OKU Laboratory

High-Speed 3D Tracking of Swimming Cell

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In the field of biology, there are many opportunities to use a microscope in order to observe microscopic objects. In general, the field of view and the focal range of microscopes is narrow. Therefore, when the moving object such as swimming cell is observed, the swimming cell moves not only to planar direction but also to depth direction of the microscope and it is difficult to keep the swimming cells under the microscope. To solve this problem, the high-speed 3D tracking for microscopic objects is demanded.

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In order to execute the high-speed 3D tracking, it is necessary to rapidly acquire 3D information of the target object. As a way of acquiring 3D information, the focusing scanning which the focal position of the microscope is scanned in the direction of optical axis, is used in our study. In recent years, the tunable acoustic gradient index (TAG) lens which the focal length can be changed at high-speed (several tens to several hundreds kilo-hertz), was developed. Thus, the focus scanning can be executed at high-speed by using TAG lens. However, TAG lens cannot stop the vibration at a specific focal length. To acquire the specific focal position with TAG lens, a short-time exposure (about 1 us) must be executed only when the lens reaches a specific focal length.

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In this study, the temporally coded exposure (TeCE) camera is developed in order to solve this problem. There are two features of TeCE camera as follows; 1)  to execute multiple times short-time exposure in arbitrary timing, 2) to accumulate the photo-electrons generated by each short-time exposure. The high-speed 3D measurement can be achieved by synchronizing the TAG lens and TeCE camera. In our study, the high-speed 3D module which coupling TeCE camera with TAG lens was developed, and it was confirmed that 3D information of swimming cell (Chlamydomonas) can be acquired in 3 milliseconds. The high-speed 3D tracking of the cell was also achieved by controlling an automated XYZ-stage based on the acquired 3D information, as shown in the movie below. The stage continuously moved the container holding the specimen so that the target was always kept at the center of the volume of view of the microscope.