Optical coherence tomography angiography
The development of angiographies has been a long time in the making with initial progress dating back to the discovery of the circulation of blood by William Harvey in 1628. The first right heart catheterisation was performed in 1929 with the eventual first angiogram procedure as we would know it today described by Mason Stone in the 1960s .
An angiogram is an image produced through the methods of angiography which is a technique used in the medical world to capture the anatomy of the blood vessels of the body such as the veins, capillaries and arteries. An angiography is typically performed to ensure healthy functioning of the heart and blood vessels by determining whether or not blood is flowing freely through the blood vessels.
The process of an angiography usually involves injecting a dye into the patient which can then be detected by an x-ray and displayed as an image. This means that the dye must be introduced into the patient’s bloodstream in an invasive procedure, generally through the use of general anesthetic and a catheter. As with any invasive procedures there are risks posed to the patients and these risks must be monitored and assessed constantly throughout the procedure to prevent them from becoming serious complications. 
Some of the other negative effects of current angiographies are less obvious but are equally, if not more, dangerous. It is known that some people do not respond well to the dyes, which can result in anaphylactic shock, used in the angiographic process . There are two main types of negative reactions to these dyes, chemotoxic and allergic. These reactions can be very difficult to quantify as they range in their appearance ranges in time, manifestation and severity. The delay in reaction time to the dyes makes calculating the number of negative reactions particularly challenging as these later negative reactions can be easily overlooked if the patient, for example, has left the medical centre . Angiographies are also unreliable when it comes to predicting the severity of a disease . This can lead to false negatives in patients who may now suffer unexpected and sudden vascular difficulties at a later date in life. On top of all of these health risks, the contrast agent which is used in the angiography can be quite expensive. Costing as much as $130 per litre , the elimination of this dye from angiographies would certainly help in reducing the cost of medical bills.
The focus of our new medical imaging technology is the eye. We are looking at better and safer ways to capture images of the blood vessels of the eyes. There are some techniques which exist today for resolving these images such as the Fluorescein Angiography. This technique was developed in 1961 . The technique used for imaging the eyes through this form of angiography is very similar to that used in any blood vessel imaging. Firstly, the fluorescein dye must be injected into the bloodstream, usually through a vein between the elbow and the shoulder to allow for fast travel time to the eye. This travel time takes roughly 10 seconds and is dependent on factors such as patient age and dye injection velocity. After about 30 seconds the concentration of fluorescein dye in the blood vessels of the eye is at its peak and the angiography is taken. In 10 minutes the dye is all but gone from the patient’s system. This technique does not require a catheter and so is much less invasive than a standard angiography. However, this does not mean that it is not without its own risks. The most prevalent risks involved with fluorescein angiographies come in the form of people’s reactions to the procedure as opposed to the procedure itself from going wrong. The typical negative reactions that patients study are nausea and vomiting which occur in 3-15% and 7% of patients respectively . Patient mortality is not a large factor to be considered with this procedure estimated to be approximately 1 in 221,781 .
Another technique which is attempting to address the same issues that we are with Optical Coherence Tomography Angiography is Doppler Ultrasound. Doppler ultrasound is a technique which makes use of the Doppler effect to help clinicians to view body tissues and fluids, such as blood, without the need for any invasive procedures.
This is a promising technique as it allows images of the body’s internals to be taken without posing any risk to the patient from invasive procedures. The technique however is not without its limits. For example, in Esophageal Doppler ultrasound there are several limitations which come into play such as, patient discomfort, large probe size, a need for frequent probe readjustment due to patient movement, operator dependence as well as the need for additional sedation to combat all of the aforementioned complaints , . These forms of discomfort for the patient can make the imaging process unreliable and inefficient which results in lost time for both parties that are involved in the procedure.
(However, the average thickness of the epidermis in the human body is 83.7 m  and is highly dependent on the part of the body examined. On the forearm the mean thickness of both epidermis and dermis in healthy adults vary from 1.19 mm to 2.12 mm on different parts of the arm .As a result,
high resolution ultrasound imaging would not be able to visualize all three layers of the skin on a different part of the body or even on the other part of the forearm. Thus, there is a need of an imaging tool with a higher resolution and adequate penetration depth. )
Description of the Emerging Technology
Optical coherence tomography angiography (OCTA) is a new non-invasive imaging technique that uses motion contrast imaging to obtain high-resolution volumetric blood flow information, generating angiographic images in a matter of seconds. Many developments and improvements have been made since the origin of this technique due to the large strides and advancements in scientific understanding and software technology since its predecessors, which is allowing OCTA to be brought to the fore.
Red blood cells travelling through a vessel during previous OCT and doppler OCT techniques caused a change in tissue reflectance. This concept brought on the development of OCTA which uses the moving particles, such as blood cells, within the biological tissues as contrast agents themselves to image blood flow. Two OCT signals are observed. Biological tissue and the vessel containing flow backscatter these two respective signals. Over time, the OCT signal relayed by the biological tissue is steady as there is no movement, however the backscattered signal from within the vessel is constantly changing over time as red blood cells (RBC) are tumbling and flowing within. Figure X provides a clear view of this phenomenon .
Figure 3 Optical coherence tomography based angiography [Invited] Reference properly Extracted from Chen et al.  Fundamental view of optical coherence tomography based angiography. Signals are sampled from five points in the A-scan, where three pixels (1, 2, and 5) are located at the static tissue, and two pixels (3 and 4) are located within a functional blood vessel. Dynamic changes in the OCT signals for pixel 3 and 4 can be observed over time while signals from pixel 1, 2, and 5 remain steady.
The differences in OCT signals received from the same location at different time points are calculated so that OCTA can distinguish the moving particles from the static tissue, and therefore able to generate flow signals and allow the visualization of microvascular networks in biological tissues without a need of intravenous dye injection.
Physics of the OCTA method
As explained briefly, OCTA employs the laser light reflectance of red blood cells to achieve a depiction of the blood vessels within different areas of the eye, thus removing the need for intravascular dyes. However the OCT scan of a patient’s scan requires multiple scans. The scan consists of multiple individual scans known as A-scans, which are then compiled into a B-scan, which provides cross-sectional structural information. With OCTA technology, the same tissue area is repeatedly imaged. Differences between these scans are then analysed, which allows zones of high flow rate (i.e. indicated by marked changes between scans) and zones with slower, or no flow at all, to be detected.  These areas of no flow will be extremely similar between successive scans. Two light sources of different wavelengths may be used for the process. A spectral domain OCT (SD-OCT) light source, with a wavelength of originally 800nm or a swept-source OCT (SS-OCT), which utilizes a longer wavelength, close to 1050nm may be used. Longer wavelengths have a deeper tissue penetrance, but a slightly lower axial resolution.[6,7] With this in mind, the SD-OCT source has also demonstrated its ability to operate in line with comparable standards at 1300nm to increase tissue penetration, sacrificing a small percentage of resolution. However, the more commercially available SD-OCT uses an 870nm wavelength which has shown excellent imaging.  OCT-A employs two methods for motion detection: amplitude decorrelation or phase variance.
Split Spectrum Amplitude Decorrelation and Speckle Variance
These two methods are categorised together as they analyse only amplitude changes in the OCT signal, ignoring any information contained in the phase. Here the method detects differences in amplitude between two different OCT B-scans.
Barton and Stromski were the first to consider detecting blood flow signal by measuring the speckle changes in OCT signal. . The first order of laser speckle statistics describes speckle at a point while the second order statistics contains the joint statistical properties of speckle at two or more points, as well as the information about the motion of scatterers [10,11]. The temporal speckle fluctuations at a point show a dependence on the mean velocity of the scatterers. Using this and the fact that the average velocity of blood flow in the skin can be indicated by the ratio of high to low frequency components of the power spectrum of speckle intensity , Barton and Stromski used the amplitude information of the OCT signal only, to successfully use speckle analysis in OCT images in-vivo. [10,13] This process was further enhanced by Mariampillai et al. [14,15] leading to the speckle variance method we use today where the variance of the amplitude fluctuations between B-scans is calculated to visualize flow given by:
where M is the number of B-scans in the acquired MB-scan series. In the locations where the flow is present, the variance is higher than in the signal originating from static tissue.
The amplitude-decorrelation method uses the correlation as a metric to detect the changes in the OCT signal. The correlation function of the discrete, complex, OCT signal is given by:
The complex correlation can be used to visualize flow , however in categorising this method ,the following approach uses only the amplitude of the OCT signal to calculate the decorrelation :
Which can be written as equation (Y)
In the areas of flow, the decorrelation of the amplitude calculated between the B-scans is higher than in the static tissue, where it is caused by the noise. A high decorrelation value, therefore, implies the presence of vessels in the imaged tissue. To improve visualization and reduce background noise from normal small eye movements, the split spectrum amplitude decorrelation technique was developed. In 2012, Jia et al.  proposed a split-spectrum amplitude-decorrelation angiography (SSADA) algorithm to extract flow signal and distinguish functional vessels. The full OCT spectrum is split into several narrower bands and the data in each newly split band is processed separately to generate several flow images for the same B-scan location. The Split spectrum method has been shown to improve the signal-to-noise ratio of flow detection [18,19]. However, the resolution in the axial direction is reduced, typically ~3 times lower resolution compared with conventional decorrelation approach. On the other hand, with a reduction of axial resolution, it diminishes the sensitivity to pulsatile bulk motion in the axial direction. The same is also true to the sensitivity of detecting flow though. By increasing the number of split-spectrums, the decorrelation signal-to-noise ratio can be improved without increasing the scan acquisition time. The optimized number for split-spectrum was found to be 9 [19,20].
The phase variance method is related to the emitted light wave properties, and the variation of phase when it intercepts moving objects, ignoring amplitude variations. The phase-variance method uses a variance of the phase differences, 〖∆∅〗_(v,m) occurring between B-scans to visualize flow [21,22,23]:
Fingler et al. carried out the first clinical study using the retinae of mice in 2008, showing that phase-contrast OCT could enable three-dimensional visualization of retinal and choroidal vasculature in vivo. Following this success, the technique was adapted for human retinal imaging. Swept-source based phase-variance OCTA system were demonstrated by Motaghiannezam et al. and Poddar et al. , enabling the visualization of vessel networks in the choroid.
Due to its ability to display both structural and volumetric blood flow information, optical coherence tomography angiography has found an application in many different fields of medicine. Main research focus is put on ophthalmology – but there is an increasing research conducted also in the field of dermatology – as well as in neuroscience  or gastroenterology .
Though OCT appliances have been widely used as an imaging tool in ophthalmology since the first
FDA-approved system in 2006 , OCTA as an extended system simultaneously gives an opportunity to quantitatively investigate microvascularization in vivo in the optic nerve region . In ophthalmology the standard procedures to assess the retinal circulation in vivo are fluorescein angiography (FA) and indocyanine green angiography (ICGA) . Opposing to both of these invasive methods, optical coherence tomography angiography does not require an injection of a fluorescent dye to the circulation system  which could lead to some serious complications , . Though it has smaller field of view, relatively more artefacts and cannot detect a leakage from a blood vessel, the acquired images by OCTA have higher resolution and can provide information about particular layers of both retinal and choroidal vasculature . In general, it is able to visualize more detailed vascularization of the optic nerve region like for instance the deep capillary networks (Figure 1) .
There are several commercially available optical coherence tomography angiography applications.
As mentioned above, in oppose to FA and ICGA procedures, OCTA does not require any dye so the risk of side effects is minimised. Moreover, it is much faster and easier to perform without an intravenous injection. OCTA applications enable non-invasive in situ examination performed by a single clinician . In 2017 there were four significant commercially available appliances performing optical coherence tomography angiography in ophthalmology. In the cross sectional study all of them were characterized by some strengths and weaknesses, but on the whole they all were affected by motion artefacts . Though there are already in use, their fully capabilities are yet to be improved and investigated.
Optical coherence tomography angiography enables expanding knowledge in a field of a retinal and choroidal anatomy but also in a pathophysiology of various eye disorders . Microvascularization in the optic nerve region for years has been regarded as a possible factor in the pathophysiology of for example glaucoma , , . Thus, nowadays microvascularization is being investigated using OCTA to propose a novel diagnostic and prognostic tool of a glaucoma disease . It was proved that OCTA vessel density give the same results in a distinguishment between healthy and glaucoma eyes in vivo as the standard retinal nerve fiber layer (RNFL) thickness measurements . By measuring capillary perfusion density non-invasively and dye-free OCTA gives very promising results in the glaucoma progression assessment and diagnosis . Apart from glaucoma, OCTA has been investigated as a new tool in evaluating the stage of a diabetic retinopathy  as well as of a non-arteritic anterior ischaemic optic neuropathy (NAION) .
Optical coherence tomography angiography has also been the subject of research in a clinic dermatology. In that field the standard diagnosis is made through observing the abnormality of the skin, studying the medical history of a patient and performing histopathological tests on a part of the abnormal tissue
in a lab . The sample is collected during an invasive biopsy, which is an inconvenient and stressful procedure for the patient .
Though there are some non-invasive techniques of skin layers imaging but either they are of high resolution (∼1m) but low penetration (∼150 m) such as confocal microscopy  or of deep penetration (∼1.2 mm) but not adequate resolution (∼20 m) like high resolution ultrasound imaging . High resolution ultrasound imaging is a preferable method as ultrasounds are harmless and the cost of the procedure is low. That technique enabled visualization not only all three layers of the skin on a part of a forearm but also microstructures such as hair follicles and sebaceous glands .
Optical coherence tomography angiography not only has a resolution of (∼1m) and penetration depth of (1-3 mm) but also gives the possibility to visualize microvascularization of the skin simultaneously . It would be highly useful to be able to perform a histopathology of the abnormality in vivo without the need of biopsy. Not to mention increased comfort for the patient and decreased time needed for an examination. Although biopsy would still be the most reliable tool in diagnosis, probably in many cases could be avoided.
The big obstacle in imaging the layers of the skin is the attenuation and scattering of the light beam by the tissue. Using an optical clearing agent could solve that problem. The recent study has shown that a mixture of fructose with thiazone and PEG400 decreased the scattering coefficient in mouse dorsal skin during OCTA examination . That caused improved blood flow imaging with increased contrast and resolution of the deeper parts of the skin. The possibility to analyse the cutaneous hemodynamics gives a chance to study and diagnose skin pathology in a faster and more reliable way.
e microvasculature up to capillary level and visualize the remodeled vessels around the acne lesion
mage microvasculature up to capillary level and visualize the remodeled vessels around the acne lesion
First use of OCT microangiography in vivo on human facial skin to assess changes in microvascularization around acne lesion was performed in 2015 . It was suggested that in the future vascular density changes could become biomarkers of human skin abnormalities and diseases. Since then the feasibility of OCT angiography in dermatology started to be vastly investigated as a diagnostic or prognostic tool in different human skin diseases.
In 2018 Deegan et al.  imaged different using OCTA pathological skin conditions. When imaging inflammatory conditions in Cutaneous Graft-versus-Host and psoriasis diseases, vascular density was increased with correlated with higher metabolism and vessel recruitment during inflammation . Moreover, they imaged changes in vascularization of nails folds at different stages of systemic sclerosis as shown on Figure 3. It was concluded that OCTA seemed to be a promising and worth further research tool to assess subsequent stages of a Cutaneous Graft-versus-Host, psoriasis and systemic sclerosis based on anatomy and microvascularization of specific regions .
Figure 5 Differences in en face vascular images of healthy (A, D), early stage scleroderma (B, E) and late stage scleroderma (C, F) nails folds. D, E, F are corresponding B-scans to the central regions of A, B, C images respectively identified by white dashed lines. Scale bar = 1 mm.
The investigation of cutaneous wound healing process also becomes possible using an optical coherence tomography angiography . The microvascularization around and inside a wound at different stages of healing can be visualised and quantified. That might enable more effective therapy strategies and assessing the direct impact of different growth factors on wound healing .
Optical coherence tomography angiography might enable precise analysis and diagnosis of the skin without tissue histopathology and uncomfortable biopsy. It is still a developing technique although very promising. Moreover, in a field of dermatology due to increasing importance of everyday appearance, people are willing to bear higher cost in order to improve their look in a more sufficient and faster way.
Optical coherence angiography remains the still evolving technique in the disease diagnosis and prognosis. In ophthalmology it is quite well developed but still the most common commercially used OCTA applications in ophthalmology are affected by the motion artefacts . In that field, probably the biggest focus in the future will be put on an attempt to minimalize their influence.
In dermatology on the other hand, there is still no reliable correlation between microvascular changes and pathology yet. That issue will probably be widely investigated in the following years. Furthermore,
the attenuation of the light beam by different layers of the tissue must be resolved. Different kinds of optical clearing agents that would increase the quality of the imaging are expected to be examined as well.
On the whole, OCTA technology gives a possibility of diagnosis for more people. For instance to those allergic to dyes used in a traditional angiography.
At the beginning the cost of OCTA procedure probably will be higher than a standard one. However, over time with more advanced applications and without a cost of a dye, the examination might become very price-affordable.
Moreover, no need of an invasive procedure like biopsy or intravenous injection will definitely increase the comfort of the medical investigation for both clinicians and patients, especially children.
Another advantage would be that the time of getting the final results or diagnosis should be much reduced. That could probably reduce waiting time for a medical appointment in public healthcare in many countries.
With further research and resolving some problems like motion artefacts optical coherence tomography angiography is likely to become a routine examination. Probably in the nearest future its applications will be considered in many more areas of medicine as an emerging diagnostic and prognostic imaging tool.
Data processing and image acquisition is currently a post-scanning, multi-step process; however, an end goal would be to streamline procedures to the point where clinicians could make accurate, real time assessments simply, without compromising on detail or volume of gatherable information.
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