Time‐resolved mapping of myocardial stiffness using 2D multifrequency spiral MR elastography with and without external vibration

Figures

• 

  FIGURE 1. Cardiac MR elastography (MRE) setup. Four air actuators, two near the sternum and two near the left pectoral region, are placed on the chest (red) and connected by plastic tubing to the pressure control box, which generates periodic compressed‐air pulses at the desired MRE frequency. MRE image acquisition with the surface coil is electrocardiogram (ECG)‐triggered in synchrony with the phase of the externally induced vibrations.

• 

  FIGURE 2. Timing diagram of the cardiac gradient‐echo spiral MR elastography (MRE) sequence. The imaging sequence over one TR is shown on the top. After detection of an R‐wave from the electrocardiogram (ECG) (red triangles), the sequence is synchronized by a short delay Δt to ensure encoding of one of six wave dynamics (time points T1…T6 during a wave period). One of four spiral k‐space segments (S1…S4) is acquired per TR with first segments S1 at first wave dynamic T1 in the R–R series of, for example, 30 images, followed by segments S2 of the same wave dynamic T1. In this way, the k‐spaces of all 30 images within an R–R interval are filled up with four spiral segments before the next wave dynamic, T2, is acquired. All six wave dynamics, T1…T6, are sampled within a 23‐s breath‐hold. This sequence with vibration frequency F1 and motion‐encoding gradients (MEG) direction D1 is then repeated for MEG directions D2 and D3 and frequencies F2 and F3, requiring a total of nine breathholds. BH, breathhold; D1…D3, directions of space diagonals of the MEG according to the imaging frame; F1…F3, external vibration frequency; Resp, respiration state; S1…S4, spiral segments in k‐space; T1…T6, time…

• 

  FIGURE 3. Normalized phase‐to‐noise ratio (PNR) based on the encoding efficiency of a rectangular motion‐encoding gradients (MEG) with balanced first momentum and 8.75 ms duration, plotted over a spectrum of mechanical frequencies. Shown are PNR for equal mechanical vibration amplitudes over the entire spectrum (purple curve) and effective PNR considering fractional encoding of exponentially damped higher frequencies according to the Voigt model as described in Rump et al. 53 (black curve, shear modulus μ = 6 kPa, viscosity η = 4.5 Pa‐s, values correspond to 2.4 m/s shear wave speed and medium tissue viscosity 54 ). Vertical lines delineate the encoding efficiencies of precisely known externally induced vibrations at 70, 80, and 90 Hz (solid lines) and the corresponding encoded frequency bandwidths (dashed lines), defined by the sequence timing for each frequency.

• 

  FIGURE 4. Time evolution of frequency‐ and encoding direction‐averaged harmonic displacement in the left ventricular (LV) myocardium at 70, 80, and 90 Hz induced by external vibration (green) and endogenous harmonics without external vibration (red) over normalized R–R interval. For comparison, the change in LV diameter is shown in blue. Group means are represented by solid lines and 95% confidence intervals by colored shaded regions. Cardiac phases of interest are demarcated in gray. The detected R‐wave peak was defined as the beginning of the isovolumetric contraction phase (IVC), whereas the maximum total displacement amplitude (marked by a cross) defined the center of the optimal phase (OP). The minimum LV diameter (marked by an asterisk) defined the center of the end‐systolic phase (ES). The center of the diastolic phase (DIA) (marked by a circle) was defined by the maximum LV diameter at end‐diastole. All intervals of interest extended over 4% (40 ms) of the normalized R–R range.

• 

  FIGURE 5. Representative MR elastography (MRE) magnitude images, displacement maps, and shear wave speed (SWS) maps in cardiac phases isovolumetric contraction (IVC), optimal phase (OP), end‐systolic (ES), and diastole (DIA) in a short‐axis view. (A) Maps obtained with vibration. (B) Maps obtained without vibration. In the top rows, MRE magnitude images are shown along with regions of interest (ROIs) placed in the LV myocardium, demarcated by yellow lines. The middle rows show the displacement component encoded by the +x + y + z MEGs at 80 Hz (real parts of the complex wave images). The bottom rows show compounded SWS maps based on three frequencies and three displacement directions. Inversion artifacts because of noise and non‐periodic cardiac motion appear as pixelated noise patterns in SWS maps (red arrows).

• 

  FIGURE 6. Time evolution of group mean shear wave speed (SWS) within the left ventricle over normalized R–R interval for test (blue) and re‐test (red). 95% confidence intervals are shown as colored shaded areas with cardiac intervals of interest demarcated in gray. (A) SWS obtained with external vibration. (B) SWS obtained from endogenous shear waves without external vibration. Optimal test–re‐test reproducibility across the R–R interval was achieved with external vibration, whereas smaller endogenous harmonic amplitudes (see Figure 4) gave rise to greater SWS variability. DIA, diastole; ES, end‐systole; IVC, isovolumetric contraction; OP, optimal phase.