(A) Schematic showing the RThAM imaging principle. We used silicone oil (Si oil) as dielectric medium in an open transmission line (TxLine) and coupled an RF field [E⃑(t)] to the biological sample (supported by a tubing of ∅1 mm) to excite thermoacoustic wave (dashed red circles). The thermoacoustic wave is coupled by deionized water (DI H₂O) contained by PE film to an ultrasound (US) transducer. Resin coating to the copper and an extra PE film are applied to avoid direct contact between the sample and TxLine. (B and C) RThAM image of two copper wires (diameter, 50 μm) formed (B) by MIP in the xy plane and (C) by a slice (indicated by the dotted gray line) in the yz plane. (D) Line profiles showing a FWHM of 23 μm in the z axis (axial) and 33 μm in the y axis (lateral) along the solid blue and dashed arrows indicated in (C), respectively. (E) A temporal RThAM signal (black curve) showing an SNR of 34 dB and its Fourier transform (FFT; blue curve) showing the frequency components and a central frequency at 47 MHz.

(A to D) Imaging of a one-cell stage embryo (¹/₂ hpf). (A) Bright-field microphotograph. (B) RThAM MIP image. (C) An overlay of RThAM MIP onto the microphotograph. (D) The profile along the arrow in (C) showing dominant RF absorption in the blastodisc versus the yolk. (E to H) Imaging of a 512-cell stage embryo (4 hpf). (E) Bright field. (F) RThAM MIP. (G) Overlay. (H) An xz-plane B-scan along the dashed orange line in (G), showing intense absorption from the blastomere region and the absorption originated from the surface of the embryo. The gray arrows indicate the B-scan analyzed in fig. S4. (I to K) Imaging of a transparent Casper zebrafish larva at 3 dpf. (I) Optical reflection-mode microphotograph of the larva in the coronal view with annotation of major anatomical features: (1) forebrain; (2) midbrain; (3) hindbrain; (4) inner ear; (5) liver, intestine, and pancreas; (6) heart; (7) swim bladder; (8) spinal cord and vessels/muscles in the trunk; and (9) yolk sac. (J) An overlay of an RThAM slice (~150 μm in thickness) onto the microphotograph (edge-filtered). (K) MIP in the yz plane. Scale bars, 500 μm.