Mohan Li

Abstract Continuous imaging of cardiac functions is highly desirable for the assessment of long-term cardiovascular health, detection of acute cardiac dysfunction and clinical management of critically ill or surgical patients 1–4 . However, conventional non-invasive approaches to image the cardiac function cannot provide continuous measurements owing to device bulkiness 5–11 , and existing wearable cardiac devices can only capture signals on the skin 12–16 . Here we report a wearable ultrasonic device for continuous, real-time and direct cardiac function assessment. We introduce innovations in device design and material fabrication that improve the mechanical coupling between the device and human skin, allowing the left ventricle to be examined from different views during motion. We also develop a deep learning model that automatically extracts the left ventricular volume from the continuous image recording, yielding waveforms of key cardiac performance indices such as stroke volume, cardiac output and ejection fraction. This technology enables dynamic wearable monitoring of cardiac performance with substantially improved accuracy in various environments.

Serial assessment of the biomechanical properties of tissues can be used to aid the early detection and management of pathophysiological conditions, to track the evolution of lesions and to evaluate the progress of rehabilitation. However, current methods are invasive, can be used only for short-term measurements, or have insufficient penetration depth or spatial resolution. Here we describe a stretchable ultrasonic array for performing serial non-invasive elastographic measurements of tissues up to 4 cm beneath the skin at a spatial resolution of 0.5 mm. The array conforms to human skin and acoustically couples with it, allowing for accurate elastographic imaging, which we validated via magnetic resonance elastography. We used the device to map three-dimensional distributions of the Young's modulus of tissues ex vivo, to detect microstructural damage in the muscles of volunteers before the onset of soreness and to monitor the dynamic recovery process of muscle injuries during physiotherapies. The technology may facilitate the diagnosis and treatment of diseases affecting tissue biomechanics.

Electronic patches, based on various mechanisms, allow continuous and noninvasive monitoring of biomolecules on the skin surface. However, to date, such devices are unable to sense biomolecules in deep tissues, which have a stronger and faster correlation with the human physiological status than those on the skin surface. Here, we demonstrate a photoacoustic patch for three-dimensional (3D) mapping of hemoglobin in deep tissues. This photoacoustic patch integrates an array of ultrasonic transducers and vertical-cavity surface-emitting laser (VCSEL) diodes on a common soft substrate. The high-power VCSEL diodes can generate laser pulses that penetrate >2 cm into biological tissues and activate hemoglobin molecules to generate acoustic waves, which can be collected by the transducers for 3D imaging of the hemoglobin with a high spatial resolution. Additionally, the photoacoustic signal amplitude and temperature have a linear relationship, which allows 3D mapping of core temperatures with high accuracy and fast response. With access to biomolecules in deep tissues, this technology adds unprecedented capabilities to wearable electronics and thus holds significant implications for various applications in both basic research and clinical practice.

Abstract Depth-of-interaction (DOI) capability is important for achieving high spatial resolution and sensitivity in dedicated organ and small animal positron emission tomography (PET) scanners. The dual-ended readout is one of the common methods that can achieve good DOI resolution. The aim of this study is to evaluate a dual-ended readout detector based on silicon photomultiplier (SiPM) and TOFPET2 application-specific integrated circuit (ASIC). The detector is based on 4 4 lutetium–yttrium oxyorthosilicate (LYSO) units, each unit contained 6 6 LYSO crystals, and the crystal size was 1 1 20 mm 3 . The four lateral surfaces of LYSO crystals were mechanically ground to W14 (surface roughness 10–14 m), and the two ended surfaces were polished (surface roughness <0.5 m). The reflector was Toray Lumirror E60, and the packing fraction of the LYSO block was 86.5%. Each LYSO unit was read out from both ends with two Hamamatsu S13361-3050AE-08 SiPM arrays. The analog output signals of SiPM were digitized by PETsys TOFPET2 ASIC and acquired by PETsys SiPM Readout System. The ASIC and SiPM were cooled by a fan and a Peltier element. To investigate the crystal resolvability, different light guide thicknesses including 0.8, 1, 1.2 and 2 mm were tested. The light guide was made of optical glass (H-K9L-Foctek Photoincs), and the size and refractive index were 6.45 6.45 mm 2 and 1.53 (at 420 nm), respectively. To characterize the detector performance at different depths, another 1 25.8 20 mm 3 single LYSO slab was used. Data were acquired at 10 depths (1, 3, …, 19 mm), and each depth had a 10 min acquisition time and about 40 thousand coincidence events. During the experiment, the SiPM temperature was controlled as 27.6 0.4 °C. The results showed that the 1.2 mm light guide offered the best crystal resolvability. The energy, coincidence time, and DOI resolution full-width at half-maximum of the detector were characterized as 15.66% 0.66%, 602.98 10.58 ps, and 2.33 0.07 mm, respectively. The good DOI resolution indicates the potential of utilizing the detector for high-resolution PET applications.

Recent progress with Cadmium Zinc Telluride (CZT) radiation sensors grown by the traveling heater method (THM) at Kromek is reported. Large volume monolithic pixelated detectors, 40×40×15 mm³ have been fabricated with good initial gamma spectroscopy response (< 2.5% energy resolution at 662 keV at room temperature without correction). After depth of interaction (DOI) correction, detector performance with < 1% energy resolution at 662 keV at room temperature has been obtained on pixelated 22×22×15 mm³ CZT detector. For medical imaging applications, 20×20×6 mm³ pixelated detectors exhibits < 3% energy resolution at 122 keV without correction. These results have been achieved via our proprietary THM crystal growth in combination with our robust device fabrication technique. Examples of progress in other areas of CZT development for gamma spectroscopy and imaging applications such as 40×40×5 mm³ cross-strip device for PET and Kromek’s general-purpose SPECT camera will also be presented.