19 Apr 2023
Using a guidewire as reference point compensates for fluctuating fluorescence.
But attenuation of the fluorescence signal is a hurdle, complicating detection of the fluorescent markers added to the bloodstream once they are attached, as intended, to the blood vessel walls.
When the NIRF detector is moved through the vessel and its distance from the wall changes, so the attenuation fluctuates and the fluorescence signal varies.
A project at the Technical University of Munich (TUM) and its Central Institute for Translational Cancer Research (TranslaTUM) has now developed a possible solution, an adaptive correction scheme effectively tailored to each imaging frame as it is collected during the intravascular procedure.
As described in Journal of Biomedical Optics, the new approach involves a guidewire, coated by a fluorescent marker and used to steer the NIRF-ultrasound catheter during an examination. This can provide a reference signal for accurate attenuation measurements during the intravascular imaging procedure itself.
"Collection of attenuated NIRF signals from the fluorophore-coated guidewire can be employed to compute the light attenuation due to blood, as a function of distance, for each frame location," noted the project in its published paper. "Combining this position-dependent blood attenuation with the NIRF detector-to-vessel wall distance, which is computed from the ultrasound images, can offer an accurate correction scheme for intravascular NIRF imaging."
Design of a feasible guidewire was partly facilitated by the recent development of suitable bio-compatible fluorescence coatings, according to the project, and the availability of these coatings could now lead to a viable solution for the broad application of this kind of correction scheme.
Straightforward clinical translation
In trials, the new method was used to image capillary phantoms, which simulate the properties of small blood vessels, and then the coronary artery of a pig's heart for an ex vivo measurement. The platform recorded a 4.5-fold improvement over uncorrected NIRF signal and restricted errors to below 11 percent for target signals, according to the project.
The correction method also maintained a mean accuracy of 70 percent in tissue experiments. These values "are in stark contrast to the accuracies obtained by other correction methods, which use average attenuation factors rather than calculating them for each frame and for the precise probe-to-vessel distances measured via intravascular ultrasound," said the team.
Future work will include improvements to the imaging speed, which in these trials was limited to 1 fps. Faster rates might be achieved by enhancing data streaming of the acquired signals, or by reducing back-end vibrations and catheter friction.
It could be relatively easy to directly incorporate this technique into clinical practice, since no major modifications to existing equipment are required. The guidewire might also be used as a reference standard for alternative intravascular fluorescence imaging modalities, as well as other optical methods beyond fluorescence, if appropriate coatings are used.