Effective motion-sensitizing magnetization preparation for black blood magnetic resonance imaging of the heart

Abstract

Purpose: To investigate the effectiveness of flow signal suppression of a motion-sensitizing magnetization preparation (MSPREP) sequence and to optimize a 2D MSPREP steady-state free precession (SSFP) sequence for black blood imaging of the heart.

Materials and methods: Using a flow phantom, the effect of varying field of speed (FOS), b-value, voxel size, and flow pattern on the flow suppression was investigated. In seven healthy volunteers, black blood images of the heart were obtained at 1.5T with MSPREP-SSFP and double inversion recovery fast spin echo (DIR-FSE) techniques. Myocardium and blood signal-to-noise ratio (SNR) and myocardium-to-blood contrast-to-noise ratio (CNR) were measured. The optimal FOS that maximized the CNR for MSPREP-SSFP was determined.

Results: Phantom data demonstrated that the flow suppression was induced primarily by the velocity encoding effect. In humans, FOS=10-20 cm/s was found to maximize the CNR for short-axis (SA) and four-chamber (4C) views. Compared to DIR-FSE, MSPREP-SSFP provided similar blood SNR efficiency in the SA basal and mid-views and significantly lower blood SNR efficiency in the SA apical (P=0.02) and 4C (P=0.01) views, indicating similar or better blood suppression.

Conclusion: Velocity encoding is the primary flow suppression mechanism of the MSPREP sequence and 2D MSPREP-SSFP black blood imaging of the heart is feasible in healthy subjects.

Fig. 1

Three different motion-sensitizing magnetization preparation (MSPREP) sequences consisting of 90_x tip-down, 180_y refocusing, and 90_-x tip-up non-selective RF pulses with a pair of a) unipolar (UNIP), b) bipolar (BIP), and c) reverse bipolar (RBIP) motion-sensitizing gradients positioned around the 180_y pulse. Note that the 180_y refocusing pulse was used to minimize off-resonance effects, leading to signal attenuation of both stationary and moving tissues due to T2 decay. Moving tissues experience further signal attenuation due to the intravoxel dephasing induced by diffusion effect and velocity encoding effect. The corresponding field of speed (FOS) and b-value are shown assuming rectangular gradient waveforms for simplicity (γ is the gyromagnetic constant). Note that BIP and RBIP have the same b-value, but BIP has infinite FOS (corresponding to no velocity encoding effect) regardless of the gradient amplitude G.

Fig. 2

The inverse relationship between FOS and b-value of a) unipolar (UNIP) and b) reverse bipolar (RBIP) MSPREP sequences. Calculations were done assuming rectangular gradient waveforms for simplicity and gradient amplitude G = 33 mT/m (refer to Fig.1 for definition of gradient amplitude and timing parameters).

Fig. 3

Schematic of the ECG-triggered segmented-k-space motion-sensitizing 2D imaging sequence. After an ECG trigger delay (TD), motion-sensitizing magnetization preparation (MSPREP) was used to dephase moving spins in the transverse plane (while leaving stationary spins unaffected except for T2 relaxation) and store the resultant velocity-modulated signals in the longitudinal magnetization. Spoiler gradients (SPOILER) destroyed remnant transverse magnetization, followed by a short delay (DISDACQ) consisting of 6 dummy repetitions (data acquisition turned off) to provide steady-state magnetization preparation for subsequent imaging. Gradient recalled echo (GRE) and steady-state free-precession (SSFP) readout was used for phantom and human imaging, respectively. In humans, TD was chosen such that imaging occurs during mid-diastole when the ventricular walls are least mobile. The described sequence is repeated until the image data is fully acquired.

Fig. 4

Cross-sectional 2D MSPREP-GRE magnitude (top row) and phase (bottom row) images of a flow phantom under laminar flow conditions (peak velocity approximately 20 cm/s) obtained with a) UNIP (FOS = 15 cm/s, b = 0.4 s/mm², b) BIP (FOS = ∞, b = 0.4 s/mm²), c) BIP (FOS = ∞, b = 10 s/mm²), d) BIP with G = 0 mT/m (FOS = ∞, b = 0 s/mm²), and e) RBIP (FOS = 15 cm/s, b = 0.4 s/mm²). An infinite FOS corresponds to no velocity encoding effect. The pixel size was 0.23 × 0.23 mm². Clearly, the dephasing effect of the MSPREP sequence is determined primarily by its FOS and not by its b-value, demonstrating that velocity encoding is the predominant dephasing mechanism for bulk flow. Unlike the BIP sequence, the UNIP and RBIP sequences modulated the longitudinal magnetization with cos(γm₁v), where γ is the gyromagnetic constant, v is the spin velocity, and m₁ is the gradient first moment. Correspondingly, spins moving at certain velocities will be inverted prior to imaging, leading to phase jumps of π in the phase image (a-e).

Fig. 5

Cross-sectional 2D MSPREP-GRE images of a flow phantom obtained with the UNIP sequence demonstrating increased dephasing and flow suppression obtained with smaller FOS (due to more rapid intravoxel dephasing) (a-b), larger pixel size dx and dy (due to increased signal averaging) (c-d), or turbulent flow condition (due to the random phases acquired by moving spins) (e).

Fig. 6

2D MSPREP-SSFP images of the SA mid view obtained with FOS = 5 cm/s (a), 10 cm/s (b), 15 cm/s (c), 20 cm/s (d), 25 cm/s (e), 30 cm/s (f) and infinity (no velocity encoding effect) (g), and without MSPREP (h). Reducing FOS increased motion sensitivity, leading to improved suppression of blood flow; however, more mobile cardiac structures such as the papillary muscles and the mid-lateral LV wall were also suppressed (a). On the contrary, increasing FOS reduced motion sensitivity, leading to insufficient suppression of slow-moving blood; however, the cardiac structures were better visualized (e-f). In the extreme case of an infinite FOS, blood signal was not suppressed (g), similar to the image acquired without MSPREP (h). The optimal FOS that maximizes the LV-to-blood CNR was approximately 15 cm/s (d). BB image obtained with the conventional DIR-FSE sequence is also shown as a reference (i). Note that both techniques provided effective suppression of the intraventricular blood and very similar depictions of the ventricular walls and the papillary muscles.

Fig. 7

Mean LV-to-blood contrast-to-noise ratio (CNR_LV-BLOOD) as the function of FOS calculated from SA and 4C MSPREP-SSFP images of healthy subjects (N = 7). The error bars represent ± standard error. The optimal FOS value was found to be approximately 15-20 cm/s for the SA basal and mid views and 10-15 cm/s for the SA apical and 4C views. The optimal FOS range (defined as the FOS values whose mean CNR was statistically the same as the maximum mean CNR as determined by t-test) is also shown for each imaging plane.

Fig. 8

Comparison of SA and 4C BB images obtained with MSPREP-SSFP (a-d) and DIR-FSE (e-h). Both techniques provided very similar depiction of the myocardium and the papillary muscles as well as good intraventricular blood suppression in the SA views. MSPREP-SSFP provided improved blood suppression compared to DIR-FSE in the 4C view.