RF heating-enhanced photoacoustic tomography

TL;DR

RF Heating-Enhanced Photoacoustic Tomography (HEPAT) maps RF absorption via temperature-dependent changes in thermomechanical properties, which enables the use of slow, inexpensive RF subsystems and provides an additional layer of contrast.

Abstract

Photoacoustic tomography (PAT) and thermoacoustic tomography (TAT) both leverage acoustic signals generated by electromagnetic absorption to noninvasively image deep tissues. PAT operates by detecting optical absorption, whereas TAT targets radiofrequency (RF) absorption, providing complementary information on tissue composition and structure. Combining these modalities into a single system promises richer contrast but remains difficult due to the expense and complexity of the RF source. Here, we show that PAT can be integrated with a low-cost RF heater and used to image both optical and RF absorption in tissue phantoms. RF Heating-Enhanced Photoacoustic Tomography (HEPAT) maps RF absorption via temperature-dependent changes in thermomechanical properties, which enables the use of slow, inexpensive RF subsystems and provides an additional layer of contrast. HEPAT therefore provides distinct, complementary contrast relative to existing photoacoustic imaging systems, expanding specificity and diagnostic power while opening new avenues for studying temperature-related tissue phenomena.

Figures (3)

• Figure 1: (A) Schematic of a HEPAT setup in which an RF source heats a sample and a pulsed laser generates a PA signal that is then detected with an ultrasound imaging system. As the sample's temperature rises, changes in its thermomechanical properties alter the PA signal, ultimately enabling additional contrasts. (B) Simulation showing a snapshot of the propagating photoacoustic pressure wave 15 $\mu$s after a single-pulse optical excitation in a tissue phantom containing a non-RF-absorbing region (polyurethane, PU) and a highly RF-absorbing region (agar). (C) Cross-sectional profiles along the dotted gray line in (B) before RF heating at $t_0$, immediately after heating at $t_1$, and after sufficient time for thermal diffusion at $t_2$. Immediately after heating, only the agar region shows a marked change in amplitude (demonstrating RF absorption contrast), whereas after thermal diffusion, both the agar and PU regions exhibit altered signals due to their temperature-dependent Grüneisen parameters. This allows us to separate PA contrast, RF absorption contrast, and $d\Gamma/dT$ (Grüneisen) contrast as shown in (D--F). (D) PA pressure before heating ($t_0$). (E) Pressure difference due to RF absorption but without thermal diffusion ($p(t_1) - p(t_0)$). (F) Pressure difference after thermal diffusion ($p(t_2) - p(t_0)$). Note that the opposite signs in $d\Gamma/dT$ contrast clearly show that the two targets are composed of different materials.
• Figure 2: Microwave oven-based HEPAT demonstrating RF absorption contrast with a US$50 addition to an existing PAT system. (A) Schematic of experimental setup. Agar (left) and silicone (right) targets are placed inside a consumer microwave oven and illuminated by a pulsed laser through an Al foil mesh window. The resulting PA signals are detected by a probe above the targets. (B) PA image of the top surface of the targets before microwave heating. (C) PA image taken after RF heating, which raises the agar's temperature while leaving the silicone largely unaffected. (D) HEPAT image, obtained by subtracting the pre-heating (B) from the post-heating (C) images. Because only the agar heated, only it showed a clear increase in signal, whereas the silicone remained unchanged. See Visualization 1 for the HEPAT image sequence acquired during heating. (E) Vertical cut profiles through the agar and silicone regions confirm that the agar's PA amplitude increases markedly, whereas the silicone signal stays nearly constant.
• Figure 3: HEPAT on specifically engineered setups demonstrating Grüneisen contrast $d\Gamma/dT$. Top row shows the images from the linear probe experiment, and the bottom row shows the images in the ring probe experiment. (A) Drawings of both setups where consumer magnetrons heat fatty (PU) and watery (agar) tissue phantoms and ultrasound probes detect the PA signals. (B, C) Photoacoustic images of each phantom acquired before (B) and after (C) RF heating and thermal diffusion. (D) HEPAT difference images (after minus before), revealing that heating decreases the PA signal in the PU target while increasing it in the agar---an effect driven by their opposite signs of $d\Gamma/dT$. (E) Vertical cross sections through the PU and agar regions confirm these opposing signal changes, illustrating clear contrast between the two materials that emulates RF absorption contrast.