Comparison of fast acquisition strategies in whole-heart four-dimensional flow cardiac MR: Two-center, 1.5 Tesla, phantom and in vivo validation study

Abstract

Purpose: To validate three widely-used acceleration methods in four-dimensional (4D) flow cardiac MR; segmented 4D-spoiled-gradient-echo (4D-SPGR), 4D-echo-planar-imaging (4D-EPI), and 4D-k-t Broad-use Linear Acquisition Speed-up Technique (4D-k-t BLAST).

Materials and methods: Acceleration methods were investigated in static/pulsatile phantoms and 25 volunteers on 1.5 Tesla MR systems. In phantoms, flow was quantified by 2D phase-contrast (PC), the three 4D flow methods and the time-beaker flow measurements. The later was used as the reference method. Peak velocity and flow assessment was done by means of all sequences. For peak velocity assessment 2D PC was used as the reference method. For flow assessment, consistency between mitral inflow and aortic outflow was investigated for all pulse-sequences. Visual grading of image quality/artifacts was performed on a four-point-scale (0 = no artifacts; 3 = nonevaluable).

Results: For the pulsatile phantom experiments, the mean error for 2D PC = 1.0 ± 1.1%, 4D-SPGR = 4.9 ± 1.3%, 4D-EPI = 7.6 ± 1.3% and 4D-k-t BLAST = 4.4 ± 1.9%. In vivo, acquisition time was shortest for 4D-EPI (4D-EPI = 8 ± 2 min versus 4D-SPGR = 9 ± 3 min, P < 0.05 and 4D-k-t BLAST = 9 ± 3 min, P = 0.29). 4D-EPI and 4D-k-t BLAST had minimal artifacts, while for 4D-SPGR, 40% of aortic valve/mitral valve (AV/MV) assessments scored 3 (nonevaluable). Peak velocity assessment using 4D-EPI demonstrated best correlation to 2D PC (AV:r = 0.78, P < 0.001; MV:r = 0.71, P < 0.001). Coefficient of variability (CV) for net forward flow (NFF) volume was least for 4D-EPI (7%) (2D PC:11%, 4D-SPGR: 29%, 4D-k-t BLAST: 30%, respectively).

Conclusion: In phantom, all 4D flow techniques demonstrated mean error of less than 8%. 4D-EPI demonstrated the least susceptibility to artifacts, good image quality, modest agreement with the current reference standard for peak intra-cardiac velocities and the highest consistency of intra-cardiac flow quantifications.

Level of evidence: 1 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2018;47:272-281.

Keywords: 4D flow cardiac MR; MR flow imaging; flow quantification; phase-contrast magnetic resonance imaging; validation.

Figure 1

Scoring of image artifacts was done on the raw 4D flow data for each directional phase contrast data (as demonstrated in first row). Case example: In this case, there is velocity aliasing artifact for 4D‐k‐t BLAST acquisition (orange arrow) (all acquisitions at velocity encoding = 150 cm/s). This velocity aliasing artifact can be corrected and does not limit quantification. 4D‐SPGR had severe phase dispersion artifact which limited quantification. Streamlines demonstrate better velocity profile for 4D‐EPI versus 4D‐SPGR (second row).

Figure 2

Image quality of mitral and aortic valve flow acquisitions using the different acceleration techniques. Both for the aortic and mitral valve, 2D PC image quality was the best. 4D‐EPI and 4D‐k‐t BLAST did not differ much in image quality. 4D‐SPGR was poorest in image quality.

Figure 3

Bland Altman analysis and scatter plots for the assessment of peak velocity using all the acceleration methods. 4D‐SPGR had maximum bias (‐22, 95% CI: ‐31–12) versus 2D PC acquisition. 4D‐EPI demonstrated least bias (‐3, 95% CI: ‐7–2) and highest correlation (R² = 0.79) to 2D PC acquisition for peak velocity assessments.

Figure 4

Scatter plots of net forward flow (NFF) correlation through the mitral and aortic valve to investigate consistency between all the four methods. 4D‐EPI demonstrated the best consistency (R² = 0.87) versus 4D‐k‐t BLAST acquisition which demonstrated least correlation (R² = 0.32) between NFF of MV and AV.