Abdominal MR elastography with multiple driver arrays: Performance and repeatability

Abstract

Purpose:

To evaluate the performance and repeatability assessing liver, spleen, and kidney stiffness with magnetic resonance elastography (MRE), using arrays of pneumatic passive drivers.

Methods:

An array of four flexible, pneumatically activated passive drivers for abdominal MRE were developed and tested in this study. Multiple MRE acquisitions were performed prospectively in a series of eleven volunteers, with activation of all combinations of the four drivers, individually and simultaneously. MRE exams were repeated three times to study within-day and between-day test-retest repeatability. Semi-quantitative evaluation of wave propagation and penetration, and quantitative assessment of tissue stiffness was conducted for liver, spleen, and kidneys.

Results:

When driver location and amplitude was sufficient to achieve necessary shear wave illumination in any given region of interest, the results showed excellent test-retest repeatability in abdominal organ stiffness with both single and multiple driver configurations. The results confirmed that multiple driver arrays provided suitable shear wave illumination over a larger region of the abdomen, allowing more reliable stiffness measurements in multiple organs. MRE assessment of the spleen was found to be prone to effects of excessive shear wave amplitude, however.

Conclusion:

A multiple driver array provides shear wave illumination over a larger region of the abdomen than obtained with a single driver, for MRE assessment of multiple abdominal organs, providing excellent test-retest repeatability in stiffness measurements. However, careful tuning of the location and amplitude of each driver is essential to achieve consistent results.

Introduction

Magnetic resonance elastography (MRE) is a phase contrast-based MRI technique for visualizing applied shear waves in tissue and then calculating quantitative mechanical properties in vivo. MRE-assessed liver stiffness measurement has been primarily used as a quantitative noninvasive biomarker for diagnosing and staging hepatic fibrosis in patients with chronic liver diseases [1]. Recent investigations have shown that splenic stiffness is another biomarker that is promising for predicting clinically significant portal hypertension and decompensated cirrhosis [2–5]. In patients with end-stage liver diseases, complications lead to reduction in perfusion of the kidneys, causing impaired renal function (hepatorenal syndrome). Therefore, simultaneous stiffness evaluation of multiple abdominal organs in patients with chronic liver diseases may be beneficial [6–8].

In current clinical practice, a single driver, secured to the lower right chest wall is used for MRE assessment of the liver. However, if the goal of the examination is to evaluate other abdominal structures as well, it may be efficient to employ more than one shear wave driver [6]. Previous phantom studies have explored using multiple drivers to get compensated wave field and reliable stiffness assessment [9–13]. However, these results remain to be verified in vivo. Another unresolved issue is the influence of multiple drivers on the repeatability of tissue stiffness assessment in vivo. Compared with 2D-MRE, advanced 3D vector MRE provides complete displacement information along three orthogonal directions and allows volumetric acquisition and more reliable stiffness calculation in organs with complex geometry and heterogeneous properties. Additionally, 3D-MRE is less operator-dependent than 2D-MRE. Therefore, the purpose of this study was to evaluate the performance of multiple drivers in vivo for simultaneous stiffness assessment with 3D-MRE in the upper abdomen, and the influence of multiple wave sources on repeatability.

Materials and Methods

Participants

This prospective volunteer human study was approved by our Institutional Review Board. Written informed consent was obtained from each volunteer. Between July 2019 and October 2019, eleven volunteers with no known liver, splenic, or renal diseases, as indicated by their medical history, were enrolled (male/female 8/3, mean age 35.6 years old, range 26 – 43 years old).

MRI/MRE acquisition

Volunteers all fasted for at least 4 hours before the examinations, which were performed on a 3T scanner (GE HealthCare, Milwaukee, WI) using a 16-channel body coil. Our imaging protocol included a gradient-echo in- and out-of-phase T1-weighted MRI and a series of 3D-MREs, covering the liver, spleen, and kidneys in one acquisition, with eight different driver configurations. For each 3D-MRE data acquisition, a single shot, flow compensated, spin-echo based, multi-slice, echo-planar-imaging (EPI) MRE sequence was used with the following imaging parameters: 60 Hz vibrations, acquisition matrix = 96 × 96, 32 slices, TR/TE = 1300/46.8 ms, slice thickness = 3.5 mm, parallel imaging acceleration factor of 2, three phase offsets, and six motion encoding directions (±X, ±Y, ±Z). Each 3D-MRE acquisition required four 12-second breath holdings at the end of expiration. Four soft, flexible acoustic passive drivers were used. Each passive driver was connected to an active driver (Resoundant, Inc., Rochester, MN). A local network was built to control the four active drivers, simultaneously activating each independently or multiple driver arrays for combined wave generation. A default setting of 50% power and zero phase delays were used during the MRE acquisitions for all the active drivers. The first passive driver (DR1) was placed over the right lower anterior chest wall at the level of the xiphisternum, centered on the middle clavicular line. The remaining three passive drivers, numbered as driver 2 (DR2), driver 3 (DR3) and driver 4 (DR4), were placed in clockwise order around the abdomen, as demonstrated in Figure 1. DR3 and DR4 were centered at the left and right kidneys, respectively. Considering the most common clinical needs in daily practice (e.g., hepatosplenic or renal assessment alone), passive drivers were activated in the following order: DR1 alone, DR2 alone, DR3 alone, DR4 alone, DR1 and DR2 simultaneously (presented as DR12 thereafter), DR1 and DR3 simultaneously (presented as DR13 thereafter), DR1 and DR4 simultaneously (presented as DR14 thereafter), and all four drivers together (presented as DR1234 thereafter). The examination was repeated three times, twice on the same day with a ten minutes off-the-table break, and the third exam was conducted on a different day within one month of the initial exams.

Fig 1.

Driver position of the four soft flexible passive drivers used for simultaneous stiffness assessment in the liver, spleen, and kidneys. (a) upper abdominal structures and their relationship with four passive drivers; (b) lower abdominal structures and their relationship with four passive drivers; (c) back view of one flexible square passive driver.

Post-processing and image analyses

The MRE data were post-processed offline using a customized 3D multimodal direct inversion algorithm to reconstruct the magnitude and wave images, as well as the elastograms (stiffness maps) [14]. Images were analyzed by the same investigator (blinded, two years of experience in abdominal MRE) using customized software. A semi-quantitative evaluation was performed to assess the depth of wave propagation and the existence of significant intravoxel phase dispersion (IVPD). In the liver, wave propagation was scored as: 1 (within the peripheral 1/3 of the liver), 2 (reaching the middle 1/3 of the liver), and 3 (reaching the inner 1/3 of the liver). In the spleen and kidneys, wave propagation was scored as 0 (no visible wave propagation throughout the organ), 1 (weak wave propagation throughout the organ), and 2 (strong wave propagation throughout the organ). Accordingly, a mean score of less than 1.5 was considered poor wave propagation for the spleen and kidneys in the final analyses. IVPD was defined as a visible signal loss on the magnitude images that usually occurs where difficult to unwrap phase wrapping in the wave images was caused by high motion amplitude. For the liver, spleen, and both kidneys, freehand regions of interest (ROIs) were manually drawn on all available slices under each driver configuration to include as much tissue as possible while avoiding the edge, artifacts, major vasculature, and renal sinus using the anatomic T1W images and MRE magnitude images as references. Those ROIs were then copied automatically to corresponding stiffness maps. Elastography confidence maps were calculated based on an assessment of signal-to-noise ratio and wave amplitude. Voxels with less than a 90% confidence level were discarded from the ROI analysis. Finally, the voxel-wise mean stiffness values and the number of voxels from all available slices were recorded for statistical analyses.

Statistical analysis

Semi-quantitative scores and tissue stiffnesses were summarized as mean ± standard deviation (SD). For each organ, the quality score and tissue stiffness differences between the single and multiple drivers were compared using paired Student’s T-tests. Spearman’s correlation was used to assess the relationship between the stiffness measurements using a single driver and multiple driver arrays. Agreement in the tissue stiffness between the single and multiple driver acquisition was analyzed in terms of the coefficient of variation (CV), and was defined as an excellent, good, moderate, and poor agreement with CV<10%, 10%<CV<20%, 20%<CV<30% and CV>30%, respectively. The overall, within-day and between-day test-retest repeatability coefficients (RC) were also calculated. All statistical analyses were performed using SPSS (IBM SPSS Statistics 25) with a significance level of 0.05.

Results

All volunteers completed at least two exams collected on the same day or on different days (missing data was caused by the subject’s unavailability and not due to technical issues). The mean time interval between the first and second visit was 5.6 days (range: 1 to 19 days). As calculated by 2-point Dixon imaging, the liver fat fraction ranged from 5.9% to 27.4% (median 9.57%, interquartile range 8.60% to 13.75%). Except for steatosis, no other incidental abnormalities in the liver, spleen, or kidney were found. The body mass index ranged from 19.81 kg/m² to 29.98 kg/m². Semi-quantitative evaluations of the MRE wave images were summarized in Table 1. Quantitative measurements of abdominal tissue stiffness using single and multiple driver configurations were compared in Table 2, and the ROI sizes (i.e., number of pixels used for the stiffness measurement) of the single and multiple driver configurations were compared in Table 3. The overall, within-day and between-day test-retest repeatability coefficients of the stiffness measurements using single and multiple driver configurations are shown in Table 4.

Table 1.

Semi-quantitative comparison of the wave propagation and the presence of significant intravoxel phase dispersion between single- and multiple- passive driver configurations.

Driver(s)	Liver			Spleen			Left kidney			Right kidney
	Wave propagation	p	Case with IVPD#	Wave propagation	p	Case with IVPD#	Wave propagation	p	Case with IVPD#	Wave propagation	p	Case with IVPD#
Single driver	1.40± 0.52 (DR1)	NA	1 (10%)	1.64±0.50 (DR2)	NA	0 (0%)	2.00±0.00 (DR3)	NA	0 (0%)	2.00±0.00 (DR4)	NA	0 (0%)
DR12	1.45±0.52	NA	1 (9%)	1.64±0.50	NA	0 (0%)	0.64±0.50	<0.001	0 (0%)	0.64±0.50	<0.001	0 (0%)
DR13	1.55±0.52	0.343	1 (9%)	2.00±0.00	0.038	5 (45%)	2.00±0.00	NA	0 (0%)	1.18±0.40	<0.001	0 (0%)
DR14	2.36±0.67	<0.001	6 (55%)	1.45±0.69	0.553	0 (0%)	1.00±0.63	<0.001	0 (0%)	2.00±0.00	NA	0 (0%)
DR1234	2.36±0.67	<0.001	6 (55%)	2.00±0.00	0.038	5 (45%)	2.00±0.00	NA	0 (0%)	2.00±0.00	NA	0 (0%)

IVPD= intravoxel phase dispersion

^#numbers are the number of volunteers with IVPD, and data in parentheses are percentages.

In the liver, wave propagation was scored as 1 (within the peripheral 1/3 liver), 2 (reach the middle 1/3 liver), and 3 (reach the inner 1/3 liver). In the spleen and kidneys, wave propagation was scored as 0 (no visible wave propagation), 1 (weak wave propagation), and 2 (strong wave propagation). NA, not applicable.

Table 2.

Mean tissue stiffness and coefficient of variation in each organ using single- and multiple- passive driver configurations.

Driver(s)	Liver			Spleen			Left kidney			Right kidney
	Mean± SD	ρ	CV%	Mean± SD	ρ	CV%	Mean± SD	ρ	CV%	Mean± SD	ρ	CV%
Single driver	1.66± 0.11 (DR1)	NA	NA	4.38±0.28 (DR2)	NA	NA	4.63±0.47 (DR3)	NA	NA	4.27±0.51 (DR4)	NA	NA
DR12	1.66±0.10	0.903*	1.47 (0.38, 2.67)	4.41±0.27	0.882*	2.35 (0.11, 4.03)	4.32±0.44	0.300	7.21 (0.92, 21.41)	4.22±0.66	0.409	5.60 (0.27, 23.31)
DR13	1.68±0.10	0.891*	1.46 (0.36, 2.86)	4.15±0.39 *	0.673*	5.89 (0.24, 11.23)	4.64±0.40	0.945*	2.34 (0.11, 6.04)	4.62±0.38*	0.718*	5.79 (0.02, 14.95)
DR14	1.66±0.10	0.818*	1.97 (0.46, 5.73)	4.49±0.42	0.509	3.82 (0.07, 16.20)	4.74±0.40	0.609*	4.44 (0.61, 12.99)	4.32±0.60	0.973*	2.16 (0.11, 4.13)
DR1234	1.67±0.12	0.842*	2.09 (0.53, 6.13)	3.99±0.28*	0.518	6.86 (0.07, 15.78)	4.65±0.42	0.909*	2.50 (0.55, 5.70)	4.27±0.58	0.936*	1.87 (0.17, 5.15)

Parentheses indicate the single default driver for each organ in the first row or the range for the coefficient of variation (CV) in the subsequent rows.

^*denotes value differs significantly from that of using the single driver configuration, or significant correlation at the spearman’s correlation analysis. SD, standard deviation; NA, not applicable.

Table 3.

Tissue volume (milliliter) for stiffness measurement with a confidence level of ≥0.90 using single- and multiple- passive driver configurations.

Driver(s)	Liver (mL)	Spleen (mL)	Left kidney (mL)	Right kidney (mL)
Single driver	175.9±27.4 (DR1)	47.3±25.5 (DR2)	31.5±10.6 (DR3)	22.6±12.5 (DR4)
DR12	181.7±35.1	47.2±24.1	39.3±11.2*	28.8±15.8 *
DR13	180.8±32.8	32.4±25.8 *	30.8±9.7	29.6±16.1 *
DR14	176.0±30.9	48.1±24.4	41.2±10.6 *	23.3±12.2
DR1234	164.3±37.2 *	28.6±23.1 *	30.5±11.1	22.3±12.3

Parentheses indicate the single default driver for each organ.

^*denotes value differs significantly from that of using the single driver configuration.

Table 4.

The overall, same-day and between-day repeatability coefficients (RCs) of stiffness measurement in the liver, spleen, left, and right kidney using single- and multiple- passive driver configurations.

	Driver(s)	Liver	Spleen	Left kidney	Right kidney
Overall RC	Single driver	13.1% (10)	16.3% (10)	24.0% (11)	22.1% (11)
	DR12	13.1% (11)	18.4% (11)	39.8% (11)	34.3% (11)
	DR13	12.4% (11)	29.4% (10)	23.2% (11)	30.5% (11)
	DR14	12.2% (11)	33.0% (11)	29.2% (11)	21.9% (11)
	DR1234	11.6% (11)	26.6% (11)	24.5% (11)	20.6% (11)
Same day RC	Single driver	10.1% (9)	13.9% (9)	25.0% (9)	15.5% (9)
	DR12	10.4% (10)	17.6% (10)	47.9% (10)	36.2% (10)
	DR13	12.1% (10)	25.2% (9)	22.3% (10)	22.0% (10)
	DR14	10.7% (9)	27.1% (9)	32.4% (9)	21.9% (9)
	DR1234	10.4% (9)	29.2% (9)	25.3% (9)	14.1% (9)
Between days RC	Single driver	13.3% (9)	12.7% (10)	19.4% (11)	22.7% (11)
	DR12	13.8% (10)	17.4% (10)	30.8% (10)	27.3% (10)
	DR13	11.5% (10)	25.4% (10)	22.8% (10)	32.2% (10)
	DR14	13.1% (10)	34.9% (10)	25.0% (10)	19.6% (10)
	DR1234	11.8% (11)	25.1% (11)	22.4% (11)	22.2% (11)

Numbers in parentheses are the exact number of volunteers for calculating the repeatability coefficient (RC).

Liver assessments

There were no technical failures in the liver MRE exams. When utilizing the multiple driver arrays, we observed greater wave propagation in the deep structures and more frequent IVPD effects in the superficial parenchyma adjacent to the vibrating driver (Table 1). Multiple driver arrays usually led to more homogeneous looking elastograms, especially under the DR14 and DR1234 arrays (Figure 2). In all subjects, there was an excellent agreement (CV≤6.1%) in the liver stiffness measurement between the single and multiple driver arrays. The overall test-retest repeatability coefficient of the liver stiffness assessment was 13.1%, 13.1%, 12.4%, 12.2%, and 11.6% when using the driver configurations of DR1, DR12, DR13, DR14, and DR1234, respectively. Best wave propagation and coverage in the hepatic tissue were shown when all drivers were activated (D1234).

Fig 2.

Illustration of a liver MRE exam using single and multiple passive driver configurations. With multiple drivers, more shear waves from different directions were introduced into the liver and, consequently, the stiffness images are more homogeneous within the entire liver.

Spleen assessments

There was one technical failure case in the spleen MRE exams. We observed that the image quality of splenic MRE was degraded under the DR13 and DR1234 configurations (Figure 3), which had substantial IVPD effects throughout the splenic tissue and resulted in incorrect phase unwrapping and consequently an invalid stiffness calculation. There was a good (CV≤11.2% for DR13, CV≤16.2% for DR14 and DR1234) to excellent (CV≤4.0% for DR12) agreement in the spleen stiffness measurements between the single and multiple driver arrays in all subjects (Table 2). The overall RC of the splenic stiffness assessment was 16.3%, 18.4%, 29.4%, 33.0%, and 26.6% when using the driver configurations of DR1, DR12, DR13, DR14, and DR1234, respectively (Table 3). Examples of successful spleen MRE exams using the single and multiple driver arrays are demonstrated in Figure 4. Poor wave propagation in the splenic tissue was shown when drivers on only the right side were activated.

Fig 3.

Illustration of a failed spleen MRE exam using the multiple driver configurations. With the confidence level set at ≥0.90, the driver configurations of DR13 and DR1234 resulted in a zero and one pixel for the measurement of splenic stiffness, respectively. The spleen also had a low number of pixels (50) for the splenic stiffness measurement when using DR3, along with signal loss immediately next to the DR3 in the magnitude image. Failure to correctly unwrap the phase wrap (see white arrows in the wave images) was also indicated in the wave images when using DR3, DR13 and DR1234.

Fig 4.

Illustration of spleen MRE using single and multiple passive driver configurations. The measured splenic stiffness varies with driver position. The repeatability of the splenic stiffness was highest when using a single driver (DR2), followed by driver configuration of DR12 when using multiple drivers.

Kidney assessments

There were no technical failures in the kidney MRE exams. We did not observe significant IVPD effects in either kidney. There was no significant difference in kidney stiffness measurements between single and multiple driver arrays, except for the right kidney with the DR13 configuration (Table 2). There was a good (CV≤15% for long distance: DR14 for left kidney, DR13 for right kidney) to excellent (CV≤6% for short distance: DR13 for left kidney, DR14 for right kidney) agreement in the kidney stiffness measurements between the single and multiple driver arrays in all subjects (Table 2). The overall RC of kidney stiffness assessment was 24.0%, 39.8%, 23.2%, 29.2%, and 24.5% when using driver configuration of DR3, DR12, DR13, DR14, and DR1234, respectively, for the left kidney. The overall RC of renal stiffness assessment was 22.1%, 34.3%, 30.5%, 21.9%, and 20.6% when using driver configuration of DR4, DR12, DR13, DR14, and DR1234, respectively, for the right kidney. We found that multiple driver arrays that did not include the default driver with the shortest distance had poor wave illumination in kidney MRE exams (Table 1 and Figure 5).

Fig 5.

Illustration of kidney MRE using single and multiple passive driver configurations. Good propagating waves were observed in each kidney only when the default driver (i.e., the driver placed directly below the organ of interest) was activated, alone or in combination with other drivers. The stiffness measurement was taken in the axial plane, and the renal sinus was excluded. The right kidney tended to have a lower stiffness compared with the left kidney when both were imaged with good waves.

Clinical applications

In one of our ongoing clinical studies, we adopted this multiple-driver configuration for assessing the mechanical properties of the liver, spleen, and kidneys in patients with suspected hepatorenal syndrome. To achieve adequate spatial resolution for different organs, DR12 was used to perform hepatosplenic MRE at conventional 60Hz, while DR34 was used to perform renal MRE at 90Hz. In addition, we increased vibration power to compensate for substantial wave attenuation, especially at high frequencies in obese patients (see an example in Supplementary Figure 1).

Discussion

When the driver location and amplitude were sufficient to achieve necessary shear wave illumination in any given region of interest, our results showed excellent test-retest repeatability in abdominal organ stiffness with both single and multiple driver configurations. The results confirmed that multiple driver arrays provided suitable shear wave illumination over a larger region of the abdomen, allowing more reliable stiffness measurements in multiple organs. MRE assessment of the spleen was found to be prone to the effects of excessive shear wave amplitude, however these could be controlled by adjusting the passive driver amplitude.

Liver MRE results in this study echoed the previous phantom studies [9–11] with combined wave generation and increased homogeneity. The repeatability coefficient (RC) of the 3D-MRE assessed liver stiffness using a single driver (13.1%) was significantly improved compared with the previous meta-analysis using 2D MRE (22%) [15]. With multiple passive drivers, the RC slightly reduced from 13.1% to 11.6%. The passive drivers used in this study are all smaller than the standard liver MRE passive driver currently used for clinical examinations. Therefore, multiple standard drivers could give benefits in clinical applications from two aspects: (i) Deep structures of the entire liver can be imaged with more uniform wave propagation resulting in a homogenous stiffness calculation; (ii) Potential applications of guided biopsy with improved coverage of tissue stiffness assessment in patients with inhomogeneous fibrosis or focal lesions to address the challenge of shadowing effects (i.e., low-amplitude wave motion and less reliable stiffness estimates behind a stiff region) [11].

Our splenic stiffness measurement was within the reported range of normal spleen stiffness (2.35 – 5.58 kPa) [16,17]. A previous study indicated that the measured spleen stiffness might vary with driver position [17]. We observed that the best RC occurs using the left anterior passive driver over the spleen (DR2, 16.3%). Overall, the RC was degraded to 29.4% and 26.3% when using DR13 and DR1234, respectively. As a relatively superficial organ with slim geometry, the splenic stiffness measurement could be affected by driver position and amplitude in two situations: (i) Poor shear wave amplitude can lead to noisy data with unreliable calculation (overall RC = 35.7% for DR1; overall RC = 33.0% for DR14). (ii) Excessive shear wave amplitude can introduce severe IVPD effects (as seen in the spleen with DR 3, DR13 and DR1234 configurations in Figure 3) with consequent signal loss and phase unwrap difficulties. Therefore, the driver position and acoustic power should be carefully selected for the spleen MRE evaluation. Therefore, two drivers on both sides of the abdominal wall should be considered if simultaneous hepatic and splenic MRE assessments are needed for liver fibrosis and portal hypertension assessments.

There is great interest in utilizing kidney stiffness to evaluate disease processes, especially in patients with chronic renal diseases or renal allografts [8]. In previous studies, a 3D acquisition along the coronal imaging plane was recommended to assess the shear stiffness of the renal cortex and medulla [7,18]. Our kidney data were collected along the axial plane in the same 3D-MRE acquisition for the liver and spleen. Results showed that the renal stiffness measurements agreed with previously reported values (right kidneys: 4.27±0.51 kPa versus 4.12±0.24 kPa at the axial plane and 4.32 ±0.59 kPa in the coronal oblique plane) [7], as well as slightly higher stiffness in the left kidneys and excellent technical repeatability (CV=10%) [18]. The multiple driver arrays yielded comparable renal stiffness measurements with similar repeatability. In clinical practice, a single driver adjacent to the target is sufficient if only one kidney is involved. Two drivers on each side of the body are needed for evaluating both kidneys. Other driver placements do not affect kidney MRE results.

In summary, liver, spleen, and kidney stiffness measurements could benefit from multiple driver arrays resulting in improved coverage, uniform wave propagation, and sustained repeatability. Careful design of the passive driver position and amplitude is essential to achieve consistent results, especially in organs with special anatomy and complex geometry. This single-center prospective human study has some limitations. First, the sample size is small because of the time-consuming eight different MRE driver configurations and repeated exams. Second, only 60 Hz mechanical vibration was evaluated, which is not an ideal frequency for assessing pancreatic stiffness (typically performed at 40Hz) and kidney stiffness (typically performed at 90Hz or higher). Third, default values of 50% acoustic power and zero phase delay were used for all four active drivers. By adjusting the acoustic power, the amount of IVPD may have been reduced in some exams. Fourth, the BMI range of volunteers is narrow, and may not represent all clinical patients (see Supplementary Figure 1 for the example application in obese patients). Last, all acquisitions were acquired with breath-holds at the end of expiration. Inconsistent breath holds and image co-registration could affect the test-retest repeatability in the stiffness measurement. Future progress in the free-breathing 3D-MRE method and hardware engineering that allows the generation of multiple frequencies, intensities, and phases can further promote the concept of simultaneous stiffness assessment of abdominal organs with MRE.

Conclusion

In brief, while using a single driver for liver MRE has been shown to provide outstanding performance in clinical practice, there is potential for further improvement using arrays of drivers. Renal MRE benefits from multiple drivers with simultaneous evaluation on the right and left kidneys and sustained repeatability. The reliability of splenic stiffness measurement is sensitive to excess shear wave illumination if a driver is closely located or has excess amplitude.

Abdominal MRE with multiple driver arrays can provide broader anatomic coverage, avoid the need for multiple acquisitions, and can provide improved image quality and technical repeatability. However, care must be taken to provide suitable positioning and amplitude of each driver to achieve technical success and reliable MRE measurement for specific applications.

Abbreviations

MRE

magnetic resonance elastography

DR1

driver number 1

DR2

driver number 2

DR3

driver number 3

DR4

driver number 4

DR12

simultaneous activation of driver numbers 1 and 2

DR13

simultaneous activation of driver numbers 1 and 3

DR14

simultaneous activation of driver numbers 1 and 4

DR1234

simultaneous activation of driver numbers 1, 2, 3, and 4

IVPD

intravoxel phase dispersion

ROI

region of interest

SD

standard deviation

CV

coefficient of variation

RC

repeatability coefficient