| Affiliation |
(AGL) Chairman, Department of Ophthalmology, The Methodist Hospital, Houston, Texas; Professor of Ophthalmology, Weill Cornell Medicine, New York City, New York; (YL) Class of 2022, Baylor College of Medicine, Houston, Texas |
| Transcript |
Magnetic resonance imaging, magnetic resonance imaging tells us exactly what it is. It is imaging using a giant magnet of resonance. What is resonance? Resonance is vibration and the resonating substrate of MRI is a proton. This proton is hydrogen, it happens to be a very good anatomic substrate for NMR, Nuclear magnet resonance, also known as MRI, because you are a lot of hydrogen. Most of your hydrogen is in water, it is the H in H2O. So each of this tiny hydrogens has a net magnet vector, a dipole moment, and under normal circumstances your hydrogens are all pointing in different directions and therefore you have no net magnetic vector. However, when you enter into the giant magnetic field of, say a 1 tesla, all of your little magnets will align inside of the giant magnetic field. At that point, radio frequency pulse can be given to impart energy to the system, that will flip the protons into the transverse plane, when we turn off this radio frequency pulse, the protons will return to their static equilibrium states that will give off energy which we can measure as signaling intensity. So the language of NMR is signal intensity, hyperintense, hypointense, and isointense. Because intensity is language of MRI, as opposed to in CT, where the parameter is density. So knowing that hydrogen and water is the anatomical substrate of MRI you can then weight the study towards time constant 1 or time constant 2. And by weighting the study all we are doing is changing the parameter of the acquisition of the signal. So things that are bright on T2 tend to have a lot of water because proton density is the determining factor on the signal intensity on T2 so T2 is the water study and because most of the pathology that we encounter in the brain brings water with it. Things that are bright on T2 are usually pathology. Now We have a problem on T2 because there is this other substance in brain that has a lot of water that is not pathologic, it's normal, and that is cerebrospinal fluid, so cerebrospinal fluid is very bright on T2 so we would like to have a sequence that can attenuate or reduce the fluid signal and we can do that using an inversion recovery sequence, so when we use inversion recovery to attenuate the fluid, in this case, the cerebrospinal fluid (CSF), we call that flair so the T2 CSF signal can be attenuated using fluid attenuation inversion recovery, also known as flair, T1 in contrast to T2 is the anatomy study and the reason it is the anatomy study is that there are some things that are just naturally bright, hyper intense, on T1, like fat. And it provides us very good differentiation in signaling intensity without giving any contrast material. However the bad part is fat is bright on T1 but it's too bright so just like CSF is too bright on T2, fat is bright on T1 so we would like use inversion recovery to attenuate the fat signal and that is called Shorter Tl inversion recovery sequence, or fat suppression or fat saturation. So fat sat, and fat suppression, STIR all of these are fancy ways for suppression fat signal and reducing signal on T1, you do not have to order T1 or T2 it just comes automatically with the study, the study is automatically weighted for T1 time constant 1 and T2, however you need to consider ordering the suppression sequences, flair for T2, and fat saturation for T1 so that we can take away the normal bright signal of normal things that are bright or dark on T1 and T2. in addition we have to give the Gadolinium contrast material, so Gadolinium is a paramagnetic material that will enhance the local magnetic field. So if we give gadolinium, it will make things look more hyperintense if there's a break down or lack of the blood brain barrier. So some normal structures in the orbit which do not have the blood brain barrier, like the extraocular muscles, will enhance, the core will enhance. But the optic nerve, which is central nervous system, does have the blood brain barrier and therefore will not enhance under normal conditions. So if we see enhancement of the optic nerve after the gadolinium we know we have pathology there, we know we have break down of blood brain barrier. And the enhancement we look for is on T1, gadolinium post contrast. So now that you know how the protons work and how the signal intensity and MRI work in the giant magnet, you should know that fast moving blood produces those signal so if the proton is moving too quickly it won't get a chance to align in the magnet fluid and it will create a void in the signal which we call a flow void. So a void in the signal created by rapidly flowing blood means that high flow arterial blood for example internal carotid is going to be dark in all sequences. It will be hypointense, dark on T1 and T2 and after gadolinium and before gadolinium it will all be dark, and that flow void is how we can tell we are dealing with arterial high flow blood. So the basis of NMR, proton, water, weighted to study T1 T2 some things are bright on T2 like water, somethings are bright on T1 like fat we can suppress the normal signal with flair like CSF on T2 and fat saturation on T1 always give the gadolinium and we can see a high fast moving blood as a flow void in all sequences. |
