Rapid acquisition technique for MR elastography of the liver

Abstract

Magnetic resonance elastography (MRE) of the liver is a novel noninvasive clinical diagnostic tool to stage fibrosis based on measured stiffness. The purpose of this study is to design, evaluate and validate a rapid MRE acquisition technique for noninvasively quantitating liver stiffness which reduces by half the scan time, thereby decreasing image registration errors between four MRE phase offsets. In vivo liver MRE was performed on 16 healthy volunteers and 14 patients with biopsy-proven liver fibrosis using the standard clinical gradient recalled echo (GRE) MRE sequence (MREs) and a developed rapid GRE MRE sequence (MREr) to obtain the mean stiffness in an axial slice. The mean stiffness values obtained from the entire group using MREs and MREr were 2.72±0.85 kPa and 2.7±0.85 kPa, respectively, representing an insignificant difference. A linear correlation of R(2)=0.99 was determined between stiffness values obtained using MREs and MREr. Therefore, we can conclude that MREr can replace MREs, which reduces the scan time to half of that of the current standard acquisition (MREs), which will facilitate MRE imaging in patients with inability to hold their breath for long periods.

Keywords: Liver MRE; Liver fibrosis; MR elastography; MRE.

Figure 1. Experimental setup

A passive driver is placed on the abdomen, on the ROI. Waves are noninvasively transmitted to the passive driver and into the subject. These waves are imaged by the MRI scanner and are used to calculate stiffness.

Figure 2. Pulse sequence diagram

a) shows the GRE based MREs pulse sequence. The polarities of the MEGs are swapped every other TR and the TR equals to 3 cycles of external motion (CEV). b) Shows the GRE based MREr pulse sequence. In MREr the polarities of the MEGs stay same for all the TR cycles. Every TR is synchronized with 1.5 CEV. For simplicity flow compensating gradients and killer gradients are not shown.

Figure 3. MRE Normal HealthySubject

Left Column: Images obtained using standard clinical GRE MRE sequence (MREs). Right Column: Images obtained using rapid GRE MRE sequence (MREr). a), d) Axial magnitude image of the liver showing the ROI (white dotted line). b), e) Snap shot of wave propagation in the liver. c, f) Stiffness map obtained using standard inversion from the scanner. The hatched region demonstrates the area with low phase difference SNR. The mean stiffness value obtained from the ROI is 1.77±0.39 kPa (MREs) and 1.79±0.39 kPa (MREr).

Figure 4. MRE Liver Fibrosis Patient

Left Column: Images obtained using standard clinical GRE MRE sequence (MREs). Right Column: Images obtained using rapid GRE MRE sequence (MREr). a), d) Axial magnitude image of the liver showing the ROI (white dotted line). b), e) Snap shot of wave propagation in the liver. c, f) Stiffness map obtained using standard inversion from the scanner. The hatched region demonstrates the area with low phase difference SNR. The mean stiffness value obtained from the ROI is 3.38±0.92 kPa (MREs) and 3.38±0.83 kPa (MREr).

Figure 5. MREs and MREr correlation

Plot of stiffness values obtained using MREs and MREr showing a linear correlation of R²=0.99.

Figure 6. Bland-Altman Analysis

Plot of difference in stiffness values obtained using MREs and MREr as function of mean stiffness values obtained using both the techniques with a mean bias of 0.03kPa.