Wavelength of photon and size of imaged object

[fred1234]

I have read the statement that 'the wavelength of the photons used in imaging an object needs to be much larger than the object itself', although I have never seen/heard a reason explained or the name of a theorem quoted. I have seen this in the description of more than one imaging modalities, X ray, MR. I assume it has to do with the probability of photon interacting with the spatially distributed atoms/molecules containing the proton/neutron/electron. Which has something to do with the oscillatory existence / non-existence of the electric and magnetic properties of the photon as it travels. And I assume that some scientist(s) that I have heard of before already have a theorem named after them that explains it.

Any ideas?

 

[jtbell]

I have read the statement that 'the wavelength of the photons used in imaging an object needs to be much larger than the object itself'

Size of my desk: about 1 meter
Wavelength of the light striking it: about 500 nanometers. (5 \times 10^{-7} meters)

I can see my desk just fine.

 

[fred1234]

The imaging modalities I am referring to, (X ray, MR), the photons predominately homogeneously interact with the proton/neutron/electron(s) of the atom/molecules throughout the whole object being imaged and not just the surface of the object that the photons reach first. For example in 1H MRI at 1Tesla the frequency of the photons used have a wavelength of about 6 meters, which is much larger than the humans that are normally the objects imaged. When 9.4Tesla is used the wavelength narrows to about 1 meter and MR imaging of humans starts to become tricky. Why? What is the properties of the interactions of the photons with the matter that have changed between 6 meters and 1 meters wavelengths that involve the size of the object? MRI of rats and mice at 9T does not involve the same difficulties. (Note: MR scientists are only brave enough to work with the rats that are much smaller than 1 meter :-)

 

[Vanadium 50]

In MRI the position information is not given by the spatial localization of the emitted radiation. The subject being imaged is in an inhomogeneous magnetic field, so the resonant frequency depends on position. Know the frequency and you know where the radiation was emitted from.

 

[granpa]

http://en.wikipedia.org/wiki/Superlens

 

[HallsofIvy]

I have read the statement that 'the wavelength of the photons used in imaging an object needs to be much larger than the object itself', although I have never seen/heard a reason explained or the name of a theorem quoted.

Are you sure you don't have this backwards? The smaller the wave length, the more precise position information. I would think that the wavelength must be much smaller than the object.

I have seen this in the description of more than one imaging modalities, X ray, MR. I assume it has to do with the probability of photon interacting with the spatially distributed atoms/molecules containing the proton/neutron/electron. Which has something to do with the oscillatory existence / non-existence of the electric and magnetic properties of the photon as it travels. And I assume that some scientist(s) that I have heard of before already have a theorem named after them that explains it.

Any ideas?

 

[PRodQuanta]

I agree with HallsofIvy. I think you have it backwards. In fact, I am sure you have heard of electron microscopes and electron beam lithography. These machines exist for this very reason: electrons have a shorter wavelength than light (they give more resolution than optical microscopes and photolithography, respectively).

Bests,
-PR

 

[fred1234]

The statement as I heard/read it was 'the wavelength has to be much larger than the object being imaged'. Note: this is object size as a whole entity and not the separation between resolvable features. This reason was given as one of the requirements for MR imaging and one of the reasons imaging at really high magnet strengths would be tricky. The wavelength used for 1H MR imaging at 1.5T(most common magnet strength used) is 4.5m, which is much larger than about 0.25m bore of the magnet used to image humans. MR imaging under these conditions is considered to be reasonably compliant due to this aforementioned statement. The wavelength at 9.4T, (currently considered a really high magnetic strength), is just under 1m, and was stated that imaging at 9.4T would be challenging due to the wavelength to size of object condition. Some, but not all, of the problems that were mentioned were: hot spots, nulls, standing waves. The resolution seems to be better at 9.4T than at 1.5T so I do not think this has to do with the standard wavelength to molecule separation restriction of achievable resolution.

Everywhere I have read this just states it without explanation of what the cause or effects related to this requirement would be. When I ask lecturers that have made this statement they appear to side step around giving any actual answer. Since I have come across this statement many times I would like to understand what is meant by it. I do not have any clue as to how the size of the object would manifest itself as a problem in relation to the wavelength. I assume that the probability of photon / proton(neutron) interaction is dependent on the wavelength somehow and this produces spatial variations in this probability, but this is only a guess.

Thanks,

Fred