Assessment of Ultrasound Field Properties and the Potential Effects on Cells

Abstract: Ultrasound is regarded a convenient and safe tool to acquire diagnostic information that we need for clinical use. For a long time ultrasound has been counted as a harmless method, but after all, there is a heating and a me-chanical impact by ultrasound exposure. This influence can reveal both positive (e.g., cell plant growth) and negative (e.g. cell death) effects. Acoustic exposure pattern changed drastically in recent years due to the rapid, technological developments in ultrasound imaging. Ultrasound imaging has become more sophisticated and new techniques are becoming more common, bringing with them not only increased diagnostic capabilities, but also potential threats as far as safety considerations are concerned. The goal of the thesis project is to analyze the ultrasound field characteristics, based on which research would be achievable in the future about how cells are affected by ultrasound exposure with different basic parameters. These parameters include excitation pressure amplitude, number of cycles in a pulse (n), pulse repetition frequency (PRF), acoustic working frequency (f), phase of ultrasound, shape of ultrasound wave (window mode). Some pilot cell experiments are also done in this project. Ultrasound-induced bioeffects on cells have been studied by many scientists, and some experiments tell us that ultrasound beams may cause serious mechanical and thermal damage on e.g. cells. Two general indices, the thermal index (TI) the mechanical index (MI) reflect information on the output level of the ultrasound machine and how a change in output would affect the likelihood of inducing a biological effect. Besides these two indices, other six parameters also are valuable to help us understand the potential threat of ultrasound applications. These parameters are peak negative pressure, peak positive pressure, spatial peak temporal peak intensity (Isptp), spatial peak temporal average intensity (Ispta), spatial peak pulse average intensity (Isppa) and output power of transducer (Wo). The above mentioned eight parameters are important in analyzing the acoustic beams. During the first phase of the experiment (acquisition of ultrasound field parameters) a hydrophone was put at the focus point of the ultrasound beam to acquire the time domain waveform signal of the ultrasound waves. By setting up f, PRF, n, phase and window mode into the computer controlled pulser (SNAP system, Ritec Inc), dif-ferent beams were sent to the hydrophone. Different combinations of basic parameters lead to 186 sets of acoustic beams. We used the hydrophone and oscilloscope to record the waveform signal respectively. Then by self-designed MATLAB software (Mathematical Computing Software, MATLAB®, Natick, Massachusetts, United States), the desired eight characteristics of acoustic field were calculated. Human chronic myelogenous leukemia cell line (K562) were exposed to defined ultrasound waves in the second phase of the experiment. Both trypan blue and resazurin viability assays were used to evaluate effect on the cells immediately after the exposure and 24 hours after the exposure. Resazurin viability assay conducted immediately after the exposure showed reduction of the cell viability up to 46% when the attenuation of amplitude is 0 dB (i.e. the output is the biggest). No cell death was induced. It also showed that after 24 hours the cells viability partially recovered to about 85%. Trypan blue assay showed nearly no cell death was induced.