2012 Proceedings - International Tissue Elasticity Conference

042 REPRODUCIBILITY STUDIES IN SHEAR WAVE ELASTOGRAPHY.
Mickael Labit 1 , Hervé Monpeyssen 2 , Olivier Lucidarme 1,3 , Laurence Leenhardt 1,2 ,
Frédérique Frouin 1 , Claire Pellot–Barakat 1 .
1 UPMC/INSERM UMR_S 678, 91 Boulevard de l'Hôpital, Paris, FRANCE; 2 Nuclear Medicine
Department, 3 Radiology Department, Pitié–Salpêtrière Hospital, 83, boulevard de l’Hôpital, Paris,
FRANCE.
Background: Ultrasonic shear wave elastography has a unique capability of providing quantitative maps
of the Young’s modulus E(kPa) of soft tissue [1]. In most applications, the degree of malignancy of
suspicious structures is directly derived from the mean elasticity value computed in manually selected
regions of interest (ROIs) about the lesion. The operator dependant steps required to measure this
elasticity index can be a source of inaccuracy and bias which need to be identified.
Aims: The purpose of this work was to study the manual steps involved in the making and measuring of
shear wave maps and to propose automatic analysis and recommendations in order to improve the
accuracy of the elasticity index and avoid biased measurements that could lead to diagnosic errors.
Methods: Ultrasonic shear wave images of a breast elastography phantom as well as of thyroid tumors
and patient livers were acquired on the (SSI) Aixplorer machine with the SL15–4 linear and SC6–1
transducers. Manual interventions performed after localizing the lesion plan include: 1) freezing a stable
frame, 2) positioning the shear wave window, 3) selecting a ROI in the tumor. For Step 1, 30 video clips of
patient livers as well as 5 clips of phantom of 10 seconds each were acquired. The average elasticity of a
central area of the shear wave window was computed throughout the clip and an automatic extraction of
the frame presenting the smallest difference with its neighbors was performed (Figure 1). To study Step 2,
the shear window was placed in 20 different locations of the B–mode frame of homogeneous areas of the
phantom. The mean elasticity according to the axial and lateral distances was computed. For Step 3, an
algorithm enabling the automatic selection of a representative ROI inside thyroid tumors and phantom
inclusions was developed, based on criteria of homogeneity and size.
Results: 1) The automatic frame selection allows the reproducible detection of the most stable elastogram
in 80% of the liver clips and 100% of the phantom clips. The 20% failure occurred for left livers where the
raw data clip quality is poor. 2) Column–like artifacts were observed when the shear window was placed
at the top of the phantom (Figure 2). An inaccuracy of up to 7kPa of the elasticity index (almost 30% of
the estimate) was observed (Figure 3). This artifact appears mainly under 6mm of depth (Figure 4) when
the shear window is at the top. This is not as strikingly observed when the shear window is deeper. 3) The
automatic ROI selection provides reproducible elasticity measures in the thyroid.
Conclusions: Automatic frame and ROI selection methods were proposed to allow intra– and inter–operator
reproducibility of elasticity measurements. Important artifacts observed when the elastography map is at
the top border of the frame suggest that measurements should be made at least 6mm from the skin
surface. An ultrasonic standoff pad could also be used in order to minimize this effect when exploring
superficial organs.
References:
[1] M. Tanter et al.: Breast Lesion Viscoelasticity and Supersonic Shear Imaging. Ultrasound in Med. and Biol.,
Vol. 34, No. 9, pp. 1373–1386, 2008.
Figure 1: Average elasticity calculated throughout
one video clip. Each step corresponds to
a renewal of the elastogram.
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Figure 3: Mean elasticity along an axial
depth of 6mm as function of
the lateral position.
Figure 4: Mean elasticity as function
of the axial position.
Figure 2: B–mode and superimposed elastograms from a homogeneous phantom area. The shear window was placed at various
locations at the top of the frame (Frames 1–3) and at 5mm from the top (Frames 4–6).
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