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

Aging: Gender Bias, Hormones and Biomarkers

Women, on the whole, live longer than men. But, in them, declining estrogen levels usher menopause. With menopause, the chances of osteoporosis (bone demineralization leading to fractures and bone pain) and cardiovascular diseases (like heart attack and arterial diseases: estrogen elevates HDL levels, the good cholesterol; and reduces LDL levels, the bad cholesterol) increase drastically. This can be alleviated by exogenous administration of estrogen, in the form of skin patch, for example. But the concomitant risks of cancer should be borne in mind. Fertility comes to an end with menopause. This actually is a blessing in disguise, since reduced fertility would reduce maternal mortality (an aged mother is more likely to suffer complications of childbirth) and would curtail the risks of fetal malformations (which rises with both maternal and paternal age).

Glucocorticoids, such as cortisol, are hormones released in response to stress, such as physical and mental stress, have been implicated in aging. Their level rises in blood with aging, and this is due to the faulty sensing mechanism in a brain area called hippocampus. Hippocampus senses glucocorticoid levels and calculates the amount of this steroid to be secreted, by negative feedback, in much the same way the 'gain' if an 'electronic amplifier' is regulated by negative feedback. Rising glucocorticoid levels with age damages this feedback control mechanism, with consequent production of still more glucocorticoids and this vicious cycle repeats. This is 'glucocorticoid cascade hypothesis' theory of aging. Catecholamines, such as adrenaline (epinephrine), nor-adrenaline (norepinephrine) are stress hormones and they raise the heart rate and overall metabolism. They initiate the 'fight and flight' response and raise the blood glucose level. They too, seem to play a role in aging.

Growth hormone (somatotropin/GH) administration restores muscle mass and vigor, and so does hormones such as DHEA, testosterone (in males); but hormone supplementations have their own risks. You might ask, how do we know that we are aging? Is there any biochemical or radiological test to quantify it? The answer is no, presently, but a few 'indices' like cross-linking of collagen, insulin resistance, free testosterone (male), DHEAS (dehydro epi androsterone-sulfate), fibrinogen level, body mass index or BMI (female), handgrip strength, graying of hair and a few others have been proposed.

Cell Membranes: Smart Wrappers

The cells in our bodies are covered by a coat called the cell membrane, which is also known as plasma membrane. This membrane is bi-layered, and is composed of phospholipids, among other things such as proteins. The interior of the cells contain water and the cells themselves bathe in an ocean of saline water (a legacy from our marine ancestors). Hence, the bilayer is fashioned in such a way that the water loving (hydrophilic/polar) portions of phospholipid molecules are in contact with water lying external and internal to it. The structure of phospholipids resemble matchsticks; the tips representing hydrophilic phosphate parts while the shafts represent lipid portions, hydrocarbon chains. Thus, it can be very clearly comprehended that the tips of these matchsticks will be oriented exteriorly and inwardly (as is seen in the picture), while the hydrocarbon ends (non polar/hydrophobic), lie buried deep within the membranes' middle portion.

This membrane is in no way static, as the picture might convey. It is highly dynamic and this gives rise to membrane fluidity, which is contributed largely by cholesterol. Imagine a pond with numerous soccer balls floating on the water. You throw a heavy object, say a stone on them. The balls will make way for the stone to go through them before it sinks. Moments later the balls will again fill the gap, the stone created, thus realigning themselves. The cell membrane possesses this property, fluidity. This is essential for the cell to change shape (RBCs change shape while negotiating small capillaries), endocytosis (a process by which cells engulf external particles), receptor conformational changes and many others.

The protein molecules that are studded/embedded within the cell membrane serve various functions. They act as receptors (molecules that interact with various ligands), structural proteins, pumps, carrier proteins (helps in ferrying molecules by facilitated diffusion), ion channels (conduits for ions), enzymes and adhesion molecules (helps cells stick/adhere to themselves and to the underlying basal lamina or basement membrane), to name a few.

Cell membranes also help maintain the polarity of the cell, both by electrical (ionic) insulation and Na+/K+ ATPase pump activity. Membranes that cover intracellular bodies (organelles), also resemble cell membranes and they serve vital functions too. For example, mitochondria generate harmful free radicals, which could wreck havoc had it not been covered by this biological membrane. This intelligent mosaic carpet thus continue to amaze us.

Treating Type2 DM

So far, we have discussed about insulin, its actions, type 1 diabetes mellitus and its treatment. In contrast to type 1 DM, type 2 DM is characterized by end organ (tissue level) insensitivity to insulin. Thus insulin is present in normal or more than normal quantity, but somehow it can not deliver its message across the cell. Obviously such a state cannot be addressed by giving more insulin, but in making the cell more responsive to its effects. This is done by administering oral hypoglycemic drugs (OHD). (Although, in complicated type2DM or when there is an added stress like a fracture, emotional trauma, surgery; insulin has to be administered, in addition).

Sulfonylurea group of drugs act by closing the SUR (Sulfonylurea) channel, which is in fact, a part of the ATP sensitive potassium channel. Closing this channel would mean build up of potassium inside the cell, thus neutralizing the electro negativity inside, making the cell depolarized. This in turn will lead to influx of calcium ions from outside, which in turn will lead to release of stored insulin. Thus these groups of drugs are also referred to as insulin releasers. Of these, short acting sulfonylureas like glimiperide or glyclazide are preferred over their long acting cousins like chlorpropamide. This is due to the adverse effect of hypo glycemia, which, naturally tend to be long and more profound with long acting sulfonylureas.

Biguanides like metformin are very helpful, specially in obese diabetics. It acts in various ways, though its exact mechanism of action hasn't yet been worked out. It is also effective in the treatment of PCOD (poly cystic ovarian syndrome). Phenformin, a biguanide, which caused fatal lactic acidosis, was withdrawn from the market.

Meglitinides are a group of drugs which act by releasing insulin, in much the same manner as the sulfonylurea drugs. Repaglinide and nateglinide are examples of this group.

Thiazolidinediones, which include pioglitazone and rosiglitazone, are novel drugs which target PPAR-gamma (peroxisome proliferator-activator receptor-gamma), an intracellular receptor.
The Tzds as they are better known, combine with the intra nuclear receptor, thereby modifying gene activation and thus bring about their actions. PPAR-gamma are located in muscle cells, fat cells, liver and other organs. Their effects take time, since they act through genetic transcription, translation etc. In addition to sensitizing the tissues to insulin, they also lower LDL, increase HDL in those taking it. Thus, they also correct, to some extent, the metabolic disorders which usually accompany diabetes.

Then there are also alpha glucosidase inhibitors, like acarbose and miglitol, which prevent the breakdown of polysaccharides in the intestine, to smaller molecules like glucose or fructose. As the intestine cannot absorb carbohydrate molecules other than simple sugars, blocking polysaccharide breakdown in the gut will thus reduce the sugar load into the blood stream.

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