Manual of Ultrasound in Obstetrics and Gynaecology Kakoli Ghosh Dastidar
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Basic Physics of Diagnostic UltrasoundChapter 1

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WHAT IS DIAGNOSTIC ULTRASOUND?
Sound, as a mode of explorative studies was first developed by Professor Langevin for the First World War to fight the U-Boat menace. In a similar technique ultrasound or sound of high frequency that is beyond audible range, was used later for diagnostic purposes in medical science. It is produced electronically by converse piezoelectric effect, a property exhibited by some elements in nature called piezoelectric crystals. Piezoelectric effect was first described by Madam Curie. But now, the transducers or ultrasound producing crystals are also manufactured artificially. Like audible sound, ultrasound waves are also mechanical-pressure waves and need a medium for propagation.
The world of present day diagnostic ultrasonography was essentially a black and white one with various shades of grey between them until recently. Recent developments in vascular flow studies by Colour Doppler has added a lot of colour to this already exciting technology.
The perfection of diagnostic superiority in real time studies lies in the correct appreciation of the various shades of black colour and their respective interpretation by an experienced sonologist.
 
PIEZOELECTRIC EFFECT
Certain crystals in nature got intrinsic property of transforming electrical energy to mechanical 3energy and vice-versa designated as converse peizoelectric effect and peizoelectric effect. Such crystals are arranged in the face of transducer. Crystals commonly used are Lead Zirconium titanate.
Terminology and principles that govern the passage of diagnostic ultrasound waves through tissues:
Ultrasound is sound whose frequency is beyond the audible range. They are mechanical pressure waves.
Frequency of ultrasound is given as number of cycles of waves passing a point per unit time (second). The higher the frequency, lesser is the tissue penetration. The speed at which ultrasound waves pass through the medium is called acoustic velocity (C) which is given by
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Acoustic velocity C = Acoustic wave length × Frequency, and its value for different media are given in Table 1.1.
Table 1.1  
Air
330 mt/sec
Water
1480 mt/sec
Fat
1450 mt/sec
Blood
1570 mt/sec
Kidney
1560 mt/sec
Soft tissue (average)
1540 mt/sec
Liver
1550 mt/sec
Muscle
1580 mt/sec
Bone
4080 mt/sec
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Acoustic waves may be longitudinal or transverse during a study. Longitudinal waves have direction of particle motion parallel to wave velocity. Transverse waves have direction of particle motion perpendicular to wave velocity. For this reason only longitudinal waves can propagate in soft tissues of body. In a wave form of transmission, the wave length is equal to repetition distance in space for a single frequency wave.
Acoustic intensity (I) is the energy propagating through a unit area in the medium normal to the direction of propagation per unit time and is given as I = mill watt/sec.
Acoustic impedance (Z) is the obstructive capacity of the particular tissue, through which the high frequency sound is being sent, towards passage of ultrasound waves through itself. It is equal to the product of tissue density and acoustic velocity and is dependant on tissue character (density P). It is extremely important in predicting magnitude of reflected echoes at tissue interface which forms the backbone for formation of sonographic picture.
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Acoustic impedance = Tissue density (gm/cm3) × Acoustic velocity (cm/sec.)
Technique for measurement of distance of anatomy or pathology within the tissue is done using 5the time taken for pulsed sound wave to travel from the source to tissue interface (which acts as a sound reflector) and back to the echo receiver. Using the acoustic velocity of the medium, the depth or distance is calculated using the formula.
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Distance = ½ Acoustic velocity × time from source to reflector and back.
 
REALM OF DOPPLER
The Doppler effect is a change in the frequency of sound emitted by a moving source. The frequency change of back scattered echo can be calculated with the Doppler shift equation:
  • Δν = Frequency change (Doppler shift Hz)
  • ν = Frequency of initial beam (Hz)
  • s = velocity of blood (m/sec)
  • v = velocity of sound (1540 m/sec)
  • θ = angle between sound beam and direction of blood flow.
The smaller the angle between the sonic beam and flow direction (θ), the greater the vector of motion towards the transducer, and greater the Doppler shift.