You are presented with multiple paper abstracts, alongside the associated literature review and chapter titles. Given the provided information, you are going to do a literature review. Please generate a coherent literature review that synthesizes the content, draws connections between different works, highlights major findings, and identifies any gaps in the existing research.

Literature Review Title: A Review on the Extraction of Quantitative Retinal Microvascular Image Feature 
Chapter Title:  Acquisition of Fundus Retinal Image. 
1. Abstract of Cited Paper (BIB001):  A high-speed spectral-domain optical coherence tomography (OCT) system was built to image the human retina in vivo. A fundus image similar to the intensity image produced by a scanning laser ophthalmoscope (SLO) was generated from the same spectra that were used for generating the OCT sectional images immediately after the spectra were collected. This function offers perfect spatial registration between the sectional OCT images and the fundus image, which is desired in ophthalmology for monitoring data quality, locating pathology, and increasing reproducibility. This function also offers a practical way to detect eye movements that occur during the acquisition of the OCT image. The system was successfully applied to imaging human retina in vivo. 
2. Abstract of Cited Paper (BIB002):  PURPOSE ::: To demonstrate high-speed, ultrahigh-resolution, 3-dimensional optical coherence tomography (3D OCT) and new protocols for retinal imaging. ::: ::: ::: METHODS ::: Ultrahigh-resolution OCT using broadband light sources achieves axial image resolutions of approximately 2 microm compared with standard 10-microm-resolution OCT current commercial instruments. High-speed OCT using spectral/Fourier domain detection enables dramatic increases in imaging speeds. Three-dimensional OCT retinal imaging is performed in normal human subjects using high-speed ultrahigh-resolution OCT. Three-dimensional OCT data of the macula and optic disc are acquired using a dense raster scan pattern. New processing and display methods for generating virtual OCT fundus images; cross-sectional OCT images with arbitrary orientations; quantitative maps of retinal, nerve fiber layer, and other intraretinal layer thicknesses; and optic nerve head topographic parameters are demonstrated. ::: ::: ::: RESULTS ::: Three-dimensional OCT imaging enables new imaging protocols that improve visualization and mapping of retinal microstructure. An OCT fundus image can be generated directly from the 3D OCT data, which enables precise and repeatable registration of cross-sectional OCT images and thickness maps with fundus features. Optical coherence tomography images with arbitrary orientations, such as circumpapillary scans, can be generated from 3D OCT data. Mapping of total retinal thickness and thicknesses of the nerve fiber layer, photoreceptor layer, and other intraretinal layers is demonstrated. Measurement of optic nerve head topography and disc parameters is also possible. Three-dimensional OCT enables measurements that are similar to those of standard instruments, including the StratusOCT, GDx, HRT, and RTA. ::: ::: ::: CONCLUSION ::: Three-dimensional OCT imaging can be performed using high-speed ultrahigh-resolution OCT. Three-dimensional OCT provides comprehensive visualization and mapping of retinal microstructures. The high data acquisition speeds enable high-density data sets with large numbers of transverse positions on the retina, which reduces the possibility of missing focal pathologies. In addition to providing image information such as OCT cross-sectional images, OCT fundus images, and 3D rendering, quantitative measurement and mapping of intraretinal layer thickness and topographic features of the optic disc are possible. We hope that 3D OCT imaging may help to elucidate the structural changes associated with retinal disease as well as improve early diagnosis and monitoring of disease progression and response to treatment. 
3. Abstract of Cited Paper (BIB003):  There are several ophthalmic devices to image the retina, from fundus cameras capable to image the whole fundus to ::: scanning ophthalmoscopes with photoreceptor resolution. Unfortunately, these devices are prone to a variety of ocular ::: conditions like defocus and media opacities, which usually degrade the quality of the image. Here, we demonstrate a ::: novel approach to image the retina in real-time using a single pixel camera, which has the potential to circumvent those ::: optical restrictions. The imaging procedure is as follows: a set of spatially coded patterns is projected rapidly onto the ::: retina using a digital micro mirror device. At the same time, the inner product’s intensity is measured for each pattern ::: with a photomultiplier module. Subsequently, an image of the retina is reconstructed computationally. Obtained image ::: resolution is up to 128 x 128 px with a varying real-time video framerate up to 11 fps. Experimental results obtained in ::: an artificial eye confirm the tolerance against defocus compared to a conventional multi-pixel array based system. ::: Furthermore, the use of a multiplexed illumination offers a SNR improvement leading to a lower illumination of the eye ::: and hence an increase in patient’s comfort. In addition, the proposed system could enable imaging in wavelength ranges ::: where cameras are not available.

Literature Review:
The term image acquisition can be defined generally as obtaining an image from any hardware based source. Image acquisition is the initial and most significant step of image processing. The performance of processing image for any intended job extensively depends on the performance of image acquisition. Retinal fundus imaging can be explained as the process of obtaining a two-dimensional representation of threedimensional semitransparent tissues of retinal microvasculature projecting on to the imaging plane and then reflecting the required amount of light  . A technique was developed in BIB001 for generating seamless, high-quality, and wide field montage which could be good for real-time photo documentation of the disc and macular abnormalities. The technique used alignment with high accuracy and blending of partially overlapped slit lamp biomicroscopic fundus image BIB001 . The study of BIB002 developed an algorithm to obtain fundus intensity image using Scanning Laser Ophthalmoscope (SLO) quality from the main spectra that were measured with spectral-domain Optical Coherence Tomography (OCT) and this algorithm provides fundus and OCT images together that can avoid the complexity of registering the fundus feature of any cross-sectional OCT image. Afterwards, this algorithm was extended to generate high contrast shadow-grams of the retinal blood vessel to facilitate the OCT data registration to the further imaging systems BIB002 . The study of BIB003 demonstrated a technique for highspeed ultrahigh-resolution 3-dimensional OCT retinal imaging with and retinal imaging protocols using Fourier domain detection. This technique used a dense raster scan pattern to obtain the three-dimensional OCT data of macula and optic disc. Retinal and interretinal layer and nerve layer thickness can also be mapped using this system BIB003 . The study of BIB004 also developed a real-time imaging approach using a single pixel camera for capturing a perfect fundus retinal image. In modern ophthalmology, there are a lot of image acquisition techniques already developed even using the smartphone. The built-in camera technology, exciting development of cloud storage, and technology for accessing electromedical history using smartphone have encouraged the physicians to use the smartphone in ophthalmic imaging and lead to a dream to have a teleophthalmology system fully based on a smartphone. Welch Allyn first developed an imaging adaptor based on smartphone attaching an iPhone with Welch Allyn Panoptic Ophthalmoscope that can take pictures of retina including the iExaminer App  . The noble design of a compact, slim, smooth, and 3D printed attachment that allows high-quality fundus image coupling smartphone to indirect ophthalmoscopy condensing lenses was reported in  . Converting an RGB color image into the grey-scale image is the first step of many image analysis workflows because in the grey-scale image the amount of information is simplified and maintained only the information related to the features of the image like edges, regions, blobs, and junctions that are needed to be analyzed  . The following transformation equation is used to convert the RGB color image into greyscale image: