Coronary stents are currently the most widely used coronary intervention for atherosclerosis in the United States. While the procedure is more than 95% successful [47
], stents have brought along several unique issues including restenosis, hyperplasia, and stent drift. The ability to visualize stents both during the stenting procedure and during postsurgery follow-up is important in order to correctly assess the stent with respect to the plaques and vessel, and also identify its apposition within the vessel wall. Immediately following a stenting procedure, it is important to determine the relation of the stent struts to the vessel wall [48
]. Ideally, the stent is deployed in contact with the lumen wall; however, malapposition can occur, resulting in the stent being detached from the wall. This can cause turbulent eddies to form in the vessel that can lead to thrombosis in the area of the stent. It is also important when monitoring the stent to determine how much restenosis has been formed around the stent struts. This distance must be determined to assess stent viability. Currently, the most common method for assessing stent position is X-ray coronary angiography/fluoroscopy [49
]. However, this procedure is problematic due to its use of ionizing radiation and possible complications in using iodinated contrast agents. Furthermore, X-ray fluoroscopy can only depict a 2-D projection that can lead to an underestimation of the lumen diameter and the stent apposition within the lumen.
Other imaging modalities such as MRI and coherence tomography (CT) have problems with metallic susceptibility artifacts, which prevent the vessel lumen from being accurately visualized relative to the stent [47
]. The use of optical CT (OCT) directly competes with these disadvantages with a resolution of 10–20 μ
m, but has severe depth limitations, allowing only a penetration depth of about 2 mm [48
]. The presence of blood flowing through the vessel limits this depth even further, requiring clinicians to flush the vessel during the imaging procedure [52
]. Furthermore, the tissue behind the stent strut becomes hidden due to scattering shadows in OCT, which prevents complete diagnosis of the stent’s relation to the vessel lumen [52
To counteract these disadvantages, we used the IVPA imaging system to image a clinical off-the-shelf stent (Cordis BX Velocity). The IVPA imaging has sufficient depth penetration, resolution, and contrast to visualize the stent and surrounding tissue. The imaging is tomographic, allowing 3-D reconstructions of the stent and vessel combined.
To test the feasibility of the IVPA system to image stents, a study was performed using a BX Velocity 5.0 mm stent embedded within the tissue-mimicking vessel. The cylindrical phantom was prepared using 8% PVA and 1% silica. The 10 mm outer diameter vessel had three different interior regions, where the inner diameter varied around the stent. These three regions were constructed so that the stent was embedded approximately 1.0 mm inside the vessel wall, deployed (adjacent to the vessel wall), and malapposed approximately 1.0 mm from the lumen wall. The malapposed region was creating a gap between the stent and the vessel wall. However, due to the molding process during phantom fabrication, the stent itself was covered with a thin film of PVA.
Imaging of the vessel and stent was performed using the prototype IVUS/IVPA benchtop system similar to the setup shown in , where, instead of an air beam, laser irradiated the vessel from the outside using an optical fiber delivering 800 nm wavelength light. This wavelength was chosen for sufficient depth penetration. For pullback-based 3-D imaging, a 1-D axis was placed under the water cuvette to move the sample along its longitudinal axis.
The cross-sectional ultrasound, photoacoustic, and combined images of the vessel phantom with the stent are shown in . With the ultrasound signal displayed at 40 dB, the photoacoustic images at 15 dB show high contrast between the stent and the background.
Fig. 10 (Left column) IVUS, (middle column) IVPA, and (right column) combined IVUS/IVPA images from the three different stent regions in the vessel. (a) Stent embedded within the vessel. (b) Stent adjacent to lumen wall. (c) Stent detached from lumen wall. Due (more ...)
Indeed, photoacoustic signal from the stent is high due to the high optical absorption of metal struts compared to the vessel that has little photoacoustic response at this wavelength. This allowed high contrast of the stent to the background tissue. The ultrasound image visualized the complete vessel including the structure and thickness of the vessel wall. Therefore, the location of the stent (IVPA image) was given in relation to the vessel (IVUS image). By scanning along the length of the vessel, the varying radial distance between the stent and the lumen wall could be assessed. In the region where the stent was embedded within the vessel [see ], the stent struts were measured embedded 0.7–1.0 mm within the vessel wall. In the region where the stent was merely adjacent to the vessel wall [see ], the image gave good qualitative agreement in visualizing the correct position of the stent to the vessel. The malapposed section was also quantitatively measured and showed that the stent was malapposed away from the lumen wall by 0.8–1.1 mm [see ].
By combining a set of 80 cross-sectional images, a 3-D image of the entire vessel wall and stent was reconstructed (see ). The structure of the stent was clearly seen in the context of the vessel structure. The transparency (alpha value) of the ultrasound image was modified such that only the photoacoustic signal could be seen, leaving only the structure of the stent [see ]. The shape and position of the stent within the vessel is easily assessed [see ]. The photoacoustic image also allowed the inner diameter of the stent to be correctly measured at 5.0 mm, the manufacturer reported size of the stent.
Fig. 11 3-D reconstructed (a) IVUS, (b) IVPA, and (c) combined images of trisectional phantom. Individual cross sections can show the position of the stent within the vessel. Photoacoustic signal alone can assess the shape of the stent in order to determine the (more ...)
demonstrated that a stent deployed into an excised sample of rabbit aorta could also be visualized in the IVPA image. Even though the tissue was irradiated externally by the laser, sufficient laser fluence penetrated the vessel wall to generate photoacoustic signals with enough contrast from the stent. Thus, IVPA imaging may be able to image stents embedded deep inside the arterial wall.
Reconstructed (a) IVUS, (b) IVPA, and (c) combined images of a stent deployed within an excised section of an atherosclerotic rabbit aorta. Stent is visible as adjacent to lumen wall.
Since in vivo IVUS/IVPA imaging may occur in the presence of optically attenuating luminal blood, we also imaged the samples in an optical scattering environment created with a mixture of low fat milk and water. The optical fiber delivering the light to the vessel was placed 0.5 cm away from the surface of the vessel. Although the photoacoustic signal intensity showed a reduction in the peak signal intensity of the stent struts due to the light attenuation of milk, the quality of the IVPA image was reduced insignificantly.
Coronary artery stents are well visualized using combined IVUS/IVPA imaging. Ultrasound imaging provides useful structural information of the vessel wall. Photoacoustic imaging utilizes the differential optical absorption of laser energy and offers high optical contrast in viewing the metal stent relative to the surrounding vessel. In the combined IVPA/IVUS images, the full structure of the phantom is visible and not obscured behind the stent struts, thus allowing one to see the apposition of the stent within the vessel wall, regardless of where the stent was located.
The use of IVUS/IVPA imaging to image stents is a natural progression as stents are commonly used to treat blood vessels that have narrowed due to atherosclerosis. Recent studies have shown that stent positioning can drift over time, bringing the need to detect stent location with respect to the site of atherosclerosis while analyzing the progression of plaque vulnerability also. Our study clearly shows that IVUS/IVPA imaging is a promising modality to image stents in vivo.