fMRI, BOLD and the Beautiful

When we want to examine the brain of a person noninvasively by Computed Tomography (CT) or MRI, we get a ‘snapshot’ of the anatomy (or pathology, if any) of the subject’s brain. We are however clueless as to its functional aspect. fMRI or Functional Magnetic Resonant Imaging allows us to do just that. The difference is not unlike a ‘still picture’ versus a ‘video of a moving train’. PET scans, previously described, also can asses the functional state of the brain.

Whenever we do a task, think, dream, memorize, speak or see things, the brain is not activated as a whole; but only certain portions of it are activated. Activation, here, means increased metabolic activity of neurons in certain areas of the brain. Naturally, these ‘metabolically active’ neurons would demand more energy which would power them. The blood supply to these areas increases as a result of this metabolically driven vasodilation. The arteries then bring in glucose and oxygen with them, with Oxygen being transported in the form of Oxyhemoglobin (oxygenated hemoglobin or HbO2). Neurons on the other hand use up the oxygen contained in the blood, thereby reducing it to de-oxyhemoglobin or simply Hb. However, the alteration in tissue perfusion exceeds the extraction of oxygen by the neurons, so the concentration of deoxyhemoglobin within ‘the areas’ decreases. This causes molecular inhomogeneities in the magnetic field.

Oxyhemoglobin is diamagnetic, meaning that they align perpendicularly to magnetic field lines. On the other hand, deoxyhemoglobin is paramagnetic, i.e. it aligns parallely and proportinately with the intensity of the magnetic field. This causes the inhomogeneity within the magnetic field (magnetic susceptibility) in the tissue sampled. This inhomogeneity is exploited in fMRI in terms of decay of transverse magnetization, T2*, with longer T2* values in HbO2 blood and shorter values in Hb (paramagnetic) blood.Since this stems from the oxygen content in blood, fMRI is also known as the BOLD ((blood oxygenation level dependent) effect.

The machine is essentially the same as the MRI machine with echo planar imaging technology that permits faster imaging due to faster gradient switching, improved algorithm and faster CPU processing power. The patient/subject is placed inside the magnetic chamber and MRI signals are acquired, Fourier transformed and corrected for artifacts. Finally the computer reconstructs a 3D fMRI image out of this.

As is obvious, we can learn about the motor areas of a patient by asking him to grasp an object or giving him any motor task and noticing which area(s) of the brain lights up. A neurosurgeon can then be cautious about not hurting these areas. Similarly, the mapping will help spare motor and other vital areas like auditory, visual and language areas from damage in radiotherapy procedures, in addition to neurosurgery. It can also detect occult Alzheimer’s disease and cognitive deficits including those of the autism spectrum and dyslexia (reading disorder).

fMRI can also be employed to ‘read peoples’ minds’, thoughts, intentions including lie detection. Watch the video below which explains how an fMRI scan is done and interpreted.


Thus the legal and forensic implications are obvious. However, in fMRI, correlation doesn't always mean causation. Whatever it may be, it seems that fMRI is very much here to stay, both in the clinics as well as in cognitive neuroscience research. It may also be combined with tractography, MRI or other diagnostic radiologic modalities.

Hardenbergh et al combined Tractography techniques with fMRI, using a technique capable of rendering multiple color-coded functional activation volumes and fiber tract bundles. Many pharmacologically active drugs have effect on memory impairment, which can be seen in ‘telltale’ fMRI scans. Sperling et al studied the effects of lorazepam (a benzodiazepine) and scopolamine (an anticholinergic drug once used as ‘truth serum’ by the CIA) on healthy volunteers and found that they did impair memory and their functional coordinates could be reproducively mapped on fMRI scans (see figure on the left). I still shudder at the thought of what happened during my PG exam when I took a benzodiazepine.

Last modified: Mar 09, 2014
Reference: Integrated 3D Visualization of fMRI and DTI tractography
Gore, J. (2003). Principles and practice of functional MRI of the human brain Journal of Clinical Investigation, 112 (1), 4-9 DOI: 10.1172/JCI200319010

The World of Tractography Where The White Matter Tracts Appear Colored

The Central Nervous System (CNS) communicates with the exterior (sensory e.g. gets visual, tactile information etc. on the one hand; and motor, performs limb movement, posture regulation etc. on the other) via the peripheral (somatic) nervous system. It also connects with the interior (our viscera or organs) via the Autonomic Nervous System. That is, it does its job in a bidirectional way: by the motor or the actuator arm, and sensory or the receptor arm. For such ‘actions’ to occur, cables of nerve fibers are laid within our body. Wouldn't it be nice if we could visualize these cables, their dispositional anatomy or any pathology that could afflict them?

Schematic of a 'peripheral' myelinated axon 

The brain and spinal cord together constitute the CNS. We also know that there are about a hundred billion neurons in the CNS. Each neuron has a cell body (soma), an axon wrapped by myelin, and many dendrites. (See figure). It is the axon that carries the information in the form of action potential. These cables (bunch of axons, called tracts) are not laid haphazardly. Nature tries to conserve space, length, energy and so on and thus the axons form into tracts in a topologically efficient way. They run up and down (also front-back and sideways) the cord to the brain, the organ that we will now concentrate upon.

Your electrical wiring to your ceiling fan would include a switch and the fan itself. The wire (cable) from the switch would ascend vertically up the wall, make a 90 degree angle, and then reach the fan horizontally up in the ceiling. Likewise, in our brain, which is made up of two hemispheres, would connect. Three broad fiber types are seen: from one hemisphere to the other (commisural fibers), restricted to one hemisphere (front to back or antero-posteriorly are association fibers) and finally vertically (up down orientation go the projection fibers).

Exploring the tracts can now be done in live animals including humans. Improvement in MRI technology has enabled us to see the tracts (tractography). Improved gradient coils, faster processors and superior software have shortened the scanning time, thereby reducing 'blur' due to organ movement (e.g. diaphragm) and patient movement. This procedure called Echo Planar Imaging (EPI) has given birth to Functional Magnetic Resonance Imaging (fMRI), Diffusion Weighted Imaging (DWI), tractography and many other diagnostic and research procedures.

Consider the neuron shown in the above picture. Water molecules in the axon (yellow) are constantly in Brownian (random) motion due to thermal energy within. Hence they tend to diffuse constantly to come to equilibrium. In most of the cerebrospinal fluid spaces these microscopic motions are equal in all directions. This is called isotropic diffusion. But in myelinated neurons, as in the white matter fiber tracts, water motion is constrained due to the fatty nature of the myelin sheath (in blue) which hinders water flow across it. This anisotropic diffusion allows the flow of more water molecules along (parallel) the direction of the nerve fiber. The apparent diffusion coefficient (ADC) is thus more along the nerve fiber. Diffusion Weighted Imaging (DWI) can capture this microscopic water flow and delineate anatomically the orientation of nerve fiber tracts.

Pyogenic abscesses hinder diffusion by virtue of their increased viscosity, a rheological property. In the early stages of acute cerebral infarction there is reduced diffusion too, giving rise to high signal intensity. However, in most pathologies of the brain the ADC is increased. Diffusion Tensor Imaging (DTI), a diffusion MRI technology, tracks fiber orientation by assigning values in ellipsoid voxels (VOlume piXEL). Ellipsoid because unlike isotropic diffusion where molecules diffuse equally in all directions, the anisotropy in the white matter tracts does not permit them to move with equal ease in all directions, and hence the pattern is that of an ellipsoid and not spherical. By connecting the long axes of all the ellipsoids, the trajectory/orientation is deduced.

Colors are added to it with respect to the three principal axes (x, y and z) and the result is a stunning tractography! (see left)



The YouTube video below describes the colorful realms of tractography and how they are used.

When fMRI and DTI are combined together, a whole new world emerges. But, I prefer to keep it on hold till I discuss Functional Magnetic Resonance Imaging.

NB: The picture of the neuron/axon shown is not representative of CNS neurons. Oligodendrocytes that form myelin in the CNS does not encircle so many times as the Schwann cells in peripheral neurons do. It has been shown here only for the sake of clarity.

Last modified: Mar 10, 2014
Reference: Diffusion Tensor Tractography: Exploring the Cost-Benefit Ratio of Incorporating CSF Suppression into Fiber Tracing Algorithms

William P. Dillon. Neuroimaging in Neurologic Disorders. In: Harrison's Principles of Internal Medicine, 17th Ed., Volume 2, McGraw Hill; 2008. p. 2491-2497.

P. Mukherjee,, J.I. Berman,, S.W. Chung,, C.P. Hess, & R.G. Henry (2008). Diffusion Tensor MR Imaging and Fiber Tractography: Theoretic Underpinnings AM J Neuroradiol DOI: 10.3174/ajnr.A1051

Relaxation in the Nuclear Microcosm

All of us want to give themselves a hard earned ‘rest’ after a “hard day’s night”, don’t we? So do the protons, perturbed by the destabilizing magnetic component of the radio-frequency pulse [which previously ‘happily’ aligned themselves to the externally applied magnetic field; one way (parallel) or the other (antiparallel)] applied at the Larmor frequency. It is like slapping an individual in a “merry go round” each time he came near a person who is paid just for slapping that person. But, when we call ‘spin’, we do not mean ‘spin’ the way we see them in a classical world. [We’ve given various names to the ‘quarks’: up, down, strange, bottom etc. depending on ‘something’ called ‘flavor’; and red, green and blue depending on ‘something’ called ‘color’.However, spin, flavor, color etc.‘in the quantum world’ have ‘no relevance’ to what we usually attribute to them in our everyday life. Things are a bit crazy in the quantum world, but I will take recourse to some ‘classical world’ analogies to make the description lucid.]

Thus, the already aligned nuclei (parallel or anti-parallel to the applied steady external magnetic field B0), has now been perturbed owing to the ‘knocking’ by the ‘magnetic component’ (B1) of the electromagnetic RF pulse. The nuclei gain energy and sway away from the perpendicular to the horizontal (90 degree) depending on how long the RF pulse is applied. So, now the nuclei behave like ‘punch-drunk’( like a person who’s been reeling due to a strong blow to the head!). Remember, that this new angular momentum is also a vector quantity having magnitude and direction. It can be resolved in terms of a horizontal component (Mxy) and a vertical component Mz. Anyway, the proton does recover from this situation, after some time, once the external RF field has stopped. Typically, Mxy component decays faster than the recovery of Mz.

The excited proton recovers in two ways and both forms occur simultaneously: (1) The excited nuclei which now have been ‘forced’ to lie horizontally (90 degree), ‘re-align’ themselves back to their ‘original position’ as they were before the RF pulse (perpendicularly towards the field of externally applied field B0); and (2) the energized protons dissipate their energies to the surrounding nuclei (horizontally) at their level. The first example, obviously, is called the (spin-lattice, or longitudinal) relaxation; while the second one, transverse relaxation (T2). There is little energy loss due to RF emission.

T1 relaxation, also known as, longitudinal relaxation or spin-lattice relaxation can be best understood if you see the following Youtube video. [The spiral trajectory, in this case, reminds me of the laser experiment I did to satisfy my lesser friends. Analogically, the trajectory would be such, if the power supply were switched off.] In T1 relaxation, the proton loses energy to the surrounding lattice, by interacting with nuclei in the lattice which are in vibrational, translational and rotational motion. Clearly, the surrounding nuclei (lattice) having the same (or nearly same) Larmor frequency will efficiently absorb energy of the excited proton, resulting in a tiny rise of temperature.

T2 relaxation (transverse or spin-spin relaxation) on the other hand, does not involve exchange of energy with the lattice.
The magnetic moments of the protons merely changes phase. Here, the nuclei exchange “quantum states” (kind of, what Einstein called ‘spooky action at a distance’): an excited nucleus (proton) will transfer its energy and relax, while the neighboring nucleus in the lower energy state that absorbs it becomes excited. This loss of phase coherence of spins can be clearly seen in this beautiful video.

It can be understood easily that T1 and T2 values would depend on the surrounding molecular environment (tissues, for example). Hence, the values differ in different tissues. Again, since Mxy decays faster, as described, it may be understood why T1 is greater than T2 (usually, T1=5T2). Both T1 and T2 contribute toward contrast in tissues. T1 relaxation time is the time needed for 63% of protons to return to their previous equilibrium state. Likewise, T2 relaxation time is the time needed for 63% of protons to become dephased owing to their interaction with nearby protons. The contrast, naturally depends on the water content of the tissues. Grey matter has about 10% more water than white matter and this creates a contrast. We can also create contrast by varying TR and TE times.

TR (Repetition Time) refers to the time gap at which consecutive RF pulses are applied; while TE (Echo Time) refers to the time delay between the applied RF pulse and its reception (echo). T1 weighted images (T1W) are produced by keeping TR and TE relatively short, while T2 weighted images (T2W) are produced by keeping TR and TE relatively long. Water molecules being relatively light spins much faster than the Larmor frequency, making energy transfer rather tough (exchanging of packets of energy becomes more efficient as the relative angular velocity narrows). Consequently, water has a long T1 time. Proteins and nucleic acids being rather heavy, spin slowly. They also have problem with energy exchanging, and thus have a long T1. Cholesterol, a medium sized molecule, precesses near the Larmor frequency, efficiently absorbing the energy and giving a small T1 value.Thus (fat) liquid cholesterol in craniopharyngiomas, a benign tumor, appears bright on T1W images (T1 being small, the rate at which RF energy is released is fast. Hence, the signal intensity in NMR is high).

Subacute hemorrhage also has shorter T1, due to the presence of paramagnetic iron in methemoglobin present in the tissue, hence high signal intensity. Cerebrospinal fluid (CSF), edema (collection of fluid in tissue space or ECF) having more water content have both long T1 & T2 relaxation time. They give low signal intensity in T1 (dark) but higher signal intensity (bright) in T2W images. T2W images are superior to their T1 counterparts in case of infarction, edema, demyelination etc. Contrast agents like the heavy metal Gadolinium, a paramagnetic substance, has been used to reduce both T1 and T2 times by introducing inhomogeneity in the magnetic field. Gadolinium is complexed (chelated) with a substance called DTPA to prevent toxic build-up inside body tissues. This gives high signal in T1W but a low signal in T2W. It (the complex) does not cross the blood brain barrier (BBB); but disruption in the BBB or parts of the brain where it is deficient (circumventricular organs), take-up the substance and affects relaxation properties.

Below is an MRI showing changes in Subacute Sclerosing Panencephalitis, a complication of measles. Note: Panels A and C are T1-weighted images; B and D are T2-weighted images. The hypointense (darker) signal on the T1-weighted image (arrow in A) and a hyperintense (bright) signal on the T2-weighted image (arrow in B) can be clearly seen.

Given all these, it can be said that relaxation parameters of nuclei have enabled us in visualizing biological tissues nonivasively, identifying chemicals spectroscopically and a lot more as we shall see later.

• References: Magnetic Resonance Imaging: David D. Stark, William. G. Bradley, Jr.

• NMR spectroscopy
• Magnetic Resonance Imaging
• Principles of NMR
• William P. Dillon. Neuroimaging in Neurologic Disorders. In: Harrison's Principles of Internal Medicine, 17th Ed., Volume 2, McGraw Hill; 2008. p. 2491-2497.
• Ian L. Pykett, Ph.D., Jeffrey H. Newhouse, M.D., Ferdinando S. Buonanno, M.D., Thomas J. Brady, M.D., Mark R. Goldman, M.D., J. Philip Kistler, M.D., & Gerald M. Pohost, M.D. (1982). Principles of Nuclear Magnetic Resonance Imaging Radiology
• Time constants

Last Modified: Aug 19, 2010

Understanding the Basic Principles of Nuclear Magnetic Resonance Imaging

Nuclear Magnetic Resonance Imaging (NMRI), better known as Magnetic Resonance Imaging (MRI) in medical parlance, is an invaluable tool in the study of the neurological system, soft tissue and musculo-skeletal system disorders. The word “Nuclear” was intentionally dropped later, as the procedure could then be wrongly interpreted by patients in relation to “ionizing radiation”, which certainly is not the case. However, the term Nuclear Magnetic Resonance (NMR) continues to be used in other (non-medical) fields of science, such as analytical chemistry, physics, biochemistry, petroleum industry, analysis of biological samples etc. In either case, the procedure and the basic principles remain the same. Paul Lauterbur was one of the pioneering inventors of this seemingly tough technological field.

Matter is made up of atoms, which in turn, are composed of negatively charged electrons orbiting around the nucleus (look at the animation of a Helium atom on the left), consisting of positively charged protons and charge-less neutrons (with the exception of Hydrogen 1H nucleus, which contains a single proton and no neutron). These subatomic particles (electron, proton etc) somehow, can not be understood in terms of shape or color; instead they are denoted by their charge, mass or spin (angular momentum). An even number of them will cancel each other’s spin [just like two revolving spheres, in touch with each other would, in a ‘classical world’ (if one rotated clockwise, the other would rotate anticlockwise, canceling any resultant spin)].

Hence, a net resultant spin would result in the nucleus only if it contained an odd (unpaired) number of protons, an odd number of neutron or both. [The concept that certain nuclear species had angular momentum was first suggested by Wolfgang Pauli, while explaining the fine structures in the Atomic spectra. In the presence of an external magnetic field, the spectral lines got split, depending on the strength of the field (Zeeman Effect).]

Since nucleons bear a net charge (owing to the protons contained), the spinning nuclei will generate a magnetic field (since moving charges generate magnetic field). Each of these charged spinning ‘spheres’, hence, may be thought of as a tiny bar magnet having a magnetic dipole (that is a north-south orientation). [Electrons, similarly, have their own angular momentum though, responsible for molecular structure which nature uses, but they are not used by humans (Milestones in Spin podcast)] When we talk about “MRI” in humans, we mean proton nuclear magnetic resonance; i.e. NMR that detects the presence of hydrogen (proton) nuclei.

Our bodies have a plentiful of Hydrogen atoms: from the water within us, in cells and in extracellular fuid, (and to a lesser extent to the adipose tissue (fat)). These charge-carrying ‘unpaired’ protons (Hydrogen nuclei) rotate around their axes, but since all are spinning in a random fashion (as there’s no coordinator of any sorts); their net spin is zero, or in other words, their net magnetic moment is zero (as shown on the left).

Understanding spins aren't easy either. But, Prof. Stephen Hawking made it quite simpler for us using the real classical world analogy of ‘playing cards’ in his famous book A Brief History of Time (follow the link to learn more about ‘spin’). Having said that, the unpaired, positively charged protons having half integer (1/2) spins, behave like magnetic dipoles; it may now be understood easily that the spinning protons (nuclei) would align themselves to an externally applied magnetic field.

Thus, in a static magnetic field, the randomly oriented ‘tiny bar magnets’ align themselves up according to the applied magnetic field. These spinning protons (nuclei) also precess (make an angle) with the applied magnetic field (Bo), much like a spinning top does when its angular momentum diminishes. An animation of a proton precessing around a field is shown on the right.

[The magnet used for this purpose employs superconductivity. In a superconductive magnet, the electromagnet coils are immersed in liquid Helium at minus 269 degree Centigrade. At such a low temperature, the coils loose ‘resistance’ to the flow of electrons, resulting in a highly stable and a very strong magnet. (However, any minute vibration in the superconducting magnet can lead to runaway Eddy current leading to a phenomenon called 'quenching', that happened in the Large Hadron Collider at CERN, collapsing the whole setup.) Normally, 1.5 Tesla magnets are used, though nowadays 7 Tesla magnets have arrived. A 1 Tesla (1 Tesla=10,000 Gauss) magnet is 20,000 times stronger than the earth’s magnetic field)]. Also, note that we are considering magnetic moments along the axis of the external field only, as far as the sum-total alignment of individual magnetic torque contributing to a 'macroscopic' magnetization (M) is concerned. This is because the transverse components of the individual spins cancel out, as is seen in the 'cone' of the above picture. 600 persons of equal power, each pulling a rope either 30 degrees Northwest or 30 degree Northeast (in a 2 dimension), will certainly cancel out the 'east-west' vector, while the Northward vector will add-up. [It is this M that produces the induction current in the receiver coil].

The protons have two choices. Either they have to align parallel or anti-parallel to the applied magnetic field (known as spin-up and spin-down position respectively). In any case, the protons only ‘partially polarize’ since they tend to ‘make an angle’ with the applied static magnetic field. Spin down position is the higher energy state while spin-up state is the lower energy state of the spinning protons (in the case of 23Na, there can be 4 spin-states instead of 2 as in 1H). (Obviously, a swimmer swimming upstream has more energy than his antiparallel counterpart.) The protons revolve (precess) around the direction of the magnetic field (Bo) at an angle, while at the same time they rotate around their own axis. Just as what happens in the solar system. [However, the upper (-1/2) and lower energy (+1/2) spin states are almost equally populated with only a very small excess in the lower energy state at room temperature. Since, there are so many of them that we finally make some headway].

Let me clarify a bit. You've seen a spinning-top rotating around its own axis. Due to Earth’s gravitational field, the top ‘maintains’ an angle (with the perpendicular/vertical), more visible when its angular momentum (speed) decreases, as it continues spinning. The top may be seen to revolve around “the perpendicular” at an angle (=‘precess’), (in addition to its rotation around its “own axis”) during its course of revolution. [Watch the Video "Introductory NMR & MRI Video 01 Precession and Resonance" to see what precession in NMR is]. This is what precession is about.

The frequency of precession is given by the Larmor relationship:
f=w/2*pi=yBo/2*pi (2*pi=360 degree)
w=angular freq. in radians per second; since there are 2*pi radians (360 degree) in a circle; we can find f, the frequency of rotation.
y is the magnetogyric (gyromagnetic) ratio, nuclear constant characteristic of every isotope. For 1H it is 42.5 MHz/T;
Bo=static magnetic field

The above equation is important, as we shall see later. Now let’s summarize what we learned so far.
Protons (nuclei) spin randomly in an atom. They tend to align with respect to an external magnetic field. These protons make an angle with the magnetic field as it goes about the magnetic field (while it also dutifully goes around itself), some parallel, and some antiparallel.

In MRI, our objective will be to disturb this alignment of protons with a dose of radio frequency pulse, in a similar way I discussed in my radio transmitter article but in a much, much bigger way. But since the ‘target’ (proton) is moving (precessing) around the field, we better ‘punched’ the target as if we were moving at the same angular velocity (so that the relative velocity was zero). Thus, when we apply the RF frequency pulses at the Larmor frequency, perpendicular to the magnetic field; the magnetic component (B1) of this electromagnetic wave temporarily knocks the protons out of alignment (see picture). If energy is absorbed by the nucleus, then the angle of precession will change. Assuming the field strength to be 1 Tesla, the protons are revolving 42.5 million times per second; it is at this frequency we give the pulse (i.e. at the Larmor frequency).

The protons are pushed out of alignment and as the pulse ends, they ‘relax’ (more on how they ‘relax’, later) back to their undisturbed ‘equilibrium’ position. This causes emission of an RF signal (the Echo) that can be picked up by the receiver coil (the same transmitter coil that produced it, in most cases); a damped oscillating wave generated, as the ‘disturbed’ magnetic moments coming back to realign with the magnetic field. Now, the problem begins. We have applied a uniform/homogeneous magnetic field (Bo). There are a lot of protons but we don’t know who’s who and residing where. That is why we also apply orthogonal magnetic field ‘gradients’ along the three (x, y, z) axes. [In a classroom, spray gradually ‘more’ yellow color in the front row and to the left than the back and to the right. In a similar way, spray blue color; hope your students don’t object. Now, every one of your students has a unique color: yellow, blue or green and with different hues]

Now that we get a decaying signal, which of high frequency; we mix it with a low frequency signal, in much the similar way as in heterodyning, to produce an ‘interferogram’. This interference map is digitized, which is called the Free Induction Decay (FID). Thus, we do find too many frequencies in ‘the low frequency map’ which occur in ‘almost’ the same time. So, what can we do?

Waka Waka! In this football World Cup 2010 at South Africa, audience seems to have a deafening organ, what they call ‘vuvuzela’. How are we going to analyze so many vuvuzelas when they are blowing at the ‘same time’? Just plot them in ‘frequency domain’ instead of ‘time domain’. Here’s Discrete Fourier Transform (DFT) which will do happily for you. [Simply put, it samples the different frequencies and plots them; not all vuvuzelas have the same frequency]

Now, that fuzzy picture of multiple frequencies has a 'spatial information' (owing to its orthogonal gradient magnetic field), contrast information (due to its ‘relaxation’ parameter), and foremost that it can be analyzed visually by humans, have enabled MRI to be a indispensable tool for the medical professional, as much as NMR has to the physicist or the discerning chemist. In MRI (NMR) it is not that important where or how energy is absorbed, but how quickly the excited protons revert back to its previous position is much more important, and hence the relevance of T1 and T2 relaxation times.

By the way, contrast depends on the t1 and t2 relaxation, the surrounding chemical environment affecting relaxation, and of course the water content of the tissues [gray matter contains 10-15% more water than white matter.]

Finally, the article wouldn't be resourceful enough if I do not post some MRI scans of the brain, this time, that of an epileptic patient (below).

(A sagittal section is obtained as the 'slice' takes a 'left to right' view (and vice versa); a coronal section means a 'front to back' view (or vice versa), and an axial slice means a virtual transverse section through the head.) Here's the picture of an actual MRI Machine below:

Naturally, the small tunnel may induce claustrophobia; the whirring acoustic noise from switched gradient coils may be troublesome to the patient; any implanted pacemaker may be subjected to interference from the electrical field resulting in dislodgement or malfunction (as in other ferromagnetic objects such as wrist watch, key rings etc.). Moreover, sudden movement by the patient may induce voltage in semicircular canal producing vertigo, a sensation of giddiness. Advances in MRI technology is happening fast. Claustrophobia may now be ameliorated with a wide bore MRI. A newly developed MRI scanner with Total Imaging Matrix (TIM) technology patients don't feel as claustrophobic, the imaging time is quick, quality of picture is better and even the acoustic noise is less (watch the YouTube video here). Whatever be the shortcomings of MRI, the benefits far outweigh the risks and it is here to stay and evolve.

References:
Magnetic Resonance Imaging: David D. Stark, William. G. Bradley, Jr.
NMR spectroscopy
Magnetic Resonance Imaging
MRI basics
Principles of NMR
William P. Dillon. Neuroimaging in Neurologic Disorders. In: Harrison's Principles of Internal Medicine, 17th Ed., Volume 2, McGraw Hill; 2008. p. 2491-2497.
Ian L. Pykett, Ph.D., Jeffrey H. Newhouse, M.D., Ferdinando S. Buonanno, M.D., Thomas J. Brady, M.D., Mark R. Goldman, M.D., J. Philip Kistler, M.D., & Gerald M. Pohost, M.D. (1982). Principles of Nuclear MagneticResonance Imaging Radiology
P.S. We will discuss T1 and T2 relaxation, fMRI, tractography and NMR spectroscopy later.

Created: Jun 24, 2010; Last modified: Mar 10, 2014

Atomic Force Microscopy: Feels The Atoms, Sees The Bonds

When it comes to viewing things on the atomic scale, one has to be very careful and innovative. To understand how an atomic force microscope works, we should better discuss a bit about its predecessor: the scanning tunneling microscope (STM). STM was invented by Binnig and Rohrer for which they got the Nobel Prize in Physics. Binnig and colleagues later went on to develop the first Atomic Force Microscope (AFM). Both AFM and STM are types of Scanning Probe Microscopy, which employs a probe that scans the sample.

STM consists of a sharp probe tip, which scans over the specimen as the adjoining picture shows. First, the probe tip is brought near the sample manually, and then the finer adjustment of maintaining probe sample distance (height) is done by piezoelectric control. A voltage applied between the tip and the sample causes electrons to tunnel from the tip to the sample. As we have seen in Ohm’s law, the tunneling current will depend on the applied potential difference (voltage bias); and the height of tip-sample separation and the local density of states (factors determining ‘resistance’). If we know two of the three unknowns, we can calculate the other, which is actually done by the computer by data acquisition.

We can do STM in two ways. We can keep the tip position (height) fixed as it scans the specimen topography (constant height mode). Here the voltage and height are both held constant, while the tunneling current varies. In constant current mode, the tip is always at a specific height over the specimen. That is, as the tip hovers over the rugged terrain of the sample surface and comes close to a raised spot, the tunneling current will increase. The increased current will be sensed, amplified and passed to the feedback electronic circuitry which will ‘lift’ the probe-tip by applying a voltage to the piezo crystal. Hence the electronic servomechanism maintains a constant tip sample distance in constant current mode. Piezoelectric crystals translate pressure changes into electricity (and vice versa) as we see in oven gas lighters and in mobile phone speakers.

But STM has its inherent drawback: the sample has to be a conductor or a semiconductor, in order for tunneling to occur. Hence, biological tissues, non conducting polymers can not be imaged. So, the need for atomic force microscopy arose. Here again, Gerd Binnig played a pivotal role.

Atomic Force Microscopy operates on a similar principle. First, let’s discuss how the music on gramophone record grooves is translated. The stylus (which overlies a piezo crystal) feels the groovy surface of the vinyl LP disc which revolves on a turntable. The mechanical vibration sets in a voltage in the piezo via the stylus. We get the surface topology in the form of music. Another great way of viewing how the AFM scans and interprets a sample is how a blind person ‘feels’ a surface by using a stick (see picture).

In AFM, a very sharp tip (made of silicon or silicon nitride) is scanned over a surface with similar feedback mechanisms that maintain the tip at a constant force (to get height information), or height (to obtain force information) above the sample surface. This sharp tip is mounted on a cantilever, a rod like structure whose other end is fixed and unmovable. As the probe tip raster scans (i.e. scans in a zigzag fashion as done in TV scanning) the surface of the sample, a laser light is made to fall on the back of the cantilever. The light gets reflected off from this side and is detected by one of a dual photodiode. The cantilever ‘tilts’ as it slopes ‘down a bump’, causing another reflection that hits the other of the dual photodiode. A differential amplifier then calculates the difference output between the two light intensities and this is proportional to the cantilever deflection. We can calculate the force (acting between the cantilever tip and the sample) using Hooke’s law (F=-kx), where F is force, k is constant for a particular cantilever, and x is the deflection of the cantilever. The amplifier difference output may be used to keep the tip in either at a constant force or at a constant height above the sample, through piezoelectric servo mechanism (feedback).

AFM is very versatile and allows many user specific modifications. It can be done in contact mode, where the tip remains in contact with the surface as it scans over the specimen. In tapping mode, the cantilever oscillates over the sample, touching the surface intermittently. Interatomic forces like van der Waals forces and electrostatic forces cause a deflection of the cantilever when the tip comes closer to the sample. In the non contact mode, the cantilever does not touch the sample, and is oscillated at slightly above its natural resonance frequency. Any long range force like van der Waals force will decrease the frequency of the vibrating cantilever, when the tip approaches the sample. There are many other variants of AFM and combination of AFM with other imaging modalities like optical microscopy, Raman spectroscopy and so on.

But Leo Gross and colleagues at IBM Research Zurich wanted another, so they went on to develop another variant of AFM. They used a tuning fork like probe. One end of the probe was near, while the other end was away from the sample. When the fork was vibrated, the prong next to the sample experienced a minute shift in frequency due to forces acting upon it. This frequency drift when compared to the other prong, gives the molecular picture.

To get the finer detail of a molecule, one had to use a sharp AFM probe tip. They linked a molecule of carbon monoxide to the tip in such a way so that the lone oxygen atom became the de facto tip. It gave the structure of pentacene in unprecedented detail. You can clearly see even the bonds between carbon atoms, just as we read in our textbooks. However, the experiment must be vibration free, thermal noise free (at a very low temperature), and should be done in vacuum.

References: Scanning tunneling microscope (Wikipedia), Atomic Force Microscope (Wikipedia),
Single molecule's stunning image (BBC)

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PET Scan: Particle Physics And Electronics in Medical Imaging

Numerous imaging modalities are there to view anatomical structures in our body. They include X rays, Ultrasound imaging, MRI and many other procedures where we can see normal or diseased tissues in our body. They tell us ‘the location’ (position) in the body the image corresponds to. If we wanted to see what were happening in these locations, we would then need to perform functional imaging techniques like PET scan or fMRI.

In Positron Emission Tomography or PET, a radioactive isotope that decays by positron emission is introduced into the body. Positron emitting radioisotopes are prepared by bombarding stable atomic nuclei by protons. Protons are speeded up in a particle accelerator called cyclotron which then impinge upon the stable nuclei, and knocks out one neutron from its nucleus. The proton now occupies the position where the ousted neutron once stayed. But this atomic configuration is unstable, so the proton now decays. It decays by emitting positron, a particle resembling an electron in all aspects except that the charge is positive and not negative. In other words, a positron is an antimatter: an anti-electron.

Many different radioisotopes are there, such as Fluorine18, Oxygen15, and Carbon11. 18F is the most commonly used isotope. It replaces hydroxyl (OH) group in molecules of interest. We can use 18F Fluorodeoxyglucose (18FDG) an analog of glucose for probing the activity of brain. Our brain uses glucose for its metabolism, so when it encounters 18FDG, it stores them. The FDG in the brain begins emitting positrons. These particles travel only a short distance before they meet nearby electrons and annihilate. Two gamma ray photons, each having 511 keV of energy are produced--- Photons because they are electromagnetic waves, and gamma ray because the frequencies correspond to the gamma ray spectrum of electromagnetic waves.

So by detecting these photons, we can find out where they came from, since we know that these photons are emitted back to back, 180 degrees apart. (They aren’t exactly collinear as their initial velocity is not zero, and some computational error always creeps in). For the detection part, we need a detection array which will convert these photons into electrons. This is done by scintillators. Bismuth germanate, Luterium oxyorthosilicate (LSO) are some of them. Photons which are incident on them produce electrons by photoelectric effect. These electrons are then guided through a vacuum tube, which has many positive electrodes (dynodes) held at successively higher voltages. These dynodes of this photomultiplier tube accelerate these electrons, which in turn knock-off more electrons from the dynode plates. Thus we get more electrons than what we started with. The signal has now been amplified and we now have a measurable current.

With advancement in detection technology, silicon avalanche photodiodes (silicon APD) has now shown promise to replace the vacuum technology (photomultiplier tube). As the name suggests, APDs work in a similar way an avalanche gains its momentum as it descends from the mountain-avalanche effect. Detecting photons aren’t sufficient. We need to detect only co-linear (coincident) photons. Each collinear photon pair (i.e. 180 degrees apart) will constitute an event. All other photons (noise) must be rejected. About 10^7 to 10^8 or more ‘events’ must be registered in order to have a good signal to noise ratio. Image faithfulness varies proportionally with the square root of the number of events.

By acquiring a large number of events, the computer software is able to determine exactly where these radioactive tracers are located. This, in our case, means the locations where the neurons are accumulating (accumulation is a function of utilization of glucose) 18FDG. Thus we get a functional map. In order to know ‘what’ these structure were, we need to combine anatomical imaging like MRI or CT with PET. This combined PET-CT or PET-MRI let us know what structures are doing how much.

PET scan is very useful in neuroscience researches, clinical diagnoses like cancer detection, receptor analyses and even watching gene expression in molecular biology.

References: A good site with animation: PET animation

Physicsworld

Last Mod: 10 Mar, 2014

How About Letting A Capsule Videograph Your Intestine?

In electronics we use integrated circuits (IC) to digitize analog signals, amplify them, and encode and transmit them where necessary. Digital logic ICs come in handy where generation of oscillator frequencies, for transmission of signals is needed.

These logic ICs come in broadly 2 types: CMOS-FET (Complementary Metal Oxide Semiconductor- Field Effect Transistor) and TTL (Transistor Transistor Logic). While TTL ICs offer higher speed, they also consume higher current and a fixed regulated voltage (typically 5V). CMOS ICs work happily anywhere between 3V and 18V. Voltage regulation is not necessary. Moreover, they consume much less current, making a battery last longer.

This is perhaps why scientists used CMOS devices in this capsule. They used a CMOS capsule (shown on the left) to film the innards of human beings. Patients were made to ingest (take orally) this capsule, which contained a tiny camera and was capable of transmitting the pictures, as it traveled through the gastrointestinal tract. It photographed noninvasively and unobtrusively. The wireless endoscope snapped and transmitted the films in several frames per second. A radio receiver was placed outside the body to pick the signals and the signals were processed. You get the picture effortlessly while the capsule toiled.

The capsule labored till the battery lasted or it was passed in the stool. The whole story sounds somewhat like voyager space probes. These little capsules are now routinely assisting surgeons in finding polyps, tumors and other pathologies like ulcerated mucosae. The picture on the left shows a real snap of small intestinal mucosal erosion (red area in the center). In a study in Imperial College, London, UK, by Hamdulay and others on Behcet syndrome, 10 out of 11 known cases showed intestinal ulcerations in this method.

May be in near future they will even perform surgeries through remote control.

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On The Pursuit Of Arterial Plaques

In patients of disorders of lipid metabolism as in metabolic syndrome, diabetes mellitus and hypercholesterolemia, cholesterol laden plaques develop in the lumen of arteries. These plaques hinder the flow of blood leading to peripheral arterial disease (PAD). The narrowing of blood vessel that occur gives rise to symptoms of intermittent claudication and rest pain. Intermittent claudication is characterized by pain of the lower extremities as the person walks a few steps. This may progress to rest pain which is continually present, irrespective of whether the person walks or not. These plaques may also restrict blood flow to the heart causing myocardial ischemia.

What happens when these plaques rupture or loosen and detach? Fatal myocardial infarction (MI) and stroke (Cerebrovascular accident) may result when the heart or the brain arteries are involved respectively. We know that dyslipidemia of diabetes predispose us greatly to atherogenous plaque formation. LDL (low density lipoprotein), particularly oxidized LDL is a grave offender in this regard. Now imaging modalities are at hand which will let us visualize these atherosclerotic events, in real time.

Inflammation by oxidized LDL and their ilks increase the production of TNF-alpha (Tumor Necrosis Factor) and IL-1 (InterLeukin-1). They in turn increase the expression of vascular cell adhesion molecule-1 (VCAM) and P selectin. These intracellular adhesion molecules kind of attract leukocytes, which bind loosely to the plaque. These are the ominous plaques seething to rupture and loosen.

Micro Particles of Iron Oxide (MPIO) targeted with anti-VCAM-1 monoclonal antibody (mAb) are now being used to probe and identify these lesions. While the monoclonal antibody will latch onto the antigen (VCAM) as a key fits onto a lock; the microparticles of iron will act as a marker when seen in NMR (Nuclear Magnetic Resonance) imaging. Also called MRI or magnetic resonance imaging, this technique detects the density and spins of protons (H+ or hydrogen nuclei). In areas where contrast is less, contrast agents are employed to get a clear picture. Now, a team from National Taiwan University has developed this technique which employs dextran-coated iron oxide nanoparticles tagged with anti-VCAM-1 to get a glimpse of whats going on inside arterial lumen. A combination of anti-VCAM-1 mAb and anti-P-selectin mAb (VCAM-MPIO-P-selectin) is also being developed.

Although we already have angiography, intravascular ultrasound, and optical coherence tomography to detect these plaques, the newer techniques will certainly throw more light on this insidious killer process within.

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Every Breath You Take, I'll Be Watching You

When The Beatles sang " What goes on in your heart? What goes on in your mind?" and The Police warned "Every breath you take", you thought they exaggerated. Now we know they didn't. Scientists have really advanced so much on picking up and interpreting faint magnetic signals, this could soon be a reality; reading and interpreting our minds, non-invasively.

As action potentials are generated in the heart or while we think, electromagnetic waves are generated. These waves are produced as a result of changing ionic fluxes in these organs. We have been capturing them through ECG, EEG and magneto-encephalograms (MEG) for quite some time. As science and technology advances, gadgets shrink, gadgets think, become more sensitive and specific.

When a electrically conducting piece of metal is kept in a magnetic field, the magnetic field will exert a force on the moving electrons that will push them to one side, creating a voltage differential in the conductor. This effect is known as Hall effect and has given the birth of many devices like Gaussmeter or magnetometer. Hall effect sensor ICs are used in many applications of electronics, requiring sensing of magnetic fields. (I found one Hall IC inside my floppy disk drive, at the periphery of the circular flywheel.) Sensing brain waves will not be easy using Hall ic though, as the device is not that sensitive.

Superconducting quantum interference devices (SQUID) are much more sensitive than the former. The intensity of brain's magnetic field just outside the skull varies from 0.1 to 1 picotesla, less than a hundred-millionth of Earth's magnetic field. SQUIDs consist of two liquid helium cooled superconductors sandwiching a piece of thin insulator, creating two Josephson junctions. The presence of any stray magnetic field gives rise to interference and is detected by SQUID probes. So sensitive is this technology that it is being used to probe biomagnetism, gravitational waves and other extremely weak phenomena. But it has its inherent weaknesses too. It is bulky, expensive, power hungry and needs to be cooled near absolute zero.

Now scientists are devising gadgets that can operate near room temperature, handy and relatively inexpensive. A cell containing an alkali gas (like rubidium) will practically transmit all of a circularly polarized laser beam, when the spin of rubidium atoms point in the same direction. An object when placed near it, will distort the spins and some laser beam will be absorbed. Such vapor cells has sensitivity in the femtotesla range. Got to be more careful next time you see your wife carrying a strange object in her hand and getting closer to you (or your head!).

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The Medical Physics Of X-Ray

Wilhelm Conrad Roentgen was passing a current through a 'partially' evacuated glass tube from an induction coil. Induction coils typically produce large make and particularly break currents due to collapsing magnetic field that produces electricity in the opposite direction. He noticed that a fluorescent screen glowed, despite the room being dark and the glass tube covered by black paper. He later noticed that this device produced some hitherto unknown 'rays' that could penetrate variety of materials. He could see through his own flesh, down to the bones, and medical application of X-ray was born. Later he captured this in a photographic plate. The photo on the left is that of Rontgen's wife's hand, still wearing the wedding ring, one of the earliest recorded X rays.

When there is a vast potential difference in a partially evacuated glass tube, electrons rush towards the anode from cathode. As they impinge on the anode (anode being positively charged, attracts electrons), which is usually made of tungsten (Wolfram), they suddenly decelerate by colliding with the electrons of the anode material. Atoms are made up of protons and neutrons (which constitute the nucleus); and electrons, revolving around them in defined orbits. The electrons have clearly defined shells, spins, orbitals so that Pauli's exclusion principle is obeyed. Upon colliding, the electron knocks out an electron from the inner shell, which jumps to a higher energy level (outer shell). It finally comes down to its original place and emits electromagnetic radiation (photon) or X-Ray in the process. Thus X rays can be said to be produced mechanically (gamma rays are the result of nuclear decay or disintegration). The frequency of radiation is dictated by the equation, e=hv; where e is the energy of the quanta, h the Planck's constant and v the frequency of radiation.

X rays are electromagnetic radiation whose frequencies are higher than ultraviolet rays, but lower than gamma rays. They can be thought of as packets of energy or photons. X rays are ionising radiations. They ionize by either of the three ways: photoelectric effect, Compton effect, or pair production. In diagnostic applications (= at low energies, 30-100keV) photoelectric effects, the process just mentioned, predominate. Photoelectric effects are proportional to the cube of the atomic number that is exposed (Z^ 3). This explains the mechanism why bones (containing calcium) contrast so well with soft tissues. At higher energies, as is employed in radiotherapy, Compton effect, whereby the incident electron transfers some of its kinetic energy (to impart) to the target electrons and the rest as a deflected, less energetic photon. At still higher energy (above1.02 MeV), the energetic electron will form 'matter-antimatter' pair in the exposed material. A positron and an electron will form, which will annihilate later (to form two photons which will fly almost 180 degrees apart, i.e. in opposite directions).

In addition to detection by photographic plates, fluoroscopy; solid state materials like lithium doped germanium or silicon can also detect x rays. Here, the photons cause the formation of electron hole pair, which can then be detected. X ray photons may be transformed into visible photons when they interact with alkali halides such as sodium iodide (NaI). This visible light may then be amplified by photomultiplier tubes. Strictly speaking, they are electron multipliers, because as x ray photons hit the halide surface, typically only a few atoms thick, electrons are emitted. These electrons are accelerated by subjecting them to a cascade of increasing positive voltages (dynodes) in a circular or venetian blind system. Photomultipliers require less x-ray exposure (due to this magnification) and are widely used in nuclear physics.

X rays have various uses in medicine, industry and science. Orthopedic situations such as fracture, joint displacement; cancer; lung parenchyma and associated illnesses like emphysema, pneumonia; calculi (stones: renal, gall stones etc); paranasal sinuses (PNS, in sinusitis) are among them. In physiology, biochemistry and many other fields, X-ray crystallography is a valuable armament in deciphering the structure of crystalline molecules. The photons diffract (scatter) as they travel through the crystal lattice, leaving its imprint in the process. One can decode the molecular structure by deciphering the 'scatterings'. Deducing the structure of DNA molecule is perhaps the best known example till date.

Natural structures can also emanate X rays. Some celestial bodies emit x rays. But don't ask me about X ray specs, I have no idea. Anyway, X-rays are not always invisible themselves. High energy x-rays, make the peculiar sensation of light produced within the eye itself, when someone looks directly into the beam.
References: Stephen M. Hahn, Eli Glatstein, "Environmental and Occupational Hazards", HARRISON'S PRINCIPLES OF INTERNAL MEDICINE, Vol.2 (15th edition) pp. 2586-2587.

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