ImiSystem

Basic Components of an Imaging System Signal(s) Generator Signal(s) Detector Signal(s) Object Signal(s) Signal(s) Processor Image DICOM Image PACS Basic Sound Characteristics Sound is mechanical energy that propagates through a continuous, elastic medium by compression and rarefaction of the particles in the medium (imagine a spring as the elastic medium) Compression is caused by a deformation induced by an external force (such as a piston) and resulting pressure of the medium Rarefaction occurs after the compression event when the piston reverses force The compressed particles transfer their energy to adjacent particles but with less local pressure amplitude A continuous back-and-forth from the piston will yield a series of compression and rarefaction events – the particles only experience very small displacements Basic Sound Characteristics Basic Sound Characteristics Like EM, sound can be defined by wave properties: Amplitude: Intensity of the wave (eg, Pressure) Wavelength (l): Distance between compressions or rarefactions or any two points that repeat the sinusoidal wave of pressure amplitude Frequency (f): Number of times the wave oscillates through one cycle each second The period is amount of time for one wave cycle (1/f) The speed of sound (c), wavelength, and frequency is related in the following equation: c = lf Infrasound: <15 Hz, Audible spectrum: 15 to 20 Hz, Ultrasound: >20Hz, Medical US: 2 to 10 MHz up to 50MHz Basic Sound Characteristics Speed of sound is also dependent on the medium that it propagates through and varies in different materials The wave speed is determined by the ratio of bulk modulus (B), which is a measure of stiffness of a medium, and the density of the medium: A highly compressible medium (eg, air) has a low speed of sound A less compressible medium (eg, bone) has a higher speed of sound The frequency is unaffected by changes in sound speed if passing through different media which means the wavelength is dependent on the medium Basic Sound Characteristics Interactions of US with Matter As US energy propagates through a medium, there are four interactions: reflection, refraction, scattering, & absorption Reflection: occurs at tissue boundaries where there is a difference in acoustic impedance of adjacent materials If an incident beam is perpendicular to the boundary, a fraction of the beam (an echo) returns directly back to the source Refraction: change in direction of transmitted US energy with a non-perpendicular incidence Scattering: occurs either by reflection or refraction by small particles within the tissue medium, causing the beam to diffuse in many directions (creates texture and gray scale in an US image – a good thing!!!) Absorption: US energy is converted to heat energy (loss of signal) Interactions of US with Matter Acoustic impedance (Z) of a material is defined: and has a special name of rayl Can be likened to the stiffness and flexibility of a spring When springs with different compressibility are connected together, the energy transfer is dependent on the stiffness A large difference in the stiffness represents a large reflection of energy (eg, spring attached to a wall) So using these properties, sound propagating through a patient such as soft tissue next to lungs (eg, air), represents a large difference in stiffness and a reflection of US energy – what happens to two tissues with similar stiffness?? The different tissue stiffness (Z) yields the differences in transmission & reflection of US energy producing the image Interactions of US with Matter: Reflection Reflection of US energy at a boundary between two tissues occurs because of the differences in Z For perpendicular incidence, the intensity reflection coefficient, RI is: Subscripts 1 & 2 represent proximal & distal tissues For a typical muscle-fat interface, approximately 1% of the US intensity is reflected and the rest is transmitted to greater tissue depths At muscle-air interface yields nearly 100% reflection and almost no further transmission beyond the air cavity So, if a transducer is placed on the skin surface and there is pockets of air between them, what happens? Acoustic coupling gel is used between transducer and the skin to eliminate air pockets Interactions of US with Matter c2>c1 Is Material 2 more or less dense? Interactions of US with Matter: Attenuation US attenuation is the loss of acoustic energy with distance traveled caused by scattering and tissue absorption Tissue absorption is converted to heat in the tissue Attenuation coefficient is the relative intensity loss per centimeter of travel for a given medium Ultrasound (US) Imaging System Ultrasound (US) Imaging System US Imaging applications require broad bandwidth but short spatial pulse length Ultrasound (US) Imaging System In the Receiver circuit, Time Gain Compensation (TGC) is a user-adjustable amplification of returning echo signals TGC attempts to correct for beam attenuation The ideal TGC curve makes all equally reflective boundaries equal in signal amplitude regardless of the depth of the boundary Ultrasound (US) Imaging System Three Echo Display Modes used for US: A-mode, B-mode, and M-mode A-mode (amplitude) is the display of amplitudes of processed information versus time (seldom used today) B-mode (brightness) is electronic conversion of the A-mode information into brightness-modulated dots proportional to the echo amplitudes M-mode (motion) uses B-mode information and records motion of echo signals from tissues over time Ultrasound (US) Imaging System Early B-mode scanners utilizing a manual articulating arm Ultrasound (US) Imaging System Current B-mode US systems with electronic scanning Larger FOV Radioactive Decay Decay occurs to a lower energy state: Mass defect = mass of constituents in atom – mass of atom Binding energy = Mass defect x c2 Lower energy state = Greater mass defect Energy released with decay Radioactive decay occurs through sequence of radioactive isotopes until a stable isotope is reached 9/20/2018 18 Types of radioactive emissions Four types of emission: Alpha particles (2 protons + 2 neutrons) Beta particle (1 electron) Positron (antimatter electron) Gamma Ray (characteristic EM photon) Nuclear medicine: Gamma Rays (Planar Scintigraphy, SPECT) Positrons (PET) 9/20/2018 19 Nuclear Medicine: SPECT Single Photon Emission CT (SPECT) Camera w/ Large Scintillation Crystal (NA(TI) Sodium Iodide) and Photomultiplier Tubes (PMT) Capture Gamma Ray Photons from the Object Signal Measurement from PMT w/ Attenuation Correction Yields Interaction Position Camera Rotated Around Object to Gather Projection Data & Reconstruction Performed (filtered backprojection) Study Lasts Between 15-20 Minutes, Sometimes Longer Single Photon Emission CT (SPECT) Single Photon Emission CT (SPECT) Only this type of interaction provides the proper signal detected. Because of the limited number of gamma ray photons detected, the image is usually low spatial resolution – but it shows FUNCTIONAL and not anatomic processes within the patient body Nuclear Medicine: SPECT Projection images are acquired similar to CT: The gamma camera is rotated around the patient either continuously or at specific angles (step-and-shoot) At each stop, projection data is acquired and the process is repeated for a full 360 degrees instead of just 180 degrees. Why? Once the projections are acquired, then a filtered backprojection is applied with a special ramp filter called a Butterworth filter The Butterworth filter can be varied based on the desired output (smoother or more spatial resolution) Nuclear Medicine: PET A positron is emitted by a nuclear transformation and scatters through matter until it loses most of its energy and annihilates with an electron resulting in two 511-keV photons emitted in exactly the opposite direction. Annihilation Coincidence Detection (ACD) forms the basis for PET imaging. 9/20/2018 25 From: http://xkcd.com/