Supplement (videos)

Transcription

1 Supplement (videos) Ruben s tube (sound): Doppler US (diagnostic use): High Intensity, focused US (therapy): Ultrasound Biophysics 2 nd semester József Orbán Dep. of Biophysics April Source of images: 1

2 ?!? How can we determine the distance of the thunderbolt? What defines the brightness of any point on the US image/screen? distance ~ time: d=vt v air = 320 m/s Sound Sound wave: longitudinal mechanical wave! (vibration) 0 Hz 20 Hz 20 khz 2 MHz US diagnostic 20 MHz f (Hz) range infra audible u l t r a lung bile bone glass/metal v (m/s) fat w blood propagation of sound requires elastic medium (particles): gas, liquid, solid BUT! vacuum 2

3 Propagation of (ultra) sound in medium Pulse-echo principle v= 1500 m/s, in water US source Propagation time: t= 2d/v Reflection at the first interphase. What can we see on screen? US machine measures the time of pulse-echo (back and forth)! Then calculates with the propagation speed in water. Propagation of (ultra) sound in medium UH forrás Water Iron Air V air = 331 m/s V water = 1550 m/s V iron = 5100 m/s f = 2000 Hz λ air = cm λ water = 77.5 cm λ iron = 2.55 m US attenuates in air (absorption). Reflection on interphase. Attenuation in 2 nd medium. I 0 = I reflected +I refracted v 1 < v 2, ρ ~ v??? What happens at the 2 nd interphase??? 3

4 Ultrasound 1. Sound pressure: The ultrasound wave exerts pressure on an objects in its direction of propagation. The pressure is directly proportional to ultrasound intensity. 2. Absorption: Energy absorption by the medium that leads to an increase in its temperature. Absorption increases with frequency and distance travelled: A(x) A 0 e x Typical frequency of devices: 8 MHz: superficial vasculature A= amplitude 4 MHz: deep vasculature = absorption coefficient x= distance (layer thickness) 2 MHz: fetal ultrasound 3. Reflection Ultrasound R(reflected) z 1 =v 1 *ρ 1 T = 1-R z 2 =v 2 *ρ 2 z 2 1 z2 R z z 1 2 z: acoustic impedance (resistance) 4. Axial resolution In order to resolve the d axial distance: t w vt w 2d d For a given frequency, the axial resolution improves with decreasing Q. For a given Q, the axial resolution improves with increasing frequency. 4

5 Resolution of ultrasound images Resolution threshold: the distance between two points that may still be distinguished by their detected ultrasound image. Axial resolution threshold: the half of the pulse length. The higher the f, the shorter the pulse. The higher the f, the higher the absorption of the tissue. The choice of the appropriate frequency is always a compromise between resolution power and detection depth. Pulse frequency high low Frequency High Low monitor Resolution of ultrasound images Lateral resolution threshold: essentially the same as the beam width US beam Always greather than the axial threshold, so the lateral resolution is worst than the axial one. Best near the focus region. 5

6 Focusing 1. Fixed focus solution: e.g. acoustic lenses Focusing 2. Electronic focusing During emission/detection. i i The focus region can be set to any desired depth. Transducers Multi element transducer, all of them contain an electronic delay unit to form the shape of US beam. Interference of the waves, maximal sound pressure at given distance. 6

7 Scanning (always with focused beam) 1. Mechanic: Single piezo crystal Sector scan out of date! 2. Electronic: Transducer array linear array curved array Several ceramic wafer side by side (pl. 512) 1D lines Shift to the next transducers Ultrasound operation modes A-mode (Amplitude modulated): Single transducer. US beam propagates in a straight line. The echo is displayed in the form of a voltage peak on an oscilloscope. B-mode (Brightness): Voltage pulse is displayed as a grayscale spot. transducer Amplitude A mode Brightness B mode US pulse 2d B-mode: scanning Reflection: tissue/bone 35% air/skin 100% gel/skin 0,1% Important to use gel! 7

8 Ultrasound modes M-mode (time Motion) Temporal display of periodically moving objects (e.g., echocardiography). Based on B mode linear scans in time. X-axis: time. Y-axis: 1D B-mode image (line). LA septum LV sys dias sys dias Mitralis stenosis time Other areas of application As an effect of US, dust can aggregate, so in factories can be applied to get rid of dust. applied to get rid of dust. Similarly, at airports it can be used for dismiss the fog Structural test of metals (holes, cracks) Sonar: determination of sea/river depth, surface scanning It can kill microorganisms, it is appropriate for sterilization 8

9 Other areas of application Therapic application US The attenuation of US is due to the absorption mainly. The absorbed vibrational energy can have - Heat effect (increased particle motion) - Non thermal effect (cavitation, cell membrane permeability change) Cavitation Formation of short-lived cavities (small bubbles) upon the breakage of intermolecular cohesion forces. Micromassage Tissue vibration with different frequences friction force heat production 9

10 Ultrasound instrument transducers Source of images: Color doppler: Toward to Transducer: warm colour From the Transducer: cold colour Slower: darker Faster: brighter Umbilical cort 10

11 Vascular Doppler Ultrasound: Doppler shift (f-f 0 ) is in the audible range Ultrasound v ' f f f0 2f0 cos v US f 0 = 8 MHz Loudspeaker f = MHz f = 5.4 khz Skin Vein Transducer f f f 0 Θ v US v v' 75cm s vus 1540m s 45 f 8MHz 0 11

12 Ultrasound - Doppler 1842: Christian Doppler Doppler ultrasound: Frequency of US reflected from a moving surface is changed relative to the original frequency: f v ' f0 1 vus f=frequency of reflected US f 0 =original frequency v US =propagation velocity of US in the medium v'=velocity of the reflecting surface Continous ultrasound should be used vus f f0 v ' 2f cos Applications: 1. Doppler echocardiography 2. Vascular Doppler, blood flow 3. Fetal ultrasound v'=velocity of blood flow v US =propagation velocity of US in the medium f-f 0 =Doppler shift =angle between US beam and the blood flow axis Ultrasound Doppler echocardiography Colour Doppler echocardiography RV LV RA LA Ao Aorta insuffitienty 12

13 2D B-scan Sequence of one dimensional B(rightness) images. Multiple reflection echo shadow artefact Stretching/conraction of area(s) on screen 3D reconstruction With a sequence of 2D B images, one can collect information from all the volume elements of a selected volume within the body. 13

14 Transducer polarity cabel other polarity cabel Image source : Generation of US: Inverse piezoelectroic effect + Piezoelectric crystals: Natural crystal: quartz Artificial crystal: ceramic wafer, PZT (lead zirconate titanate) Direct piezoelectric effect (mechanic deformation charge separation electric potential) Inverse piezoelectric effect (alternating voltage movement of ions/charges mechanic deformation US) 14

15 Ultrasound pulse Skin 1 ms 1 khz Pulse separation time Propagation speed of US in soft tissues is 1450 m/s transducer lifetime of a pulse μs Ultrasound tomography - basics Why don t we use US for total body section? The answer is given by the properties of US: absorbance, resolution, detection depth falciform ligament FL gastrolienal ligament GL lienorenal ligament LR lesser omentum LO 15