A feasibility study of a novel spectral method using radiofrequency ultrasound data for monitoring laser tissue ablation
Introduction
The recent development of minimally invasive therapeutic tools and procedures can significantly change the approach to any early treatment of disease, thus improving the patient’s quality of life and significantly cutting management costs. Heat ablation represents one the most effective minimally-invasive therapies [1], [2], [3], [4]. Today, the most widely methods used in clinical practice for heat ablation therapy are radiofrequency ablation (RFA), microwave ablation (MWA), laser interstitial thermal therapy (LITT) and high intensity focused ultrasound (HIFU). [5], [6], [7], [8], [9], [10], [11], [12]
Although the advantages and disadvantages of one technique with respect to the other ones are still under investigation in several fields [1], [4], [5], [6], [7], [8], [9], [10], [11], the LITT has shown peculiar features. Indeed, the intrinsic characteristics of laser light combined with tissue-penetration control, depending on the exploited wavelength, allow an optimal penetration associated with a lower temperature gradient throughout the ablation zone, thus granting less risk of carbonization and vaporization of tissue [12], [13], [14]. Furthermore laser ablation achieves good results in the treatment of tumours smaller than 2 cm thanks to its lower complication rates in respect of other techniques [12], [15], [16].
The success of ablation therapy is highly dependent on real-time monitoring of the ablation extent and monitoring the treated area is of vital importance to avoid necrosis of healthy tissue and ensure a complete treatment of the target area [17], [18], [19], [20], [21], [22]. For this reason, real-time monitoring must have some important requirements, such as high frame/rate, minimal invasive standard and a user-friendly interface for the physician. Currently, non-invasive monitoring techniques have been used exploiting magnetic resonance imaging (MRI), computed tomography (CT) and also ultrasound imaging [23], [24], [25]. Despite their spatial high-resolution [26], [27], [28], MRI and CT-based ablation monitoring have several disadvantages, such as requiring specialized equipment for compatibility, together with limitations in real-time application. Meanwhile, ultrasound equipment has garnered interest [25], [29] thanks to its wide availability, relatively low cost, portability and easiness to be used without adding any significant constraints. However its chief failing consists in difficulty in identifying the treated area by using conventional B-mode images, and sometimes, in overestimation of the ablation extension [30]. Several processing techniques such as the study of the pixel shift or ultrasound elastography [31], [32], [33], [34], [35], [36], [37], [38] have been applied to augment conventional images for monitoring the ablation zone. The recent review article by Lewis [25] presents an extended overview of the different techniques for processing the ultrasound signal as to ablation monitoring and it highlights both the important improvements already achieved and the persistent difficulty in correctly correlating the temperature value with the effective tissue damage extension. The paper also mentions several algorithms based on spectral analysis of the radiofrequency (RF) ultrasound signal, which has opened up further potential in the use of ultrasound for tissue differentiation, so as to support physicians in early diagnosis and monitoring before, during and after mini-invasive operations [30], [39]. Indeed, the ultrasound RF echo signal from soft tissue is the result of a close interaction between the mechanical energy of the wave and the structure it goes through [40].
In order to gain additional information for tissue characterization and differentiation purposes, it is essential to not only preserve the amplitude of the RF signal, as performed by the conventional ultrasound scanners for representing the B-mode images, but analyse the entire content of the signal: amplitude, phase and frequency. Therefore, out of the spectral features of this signal, it is possible to extract information concerning the encountered structures.
In this study, the proposed processing method, called TUV (Thermotherapy Ultrasonic View), analyses the evolution of spectral parameters in an N-dimensional hyperspace, as defined in [41], [42], [43] before, during and after laser ablation, in order to evaluate the correlation with the structural alterations suffered by the tissue. Preliminary processing has been performed on a dataset of RF signals from four ex- vivo prostatic glands with Benign Prostatic Hyperplasia (BPH) in order to distinguish the treated area from the surrounding tissues and monitor the evolution of the damage.
Section snippets
Analysis algorithm TUV
The TUV investigation method proposed in this work is based on the study of spectral coefficients in N dimensional hyperspace generated as explained in details in the authors’ previous papers [41], [42], [43]. The signal processing algorithm previously presented and called HyperSPACE [42], is able to characterize biological tissue in different physiological conditions by means of the RF ultrasound signal. Already tested for breast nodule characterization, it has obtained a sensitivity of 92.2%
Results
In order to visualize the variations of clusters related to the ablated areas of the analysed samples, three equal sized regions have been considered at increasing distances from the irradiation source and have been represented in one of the possible 3D projection of the N dimensional hyperspace. For the sake of clarity, the three different schemes for windows positioning are described in Fig. 6. All the schemes refer to a transversal section of a prostatic gland having the fiber inserted, so
Discussion
The spectral algorithms, previously developed by the authors and based on the RF signal, have been adapted to detect the changes occurring when tissue undergoes laser ablation. The TUV method has proven to be a potential support for monitoring the tissue damage inflicted by laser treatment, as it detects the different reactions of the spectral coefficients depending on different factors: the distance from the optical fiber, the power, the energy and the duration of the intervention, as shown in
Conclusions
The TUV method based on the spectral analysis of the RF ultrasound signal has been applied to monitor tissue modifications induced by laser ablation treatment on prostatic tissue. The preliminary experimentation, performed on four ex-vivo prostatic glands and seven treatments, has demonstrated that the clusters of spectral coefficients, related to the tissue undergoing laser radiation, changed their shape and position in the N dimensional spectral hyperspace. By representing these changes on
Acknowledgments
Our word of thanks goes to Elesta S.r.l (El-En group) in Calenzano, Florence for their technical support and collaboration. We also wish to express a heartfelt thankyou to Prof. Marco Carini and Dr. Alfonso Crisci of the Urology Clinic I of the University of Florence for their collaboration. Finally, we wish to say thanks to the person who has allowed the existence of this work with his thirty years of successful researches, both in the field of ultrasounds and laser sources applied to
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