Wideband late gadolinium enhancement (LGE) MRI was recently proposed for scar imaging in patients with implantable cardioverter defibrillators (ICDs)^1-3. Prior to wideband LGE, scar imaging in ICD patients was difficult due to the appearance of hyperintensity artifacts resulting from severe off-resonance induced by the ICD, which can obscure scar^1,2. Wideband LGE uses a wideband inversion pulse with a spectral bandwidth (BW) of 3.8kHz to replace the conventional inversion pulse (typical BW≈1kHz), and successfully eliminates hyperintensity artifacts in the majority of ICD patients^1-3.

However, the wideband inversion pulse can only invert spins in the range of ±1.9kHz, whereas the off-resonance in ICD patients’ myocardium could be as large as 6kHz1. To counteract off-resonance larger than 1.9kHz, the center of the frequency sweep of the wideband inversion pulse was shifted by ±1500Hz (depending on the off-resonance polarity). However, this requires repeat scans. In some patients, the off-resonance varies across the myocardium, being <1.9kHz in some slices and >1.9kHz in other slices. Furthermore, we have observed a case where the wideband inversion pulse with +1500Hz frequency shift did not completely eliminate the hyperintensity artifact (Fig1).

These problems may be solved by increasing the BW of the wideband inversion pulse beyond 3.8kHz; however, B1 amplitude of this pulse would then be too high to implement. We present a different wideband adiabatic inversion pulse with 8kHz BW which can resolve the above mentioned issues.

The previously reported wideband inversion pulse of BW 3.8kHz was designed using an adiabatic hyperbolic secant (HS) inversion pulse^1,3. This pulse required a peak B1 amplitude of 19μT. To achieve a BW of 6 kHz with the HS pulse would require an optimal B1 amplitude of 28μT, which is beyond hardware capability. We designed our 8kHz BW pulse using a hyperbolic secant 4 (HS4) adiabatic inversion pulse4,5. The HS4 pulse requires a lower peak B1 amplitude than an HS pulse of similar BW. In our pulse design, we scaled the frequency modulation function^4,5 to achieve a frequency sweep of 8kHz. We used a Bloch simulation to study the inversion profile and determine the peak B1 amplitude required.

The HS4 wideband inversion pulse was implemented in an inversion-recovery spoiled gradient-echo LGE sequence with TR/TE=3.9/1.5ms, flip angle=25°, readout BW=500Hz/pixel, matrix: 144x256, resolution=1.4x1.9mm, slice thickness=8mm. The HS4 wideband LGE sequence was tested in a phantom, 2 non-contrast healthy volunteers and 2 patients with ICDs who had been referred to cardiac MRI prior to ventricular tachycardia ablation. All ICD patient scans were carried out under safety guidelines published in the literature for MRI of ICD patients^1-3. As a comparison, images in the same slice positions were obtained using the original HS wideband LGE sequence with frequency offsets of 0Hz and ±1500Hz. In the phantom scans, an ICD was placed at 4cm from the phantom. The LGE sequence was used to suppress the signal from the phantom contents. In the healthy volunteers, an ICD was attached to the body coil near the left shoulder of the volunteer to reproduce the myocardial off-resonance of ICD patients. All scans were done on a 1.5T Siemens (Erlangen, Germany) scanner. Images were studied to determine the extent of the hyperintensity artifacts.

Bloch simulation results of the wideband HS4 pulse are shown in Fig2. The HS4 pulse required a peak B1 amplitude of 19µT to produce inversion efficiency of 93% (Fig2C). Fig. 2D shows the adiabatic behavior of the HS4 pulse. A minimum B1 amplitude of 13.2µT is required to produce 80% inversion efficiency.

Phantom results are shown in Fig3. The HS4 wideband pulse reduces artifacts more effectively than the HS pulse. Fig4 shows images from a healthy volunteer. Fig4C shows that the HS4 wideband pulse is more successful in eliminating artifacts than the HS pulse (with or without frequency offset).

Fig5 shows images from an ICD patient. The HS pulse with no frequency offset produced hyperintensity artifacts, which resembled scar, in apical slices but not in basal slices (Fig5A&B). Using the HS4 wideband pulse, the hyperintensity artifact was removed, and no other artifact was generated.

We present a wideband inversion pulse designed with an HS4 adiabatic pulse with 8kHz BW. This pulse leads to further reduction of hyperintensity artifacts in LGE over the HS wideband pulse and will reduce the necessity of repeated scans with frequency offsets. The HS4 pulse may also reduce false positives in scar identification in ICD patients. The HS4 wideband pulse should have important applications in ICD patients for 2D LGE as well as 3D LGE for ventricular and atrial scar imaging and T1 mapping.