Ultrasound Shear Wave Elastography: Variations of Liver Fibrosis Assessment as a Function of Depth, Force and Distance from Central Axis of the Transducer with a Comparison of Different Systems

Abstract

We evaluated variation in fibrosis staging caused by depth, pre-load force and measurement off-axis distance on different ultrasound shear wave elastography (SWE) systems prospectively in 20 patients with diffuse liver disease. Shear wave speed (SWS) was measured with transient elastography, acoustic radiation force impulse (ARFI) and 2-D shear wave elastography (SWE). ARFI and 2-D-SWE measurements were obtained at different depths (3, 5 and 7 cm), with different pre-load forces (4, 7 and 10N and variable) and at 0, 2 and 4cm off the central axis of the transducer. A single, blinded pathologist staged fibrosis using the METAVIR system (F0-F4). Area under the receiver operating characteristic curve was charted to differentiate significant fibrosis (F ≥ 2). Depth was the only factor found to influence ARFI-derived values; no acquisition factors were found to affect 2-D-SWE SWS values. ARFI and 2-D-SWE for diagnosis of significant fibrosis at a depth of 7cm along the central axis had good diagnostic performance (areas under the receiver operating characteristic curve: 0.92 and 0.82, respectively), comparable to that of transient elastography. Further investigation of this finding will likely be of interest.

Keywords: Depth; Liver fibrosis; Off-axis; Pre-load force; Shear wave; Ultrasound elastography; Variation.

Fig.1

Force-sensing Attachment for Ultrasound Probe and force measurement system. A: SolidWorks rendering of the force-measuring probe. B: Photograph of the complete system. The ultrasound probe cable is Velcro-strapped to the device cable to provide strain relief. A steel enclosure houses the electronic components while the laptop displays and records the probe contact state

Fig.2

ARFI images of one patient with F4 fibrosis (METAVIR): A, depth=3 cm; B, depth=5 cm; C, distance off-axis=2 cm; D, distance off-axis=4 cm

Fig.3

D-SWE images of one patient with F4 fibrosis (METAVIR): A, depth=3 cm; B, depth=5 cm; C, distance off-axis=2 cm; D, distance off-axis=4 cm

Fig.4

AUROC for the predictive value of TE and ARFI performed at different depths for the presence of significant fibrosis (F≥2 METAVIR): TE and ARFI were all obtained along the central axis of the transducer, ARFI3cm, ARFI5cm and ARFI7cm were measurements made at the depth of 3 cm, 5 cm and 7 cm from the skin, respectively

Fig.5

AUROC for the predictive value of TE and 2D-SWE performed at different depths for the presence of significant fibrosis (F≥2 METAVIR): TE and 2D-SWE measurements were all obtained with variable pre-load force and along the central axis of the transducer, SWE3cm, SWE5cm and SWE7cm were measurements made at the depth of 3 cm, 5 cm and 7 cm form the skin, respectively

Fig.6

AUROC for the predictive value of ARFI performed with different pre-load forces for the presence of significant fibrosis (F≥2 METAVIR): ARFI were all obtained at a depth of 5 cm from the skin and along the central axis of the transducer, ARFIvariable, ARFI4N, ARFI7N and ARFI10N were measurements made with variable, 4 N, 7 N and 10 N pre-load force, respectively.

Fig.7

AUROC for the predictive value of 2D-SWE performed with different preload forces for the presence of significant fibrosis (F≥2 METAVIR): 2D-SWE were all obtained at a depth of 5 cm from the skin and along the central axis of the transducer, SWEvariable, SWE4N, SWE7N and SWE10N were measurements made with variable, 4 N, 7 N and 10 N preload force, respectively.

Fig.8

AUROC for the predictive value of ARFI performed at different distances from the central axis of the transducer for the presence of significant fibrosis (F≥2 METAVIR): ARFI were all obtained at a depth of 5 cm from the skin and with variable preload force, ARFIcen0cm, ARFIcen2cm, ARFIcen4cm were measurements made at 0 cm, 2 cm and 4 cm from the central axis of the transducer, respectively.

Fig.9

AUROC for the predictive value of 2D-SWE performed at different distances from the central axis of the transducer for the presence of significant fibrosis (F≥2 METAVIR): 2D-SWE were all made at depth of 5 cm from the skin and with variable preload force, SWEcen0cm, SWEcen2cm and SWEcen4cm were measurements made at 0 cm, 2 cm and 4 cm from the central axis of the transducer, respectively.

Fig. 10

Relation between ARFI (at depth of 7cm form skin), 2D-SWE (at depth of 7cm form skin) and TE value and fibrosis stage: ARFI and 2D-SWE were measurements made at 7cm form skin.