Absolute SWE stiffness measurements (kPa) of normal thyroid tissue,
hyperplastic nodules and papillary carcinoma vary depending on the resting pressure applied by the transducer.
The differences in SWE stiffness results between papillary carcinoma and hyperplastic nodules are greater at higher levels of compression than at lower compression levels. In our study,
there was no significant difference in mean SWE stiffness values between papillary carcinoma and hyperplastic nodules at minimal compression level,
which refers to minimal contact of the transducer with the skin (evidenced by a thick US gel layer being maintained between the transducer and skin).
This finding is important as appears to contradict the advice that SWE should be performed with minimal compression of the skin in order to standardize the technique and prevent the operator from applying excessive precompression inadvertently.
Indeed,
this mechanism may also account for the discrepancy in published SWE stiffness values of benign and malignant nodules between different studies [13].
In this regard,
in separate studies,
Sebag et al.
and Veyrieres et al.
documented similar stiffness readings,
150 ± 95 kPa (mean ± SD) and 115 ± 60.4 kPa for malignant nodules,
and 36 ± 30 kPa and 41 ± 25.8 kPa for benign nodules,
respectively.
However,
in another study using identical SWE technology,
Bhatia et al.
documented a much lower median stiffness of 43.1 kPa,
(range 12.2±187.5 kPa) for malignant nodules and 26.2 kPa (range 7.4±132.0 kPa) for benign nodules.
Extrapolating the present study findings,
it is possible that Bhatia et al.
had performed SWE using minimal compression,
where as Sebag et al.
and Veyrieres et al.
had performed SWE using higher levels of compression.
There are several limitations of the present study:
First,
the study sample is small,
especially of papillary carcinomas,
although the preliminary results are already informative and this study is ongoing.
Second,
we did not measure actual compression force applied but instead measured a surrogate marker,
axial strain.
In practice,
applying a fixed force and determining the precise stress applied to tissues in vivo is extremely challenging,
partly due to many factors that can influence stress distribution such as stress decay and boundary conditions. Nevertheless,
our study is the first step towards objective assessment the effects of different compression levels on stiffness measurements in the thyroid.
Third,
SWE results for measurements acquired in the longitudinal and transverse planes through the thyroid were pooled,
although it is theoretically possible that SWE plane may influence SWE measurements due to tissues displacing or behaving differently under different axes of compression,
either due to intrinsic tissue anisotropy or due to differences in mobility/stiffness of the neighbouring tissues such as trachea and carotid vessels. We are currently collecting more data to allow subgroup analysis according to acquisition plane.
Finally,
we acquired data at different SWE compression levels that were assigned arbitrarily by the operator,
albeit with visual guidance from the corresponding image),
which may or may not be comparable to the range of compression levels applied by most operators during elastography.
Consequently,
we do not know if the effect of different levels of pre-compression shown by our study is relevant to routine clinical practice.
We are in the process of investigating this question in another related study.
To conclude,
this preliminary study has shown that papillary carcinoma and hyperplastic nodules differ in their rate of increase of SWE stiffness for given increases in pre-compression,
such that papillary carcinomas show a greater increase in stiffness.
This variation may account for discrepancies in SWE stiffness values as well as accuracy results reported in different studies,
and highlights the fact that thyroid SWE is potentially highly operator dependent.
Further research is required to evaluate the influence of compression levels applied by operators on SWE in the thyroid,
in order to standardize the technique before it can be considered for use in routine clinical practice.