2 -1 Ultrasound Physics Nature of Ultrasound

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2 -1 Ultrasound Physics

Nature of Ultrasound Definition of sound "Sound is the sensation perceived by the sense of hearing". – general definition “Sound is mechanical energy transmitted by pressure wave in a material medium”- Medical definition

Nature of Ultrasound What is sound? What is a sound wave? A sound wave may be defined as a disturbance or variation that transfer energy progressively from point to point in a medium It is cyclic variations of molecules of matter which permit the energy in a sound wave to propagate through the medium Particle vibration Direction Of propagation Longitudinal waves

Frequency range Infrasound : Less than 20 Hz Audible sound : 20 Hz to 20 k. Hz Ultrasound : greater than 20, 000 Hz(20 k. Hz) Clinical imaging ultrasound frequency range : 2 MHz to 10 or 12 MHz

Sound spectrum Infrasound

Propagation speed What determines the speed of sound? ? 1. Density ( C α 1/Density) 2. Compressibility ( C α 1/Compressibility ) 3. Stiffness ( C α stiffness) 4. Frequency 에 independent

Velocity of sound is constant Air : 330 m/sec = 0. 33 mm/μs Fat : 1460 m/sec Mercury : 1450 m/sec Water : 1480 m/sec Soft tissue : 1540 m/sec = 1. 54 mm/μs Muscle : 1600 m/sec Lead : 2400 m/sec Skull bone : 4080 m/sec Aluminum : 6400 m/sec

What is Piezioelectric effect Pressure electricity generation Vibration

What is PZT ? PZT is lead zirconate, a man made ceramic material which is given the piezoelectric properties by the polarization process. Modern ultrasound transducers utilize PZT

Transducer and Probe Lens Cover Coupling layers Composite PZT Backing

What is Curie temperature ? The Curie temperature is the temperature at which PZT loss its piezoelectric property. The Curie temperature for PZT is 300 to 400 degrees C or 600 to 700 degrees F. Ultrasound transducers should never be heat sterillized

Damping Block 1. Purpose : to shortest pulse possible since this will improve axial resolution. ( eliminate the noise ) 2. Optimal impedance : PZT 와 같다. 3. Damping block -> shorter pulse -> SPL and PD reduced -> Axial resolution = SPL/2 Lens Cover Coupling layers Composite PZT Backing

Near Zone & Far Zone 1. Near Zone ( Fresnel Zone) 2. Far Zone (Fraunhofer Zone)

Near Zone Length 1. NZL(mm) = Transducer diameter²/ wavelength Transducer diameter와 frequency가 증가하면 할 수록 NZL 증가한다.

Focusing 1. Focusing is a way reducing the beam diameter in near zone 2. Improve lateral resolution in near field 3. Focusing does not increase the near zone length. 4. Focal lenghth, Focus(Focal point), Focal Zone Focal distance lens Focal point Focal zone External or mechanical focusing with an acoustic lens

What is Resolution? ? Spatial Resolution • Lateral Resolution • Axial Resolution • Elevational Resolution Contrast Resolution Temporal Resolution Frame rate

Axial (Range) resolution Define : Axial resolution is ability of an ultrasound transducer to detect two closely space structures which is parallel to direction of sound travel 동의어(LARRD) = Longitudinal, Axial, Range, Radial, Depth resolution Axial resolution(mm) = SPL/2

Axial (Range) resolution Axial resolution(mm) = SPL/2 SPL = Number of cycle * λ

How may axial resolution be improved? 1. Effectively damping which reduces the ringing of the element which decreases spatial pulse length and pulse duration 2. Increase transducer frequency = Reduce wave length, SPL, PD. .

Lateral resolution Define : Lateral resolution is ability of an ultrasound transducer to detect two closely space structures which lie perpendicular to direction of sound beam 동의어(LATA) = Lateral, Angular, Transverse Azimuthal resolution Lateral resolution(mm) = Beam Diameter(mm)

How may lateral resolution be improved? Decreasing beam diameter 1. Increasing the transducer diameter will extend the near zone length and decrease the beam diameter overall 2. Increase transducer frequency will extend the near zone length and decrease the beam diameter overall 3. Focusing Decreases the beam diameter in the near zone, Multi-focusing improve lateral resolution but decreases frame rate 4. Dynamic aperture

Lateral resolution Limited Lateral Resolution

Lateral resolution Good Lateral Resolution

Lateral resolution L J

Resolution 3. Elevational Resolution Conventional Beamforming 3 D Beamforming Elevation plane fixed focus Less Far field focus Matrix Array Technology 3 D Beamforming supporting Active Matrix Array Technology. Ultrasound signals actively focused in the elevation dimension. Better focus = better Resolution, Homogeneity & Sensitivity

Contrast Resolution • Contrast resolution is the ability to distinguish subtle differences in similar tissues. • Grayscale maps depicting 256 shades of gray are used to display contrast.

Temporal Resolution Fast frame rates = Temporal Resolution = Anatomical Accuracy

Temporal Resolution Define : Temporal resolution is ability of an ultrasound machine to depict rapidly moving structures accurately Determined by Frame rate (FPS… )

Temporal Resolution Volume rate static 3 D Real. Time 4 D

What is scan line& frame 1. A scan line represent one line of a frame, In general , one pulse equals one scan line. 2. A frame is a collection(between 100 -150)of scan lines 3. The standard frame rate is 30 frame per second

Line Density : Image is consisted by data of Scan. Line. Density is a parameter for density of Scan. Line More Scanline makes more detailed(better lateral resolution) image, but Frame. Rate becomes slow. Line. Density: High Line. Density: Low more scanline in frame (low Frame. Rate) less scanline in frame(high Frame. Rate) better lateral resolution worse lateral resolution

How may temporal resolution be improved? 1. Decreasing sector width size(FOV) 2. Decreasing Image Depth 3. Increasing frame rate

Mechanical Probes Mechanical transducers require motion for creation of the ultrasound beam. These are rarely used now. Examples of mechanical methods are: • Multiple elements mounted on a rotating wheel • Oscillating single elements • Oscillating Mirror

Type of Transducer

Linear Probes consist of a collection of elements arranged one after the other in a straight row. In a linear array, each scan line is formed by firing a small subgroup of elements

Convex Probes Curvilinear are linear arrays with a convex surface. Also called convex array transducers, curvilinear transducers usually produce an image as shown.

Phased array Probes Phased array transducers have rectangular elements arranged side by side. Also called sector arrays, these rely on electronic beam steering to sweep sound beams. Voltage pulses are applied to all the elements, but in phase, so the sound pulse may be sent out in a certain direction. Sector probes have a small footprint, and hence are suitable for cardiac applications. This transducer produces a sector or pieshaped image.

Sector phased array :

PRF ( Pulse Repetition Frequency ) PRF is frequency is the number of pulse emitted by the transducer per second. Formula : PRF( k. Hz) = 1/PRP(ms) The sonographer can affect PRF by changing the depth. PRF 와 depth 는 반비례 Nyquist limit = PRF/2

< Receiver functions -Amplication> Gain System gain controls the degree of echo or brightness of the image. The gain is measured in decibels, an arbitrary measurement of sound amplitude. Overall gain : amplification of received signal

Image adjustments: Gain

Receiver functions -compensation> TGC( Time Gain Compensation ) The DGC attempts to compensate for the acoustic loss by absorption, scatter and reflection and to show structures of the same acoustic strength with the same brightness no matter what the depth

< Receiver functions -compensation> TGC( Time Gain Compensation ) Echointensity Depth Without Time Gain Compenstion TGC (Time Gain Compensation) With Time Gain Compensation

< Receiver functions-Compression> Dynamic range 1. Define : echo amplitude range the ratio of the strongest amplitude to the weakest amplitude signal thant a particular component of the ultrasound instrument is capable of processing 2. Unit : d. B SIGNAL SATURATION LEVEL ULTRASONIC SIGNALS DYNAMIC RANGE REJECT LEVEL NOISE LEVEL ZERO SIGNAL LEVEL Dynamic range

< Signal processing > Dynamic range 1. 20 log voltage 1(largest signal amplitude) / voltage 0(smallest signa amplitude) Narrow Wide Dynamic Range Which photo gives a better representation of the baby? Which photo gives enough sensitivity to detect a tear on the baby’s face?

< Receiver functions-Rejection> Rejection eliminates low level(noise) signal(low amplitude). Sonographer can control (2) (1) A B (1) C (2)

Display modes A-mode B-mode M-mode C-mode

A-mode 1. A(amplitude) mode displays returning echoes as spikes 2. A-mode provides infromation then about reflector strength and distance = go-return time 과 amplitude 로 표현 Amplitude Distance A-mode Alp-mode If you trouble remembering what an A-mode display looks like, just think of the Alps!

B-mode 1. B-mode display returning echoes as bright dots 2. Advantage : it provide anatomic cross sectional information as well as information concerning reflector motion Echoes(d B) grey scale assignment

M-mode 1. M-mode(T-M mode) is a b-mode operation which displays the depth of structures over time. 2. Advantage : it provide accurate information of reflector motion, provides excellent temporal resolution 3. Disadvantages: Poor spatial information (only one dimensional)

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