This opinion artcile highlights the acoustic-resolution photoacoustic microscopy (AR- PAM) which can noninvasively image and monitor stents through skin in various clinically relevant scenarios. Imaging at multiple wavelengths allowed discrimination between stent and simulated plaque. It is underlined with emphais that AR-PAM is a promising label-free tool for monitoring stent status in shallow vessels like carotid and femoral arteries, though further optimization and validation are needed.

The treatment of arterial occlusions in a variety of anatomical areas, such as the coronary, renal, and peripheral arteries, has been transformed by the widespread use of intravascular stenting, which uses devices constructed of sophisticated metallic alloys and polymers. In order to avoid the initial artery collapse and elastic rebound that frequently occur after balloon angioplasty, stents are essential. Despite their therapeutic success, a number of complications, including structural problems such stent fracture and malapposition and in-stent restenosis (ISR), where new tissue growth narrows the artery lumen, can reduce the long-term efficacy of stents. As a result, post-implantation surveillance must be accurate and ongoing. X-ray angiography, computed tomography (CT), and magnetic resonance imaging (MRI) are among the well-established methods that are largely used in current clinical monitoring. Despite their effectiveness, these techniques have inherent drawbacks. For example, MRI has trouble with metallic artifacts and is frequently non-diagnostic when ferrous implants are present, while X- ray-based modalities need ionizing radiation and contrast chemicals. Photoacoustic Imaging (PAI) [1-5], an emerging area, offers a compelling alternative. The possibility of Photoacoustic Microscopy (PAM), a high-resolution PAI variation, for the non-invasive monitoring of stents in relatively superficial vessels is explicitly investigated in this opinion article. PAM offers a new and clinically promising method for identifying serious post- stenting problems by providing high-resolution imaging without the requirement for ionizing radiation or invasive procedures.

Photoacoustic imaging (PAI) is a cutting-edge technique used to visualize the structure and function of biological tissues. It works by shining pulsed laser light onto tissues, where certain molecules (like haemoglobin or melanin) quickly absorb the light energy. This energy absorption causes a rapid, tiny increase in temperature, leading the tissue to expand slightly and produce pressure waves—essentially sound waves. Specialized ultrasound detectors pick up these acoustic signals and convert them into electrical ones, which are then processed by advanced computer algorithms to create detailed images [see Figure 1]. 

The unique advantage of PAI is its ability to use the natural pigments found in tissues, such as haemoglobin, to produce high-contrast images without needing any special dyes or ionizing radiation. By adjusting the laser's wavelength, scientists can highlight different tissue components, making it possible to distinguish between arteries and veins or monitor oxygen levels in the blood. This makes PAI useful for diagnosing diseases and tracking their progression in a non-invasive way [1-5].

Recent advances have improved the speed, sensitivity, and accuracy of PAI systems. Despite its benefits, PAI faces several challenges, including the need for specialized equipment, limits to how deep it can image, and the complexity of handling differences in tissue properties. Ongoing research continues to address these technical hurdles, advancing PAI's role in biomedical imaging and potential future clinical application.

                                                      Figure 1: Schematic of Photoaccoustic Microscopy. Reproduced from [1] Stent overview

In principle, they are tiny nets of tubes. They are composed of either metallic alloy or polymer. Their insertion usually applies to arteries that are already blocked by plaque. These stents protrudes open that blockage. Angioplasty is the medical procedure via which the stents are inserted. The main objective of the placement of the stent is to increase blood flow to the heart which in turn helps reduce heart attack. They are in general placed much deeper, inside hollow structures of the body such as arteries, veins, or ducts, to keep them open and restore the flow of blood or other fluids. The depth of their insertion varies greatly depending on the target location. A coronary stent can last for a long time if the subject adheres to proper medication and lifestyle. However, there may arise restenosis which needs medical attention. In such scenarios, there is need for monitoring of stent health which can be short-term or long term. This in turn provides vital information regarding compression, fractures as well as malapposition. Towards diagnosis of these status, there are traditional procedures such as X-ray angiography, X-ray fluoroscopy, X-ray computed tomography (CT) having considerable demerits such as use of ionizing radiation. In similitude to these, intravascular ultrasound also requires incision and anesthesia. As such, there must be some non-invasive alternatives which can facilitate the effective monitoring of stent status [7-10].

3.1In such scenarios, the PM can be tailored to sense the status of stent circumventing the demerits that are prevalent in traditional techniques. By picking up acoustic waves from materials when shone with light with suitable wavelength, it is possible to map them. Accordingly, researchers have reportedly discovered that photoacoustic microscopy can noninvasively image a stent through the skin as a way to monitor stent placement for any issues [11]. The reported approach is more advantageous than its competitors. It is invasive as well as devoid of ionizing radiation or contrast agents.

3.2 Implementation

As reported in [11], acoustic-resolution photoacoustic microscopy [AR-PAM] utilizes optical parametric oscillator which is wavelength dependent. As such, tunable pulsed laser of wavelength (670 nm and 1210 nm) in conjugation with a beam splitter, photodiode and a 50- MHz flat ultrasound transducer with an acoustic lens have been used in the design. The scheme has been tested in multiple scenarios. This includes fractures, compression, movement of overlapped stents and deposition of plaque—mimicked by butter—after stenting. Because of use of two distinct wavelengths, the butter mimicking lipid and the stent under mouse skin absorb light differently which can be sensed via transducer conjugated the acoustic lens in both lablel free and non-invasive mode. This is the beginning of the use of PI as an alternative non-invasive imaging for effective monitoring after its placement.

The short communication appraises a very effective tool for monitoring stent under through the use of AR-PAM. Although in its nascent stage, there are plenty of scopes of the evolution of this technique. The ex-vivo conditions applied to this technique can be further extended to in-vivo scenarios. These will in turn feasibly help to optimize the tradeoff between imaging depth and resolution for different locations—thereby paving way for a robust non-invasive tool for scanning stent under skin.

There is no conlict of interest