Cardiologists use intravascular near-infrared fluorescence (NIRF) imaging with intravascular ultrasound (IVUS) and other imaging modalities to examine blood vessels and assess cardiovascular health. Although NIRF and IVUS, used in tandem, provide a powerful technique for detecting conditions such as plaque buildup in arteries, variable attenuation of blood inside vessels can interfere with the accuracy of the NIRF measurements. An approach developed by researchers at the Technical University of Munich (TUM), in which a fluorophore-coated guidewire is used to steer the NIRF-IVUS probe, measures blood attenuation in real time during an intravascular NIRF examination and corrects signal intensities to allow accurate measurement of blood attenuation in the form of correction factors, on a frame-by-frame basis. This approach could enable greater accuracy in near-infrared (NIR) cardiovascular imaging, helping to improve health outcomes for people with cardiovascular disease, the researchers said. NIRF imaging capitalizes on the low-light attenuation in tissue in the NIR region to also operate through blood. However, as the distance between the NIRF detector and the blood vessel wall changes during intravascular imaging pullbacks, this attenuation varies, leading to corresponding fluorescence signal variation. Accurate quantification therefore requires a way to account for the blood attenuation at the wavelength of operation and the distance between the NIRF detector and the vessel wall that contains the fluorescence source. The researchers coated their guidewire with a known concentration of fluorescent particles; because the guidewire is always visible to the NIRF probe, this step ensured that the signal on the guidewire would be able to provide an indirect measure of blood attenuation during imaging. The distance between the NIRF probe and the guidewire, and between the NIRF probe and the blood vessel wall, was determined using IVUS. The researchers used a simple calibration procedure to calculate a correction factor for the fluorescence signal measured at the blood vessel wall. They collected the attenuated NIRF signals from the fluorophore-coated guidewire and used the signals to compute the light attenuation due to blood, as a function of distance, for each frame location. They then combined this position-dependent blood attenuation with NIRF detector-to-vessel-wall distance, computed on the IVUS images, to achieve an accurate correction scheme for intravascular NIRF imaging. According to professor Vasilis Ntziachristos, the demonstration provided an adaptive correction scheme that was tailored to each patient as well as each imaging frame collected during the imaging procedure. The variable fluorescence attenuation of blood has been a hindrance to near-infrared fluorescence (NIRF) measurements in cardiovascular imaging. Researchers at TUM devised an innovative correction method, in which the guidewire is coated with a fluorescent agent and used as a reference standard in each frame, leading to a much higher level of accuracy. Courtesy of Rauschendorfer et al., doi: 10.1117/1.JBO.28.4.046001. The researchers used a small NIRF-IVUS system to test their technique on a porcine model and on capillary phantoms that simulated the properties of small blood vessels. They recorded an improvement over uncorrected NIRF signals of up to 4.5-fold and recorded errors of less than 11% for target signals acquired at distances up to 1 mm from the probe system. The correction method maintained a mean accuracy of 70% in tissue experiments. These values contrast with the values obtained by other correction methods, which use average attenuation factors rather than calculating the attenuation factors for each frame and for the precise probe-to-vessel distances measured via IVUS. The researchers believe that it should be relatively easy to directly incorporate their technique into clinical practice, since no major modifications to existing equipment are required for the method. In addition, the coated guidewire could be used as a reference standard for other intravascular fluorescence imaging modalities. With the appropriate coatings, it could also be used for optical methods that do not involve fluorescence. The research was published in Journal of Biomedical Optics (www.doi.org/10.1117/1.JBO.28.4.046001).