Variable flip angle balanced steady-state free precession for lower SAR or higher contrast cardiac cine imaging

Feasibility of Three-Dimensional Balanced Steady-State Free Precession Cine Magnetic Resonance Imaging Combined with an Image Denoising Technique to Evaluate Cardiac Function in Children with Repaired Tetralogy of Fallot

Objective

To investigate the feasibility of cine three-dimensional (3D) balanced steady-state free precession (b-SSFP) imaging combined with a non-local means (NLM) algorithm for image denoising in evaluating cardiac function in children with repaired tetralogy of Fallot (rTOF).

Materials and Methods

Thirty-five patients with rTOF (mean age, 12 years; range, 7–18 years) were enrolled to undergo cardiac cine image acquisition, including two-dimensional (2D) b-SSFP, 3D b-SSFP, and 3D b-SSFP combined with NLM. End-diastolic volume (EDV), end-systolic volume (ESV), stroke volume (SV), and ejection fraction (EF) of the two ventricles were measured and indexed by body surface index. Acquisition time and image quality were recorded and compared among the three imaging sequences.

Results

3D b-SSFP with denoising vs. 2D b-SSFP had high correlation coefficients for EDV, ESV, SV, and EF of the left (0.959–0.991; p < 0.001) as well as right (0.755–0.965; p < 0.001) ventricular metrics. The image acquisition time ± standard deviation (SD) was 25.1 ± 2.4 seconds for 3D b-SSFP compared with 277.6 ± 0.7 seconds for 2D b-SSFP, indicating a significantly shorter time with the 3D than the 2D sequence (p < 0.001). Image quality score was better with 3D b-SSFP combined with denoising than with 3D b-SSFP (mean ± SD, 3.8 ± 0.6 vs. 3.5 ± 0.6; p = 0.005). Signal-to-noise ratios for blood and myocardium as well as contrast between blood and myocardium were higher for 3D b-SSFP combined with denoising than for 3D b-SSFP (p < 0.05 for all but septal myocardium).

Conclusion

The 3D b-SSFP sequence can significantly reduce acquisition time compared to the 2D b-SSFP sequence for cine imaging in the evaluation of ventricular function in children with rTOF, and its quality can be further improved by combining it with an NLM denoising method.

Evaluation of a Workflow to Define Low Specific Absorption Rate MRI Protocols for Patients With Active Implantable Medical Devices

BACKGROUND

MRI exams for patients with MR-conditional active implantable medical devices (AIMDs) are contraindicated unless specific conditions are met. This limits the maximum specific absorption rate (SAR, W/kg). Currently, there is no general framework to guide meeting a lower SAR limit.

PURPOSE

To design and evaluate a workflow for modifying MRI protocols to whole-body SAR (WB-SAR ≤0.1 W/kg) and local-head SAR (LH-SAR ≤0.3 W/kg) limits while mitigating the impact on image quality and exam time.

STUDY TYPE

Prospective.

POPULATION

Twenty healthy volunteers on head (n = 5), C-spine (n = 5), T-spine (n = 5), and L-spine (n = 5) with IRB consent.

ASSESSMENT

Vendor-provided head, C-spine, T-spine, and L-spine protocols (SAR_RT ) were modified to meet both low SAR targets (SAR_LOW ) using the proposed workflow. in vitro SNR and CNR were evaluated with a T₁ -T₂ phantom. in vivo image quality and clinical acceptability were scored using a 5-point Likert scale for two blinded readers.

FIELD STRENGTH/SEQUENCES

1.5T/spin-echoes, gradient-echoes.

STATISTICAL ANALYSIS

In vitro SNR and CNR values were evaluated with a repeated measures general linear model. in vivo image quality and clinical acceptability were evaluated using a generalized estimating equation analysis (GEE). The two reader's level of agreement was analyzed using Cohen's kappa statistical analysis.

RESULTS

Using the workflow, SAR limits were met.

LH-SAR

0.12 ± 0.02 W/kg, median (SD) values for LH-SAR were 0.12 (0.02) W/kg and WB-SAR: 0.09 (0.01) W/kg. Examination time did not increase ≤2x the initial time. SAR_RT SNR values were higher and significantly different than SAR_LOW (P < 0.05). However, no significant difference was observed between the CNR values (value = 0.21). Median (IQR) CNR values were 14.2 (25.0) vs. 15.1 (9.2) for head, 12.1 (16.9) vs. 25.3 (14.2) for C-spine, 81.6 (70.1) vs. 71.0 (26.6) for T-spine, and 51.4 (52.6) vs. 37.7 (27.3) for L-spine. Image quality scores were not significantly different between SAR_RT and SAR_LOW (median [SD] scores were 4.0 [0.01] vs. 4.3 [0.2], P > 0.05).

DATA CONCLUSION

The proposed workflow provides guidance for modifying routine MRI exams to achieve low SAR limits. This can benefit patients referred for an MRI exam with low SAR MR-conditional AIMDs.

LEVEL OF EVIDENCE

1 Technical Efficacy Stage: 5 J. Magn. Reson. Imaging 2020;52:91-102.

Highly-accelerated self-gated free-breathing 3D cardiac cine MRI: validation in assessment of left ventricular function

OBJECTIVE

This work presents a highly-accelerated, self-gated, free-breathing 3D cardiac cine MRI method for cardiac function assessment.

MATERIALS AND METHODS

A golden-ratio profile based variable-density, pseudo-random, Cartesian undersampling scheme was implemented for continuous 3D data acquisition. Respiratory self-gating was achieved by deriving motion signal from the acquired MRI data. A multi-coil compressed sensing technique was employed to reconstruct 4D images (3D+time). 3D cardiac cine imaging with self-gating was compared to bellows gating and the clinical standard breath-held 2D cine imaging for evaluation of self-gating accuracy, image quality, and cardiac function in eight volunteers. Reproducibility of 3D imaging was assessed.

RESULTS

Self-gated 3D imaging provided an image quality score of 3.4 ± 0.7 vs 4.0 ± 0 with the 2D method (p = 0.06). It determined left ventricular end-systolic volume as 42.4 ± 11.5 mL, end-diastolic volume as 111.1 ± 24.7 mL, and ejection fraction as 62.0 ± 3.1%, which were comparable to the 2D method, with bias ± 1.96 × SD of -0.8 ± 7.5 mL (p = 0.90), 2.6 ± 3.3 mL (p = 0.84) and 1.4 ± 6.4% (p = 0.45), respectively.

CONCLUSION

The proposed 3D cardiac cine imaging method enables reliable respiratory self-gating performance with good reproducibility, and provides comparable image quality and functional measurements to 2D imaging, suggesting that self-gated, free-breathing 3D cardiac cine MRI framework is promising for improved patient comfort and cardiac MRI scan efficiency.

Interventional CMR: Clinical applications and future directions

Interventional cardiovascular magnetic resonance (iCMR) promises to enable radiation-free catheterization procedures and to enhance contemporary image guidance for structural heart and electrophysiological interventions. However, clinical translation of exciting pre-clinical interventions has been limited by availability of devices that are safe to use in the magnetic resonance (MR) environment. We discuss challenges and solutions for clinical translation, including MR-conditional and MR-safe device design, and how to configure an interventional suite. We review the recent advances that have already enabled diagnostic MR right heart catheterization and simple electrophysiologic ablation to be performed in humans and explore future clinical applications.

Free-breathing variable flip angle balanced SSFP cardiac cine imaging with reduced SAR at 3T

PURPOSE

To develop a free-breathing variable flip angle (VFA) balanced steady-state free precession (bSSFP) cardiac cine imaging technique with reduced specific absorption rate (SAR) at 3 Tesla.

METHODS

Free-breathing VFA (FB-VFA) images in the short-axis and four-chamber views were acquired using an optimal VFA scheme, then compared with conventional breath-hold constant flip angle (BH-CFA) acquisitions. Two cardiac MRI experts used a 5-point scale to score images from healthy subjects (N = 10). The left ventricular ejection fraction, end diastolic volume (LVEDV), end systolic volume, stroke volume (LVSV), and end diastolic myocardial mass (LVEDM) were determined by manual contour analysis for BH-CFA and FB-VFA. A pilot evaluation of FB-VFA was performed in one patient with Duchenne muscular dystrophy.

RESULTS

FB-VFA SAR was 25% lower than BH-CFA with similar blood-myocardium contrast. The qualitative FB-VFA score was lower than the BH-CFA for the short-axis (3.1 ± 0.5 versus 4.3 ± 0.8; P < 0.05) and the four-chamber view (3.4 ± 0.4 versus 4.6 ± 0.6; P < 0.05). The LVEDV and the LVSV were 5% and 12% larger (P < 0.05) for FB-VFA compared with BH-CFA. There was no difference in LVEDM.

CONCLUSION

FB-VFA bSSFP cardiac cine imaging decreased the SAR at 3T with image quality sufficient to perform cardiac functional analysis. Magn Reson Med 76:1210-1216, 2016. © 2015 Wiley Periodicals, Inc.

Dual echo positive contrast bSSFP for real-time visualization of passive devices during magnetic resonance guided cardiovascular catheterization

BACKGROUND

Cardiovascular magnetic resonance (CMR) guided cardiovascular catheterizations can potentially reduce ionizing radiation exposure and enable new interventions. Commercially available paramagnetic X-Ray devices create a small signal void in CMR images, which is ambiguous and insufficient to guide catheterization procedures. This work aims to improve real-time CMR of off-the-shelf X-Ray devices by developing a real-time positive contrast sequence with color overlay of the device onto anatomy.

METHODS

A dual-echo bSSFP sequence was used to generate both a dephased positive contrast image and bSSFP image simultaneously. A variable flip angle scheme was implemented to reduce the specific absorption rate (SAR) and hence device heating. Image processing was used to isolate the device from background signal, and the device was overlaid in color on the anatomy, mimicking active device visualization. Proof-of-concept experiments were performed using a commercially available nitinol guidewire for left heart catheterization in Yorkshire swine.

RESULTS

The dual echo pulse sequence generated a temporal resolution of 175 ms (5.7 frames/second) with GRAPPA acceleration factor 4. Image processing was performed in real-time and color overlay of the device on the anatomy was displayed to the operator with no latency. The color overlay accurately depicted the guidewire location, with minimal background contamination, during left heart catheterization.

CONCLUSIONS

The ability to effectively visualize commercially available X-Ray devices during CMR-guided cardiovascular catheterizations, combined with safe low-SAR pulse sequences, could potentially expedite the clinical translation of interventional CMR.

Fast 3D T2 -weighted imaging using variable flip angle transition into driven equilibrium (3D T2 -TIDE) balanced SSFP for prostate imaging at 3T

PURPOSE

Three-dimensional (3D) T2 -weighted fast spin echo (FSE) imaging of the prostate currently requires long acquisition times. Our objective was to develop a fast 3D T2 -weighted sequence for prostate imaging at 3T using a variable flip angle transition into driven equilibrium (T2 -TIDE) scheme.

METHODS

3D T2 -TIDE uses interleaved spiral-out phase encode ordering to efficiently sample the ky -kz phase encodes and also uses the transient balanced steady-state free precession signal to acquire the center of k-space for T2 -weighted imaging. Bloch simulations and images from 10 healthy subjects were acquired to evaluate the performance of 3D T2 -TIDE compared to 3D FSE.

RESULTS

3D T2 -TIDE images were acquired in 2:54 minutes compared to 7:02 minutes for 3D FSE with identical imaging parameters. The signal-to-noise ratio (SNR) efficiency was significantly higher for 3D T2 -TIDE compared to 3D FSE in nearly all tissues, including periprostatic fat (45 ± 12 vs. 31 ± 7, P < 0.01), gluteal fat (48 ± 8 vs. 41 ± 10, P = 0.12), right peripheral zone (20 ± 4 vs. 16 ± 8, P = 0.12), left peripheral zone (17 ± 2 vs. 12 ± 3, P < 0.01), and anterior fibromuscular stroma (12 ± 4 vs. 4 ± 2, P < 0.01).

CONCLUSION

3D T2 -TIDE images of the prostate can be acquired quickly with SNR efficiency that exceeds that of 3D FSE.