An ICCD camera-based time-domain ultrasound-switchable fluorescence imaging system

Abstract

Fluorescence imaging in centimeter-deep tissues with high resolution is highly desirable for many biomedical applications. Recently, we have developed a new imaging modality, ultrasound-switchable fluorescence (USF) imaging, for achieving this goal. In our previous work, we successfully achieved USF imaging with several types of USF contrast agents and imaging systems. In this study, we introduced a new USF imaging system: an intensified charge-coupled device (ICCD) camera-based, time-domain USF imaging system. We demonstrated the principle of time-domain USF imaging by using two USF contrast agents. With a series of USF imaging experiments, we demonstrated the tradeoffs among different experimental parameters (i.e., data acquisition time, including CCD camera recording time and intensifier gate delay; focused ultrasound (FU) power; and imaging depth) and the image qualities (i.e., signal-to-noise ratio, spatial resolution, and temporal resolution). In this study, we also discussed several imaging strategies for achieving a high-quality USF image via this time-domain system.

In vivo ultrasound-switchable fluorescence imaging using a camera-based system

“…Thus, the thermal relaxation time constant is 0.446 s, which is longer than our ultrasound exposure time of 0.4 s. (2) By setting the camera exposure time after the FU pulse ended we avoided a possible motion artifact from ultrasound-tissue vibration. (3) According to our previous work [28], by setting an appropriate FU pulse duration (i.e., 0.4 s) and a long camera exposure time (i.e., 1.0 s) we could achieve a high signal-to-noise ratio (SNR) while at the same time maintaining a good spatial resolution. Meanwhile, we selected a time interval = 30.0 s between each USF signal acquisition because this interval should be sufficient for the heated tissue region to cool down (through thermal diffusion) so that it would not affect the next scan, as investigated in our previous work.…”

“…The spatial resolution of a USF image is determined by the size of thermal focal volume generated by the FU beam. [28] It depends on several parameters, such as ultrasound pulse duration, power, focus size and tissue's acoustic (such as absorption coefficient and its nonlinearity) and thermal (such as diffusion coefficient) properties. [18] Because our previous works have quantified the relationship between ultrasound input and spatial resolution in several tissue models.…”

“…[18] Because our previous works have quantified the relationship between ultrasound input and spatial resolution in several tissue models. [18,22,27,28], we did not repeat it in this work considering we used the same ultrasound transducer as in our previous studies. It is worth mentioning that in this work the spatial resolution varied in each experiment because we adopted different FU power (or driving voltage).…”

“…A higher FU power provided a better SNR and a worse resolution. [28] Because the living tissues (e.g., liver and kidney) usually have a lower ultrasound absorption coefficient than ex vivo tissue samples or phantoms as they contains more fluids including blood, we adopted higher Vpps. In addition, increasing imaging depth also degrades SNR.…”

“…[13][14][15][16] Another technique is aimed at confining fluorescence emission only in a small volume through the interference of a FU and only switching on the fluorophores in the focal volume, so called ultrasound-switchable fluorescence (USF). [17][18][19][20][21][22][23][24][25][26][27][28] USF adopts a unique fluorescent contrast agent whose fluorescence can be switched on when temperature rises above a threshold (T th ). [19][20][21]23] When a FU beam is delivered into tissue, it induces a temperature rise in its focus.…”