2C-2 Pulse Repetition Rate Excitation of Contrast Agents

This paper presents a new bubble excitation mode, in which larger bubbles are stimulated to oscillate by a sequence of short high-frequency ultrasound (US) pulses. By tuning both the pressure burst and its repetition frequency, ultrasound contrast agent bubbles can achieve significant radial excursion at their apparent resonance frequency. The excitation method can be used in situations where resonance frequencies of the bubbles are widely separated from the applied imaging frequencies. Following the excitation, the vibration of the bubbles can in principle be detected with imaging techniques like pulse inversion and power modulation. The excitation method, and its underlying mechanisms, was investigated in both optical experiments and simulations. Encapsulated bubbles (Definityreg, Bristol-Myers Squibb MI, N. Billerica, MA) were excited with short (<0.3 ms) bursts with a centre frequency of 10 MHz. The radial motion of single bubbles was captured by the Brandaris-128 fast framing camera running at 15 million frames per second. When the pulse repetition frequency of the bursts equaled the apparent resonance frequency, about 1.1 MHz for a 6.2 mm diameter bubble, the radial excursion builds up after every pulse. The radial excursion also showed a quadratic relation with the transmission amplitude