Chapter 4

Mark D. Herbst, M.D., Ph.D.

Magnetization

• MRI depends on magnetization of tissues

• Temporary magnetization occurs when the hydrogen nuclei are laingned in one direction more than other directions

• The sum of all the individual spin vectors is the net magnetization vector.

Maximum magnetization depends on 3 things

• Concentration of magnetic nuclei (PD)

• Magnetic sensitivity of the nuclide (1.0 for H-1)

• Magnetic field strength (B0)

MR Image

The MR image shows different tissues as different levels of gray because the magnetization of each tissue is different when the “picture is snapped.”

The level of magnetization at any given time in a tissue depends on several factors, the most important being T1, T2, and PD.

Tissue Magnetization

• Occurs initially a within several seconds of placing the patient in the magnet.

Magnetization Direction

• Two main directions: – Longitudinal (along the z direction, same as

the magnetic field of the magnet)– Transverse (in the xy plane)

Flipping or tipping

• When the net magnetization vector is subjected to RF at the Larmor frequency, it begins to tilt out of alignment with the magnetic field direction of the magnet (z) and toward the xy plane.

• This tilt is called the flip angle or tip angle and is usually 90 or 180 degrees, but can be any angle.

Terms to know

• Excitation-any time the equilibrium condition is disturbed by an RF pulse

• Equilibrium-protons in the magnet are undisturbed by RF long enough to achieve the maximum magnetization in the z direction

• Relaxation- process of going back to equilibrium after excitation

• Saturation-when no signal can be obtained from tissue because it has absorbed all the RF it can (like a saturated sponge) and both energy levels are equally populated.

T1 relaxation

• Describes the regrowth of the net magnetization vector in the z direction

• T1 is defined by the time it takes for it to regrow 63%

T2 Relaxation

• Describes the decay of magnetization in the xy plane (dephasing)

• Defined as the time it takes for magnetization in the xy plane to decay 63%

• Dephasing is caused by spin-spin interactions or by diffusion through magnetic field gradients

Effect of B0 on relaxation times

• No effect on T2

• Increased T1 with increasing B0

Transverse magnetization

• Produces contrast between tissues

• Generates RF collected by receive coils (signal, echoes) to be converted to images

Magnetic susceptibility

• Diamagnetic-negative or low magnetic susceptibility (air, cortical bone)

• Paramagnetic (have unpaired electrons like ions of gadolinium, manganese, iron, chromium; nitroxide, molecular oxygen; magnetite (a superparamagnetic iron oxide); cause marked decrease in T1, mild decrease in T2)

• Ferromagnetic-metallic iron, ferritin