MRI, deep body photography: Chapter 3

In the previous articles no.1 and no.2 in this short series, I have written about what NMR is and the factors influencing it. Now I hope this article is enough to cover up all the remaining minimum stuff one must know to know (that’s a double ‘know’) the working of MRI.

Let me tell you a bit about relaxation processes before I try to build an MRI machine based on the knowledge I so tried to share in this short series.

So when we hit the nucleus in the lower energy state with the photons of the appropriate energy, the nucleus jumps into higher levels. This higher level nucleus loses this gained energy after some time, the time interval which can be called as its lifetime and gets into the lower energy states. But direct spontaneous emission process is found to be less probable than other processes of losing energy.

There are two most probable relaxation processes. One is where this nucleus gives the energy to nearby nucleuses having resonances frequencies near to its frequency. Other is to lose it in the form of heat energy. These processes are important in understanding the environment of the nucleus which was our aim from the beginning.

Now the overall part of the sample under consideration is having two axes. One along the direction of the external magnetic field and the other perpendicular to it. The magnetization along the external field is called longitudinal field which is a function of the energy of the tissue, remember the energy orientations in the external field. And the one perpendicular to it is called transverse field which is a function of entropy.

Now let’s build an MRI machine….

• We need a strong magnet, strong as incapable of producing field 10000 times powerful than that of Earth,
• We need a radio frequency source,
• We need a radio frequency receiver, and finally
• We need a frequency analyser.

For the magnet, I can use superconducting coils. But then this magnetic field covers a wide range of tissue. For the scan to be powerful, we add an extra layer of magnets capable of producing field in 3 perpendicular axes, these are called gradient axes. Gradient axes because they produce field as a gradient of the spatial location, so the excitation frequencies of every nucleus are spatial location dependant. Thus enabling us to the precise select certain region inside which the field is the same. We can select a certain location, ie., a plane and work on it.

If the field gradient is in the z-direction, we can select a 2d XY plane to work on. Similarly for the other two directions. By the combination of gradient fields in 3 directions, we can map out the 3-d structure of a body.

Now between the gradient coils and magnet, we place Radiofrequency source, the source of photons we need to excite the nuclei.

Then we add a scanner or a radio frequency receiver outside everything. Thus, we are able to make an MRI machine. But we didn’t really have an insight into how it really works!

First, we switch on the powerful magnet, Thus getting the target nuclei in our body-oriented along the field. But which specific nuclei are we speaking about? Not a particular one. A full scan is done with a single nucleus in mind. Since body 72% of water, we can make use of hydrogen nuclei, which is spin active. But sometimes, to get specific scans, we can inject a non-toxic dye containing a spin active nuclei into the region under test. All we need is to have a nice population of spin active nuclei.

Now the nuclei are oriented based on the entropy and energy values, giving a resultant longitudinal and transitive fields. (The nuclei with a spin in other than longitudinal direction rotates along the longitudinal axis with certain frequency) We now switch on gradient magnets to select the region under study. Then according to the nuclei and the magnetic field values, we switch on the RF source to send pulses thus exciting the nuclei and now the resultant field is not along the external field, instead, it is in some arbitrary direction.

This is where the game begins. Now after you switch off the source, the nuclei’ relaxation process begins. The relaxation process along the longitudinal axis is a function of energies and it occurs with a time constant T1 (let's say). It is rather a slow process when compared to the relaxation process along a transitive direction which is a function of entropy, where the process occurs with a time constant T2. Since most of the nuclei that result in the transverse field are actually revolving at Larmor frequency, their magnetic field is not in a single direction but rather rotating.

So the scanners are basically coils, We know that according to Lenz law, the coil induces a current in order to oppose the incoming magnetic flux. Here the scanners catch the flux from the varying Transverse magnetic. This produces a current signal in the scanner coils. This signal is called Free Induction decay, simply FID. An FID signal is as shown in the figure below. And the decay is oscillating from + to — since the nuclei are also rotating with Larmor frequency in the process. And the decay profile is also oscillating with Larmor frequency, which is exactly equal to the resonance frequencies.

How to study A MRI image?

From the decay profile, one can detect the presence of nuclei. Based on the expected data, we expect to receive certain frequencies. An MRI scans point to point and the received signals are laid out as an image with a change in contrast of images specifying different frequencies. So if there is a slight difference, in contrast, we can conclude the presence of some tumour or abnormality. We learnt that these frequencies are dependent on the surrounding of the second article.

Now we learnt how an MRI works at least roughly.

So MRI is a non-invasive technique which is done using non-harmful radiation, used to study anatomy and physiology of the human body. And 2003 Nobel prize in medicine is awarded to Paul Lauterbur of the University of Illinois at Urbana-Champaign and Sir Peter Mansfield of the University of Nottingham for their “discoveries concerning magnetic resonance imaging”.

Thus I conclude the short series of articles on how MRI works. Critics are very much welcomed to state their views, and the people with doubts can ask me questions in the comment section

Originally published at http://batmanview.wordpress.com on November 14, 2016.