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Unit II. Image formation and acquisition principles. Part II Major imaging modalities Dr. Felipe Orihuela-Espina Outline Fundamental models of image formation Kinds of radiation and imaged properties The imaging system Point spread function Imaging filters: Monochromatic, colour, multi-spectral and hyperspectral images Resolution (pixel, spatial, radiometric/magnitude,spectral, temporal, superresolution) Image quality and uncertainties in image formation (digitization, quantum efficiency, metamerism, calibration, CNR, SNR) Major imaging modalities Magnetic Resonance Imaging Optical Imaging X-Ray (X-Ray) CT (X-Ray) Fluoroscopy Coherent Tomography (OCT) Diffuse Optical Imaging (NIRS) Microscopy Confocal imaging One and two-photon imaging Electrical and magnetic imaging EEG/MEG EMG ECG Ultrasound © 2015. Dr. Felipe Orihuela-Espina 2 Some references Very nice slides on image acquisition systems including CT, MRI, Ultrasound, PET, SPECT http://webpages.uncc.edu/krs/courses/6010/m edvis/imaq1a.pdf You may find some good MRI examples at: http://www.mrtip.com/serv1.php?type=db1&dbs=t2%20weig hted%20image © 2015. Dr. Felipe Orihuela-Espina 3 MAJOR IMAGING MODALITIES © 2015. Dr. Felipe Orihuela-Espina 4 ELECTRICAL AND MAGNETIC IMAGING © 2015. Dr. Felipe Orihuela-Espina 5 Electrochemistry Electrochemistry is the study of reactions in which charged particles (ions or electrons) cross the interface between two phases of matter* typically a metallic phase (the electrode) and a conductive solution, or electrolyte. Source: [http://www.chem1.com/acad/webtext/elchem/ ec1.html] *A phase of matter is a region of space (a thermodynamic system), throughout which all physical properties of a material are essentially uniform. Source: [Wikipedia:Phase_(matter)] © 2015. Dr. Felipe Orihuela-Espina 6 Electrodes An electrode is a collector or emitter of electric charge or of electric-charge carriers. Often, just a metal strip When an electrode is placed in a solution (electrolyte), the electrode acquires either a positive or negative charge with respect to the solution. This potential difference is called electrode potential. Figure from: [http://www.askiitians.com/iit-jeechemistry/physical-chemistry/electrodepotential.aspx] © 2015. Dr. Felipe Orihuela-Espina 7 Electrodes An electrode reaction or electrode process refers to the net oxidation or reduction process that takes place at an electrode. In the case of oxidation: 1. Anions in the electrolyte will flow to 2. 3. the interface boundary. Cations in the electrolyte will flow away from the interface boundary. To counteract this, electrons in the electrode will flow away from the interface boundary creating a current in the electrode. Oxidation: Ions pass from the electrode into solution leaving a negative charge on the electrode. Reduction: Ions pass from the solution to the electrode leaving a positive charge on the electrode Figure from: [http://soundlab.cs.princeton.edu/learni ng/tutorials/sensors/node10.html] © 2015. Dr. Felipe Orihuela-Espina 8 Electrodes Any electrode may act as anode or cathode depending on the direction of the flow of ions. When electrode is negatively charged with respect to solution, i.e., it acts as anode. Oxidation occurs. The anode is the electrode where oxidation reactions take place When electrode is positively charged with respect to solution, i.e., it acts as cathode. Reduction occurs. The cathode is the electrode where reduction reactions take place © 2015. Dr. Felipe Orihuela-Espina 9 Electrodes To measure a potential difference (voltage) we need 2 electrodes; one acting as a reference. Figure from: [http://web.nmsu.edu/~kburke/Instrume ntation/IS_Electrod.html] © 2015. Dr. Felipe Orihuela-Espina 10 Electrophysiology Electrophysiology is the study of electric activity in biological tissues and bodies. It involves measurements of voltage change or electric current on a wide variety of scales from single ion channel proteins to whole organs In electrophysiology an electrode acts transducer between the ionic transport of the nerve and the electron flow in copper wire. Its variants are standard in several procedures e.g. ECG or EKG for cardiac activity monitoring, EEG for temporal resolution in neuroimage, EMG for muscle activity. © 2015. Dr. Felipe Orihuela-Espina 11 Electrophysiology Electric and magnetic imaging senses the electromagnetic activity that the body uses to transmit information through the nervous system (NS). Since it monitors activity of neurons is a direct measure of NS activity including the brain. Since the source is the body itself, there is no external irradiation. © 2015. Dr. Felipe Orihuela-Espina 12 Nervous system Tissues are groups of cells that work together to carry out a certain task within an organism An organ is a group of tissues that work together An organ system is a group of organs that work together to perform one or more functions. Definitions from wikipedia (several pages) © 2015. Dr. Felipe Orihuela-Espina 13 Nervous System The nervous system is the organ system responsible for the transmission and reception of signals between different parts of a body. The system is composed mainly of two types of cells: Neurons – responsible for information transmission Glial cells – responsible for homeostasis and protection © 2015. Dr. Felipe Orihuela-Espina 14 Nervous system The neuron is the basic cell of the NS*. Neurons specialized in information transmission and they do this in two forms: Chemical Electrical Figure from: [Wikipedia: Complete_neuron_cell_diagram_en.svg] *There are as many as 10,000 specific types of neurons responsible for different tasks in the human brain. Mainlly they can be coarsely classified in: motor neurons (for conveying motor information), sensory neurons (for conveying sensory information), and interneurons (which convey information between different types of neurons). Source: [Stufflebeam R “Neurons, Synapses, Action Potentials, and Neurotransmission” http://www.mind.ilstu.edu/curriculum/neurons_intro/neurons_intro.php] © 2015. Dr. Felipe Orihuela-Espina 15 Electrophysiology The cell membrane of the axon and soma contain gated ion channels that allow the neuron to generate and propagate an electrical signal (an action potential). These signals are generated and propagated by chargecarrying ions. Ions, not electrons, are the carriers of current in the nervous system. Depolarization and resting state Figures from: [science.education.nih.gov] © 2015. Dr. Felipe Orihuela-Espina 16 Electrophysiology There are four main types of gated channel. Voltage-gated channels Figure from: [http://7e.biopsychology.com/step03.01. html] (open/close in response to a change in membrane potential) Chemically-gated ion channels (open/close in response to binding with an extracellular chemical messenger (neurotransmitter)) Mechanically-gated ion channels (open/close in response to physical stimuli) Thermally-gated channels (open/close in response to temperature changes) We won’t see them in detail © 2015. Dr. Felipe Orihuela-Espina 17 Electrophysiology ☞ Our knowledge of ion channels is still limited They are too tiny to be seen in detail, even with the electron microscope. Nanoscopy may soon sort out this limitation. We do not know for sure if there are separate channels for the different ions (Na, K), or one channel with different permeability. To know more: The origin of the resting membrane potential An interactive presentation. A bit tough on chemistry. http://www.st-andrews.ac.uk/~wjh/neurotut/mempot.html © 2015. Dr. Felipe Orihuela-Espina 18 Electrophysiology Action potentials are brief, rapid, large changes in the membrane potential in which the potential actually reverses. Action potentials are propagated/conducted along the axon of a neuron without losing their strength (nondecremental conduction) Source and figure from: [https://dundeemedstudentnotes.wordpress. com/category/nervoussystem/physiology/page/2/] © 2015. Dr. Felipe Orihuela-Espina 19 Electrophysiology In the resting state, potassium is concentrated on the inside of cells. Depolarization and resting state Figure from: [http://peer.tamu.edu/curriculum_modules/OrganSystems/module_5/whatweknow2.htm] © 2015. Dr. Felipe Orihuela-Espina 20 Electrophysiology Step 1. A resting (diffusion) membrane potential normally exists through the efflux of positive-charged (potassium) ions maintaining an electrochemical equilibrium of –75 mV. Depolarization and resting state Figure from: [http://peer.tamu.edu/curriculum_modules/OrganSystems/module_5/whatweknow2.htm] © 2015. Dr. Felipe Orihuela-Espina 21 Electrophysiology Step 2. When a neuron is stimulated, molecular receptors in its membrane open. Depolarization and resting state Figure from: [http://peer.tamu.edu/curriculum_modules/OrganSystems/module_5/whatweknow2.htm] © 2015. Dr. Felipe Orihuela-Espina 22 Electrophysiology Step 3. Sodium ions move in because their is an electrical pulling force (inside is negative and sodium ions are positive) and an osmotic force (sodium is more concentrated on the outside). Depolarization and resting state Figure from: [http://peer.tamu.edu/curriculum_modules/OrganSystems/module_5/whatweknow2.htm] © 2015. Dr. Felipe Orihuela-Espina 23 Electrophysiology Step 4. As sodium rushes in, the inside becomes positive, and that forces out potassium. Depolarization and resting state Figure from: [http://peer.tamu.edu/curriculum_modules/OrganSystems/module_5/whatweknow2.htm] © 2015. Dr. Felipe Orihuela-Espina 24 Electrophysiology Figure from: [https://dundeemedstudentnotes.wordpress.com/category/nervoussystem/physiology/page/2/] © 2015. Dr. Felipe Orihuela-Espina 25 Electrophysiology ☞ The transmission of the action potentials along the axon is a complicated process. It may occur by means of contiguous conduction (in non-myelinated fibers) or by salitatory conduction (in myelinated fibers). After an action potential has occurred, there is a transient negative shift, called the afterhyperpolarization or refractory period. This mechanism prevents an action potential from traveling back the way it just came. To know more: Grace and Bunney (1983) Neuroscience 10(2):317-331 © 2015. Dr. Felipe Orihuela-Espina 26 Electrophysiology Neurotransmission (or synaptic transmission) is communication between neurons as accomplished by the movement of chemicals or electrical signals across a synapse. Source: [Stufflebeam R “Neurons, Synapses, Action Potentials, and Neurotransmission” http://www.mind.ilstu.edu /curriculum/neurons_intr o/neurons_intro.php] Figure from: [imgarcade.com] © 2015. Dr. Felipe Orihuela-Espina 27 Electrophysiology Figure from: [https://www.studyblue.com/notes/note/n/exam-2/deck/1245341] © 2015. Dr. Felipe Orihuela-Espina 28 To be able to measure electrical activity of the body “non-invasively”* from the outside; electrodes are placed over the skin. decrease the skin impedance (mainly from the stratum corneum) to facilitate the transduction of the ionic currents This may be achieved by using conductive gels Figure from: [http://soundlab.cs.princeton. edu/learning/tutorials/sensors /node10.html] It is necessary to Figure from: [imgarcade.com] Electrophysiology through the skin * Electrolyte conductive gels are often abrasive; they remove dead cells. No the so called dry electrodes operate gel-free. [Lopez-Gordo et al (2014) Sensors 14, © 2015. Dr. Felipe Orihuela-Espina 29 12847-12870] EEG Electroencephalography (EEG) is the recording of electrical activity along the scalp. EEG is the oldest of neuroimaging modalities dating back to 1924 when Hans Berger described the alpha waves of the human brain*. Regarded as the gold standard when it comes to temporal resolution. * Berger was not the first to notice electrical activity through the skull; that occurred even earlier with the work of Richard Caton in 1875 who work with dogs and apes and possibly was recording spontaneous activity). Note that Caton’s work is even earlier than X-Rays discovery! © 2015. Dr. Felipe Orihuela-Espina 30 EEG EEG activity reflects the summation of the synchronous activity of thousands or millions of neurons that have similar spatial orientation. The electric potential generated by an individual neuron is far too small to be picked up by EEG. The EEG signal originates mainly from cerebral cortex pyramidal cells because of their orientation relative to the cortical surface If the cells do not have similar spatial orientation, their ions do not line up and create waves to be detected. Schema of electric dipole, ionic currents and differential measure Figure from [Lopez-Gordo et al (2014) Sensors 14:12847-12870] © 2015. Dr. Felipe Orihuela-Espina 31 EEG “The origin of cerebral potentials is based upon the intrinsic electrophysiological properties of the nervous system. Identifying the generator source(s) and electrical field(s) of propagation are the basis for recognizing electrographic patterns that underly the expression of the “brain waves” as normal or abnormal.” Source: [Tatum IV, W. O.; Aatif, M. H.; Benbadis, S. R. & Kaplan, P. W. Handbook of EEG interpretation. Demos Medical Publishing, 2008, 276 pp.] © 2015. Dr. Felipe Orihuela-Espina 32 EEG © 2015. Dr. Felipe Orihuela-Espina Figure from: [www.ecvv.com] Figure from: [www.brainmaster.com] 33 EEG The right-leg driver (DRL) circuit eliminate electromagnetic interference noise originated by the body acting as an antenna. If this is not present; a Faraday cage may also be used. © 2015. Dr. Felipe Orihuela-Espina 34 EEG Figure from: [http://www.psych.nmsu.edu/~jkroger/lab/EEG_Introduction.html] © 2015. Dr. Felipe Orihuela-Espina 35 EEG Figure: [Jurcak2007] Figure: [Self elaboration from LACCIR project] © 2015. Dr. Felipe Orihuela-Espina 36 EEG Figure: [Self elaboration from LACCIR project] © 2015. Dr. Felipe Orihuela-Espina 37 EEG: Information processing Perceptual analysis Deployment of covert attention Sensory process Object recognition and categorization Stimulus evaluation and context updating (working memory) Error processing and reinforcement learning Not understood Brain information transformation: From raw sensory input to behavioural response (in a visual search task) Figure modified from [WoodmanGF2010] 24/05/2017 INAOE 38 MEG Magnetoencephalogra phy (MEG) records magnetic fields produced by electrical currents occurring in the brain. The MEG (and EEG) signals derive from the net effect of ionic currents flowing in the dendrites of neurons during synaptic transmission. Figure: [Wikipedia:Magnetoencefalografía] © 2015. Dr. Felipe Orihuela-Espina 39 MEG Figure from: [http://www.slideshare.net/Anuj0909/magnetoencephalography-by-anuj-malik] © 2015. Dr. Felipe Orihuela-Espina 40 MEG Figure from: [Kim et al (2013) Korean J Pediatr 2013;56(10):431-438] © 2015. Dr. Felipe Orihuela-Espina 41 MEG Figure from: [Foley et al. (2014) Epilepsy & Behavior 30:38-42] © 2015. Dr. Felipe Orihuela-Espina 42 ECG or EKG Electrocardiogram (e-lek-tro-KAR-de-ogram), ECG or EKG records the heart's electrical activity. An ECG shows: How fast your heart is beating Whether the rhythm of your heartbeat is steady or irregular The strength and timing of electrical signals as they pass through each part of your heart Source: [http://www.nhlbi.nih.gov/health/healthtopics/topics/ekg] © 2015. Dr. Felipe Orihuela-Espina 43 ECG With each heartbeat, an electrical signal spreads from the top of the heart to the bottom. As it travels, the signal causes the heart to contract and pump blood. Figure from: No longer available [http://ecgguru.com/ecg/ecg-waveformillustration] Figure from: [http://drmedicineguide.blogspot.mx/2013/03 /ecg-electrode-placement.html] © 2015. Dr. Felipe Orihuela-Espina 44 ECG Figure from: [http://www.swharden.com/blog /2009-08-14-diy-ecg-machineon-the-cheap/] Figure from: [http://www.nhlbi.nih.gov/health/healthtopics/topics/hb/understanding/] © 2015. Dr. Felipe Orihuela-Espina 45 ECG Electrode positioning in ECG obeys to sensitivity in detecting myocardial infarction. A classical ECG (small picture) has 10 electrodes (12 leads systems) but other montages (large picture) are possible Small figure from: [nuclearcardiologyseminars.com] Large figure from: [http://drmedicineguide.blogspot.mx/2013/03/ecg-electrode© 2015. Dr. Felipe Orihuela-Espina 46 placement.html] EMG Electromyography (EMG) recording the electrical activity produced by skeletal muscles (those voluntarily controlled) When muscles contract, summation of motor neurons action potentials give off an electrical burst strong enough to be detected by an electrode placed on the skin. It uses needle electrodes (better SNR) or pads (less invasive) © 2015. Dr. Felipe Orihuela-Espina 47 EMG Figure from: [www.protokinetics.com] Figure from: [http://www.lookfordiagnosis.com/mesh_info.php?te rm=Electromyography&lang=1] © 2015. Dr. Felipe Orihuela-Espina 48 EMG Figure from: [http://www.a-tech.ca/subcat.php?id=106] © 2015. Dr. Felipe Orihuela-Espina 49 EMG EMGs of the human soleus, bipolar surface electrodes. (a) Very slight contraction, separate motor unit potentials are visible. (b) Strong contraction, showing an ‘interference pattern’. The individual motorunit potentials are no longer discernible. Note the different voltage scales. Figure from: [Hof AL (1984) Human Movement Science 3:119-153] © 2015. Dr. Felipe Orihuela-Espina 50 EMG Figure from: [De Luca CJ (1997) Journal of Applied Biomechanics, 13 (2): 135-163] © 2015. Dr. Felipe Orihuela-Espina 51 ULTRASOUND © 2015. Dr. Felipe Orihuela-Espina 52 Disclaimer ☞ This section is roughly based on University of Washington at Seattle’s course on “Bioengineering 508 Physical Aspects of Medical Imaging ” by Prof. Paul Kinahan and colleagues. You can find the original presentation here: http://courses.washington.edu/bioen508/Lectu re6-US.pdf © 2015. Dr. Felipe Orihuela-Espina 53 Ultrasound Remember: Ultrasound waves: …are mechanical pressure waves (need an elastic medium to propagate) …propagate longitudinally Figure from [http://www.olympus-ims.com/en/ndttutorials/flaw-detection/ultrasound/] An elastic medium is one that deforms under force. region of increased pressure on a sound wave is called a compression (or condensation). A region of decreased pressure on a sound wave is called a rarefaction (or dilation) Figure from [http://physics.stackexchange.com/questions/123471/compressionvs-rarefaction-in-sound-waves] © 2015. Dr. Felipe Orihuela-Espina 54 Ultrasound Remember that as a wave; it still complies with the relation: c=fλ Figure from [University of Washington Course on “Bioengineering 508 Physical Aspects of Medical Imaging ”] © 2015. Dr. Felipe Orihuela-Espina 55 Ultrasound Remember that as a wave; it still complies with Snell’s law Figures from [University of Washington Course on “Bioengineering 508 Physical Aspects of56 © 2015. Dr. Felipe Orihuela-Espina Medical Imaging ”] Ultrasound Remember that as a wave; it still complies with Attenuation as per Beer-Lambert law Becuase of attenuation the intensity of echoes decrease. Attenuation measurements can be made by examining the exponential decay of multiple back surface reflections. Figure from: [https://www.ndeed.org/EducationResources/CommunityColl ege/Ultrasonics/MeasurementTech/attenua tionmeasure.htm] © 2015. Dr. Felipe Orihuela-Espina 57 Ultrasound Figure from: [http://galleryhip.com/ultrasoundmachine.html] Figure from: [medstandard.org] © 2015. Dr. Felipe Orihuela-Espina 58 Ultrasound Basic idea: 1. Irradiate sound waves 2. 3. 4. into the sample. Sound waves travel at different speed depending on the medium through which it propagates. Waves are reflected at the interfaces between the different tissues The return time of the waves tells us of the depth of the reflecting surface Figure from [University of Washington Course on “Bioengineering 508 Physical Aspects of Medical Imaging ”] © 2015. Dr. Felipe Orihuela-Espina 59 Ultrasound Figure from [Coletta, Chapter 16 “Mechanical Waves: Sound”] © 2015. Dr. Felipe Orihuela-Espina 60 Ultrasound Propagation of sound waves in tissues: An echo is a wave sound that reflects as the medium changes. Echoes can be: Specular echoes*: Originate at smooth boundaries from relatively large media Intense and angle dependent Reflection from large surfaces Figure from [University of Washington Course on “Bioengineering 508 Physical Aspects of Medical Imaging ”] * The name of specular echoes is ambiguous; as they refer to all reflection echoes whether specular or not © 2015. Dr. Felipe Orihuela-Espina 61 Ultrasound Propagation of sound waves in tissues: An echo is a wave sound that reflects as the medium changes. Echoes can be: Scattered echoes: Originate from objects that are size of wavelength of smaller Weak and less angle dependent Reflection from small surfaces Figure from [University of Washington Course on “Bioengineering 508 Physical Aspects of Medical Imaging ”] © 2015. Dr. Felipe Orihuela-Espina 62 Ultrasound The bulk modulus of a substance is its resistance to uniform compression. It is defined as the ratio of the infinitesimal pressure increase to the resulting relative decrease of the volume. Its SI unit is the Pascal. Figure from: [http://www.engineeringtoolbox.com/bulk-modulus-elasticity-d_585.html] © 2015. Dr. Felipe Orihuela-Espina 63 Ultrasound The bulk modulus of a substance: where V is the initial volume. And since density ρ is the mass per unit of volume; Figure from: [http://www.engineeringtoolbox.com/bulk-modulus-elasticity-d_585.html] © 2015. Dr. Felipe Orihuela-Espina 64 Ultrasound Compressibility is a measure of the relative volume change of a fluid or solid as a response to a pressure. Compressibility is the inverse of the bulk modulus. ☞ Watch out! Sometimes compressibility is denoted as which can be confusing. © 2015. Dr. Felipe Orihuela-Espina 65 Ultrasound The speed of sound c in a certain medium depends on the medium density ρ and elasticity or stiffness represented by the bulk modulus and can be approximated by: © 2015. Dr. Felipe Orihuela-Espina 66 Ultrasound ☞Beware! It’s a bit more complicated… …since it depends on compressibility (a relation of pressure and volume), it also depends on temperature but you are unlikely to see this level of detail in an introductory text. You may know the classical gas state equation: PV=nRT For solids, there is also a state equation relating P, V and T, but it is rather more complicated. Figure from: [Nasoni et al (1980) Ultrasonics symposium, 10771082] © 2015. Dr. Felipe Orihuela-Espina 67 Ultrasound generation Piezoelectric effect Piezoelectricity means electricity resulting from pressure*. The piezoelectric effect is the capacity of certain solid materials to accumulate electric charge in response to applied mechanical stress or viceversa. Externally apply mechanical stress and generate internal electricity Externally apply electricity and generate internal mechanical strain First demonstration of the piezoelectric effect was in 1880 by siblings Pierre Curie (Nobel Prize Winner) and Jacques Curie Figure from: [Wikipedia: Piezoelectricity] ☞ For further details,check back up slides. *In greek, piezo means squeeze. © 2015. Dr. Felipe Orihuela-Espina 68 Ultrasound generation Ultrasound generators are transducers based on the piezoelectric effect. Apply electricity and get a mechanical strain (sound) Simple ultrasound generator. The SPKR is a piezoelectric material. Figure from: [http://www.discovercircuits.com/U/ultrasonic. htm] © 2015. Dr. Felipe Orihuela-Espina 69 Ultrasound generation Typically, the active element of the transducer is a thin piezoelectric ceramic or piezocomposite element. The active element, often referred to as the crystal, is: protected from damage by a wearplate or acoustic lens, and backed by a block of damping material that quiets the transducer after the sound pulse has been generated. Dual transducers differ in that they have separate transmitting and receiving elements separated by a sound barrier. Source: [http://www.olympus- ims.com/en/ndt-tutorials/flawdetection/generating-ultrasound/] Figure from: [http://www.olympusims.com/en/ndt-tutorials/flawdetection/generating-ultrasound/] © 2015. Dr. Felipe Orihuela-Espina 70 Ultrasound generation Figure from: [University of Washington Course onDr. “Bioengineering 508 Physical Aspects of Medical Imaging © 2015. Felipe Orihuela-Espina 71 ”] Ultrasound image formation 1. 2. The ultrasound machine irradiates high frequence sound pulses to tissue. The sound waves interact with matter. During this interaction it hits boundaries between tissues Some sound waves will be reflected others refracted to continue travelling deeper in the matter 3. Reflected waves who return to tissue surface are detected by the probe and passed to a CPU. The probe again capitalises on the piezoelectric effect to convert back sound to electricity. 4. The CPU estimates the distance from the probe to the tissue boundaries using echoes times. In the most basic ultrasound image pixel intensities are proportional to echoes strength. Distance is represented along the Y axis. Figure from: [University of Washington Course on “Bioengineering 508 Physical Aspects of Medical Imaging ”] © 2015. Dr. Felipe Orihuela-Espina 72 Ultrasound image formation Echo Display Modes: A-mode (amplitude): display of processed information from the receiver versus time Speed of sound equates to depth B-mode (brightness): Conversion of A-mode information into brightness-modulated dots M-mode (motion): uses B-mode information to display the echoes from a moving organ Source and Figure from: [University of Washington Course onDr. “Bioengineering 508 Physical Aspects of Medical Imaging © 2015. Felipe Orihuela-Espina 73 ”] Ultrasound image formation Source and Figure from: [University of Washington Course on “Bioengineering 508 Physical Aspects of Medical Imaging ”] © 2015. Dr. Felipe Orihuela-Espina 74 Ultrasound Mechanical absorption coefficient of a material is generally dependent on frequency. A common model for this dependency is: Where α is the attenuation coefficient and a=8.7. Often; b~1 With this model, we can estimate the depth of penetration* of the sound wave as a function of frequency and its attenuation in different tissues. * Depth of signal is related to reflection time, so as time progresses, the signal will be increasingly attenuated. Time-dependent attenuation causes severe signal loss if not compensated. Thus, all systems nowadays are equipped with circuitry that performs time-gain compensation (TGC), a time-varying amplification. Source: [University of Washington Course on “Bioengineering 508 Physical Aspects of Medical Imaging ”] © 2015. Dr. Felipe Orihuela-Espina 75 Ultrasound image formation In ultrasound, spatial resolution is proportional to sound frequency. Higher frequencies yield better image resolution. …but the price to pay is lower penetration. Lower frequencies penetrate deeper in the tissues. …but don’t be fooled! There is no need to compromise. Use broadband irradiation and get the best of both Figure from: [http://www.usra.ca/transducer.php] worlds! © 2015. Dr. Felipe Orihuela-Espina 76 Ultrasound Ultrasound image of a fetus in the womb, viewed at 12 weeks of pregnancy. Figure from: [Wikipedia:Ultrasound] © 2015. Dr. Felipe Orihuela-Espina 77 3D/4D Ultrasound Figure from: [imgarcade.com] © 2015. Dr. Felipe Orihuela-Espina 78 HD Ultrasound Figure from: [https://www.youtube.com/watch?v=BD7quHKgEuk] © 2015. Dr. Felipe Orihuela-Espina 79 Ultrasound: Echocardiography Figure from: [https://www.youtube.com/watch?v=4v BJoWP-zBM] For the analysis of valves and blood flow, ultrasound Doppler is used Figure from: [http://www.ntnu.edu/isb/echocardiogra phy] © 2015. Dr. Felipe Orihuela-Espina 80 Ultrasound Thyroid Liver Spine All figures from: [http://www.oit.edu/academics/degrees/diagnostic-medical-sonography] © 2015. Dr. Felipe Orihuela-Espina 81 Ultrasound A excellent introductory slideshow: http://www.slideshare.net/KhalisKarim/physicultrasound © 2015. Dr. Felipe Orihuela-Espina 82 THANKS, QUESTIONS? © 2015. Dr. Felipe Orihuela-Espina 83 BACK UP © 2015. Dr. Felipe Orihuela-Espina 84 Piezoelectric effect The piezoelectric effect results from the change of polarization when applying a mechanical stress. The change in polarization appears as a variation of surface charge density upon the object faces Figure from: [http://www.cosmic-energy.org/?page_id=771] © 2015. Dr. Felipe Orihuela-Espina 85 Piezoelectric effect Figure from: [http://global.kyocera.com/fcworld/charact/elect/piezo.html] © 2015. Dr. Felipe Orihuela-Espina 86 Piezoelectric effect Figures from [University of Washington Course on “Bioengineering 508 Physical Aspects of Medical Imaging ”] © 2015. Dr. Felipe Orihuela-Espina 87 Ultrasound Impedance is the product of the density of a material ρ and the speed of sound c in that material © 2015. Dr. Felipe Orihuela-Espina 88