A tailored passive driver for liver MRE in pediatric patients

Abstract

Objectives: Magnetic resonance elastography (MRE) is increasingly used in the pediatric population for diagnosis and staging of liver fibrosis. However, the MR-compatible driver and sequences are usually those used for adult patients. Our feasibility study aimed to adapt the standardized adult MRE passive driver and vibrational parameters to a pediatric population.

Methods: We designed an elliptic passive driver shaped on a torus equipped with an elastic membrane and adapted to children's morphologies. As a first step, eight children (aged 8-18 years) were enrolled in a prospective pilot study aiming to determine the threshold vibrational amplitude for MRE using a custom passive driver, based on phase aliasing assessment and the occurrence of signal void artifacts on magnitude MR images. In the second step, the practicality and the consistency of the custom driver were assessed in a further 11 pediatric patients (aged 7-18 years). In the third step, we compared our custom driver vs. the commercial driver on six adult volunteers, in terms of the reliable region of interest area within the acquired MRE slices, the shear wave maps' quality, and measured stiffness values obtained.

Results: Based on pediatric patient data, the threshold vibrational amplitude expressed as percentage of maximum output was found to be 0.4 and 1.1 times the body weight (kg) at 40 and 60 Hz frequencies, respectively. In comparison to the commercial passive driver, the custom driver improved threefold the contact with the body surface, also enabling a more comfortable examination as self-assessed by the volunteers.

Conclusions: Our custom driver was more comfortable for the volunteers and was able to generate more homogenous shear waves, yielding larger usable hepatic area, and more reliable stiffness values.

Keywords: MRE workflow; liver stiffness; patient-specific vibrational excitation; pediatric MRE; toroidal passive driver.

Figure 1

(A) picture of the commercial Resoundant MRE passive driver. (B) Illustration of the custom passive driver, shown as a computer model, without its biocompatible membrane. (C) Front view of the custom driver equipped with its membrane. The arrow indicates the seal. (D) MPR sagittal oblique reconstructed plane from 3D breath-hold data. The arrow indicates the position of the marker, confirming the correct positioning of the passive driver. (E) Fat-suppressed T1w MR images of water-filled commercial driver vs. the custom driver (F) on the same volunteer. The yellow dotted line highlights the transmitting contact surface for the vibrational excitation. MRE, magnetic resonance elastography; MR, magnetic resonance; MPR, multiplanar reformation.

Figure 2

Illustration of magnitude images acquired the TSE MRE sequence (A,B) and corresponding shear wave (C,D) for case with phase aliasing at 40 Hz (A,C) and case without phase aliasing at 60 Hz (B,D). The white arrow shows signal void artifacts caused by overstated amplitude of the vibrational excitation. (D) corresponds to a threshold amplitude. (E) Determination of the threshold amplitude using the BM, BMI, and BSA as a function of the elastographic frequency. Red color corresponds to phase aliased cases and blue color corresponds to nonaliased ones using 30% and 50% of the output amplitude. The black arrows indicate the highest ratio before phase aliasing for 40 and 60 Hz. One patient had morbid obesity, and the amplitudes of 30% and 50% were too low to perform a reliable MR elastography at 60 Hz. MRE, magnetic resonance elastography; BM, body mass; BMI, body mass index; BSA, body surface area; A, Amplitude; TSE, turbo spin echo.

Figure 3

Illustration of Syngo.via workflow for postprocessing. (A) 3D Dixon T1w anatomic image of the liver in transverse plane acquired before MRE, (B) nearest anatomical MRE plane superimposed with the rROI, and (C) image fusion between stiffness map, 95% CI map, and rROI. (D) rROI overlapped on one shear wave instance. MRE, magnetic resonance elastography; CI, confidence interval; rROI, representative ROI.

Figure 4

(A) Comparison of the wave propagation in the liver using the commercial passive driver and (B) the custom one, same volunteer. The white arrows highlight a regular wave pattern. (C) Illustration of the stiffness map and 95% CI obtained with the commercial driver, including a hot spot (see arrow), and (D) with the custom driver, artifact-free. Color map range 0–8 kPa. CI, confidence interval.

Figure 5

(A) Relative rROI area as a fraction of the entire liver averaged over the four slices, expressed in %, for different excitation amplitudes, using the custom passive driver and the commercial one in adult volunteers. (B) Stiffness values of the rROI area-weighted (kPa). rROI, representative ROI.