# Recent Developments in Instrumentation of Functional Near-Infrared Spectroscopy Systems
## 4 Frequency-Domain fNIRS Instrumentation
In fact, the spatial resolution of the optical tomography is limited by the signal-to-noise ratio in the order of 20% of the imaging depth [80]. Figure 12: Schematic representation of NIR light penetration for both low and high modulation frequency. The figure illustrates two detected signals successively along with the possible photon paths “banana shape” in different layers with various absorption coefficient and reduced scattering coefficient. For breast imaging applications and to overcome the spatial resolution challenge, several groups have studied the co-registration of FD-NIRS optical tomography with other high-resolution imaging modalities such as MRI, ultrasound, and mammography [81; 82; 83; 84]. In this approach, high spatial resolution images are used to guide the optical functional imaging with high localization accuracy. One research group has investigated the co-registration of mammographic x-ray images and optical breast imaging. The functional information provided by FD-NIRS optical tomography and the anatomical information provided by mammography imaging offers information that neither mammography nor optical imaging is enough single-handedly [83]. Once the optical scanning is completed, the optical probe shown in Figure 14a is removed to allow the breast to be compressed for the mammography scanning. The pressure pain associated with mammography scanning is the main disadvantage of this approach. Alternatively, the MRI-guided optical imaging has been investigated to image adipose and fibroglandular breast tissue by another group [82]. In this approach, the optical probe is a circular geometry consists of six laser diodes emit light at two wavelengths 660 and 850 nm modulated at 100 MHz as presented in Figure 14c. For each source illumination, measurements are collected from 15 locations with photomultiplier tube (PMT) detectors. Unlike the mammography and optical breast imaging approach, the MRI and NIR data are acquired simultaneously. The bulk size and the high cost of MRI systems are real challenges for this technique. Figure 14: Co-registration of FD-NIRS optical tomography with (**a**) mammography (Reproduced from reference [85]); (**b**) ultrasound (Reproduced from reference [84]); and (**c**) with MRI for breast imaging [82]; Copyright (2006) National Academy of Sciences, U.S.A.  
Figure 13: Block diagram of a conventional dual-phase lock detection system. The ultrasound (US)-guided optical imaging approach was also investigated [76; 84]. In this approach, the co-registration of the B-scan ultrasound images are utilized to improve the localization of breast tumor, while the optical imaging provides optical absorption information of the tumor vasculature. In the flat surface optical probe geometry illustrated in Figure 14b, the probe consists of 9 source locations and 14 PMT detectors. The NIR light is emitted by four laser diodes of wavelengths 730, 785, 808, and 830 nm modulated at 140 MHz. The B-scan ultrasound probe is located at the center of the optical probe, where it is surrounded by NIR sources and detectors. The ultrasound is considered safe for the patients, its cost is relatively low, and it is a movable system. Figure 14 depicts the three different approaches aimed for breast imaging. In the last five years, there has been renewed interest in improving the fNIRS technology itself leading to new advantages associated with FD and potentially new applications. [PERSON] and his team have recently developed an ultrafast frequency-domain diffuse optics system with a deep neural network (DNN) processing method to measure the optical properties [86]. The DNN is used to replace the time-consuming Levenberg-Marquardt iterative algorithm which was adopted to fit the calibrated amplitude and phase measurements to an analytical forward model. In contrast to the iterative algorithm, the DNN is 3-5 orders of magnitude faster to estimate the optical properties of measured tissue.