High frequency ultrasound imaging using optics.

Abstract

Dynamically focused and steered high frequency ultrasound imaging systems require arrays with fine element spacing, wide bandwidths and large apertures. However, these characteristics are difficult to achieve at frequencies greater than 30 MHz using conventional array construction methods. Optical schemes offer a solution. An optical detection array was built with near optimal resolution over a wide depth of field, demonstrating the potential for high frequency ultrasound imaging using optical methods. As an initial test of the overall imaging capabilities of the system, wire targets and tissue mimicking phantoms were imaged with 10-50 MHz ultrasound. Using a 100 x 100 element (i.e., 10,000 elements) 10 MHz optical array, select image planes of a tissue phantom were reconstructed, demonstrating potential 3-D imaging. The finest resolution was less than 50 $\mu$m, produced with a 300 element, 50 MHz system. A possible application is in pathology, where 2-D or 3-D fine resolution imaging can be performed in situ without the need for biopsies. Optical systems have lower sensitivity than their piezoelectric counterparts, limiting their widespread use in ultrasound imaging. A solution is optical feedback detection. These methods, including active optical detection and etalon sensing, are presented and demonstrate enhanced sensitivity while preserving the benefits of traditional optical detection. An active detection system consists of a Neodymium-doped glass waveguide laser with an optical demodulation system. The waveguide cross-section, or equivalently the optical element size, is approximately 3 $\mu$m square, ideal for phased array imaging up to 500 MHz. At 10 MHz, the active detector sensitivity is 10$\rm\sp{-8}nm/(Hz)\sp{l/2}$ comparable to piezoelectric detection and two orders of magnitude greater than that of traditional passive optical detection. The etalon detection system consists of a high finesse optical cavity, external probe laser and optical intensity detector. Ultrasound signals change the etalon cavity length, altering resonance conditions. Intensity variations of the reflected optical beam represent the incident ultrasound signal. The sensitivity of the etalon sensor is comparable to active detection, and may be more efficient at high ultrasound frequencies ($>$30 MHz) due to reduced cavity length ($<$4mm) and parallel processing capabilities.