Usg physics for a non radiologist an overview

Basics of ultrasound and physics for a non radiologist

1. ULTRASOUND PHYSICS

2. INTRODUCTION sound wave ---the propagation of mechanical energy through a medium Pressure wave with rarefaction and compression Human hearing range 20Hz to 20000Hz <20 Hz -- infrasound > 20000Hz -- ultrasound For diagnostic applications from 2 to 15 MHz

3. BASIC ACOUSTICS

4. Transverse waves-- waves travelling perpendicular to directions of particle movement Used in shear wave elastography Longitudinal waves-- waves along the directions of particle movement Used in conventional ultrasound imaging and doppler Propagation velocity determined by resistance, density, stiffness of medium

5. 3 parameters describe the strength of a wave: amplitude, power and intensity Amplitude -- size of the wave and is defned graphically as the difference between the average and maximum values of the wave Power -- energy generated per unit time (Joules/second) and is expressed in Watts Intensity represents the concentration of energy in a cross-section of a sound beam power /cross-sectional area of the sound wave

6. Acoustic velocity/ propagation velocity(c) is the speed at which sound waves travel through a medium directly proportional to the density and stiffness of the medium The velocity is fastest in solids and slowest in air The average speed of propagation of ultrasound in body tissue is about 1540 m/s

7. Propagation velocity c = f λ

8. Acoustic impedance is the frequency-dependent resistance that an ultrasound beam encounters as it passes through a tissues Z = ρ ∙ c, where Z = acoustic impedance, ρ = density of the medium, c = speed of sound in the medium. The difference in acoustic impedance between two tissues infuences the amplitude of the returning echo. The amount of reflection or backscatter is determined by the diference in the acoustic impedances of the materials forming the interface.

9. INTERACTIONS OF ULTRASOUND WITH TISSUES Reflection Transmission Attenuation Scattering

10. Reflection -- phenomenon in which a part of the energy is sent back to the medium from which the energy originates Refraction is the change of direction of sound while crossing the interface between two media Attenuation-- is the amplitude of the sound waves decreases with increasing depth of penetration in the body attenuation is directly related to the frequency of the ultrasound beam Scattering -- is the redirection of sound waves in different directions caused due to interaction with a rough surface or small reflector

11. RESOLUTION Resolution is the ability to distinguish between two structures that are positioned close to each other. Resolution depends on the frequency of ultrasound, higher frequency gives better resolution Resolution can be classifed as spatial and temporal.

12. Spatial Resolution is the ability of ultrasound to distinguish between two objects lying side by side. It can be of two types, axial and lateral.

13. Axial resolution --two structures lying along the ultrasound beam axis as separate and distinct Affected by the frequency of the beam The best achievable axial resolution is 0.5λ Lateral resolution -- two structures lying perpendicular to the beam axis, structures lying side by side. It is affected by the beam width Temporal resolution is the ability to precisely locate moving structures at given time instants. depends on the processing speed and refresh rate of the USG machine

14. ULTRASOUND TRANSDUCER Quartz, lithium niobate and tourmaline

15. ECHOGENECITY hypoechoic , e.g. muscles hyperechoic , e.g. fascia, bones, pleura anechoic , e.g. liquid flled cavity such as blood vessels, pleural effusion

16. ULTRASOUND MODES There 3 basic modes Amplitude-mode --A line through the target is scanned after which amplitudes of the ultrasound wave echoes are returned to the transducer utilized to measure the corneal thickness in ophthalmology. This one dimensional information can be utilized to infer depth of a structure

17. Brightness-Mode --utilized to detect static structures and appreciate anatomy. Amplitudes from a returning ultrasound wave are displayed as points with differing brightness. The brightness of a point represents the strength of the return wave. Once all the echoes from the subsequent transmitted pulses have returned, a complete 2D Brightness mode image is displayed

18. Motion-Mode -- is utilized to image rapidly moving structures such as cardiac valves or vessel walls. motion mode utilizes repeated emission of an ultrasound beam in a stationary location to gain information from moving structures at different time points. Information is displayed along a time axis which describes the motion of the structure at varying time points

19. TYPES OF TRANSDUCERS There are four main types of transducers utilized to image the body: curvilinear , phased array , linear array and endocavitary Curvilinear transducers are ideal for abdominal imaging. They have excellent tissue penetration which allows for imaging of deeper structures. Typical frequency range is from 2–5 MHz

20. Linear array transducers are best for imaging superfcial structures such as muscles, nerves, vasculature or soft tissues. They produce a rectangular image, high frequency and low penetration Typical frequency range is from 5–10 MHz

21. Phased array transducers are best utilized to image through small regions such as between ribs They are commonly used in cardiac imaging A pie-shaped feld of view is created utilizing electronic beam steering. Typical frequency range is from 2–7 MHz

22. Endocavitary transducers are placed inside a body cavity and are primarily used for obstetric, gynecologic or otolaryngology applications. They produce wide angle images up to 180 degrees. Typical frequency range is from 8–13 MHz

23. TRANSDUCER POSITION All transducers have a position indicator which corresponds to the marker on the image screen Standard ultrasound imaging planes include the transverse (also known as axial), sagittal and coronal planes

24. TIME GAIN COMPENSATION operator-controlled amplifcation technique to make up for the sound attenuation as ultrasound waves travel through tissue. must be manually adjusted for each tissue type to be scanned and manipulated for best image optimisation Sliders knobs-- controls the gain for a specifc depth, which gives a well-balanced image the sliders are called near field TGC and far-field TGC

25. PRACTICAL ASPECTS For image optimization, the following points should be kept in mind • Frequency of the transducer • Depth adjustment • Gain • Focus • Compound imaging use

26. Frequency --higher frequency is usually selected for superfcial interventions that require greater resolution With decreasing frequency, tissues at greater depth can be imaged, at the cost of resolution Depth --Depth of the image has to be adjusted to the depth of the object to be blocked or the depth of the intervention endpoint The depth selected should be at least a few centimetres more than the depth of the target needle overshoots the target it can be seen

27. Decreasing the depth increases magnifcation and vice versa For superfcial blocks, by decreasing depth setting, greater details can be appreciated Gain -- the gain of function is used to increase overall screen brightness. An optimum gain should be used to obtain the best possible contrast between the muscles and the connective tissue (fascia) for a nerve block because usually the nerves produce echo that is similar to the connective tissue. While TGC controls are used to modify the gain at different depths, overall gain can be adjusted using the gain button

28. Focus -- focus of the ultrasound image is the narrowest point of the ultrasound beam. It is the point where the image resolution is best. Modern ultrasound machines have electronic focus adjustment capacity. It is best to place the focus just at the level of or slightly below the object to be viewed for optimum image quality

29. NEEDLING TECHNIQUES In Plane --the needle is placed parallel to the transducer. The needle shaft and tip are both visible The quality of needle visualisation depends on the angle of entry of the needle Flatter the angle (acute angle), the better is the image. To reach deeper objects, a more perpendicular angle is required

30. Out of Plane --the needle is placed perpendicular to the transducer probe. The tip of the needle may be diffcult to locate accurately in this approach and the use of echogenic tip needles is advised It is important to observe tissue displacement by the advancing needle tip as often the needle tip may not be seen Actual needle to nerve contact can be verifed by nerve stimulation and pattern of local anaesthetic spread.

31. BIOEFFECT AND SAFETY Thermal Index (TI) --It is the transducer acoustic power divided by the estimated power needed to increase tissue temperature by 1 degree. Mechanical Index (MI) --It is the peak rarefactional pressure divided by the square root of the centre frequency of the pulse bandwidth. The relative likelihood of thermal and mechanical hazard is indicated by TI and MI TI or MI >1.0 is dangerous