Recent Advances in Contrast-Enhanced near Infrared Diffuse Optical Imaging of Diseases Using Indocyanine Green

Due to its spectral and binding properties, the well-tolerated near-infrared absorbing and fluorescent dye indocyanine green (ICG) has found various applications in diffuse optical imaging of diseases in humans. Here, we review applications of this dye for the detection of breast cancer, for perfusion monitoring of the brain in ischaemic stroke and during cardiac surgery, and for the detection of rheumatoid arthritis in finger joints. We give a short overview of the properties of ICG which are relevant for diffuse optical imaging and summarise clinical applications of the dye. Then we describe the experimental methods used for contrast-enhanced imaging of breast cancer with ICG, and we discuss the results of studies on patients with breast lesions. Generally, after intravenous application an increased concentration of ICG in breast carcinomas was observed. In part, differences in ICG pharmacokinetics between malignant lesions, benign lesions and healthy tissue were reported. In a recent study, fluorescence measurements revealed significant differences in extra-vasation of ICG between malignant and benign breast lesions. In brain imaging, ICG has been used as a tracer to assess perfusion of the cortex. Measurements on stroke patients have been performed by comparing the arrival times of a bolus in the affected and in the healthy hemisphere of the brain. Furthermore, ICG boluses have been applied to monitor cerebral blood flow during cardiac surgery. We review the results of the available studies based on either continuous wave light sources, frequency domain techniques, or time domain techniques, and we discuss the depth selectivity of these methods. In one of the studies, fluorescence detection of ICG was investigated and compared to absorption detection used so far. When employed for the detection of rheumatoid arthritis in finger joints, the time course of the ICG distribution in the tissue after intravenous bolus injection was recorded by measuring ICG fluorescence in reflection geometry. We describe the features of the experimental technique and discuss the results of clinical studies performed so far. Finally, we summarise the properties of ICG exploited as contrast mechanisms in diffuse optical imaging.