sound in medicine

??Infrasound
??< 20 Hz
??Earthquake, atmospheric pressure changes, blower in ventilator
??Not audible
??Headaches and physiological disturbances
??Sound
??20 ~ 20,000 Hz
??Audible
??Ultrasound
??> 20 kHz
??Not audible
??Medical imaging, blood flow measurements, etc.
General Properties of Sound
??Sound:
1-Mechanical disturbance or vibration in a gas, liquid, or solid.
2-Travels from a source with some definite velocity.
? 3-Vibration ??local increase (compression) or decrease (rarefaction) of pressure relative to atmospheric pressure.
4- Longitudinal wave: pressure changes in the same direction as wave.
5- Compression and rarefaction: density changes by displacements of atoms and Molecules.

*The relationship between the frequency of vibration ? of the sound wave, the wavelength ? , and the velocity ? of the sound wave is
? = ??
*The intensity ? of a sound wave is the energy passing through 1 m2/sec, For a plane wave? Is given by
?=1/2 ??A2(2??)2=1/2?(A?)2
Where ? is the density of the medium , ? is the velocity of sound , ? is the frequency, ? is the angular frequency, which equals (2??), A is the maximum displacement amplitude of the atoms or molecules from the equilibrium position ,and ? is the specific acoustic impedance .
*The intensity can also be expressed as:
?=?02/2? ? ?0=?(2 ? ?) ?0=The maximum change in pressure.
H.W: The maximum sound intensity that the ear can tolerate at 1000Hz is approximately (1w/m2). a) What is the maximum displacement in air corresponding to this intensity?
b) The faintest sound intensity the ear can hear at 1000 Hz is approximately 10-12 W/m2. What is A under these conditions?
We can use ratios between cases a and this case.
This displacement is smaller than the diameter of the hydrogen atom,
C) Calculate the sound pressures for cases a and b?

??Sound intensity
?Reference sound intensity and pressure
?I0 = 10-16 W/cm2, P0 = 2?10-4 dyne/cm2
?Barely audible at 1000 Hz by a person with good hearing
?Maximally intense sound without pain = 120 dB

R/A0 = ?2 -?1/?1 + ?2
For sound wave in air hitting the body, ?1 is the acoustic impedance of air and ?2 is the acoustic impedance of tissue.
If ?1 = ?2 ? no reflection and transmission to sound, medium is complete.
If ?2 ? ?1 ? sound wave passes through tissue. There is some loss of energy due to frictional effects.
?
??Absorption of sound energy
?Amplitude at depth of x = A(x) ??A0e?a x
?a = absorption coefficient in cm-1 at a particular frequency
?Intensity, I ??A2
I (x) ??I0 e???ax
?The HVT (half-value thickness) is the tissue thickness needed to decrease I0 to I0/2
? -High absorption in the human skull.
- Absorption increases, as sound frequency increases ??there is maximal
Frequency that can be used in the human body
??Divergence (spreading out) of sound
?Decreases intensity
?If point source, intensity decreases by the inverse square law (I ??1/x2)
Where x is the distance from the source to the measuring point.
The Stethoscope
??Hearing aid for sounds from the heart and lungs (Fig.1)



?Mediate auscultation: The act of listening to the sound of the heart and lungs using a stethoscope.
??Modern stethoscope: bell, tube, ear pieces (Fig.2).




??Open bell: small air volume is desirable
?The open bell is an impedance matcher between the skin and the air and accumulated sounds from the contacted area. The skin under the open bell behaves like a diaphragm. The skin diaphragm has a natural resonant frequency at which it most effectively transmits sounds; the factors controlling the resonant frequency are similar to those controlling the frequency of a stretched vibrating wire. The tighter the skin is pulled, the higher its resonant frequency. The larger the bell diameter, the lower the skin s resonant frequency.
? Closed bell with diaphragm: better for lung sounds, which are of higher frequency than heart sounds, small air volume is desirable.
??Tube: 20 cm long, 0.3 cm diameter
??The volume of the tubes should also be small, and there should be little frictional loss of sound to the walls of the tubes. The small volume restriction suggests short, small diameter tubes, while the low friction restriction suggests large diameter tubes. If the diameter of the tube is too small, frictional losses occur, and if it is too large, the moving air volume is too great; in both cases the efficiency is reduced.
??Ear pieces: must fit snugly in the ear to minimize air leakage.

Ultrasound Pictures of the Body
??Bats and porpoises emit ultrasound (30 ~ 100 kHz) and listen to the echoes to navigate: delay time of echo ??distance to the object
??SONAR (Sound Navigation and Ranging) it was discovered during World War II, where the sound wave pulse is sent out and reflected from object.
From the time required to receive the echo and the known velocity of sound in water, the distance to the object can be determined.
- To obtain diagnostic information about the depth of structures in the body, we send pulses of ultrasound into the body and measure the time required to receive the reflected sound (echoes) from various surfaces in it. This procedure is called the A scan method of ultrasound diagnosis.

??A scan
?- Pulse transmission: a few ?s long pulse with 400 ~ 1000 pulse/s.
? -Two medium with different acoustic impedance ??reflection.
A scan procedure, echoencephalography has been used in the detection of brain tumors. Pulses of ultrasound are sent into a thin region of the skull slightly above the ear and echoes from the different structures within the head are displayed on oscilloscope, the usual procedure is to compare the echoes from the left side of the head to those from the right side and to look for a shift in the midline structure. A tumor on one side of the brain tends to shift the midline toward the other side, a shift of more than 3mm for adult or 2mm for child is considered abnormal.
- Application of A scans in ophthalmology can be divided into two areas:
One is concerned with obtaining information for use in the diagnosis of eye diseases.
The second involves biometry, or measurements of distances in the eye.
At the low power levels used, there is no danger to the patient s eye. Ultrasound frequencies of up to 20MHz are used. These high frequencies can be used in the eye to produce better resolution since there is no bone to absorb most of the energy and absorption is not significant because the eye is small.
-Ultrasound diagnostic techniques provide information about the deeper regions of the eye and are especially useful when the cornea or lens is opaque. Tumors, foreign bodies, and detachment of the retina (the light-sensitive part of the eye) are some of the problems that can be diagnosed with ultrasound. With ultrasound, it is possible to measure distances in the eye such as lens thickness, depth from cornea to lens, the distance to the retina, and the thickness of the vitreous humor.

??B scan
The B scan method is used to obtain two-dimensional views of parts of the body .The principles are the same as for the A scan except that the transducer is moved. As a result, each echo produces a dot on the oscilloscope at a position corresponding to the location of the reflecting surface. A storage oscilloscope is usually used so that a lasting image can be formed and a photograph can be made.
B scan provide information about the internal structure of the body. They have been used in diagnostic studies of the eye, liver breast, heart, and fetus. They can detect pregnancy as early as the fifth week and can provide information about uterine anomalies.


Ultrasound to Measure Motion:
Two methods are used to obtain information about motion in the body with ultrasound:
M (motion) scan.
Used to study motion of the heart and heart valves.
The M scan combines certain features of the A scan and B scan. The transducer is held stationary as in the A scan and the echoes appear as dots as in the B scan of the mitral valve and pericardial effusion can be detected with an M scan.
2-?Doppler technique
-Used to measure blood flow. -Detect motion of fetal heart, umbilical cord, and placenta in order to establish fetal life during the 12 to 20 week of gestation.

H.W.: Explain the Doppler Effect?

Physiological Effects of Ultrasound in Therapy
? Low intensity ultrasound levels used for diagnostic work (0.01 W/cm2 average power and 20W/cm2 peak power)???no harmful effects are observed. As the power is increased, ultrasound becomes useful in therapy. Ultrasound is used as a deep heating agent at continuous intensity levels of about 1 W/cm2 and as a tissue-destroying agent at intensity levels of 103 W/cm2.
The primary physical effects produced by ultrasound are temperature increase and pressure variation. The primary effect used for therapy is the temperature rise due to the absorption of acoustic energy in the tissue. Ultrasound diathermy complements deep heating electromagnetic diathermy.
Ultrasound waves differ completely from electromagnetic waves, they interact with tissue primarily by microscopic motion of the tissue particles. As a sound wave moves through tissue, the regions of compression and rarefaction cause pressure differences in adjacent regions of tissue. Stretching occurs in these regions, if the stretching exceeds the elastic limit of the tissue, tearing results. This is why an eardrum can be ruptured by a very intense sound source. In physical therapy the typical intensity is about 1 to 10 w/cm2 and the frequency is about 1 MHz .
-Intense ultrasound waves can change??water into H2 and H2O2 and??rupture DNA molecules.
***Negative pressure in the tissue during rarefaction can cause dissolved gas to come out of solution and form bubbles. This forming of bubbles called cavitation, can break molecular bonds between the gas and tissue. The collapse of the bubbles releases energy that can also break bonds. Free radicals produced during the breaking of bonds can lead to oxidation reaction. At power levels of 103 w/cm2 it is possible to selectively destroy tissue