Non-contact acoustic radiation force impulse microscopy via photoacoustic detection for probing breast cancer cell mechanics

Abstract

We demonstrate a novel non-contact method: acoustic radiation force impulse microscopy via photoacoustic detection (PA-ARFI), capable of probing cell mechanics. A 30 MHz lithium niobate ultrasound transducer is utilized for both detection of phatoacoustic signals and generation of acoustic radiation force. To track cell membrane displacements by acoustic radiation force, functionalized single-walled carbon nanotubes are attached to cell membrane. Using the developed microscopy evaluated with agar phantoms, the mechanics of highly- and weakly-metastatic breast cancer cells are quantified. These results clearly show that the PA-ARFI microscopy may serve as a novel tool to probe mechanics of single breast cancer cells.

Keywords: (170.0180) Microscopy; (170.5120) Photoacoustic imaging; (170.7170) Ultrasound.

Fig. 1

PA-ARFI system. (a) Schematic of a PA-ARFI system and its photographs. (b) Triggering sequences for the PA-ARFI method.

Fig. 2

Characteristics and alignment of the press-focused 30 MHz single element LiNbO3 transducer. (a) Photographic image of the transducer. (b) Pulse-echo characteristics of the transducer. (c) Lateral beam profile of a transducer. (d) Peak acoustic pressures at indicated voltage inputs to the transducer were measured by using a hydrophone. (e) Alignment of a laser beam (below) to the transducer’s focus was made by using a 6µm tungsten wire targets aligned along the horizontal (upper-left) and vertical (upper-right) direction at the center of the image and fluorescence imaging of 10 µM Rhodamine B solution. The scale bars indicate 50 μm.

Fig. 3

Evaluation of PA-ARFI microscopy with agar phantoms. (a) Target position measured using PA-ARFI microscopy versus real target position. (b) Geometry of a tissue mimicking agar phantom. Photograph of the phantom (upper) and phantom layers (lower) consisting of a FCNT (a’), agar gel (b’), and substrate layer (c’). The scale bar indicates 5 mm. (c) Comparison of between measured Young’s Moduli and calculated Young’s moduli of phantoms. (d) Displacements of FCNTs in phantoms with the indicated Young’s moduli, 0.7 and 1.7 kPa, due to acoustic radiation forces at the given voltage inputs to the transducer (2, 10, 20, 30, and 40 V_pp). (e) Temporal displacement changes of target FCNTs in the indicated phantoms at the driving voltage = 40V_pp (representative ones). (f) Temporal displacement of PA signals before and after ARFI application on 1.7 kPa phantom (driving voltage: 40 V_pp). An arrow indicates the displacement direction.

Fig. 4

Membrane displacement of different types of cancer cells due to applied acoustic radiation force (input voltage: 40 Vpp). (a) Diagram of PA-ARFI microscopy of cells. After positioning of a target cell labeled with FCNTs at the center of the image field of view (left), acoustic radiation force impulses were applied onto the cells (middle) and followed by tracking of cell membrane displacements (right). (b) Mean cell membrane displacement of MDA-MB-231 (n = 30), SKBR3 (n = 21), and MCF-7 (n = 10) cells (*: p-value < 0.01). (c) Histogram of membrane displacements of each cell type (red: MDA-MB-231, blue: SKBR3, green: MCF-7). The dotted graphs represent Gaussian distributions. (d) Temporal displacement changes of FNTs attached to the membrane of target cells (representative ones).