Understanding Ultrasound Focus and Intensity in Tissue Exposure

[chirhone]

Is ultrasound focus just like light rays where the intensity is strongest in the focus? How much is the exposure when the tissue has only a partial ultrasound beam passing (like cross section, imagine 1/4 of beam sides) through it. For example. The focus of the ultrasound is deep in the kidney at center. But the sides of the kidney is still exposed to part of the ultrasound beam. What intensity is it in comparison to the focus?

 

[sophiecentaur]

Is ultrasound focus just like light rays where the intensity is strongest in the focus? How much is the exposure when the tissue has only a partial ultrasound beam passing (like cross section, imagine 1/4 of beam sides) through it. For example. The focus of the ultrasound is deep in the kidney at center. But the sides of the kidney is still exposed to part of the ultrasound beam. What intensity is it in comparison to the focus?

The power flux across the beam will divide up in the same way for all 'beams'. Focussing a beam will mean that the highest flux density will be in the focal point / region. However, there is a difference between US and, for example X Ray technology. The Energy in an X Ray beam will interact with all tissue it comes into contact with because it all has a very similar impedance (same amount of water and same basic cells).

In radiotherapy, the focussing needs to be much more extreme. X rays cannot be easily focused with any lens or mirror so the focussing has to be done 'mechanically'.For instance, they often use three or more directions sequentially from the same source (e.g. strange down into the abdomen and from either side, with the target where they intersect.)

The Cyberknife uses multiple (dozens of?) beams and the beams follow the
movement of the organ being treated by using Ultrasound (?) during the treatment session
The energy in a US beam can be focussed (the US images of internal images are quite high res) and mostly pass through tissue with not too much loss / reflection so not much energy will be absorbed locally by living tissue.

But when a beam of US energy encounters a kidney stone, the material is very different and there will be a strong reflection. The momentum of the beam will be transferred to the stone and it will be mechanically moved and fractured. So it has to be a much more selective technology than Radiotherapy.

 

[HallsofIvy]

Is ultrasound focus just like light rays where the intensity is strongest in the focus?

That is pretty much what "focus" means. When your teacher tells you to "focus" he/she does not mean "pay less attention"!

 

[sophiecentaur]

That is pretty much what "focus" means. When your teacher tells you to "focus" he/she does not mean "pay less attention"!

Sorry, I was just looking out of the window. What did you say??