Synthetic multi-contrast late gadolinium enhancement imaging using post-contrast magnetic resonance fingerprinting

Abstract

Late gadolinium enhancement (LGE) MRI is the non-invasive reference standard for identifying myocardial scar and fibrosis but has limitations, including difficulty delineating subendocardial scar and operator dependence on image quality. The purpose of this work is to assess the feasibility of generating multi-contrast synthetic LGE images from post-contrast T₁ and T₂ maps acquired using magnetic resonance fingerprinting (MRF). Fifteen consecutive patients with a history of prior ischemic cardiomyopathy (12 men; mean age 63 $\pm$ 13 years) were prospectively scanned at 1.5 T between Oct 2020 and May 2021 using conventional LGE and MRF after injection of gadolinium contrast. Three classes of synthetic LGE images were derived from MRF post-contrast T₁ and T₂ maps: bright-blood phase-sensitive inversion recovery (PSIR), black- and gray-blood T₂ -prepared PSIR (T₂ -PSIR), and a novel "tissue-optimized" image to enhance differentiation among scar, viable myocardium, and blood. Image quality was assessed on a 1-5 Likert scale by two cardiologists, and contrast was quantified as the mean absolute difference (MAD) in pixel intensities between two tissues, with different methods compared using Kruskal-Wallis with Bonferroni post hoc tests. Per-patient and per-segment scar detection rates were evaluated using conventional LGE images as reference. Image quality scores were highest for synthetic PSIR (4.0) and reference images (3.8), followed by synthetic tissue-optimized (3.3), gray-blood T₂ -PSIR (3.0), and black-blood T₂ -PSIR (2.6). Among synthetic images, PSIR yielded the highest myocardium/scar contrast (MAD = 0.42) but the lowest blood/scar contrast (MAD = 0.05), and vice versa for T₂ -PSIR, while tissue-optimized images achieved a balance among all tissues (myocardium/scar MAD = 0.16, blood/scar MAD = 0.26, myocardium/blood MAD = 0.10). Based on reference mid-ventricular LGE scans, 13/15 patients had myocardial scar. The per-patient sensitivity/accuracy for synthetic images were the following: PSIR, 85/87%; black-blood T₂ -PSIR, 62/53%; gray-blood T₂ -PSIR, 100/93%; tissue optimized, 100/93%. Synthetic multi-contrast LGE images can be generated from post-contrast MRF data without additional scan time, with initial feasibility shown in ischemic cardiomyopathy patients.

Keywords: cardiomyopathy; late gadolinium enhancement; magnetic resonance fingerprinting; synthetic imaging; tissue characterization.

Figure 1:

Diagram of the synthetic multi-contrast LGE framework. Quantitative post-contrast ${\mathrm{T}}_1$ and ${\mathrm{T}}_2$ maps are collected using MRF. Regions of interest are manually drawn to measure the mean ${\mathrm{T}}_1$ and ${\mathrm{T}}_2$ within viable myocardium and the left ventricular blood pool. These measurements are used to calculate optimal sequence parameters for the synthetic images. Synthetic images are generated either using steady-state signal equations (for PSIR images) or Bloch equation simulations (for ${\mathrm{T}}_2$-prepared PSIR images and tissue-optimized images).

Figure 2:

Generation of synthetic PSIR and ${\mathrm{T}}_2$-prepared PSIR LGE images. (a) Synthetic PSIR images were calculated assuming an instantaneous acquisition after an inversion pulse, specified by the inversion time (TI). (b) The longitudinal magnetization (M_z) for viable myocardium, blood, and scar is plotted as a function of TI. The red point indicates the TI needed to null viable myocardium. (c) Synthetic PSIR images are shown over a range of TI times. The image with the best nulling of viable myocardium is highlighted in red with TI = 200 ms. (d) Dark-blood images were generated by simulating a ${\mathrm{T}}_2$-prepared PSIR sequence consisting of a ${\mathrm{T}}_2$-prep pulse with duration ${\mathrm{T}}_{\mathrm{P}}$ milliseconds followed immediately by an inversion pulse. (e) M_z is plotted as a function of TI (assuming a fixed ${\mathrm{T}}_{\mathrm{P}}$) for viable myocardium, blood, and scar. The parameter $\delta$ controls the contrast between blood and viable myocardium, with $\delta <0$ causing blood to appear darker than viable myocardium, and vice versa for $\delta >0$. (f) Synthetic ${\mathrm{T}}_2$-PSIR images with different blood/myocardium contrasts are shown. The ${\mathrm{T}}_{\mathrm{P}}$ and TI values that yielded the desired contrast, as controlled by $\delta$, are given below each image and were determined using Equation 3.

Figure 3:

Generation of synthetic tissue-optimized LGE images. (a) A sequence of n variable flip angles (FA) and repetition times (TR) is optimized to maximize differences in the longitudinal magnetization (M_z) among viable myocardium, blood, and scar after the n^th TR. This study uses n=5 excitation pulses. (b) The optimized flip angles and TRs and a plot of the longitudinal magnetization are shown from one patient, along with (c) the resulting synthetic image.

Figure 4:

Post-contrast MRF ${\mathrm{T}}_1$ and ${\mathrm{T}}_2$ maps; synthetic bright-blood (PSIR), dark-blood (${\mathrm{T}}_2$-PSIR), and tissue-optimized LGE images; and reference LGE images are shown from four patient scans. (a,b,c) The presence of focal ischemic scar was confirmed on the reference LGE scan for patients 1–3. (d) No scar was visible on either the reference LGE image or the synthetic multi-contrast images for patient 4.

Figure 5:

Contrast between (a) viable myocardium versus scar, (b) blood versus scar, and (c) viable myocardium versus blood in the synthetic multi-contrast LGE images. Contrast was quantified as the absolute difference in mean signal intensities between ROIs in two tissues. Statistical significance (p<0.05) is indicated by an asterisk.

Figure 6:

(a) Image quality ratings for reference LGE images and synthetic PSIR, black-blood ${\mathrm{T}}_2$-PSIR ($\delta <0$), gray-blood ${\mathrm{T}}_2$-PSIR ($\delta >0$), and tissue-optimized LGE images. Statistical significance (p<0.05) is indicated by an asterisk. (b) Sensitivity, (c) accuracy, and (d) specificity are reported for the detection of myocardial scar on a per-patient and per-segment basis for the synthetic images. Per-patient specificity is not shown since only two patients did not have myocardial scar based on the reference LGE images.