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Improvements in high resolution ultrasound for postoperative investigation of capillary microperfusion after free tissue transfer.

INTRODUCTION
High resolution ultrasound (US) techniques as implemented in the latest generation of US machines provide imminently better resolution compared to previous high resolution models. This improvement is based on advanced transducer technologies as well as updated post-processing procedures. Furthermore, matrix linear transducers providing frequencies from 6 to 15 MHz are now available. The aim of the study was the evaluation of these new techniques for the immediate postoperative investigation of microcirculation after free tissue transfer by supplemental use of Contrast-Enhanced Ultrasound Imaging (CEUS).


PATIENTS AND METHODS
To this end, we investigated 12 patients who underwent free tissue transfer in order to cover tissue defects in various body regions. We utilized the new GE Logiq E9 equipped with a linear 6-9 MHz and a matrix 6-15 MHz probe as well as the GE Logiq 9 with the previous version of the linear 6-9 MHz probe. Both machines provide the modalities of SRI, Cross Beam and THI. The perfusion curves were quantitatively analyzed using digital cine sequences (Qontrast, Bracco, Italy). Furthermore, two independent investigators evaluated the digitally recorded images with respect to the resolution of details based on a scale ranging from 0 to 5, and after application of 2.4 ml SonoVue (Bracco, Italy), evaluated the image quality regarding the representation of tissue perfusion.


RESULTS
None of the free flaps showed clinical or laboratory signs of flap failure during the hospital stay. Several flaps showed typical perfusion patterns relating to the flap type. The combination of SRI, Cross Beam and THI allows, in most cases, a more exact differentiation of tissue graft outlines and tissue composition, in particular the tissue texture, compared to the use of B-scan only. In addition, the high resolution matrix technology combined with the broader spectrum of 6-15 MHz considerably improves the representation of image details compared to multifrequency probes with 6-9 MHz. The use of updated post-processing procedures as well as new transducer technologies in CEUS also results in improved resolution and thus achieves a higher score compared to previous models.


CONCLUSION
At present, these new US technologies combined with the updated 6-9 MHz probe provide the optimal assessment of perfusion in cutaneous, subcutaneous and deeper tissue layers. The additional use of new multifrequency 6-15 MHz matrix probes improves the resolution in the B-mode to an even higher degree.