Medical Physics Photoacoustic and Ultrasound

Question: Discuss about the Medical Physics Photoacoustic and Ultrasound. Answer: Introduction: In human tissue, confocal microscopy has the potential of penetrating deeper inside the tissue. Experimental studies suggest that for human skin confocal images has the lateral resolution 0.5-1.0 m along with axial resolution of 3-5 m of section thickness. A maximum depth of 350 m over the field of view having range of 160-800 m is possible with the imaging (Assayag et al. 2013). The dominant physical mechanism that limits this imaging technique consists of Raman spectroscopy. Sample identification and quantification may be done through Raman spectroscopy and molecular vibrations. It is generally utilized for the purpose of observing vibrational, rotational and other low frequency modes in a system. It is dependent upon inelastic scattering that utilizes the monochromatic light for dispersion from a laser source in the range of visible, near infrared or near ultraviolet rays. Interaction of the laser light with molecular vibrations or other excitations present in the system serve to cause shifting of the energy pertaining to the laser photons. Thus the opacity of the material to be examined and the free working distance concerning the objective lens determines the depth of penetration underneath the surface. Diffraction limited axial focus dimension is the primary determinant for optical sectioning. Hence the limitation to optical resolution is provided by the diffraction of the visible light wavefronts as they travel the path of circular aperture located at the rear focal plane of the objective (Fouquet et al. 2015). Scattering in human tissue may be effectively minimized and avoided by the application of fluorescence anisotropy imaging technique. It consists of the non-invasive measurement of the biological tissue function and metabolism by means of directing the polarized light onto the tissue by means of stimulating the emission of fluorescence because of the presence of one or more endogenous fluorophores inside the tissue thereby allowing for the measurement of the fluorescence anisotropy (Zuckerman 2015). The phenomenon accounting for the light emission through a fluorophore of varied intensities along different axes of polarization is generally termed as fluorescence anisotropy. The extent of polarization as brought about by virtue of emission is generally referred to as anisotropy. In biological systems this technique has wide range of applications by means of offering quanitification for protein denaturation, protein association with other molecules, measurement of the dynamics of the pr otein, estimation of the viscosities of the membrane, investigating the influence on membrane fluidity. Rotational motions and resonance energy transfer seem to positively impact the imaging technique while factors like light scattering, reabsorption, misaligned polarizers in conjunction with homo resonance energy transfer serve for decreasing the anisotropy. The design of the imaging optical instrument consists of an inverted microscope, two calcite prisms carefully placed in the excitation and emission optical paths as well as a CCD camera that are effectively utilized to study the unique blinking effect concerning the quantum dots as well as the binding of the fluorescently labeled molecules with the proteins, DNA, RNA in cells (Devauges et al. 2012). The X-ray CT refers to a nondestructive method meant for observing the internal characteristics relevant to the solid objects, and in order to procure digital information regarding their 3-D geometries and pertinent properties. Directing the X-rays at an object from several orientations and measurements carried out with respect to the decr4ase in intensity along a series of linear paths forms the basis in tomographic imaging. The reduction is supported by Beers law that denotes the intensity decrement as a function of X-ray energy, material linear attenuation coefficient and path length. After that following a distinct algorithm reconstruction of the distribution of X-ray attenuation in the volume under consideration is conducted. Although the X-ray Ct technique possesses a number of strengths, yet there are certain limitations that restrict the smooth operation of the instrumentation. Resolution in X-ray Ct is normally limited to approximately 1000-2000X of the cross section diamete r of the object and high resolution is generally conferred by small objects. Blurring of the material boundaries is normally conferred by finite resolution. The main reason behind this limitation is offered by the low energy X-rays that are not capable of penetrating into the large geological specimens thereby decreasing the resolving power. Disparate attenuation contrasts also account for poor imaging in certain cases. Further polychromatic X-rays pose to complicate the calibration of gray levels to the attenuation coefficients (Cnudde and Boone 2013). In biomedicine, photoacoustic imaging system has emerged as a novel imaging technique to study the effects in researches of fundamental, preclinical and clinical nature. The working principle for this technique consists of the application of photoaxcoustic effect. Molecular vibrations because of transient thermal absorption are responsible for generating ultrasound in soft tissues with the aid of an ultrasound pulsed laser having the power limited well below the limit of safety. Reconstruction of the initial pressure orientation in the object accounts for the visualization of the inner structure. In simple words this imaging technique takes into consideration the sound produced by the light. This technique combines the essential properties encompassing high resolution, optical contrast and deep penetration ability in comparison to other optical imaging methods by virtue of utilizing the less scattering of ultrasound in case of the biological structures compared to that of protons (Ch en et al. 2014). Photoacoustic imaging relying on non-invasive imaging procedure is beneficial for functional, structural as well as molecular imaging. Conversion between light and acoustic waves following absorption of electromagnetic waves in combination with localized thermal excitation that accounts for the photoacoustic effect forms the basis of this method. Thus it can be said as an effective amalgamation of optical absorption combined with ultrasonic wave propagation making use of the high contrast and high resolution of optical imaging and ultrasound imaging respectively (Wang and Hu 2012). In case of clinical studies offering screening for melanoma cancer photoacoustic imaging and more specifically photoacoustic microscopy coupled with photoacoutic tomography is utilized to facilitate the measurement of melanoma depth. The purpose of such technique emphasizes on clinical diagnosis, prognosis as well as surgical planning for patients with melanoma (Zhou et al. 2014). The photoacoustic tomography consists of illuminating a semitransparent sample by means of an expanded laser beam thereby causing brightening of the entire volume of the sample. The spatial variation in local absorption accounts for the production of ultrasonic waves that are in turn recorded by means of an ultrasonic transducer. After that, the movement of the transducer around the sample or through usage of an array of transducers, a set of data relevant to pressure curves is obtained. Thus by virtue of using appropriate and proper reconstruction algorithms, the absorption of light as occurring within the sample may be reconstructed. Contrarily, for photoacoustic microscopy focusing of the laser beam is restricted to a smaller volume thereby accounting for greater and better axial and optical resolution. Thus depth information of the melanoma cells may be acquired by evaluating the runtime of the acoustic waves. Characterization of tumor angiogenesis due to high resolution and high contrast supported by high resolution and high contrast property thus renders efficacy of this technique employing integration of detector with sound light coaxial or confocal design in addition to flexible coupling mode (Wang et al. 2016). Photoacoustics comprise of a vast range of biomedical applications that are applicable to clinical studies also to offer diagnosis and cure in certain cases to the patients. One such application of photoacoustics that is not possible with ultrasound alone refers to its use in dosimetry during thermal therapy. As per the research findings, thermal therapy has been recognized as a novel and effective cancer treatment by means of which the body tissues are exposed to high temperatures that are usually capable of damaging and killing the cancer tissues thereby incurring minimal injuries to the normal tissues. Referring to a recent study where both in vitro and in vivo mapping with respect to drug release was done after conducting laser ablation thermal therapy by means of doxorubicin loaded hollow gold nanoshells utilizing fluorescence and photoacoustic imaging showed promising results to estimate Dox release and monitor temperature. In this context, the photoacoustic imaging served as a n important indicator for detecting the elevation in tumor temperature. The principle of photothermal energy is utilized in the process that consists of conversion of the light energy to heat by tumor specific absorbers. In course of the therapeutic procedure thermal maps are computed by virtue of monitoring the temperature induced changes as occurred in photoacoustic signal (Lee et al. 2013). Fourier domain optical coherence tomography Fourier domain optical coherence tomography (OCT) requires reference arm and optical way. The length varies between reference reflection and sample is encrypted by interferometric fringes frequency as a functional source of spectrum. Fourier domain is of two types like Spectral domain OCT and Swept source OCT (Li et al. 2012). Spectral domain OCT can use a grating to disperse spatially spectrum across array type detector. Advantages: It helps to provide a huge amount of data exponentially. With the huge data one can watch the entire macula and make clear 3D images. 3D imaging: SD OCT helps to scan the 3D images to generate information from the image. It helps to check the relationship between macula and vitreous. By moving the angle of SD OCT, one check the 3D image from different perspective (Podoleanu 2014) Disadvantages: The main disadvantage of SD OCT is the movement of mirror, which is present on the reference arm and the speed is also very slow. However, the A scan depends on the speed of the SD optical coherent tomography (Li et al. 2012). Explanation of A-Scan A scan is mainly used to collect the ultrasonic data. A scan shows the ultrasonic energy that is received. To illustrate the A scan presentation, the initial pulse can e generated by transducer to represent the signal IP (Kraus et al. 2012). The signal IP is set at zero. With the scanning of transducer along the surface of that part, another signal appears on screen at different time. If transducer is in the left position, the IP signal and A signal reflect form the surface A. This will be watched on trace. A scan display with many instruments allows signals to display the natural radio frequency form to rectify the RF signal. In A scan presentation, comparison of signal amplitude can estimate the relative discontinuity size. Signal position can determine the reflector depth on horizontal sweep (Kraus et al. 2012). Transducer can be scanned properly from back wall of the signal, BW appears lately in time. BW shows the travelled sound to reach the surface. Explanation of B and C Scans B scan presentation is a cross sectional profile view of test specimen. In B scan, the fight time of energy sound displays the horizontal axis. From B scan, the reflector depth and approximate dimension in scan can be obtained (Mathieu et al. 2013). By establishing trigger gate on A scan, the B scan can be produced typically. When the intensity of the signal is greater to trigger gate, specific point made on B scan. Sound reflecting trigger the gate form back wall of specimen and smaller reflector also helps it within material. C scan presentation gives a plan to view the size and location of test specimen. The image plane is parallel to transducer scan pattern (Suvarna et al. 2014). C scan presentation is made with automated information acquisition system like immersion scanning of a system that is controlled by computer. Mainly the data collection gate is made on the A scan and at regular interval the signal time of flight is recorded. The signal amplitude displayed the gray shades or colour for every position to record the data. The C scan presentation can give an image of feature, which reflect the sound and scatter it within or on surface of test piece (Suvarna 2014). Limitation of optical coherence tomography Optical coherence tomography has some limitation like poor penetration power and disability for measuring plaques. The location is aorto-ostial location, which is difficult for assessing the technological current stages (Podoleanu 2014). Another limitation of optical coherence tomography is penetration depth. Media opacities limits the OCT by dense cataracts. It leads to vitreous haemorrhage and errors in the RNFL. The retinal layer segmentation also became affected by optical coherence tomography. Every scan can be taken more in ranges and also in focus. It should be examined for the motion artefacts and blinks. The computer software with the correlation of OCT is axial motion. OCT is unable to visualise if the central nervous number is active. The neovascular network became inactive due to active CNV (Li et al. 2012). In isolation, OCT images may not be interpreted. It can be correlated with red free optical coherence tomography fundus image. It can include photography or ophthalmo scopy. Scanning circle of the optic disc can be difficult in service users with abnormal disc outline. Light scattering in human tissue may be reduced by means of adopting a number of imaging techniques. One such technique is near infrared (NIR) imaging and spectroscopy method. It is capable of monitoring the changes in the biological tissues by means of utilizing the light in the near infrared region and belonging to the electromagnetic spectrum from about 650 nm to 950 nm. Functional brain activity monitoring is carried out by means of NIR imaging and spectroscopy (Boas and Franceschini 2009). Measurement of optical absorption is detected by means of NIR through detection of specific chemical bonds such as O-H, N-H and C-H. The source of illumination is generally provided by the quartz halogen lamps and a two dimensional array is responsible for capturing the images. The array eliminates the need for moving the sample relative to the position of the detector. The inelastic scattering of light is utilized in this imaging technique where the laser light interacts with the molecular vi brations, photons or other excitations in the system that culminate in the shifting of the energy of the laser photons that in turn provide information regarding the vibrational modes of the system (Ferrari, Mottola, and Quaresima 2004). The prime advantages of using this method of NIR imaging and spectroscopy include lower need for minimal to no requirement for sample preparation. Deeper sample penetration is another benefit of this method. It is capable of measuring simultaneous measurements of various constituents and having wide range of application. High scan speed and high resolution are other utilities (Manley 2014). Applications of NIR include assessment of a wide range of constituents like organic and inorganic materials. Both quantitative as well as qualitative results may be obtained by virtue of NIR without having any phase constraints. NIR is applicable to a safe and rapid working environment devoid of any destruction or contact. Majority of the active ingredients in pharmaceutical industry as well as excipients are capable of absorbing NIR radiation thereby complementing the assay pertaining to the active ingredient through provision of homogeneity information related to all mixture components. Objects remaining in the dark areas may be evaluated by means of NIR radiation. Portability of the device is another advantage of NIR imaging. Minimal assay time in conjunction with less time requirement associated with powder blend homogeneity. Gas chromatography and high performance liquid chromatography applications generally take resort of NIR imaging in using chemicals and producing chemical waste (Scholkmann et al. 2014). Unlike any other analytical method, NIR also has its own limitations. Firstly high cost range of the equipment is a major disadvantage and often it poses difficulty in correctly interpreting the results when directly placed upon certain objects. Further variations in emissivities and reflections generally hinder the accurate temperature measurement. It is only possible to directly detect the surface temperature by virtue of utilizing the NIR technology. Maintenance of sensors at the required temperature also become difficult while thermal image interpretation also seems to be difficult. The molecular weight of the substance cannot be estimated by applying the NIR method. Beers law of complexity spectra are frequently not followed in case of NIR application. For each sample separate set of calibrations are necessary to conduct the measurements that account for high expenditure of the equipment. Moreover the relative positions of the various functional groups in a molecule are generall y not provided through NIR. Distinction of pure and mixture compound is not possible by means of NIR technology (Weitzel et al. 1996). References Assayag, O., Grieve, K., Devaux, B., Harms, F., Pallud, J., Chretien, F., Boccara, C. and Varlet, P., 2013. Imaging of non-tumorous and tumorous human brain tissues with full-field optical coherence tomography.NeuroImage: Clinical,2, pp.549-557. Boas, D. and Franceschini, M.A., 2009. Near infrared imaging.Scholarpedia,4(4), p.6997. Chen, Z., Yin, J., Zhou, Q., Hu, C., Yang, H.C., Chiang, H.K. and Shung, K.K., The Regents Of The University Of California, 2014. Ultrasound guided optical coherence tomography, photoacoustic probe for biomedical imaging. U.S. Patent 8,764,666. Cnudde, V. and Boone, M.N., 2013. High-resolution X-ray computed tomography in geosciences: A review of the current technology and applications. Earth-Science Reviews, 123, pp.1-17. Devauges, V., Marquer, C., Lcart, S., Cossec, J.C., Potier, M.C., Fort, E., Suhling, K. and Lvque-Fort, S., 2012. Homodimerization of amyloid precursor protein at the plasma membrane: a homoFRET study by time-resolved fluorescence anisotropy imaging. PLoS One, 7(9), p.e44434. Ferrari, M., Mottola, L. and Quaresima, V., 2004. Principles, techniques, and limitations of near infrared spectroscopy.Canadian journal of applied physiology,29(4), pp.463-487. Fouquet, C., Gilles, J.F., Heck, N., Dos Santos, M., Schwartzmann, R., Cannaya, V., Morel, M.P., Davidson, R.S., Trembleau, A. and Bolte, S., 2015. Improving Axial Resolution in Confocal Microscopy with New High Refractive Index Mounting Media.PloS one,10(3), p.e0121096. Kraus, M.F., Potsaid, B., Mayer, M.A., Bock, R., Baumann, B., Liu, J.J., Hornegger, J. and Fujimoto, J.G., 2012. Motion correction in optical coherence tomography volumes on a per A-scan basis using orthogonal scan patterns.Biomedical optics express,3(6), pp.1182-1199. Lee, H.J., Liu, Y., Zhao, J., Zhou, M., Bouchard, R.R., Mitcham, T., Wallace, M., Stafford, R.J., Li, C., Gupta, S. and Melancon, M.P., 2013. In vitro and in vivo mapping of drug release after laser ablation thermal therapy with doxorubicin-loaded hollow gold nanoshells using fluorescence and photoacoustic imaging. Journal of Controlled Release, 172(1), pp.152-158. Li, Y., Tan, O., Brass, R., Weiss, J.L. and Huang, D., 2012. Corneal epithelial thickness mapping by Fourier-domain optical coherence tomography in normal and keratoconic eyes.Ophthalmology,119(12), pp.2425-2433. Manley, M., 2014. Near-infrared spectroscopy and hyperspectral imaging: non-destructive analysis of biological materials.Chemical Society Reviews,43(24), pp.8200-8214. Mathieu, C., Hare, R.D., Jones, D.N., Babiak, P. and Neumann, C.S., 2013. Factor structure of the B-Scan 360: A measure of corporate psychopathy.Psychological Assessment,25(1), p.288. Podoleanu, A.G., 2014. Optical coherence tomography.The British journal of radiology. Scholkmann, F., Kleiser, S., Metz, A.J., Zimmermann, R., Pavia, J.M., Wolf, U. and Wolf, M., 2014. A review on continuous wave functional near-infrared spectroscopy and imaging instrumentation and methodology.Neuroimage,85, pp.6-27. Suvarna, R., Arumugam, V., Bull, D.J., Chambers, A.R. and Santulli, C., 2014. Effect of temperature on low velocity impact damage and post-impact flexural strength of CFRP assessed using ultrasonic C-scan and micro-focus computed tomography.Composites Part B: Engineering,66, pp.58-64. Wang, L.V. and Hu, S., 2012. Photoacoustic tomography: in vivo imaging from organelles to organs. Science, 335(6075), pp.1458-1462. Wang, Y., Xu, D., Yang, S. and Xing, D., 2016. Toward in vivo biopsy of melanoma based on photoacoustic and ultrasound dual imaging with an integrated detector. Biomedical optics express, 7(2), pp.279-286. Weitzel, L., Krabbe, A., Kroker, H., Thatte, N., Tacconi-Garman, L.E., Cameron, M. and Genzel, R., 1996. 3D: The next generation near-infrared imaging spectrometer.Astronomy and Astrophysics Supplement Series,119(3), pp.531-546. Zhou, Y., Xing, W., Maslov, K.I., Cornelius, L.A. and Wang, L.V., 2014. Handheld photoacoustic microscopy to detect melanoma depth in vivo. Optics letters, 39(16), pp.4731-4734. Zuckerman, R., 2015. Method for the non-invasive measurement of tissue function and metabolism by determination of steady-state fluorescence anisotropy. U.S. Patent 9,215,982.