ZCE 537/4 Ultrasound and Magnetic Resonance Imaging PDF

ZCE 537/4 ULTRASOUND AND MAGNETIC RESONANCE IMAGING ULTRASONIC FIELD AND ITS INTERACTIONS NURSAKINAH SUARDI [email protected]  INTRODUCTION Ultrasound (high frequency sound) is a very widely used medical imaging modality. Low levels...

ZCE 537/4 ULTRASOUND AND MAGNETIC RESONANCE IMAGING ULTRASONIC FIELD AND ITS INTERACTIONS NURSAKINAH SUARDI [email protected]  INTRODUCTION Ultrasound (high frequency sound) is a very widely used medical imaging modality. Low levels of ultrasound energy (generally in the form of short pulses) are transmitted into the body. As the ultrasound travels through the body it interacts with the tissues, creating a series of echoes. The machine can detect these echoes and process them to produce an image of the patient’s anatomy. It can also analyse the echoes to acquire information about blood flow and tissue movement. 2 To use ultrasound competently, you must understand both the underlying physical principles and the technology of diagnostic ultrasound machines.  INTRODUCTION Ultrasound has significant limitations. Limited ability to penetrate deep into the body due to attenuation of the ultrasound energy as it passes through tissues. Ultrasound is severely attenuated when it encounters air or bone. Ultrasound images contain artifacts This means that the acoustic window through which many body regions can be scanned is limited. An extreme example is echocardiography, where the user must find windows that allow the ultrasound to pass through to the heart while avoiding the ribs and lungs. 3  INTRODUCTION Ultrasound is operator dependent. Operator must be aware of these limitations and must be able to optimize both their scanning technique and the equipment settings to minimize their impact. The operator must recognise artifacts during the examination and minimise their negative impact. Measurements of structures must be made using correct scanning and measurement technique to reduce the impact of artifacts on their accuracy. 4  ULTRASOUND WAVE CHARACTERISTICS 5  ACOUSTIC WAVES Sound is mechanical energy that is transmitted by pressure waves through a medium. The frequency of these waves is the number of vibrations (cycles of motion) that particles in the medium make per second. Ultrasound = mechanical waves with Humans can detect sound with frequencies higher than humans can frequencies in the range of 20 to 20,000 detect (i.e., >20,000 Hz) Hz. 6  MECHANICAL ENERGY Mechanical energy (kinetic energy or potential energy) is the energy of either an object in motion or the energy that is stored in objects by their position. 1. When a bow is pulled, it stores energy. 2. When released, the bow uses its stored energy and pushes the arrow to its trajectory. 3. Thus, the bow works on the arrow at the expense of its mechanical energy. 7  HOW IS SOUND ENERGY CONVERTED TO MECHANICAL ENERGY? The sound waves generate a vibration in the molecules due to the oscillation of the particles to and fro (a constant movement backwards and forwards or from side to side). This vibrational energy is responsible for the conversion of energy into mechanical energy doing some work. 8  12.2. ULTRASONIC PLANE WAVES 12.2.1. One-Dimensional Ultrasonic Waves A pressure plane wave, p (x,t), propagating along one spatial dimension, x, through a homogeneous, non- attenuating fluid medium can be formulated starting from Euler’s equation and the equation of continuity :    1  p ( x, t ) +  o u ( x, t ) = 0 p ( x, t ) + u ( x, t ) = 0 x t t  x o is the undisturbed mass density of the medium  is the compressibility of the medium (i.e., the fractional change in volume per unit pressure in units of Pa−1) u(x,t) is the particle velocity produced by the wave IAEA Diagnostic Radiology Physics: a Handbook for Teachers and Students – chapter 12,9  VOCAL CORD The sound that is generated from our mouth is due to the vibrations of the vocal cord. These vibrations are carried by the molecules present in the mouth and the motion of the tongue and lips helps to coordinate the sound produced. 10  PROPAGATION The pressure changes of vibrating molecules are transferred mechanically to neighboring molecules in a process called "propagation". This process requires an elastic medium, unlike electromagnetic radiation which can occur in a vacuum. This medium can be a solid, liquid, or gas. Obviously, different media propagate sound differently. 11  LONGITUDINAL WAVES Longitudinal waves are waves where the displacement of the medium is in the same direction as the direction of the travelling wave. 12  COMPRESSION AND RAREFACTION 13  VELOCITY Velocity, v (m.s-1) of sound: The distance travelled by the sound per unit time. v = λf where, λ is the wavelength (mm or µm) f is the frequency (s-1 or Hz) Dependent on the compressibility and density of the propagation medium. 14  VELOCITY What Determines Acoustic Velocity Density (ρ) = mass per unit volume. In general, a greater density impedes sound propagation. 1 𝑐 ∝ √ρ Compressibility (K) = fractional decrease in volume when a pressure is applied to a substance. The easier it is to reduce the volume of a medium, the greater its compressibility. In general, greater compressibility impedes sound propagation.· 1 𝑐∝ √𝐾 15 Bulk modulus (B) = a measure of the stiffness of a medium and its resistance to being compressed  VELOCITY Compressibility: Inversely proportional to velocity. E.g. sound propagates faster in solids (hardly compressed) than gases (easily compressed). Density: Inversely proportional to velocity. E.g. sound propagates slower in denser materials (have greater inertia). 16  But, I Thought Sound Travels Faster In Solids! While independent variables, increases in density often results in less change than decreases in compressibility. Compressibility is superior than the density in affecting the speed of sound. Bulk modulus Tissue stiffness / resistance to 𝑒𝑙𝑎𝑠𝑡𝑖𝑐 𝑝𝑟𝑜𝑝𝑒𝑟𝑡𝑦 compression 𝑐= 𝑖𝑛𝑒𝑟𝑡𝑖𝑎𝑙 𝑝𝑟𝑜𝑝𝑒𝑟𝑡𝑦 Density How tightly packed the particles 17 are in medium 18 19  WAVELENGTH The wavelength, λ (mm or µm): v λ= f Wavelength: distance between two bands of compression or rarefaction. Wavelength is inversely related to frequency Note: the frequency of the sound wave, which is determined by the source, remains constant. 20  Asynchrony results in: Compressions (Condensations) High concentration High pressure High density Rarefactions Low concentration Low pressure Low density 21  COMPRESSION AND RAREFACTION You can think of vibrating particles as individual pendulums which behave like waves: A =Ao sin (2ft) Oscillating particles combine to form high pressure regions called "compression," separate to form low pressure regions called "rarefaction.“ 22 23  HOW TO DETERMINE WHICH REGION IS COMPRESSION OR RAREFACTION 24  WAVELENGTH Generation of US Beam Produce cylindrical beams Width of beam smaller than sound head As pass through tissue there is some divergence Larger the sound head more collimated the beam More focused Smaller more divergent : 1 MHz > divergence than 3 MHz 25  WAVELENGTH The sound acoustic variables include: Pressure Density Temperature Particle motion. 26  AMPLITUDE Amplitude (dB) of an ultrasound pulse: Range of pressure excursions, related to the energy content of the wave. In diagnostic applications, it is necessary to know the relative amplitude of the ultrasound pulses: 𝐀𝟐 Relative amplitude (ratio) = 𝐀𝟏 𝐀𝟐 27 Relative amplitude (dB) = 20 log 𝐀𝟏  FREQUENCY The frequency, f (s-1 or Hz) of sound is the number of cycles the wave oscillates per second. v f= λ Since v remains constant within a given medium, 𝟏 f∝ 𝛌 Period, T (s): the duration of a cycle. 1 28 T= f 29  FREQUENCY Changing frequency may mean changing sound head Frequency determines depth of penetration The higher the frequency the better the resolution. 30  INTENSITY AND POWER As an ultrasound wave passes through a medium, it transports energy through the medium. The rate of energy transport is known as “power.” Medical ultrasound is produced in beams that are usually focused into a small area, and the beam is described in terms of the power per unit area, defined as the beam’s “intensity.” 31  INTENSITY AND POWER Intensity, I (mW.cm-2) The intensity of an ultrasonic beam at a point is the rate of flow of energy through unit area perpendicular to the beam at that point. Proportional to the square of amplitude. Determines the sensitivity of the instrument, i.e. the number and sizes of echoes recorded. 32  INTENSITY AND POWER Power, P (W, mW) Rate of flow of energy through the whole cross-section of the beam. [Ultrasonic power] = [Ultrasonic Intensity] [Beam cross-sectional area] P=Ia [Equation 1.6] Beam area is determined in part by the size and operating frequency of the transducer. 33  INTENSITY AND POWER Power (W): rate of energy transfer. Intensity, I (W.cm-2): rate at which power passes through a specific area. Rate at which ultrasound energy is applied to a specific tissue location within a patient’s body. Increases with amplitude. Must be considered in terms of biological effects and safety. 34 35  INTENSITY AND POWER Relative intensity and pressure levels are described as a logarithmic ratio, the decibel (dB). The relative intensity in dB for intensity ratios is calculated as 𝐈𝟐 Relative Intensity = 10 log 𝐈𝟏 where I1 and I2 are intensity values that are compared as a relative measure, and “log” denotes the base 10 logarithm. In diagnostic ultrasound, the ratio of the intensity of the incident pulse to that of the returning echo can span a range of one million times or more. 36 The logarithm function compresses the large and expands the small ratios into a more manageable number range.  INTENSITY AND POWER If incident intensity is one million times greater than returning echo intensity, it's a 60 dB loss. If it's 100 times greater, it's a 20 dB loss. 37  PRIMARY MEASURES OF INTENSITY Spatial Peak Intensity (SPI) Highest value occurring over time Range from.25 – 3.0 W/cm2 Spatial Average Intensity Beam is not uniform Describes how much energy passing through sound head = power output (Watts) /ERA 38  EFFECTIVE RADIATING AREA (ERA) Portion of the sound head that actually produces sound wave Dependent on surface area of crystal Close to sound head size as possible Example 6 watts/ 4 cm2 = 1.5 W/cm2 Therapeutic SAI = 0.25 -2.0 W/cm2 39  PRIMARY MEASURE OF INTENSITY Temporal Peak Intensity, TPI Maximum intensity during on period in W/cm2 Temporal Average Intensity, TAI Only important with pulsed US Averaging intensity during on/off periods 40  BEAM NON-UNIFORMITY RATIO (BNR) The Beam Non-Uniformity Ratio (BNR) is a measure used to quantify the uniformity of the intensity of an ultrasound beam. It's an important parameter in ultrasound imaging and therapy because it indicates how evenly the energy is distributed across the beam profile. A high BNR can potentially lead to hot spots or areas of excessive heating in therapeutic ultrasound applications, which could cause tissue damage. 41  BEAM NON-UNIFORMITY RATIO (BNR) Amount of variability of intensity within the US beam Due to imperfections in the crystal Highest peak intensity : SAI = BNR 1:1 – desired but unattainable 2:1-6:1 – acceptable 8:1 – unacceptable Frequency, ERA, BNR must be written on the machine 42  CHOOSING AN INTENSITY No definitive Rules Use lowest possible intensity at the highest frequency that will transmit energy to tissues Patient tolerance Next table only applies to CW US Any adjustment in intensity must be adjusted in treatment time 43  RATE OF HEATING PER MINUTE Tx Intensity 1 MHz 3 MHz (W/cm2) (C degrees) (C degrees) 0.5 0.04 0.3 1.0 0.2 0.6 1.5 0.3 0.9 2.0 0.4 1.4 44  ULTRASONIC ENERGY US waves are capable of: Reflection Refraction Absorption The area exposed to the sound waves is limited to the size of the sound head Treatment Duration Depends on: The size of the area treated The output intensity 45 Therapeutic goals Generally treat for 10 to 14 days then re-evaluate athlete  ULTRASOUND INTERACTIONS 46  INTRODUCTION Diagnostic ultrasound gives useful information precisely because it interacts strongly with soft tissue. The major types of interaction are: attenuation; reflection; scattering; refraction. 47  ACOUSTIC IMPEDANCE Acoustic impedance (Z) is a physical property of tissue which describes how much resistance an ultrasound beam encounters as it passes through a tissue. The acoustic impedance (Z) of a material is defined as Z=ρc where ρ is the density in kg/m3 and c is the speed of sound in m/s. The SI unit for acoustic impedance is kg/(m2s). If the density of a tissue increases, impedance increases. Similarly, but less intuitively, if the velocity of sound increases, then impedance also increases. 48  ACOUSTIC IMPEDANCE Soft tissue adjacent to air-filled lungs represents a large difference in acoustic impedance; thus, ultrasonic energy incident on the lungs from soft tissue is almost entirely reflected. When adjacent tissues have similar acoustic impedances, only minor reflections of the incident energy occur. Acoustic impedance gives rise to differences in transmission and reflection of ultrasound energy. 49 50  ATTENUATION Ultrasound attenuation is the loss of acoustic energy with distance traveled Caused mainly by scattering and tissue absorption of the incident beam. Absorbed acoustic energy is converted to heat in the tissue. The attenuation coefficient, μ, expressed in units of dB/cm, is the relative intensity loss per centimeter 51 of travel for a given medium.  ATTENUATION The amount of attenuation is calculated as the ratio of the initial ultrasound intensity (I1) to the final intensity (I2). 52  ATTENUATION 53  REFLECTION The reflection of ultrasound energy at a boundary between two tissues occurs because of the differences in the acoustic impedances of the two tissues. The reflection coefficient describes the fraction of sound intensity incident on an interface that is reflected. 54  TRANSMISSION The fraction of the incident energy that is transmitted across an interface is described by the transmission 55  REFRACTION If the velocity of ultrasound is higher in the second medium, then the beam enters this medium at a more oblique (less steep) angle. This behavior of ultrasound transmitted obliquely across an interface is termed refraction. The relationship between incident and refraction angles is described by Snell’s law: 56  REFRACTION If the velocity of ultrasound is higher in the second medium, then the beam enters this medium at a more oblique (less steep) angle. Refraction is a principal cause of artifacts in clinical ultrasound images. 57 58  SCATTERING A change in the direction of motion of a particle because of a collision with another particle. The word scattering describes the interaction of ultrasound with small structures such as red blood cells and capillaries. It differs from reflection in two important ways scattered energy is distributed in all directions, whereas reflected ultrasound goes in a single direction; the scattered energy is generally much weaker than reflected energy and so the echoes due to scattering are generally displayed in the image as low- to mid-level grey tones. 59  SCATTERING 60  COPYRIGHT All notes, video, exercises, assignments are Copyright @ [nursakinah & 2023]. Any illegal reproduction of this content will result in immediate legal action. 61