Procedures and Surgery
Ultrasound scan
Ultrasound Scan
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Fundamental of Ultrasound Physic

Diagnostic Medical Ultrasonography

Imaging technique is used to visualize soft tissue structures of the body, by using ultrasonic sound waves.

This scanning procedure is normally done by a qualified medical sonographer who has experience in ultrasound scanning for over past few years by using a high resolution ultasound machine.

Ultrasonography imaging is widely used in medicine in order to perform both diagnosis and therapeutic medical procedures. Sonographers are medical professionals who perform scans for diagnostic purposes. Sonographers typically use a hand-held probe (called a transducer) that is placed directly on and is moved over the patient.

Role of a Medical Sonographer

  • The medical sonographer is a person who is in-charge and is responsible to perform diagnostic ultrasound scanning either requested or referred by doctor and under the supervision of a physician.
  • They are specially trained and have the ability (skills and great ultrasound techniques)  to use the ultrasound technology to obtain and capture image of soft tissue structures and organs in different parts of human body. The organs whether normal or disease that can be detected easily by the sonographers are :
  • Liver
  • Pancreas
  • Kidney
  • Heart
  • Vascular
  • Uterus
  • Ovary
  • Breast
  • Thyroid
  • Fetus (baby) & placenta
  • They work directly with the patients and review their medical history then correlates their sonographic findings with the patient’s clinical findings and provide the physician with information and data that will help in the evaluation and detection of abnormalities and diseases.

Understanding Fundamental Physics of Ultrasound

Ultrasound uses high frequency sound waves which is inaudible to humans. Audible sound wave is between 20 - 20,000 Hz. Pulses of ultrasound is between 1 and 30 M Hz.  

Sound waves do not travel in vacuum. Instead, the propagation of sound wave is made through a particular medium. Speed of sound wave is depends on both the density and compressibility of that particular medium.

Different soft tissues in human body have different speed of sound.  

Speed of Sound
bone 3500m/ sec
blood 1560m/ sec
fat 1460m/ sec
muscle 1600m/ sec
air 330m/ sec

Because of the difference in speed of sound in various tissue, ultrasound system has made as assumption that all the soft tissue has a single figure of speed of sound. Therefore an average speed of sound for soft tissue is 1540 m/sec.

Principles of ultrasound used in medical ultrasonography

  • Ultrasound can be directed as a beam.
  • Ultrasound follows strictly the laws of reflection and refraction.

Limitations of ultrasound

  • Ultrasound propagates poorly through a gas/bone structure medium.
  • The amount of ultrasound reflected depends on the acoustic mismatch between two interfaces / medium which may not be present in all organs.

Main differences between Ultrasound and X-rays


Diagnostic Ultrasound


Wave type

longitudinal mechanical waves (no radiation involves)

electromagnetic waves (radiation involves)

Transmission requirements

elastic medium

no medium


depends on the medium through which it propagates

It is relatively constant: 299,792.456.2 m/s

The above illustrates major differences between diagnostic ultrasound and X-ray radiology. Because of these differences, ultrasound has the advantage in soft tissue disease diagnosis compare to X-ray.

Superficial structures such as muscles, tendons, testes, breast and the neonatal brain are imaged at a higher frequency (7-18 MHz), which provides better axial and lateral resolution. Deeper structures such as liver and kidney are imaged at a lower frequency 1-6 MHz with lower axial and lateral resolution but greater penetration.

Pulse Echo Technique

Medical ultrasound used reflected sound to display organ images. Ultrasound pulses are introduced into the body and when these pulses meet interfaces in the body, these pulses return back to the transducer as echoes. These echoes are representative  of interface in the body. The echoes are processed and are then displayed in the screen in greyscale (grey colour). Structures that appear white, represents strong reflector, while structures that appear black represent poor reflector.   For example, when the sound beam pass through water, it will be displayed as black and when it pass through bone it will be displayed as white.

Sound Waves Parameters
Sound Waves are describe in parameters such as period, intensity, wavelength, velocity, phase, power, amplitude, frequency. 

Period - Time taken for a cycle to occur. Period is inverse of frequency. As the frequency increases, the period decreases. Conversely, as the frequency decreases, the period increases.                                     

                                         T  = 1 / f                                            T - period,  f - frequency


Power - rate of energy being transfer. Power is proportional to the wave amplitude squared.                     

                                      Power = amplitude2                        Unit - Watts, W       


Intensity - define as power per unit area 

                                       I    =    Power (W) / Area (cm2)      Unit - W/cm2


Wavelength - Distance of a cycle. It is determined by the frequency of a wave and the speed of propagation in the medium.

                                              wavelength (λ ) = propagation speed (c) /  frequency (f)     


Frequency - Number of cycles per second.
                       Frequency is inverse of period
                       1 Hertz = 1 cycle per second  

                                       f    =       1 / T                                   f- frequency,  T- period 


Amplitude - Strength of a sound wave