Photoacoustic tomography, a medical imaging technology that turns light into sound to reveal what is happening deep inside the body, may be approaching a crucial turning point. A comprehensive analysis by researchers at the University of Birmingham and University College London has mapped the performance of 82 ultrasound detectors used in photoacoustic imaging, identifying which technologies are currently best suited to deep-tissue diagnosis and which could define the next generation of high-resolution scanners.
Published in Nature Reviews Methods Primers, the study provides the first standardised comparison of detector sensitivity using a measurement known as noise-equivalent pressure, or NEP. In practical terms, NEP describes the weakest acoustic pressure a detector can distinguish from its own background noise. A lower NEP means that a detector can hear fainter ultrasound signals, an important advantage in photoacoustic tomography because signals become dramatically weaker as they travel through tissue.
Photoacoustic tomography works by combining the molecular sensitivity of optical imaging with the depth capability of ultrasound. During a scan, short pulses of laser light are delivered into the body. Molecules such as haemoglobin absorb specific wavelengths of this light, causing a minute and rapid rise in temperature. The resulting thermal expansion generates ultrasound waves that travel through surrounding tissue. Detectors placed around or on the body capture those waves, and a computer reconstructs them into images showing blood vessels, tumours, oxygen levels and other indicators of tissue function.
The method is attracting intense interest because it can reveal biological information that conventional ultrasound cannot provide while avoiding the ionising radiation used in techniques such as X-ray computed tomography. By selecting different laser wavelengths, researchers can distinguish oxygen-rich from oxygen-poor blood, potentially allowing clinicians to observe changes associated with cancer, inflammation, vascular disease and neurological disorders. Yet the technology faces a fundamental physical challenge: light becomes more diffuse as it penetrates tissue, while the sound generated at depth is absorbed and scattered before it reaches the detector.
The new review examined four major detector families: ceramic piezoelectric devices, polymer piezoelectric devices, capacitive micromachined ultrasonic transducers, known as CMUTs, and optical ultrasound sensors. Although all four technologies convert acoustic pressure into a measurable signal, they do so in very different ways. Piezoelectric detectors generate an electrical charge when compressed by an incoming sound wave. CMUTs detect changes in the capacitance of tiny vibrating membranes. Optical sensors instead measure sound-induced changes in light, often through the movement or deformation of a miniature optical resonator.
The analysis found that large ceramic piezoelectric detectors currently offer the strongest performance for many deep-imaging applications. Their relatively large sensing surfaces and efficient response to low-frequency ultrasound make them particularly effective when sound arrives directly at the detector. Low-frequency waves generally travel farther through tissue than high-frequency waves, although they carry less fine structural detail. This balance makes ceramic detectors attractive for applications such as breast imaging, where the objective may be to detect abnormalities several centimetres beneath the skin.
Detector geometry also matters. A large element can collect more acoustic energy, improving sensitivity, but it may not pinpoint the precise direction from which the sound arrived. Smaller elements provide better spatial sampling and can be arranged into dense arrays capable of producing detailed images. The problem is that reducing the size of an electrical detector often reduces the signal it receives, making the device noisier and less sensitive. This trade-off has shaped the development of photoacoustic systems: the detectors most capable of hearing faint signals are not always the ones best suited to resolving tiny structures.
Optical ultrasound sensors offer a potential way around that limitation. Because they detect sound through changes in light rather than relying on a conventional electrical element, they can be fabricated at dimensions of less than 100 micrometres while retaining high sensitivity. Such miniature sensors could be placed close together in large numbers, allowing scanners to capture the complex wave patterns needed to resolve capillaries and other microscopic structures. The review suggests that optical detectors are already among the most promising options for high-resolution imaging of small blood vessels and could become increasingly important in deep tissue if their sensitivity continues to improve.
Polymer piezoelectric detectors occupy a different position in the technology landscape. Compared with many ceramic devices, they can provide broader frequency coverage, allowing them to respond to a wider range of acoustic wavelengths. That flexibility may help researchers capture both relatively deep signals and the higher-frequency components that contain fine image detail. Polymer detectors can also be mechanically flexible, making them suitable for specialised probe designs and curved imaging surfaces. Their broader bandwidth, however, does not automatically translate into the best sensitivity for every application, particularly when the target lies deep inside the body.
CMUTs, which are produced using microfabrication techniques similar to those used in semiconductor manufacturing, showed sensitivity comparable with some of the strongest detector technologies in the review. Their small size, potential for integration with electronics and compatibility with dense arrays make them appealing for compact clinical scanners. However, the researchers found that published CMUT measurements often lacked the detail needed for a reliable comparison. Differences in calibration, experimental conditions and reporting practices made it difficult to determine how the devices would perform under equivalent circumstances.
That inconsistency is one of the central findings of the study. Comparing detectors has traditionally been complicated because laboratories have used different definitions of sensitivity, different acoustic frequencies and different measurement arrangements. Some reports describe peak pressure, others use average pressure, and still others do not fully explain the noise environment or the direction of incoming sound. By assembling available data into a common NEP framework, the researchers have created a performance map intended to make comparisons more meaningful for engineers, manufacturers and biomedical researchers.
“There is no single ‘best’ ultrasound detector for all photoacoustic imaging applications,” said lead author James Guggenheim of the University of Birmingham. Deep-tissue imaging currently benefits from highly sensitive ceramic detectors, he explained, while optical ultrasound sensors are especially strong when the goal is to image tiny blood vessels at high resolution. The long-term competition between the technologies may therefore depend less on replacing one detector type with another than on designing systems that combine their advantages.
The next generation of photoacoustic scanners will need more than sensitive individual detectors. Clinical systems must contain large numbers of closely spaced elements, maintain precise timing across many channels and process enormous volumes of data in real time. Building such arrays is technically demanding, particularly when each element must be small enough to achieve high resolution but sensitive enough to detect signals from deep tissue. The electronics, optical components, acoustic coupling materials and reconstruction algorithms must also work together without adding excessive noise or making the equipment impractical for hospitals.
The researchers argue that progress will depend on improving detector sensitivity, developing dense small-element arrays and adopting consistent standards for measuring performance. If those challenges can be solved, photoacoustic tomography could move closer to providing clinicians with a single examination that combines anatomical detail with functional information about blood oxygenation and tissue physiology. The review does not identify one technology as the universal winner. Instead, it shows that the future of photoacoustic imaging may belong to carefully matched detector designs—and potentially to optical sensors capable of hearing both the faintest deep-tissue signals and the smallest structures in the human body.
Subject of Research: Ultrasound detectors used in photoacoustic tomography
Article Title: Noise-equivalent pressures of ultrasound detectors used in photoacoustic tomography
News Publication Date: 6 August 2026
Web References: https://www.nature.com/articles/s43586-026-00510-6
References: James A. Guggenheim, Dylan M. Marques, Thomas J. Allen, Olumide O. Ogunlade, and Paul C. Beard, “Noise-equivalent pressures of ultrasound detectors used in photoacoustic tomography,” Nature Reviews Methods Primers.
Keywords: Photoacoustic tomography, ultrasound detectors, medical imaging, optical ultrasound sensors, piezoelectric detectors, ceramic detectors, polymer detectors, CMUTs, noise-equivalent pressure, deep-tissue imaging, blood vessels, tumour imaging, biomedical engineering
