Photoacoustic Imaging For Medical Diagnostics

Introduction to Medical Imaging

Discuss about the Photoacoustic Imaging for Medical Diagnostics.

Medicine imaging is study of organs and body parts that are under layer of skin, this study provides knowledge about the problem in body that cannot be seen with naked eyes early detection of the problem is possible with help of imaging, medicine imaging has contributed maximum towards maximum science. This has helped many people to get better treatment from dangerous diseases (WHO, 2016).

There are several methods used for imaging in medicine.

Microscopy- Any object that cannot be seen with naked eye. Microscopes help to magnify the objects very clearly by collection of scattered radiation or other signals that are helpful in forming images. There are various microscopies known such as optical, electron and scanning probe microscopy (Goodhew, 2001).

X rays- use of x ray is the oldest technique in medical imaging, it is form of electromagnetic, ultraviolet infrared radiation, visible light and microwave, usually x rays are commonly used in medical application, for imaging in medical to diagnose health related problem (Lucas, 2015).

Sonography- sonography is also known as ultrasound, second and oldest most used in this area,

Computed tomography ( CT)- CT scan make use of computer with x ray from different angle to scan body, CT is always referred with x ray, there are many other type of computed tomography used like PET, SPECT.

Magnetic resonance imaging (MRI)- Formation of organ parts and body parts outside the body with help of magnetic wave and radio waves, this instrument help to see problem inside body especially in complexes organs such as brain, chest, blood vessels, spine and abdomen (WebMD, 2005).

Positron emission tomography (PET) - This is other type of computed tomography used very frequently today, scanning of body parts are done with help of positron emission and formation of body structures.

Functional MRI (FMRI) – This is very new technology used, it provides scanning of flow of blood in brain by using MRI.

These are few medicine imaging used, the purpose of using these techniques is to know about the functions in body, before all this methods had come into use the diagnosis of diseases that required surgery to know disorders by opening the cavity.

Confocal microscopy reduces blurring of image from light scattering, this also provides effective resolution and thick specimens get clear examination and magnification can be adjusted electronically. 0- 1000 beneath specimen surface, it is used for three- dimensional fluorescence imaging but confocal has some limitation, when the light generate fluorescence throughout the sample in that case confocal can damage whole specimen and even can bleach (Grand & Bonfig, 2016). There is an alternative for confocal microscopy that is two-photon excitation microscopy the main advantage of using this is that it provides three dimensional and deep tissue imaging. Confocal microscopy is limited by sample scattering because of penetration depth. In medical imaging the use of instrument is very high day by day the improved version is modified. Confocal microscopy is more advanced than converstional that is very improved and used in making out the problem; it is used in measure corneal thickness (Popp & Valentine, 2003).

Different Techniques for Medical Imaging

Optical imaging has advantages but indeed of having advantages it also has limitation, to avoid scattering of light in In vivo in almost all living tissue the elastic scattering of light will occur, that will give very limited ability of formation of high resolution image formation. Optical imaging has huge advantage in medical science if and only the limitations that can occur are due to penetration power of these into living tissue. In human body there are lot of substance that cause major penetration deep into living tissues that are water, hemoglobin (oxy & deoxy) and melanin, these substances causes absorption of light at wavelength between 690 and 1000 nm, in this range the light used provide main factor of limiting photons crossing thick tissue, example of thick tissue is human brain in this the tissue not only absorb but also scatter light. Different tissue has different scattering properties of light. The substance such as blood and cerebrospinal fluid these scatter very little amount of light and white matter that scatter very high amount of light. Optical imaging provide interaction of photon with human tissue,

Figure showing three electromagnetic transmissions. The graph is plotted with attenuation on y axis and wavelength on x axis. The graphical representation of all the medical image are drawn with reference to wavelength optical depth that provide justification of electronic absorption is at higher peak and there are many electromagnetic radiation at different wavelength.

In this figure, it clearly shows about frequency and wavelength in ultrasound. Penetration and resolution intersect at a point that is clearly mention to be 0.62 source by: Pagkaliwagan

 In this graph, the entire devices are plotted. This indicates about the maximum resolution of MRI after nuclear.

 X ray CT has limits in term of resolution that is about 1000-2000x the object in cross section diameter, only small object give high resolution, if required for finest resolution that can cause blurring of material boundaries that means the boundaries of materials get blurred if finest resolution is used (Ketcham, 2016). If the object is too thick, dense or large it would absorb maximum energy. The use of high resolution x ray computed tomography for small animals. This is also very effective for soft tissue, example- thorax of mouse with resolution image obtain is 90 kVp x ray source. X ray ct is merge of ct scan and x ray properties that provide more effective result, it provide more accurate image resolution in giving the accurate information about the problem and disorder in body. Using of medical image on high level provide harmful effect on human body by giving harmful radiation that actually harm the body organ, taking example in cancer the use of radiation cause severe effect on body. That too has very adverse effect on the biological system for long duration. Exposure to radiation cause very adverse effect but it has very important for studying the organ parts to actually get clear idea about the body disorder and proper treatment can be provided.

Limitations of Optical Imaging

The operation of a photoacoustic imaging system that inform about conversion of light to sound this is specifically called photoacoustic effect it was introduced by Alexander Graham Bell in 1880 (Bayer, Luke & Emelianov, 2013). Tissue is exposed to high energy, using this technology the result obtained is deeper penetration in tissue, it is more advance than traditional optical imaging techniques,

In this figure, the diagram complete process of photoacoustic, this diagram provides complete explanation about the use and process of instrument. Experiment is conducted on a mouse that is placed in water tanker with temperature of 37 degree; anesthesia is given from other side.

Tissue is exposed to pulse near infrared laser. The tissue that contains hemoglobin, lipid, water and collagen absorbs light and undergo thermoelastic expansion. In the process, materials absorb light energy, and the light is converted to heat when material is heated, expand in size due to thermoelastic properties: it generate pressure wave. The term photoacoustic specifies the conversion of light into heat that results in formation of sound waves.

In experiment conducted by Bell, he explained that darker fibers produced louder sounds than lighter fibers; it is used in generation of medical images of biological chromophores typically present in tissues.

The use of photoacoustic effect is very much in medical field in diagnosis of vasculature cancer cardiovascular and tissue engineering scaffolds.

To detect Melanoma and determine tumor depth, handheld photoacoustic microscopy (PAM) is developed, melanoma is malignant tumor of melanocytes this is type of cancer and today this is fifth most common cancer in United States, in 2013 nearly 76,690 men and women were reported with this cancer (Zhou, 2014). Melanoma cancer is rare type of cancer but very serious than other type of skin cancer. In short this cancer occurs on skin, when skin is exposed maximum to ultraviolet rays therefore protection of skin is required (American cancer society, 2016).

Measuring the thickness of melanoma by photoacoustic imaging

Both photoacoustic and ultrasound imaging are used in measuring the thickness of melanoma at different stages. Melanoma based on thickness is classified into four different stages

T1 (<1.0mm) T2 (1.01-2.0 mm) T3 (2.01-4.0 mm) T4 (>4.0 mm) earlier the measurement method used was sonography. Photoacoustic has taken over sonography because of poor definition given by sonography. Major disease such as cancer are diagnose by medical imaging and mainly by use of photoacoustic effect to get best result and image quality. This specific cancer is very harmful for body and even causes death of human therefore proper diagnosis of melanoma which is skin cancer is important on accurate time treatment is provided, but for the treatment diagnose is initial part. The study of this cancer is done by photoascoustic, this brief clear idea about stages of cancer and how large or big that cancer has, in melanoma there are four stages that clearly have different measurement and thickness therefore the penetration of radiation through the lesion is required.

Principles of Photoacoustic Imaging

Tissues shape size and distribution play very important role in scattering of light. Considering photoacoustics that has benefits over ultrasound. Both photoacoustics and ultrasound are used to image cancerous tissues. Now photoacoustic imaging has capacity of detecting bioconjugated gold nano particles in cancerous tissues, ultrasound has ability to image photothermal therapy by regulating temperature. In hypoechoic tumors temperature mapping is done by photoacoustic but it is not possible with ultrasound (Wang, 2009). Photoacoustics is more advances than ultrasound because it use light waves to enter into tissue and well study for the cancer tumors, ultrasound is used in minor area such as in pregnancy that shows photoacoustic is more used in screening of medical image than ultrasound. Penetration of radiation in tissue is required so that accurate result regarding the disorder can be achieved. Penetrations of radiation depend upon structure and complexicity of tissue, as discussed earlier about the complex formation of tissue that affects the scattering of light and penetration.

Optical coherence tomography is biomedical imaging technique with its highly outstanding axial resolution. The large tissue area are screen and diagnosed using this Fourier domain OCT, and focal biopsy. The advantages and application for this technology is endoscopic and laparoscopic, used more specifically are many hospital to conduct endoscopic, providing high resolution images. With medical application it also has non-medical application too.

This figure represents complete flow of FD OCT (Rolland, Lee, Johnson et.al 2006).

Advantages of using Fourier domain OCT is that it provide penetration into tissues. This technology provide the complete imaging and morphology of ophthalmologic, tomography of human retina is obtain from this modified technologies, this provide complete and wide use in study of human retina.

A- scan this is commonly referred to a scan ultrasound biometry. A scan is used in optometry and ophthalmology, this provides data on length of eye and this is common sight disorders. The main use of A-scan is to detect the size and ultrasound in eye, it measures the eye, A scan is useful in correct lens implant for cataract surgery, it also help to identify issue with eyes; example of eye diseases are tumors, foreign substance, detachment of retina, glaucoma, cataracts. Both A scan and B scan are for obtaining eye problem, in a scan ultrasonography very thin sound beam are emitted that passes through eye, if angle of incidence is considered in a scan the size and shape of surface is also affect that reflection. In particular about eye the shape and size are completely different in regarding to physiological structure, eye is considered to be very complexes organ, in reference to absorption ultrasound, it is absorbed by every medium through it passes there is greater absorption in dense medium, limitation of b scan is that it is only performed on open eye not on close eye, therefore the application of b scan is limited for small children. Use of a scan is done basically for the adjustment of eye lens before the surgery is conducted; the measurement of eye lens is done with use of a scan (Pietrangelo, 2015).

Applications of Photoacoustic Imaging

B scan and c scan, in study of b scan it is used in same as in a scan application for obtaining imaging for eye, a scan is done for the preparation of lens before surgery and b scan is also known as brightness scan, it is used in detecting the problem related to eyes, in case of children and worst condition like accident b scan is performed with close eyelid by applying gel over the lid and the screening is then performed, this is useful in obtaining tumors result if present in anterior orbit of eyes (Waldron, 2016).

C scan is done to determine the normal condition of orbital fat and optic nerves present in orbit of eye, both b scan and c scan provide study of eye, the ultrasound of eye is done by these both techniques, but difference here come is about the function of both the techniques, b scan carry the function of scanning eye. This is performed to study eye when there is no view because of opaque medium, basically the disorder of eye. B scan has high resolutions that actually develop for high quality of images. When the sound waves frequency increases, resolution also increase and then depth of penetration decrease, to obtain best result of resolution the examiner should adjust in saline.

Optical coherence tomography, it is new medical imaging with resolution in range and depth of imaging in mm range. OCT is based on principle of low coherence interferometer. Used in various biological application including gastro, cardiology tumor margining. Optical coherence tomography provides cross section images of retina it is very difficult to obtain images of choroid in any of the individual. OCT has both character and feature of microscopy and ultrasound. But currently OCT technologies is most advance that uses axial resolution range from 1 to 15 micrometer nearly 10 to 100 times better than ultrasound images (Drexeler, 2008, pp 4).

In biological tissue the formation of tissue is very complex that stop the penetration of light radiation into tissue, living tissue are made of very complex substances, a tissue is formed with many complex not only one complex, many polymers water complex of blood component all this give very complex formation of tissue and that eventually stop the maximum penetration into tissue, for deep penetration of any radiation into tissue, a instrument was developed.

One method of reducing light scattering in human tissue, before getting into detail answer one must have idea about light scattering, scattering of light into any medium depend upon the structure of that material, if the material is made of simple water component the scattering of light is more and in turbid solution the scattering less therefore in human tissue there are many component that make whole process very complex, light scattering in human tissue is complex process because of complexity of tissue structure, the method describe is Monte Carlo, this method is used for recording photon absorption and scattering, this specify the light transporting in tissue, methods used are calculation from Mie theory, the wavelength increase with decrease in scattering coefficient of tissue. Absorption coefficient is affected by the presence of chromophores; they are the tissue components that absorb light (Sandell, 2012). There are many agent used in reducing the scattering of light that is Monte Carlo, fiber optic methods, Monte Carlo is very effective method than fiber optic (Johns, 2005). Example the use of glycerol on in vitro on skin tissue is evaluated by OCT method (Vargas, Chan, Barton 1999).

Conclusion

Light scattering in tissue has advantages; the spectral slop and scattering intensity appear to be correlated but particularly in normal skin. The pigment present in skin called melanin does not affect scattering of light, it mainly affect diffusion (Zonios & Dimou, 2008). There is less scattering of tissues in organ like skin, brain and vessel. Weakly scattering such as cornea lens (eye tissues) is highly transparent (Tuchin, 2007). Light scattering can very quickly detect cancer cell or tumors given by journal in optical of society of America (Fiorentino, 2015).

The disadvantages of using the method of light scattering is that it provide less scattering of light in denser medium than in medium like water. Considering the medium of tissue that consists of many substances like blood, water, protein that consist of all these material it enhance the scattering of light. Scattering of light is depended on small spherical bodies (Boustany, Boppart & Backman, 2012).  

Reference

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