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Foundations of Medical Ultrasound Imaging
Ultrasounds in some drawings :


Refraction




Reflection and transmission



Plane wave





Spherical wave

In this page, we can try to explain the foundations of medical imaging by ultrasounds. For that, we will explain at first what is an ultrasound signal to after understand the basics principle, how we can make an image with those ultrasounds. To finish, we will see different possibilities offered by this method.

So, let's begin by this question :

What is Ultrasound ?

Ultrasounds can be defined like this : 
  • Frequency of ultrasounds are higher than the upper limit of hearing which is about 20kHz but this definition  is not very clear,  the  frequency  range  stay  very huge.
             For application to diagnosis, the range of frequency is from              2MHz to 10 MHz.

Now, what about physics of ultrasounds in the body ?

  • Velocity of propagation :
    • About 1540[m/s] in human body
    • Each tissue has its own velocity.
We can note that ultrasonic diagnostic equipment assumes that sound velocity is constant in the body but this assumption causes artifacts in echo image.
  • Way of propagation :
    • Two way for the wave to propage : There are sperical and plane waves.
    • In practice, the ultrasound is at first, a plane wave and it becomes a spherical wave, like it is explained in this schema.

In practice
  • Wavelength : About 0.437[mm] in the body (3.5MHz)


Basic principle of ultrasound echo imaging  :

This plan can resume this principle :



Ultrasonic waves are emitted by the ultrasonic probe. They propagated in body tissues and each time the environment change, a part of wave is reflected. The probe can receive those echoes. And by analyzing these echos, we can make an image of propagation environment of ultrasonic wave.

Many methods permit to improve how we make the waves to propagate. To obtain an image, we need ultrasonic beam. But, in human body, ultrasonic waves are very spread and they propagate as spherical waves. So we need to focusing waves. There are two main method for this. We can by example focusing wave energy in one point (the focal point) by using acoustics lens or "electronic lens", more precisely, electronic focusing. We can note that electronic focusing can be used in elission or in recepetion. These two methods matched to the two schemes following :


Acoustis Lens


"Electronic Lens"  respectively in emission and in reception

Now, take a quick look to  use of this method.

Different scanning techniques :

Several scanning techniques exist. By changing the probe form or use different control beam for the wave, we can observe different things. Just see examples below :


We can summarize those methods with table following :

Element Arraylinearconvexlinearannular
Control of beam directionSwitched Array methodPhased Array methodMechanical
ScanlinearOffset sectorsector
Probe formlinearConvexsector
Region of imagethyroid; breastAbdominal regionsector
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