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Author Topic: How does Ultrasound work?  (Read 1661 times)

Offline heavyd2g

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How does Ultrasound work?
« on: 08/03/2013 21:13:54 »
Hi, just curious about this.

When watching an ultrasound showing a fetus for example, how does it stop and show us an image at just the right depth?  Why doesn't it bounce back sooner and show a load of flesh or later and show the adults spine?  Very smart tech but I'd like to know how we can make it hit a certain depth.

Many thanks,


Paul


 

Offline RD

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Re: How does Ultrasound work?
« Reply #1 on: 09/03/2013 03:58:24 »
Only some of the sound is reflected at a boundary layer, some is transmitted to the next layer. 

Only displaying the reflections which occur within a certain time interval would limit the display to a certain depth.
« Last Edit: 15/03/2013 08:14:52 by RD »
 

Online evan_au

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Re: How does Ultrasound work?
« Reply #2 on: 09/03/2013 10:08:16 »
Ultrasound does penetrate deep into the body, but the signal is attenuated the further down it progresses - and the reflection is further attenuated the farther it has to return. So the signal effectively becomes too fuzzy beyond a certain depth.

To maximise the quality of the image, the following steps are taken:
  • A gel is smeared on the skin. This ensures maximum energy transfer between the ultrasound transmitter/receiver and the body, in both directions.
  • The mother is required to drink an uncomfortably large amount of liquid in the hour before the ultrasound (and then banned from urinating). This ensures a full bladder, and a good dark background for the image of the baby.
  • The ultrasound technician selects the area of the image that is of most interest, hiding the layers of overlaying skin and fat, as well as the underlying layers of bladder and bone. 
 

Offline syhprum

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Re: How does Ultrasound work?
« Reply #3 on: 12/03/2013 21:51:47 »
I think the title should be "how does ultrasound scanning work".
 

Offline techmind

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Re: How does Ultrasound work?
« Reply #4 on: 14/03/2013 23:29:58 »
They use sound frequencies of a few megahertz (wavelength = speed of sound (around 1500m/s in water) / frequency.
The spatial resolution can then be of the same order as the wavelength.

The image you see on the screen is effectively like a ship's "radar" picture, but using very shortwave sound waves rather than microwave radio-waves. Each boundary layer between different tissues/materials with slightly different refractive indices causes some degree of 'reflection' of the signal.
 

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Re: How does Ultrasound work?
« Reply #4 on: 14/03/2013 23:29:58 »

 

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