Preliminary Results on the Feasibility of Using ARFI/SWEI to Assess Cutaneous Sclerotic Diseases

Abstract

In this study, acoustic radiation force impulse (ARFI) and shear wave elasticity imaging (SWEI) were applied to the skin to investigate the feasibility of their use in assessing sclerotic skin diseases. Our motivation was to develop a non-invasive imaging technology with real-time feedback of sclerotic skin disease diagnosis. This paper shows representative results from an ongoing study, recruiting patients with and without sclerosis. The stiffness of the imaged site was evaluated using two metrics: mean ARFI displacement magnitude and bulk shear wave speed inside the region of interest (ROI). In a subject with localized graft versus host disease (GVHD), the mean ARFI displacement inside sclerotic skin was 61% lower (p < 0.01) and shear wave speed 128% higher (p < 0.005) compared to those in normal skin-indicating stiffer mechanical properties in the sclerotic skin. This trend persisted through disease types. We conclude ARFI and SWEI can successfully differentiate sclerotic lesions from normal dermis.

Keywords: Acoustic radiation force; Acoustic radiation force impulse imaging; Cutaneous sclerotic diseases; Shear wave speed.

Figure 1

The left and right plots are B-mode images of sclerotic and contralateral normal upper back sites from a study subject. When sclerosis is accompanied by atrophy, disease severity can be indicated by the change in thickness of the dermis; however, normal B-mode images do not provide any information on stiffness changes.

Figure 2

The left and right plots are B-mode images of sclerotic and contralateral normal upper back sites from a study subject. Atrophy can often not occur in sclerosis, making B-mode imaging alone not useful in characterizing sclerosis.

Figure 3

Mean and standard deviation of displacements inside the Region of Interest (ROI) of a healthy subject’s upper back. The Acoustic Radiation Force (ARF) push occurs at time 0 (red arrow) and maximum mean displacement occurs at the second track time (black arrow). The displacement at the first track time after the push is comparable to maximum displacement; however, the standard deviation of the second track time is slightly lower due to decrease in shearing of the tissue as it is recovering from the push. The displacement amplitude quickly decreases after the time of maximum displacement.

Figure 4

B-mode image of the upper back skin (left), Acoustic Radiation Force Impulse (ARFI) displacement field in copper colormap overlaid on top of the corresponding gray B-mode image (middle), and differentiated displacement profile of shear wave through time (right) of a valid acquisition. The yellow box represents the region of interest (ROI) where the stiffness metrics are taken from. The color-bar unit of the middle image is [µm] and of the right image is [µm/ms]. Layers of the skin, dermis and subcutaneous layer, are well delineated by the difference in B-mode brightness and often by the difference in ARFI displacement magnitude. On the right plot, the slope of the white line represents the shear wave speed determined by Radon sum transformation. In this acquisition, the shear wave speed was 3.48 m/s.

Figure 5

Shear wave speed and Acoustic Radiation Force Impulse (ARFI) displacement comparison in contralateral right/left pairs in different body locations of a normal control study subject. The top plot shows the range of shear wave speeds reconstructed in left and right pairs of normal skin on various body sites. The bottom plot shows corresponding mean ARFI displacement inside the Region of Interest (ROI). Each box represents distribution of 3–5 individual estimates at an imaging location (total valid acquisition of 55 for this subject). All acquisitions were made in a single imaging session. The red circles indicate cases in which the stiffness metrics inside left and right contralateral sites were significantly different.

Figure 6

The distribution of ratio values of shear wave speed (SWS) and Acoustic Radiation Force Impulse (ARFI) displacement magnitude measurements inside each pair of left and symmetric right sides of healthy skin in a single patient. Data correspond to those shown in Figure 5. The ratio was taken by normalizing the metrics from the right side of the body to by those from the left. The distributions of the ratios taken from each pair were plotted as above. Median and interquartile range (IQR) of these ratios were (1.01, 0.01) and (1.02, 0.10) for SWS and ARFI displacement, respectively. The mean and standard deviation p-values for t-test between each pair of normal right and left stiffness metrics were 0.68±0.36 and 0.22±0.31 for SWS and ARFI displacement, respectively. For both SWS and ARFI displacement magnitude, there was no statistical difference.

Figure 7

Shear wave speed and Acoustic Radiation Force Impulse (ARFI) displacement comparison in contralateral normal/sclerotic pairs in different body locations of a study subject with Graft versus Host Disease (GVHD). This patient was imaged through five imaging sessions at three month intervals. Acquisitions made in the same location but different imaging sessions were considered to be from a “different” location, and therefore, there are repeated locations on the x-axis. The numbers noted in location names represent the imaging sessions for the acquisition. Blank locations indicate unsuccessful acquisitions at a given body site. The boxes in the top plot represent distribution of shear wave speed (SWS) estimations over 106 acquisitions; boxes in the bottom plot represent distribution of ARFI displacement level inside the region of interest (ROI) over 74 acquisitions.

Figure 8

The difference in skin thickness between healthy and sclerotic skin (top) and normalized shear wave speed (SWS) comparison in contralateral normal/sclerotic pairs in difference body locations of a study subject with Graft versus Host Disease (GVHD). The top plot shows the difference in skin thickness between sclerotic and contralateral healthy normal skin. The bottom plot is a modified analysis of the top plot of Figure 7, replacing measured SWS inside sclerotic skin to SWS normalized to respective skin thickness.

Figure 9

Ratios of shear wave speed (SWS) and Acoustic Radiation Force Impulse (ARFI) displacement magnitude measurements inside the sclerotic and contralateral healthy skin. SWS ratio using measured SWS is shown in the left plot (corresponding to data shown in Figure 7) and SWS ratio using skin thickness normalized SWS is shown in the right plot (corresponding to data shown in Figure 8). Median and interquartile range (IQR) of these ratios was (2.28, 0.60), (0.34, 0.19), (1.92, 0.68), (0.34, 0.19) from left to right for SWS, ARFI displacement, SWS, and ARFI displacement respectively.

Figure 10

Shear wave speed and Acoustic Radiation Force Impulse (ARFI) displacement comparison in contralateral normal/sclerotic pairs in different body locations of a study subject with morphea. This patient was imaged through three imaging sessions at three month intervals. Each box represents the distribution of 3–5 individual acquisition for an imaging location. There were a total of 38 acquisitions for this subject.

Figure 11

The difference in skin thickness between healthy and sclerotic skin (top) and normalized shear wave speed (SWS) comparison in contralateral normal/sclerotic pairs in difference body locations of a study subject with morphea. The top plot compares skin thickness in sclerotic and contralateral healthy normal skin. The bottom plot is a modified analysis of the top plot in Figure 10, replacing measured SWS inside sclerotic skin to skin thickness normalized SWS.

Figure 12

Ratio of shear wave speed (SWS) and Acoustic Radiation Force Impulse (ARFI) displacement magnitude measurements inside the sclerotic and contralateral healthy skin. SWS ratio using measured SWS is shown in the left plot (corresponding to data shown in Figure 10) and SWS ratio using skin thickness normalized SWS is shown in the right plot (corresponding to data shown in Figure 11). Median and interquartile range (IQR) of these ratios was (1.24, 0.19), (0.81, 0.04), (3.11, 0.28), (0.81, 0.04) from left to right for SWS, ARFI displacement, SWS, and ARFI displacement, respectively.

Figure 13

Longitudinal consistency of shear wave speed (SWS) and Acoustic Radiation Force Impulse (ARFI) displacement level of the healthy “normal” locations. This subject was imaged in three separate sessions at three months intervals. The top plot shows estimated SWS of the same location from three different imaging sessions. “r” or “l” noted next to the location name refers to the side of the body where the acquisitions were made. The bottom plot shows change in ARFI displacement magnitude monitored through time. 5 out of 7 locations showed consistency in SWS (circled in blue), whereas only 1 out of 7 locations (circled in blue) showed consistency in ARFI displacement level.

Figure 14

Ratio of shear wave speed (SWS) and Acoustic Radiation Force Impulse (ARFI) displacement magnitude measured between consecutive imaging sessions. Median and interquartile range (IQR) of these box plots are (0.98, 0.18) and (1.06, 0.41) for SWS and ARFI displacement, respectively.