CMR. Quantitative comparison was performed based on left ventricular segmentation.
Results
Numerical simulations showed that 3D‐SWIG both required smaller steps between successive readouts and achieved better k‐space sampling uniformity after binning than either the double golden angle or spiral phyllotaxis trajectories. In vivo evaluation showed that measurements of left ventricular ejection fraction calculated from a 48 heart‐beat free‐breathing 3D‐SWIG acquisition were highly reproducible and agreed with breath‐held 2D‐Cartesian cine (mean ± SD difference of −3.1 ± 3.5% points).
Conclusions
The 3D‐SWIG acquisition offers a simple solution for highly improved k‐space uniformity after physiological binning. The feasibility of the 3D‐SWIG method is demonstrated in this study through whole‐heart cine imaging during free breathing with an acquisition time of less than 1 min.
Purpose
We propose a novel generalization of the three-dimensional double-golden-angle profile ordering, which allows for whole-heart volumetric imaging with retrospective binning and reduced eddy ...current artifacts.
Methods
A novel theory bridging the gap between the three-dimensional double golden-angle trajectory, and the two-dimensional tiny-golden-angle trajectory is presented. This enables a class of double golden-angle profile orderings with a smaller angular distance between successive
k
-space readouts. The novel profile orderings were evaluated through simulations, phantom experiments, and in vivo imaging. Comparisons were made to the original double-golden-angle trajectory. Image uniformity and off-resonance sensitivity were evaluated using phantom measurements, and qualitative image quality was assessed using in vivo images acquired in a healthy volunteer.
Results
The proposed theory successfully reduced the angular step while maintaining image uniformity after binning. Simulations revealed a slow degradation with decreasing angular steps and an increasing number of physiological bins. The phantom images showed a definite improvement in image uniformity and increased robustness to off-resonance, and in vivo imaging corroborated those findings.
Conclusion
Reducing the angular step in cardio-respiratory-binned golden-angle imaging shows potential for overcoming eddy current-induced image artifacts associated with 3D golden-angle radial imaging.
Purpose
To develop a high temporal resolution phase‐contrast pulse sequence for evaluation of diastolic filling patterns, and to evaluate it in comparison to transthoracic echocardiography.
Methods
A ...phase‐contrast velocity‐encoded gradient‐echo pulse sequence was implemented with a sector‐wise golden‐angle radial ordering. Acquisitions were optimized for myocardial tissue (TE/TR: 4.4/6.8 ms, flip angle: 8º, velocity encoding: 30 cm/s) and transmitral flow (TE/TR: 4.0/6.6 ms, flip angle: 20º, velocity encoding: 150 cm/s). Shared velocity encoding was combined with a sliding‐window reconstruction that enabled up to 250 frames per cardiac cycle. Transmitral and myocardial velocities were measured in 35 patients. Echocardiographic velocities were obtained with pulsed‐wave Doppler using standard methods.
Results
Myocardial velocity showed a low difference and good correlation between MRI and Doppler (mean ± 95% limits of agreement 0.9 ± 3.7 cm/s, R2 = 0.63). Transmitral velocity was underestimated by MRI (P < .05) with a difference of −11 ± 28 cm/s (R2 = 0.45). The early‐to‐late ratio correlated well (R2 = 0.66) with a minimal difference (0.03 ± 0.6). Analysis of interobserver and intra‐observer variability showed excellent agreement for all measurements.
Conclusions
The proposed method enables the acquisition of phase‐contrast images during a single breath‐hold with a sufficiently high temporal resolution to match transthoracic echocardiography, which opens the possibility for many clinically relevant variables to be assessed by MRI.
The aims of this study were to develop a comprehensive cardiovascular magnetic resonance (CMR) approach to diastolic dysfunction (DD) grading and to evaluate the accuracy of CMR in the diagnosis of ...DD compared with echocardiography.
Left ventricular DD is routinely assessed using echocardiography.
Consecutive clinically referred patients (n = 46; median age 59 years; interquartile range: 46 to 68 years; 33% women) underwent both conventional echocardiography and CMR. CMR diastolic transmitral velocities (E and A) and myocardial tissue velocity (e′) were measured during breath-hold using a validated high–temporal resolution radial sector-wise golden-angle velocity-encoded sequence. CMR pulmonary artery pressure was estimated from 4-dimensional flow analysis of blood flow vortex duration in the pulmonary artery. CMR left atrial volume was measured using the biplane long-axis area-length method. Both CMR and echocardiographic data were used to perform blinded grading of DD according to the 2016 joint American and European recommendations.
Grading of DD by CMR agreed with that by echocardiography in 43 of 46 cases (93%), of which 9% were normal, 2% indeterminate, 63% grade 1 DD, 4% grade 2 DD, and 15% grade 3 DD. There was a very good categorical agreement, with a weighted Cohen kappa coefficient of 0.857 (95% confidence interval: 0.73 to 1.00; p < 0.001).
A comprehensive CMR protocol for grading DD encompassing diastolic blood and myocardial velocities, estimated pulmonary artery pressure, and left atrial volume showed very good agreement with echocardiography.
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Background
Pulmonary embolism (PE) is a common cause of death with an incidence of approximately 1–2 cases per 1000 inhabitants in Europe and the United States. Treatment for PE is the administration ...of anticoagulants for at least three months.
Purpose
To assess the feasibility of following the resolution rate of PE over time using repeated imaging with a non-contrast-enhanced magnetic resonance imaging (MRI) protocol.
Material and Methods
Patients (n = 18) diagnosed with acute PE via computed tomography pulmonary angiography (CTPA) underwent non-contrast-enhanced MRI at two tertiary hospitals. The first MRI was performed within 36 h of CTPA, with follow-up at one week, one, three, and six months. The MRI sequence used was a non-contrast-enhanced standard two-dimensional steady-state free precession under free-breathing and without respiratory or cardiac gating. All MRI scans were then compared to the initial CTPA. The emboli were assessed visually for location and size, and clot burden was calculated using the Qanadli score.
Results
MRI revealed complete resolution in seven cases at one week, in five cases at one month, and in three cases at three months. The most significant resolution of emboli occurred within the first few weeks, with only 10% of the diagnosed emboli persisting at the one-month examination.
Conclusion
The use of MRI imparts the ability to visualize PE without radiation and thus allows multiple examinations to be made, for example in studies investigating the resolution of PE or the evaluation of drug effect in clinical trials.
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