Determination of optimal imaging mode for ultrasonographic detection of subdermal contraceptive rods: comparison of spatial compound, conventional, and tissue harmonic imaging methods

Abstract

Objective: To determine which mode of ultrasonography (US), among the conventional, spatial compound, and tissue-harmonic methods, exhibits the best performance for the detection of Implanon® with respect to generation of posterior acoustic shadowing (PAS).

Materials and methods: A total of 21 patients, referred for localization of impalpable Implanon®, underwent US, using the three modes with default settings (i.e., wide focal zone). Representative transverse images of the rods, according to each mode for all patients, were obtained. The resulting 63 images were reviewed by four observers. The observers provided a confidence score for the presence of PAS, using a five-point scale ranging from 1 (definitely absent) to 5 (definitely present), with scores of 4 or 5 for PAS being considered as detection. The average scores of PAS, obtained from the three different modes for each observer, were compared using one-way repeated measure ANOVA. The detection rates were compared using a weighted least square method.

Results: Statistically, the tissue harmonic mode was significantly superior to the other two modes, when comparing the average scores of PAS for all observers (p < 0.00-1). The detection rate was also highest for the tissue harmonic mode (p < 0.001).

Conclusion: Tissue harmonic mode in uS appears to be the most suitable in detecting subdermal contraceptive implant rods.

Keywords: Implanon; Spatial compound; Subdermal contraceptive; Tissue harmonic; Ultrasonography; Wide focal zone.

Fig. 1

Transverse ultrasonography image of Implanon device in conventional mode obtained from 32-year-old female patient. Echogenic spot (short arrow) indicates Implanon rod. Note unexpectedly ambiguous posterior acoustic shadowing (PAS) (arrowheads) cast posterior to Implanon. Linear bar seen on right side of image (long arrows) corresponds to width of focus depth, indicating that this image was obtained using wide focal zone.

Fig. 2

Illustration of ultrasonography (US) examination of Implanon. US examination is performed in transverse plane with respect to Implanon device for ease of detection. Implanon is generally inserted at medial aspect of arm, along its long axis (dotted line). Hence, US probe (gray box) is initially placed at medial aspect of arm in respective transverse plane.

Fig. 3

Comparison of conventional (CV) (A), spatial compound (SC) (B) and tissue harmonic (TH) (C) modes in terms of posterior acoustic shadowing (PAS) generation in images obtained from 30-year-old female patient, using wide focal zone. PAS (arrowheads) appears discrete in TH mode (C), but is ambiguous or nearly absent in CV (A) and SC (B) modes (dotted boxes on right in images are markers denoting location and width of focus).

Fig. 4

Influence of focus width on posterior acoustic shadowing (PAS) generation tested with focus fixed at depth of Implanon. Images obtained from set wide focal zone (A, C, E) and from set single focus (B, D, F) are presented. In spatial compound (C, D) and tissue harmonic (E, F) modes, PAS does not appear to be significantly different between images obtained, using different focus widths. However, in conventional mode, length of PAS (arrowheads) was markedly shorter in image with wide focal zone (A) than in that with single focus (B) (dotted boxes on right in images are markers denoting location and width of focus. Arrows indicate center of focus).