How Ultrasound Measurements are used in Medical Diagnosis

Introduction

The application of ultrasonic in medical field is based on sonar principle as used by bats, anglers with fish detectors, and ships at sea. This is a totally non-invasive procedure. Acoustic waves are easily transmitted in water and are reflected from an interface according to change in the acoustic impedance.

All tissues of our body are made of water which can transmit acoustic waves easily. Ultrasound can be used for obtaining images of internal organs by sending high frequency sound waves into the body. The reflected sound waves (returning echoes) are recorded and processed to reconstruct real-time visual images by the computer. The returning sound waves (echoes) reflect the size and shape of the organ and also indicate whether the organ is solid, fluid or something in between.

Unlike X-rays, Ultrasound requires no exposure to ionization radiation. It is also a real-time technique that can produce a picture of blood flow as it is taking place at the very moment of imaging.

Ultrasound is an important diagnostic tool in medicine. It is used to obtain images of a range of organs in the abdomen; including the liver, kidney, spleen, pancreas, bladder, major vessels and the foetus during the pregnancy. It is used to detect cysts, tumours and cancers in these organs. The greatest advantage that it is a non-invasive technique which does not involve injecting a dye or subjecting a patient to harmful X-rays.

Different soft tissues can be seen with excellent clarity in an ultrasonic image. The power levels of 1 watt/cm² of acoustic energy do not cause any damage to any tissue, either immediately or on a cumulative basis.

The Physics behind Ultrasound Imaging

Ultrasonic waves are sound waves with frequencies greater than 20 KHz. They require a medium for transmission. They can be focused into a beam which obeys the laws of reflection and refraction. Ultrasonic waves may be longitudinal, transverse or shear. For medical diagnostic applications, only the longitudinal mode of wave propagation is used.

When a beam of ultrasonic wave passes from one medium to another, a portion of sonic energy is reflected and the remaining is refracted. The amount of energy reflected depends upon the difference in density of the two media and the angle of incidence. If the difference in densities is very large and if the angle of incidence is near normal to the interface, a greater portion of the energy is reflected.

If the difference in densities between the two media is very large e.g. tissue and bone, or tissue and gas, all the energy is reflected and the ultrasound beam will not continue through the second medium. Therefore, when ultrasound is applied to the body, an airless contact of the tissue with transducer is made by using an aqueous gel or a water bag between the transducer and the skin. The density of water is 1 g/cc while that of bone is 1.77 g/cc.

The velocity of sound through any medium depends upon the following:

• Density of the medium
• Elasticity of the medium
• Temperature

The velocity of ultrasound wave through water is 1529 m/sec, through soft tissue it is 1550 m/sec, and through bone it is 3360 m/sec. The propagation of the wave depends upon the elastic properties of the medium. The average speed (c) of the ultrasound wave in biological tissue is given by:

Where β = modulus of elasticity and ρ = density of the medium through which the ultrasound wave is traveling through

Every material has acoustic impedance. Acoustic impedance or characteristic impedance is given by:

= ρv

Where

ρ = density of the medium

v = velocity of sound through it

                                           = cf^β

Where c = constant of proportionality

              f = frequency

              β = exponential term determined by the properties of material

This means the attenuation increases with increase in frequency. Thus, as the frequency rises, the wave can penetrate a smaller distance. Therefore, for deep penetrations, lower frequencies are used. Frequencies used range from 1 MHz to 15 MHz, at 2 MHz; distinct echoes can be recorded from interfaces 1 mm apart. Another key term is the half value layer. It is the depth of penetration at which the ultrasound energy is attenuated to half the applied amount.

Ultrasonic waves exhibit the same physical properties as the audible sound waves but they are particularly preferred in situations due to any one or more of the following reasons:

• Ultrasonic waves can be easily focused i.e. they are directional and beams can be obtained with very little spreading.
• By using high frequency waves which are associated with shorter wavelength, it is possible to investigate the properties of very small structures. It is particularly true in the detection of defects where the wavelengths utilised should be of the same order as the dimensions of the defect.
• They are inaudible and are suitable for applications where it is not advantageous to employ audible frequencies.
• Information obtained by ultrasound, particularly in dynamic studies, cannot be acquired by any other more convenient technique.

Wavelength and Frequency

Ultrasonic follow the general wavelength and frequency relationship given by:

v = fλ

Where v = propagation velocity of sound

               f = frequency or number of cycles which pass any given point in unit time

              λ = wavelength i.e. distance between any two corresponding points on consecutive cycles.

Ultrasonic frequencies employed in medical measurement applications range from 1 to 15 MHz; this range also corresponds to radio frequencies, however, there is an important basic difference between radio frequency and ultrasonic energy. Ultrasonic waves are transmitted as mechanical vibrations whereas radio frequency energy would be in the form of electromagnetic radiations. No medium is necessary for propagation of energy and it would therefore, pass even through vacuum; but ultrasonic waves will only pass through a medium.

Velocity of Propagation of Ultrasonic waves

Ultrasonic energy is transmitted through a medium as a wave motion, and therefore no net movement of the medium is expected to occur.

The velocity of propagation of the wave motion is determined by the density of the medium it is traveling through and the stiffness of the medium. At any given temperature and pressure, the density and stiffness of the biological substances are relatively constant and therefore the sound velocity in them is also constant.

The knowledge of velocity of sound in a particular medium is important in calculating the depth to which the sound wave has penetrated before being reflected. If we measure the time taken by the ultrasonic wave to move from its source through a medium, reflect from an interface and return to the source, then the depth of penetration is given by:

The velocity of ultrasound in all body tissues is almost constant. Therefore, the depth of penetration can be read directly from the position of the echo pulse on the calibrated time axis of the oscilloscope trace.

Absorption of Ultrasonic Energy

The reduction of amplitude of ultrasonic beam while passing through a medium can be due to its absorption by the medium and its deviation from the parallel beam by reflection, refraction, scattering and diffraction etc. The relative intensity and the attenuation of an ultrasonic beam is expressed in decibels (dB) and the absorption coefficient α is normally quoted in dB/cm. In soft tissues α depends strongly on the frequency and therefore, for a given amount of energy loss, the lower frequency ultrasonic signal would travel more than the higher frequency signal.

Quantitatively, the average value of sound absorption in soft tissues is of the order of 1 dB/cm/MHz; the table below shows the velocity of ultrasound, characteristics, impedance and absorption coefficient in various materials.

Doppler Effect

The frequency of the reflected ultrasound wave is increased or decreased by a moving interface like blood.

Where

Δf = change in frequencies of the reflected wave

v = velocity of the interface

λ = wavelength of the transmitted ultrasound

The frequencies increase when the interface moves towards the transducer and decreases when it moves away. This increase or decrease is measured.

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Basic modes of Trasmission of Ultrasound

The modes of transmission of ultrasound most commonly used in diagnostic medical applications are:

1. Pulsed Ultrasound: In this mode, energy is transmitted in short bursts (pulses) at a repetitive rate ranging from 1 to 12 KHz. The pulse duration is kept about one micro-second. The time and intensity of returning echoes depend upon the distances and the reflecting capacities of the targets. These are displayed as a function of time after transmission and this time is proportional to the distance from the source to the target. Any movement of the target or interface with respect to time can also be displayed.
2. Continuous Doppler: A continuous ultrasound beam is transmitted and a separate transducer is used to receive the returning echoes from the targets. The frequency shifts in the echoes due to motion of the targets are detected which help in determining the average velocities of different targets as the functions of time. Two transducer crystals are used in this method. One is used for transmission and the other for detecting echoes. The continuous Doppler method is used in blood flow measurement where the distance between the source and the target is unimportant.
3. Range Gated Pulsed Doppler: It is a refinement of the Pulsed Doppler method. A gating circuit is used which permits measurement of the velocity of targets at a specific distance from the source. The velocity of the target at a decided distance can be measured as a function of time. This method helps in measuring the velocity of blood not only as a function of time but also a function of the distance from the blood vessel wall.