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Ultrasound is a non-invasive imaging modality in which image of the internal anatomical structure is obtained in the form of dotted structures with the help of frequency of 2-20MHz which is beyond the audible range of human being. Because of high frequency the penetration power of ultrasonic waves is very high and it can penetrate into deeper body parts.

These high frequency sound waves are produced by transducer. This transducer acts as both transmitter and a receiver of sound waves. They have piezo electric crystals which are align in proximal and distal rows.

Physics of ultrasound:

Piezo electric crystals: Piezo electric crystals are those substances which vibrate vigorously when electric current is supplied to them. When electricity applied to the transduced the Piezo electric crystals start vibrating in this way the electrical energy is converted into mechanical energy. Due to vibration high frequency sound waves are produced. Therefore, converting the mechanical energy in sound waves passes through the different interfaces of internal anatomical structure and absorbed/scattered according to the density of the organ and then these sound waves are again reflected back in the form of echoes which are align receive by the same transducer. This transducer converts the sound energy into electrical signals in term are converted into digital convertor and these digital signals are displayed on oscilloscope in the form of dotted anatomical structures.

Ultrasound is a non-invasive imaging mortality and does not shows any adverse biological effect of its clinical applications and therefore it can be safely used for all age groups and also for fetal evolution and neo-natal neuron’s studies it gives excellent soft tissues contrast, resolutions and has wide clinical applications in gynecology, obstetrics, pediatrics and cardiac imaging.

Wave length: The distance between two crest and trough of a sound wave particularly between two points is known as wavelength. Wavelength is a very specific characteristic sound as it determines the rate of propagation of sound waves through a medium. Smaller the wavelength larger the frequency of the sound waves and therefore greater is the intensity or the penetration power of the sound energy.

Frequency: Number of cycle per second repeated by the sound waves is known as frequency both wavelength and frequency of the sound wave determines the velocity of sound along the medium of propagation.

Acoustic impedance: Acoustic impedance is a very common terminology used in ultrasound in common term it is defined as the opposition or resistance offered by the system or the system medium through which the sound waves are travelling. It results in production of an acoustic pressure in the system.

Acoustic enhancement: It is also known as posterior enhancement and it refers to the increased echoes deep to the structure that transmit the sound waves exceptionally well. This is the most common phenomenon in case of body structures filled with fluid like gall bladder, hydrated cyst, urinary bladder etc.

Resolution: In general term resolution is the ability of an imaging mortality to differentiate between the adjacent body structures. In ultrasonography it is commonly reflexed depth or linear resolution and it is parallel to the direction of the ultrasound beam and therefore it is not affected by the depth of scanning.

Various modes of ultrasound:

Generally there are three modes of ultrasound which used in clinical application. These are as follows:

  1. A mode/amplitude mode of ultrasound: Amplitude is defined as the maximum height of the crest or the maximum depth of trough. It is denoted by ‘A’. The amplitude mode of ultrasound is the display of the amplitude spikes of different height. The most common clinical application of amplitude mode ultrasound is in ophthalmic studies like optic neuroma, optic glioma and other diseases of optic canal. The amplitude mode of ultrasound represents the height of sound signal produced through various tissues texture. A mode has two axes namely X & Y where in the x-axis represents the depth and the y-axis represent the amplitude of the sound waves.
  2. B mode/brightness mode of ultrasound: The brightness mode is the most common form of ultrasound which is used in clinical application. It is the display of 2D map of the brightness data on the oscilloscope. It is represented by X & Z axis and there is no Y-axis. The x-axis represents the depth whereas the Z-axis represents the ultrasound waves the brightness mode of ultrasound produces image in the form of bright dots. Unlike amplitude mode it does not shows the vertical spikes and therefore the brightness in this mode depends upon the intensity of the amplitude.
  3. M mode/motion mode of ultrasound: Most of the ultrasound imaging of the moving body part like blood flow imaging, cardiac imaging, fetal cardiac imaging or using the motion mode. The motion mode uses real time image capturing of the moving body part.

Various types of Transducers:

Transducers are the most important component of ultrasound unit and therefore selection of the appropriate transducer according to the demand of scanning is very important for better image quality and proper diagnosis. Apart from selection acute angle of application of the transducer on body of the patient also plays an important role. Various types of transducer in ultrasonography depending upon different parameters like shape of the transducer, field of view, alignment of piezo electric crystals, frequency range and size of the transducer. Based on these following categories of ultrasound transducer are available.

  1. Linear transducer
  2. Curvilinear or convex transducer
  3. Phased array transducer
  4. Endo-cavital or endo-cavitary transducer

Apart from the above major categories there are some other types of probes for specific use like:

  1. Trans-esophageal transducer
  2. Trans-vaginal transducer
  3. Pencil transducer etc.

Some special kind of transducers is also used for surgical interventions like laparoscopy and Doppler imaging.

  1. Linear transducer: The linear transducers are high frequency transducers and commonly used for evaluation of superficial structures and blood vessels. They produce rectangular shape of ultrasound beam and their near field resolution is better. It has wide clinical application in the examination of tendons, superficial blood vessels, breast tissues, thyroid and ligaments of the synovial joints. Application of linear transducer depends on whether it is being used for 2D imaging or 3D imaging. Therefore there are two subcategories of linear transducer. The 2D linear transducer has a wide footprint but the frequency range is 7.5-11.5 MHz
  2. Curvilinear transducer/convex transducer: It is also known as convex transducer because of shape of its application surface and the beam. Convex transducer has wide footprint and lower frequency range. It is commonly used for in-depth examination like trans-abdominal examination, trans-vaginal and trans-rectal examination, extra peritoneal organs. Convex transducer has also two subcategories and selection depends upon depth of the area of interest. 2D convex transducer has a wide field range of 3.5-6.5MHz and commonly used for scanning of intra-abdominal organs. The 3D convex transducer has a less widened footprint and frequency range of 4.5-7.5MHz, it is ideal for the clinical scanning of retroperitoneal organs. Apart from this there is a subcategory of convex transducer known as micro-convex. It has small footprint and lower frequency range and commonly used for neo-natal range and pediatric studies.
  3. Phased array transducer: Phased array transducer is specially designed to scan between the ribs or inter costal spaces such as in case of cardiac imaging of the superficial surface of liver. This type of transducer has a small footprint and a central frequency ranging from 2MHz-7.5MHz. The beam point is narrow but it could expand depending upon the applied frequency. Phased array transducer is having a poor near field resolution.

Focal zone: The area where the ultrasound beams width is narrowest.

Near field: The area between the actual surface of the transducer or beam point and focal zone.

The phased array transducer can also be used for abdominal and brain scanning.

  1. Pencil beam transducer: These are also known as CW-Doppler transducer and these are frequently use to the blood flow and speed of sound waves in blood stream this is commonly used in diagnosis of varicose vein. These transducers have a small footprint and a low frequency range of 2-8MHz
  2. Endocavitary transducer: these are commonly used in TVS-trans vaginal scanning and TRS-trans rectal scanning. Typically they have a small footprint and a central to high frequency range of 3.5-11.5MHz.These are very useful in diagnosis of intrauterine complications like endometrium thickness/bulging, cervical cancer.
  3. Trans-esophageal transducer: this is also a kind of pointed transducer it has a narrow footprint and use for evaluation of internal examination. The frequency range is central to high ranging between 3-10.5MHz. It is commonly used for cardiac evaluation through the esophagus to obtain better information about the mitral stenosis.

Apart from the above transducer there are certain transducers especially designed for specific purposes like laparoscopic transducers used in laparoscopic surgery and biopsy transducer which are used in ultrasound guided biopsies.