Question: Regading Magnetic resonance imaging, what process of decay occurs under the influence of RF
About the article "Magnetic resonance imaging", section "T1rho MRI".

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T1ρ (T1rho): Molecules have a kinetic energy that is a function of the temperature and is expressed as translational and rotational motions, and by collisions between molecules.
The moving dipoles disturb the magnetic field but are often extremely rapid so that the average effect over a long time-scale may be zero.
However, depending on the time-scale, the interactions between the dipoles do not always average away.
At the slowest extreme the interaction time is effectively infinite and occurs where there are large, stationary field disturbances (e.g., a metallic implant).
In this case the loss of coherence is described as a "static dephasing".
T2* is a measure of the loss of coherence in an ensemble of spins that includes all interactions (including static dephasing).
T2 is a measure of the loss of coherence that excludes static dephasing, using an RF pulse to reverse the slowest types of dipolar interaction.
There is in fact a continuum of interaction time-scales in a given biological sample, and the properties of the refocusing RF pulse can be tuned to refocus more than just static dephasing.
In general, the rate of decay of an ensemble of spins is a function of the interaction times and also the power of the RF pulse.
This type of decay, occurring under the influence of RF, is known as T1ρ.
It is similar to T2 decay but with some slower dipolar interactions refocused, as well as static interactions, hence T1ρ≥T2.
 Magnetic resonance imaging (MRI), a medical imaging technique *Functional magnetic resonance imaging (fMRI), an MRI technique to measure brain activity *Magnetic resonance neurography (MRN), an MRI technique to image nerves *Cardiac magnetic resonance imaging (CMR), an MRI technique to assess the cardiovascular system *Magnetic resonance angiography (MRA), an MRI technique to image blood vessels *Magnetic resonance cholangiopancreatography (MRCP), an MRI technique to image biliary and pancreatic ducts *Endorectal coil magnetic resonance imaging, an MRI technique to image the area surrounding the rectum *Delayed Gadolinium Enhanced Magnetic Resonance Imaging of Cartilage (dGEMRIC), an MRI technique to image cartilage  *Magnetic resonance elastography (MRE), an MRI technique to measure tissue stiffness *Interventional magnetic resonance imaging (IMRI) a technique using MRI to guide medical interventions *Magnetic resonance spectroscopic imaging, an MRI technique to gather cellular activity and metabolic information Magnetic resonance therapy, a proposed treatment based on the principles of magnetic resonance  
Magnetic Resonance Elastography allows measurement of the propagation of shear waves in tissue using magnetic resonance imaging. 
Spin–lattice relaxation in the rotating frame is the mechanism by which "Mxy", the transverse component of the magnetization vector, exponentially decays towards its equilibrium value of zero, under the influence of a radio frequency (RF) field in nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI).
It is characterized by the spin–lattice relaxation time constant in the rotating frame, T1ρ.
It is named in contrast to "T1", the spin-lattice relaxation time.T1ρ MRI is an alternative to conventional T1 and T2 MRI by its use of a long-duration, low-power radio frequency referred to as spin-lock (SL) pulse applied to the magnetization in the transverse plane.
The magnetization is effectively spin-locked around an effective B1 field created by the vector sum of the applied B1 and any off-resonant component.
The spin-locked magnetization will relax with a time constant T1ρ, which is the time it takes for the magnetic resonance signal to reach 37% (1/e) of its initial value, formula_1.
Hence the relation:formula_2, where tSL is the duration of the RF field. 
The measurement of the naturally occurring magnetic fields produced by the brain's electrical activity is called magnetoencephalography.
This method differs from magnetic resonance imaging in that it passively measures the magnetic fields without altering the body's magnetization.
However, data from MEG and MRI can be combined to create images that approximately map the estimated location of the natural magnetic fields.
This composite imaging process is called magnetic source imaging (MSI).