What is the difference between T1 and T2 mri scans?

Wikipedia has some stuff to get you going: http://en.wikipedia.org/wiki/MRI#Basic_MRI_scans

I will answer the easiest first: “What are the pros and cons of each technique?”
The general answer to this is that T1 scans are better for anatomy, and T2 are better for pathology, since T2 highlights fluid and many pathologies (including almost all traumatic or neoplasmic brain pathologies) are accompanied by oedema. However this rule is not always correct, because gadolinium contrast can only be imaged by T1, not T2.

“What actually happens to the particles in the brain when each one is performed?”
MRI physics is insanely hard, but I will do my best to summarise it for you. An MRI scanner works by applying a strong magnetic field to the body, which aligns all the protons along a single axis. A radio frequency (RF) pulse is then emitted, which knocks the protons off this axis. The magnetic field causes them to return to alignment, and in the process they emit RF waves which are then detected to form the image.

“What are the methodologically differences in acquiring them both?”
There are two types of relaxation; longitudinal and transverse. T1 imaging uses longitudinal relaxation signals to form its image, and T2 uses transverse relaxation signals.
When the protons are in equilibrium, there is a small discrepancy between spin ‘up’ and spin ‘down’ protons (this is quantum spin). If a 90 degree RF pulse is sent out, we can change this so there is no discrepancy, and there are equal numbers of spin up and spin down. When the pulse is turned off, the discrepancy returns. The signal from the few protons that switch back forms a T1 image.
The main magnetic field causes all the protons in the field to be aligned, but they retain their individual precession frequency. When an RF pulse is applied, they begin to precess in phase with each other, and a signal can be detected. When the pulse is switched off, they slowly return to their previous precession frequency, which is dictated by their binding in their respective atoms. As they lose phase the signal drops, and the time taken for the signal to disappear is measured and used to form a T2 image.

I’m not sure I fully comprehend what I just read, but why does longitudinal relaxation occur more in anatomy than pathology? Does it accurately allow the distinction between the two? Does the pathology alter the relaxation signals that much? Or put differently, does the alteration of the signals vary by degree of pathology? Given what I see on imaging of T1 and T2 films, I guess I need to pay closer attention. When ordering imaging which involves trauma, is it necessary or correct to order both T1 & T2 or would T2 only be adequate. Is there a substantial cost difference if I order just T2 vs both? Thank You.

@occupant1 The reason that T2 is superior to T1 for demonstrating many pathologies is due to the high signal given by water in T2 images (as you can see from the bright CSF in any standard T2 brain or spinal sequence). Most common pathologies are associated with a change in local water content, such as oedema surrounding a brain tumour. T1 receives less signal from water, and therefore won’t show these changes. However it is superior for demonstrating the anatomical changes associated with a particular pathology. In the case of a brain tumour, the T2 scan will clearly show the tumour and the effect on surrounding tissue, but any surgical planning will likely be done from the more anatomically precise T1 scans. The pathology doesn’t alter the relaxation signals as such, but it alters the tissue composition, and therefore the signal obtained from the affected region.

In a trauma situation, I would advise looking to CT first if there is any suspicion of a bleed. CT is much faster, and is perfectly adequate for showing fresh blood – you don’t want to lose a patient while you wait for MRI’s prettier pictures. That said, if an MRI is indicated, the selection of sequences should be tailored to the clinically suspected pathology. This will usually include a standard T1 and T2, as well as maybe half a dozen more advanced sequences.