[Front cover]

The cover images show High Frequency Subharmonic Imaging of the Mouse Heart. A nonlinear microbubble imaging system has been developed and evaluated for transmit frequencies in the 10 to 50 MHz range. At high frequencies, both subharmonic and ultraharmonic imaging show good performance in terms of tissue suppression, while second harmonic imaging performs poorly due to nonlinear propagation. These images provide an example of the potential for nonlinear microbubble imaging at a 20 MHz transmit frequency. The contrast agent employed was Definity, which has been previously shown to have substantial nonlinear activity at high frequencies. The upper panel shows a fundamental mode short axis image of a mouse heart and surrounding tissue immediately after the injection of contrast agent. The lower panel is an image in subharmonic mode (10 MHz), where is can be seen that tissue signals have been suppressed leaving a clear picture of agent contained within the left ventricle (LV), and in larger collateral vessels. The spatial scale is 1 mm between large hash marks. See article on page 65.