ULTRASOUND In MEDICINE

1. ULTRASOUND In MEDICINE Dr/Aida Radwan

2. Chapter (1) ULTRASOUND WAVES

3. Most of the information about our physical surroundings comes to us through our senses of hearing and sight المشاهده. In both cases we obtain information about objects without being in physical contact with them.

4. The information is transmitted to us in the first case by sound, in the second case by light. Although sound and light are very different phenomena, they are both waves.

5. A wave can be defined as : a disturbance اضطراب that carries energy from one place to another without a transfer of mass. The energy carried by the waves stimulates our sensory mechanisms.

6. Properties of Sound Sound is a mechanical wave produced by vibrating bodies. For example, when an object such as a tuning fork or the human vocal cordsالاحبال الصوتية is set into vibrational motion, the surrounding air molecules are disturbed and are forced to follow the motion of the vibrating body.

7. The vibrating molecules in turn transfer their motion to adjacent molecules causing the vibrational disturbance to propagate away from the source.

8. When the air vibrations reach the ear, they cause the eardrum طبلةالاذن to vibrate; this produces nerve impulses that are interpreted تفسر by the brain. All matter transmits sound to some extent, but a material medium is needed between the source and the receiver to propagate sound.

9. This is demonstrated by the well-known experiment of the bell in the jar. When the bell is set in motion, its sound is clearly audible. As the air is evacuated from the jar, the sound of the bell diminishes and finally the bell becomes inaudible.

10. The propagating disturbance in the sound-conducting medium is in the form of alternate compressions and rarefactions of the medium, which are initially caused by the vibrating sound source.

11. These compressionsand rarefactionsare simply deviations in the density of the medium from the average value. In a gas, the variations in density are equivalent to pressure changes.

12. Two important characteristics of sound are intensity, which is determined by the magnitudeقيمة of compression and rarefaction in the propagating medium, and frequency, which is determined by how often the compressions and rarefactions take place.

13. Frequency is measured in cycles per second, which is designated by the unit hertzafter the scientist Heinrich Hertz. The symbol for this unit is Hz. (1 Hz 1 cycle per second.)

14. The properties of sound can be explained in terms of simple sinusoidal vibrations such as would be set up by a vibrating tuning fork. This type of simple sound pattern is called a pure tone.

15. the distance between the nearest equal points on the sound wave is called the wavelengthλ. The speed of the sound wave (c) depends on the material that propagates the sound.

16. In air at 20◦C, the speed of sound is about 3.3×104 cm/sec, and In water it is about 1.4×105 cm/sec. In general, the relationship between frequency, wavelength, and the speed of propagation is given by the following equation: c =λ f f= c/ C= speed f = frequency  = wavelength

17. The pressure variations due to the propagating sound are superimposed on the ambient air pressure. Thus, the total pressure in the path of a sinusoidal sound wave is of the form P =Pa+Posin 2πft where Pais the ambient air pressure الضغط الجوي المحيط (which at sea level at 0◦C is 1.01× 105Pa = 1.01×106dyn/cm2)

18. Pois the maximum pressure change due to the sound wave, and f is the frequency of the sound. The amount of energy transmitted by a sinusoidal sound wave per unit time through each unit area perpendicular to the direction of sound propagation is called the intensityIand is given by

19. Here ρis the density of the medium, Pois the maximum pressure change due to the sound wave and cis the speed of sound propagation.

20. Some Properties of Waves The propagation of sound waves is a longitudinal wave because the motion of the molecules in the medium is parallel to the direction of wave propagation.

21. A wave with a frequency between about 20 and 20,000 Hz is a sound wave that is audible to the human ear. An infrasonic wave is a sound wave below 20 Hz; it is not audible to the human ear. An ultrasound (or ultrasonic) wave has a frequency greater than 20,000 Hz and is also inaudible.

22. In clinical diagnosis, ultrasound waves of frequencies between 1 and 20 MHz are used. As a longitudinal wave moves through a medium, molecules at the edge of the wave slide past ينزلق بعيدا one another.

23. Resistance to this shearing effect causes these molecules to move somewhat in a direction away from the moving longitudinal wave. This transverse motion of molecules along the edge of the longitudinal wave establishes shear waves that radiate transversely from the longitudinal wave.

24. In general, shear waves are significant only in a rigid medium such as a solid. In biologic tissues, bone is the only medium in which shear waves are important.

25. The maximum height of the wave cycle is the amplitude of the ultrasound wave.

26. In most soft tissues, the velocity of ultrasound is about 1540 m/sec. When two waves meet, they are said to “interfere” with each other . There are two extremes مدي of interference التداخل .

27. In constructive interference the waves are “in phase” (i.e., peak meets peak). In destructive interference the waves are “out of phase” (i.e., peak meets valley).

28. Waves undergoing constructive interferenceadd their amplitudes, whereas waves undergoing destructive interference may completely cancel each other.

29. Waves can exhibit interference, which in extreme cases of constructive and destructive interference leads to complete addition (A) or complete cancellation (B) of the two waves.

30. Characteristics of an ultrasound wave.