After the technique of MR imaging has been refined, the process by which images are generated is controlled by a set of parameters that allow certain materials and tissues to be tickled into contrast.  The parameters are {TR,TE,TI},  the RF repeat, the Echo repeat, and the Inversion recovery times, respectively.  The set of parameters plays off two essential intrinisic characteristics of mediums in order to differentiate them. The characteristics  {T1,T2} are simply time parameters indicating quantum mechancial relaxation times specific to certain materials; spin-lattice relaxation and spin-spin relaxation, respectively.  A third characteristic is the proton density of a material.  The strength of an MR signal is proportional to this number.

T1 Weighted:   Utilizes the difference between the spin-lattice relaxation times of two materials by pinning the 90 degree RF repeat signal to a time interval (TR) that facilitates signal detection when the difference between the two T1 relaxation curves is maximal.

T2 Weighted:  Utilizes the difference between the spin-lattice relaxation times of two materials by pinning the 180 degree RF echo signal to a time interval (TE) that facilitates signal detection when the difference between the two T2 relaxation curves is maximal.

Proton Density:  Utilizes the difference in proton densities of two materials when T1 and T2 characteristics are similar.  By suppressing the signal effects of T1 and T2 (dis)similarities, signal strength disparities are isolated to proton density differences.
 

The table below shows MRI images of the brain and the abdomen using each of the three weighting techniques to optimally contrast a tumor from surrounding tissue.
 

	Brain MRI	Abdomen MRI
T1 Weighted
T2 Weighted
Proton Density Weighted

The scope of MR imaging is wide, from tissue/tissue differentiation to tissue/tumor differentiation.  The case for this experiment is the differentiation of a tumor from surrounding tissue.  In this example it is assumed that it is not known where the tumor is, but its characteristics are known.  Typically, tumors have a known range of anamylous charactertics.  The idea is to maximize the constrast between the tumor and all other surrounding tissue.  The table below gives the characteristic of the materials and the tumor used in all experiments.
 

Protocols:

T1 Weighting:
Brain (TR=912ms,TE=24ms)
Abdomen (TR=900, TE=24ms)

In order to differentiate the tumor by weighting T1 effects, the relaxation curves of all the tissues must have a distinguishable difference from the tumor for some appropriate TR.  Thus a TR was chosen such that all tissues showed a maximal or near maximal difference from the tumor relaxation time.  This was done by letting TR vary from 50ms to 3000ms and plotting the spin-lattice relaxation times for each tissue and eyeballing a solution that meets the prescribed conditions.  The chosen TR pinpoints a periodic time when all relaxation curves are distinguisble.  Note that TE was chosen very small as to suppress T2 characteristics, but long enough to insure proper signal recovery.  The results can be seen in Table 1.
 

T2 Weighting:
Brain (TR=2000ms,TE=15ms)
Abdomen (TR=2000ms,TE=61ms)

A similar approach was taken here by letting TE vary from 10ms to 120ms and finding appropriate times where all relaxation curves were a distinguishable distance away from the tumor.  TR was chosen such that differences were minimized, i.e., spin-relaxation curves have all mostly decayed.  The results can be seen in Table 1.

Proton Density:

In order for this weighting scheme to be valid, there must be an appreciable difference in the proton densities of the tissues and of the tumor.  To illustrate this phenomenon I changed the proton density of the tumor to 3.0 to accentuate the differentiation capabilities of this weighting protocol.  The results can be seen in Table 1.

	Brain MRI	Abdomen MRI
TI Weighted Optimal
TI Weighted Enhanced

The inverse recovery protocol is a method used by which spin-lattice relaxation curves are manipulated by pulsing a 180 degree RF field to effectively reverse the decay.  When the proper ratio is pinpointed it has the effect of suppressing the signal of tissues with a particular T1 charactersitic.  Thus in the case of broad differentiation of different materials, this technique is not very efficacious, unless it is of interest to suppress the signal of the tumor.  In this scope it doesn't give particularly meaningful information.

White Matter vs. Grey Matter		White Matter vs. CSF
Protocol	T1 Weighted TR=850,TE=24ms Traps the maximum distance between spin-lattice relaxation decays.		T2 Weighted TR=2000ms,TE=90ms This protocol exploits the huge difference in T2 times, allowing the white matter to decay while still detecting spin-spin relaxation signal from the CSF.
Liver vs. Kidney		Liver vs. Spleen
Protocol	T2 Weighted TR=2000ms,TE=40 Exploits the slight difference in T2 times by letting T2 for the liver nearly decay while leaving enough spin-spin relaxation signal from the kidney present		T2 Weighted TR=2000ms,TE=40 Exploits the slight difference in T2 times by letting T2 for the liver nearly decay while leaving enough spin-spin relaxation signal from the spleen present.

White Matter vs. Gray Matter		Liver vs. Kidney
Protocal			Inverse Recovery TI=150 TR=490 Effectively matches liver T1
Liver vs. Spleen
	Inverse Recovery TI=150 TR=490 Effectively matches liver T1