• Piezoelectric effect
  • Pulse echo principle

Piezoelectric effect

  • An electric current passes through a cable to the transducer and is applied to the crystals, causing them to deform and vibrate. This vibration produces the ultrasound beam.

pulse-echo principle, 

  • Explains how the image is generated.
  • Ultrasound waves are produced in pulses, not continuously, because the same crystals are used to generate and receive sound waves, and they cannot do both at the same time.
  •  In the time between the pulses, the ultrasound beam enters the patient and is bounced or reflected back to the transducer.
  • These reflected sound waves, or echoes, cause the crystals in the transducer to deform again and produce an electrical signal that is then converted into an image displayed on the monitor.
  • The transducer generally emits ultrasound only 1% of the time; the rest of the time is spent receiving the returning echoes.

Interaction of ultrasound with matter

  1. Reflection – occur at tissue boundary and it create ultrasound image
  2. Refraction – change in direction of the transmitted ultrasound energy.
  3. Scattering – occur by both reflection and refraction
  4. Attenuation/Absorption – conversion of acoustic energy into thermal energy in the medium.

R = [(Z2 – Z1) ÷ (Z2 + Z1)]2 

Z= acoustic impedance is a physical property of tissue. It is describe how much resistance an ultrasound beam encounters as it passes through a tissue

R = resistance coefficient


  • 2D
  • 3D


  • two-dimensional scans are then combined by specialized computer software to form 3D images
  • Early detection of cancerous and benign tumors
  • examining the prostate gland for early detection of tumors
  • looking for masses in the colon and rectum
  • detecting breast lesions for possible biopsies
  • Visualizing a fetus to assess its development, especially for observing abnormal development of the face and limbs

Doppler ultrasound

  • the object reflecting the ultrasound waves is moving, it changes the frequency of the echoes,
  • creating a higher frequency if it is moving toward the probe
  • lower frequency if it is moving away from the probe.
  • A Doppler ultrasound is a noninvasive test that can be used to estimate the blood flow through your blood vessels by bouncing high-frequency sound waves (ultrasound) off circulating red blood cells.


  • ultrasound examination can be displayed in a variety of ways, called modes.
  • three types of USG mode
  • A-mode – AMPLITUDE OF THE REPLICATED WAVE – about the depth of structure and the amplitude of the returning echo
  • B-mode – BRIGHTNESS MODE – 2d information about the cross section.
  • M-mode – MOTION MODE – evaluate motion and timing, and can document tissue movement in a still image when the recording of a clip is not feasible.


  • To detect and measure distance of the object.
  • Helps in non-invasive painless treatment.
  • Preferred method to diagnose complications of unborn babies.
  • Ultrasound frequencies are unaffected by direct sunlight, fluorescent tubes and other sources of light.
  • Useful to acquire 3d image.


  • Ultrasound has limited use in the head and neck region because sound waves are absorbed by bone. Its use is therefore restricted to the superficial structure.
  • Lower resolution
  • Equipment expense
  • Less effective due to obesity ( subcutaneous fat ), gas ( gastric or intestinal),
  • Can’t penetrate bones and gas.
  • Do not provide direction and velocity of flow.