Glossary#

What is a magnetic gradient?#

From https://mriquestions.com/what-is-a-gradient.html

“Whenever a magnetic field differs in magnitude or direction between two points in space, a magnetic gradient is said to exist. The gradient (G) is defined as change in field (ΔB) divided by change in distance (Δs).”
“A frequent misconception about gradient fields is that the x- and y-gradients somehow skew or shear the main (Bo) field transversely. That is not the case as is shown in the diagram to the right. The x- and y-gradients provide augmentation in the z-direction to the Bo field as a function of left-right or anterior-posterior location in the gantry. The x- and y-gradients (ideally, at least) do not produce substantial components within the bore perpendicular to Bo.”

What is a gradient coil?#

From https://mriquestions.com/arent-gradients-coils.html

“gradient coils [are] a set of electromagnets embedded in the body of the MR magnet assembly. When an electrical current is passed through these coils, the main magnetic field is focally distorted in certain places, creating magnetic gradients of the first kind. Gradient coils alter the main magnetic field in a predictable manner in the x-, y-, and z-directions and are used to spatially encode the MR signal”

What is shimming?#

From https://mriquestions.com/why-shimming.html

“Shimming is the process by which the main magnetic field (Bo) is made more homogenous. Shimming may be passive, active, or both. In passive shimming small pieces of sheet metal or ferromagnetic pellets are affixed at various locations within the scanner bore. In active shimming, currents are directed through specialized coils to further improve homogeneity.”

What are the coils in a scanner?#

From https://mriquestions.com/many-kinds-of-coils.html

  • Main coils: Produce B_0

  • Shim coils: improve homogeniety

  • Gradient coils: (spatial) imaging

  • RF (radio frequency) coils: transmit B_1 field

  • Patient coils: Receive MR signal

What is a flip angle?#

From https://mriquestions.com/rf-coil-functions.html

By adjusting the magnitude or duration of these B1 [RF] pulses, the nuclear spin system can be rotated by variable flip angles, such as 90º or 180º.

How does the receiver coil work?#

“The oscillating net magnetic flux from the excited spin system can be captured by the [receiver] coil in which an induced electric current is generated. This current is then amplified, digitized, and filtered to extract frequency and phase information. “