OPTO-ACOUSTIC BREAST IMAGING
Opto acoustic breast imaging is a new generation diagnostic tool currently in the clinical trial stage for diagnosis of malignant or benign breast lumps. According to a European journal of breast cancer around 4 million new cases of breast cancer are detected every year with almost 80 percent of them turning out to be benign on laboratory testing. It is difficult for radiologists to rule out malignancy at the imaging stage. The cost of conducting surgical or core needle biopsy for every suspicious lump outweighs the results especially in women coming from struggling economies. A low cost diagnostic tool such as opto acoustic imaging can help in the timely detection of breast cancer in the early stages and also help prevent and control the increasing mortality due to this disease in developing as well as developed countries.
About Opto Acoustic Breast Imaging
Opto acoustic breast imaging or photo acoustic imaging is the promising new diagnostic tool that can help solve many of the problems listed above. Opto acoustic image can be considered as an ultrasound image in which the contrast depends on the light absorption properties of the tissue and not on the mechanical and elastic properties of the tissue. As a result, it offers greater specificity than conventional ultrasound imaging. It has the ability to detect haemoglobin, lipids, water and other light-absorbing chromophores, with greater penetration depth than purely optical imaging modalities, which rely on ballistic photons. The opto acoustic imaging tool can also provide real time information about the tissue temperature, blood flow and oxygen content along with anatomy of the smaller arteries and capillaries. These structures can be visualized under high resolution and the images can be enlarged from micrometers to centimeters with scalable spatial resolution. In Opto acoustic imaging, laser wavelengths in the visible and near-infrared (NIR) part of the spectrum between 550 and 900 nm are most commonly used. The near infrared spectral range 600–900 nm has the greatest depth of penetration which extends up to several centimetres. 
This low cost technology is the result of widespread efforts to develop a tool that can be used as a replacement for biopsy. Opto acoustic imaging is not first line screening tool. Instead it can help do away with unwanted biopsies that are costly and time consuming. It is minimally invasive technique that can help diagnose whether the breast lump is benign or malignant with high accuracy. Although the diagnostic tool is still in the clinical trial stages, opto acoustic breast imaging promises to be a boon for curtailing deaths, due to breast cancer worldwide.
Opto-acoustic imaging is a perfect combination of light and sound. It is a blend of optical imaging and ultrasound. This functional imaging device sends pulses of short wavelength infrared light which is considered safe for the human body. The infrared light penetrates the breast tissue and produces sound inside the breast tissue. These sound waves are picked up by the transducer head that is placed over the breast being examined. The sound waves are then analyzed for their intensity and density. Different tissues absorb infrared light differently and the sound waves produced differ depending on the tissue density and also on the tissue health. This is what forms the basis of ultrasound technology and opto acoustic breast imaging. 
The healthy and diseased tissue vary in their metabolic rates, oxygen uptake and hemoglobin concentration in the blood vessels to that tissue. Studies have revealed that the tumor tissue has a higher metabolic rate as compared to healthy tissue. The pattern of contrast agent uptake can help to identify the healthy tissue from a diseased tissue. This difference forms the basis of any diagnostic tool that uses a contrast agent. The area of deoxygenated blood shows up as a brightly colored area in contrast to its surrounding which helps to localize the cancerous mass. Cancerous mass is known to deplete the blood of oxygen more than the healthy tissue. The pattern of light absorption and sound wave generation is used to create a map of oxygenated and deoxygenated blood and hemoglobin concentration that can help to distinguish between malignant and benign tissue. The wavelengths or the color of laser light chosen is based on their differential absorption by oxygenated and deoxygenated blood. 
Opto acoustic imaging is a functional imaging technique that can identify this difference in tissue by measuring the amount of oxygenated and deoxygenated blood used by the diseased tissue without the use of a contrast agent. The technology is a blend of new photo imaging technology and conventional ultrasound to produce functional and anatomical images of the breast respectively. The presence or absence of two main factors angiogenesis (formation of new blood vessels) and deoxygenation of circulating blood can help radiologists to confirm or negate a malignancy. 
About the procedure
The procedure involves directing near infrared light of 2 different wavelengths between 550nm to 900nm at the site of suspected malignancy. The breast is subjected to laser light. In case the lump is cancerous it will absorb more light than surrounding healthy tissue and get heated.  This will produce a momentary expansion of the tissues that will be propagated as broadband low amplitude sound waves. These sound waves are then picked up by a single sensor or array of sensors arranged in the transducer head that is placed on the breast. The sound waves picked up by the ultrasound transducer head are relayed to the computer to help generate two dimensional images of the area of interest.
By measuring the time of arrival of the acoustic waves and knowing the speed of sound, an image can be reconstructed in a way similar to a conventional pulse-echo ultrasound image formed. An important difference between conventional ultrasound (US) and photo acoustic image formation is that with the former, localization can be achieved by focusing the transmit beam as well as the receive beam. Whereas in photo acoustic image formation transmit beams can get scattered due to different tissue density beyond 1 millimetre because of which only the receive beam can be focussed. The signals are analyzed and reconstructed to develop brightly colored high resolution images.
Although photo acoustic and ultrasound image formation and resolution are essentially the same, the bases of image contrast are fundamentally different. An ultrasound image represents differential acoustic impedance mismatch between target tissues. Ultrasound image contrast therefore, depends on the mechanical and elastic properties of tissue. An opto or photo acoustic image represents the initial pressure distribution produced by the absorption of the light energy. This image can be taken to be proportional to the absorbed optical energy distribution, which depends on the optical absorption and scattering properties of the tissue. In fact, the property of optical absorption dominates and thus opto- acoustic image contrast is often said to be ‘absorption-based’.
As a consequence, photo acoustic imaging can provide greater tissue differentiation and specificity than ultrasound because differences in optical absorption between different tissue types are much larger than those in acoustic impedance. For example the strong preferential optical absorption of haemoglobin makes photo acoustic imaging particularly well suited for visualising the microvasculature, which can be difficult to visualize with traditional ultrasound owing to the weak echogenicity of micro blood vessels. 
Pro’s and Con’s compared to other diagnostic tools
There are many other methods used to screen and diagnose breast cancer, with each one having one or more major drawbacks. Few common diagnostic tests include blood marker test, breast MRI, CT scan, PET, Chest X-ray, mammogram and ultrasound. Mammogram and ultrasound are the more commonly used tests.
Let us understand the merits and demerits of these tools. During a mammogram, the breast to be examined is compressed between two plates that are attached to a camera. The camera takes two pictures from two different directions. The procedure is repeated on the other breast too. The test is time consuming as it requires 20 minutes for each breast. Also the test can give false positive test in young women who have dense breast, active milk glands and a lot of glandular tissue. Sometimes the tumor may get missed because of dense breast tissue leading to false negative test. It is more accurate in older women. The procedure is uncomfortable and painful at times. During an ultrasound examination, high frequency sound waves are targeted at the breast. The reflected waves are converted into meaningful images. Though the test doesn’t involve any ionization radiation, it can only detect whether the mass is solid or fibroadenoma or a cyst. It cannot determine whether the lump is malignant or not. 
Advantages of Opto acoustic breast imaging technique
All the above demerits are absent in the opto acoustic imaging technique. It is a non-invasive, cost effective and easy to administer functional imaging test. It does not require appointing highly trained personnel. Radiology technicians can be trained in a very short time. The diagnostic tool is safe for the patient as well as the technician as no ionizing radiations are emitted. There is no need to use a contrast agent or dye. The procedure does not cause any discomfort to the patient and is not time consuming too.
Another benefit of using functional imaging like opto acoustic imaging is that the results are accurate and precise. Since the laser light travels deep, it can help the technician to analyze the location and size of the lesion, which can be added to the results from ultrasound, to get a clear picture. The ultrasound imaging helps to identify the shape of the lesion. The opto acoustic imaging can help to analyze the mass and the area under concern. In addition opto-acoustic imaging is highly specific and sensitive which helps in providing accurate diagnosis. It uses the specificity of light imaging and sensitivity of ultrasound testing. The images obtained are also high resolution ones which make the diagnosis easy. The tool is sensitive enough to pick up very small tumors also. Ongoing trials have also showed that opto acoustic imaging has a very high detection rate and can detect tumors as small as 2 mm. In fact it can detect tumors smaller than this which means highly malignant tumors can be detected very early. 
The test can be performed in younger as well as older women with almost no chance of false positive or false negative test. The breast tissue density, presence of milk glands and glandular tissue does not affect the accuracy of the test. It can not only detect a lump with precision but can also differentiate a malignant growth from a benign one.  The investigation procedure is comfortable for the patients as they can lie down during the procedure. The array of sensors can be arranged over the area of interest in fixed position. There is no unnecessary compression and discomfort felt by the patient.
The current clinical trials conducted for this technology reveal that opto acoustic breast imaging can help separate a group of people having a breast lump but not requiring biopsy from another group having breast lump and requiring biopsy. If these trials are successful and consistent it can help to eliminate the need for unnecessary biopsies thus saving big on diagnostic costs of breast cancer.  Researchers are also trying to establish positive correlation between the images obtained and stage of the cancer identified on laboratory analysis. 
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