Introduction

There are a wide variety of kidney diseases, many of which are life threatening and could result in death. The current non-invasive methods used for diagnosing kidney diseases such as Ultrasound, CT and MRI are based on observing structural changes in kidneys which occurs after the disease has progressed. Magnetic Resonance Elastography(MRE) is an imaging-based alternative to palpation^[1] which can show the change in stiffness when the disease is still progressing. Diffusion Tensor Imaging(DTI) provides information about diffusion of water molecules, which is useful in diagnosing different diseases based on diffusion coefficient and anisotropy of the tissue. It also provides us information about the tissue anisotropy by the Fractional Anisotropy value. The kidney function changes during pathological states and this is reflected by changes in the apparent diffusion co-efficient(ADC) and fractional anisotropy (FA) values^[2]. The aim of this study is to show the reproducibility of SE-EPI MRE and also to correlate the stiffness obtained from SE-EPI MRE to ADC, FA values obtained from DTI as none of the previous studies have investigated it.

Methods

All imaging was performed using a 3T MRI scanner (Tim Trio, Siemens Healthcare). Written informed consent was obtained from all healthy volunteers (n=4, age range:22-29 years). Coronal slices were obtained using SE-EPI MRE pulse sequences. Volunteers were laid supine with head first in the MR scanner as shown in figure 1. 60 Hz external vibrations were introduced through a soft driver with one pad for each kidney [3]. The driver pads were placed on the posterior side between the sternum and the umbilicus. For reproducibility study after the first scan the volunteers were asked to leave the scan room and repositioned for a repeat scan with the same imaging parameters Imaging parameters for SE-EPI MRE included:FOV=500x500mm², TR=750ms, TE=40.7ms, EPI-factor=63 matrix size=128x128 reconstructed to 256x256, slice thickness=6mm, number of slices=5, MRE phase offsets=4. All slices were acquired in a 18 sec breath hold. Motion encoding gradient of 60Hz was applied separately in the x, y and z directions to encode in-plane and through plane displacement fields for SE-EPI MRE. MRE images were masked to obtain the kidney region and a 4^th order Butterworth bandpass directional filter(10-40 waves/FOV) was applied to remove longitudinal component of motion and reflected waves. Finally, 3D local frequency estimation(LFE) was performed to obtain weighted stiffness map using MRE-Lab(Mayo Clinic, Rochester, MN). Imaging parameters for DTI included:FOV=500x500mm², TR=1000 ms, TE=67ms, slice thickness=6mm, number of averages=1, b-value=50,400 and 1000s/mm², matrix size=128x128 reconstructed to 256x256, number of slice=5 and diffusion directions=6. All slices were acquired in a 21 sec breath hold. The ADC and FA maps obtained from the scanner were then used to draw ROI on both kidneys individually to get ADC and FA values for cortex and medulla for all 4 healthy volunteers.

Results

Figure 2 displays the magnitude image in one of the volunteers, and snapshot of one of the time points of wave propagation in three spatial directions x, y and z and the corresponding 3D weighted stiffness map for initial and the repeat scan respectively. Mean stiffness of kidneys across all 4 healthy volunteers was found to be 3.497±0.66 kPa for left kidney and 3.817±0.71 kPa for the right kidney for initial scan and 3.535±0.71 kPa for left kidney and 3.841 ± 0.77 kPa for the right kidney for the repeat scan. Figure 3 shows the correlation between SE-EPI MRE scan 1 and scan 2 for left and right kidneys. The mean ADC value was observed to be 2.17±0.26x10-3 mm²/s for left kidney and 1.857±0.15x10-3 mm²/s for right kidney. Similarly, mean FA value was observed to be 0.482±0.13 for left kidney and 0.49±0.12 for right kidney. These values fall within the range of values reported earlier ^[4] .Figure 4 shows the correlation between stiffness observed using SE-EPI MRE sequence and ADC value as well as SE-EPI MRE sequence and FA value obtained from the DTI scan. Figure 5 shows correlation of stiffness with ADC and FA values of cortex and medulla for both kidneys.

Acknowledgements

NIH-NHLBI: R01HL124096