Conventional MRI imaging systems use uniform magnetic fields but in order to have higher resolution we must have bigger or better magnets. One of the methods is using structured magnetic patterns instead; how does this enhance the imaging?
Dear Daiki
Thanks for asking.
I was watching a TED talk by Mary Lou Jepsen about reading images from our brains. You can find it in the link below if you are interested.
https://www.ted.com/talks/mary_lou_jepsen_could_future_devices_read_images_from_our_brains/transcript#t-567245
Here's a summary the first paragraph is additional so you can skip it if you don't have time.
[This talk first presented a study that showed the FMRI results of looking at an image and imagining it.It claimed that there is almost nearly zero difference between the brain activity while looking at an object and imagining it. Based on this research they showed several hours of youtube videos to individuals and recorded their brain activity then participants were shown new videos with different people and etc. Next the brain scans were given to a computer that used the previous brain scan data to decode the new one .The computer showed what it thought the individuals were actually seeing and it was kinda similar but blurry and the presenter suggests that the solution for actually getting useful images is to enhance the resolution of the imaging technique. (FMRI)]
She then explains: (Here I copied the talk's transcript for more clarity)
Instead of bigger magnets, let's make better magnets. There's some new technology breakthroughs in nanoscience when applied to magnetic structures that have created a whole new class of magnets, and with these magnets, we can lay down very fine detailed magnetic field patterns throughout the brain, and using those, we can actually create holographic-like interference structures to get precision control over many patterns, as is shown here by shifting things. We can create much more complicated structures with slightly different arrangements, kind of like making Spirograph.
Most of the recent advances in resolution have actually come from ingeniously clever encoding and decoding solutions in the F.M. radio frequency transmitters and receivers in the MRI systems. Let's also, instead of a uniform magnetic field, put down structured magnetic patterns in addition to the F.M. radio frequencies. So by combining the magnetics patterns with the patterns in the F.M. radio frequencies processing which can massively increase the information that we can extract in a single scan.
I couldn't find any articles about how this works and I was hopping to find the answer here.
I am not an expert in your sector. Perhaps, the use of different magnetic field frequency can be used in materials control to obtain different scan deep and/or resolution. So you can start with a superficial scan with high frequency and high resolution, stop, acquire and rescan with lower freq.
Well Dr. Jepsen was a bit vague in her TED talk, but I don't think she was really talking about MRI when she was talking about structured fields, rather she was talking about alternative ways to measure brain function. According to her company's website, https://www.openwater.cc/technology, they are developing a device based on near-infrared spectroscopy (NIRS), which is sensitive to blood oxygenation like fMRI. NIRS is also used to study brain function, but historically the spatial resolution has been poor. The website says they are using interference patterns to greatly improve the spatial resolution and I think that she was hinting at this in her TED talk.
But we do structure fields in MRI all the time and in increasingly complex ways.
Dr. Jepsen did mention in her TED talk about parallel transmission in MRI. Ideally you want the B1+ field to be uniform (i.e. the same flip angle everywhere). But at higher magnetic fields the B1+ field becomes distorted, which leads to dark spots in images. B1 shimming (or RF shimming), uses an array of transmit coils, where the phase, amplitude, or even the shape of the RF pulses are varied across the array in order to produce a uniform B1+ field in the tissue. See for example Article Slice-Selective RF pulses for In-vivo B1 Inhomogeneity Mitig...
B1 shimming is basically creating a structured B1+ field in order to achieve a uniform flip angle over the image.
Of course normal B0 shimming is doing the same sort of thing for the static field. The body distorts the static field giving it structure and by adjusting the current in each of the shim coils we are structuring the applied field to offset the distorting caused by the body. Shim coils for normal B0 shimming are based on spherical harmonics, but the distortions we are trying to shim out are often more complex and recently several groups have been working on shim coil arrays with a large number of small coils more generally placed, making it easier to create more complex structure in the static field. See for example Article Magnetic Field Modeling with a Set of Individual Localized Coils
Also, normally in MRI we use linear magnetic field gradients, so we have a linear relationship between space and frequency, but recently a couple of groups have been developing methods for using gradients with more complicated shapes (structured gradients) in order to optimize parallel imaging reconstruction methods, or to alter gradient coil perfromance requirements. See for example
Article O-Space Imaging: Highly Efficient Parallel Imaging Using Sec...
Article Parallel imaging in non-bijective, curvilinear magnetic fiel...
All of these are examples of using structured magnetic field patterns to improve MRI images.
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