Magnetic nanorods double OCT image contrast in ex vivo tissue study

In experiments on chicken breast tissue, described in a research article published in Frontiers of Optoelectronics, the aligned nanorods more than doubled both the contrast-to-noise ratio and the signal-to-noise ratio of OCT images compared with tissue containing no nanoparticles.

The study, carried out by Seyyede Sarvenaz Khatami, Mohammad Ali Ansari and Behnam Shariati Bein Kalaee with Valery V. Tuchin, focused on a factor that is often overlooked in nanoparticle-enhanced imaging: orientation. Non-spherical nanoparticles such as nanorods tend to settle into random orientations after injection into tissue. Because the backscattering efficiency that generates the OCT signal depends strongly on nanoparticle orientation, that randomness works against image quality. The team’s solution was to use a small solenoid, generating an 18 millitesla field from a 12-volt direct-current supply, to align magnetite (Fe3O4) nanorods inside the tissue either horizontally or vertically before each imaging session.

The nanorods themselves were synthesized by a one-step hydrothermal and reduction process. A ferric chloride precursor solution was held at 100 degrees Celsius in a Teflon-lined autoclave for either 7 or 12 hours, producing beta-FeOOH precursors that yielded final nanorods of approximately 300 nanometers with an aspect ratio of about 6, or 150 nanometers with an aspect ratio of about 3. After functionalization with oleic acid and reduction at 300 degrees Celsius under a mixture of 90 percent argon and 10 percent hydrogen, the particles were purified and suspended in water at a concentration of 5 milligrams per milliliter. Dynamic light scattering measured hydrodynamic sizes of 54 and 153 nanometers for the shorter rods and 57 and 310 nanometers for the longer ones.

The tissue platform was deliberately standardized. Fresh chicken breasts from 45-day-old birds were cut into uniform samples of 2 by 1.5 by 0.2 centimeters, averaging 1.15 grams, all taken from the same region, cut in the same direction to keep muscle fibers aligned, and held at 4 degrees Celsius throughout. Preliminary OCT imaging verified tissue homogeneity before each experiment. Imaging was performed with a swept-source OCT system operating at a wavelength of 1050 nanometers, a choice supported by spectrophotometry showing low absorption and relatively high reflectance from the nanorods near that wavelength, which favors strong backscatter.

The central result concerns contrast. With nanorods absent, the contrast-to-noise ratio of the OCT images was approximately 3.3. With 300-nanometer nanorods aligned vertically by the magnetic field, it rose to approximately 8.6, an improvement of roughly 160 percent and more than a twofold increase over nanoparticle-free tissue. Vertically aligned nanorods produced about 60 percent better contrast than randomly oriented ones. Signal-to-noise ratios told a consistent story: baseline tissue without contrast agent measured about 12 decibels, rising by 3 and 4 percent just 15 minutes after injection of the 150- and 300-nanometer rods respectively, by 10 and 31 percent with horizontal alignment, by 28 and 53 percent in the random state, and by 66 and 114 percent with vertical alignment. For every condition, five OCT images were analyzed and one-way ANOVA with Bonferroni post-hoc testing confirmed significant differences between groups, at p below 0.03 for contrast-to-noise ratio and p below 0.05 for signal-to-noise ratio.

Size mattered as well as orientation. Under otherwise identical conditions, the 300-nanometer rods enhanced the signal-to-noise ratio by at least 18 percent more than the 150-nanometer rods, consistent with earlier work showing that larger nanorods generate stronger OCT signals. The authors attribute the orientation dependence to surface plasmon resonance: vertical alignment strongly excites longitudinal plasmons along the rod axis, enhancing scattering and absorption, whereas horizontal alignment mainly excites lower-energy transverse plasmons with weaker optical effects. The alignment itself proved remarkably fast, taking less than a minute, although prolonged field exposure risks dragging the rods toward the solenoid, so timing had to be controlled.

The second innovation addressed a practical bottleneck that has long limited nanoparticle-based OCT: distribution time. Uniform dispersion of injected nanoparticles in tissue typically takes tens of minutes, and previous reports describe silver nanoparticles taking from several hours to several days to distribute in rat tissue. In this study, 45 minutes was needed for the iron oxide rods to spread evenly without assistance. By applying a 1.7-megahertz continuous ultrasound module, at an intensity of roughly 4 watts per square centimeter, for just 5 minutes after injection, the team achieved a distribution comparable to the 45-minute spontaneous process, roughly a ninefold acceleration. Acoustic streaming and mechanical agitation are thought to drive this effect by boosting convective transport and opening pathways through interstitial spaces.

Several supporting observations strengthen the result. The ultrasound intensity sits within the standard 1-to-10-watt-per-square-centimeter range of commercial diagnostic sonography systems, OCT images showed no tissue damage or change after sonication, and transmission electron microscopy confirmed that the nanorods themselves suffered no deformation or fracture under the same exposure. Notably, for the 150-nanometer rods, the signal-to-noise ratio after 5 minutes of ultrasound actually exceeded the value obtained after 45 minutes without ultrasound, suggesting faster distribution of the smaller particles, whereas the larger 300-nanometer rods did not show this advantage, consistent with their slower distribution rate. One nuance emerged in the quantitative table: vertically aligned nanorods without ultrasound yielded a higher signal-to-noise ratio than the combined ultrasound-plus-vertical-alignment condition, which the authors suggest may reflect ultrasound-induced disorientation of the aligned rods.

The team also extracted the group refractive index of the tissue from shifts in the OCT peak positions, a parameter with potential value for distinguishing healthy from diseased tissue. Chicken breast tissue alone measured 1.410 by this method, but in the presence of the nanorods the value rose to between 1.682 and 1.692 depending on orientation, with an uncertainty of plus or minus 0.002. The vertical state gave the highest values, 1.689 for the 150-nanometer rods and 1.692 for the 300-nanometer rods, followed by the random and then the horizontal states, an ordering the authors link to the proportion of light interacting with the rods along the propagation path.

Important caveats frame these findings. All experiments were performed ex vivo on chicken breast tissue, an established but simplified model, and the authors themselves note that translating the technique to living tissue faces a directional constraint: while the ultrasound module can irradiate an excised sample from any angle, in vivo application would generally be limited to transdermal delivery from the tissue surface, restricting how the waves can be used to guide nanoparticles at depth. The paper’s broader claims about dermatological diagnostics, dental imaging, neuroimaging and vascular evaluation are presented as outlook rather than demonstrated results. Even so, the combination of magnetically switchable contrast, a ninefold reduction in preparation time and safety margins consistent with standard diagnostic ultrasound marks a methodological advance for contrast-enhanced OCT, one whose clinical value will now depend on validation in living systems.

Subject of Research: Magnetically oriented Fe3O4 nanorods as contrast agents for enhanced optical coherence tomography in ex vivo chicken breast tissue

Article Title: Optical coherence tomography with enhanced contrast using oriented magnetic nanorods

Article References: Khatami, S. S., Ansari, M. A., Shariati Bein Kalaee, B., & Tuchin, V. V. (2025). Optical coherence tomography with enhanced contrast using oriented magnetic nanorods. Frontiers of Optoelectronics, 18(1), Article 24. https://doi.org/10.1007/s12200-025-00167-1

Image Credits: AI Generated

DOI: 10.1007/s12200-025-00167-1

Keywords: optical coherence tomography, iron oxide nanorods, magnetic alignment, contrast-to-noise ratio, signal-to-noise ratio, ultrasound-assisted delivery, magnetite nanoparticles, chicken breast tissue, group refractive index, ex vivo imaging, swept-source OCT, surface plasmon resonance