Hybrid adiabatic-rectangular pulse train for effective saturation of magnetization within the whole heart at 3 T

Abstract

Uniform T(1)-weighting is a major challenge for first-pass cardiac perfusion MRI at 3 T. Previously proposed adiabatic amplitude of radiofrequency field (B(1))-insensitive rotation (BIR-4) pulse and standard and tailored pulse trains of three nonselective pulses have been important developments but each pulse has limitations at 3 T. As an extension of the tailored pulse train, we developed a hybrid pulse train by synergistically combining two nonselective rectangular radiofrequency pulses and an adiabatic half-passage pulse, in order to achieve effective saturation of magnetization within the heart, while remaining within clinically acceptable specific absorption rate limits. The standard pulse train, tailored pulse train, hybrid pulse train, and BIR-4 pulse train were evaluated through numerical, phantom, and in vivo experiments. Among the four saturation pulses, only the hybrid pulse train yielded residual magnetization <2% of equilibrium magnetization in the heart while remaining within clinically acceptable specific absorption rate limits for multislice first-pass cardiac perfusion MRI at 3 T.

Figure 1

Pulse sequence diagrams: (a) standard pulse train, (b) tailored pulse train, (c) hybrid pulse train, and (d) BIR-4 pulse train. The spoiler gradients are applied before and after each saturation pulse to dephase the transverse magnetization. The crusher gradients in between RF pulses are cycled to eliminate stimulated echoes. These diagrams are drawn to approximate proportions but not to exact scale. G_SS: slice-select gradient; G_PE: phase-encoding gradient; G_FE: frequency-encoding gradient. The RF pulse characteristics are summarized in Table 1.

Figure 2

(Top row) Simulated M_ZR maps as a function of B₀ and B₁⁺ scales. Compared with the (a) standard pulse train and (b) tailored pulse train, both the (c) hybrid pulse train and the (d) BIR-4 pulse train yielded considerably lower M_ZR maps. The resulting RMS M_ZR measurements were 0.136, 0.055, 0.013, and 0.012 for the standard train, tailored train, hybrid train, and BIR-4 pulse train, respectively. (Bottom row) M_ZR maps of the phantom as a function of B₀ and B₁⁺ scales. Consistent with the simulation results, the corresponding phantom M_ZR maps were in good agreement, and both the (g) hybrid pulse train and (h) BIR-4 pulse train produced considerably lower M_ZR maps than the (d) standard and (e) tailored pulse trains. The resulting RMS M_ZR measurements were 0.164, 0.053, 0.035, and 0.016 for the standard train, tailored train, hybrid train, and BIR-4 pulse train, respectively. The M_ZR maps are displayed with identically narrow grayscales (0 – 0.4 in dimensionless units) to bring out regional variations.

Figure 3

(Top row) Numerically simulated profiles of M_ZR as a function of B₁⁺ scale: (a) −65 Hz, (b) 0 Hz, and (c) 65 Hz off-resonance. (Bottom row) Corresponding profiles of M_ZR in the phantom as a function of B₁⁺ scale: (d) −65 Hz, (e) 0 Hz, and (f) 65 Hz off-resonance. Note that the standard pulse train yielded lower M_ZR than the tailored pulse train for B₁⁺ scales > 0.72, and vice versa for B₁⁺ scales < 0.72. In contrast, the hybrid pulse train and BIR-4 pulse train yielded considerably lower M_ZR throughout the B₁⁺ scale.

Figure 4

Representative set of normalized SR images from one volunteer that compares the performance of four saturation pulses. The standard pulse train performed well within the LV (dark regions) but poorly within the RV (bright regions). The tailored pulse train performed similarly within the whole heart, but at the expense of producing medium level of M_ZR values (gray regions and dark bands) within both ventricles. In contrast, both the hybrid RF pulse train and BIR-4 pulse train yielded uniformly minimal M_ZR within both ventricles (dark regions). The normalized images are displayed with identically narrow grayscales (0 – 0.2 in dimensionless units) to bring out regional variations.

Figure 5

Representative repeated acquisitions of non-contrast T₁-weighted images over multiple heart beats: (first row) standard train, (second row) tailored train, (third row) hybrid train, and (fourth row) BIR-4 train. Both the standard and tailored pulse trains yielded residual signal compared with the hybrid and BIR-4 pulse trains. The T₁-weighted images are displayed with identically grayscales (200 – 1000 in arbitrary units) to bring out regional variations.

Figure 6

Representative first-pass cardiac perfusion images at (first column) pre-contrast, (second column) peak blood enhancement, and (third column) peak myocardial wall enhancement, using the hybrid pulse train as the saturation pulse: (first row) apex, (second row) mid-ventricular, and (third row) base. (Fourth column) The corresponding plots of normalized T₁-weighted signal for each of 16 segments as a function of cardiac cycle. These normalized signal-time curves show good agreement between segments, suggesting that the hybrid pulse train had produced uniform T₁-weighted signal. The T₁-weighted images are displayed with identically grayscales (200 – 1500 in arbitrary units).

Figure 7

Simulated plots of S/S₀ as a function of T₁ for the LV: (a) standard pulse train, (b) tailored pulse train, (c) hybrid pulse train, and (d) BIR-4 pulse train. The standard pulse train yielded relatively accurate S/S₀, whereas the tailored pulse train yielded less accurate S/S₀. Compared with the standard and tailored pulse trains, both the hybrid pulse train and BIR-4 pulse train produced more accurate S/S₀. Solid line represents S/S₀ calculated with mean in vivo M_ZR. Upper and lower dotted lines represent S/S₀ calculated with mean+SD and mean−SD in vivo M_ZR, respectively.