Application of microwave beams

[lost_in_space]

hi

i would like to ask a question concerning the application of microwave beams.

First: a microwave beam can be thought of as a laser beam but the light it emitts is in the frequency range of microwaves (see Maser)...

I was wondering why such beams are not used for medical imaging, since at the frequnecy of microwaves such a beam has a very large skin depth... mor precisely: i guess that such a beam would cross human body similar than an x-ray beam does...
So in principle it should be possible to build a CT devisc with microwave beams instead of x-rays. however, i can find no literature in the internet that confirms my thoughts... so i would like to know where i am going wrong.


thx

 

[f95toli]

Part of the problem is the wavelength; there is not much point in having a medical probe with a resolution of a few cm (you can get around that using near field probes etc, but that is not useful for medical imaging).
Imaging with microwaves becomes practical at higher frequencies than is used for e.g. wifi, from say a couple of hundred GHz up to about a THz (and above, but then you are in the far-infrared region).

 

[lost_in_space]

Part of the problem is the wavelength; there is not much point in having a medical probe with a resolution of a few cm (you can get around that using near field probes etc, but that is not useful for medical imaging).
Imaging with microwaves becomes practical at higher frequencies than is used for e.g. wifi, from say a couple of hundred GHz up to about a THz (and above, but then you are in the far-infrared region).

Thank you for the answer !

So if i stick with the CT example: one usually measures the intensity of the beam after it has crossed a body. How ist this this intensity loss related to the wavelength of the used beam.. i appologize for my english but what to you mean by wifi ?

 

[f95toli]

The "resolving power" ("resolution") of any light source is (roughly) given by its wavelength. This is for example why deep-UV (short wavelength) is used to make small things in the semiconductor industry and why scanning electron microscopes can resolve much smaller things than an optical microscope (the wavelength of electrons is much smaller than that of light).

see
http://en.wikipedia.org/wiki/Angular_resolution

 

[lost_in_space]

The "resolving power" ("resolution") of any light source is (roughly) given by its wavelength. This is for example why deep-UV (short wavelength) is used to make small things in the semiconductor industry and why scanning electron microscopes can resolve much smaller things than an optical microscope (the wavelength of electrons is much smaller than that of light).

Thank you again


I know that the resolution limit is bounded by the wavelength in optics. However, i didt not understand how this argument applies to computed tomography.

For example: as an xray beam propagates through the body its intensity loss is determined by the density of the objects it travels through. I did not see why this argument should not work for a microwave beam as well...

but i guess now the following:

the intensity loss of a beam depends on "how much of its energy is absorbed" by the objects it runs through, and the amount of absorbed energy certainly is frequency dependent...

however, i found literature on Computerized Tomography that uses microwaves: it is called
"chirp pulse microwave computerized tom." , i have not found out yet what they are actually imaging

 

[davenn]

I know that the resolution limit is bounded by the wavelength in optics. However, i didt not understand how this argument applies to computed tomography.

Because it applies to any wavelength/freq from radio waves, through light to Xrays. As you go higher in freq, the resolution gets better.
Basically, Xrays are used in preference to microwaves because of 1) their much higher resolution and 2) greater penetration through the body

Dave

 

[sophiecentaur]

I think Xrays are still used, on many occasions, because beople are just used to them, despite the obvious hazards. I am always surprised that ultrasound is not used as a matter of course where the tissue is not encased in bone.

Microwaves of all frequencies are just not suited to tissue scanning because they are absorbed too much by 'wet' tissue.

 

[davenn]

I think Xrays are still used, on many occasions, because beople are just used to them, despite the obvious hazards. I am always surprised that ultrasound is not used as a matter of course where the tissue is not encased in bone.

Microwaves of all frequencies are just not suited to tissue scanning because they are absorbed too much by 'wet' tissue.

Agreed :) personally I prefer MRI much better resolution over ultrasound, without the harmful effects of Xrays. Dang... recently gone through 12 months of regular MRI's, Xrays, ultrasounds and even a full body bone scan.
The bone scan was the worst hahaha I "glowed in gamma rays" for almost a week after being injected with Technetium99m, my geiger counter went crazy for days haha

Dave

 

[lost_in_space]

I think Xrays are still used, on many occasions, because beople are just used to them, despite the obvious hazards. I am always surprised that ultrasound is not used as a matter of course where the tissue is not encased in bone.

Microwaves of all frequencies are just not suited to tissue scanning because they are absorbed too much by 'wet' tissue.

hello sophie,

thank you for the answer.

what is measured in Computed tomography the intensity loss of a beam after it has crossed the body. Then it is assumed that this value is proportional to the line intagral of a density function... and after all the answers to my question i have read here... i think that this density function is frequency dependend... (you image another density function for microwaves that for x rays) in the microwave frequency domain this density functuion does not have much details ( different tissues or whatever schow a very similar absorption behaviour) which explains why the resolution is so bad...

 

[f95toli]

That might be part of it; but you are still missing the point:the resolution is limited by the wavelength. It does not matter HOW/WHAT you measure, you can never resolve details much smaller than the wavelength. It might help if you realize that you can't make a beam "sharper" than about the wavelength: meaning there is no way to scan an area of say 1x1mm^2 with a microwave beam since the "spot size" of the beam will always be bigger than this (unless you use a near field probe; in which case you can go a to a few micron if you are lucky).
The is always true for light no matter what the wavelength is.

 

[lost_in_space]

That might be part of it; but you are still missing the point:the resolution is limited by the wavelength. It does not matter HOW/WHAT you measure, you can never resolve details much smaller than the wavelength. It might help if you realize that you can't make a beam "sharper" than about the wavelength: meaning there is no way to scan an area of say 1x1mm^2 with a microwave beam since the "spot size" of the beam will always be bigger than this (unless you use a near field probe; in which case you can go a to a few micron if you are lucky).
The is always true for light no matter what the wavelength is.

thank you,


the last example you gave is very nice. certainly, two points that are closer than then the diameter of the spot size of the beam cannot be distinguished by it...


one more question: does this near field contein thes "evanescent waves" which are damped exponentially ?