Gradients Explained

Gradients in MRI:

  1. Main Magnetic Field (B₀):

    • MRI scanners have a strong main magnetic field (B₀) along the z-axis. This field aligns hydrogen protons in the body and determines their precessional frequency.

  2. Purpose of Gradients:

    • Gradients are additional magnetic fields superimposed on the main field.

    • They vary in strength along specific directions (x, y, z axes) and are used for spatial encoding. This means they allow us to determine where in the body the MRI signal comes from.

  3. How Gradients Work:

    • Important: protons precess frequency is determined by the strength of the B field. By applying gradients to the B field, protons at different points recess at different frequencies.

    • Gradient coils are electromagnets that create a magnetic field that either adds to or subtracts from the main magnetic field.

    • For example:

      • If a gradient increases the field on one side, protons in that area precess faster.

      • If the field decreases on the other side, protons there precess slower.

    • This creates a gradient in precessional frequencies along the axis where the gradient is applied.

There are two different coils that apply their own fields and gradients. The first coil applies a field that oppose B_0 and the second coil applies a field that strengthens B_0. Both of these act in the z - axis. Protons will precess at different frequencies due to the changing strengths of the B field. Gradients can also be applied along perpendicular axis.

Why is this useful? When the RF pulse is applied to the protons at a specific frequency, only protons precessing at that frequency will be tilted, and consequently produce a signal. This means the signal only comes from a specific set or slice of protons that are experiencing a certain magnitude of B field, that causes it to precess at the same frequency of the pulse. These gradients can be shifted so each pulse would return a different slice.

  1. Why Gradients Are Important:

    • Each gradient (x, y, or z) changes the magnetic field in a specific plane. This means the MRI system can isolate signals from:

      • A single slice of the body (z-gradient, for slice selection).

      • A specific point within the slice (x- and y-gradients, for localization within the slice). Basically, by applying multiple gradients in different axis, we can target a specific area with a specific frequency, experiencing a specific field strength.

  2. Turning Gradients On and Off:

    • Gradients are rapidly switched on and off (e.g., in milliseconds) to manipulate the field in controlled ways.

    • This allows the scanner to encode spatial information into the MRI signal based on changes in precessional frequency and phase.

  3. Superposition of Fields:

    • The gradient fields do not replace B₀ but are added to it. The direction of B₀ remains unchanged; only its strength varies depending on the gradient.

This process of applying gradients is essential for constructing detailed MRI images by determining where in the body the detected signal originates.