MRI Magnetic Resonance Imaging

Definition A non-ionizing technique with full three dimensional capabilities, excellent soft-tissue contrast, and high spatial resolution.

MRI Facts Whole body imager = $1.5 million Slower then x-ray and CT – Takes between 3 and 10 minutes PER scan

MRI Basics The person is placed inside a strong magnet. – This produces a static magnetic field typically more than 10,000 times stronger than the earth’s magnetic field. Protons inside the body is considered a magnet. – Remember angular momentum? – MRI signal is received by these protons in the body (water and lipids)9

Nuclear Spin

Nuclear Spin (spin echo effect) When a magnetic field is applied to the protons – Most will align parallel with the field – Some will align anti-parallel with the field This creates a net magnetization

So here is how it all works… The magnet is turned on, creating the net magnetization This magnetization needs to be disturbed, so… – An RF pulse is created by the coil – A 90 degree pulse will throw the protons 90 degrees A signal is collected as the protons realign with the magnetic field The material they are contained in will determine how quickly they are able to realign – This is why there are different shades of grey

MRI Instrumentation

Types of Slices Depending on which slice of the body we want to see, a different RF pulse is applied – Coronal – Sagittal – Axial

T1 and T2 T1 = How long for protons to realign or “relax” T2 = How long for protons to fall out of phase Fat: – T1 = 280 ms, T2 = 80 ms Muscle – T1 = 870 ms, T2 = 45 ms Cerebrospinal fluid – T1 = 2400 ms, T2 = 160 ms

Functional MRI (fMRI) Based on the increased blood flow to a location that accompanies brain activity – Great for specific task or sensory based activity