This work was initially motivated by one of our recent studies in which nanoparticle presence was shown to increase the effect of ultrasound on biological specimens. Ultrasound with nanoparticles was shown to be more effective at destroying cultured cancerous cells relative to normal ones (Nanoparticle-Assisted Ultrasound Therapy (NAUT)) [1]. In mice, implanted tumors were more effectively shrunk with ultrasonic exposure with nanoparticle injection into the circulation relative to controls [2]. Similarly, mice that received intra-tumoral injections of nanoparticles prior to ultrasound exposure had their tumors raised to temperatures, based on thermometer-type measurements, higher than those receiving sham injections [3]. Magnetite nanoparticle agglomerates were shown to allow high-intensity focused ultrasound to destroy tumor spheroids, relative to controls with no agglomerates, with the cause attributed to increased inertial cavitation [4].
Contrast agents such as perfluorocarbon microparticles have been broadly used for ultrasound imaging and drug delivery [5]. The enhancement of the contrast agent depends on backscattering, beam attenuation, and difference in speed of the sound. Particle size and material are two key factors for being a good contrast agent. Nevertheless, microbubble contrast agents are limited by the stability and the large size to get through small blood vessel especially the vessels in the tumors are typically between 380 nm and 780 nm [6]. In general, the scattering cross-section for a single bubble, σ, is proportional to the sixth power of the radius. Small nanoparticles are not in favor for enhancement. Reduction in diameter by a factor of 10 could lower the cross section by six orders of magnitude. Furthermore, the bubble resonance frequency is inversely proportional to the size; this would require very high frequency transducers. Nevertheless, nanoparticle, in principle can be a better ultrasound contrast material due to the impedance mismatch between solid and soft tissue. [7] Nanobubble and nanodroplet as contrast have been pursued with most works at several hundred nanometer region. In HIFU (high-intensity focused ultrasound), magnetic nanoparticle presence reduces the required time and power needed to generate a given lesion [8]. Nanoparticles are a form of sonosensitizer, which are defined as something which concentrates ultrasonic effects where they are located. Few works on smaller nanoparticles with ultrasound have been studied. In this work, we give a clear evidence that solid nanoparticles in the gel to be irradiated with ultrasound can clearly lead to the temperature increase. This rise of temperature can cause the selective killing of cancer cells than its counter normal cells.
Polystyrene nanoparticles have been used extensively to study characteristics and functions of cells and cell membranes. For instance, their interactions with hepatocytes and hepatocyte cell lines [9], ovarian cancer cells [10], astrocytoma cells [11], kidney epithelial cells [12–13], fibroblasts [12–14], gastric adenocarcinoma [15], neurons and microglial cells [16], as well as colon [17] and lung [18] cancer cell lines have already been reported. Nanoparticles suspended in liquid cause the liquid temperature rise upon insonation to be greater than that of particle-free controls [21–23]. In tissue-mimicking phantoms, nanoparticle presence caused increased heating with ultrasound, and the increased heating is different depending on the nanoparticles used [21–22]. To date, only a couple of measurements have been pursued. Nearly all of them obtained temperature by using embedded thermocouples, which are limited to measuring temperatures at specific locations. In addition, they are subject to measurement artifact caused by their presence [24]. One exception to such is the study [24] by Józefczak and colleagues, which used an infrared camera to record temperature rises in agar phantoms with embedded magnetic nanoparticles. This approach, however, is limited to examining temperatures reached at the phantom surface. Temperatures in the phantom interior cannot be measured using this approach.
The agarose-based tissue mimicking gel is the most commonly used ultrasound imaging phantom material [25]. It was developed by Madsen and colleagues [26], and provides a platform for all sorts of ultrasonic studies. These studies include imaging system accuracy [27], nonlinear propagation in histotripsy [28], and thermocouple-based measurements of heating arising from a focused ultrasound transducer [29]. Its properties resemble those of soft tissue, and therefore provide a simple platform on which to conduct ultrasonic studies. Thermochromicity is a property specific to some materials in which the material changes color upon changes in temperature. A material can be one particular color over a temperature range, and once outside that range, change to a different color. It can be reversible or irreversible; a reversible material reverts to its original color upon being placed back within its original temperature range, while an irreversible material maintains its new color regardless of subsequent temperature changes.
A pioneering use of thermochromic materials to measure physiotherapy ultrasound fields was performed circa 1997 [30]. Recently reported thermochromic phantoms include those based on polyacrylamide, with one formulation that changes color from blue to colorless upon heating [31] and another that changes color from white to magenta [32–33]. An older formula, based on the addition of bovine serum albumin (BSA) and pH control, produces a phantom with transition temperature ranging from 50–60 oC [34]. Silicone phantoms have also been made to be thermochromic [35]. Two unusual phantom types have been produced that can indicate temperature ranges rather than a simple binary indication of being above or below the transition temperature, with the material by Iwahashi and colleagues [36] being reversible and that by Kim and colleagues [37] being irreversible. These phantom types have not been, to date, applied toward nanoparticle presence and their related additional heating. To obtain temperatures in thermochromic phantom interiors, one would section an irreversible phantom post-exposure. In this work, we report the detection of increased ultrasonic heating of nanoparticle-bearing tissue-mimicking gel with embedded nanoparticles using a thermochromic gel phantom material. Being able to estimate the entire altered temperature field when nanoparticles are present is useful in evaluating the effectiveness of the particles present as thermally based sonosensitizers. Issues with thermocouples are avoided, and a complete three-dimensional map of the ultrasonic field’s heating effects can be obtainable with a single ultrasonic exposure.