Introduction - Quantitative MRI mOOC

The main magnetic field, also called the B₀ field, plays a crucial role in MRI. It dictates the precessional frequency of the spins and sets-up the bulk magnetization, which plays an important role in the image signal-to-noise ratio. Moreover, the radio frequency coils, tuned to the B₀ field, are responsible for flipping the spins in the transverse plane and for acquiring the signal. However, imaging reconstruction techniques assume a perfectly homogeneous B₀ field to reconstruct the signal from k-space data. An inhomogeneous B₀ field can lead to image artifacts such as signal loss, distortions Jezzard & Balaban, 1995, poor fat saturation Anzai et al., 1992 and many other image artifacts. In extreme cases, it can completely hinder the ability to create an image. B₀ inhomogeneities are also problematic for MR spectroscopy (MRS), because they widen the spectral linewidth.

When a subject is introduced in the scanner, the static B₀ field can be rendered more homogeneous through a technique called active shimming. Active shimming sends the appropriate amount of current through specific gradient and shim coils, in order to generate a magnetic field that will compensate for the existing (inhomogeneous) magnetic field. This procedure requires precise and accurate mapping of the B₀ field. B₀ maps show the difference between the current field and the expected field, and are typically displayed in units of magnetic field strength (Tesla [T]), precessional frequency (Hertz [Hz]) or in parts per million (ppm). Eq. 5.1 can be used to convert from the different units.

\frac{f-f_{0}}{f_{0}}\ast10^{6}=\frac{\Delta f}{f_{0}}\ast10^{6}=\frac{B-B_{0}}{B_{0}}\ast10^{6}=\frac{\Delta B}{B_{0}}\ast10^{6}=\delta_{ppm}

(5.1)

where $f$ , $f_{0}$ and $\Delta f$ represent the actual precessional frequency, the Larmor frequency and the difference between current and the Larmor frequency ( $f-f_{0}=\Delta f$ ) respectively. B, B{sub}0 and Δ B all have similar interpretations as $f$ , $f_{0}$ and $\Delta f$ but are in units of field strength (T). The relationship between frequency and field strength can be found using the well known Larmor equation ( $f=\gamma B / 2\pi$ ). $\delta_{ppm}$ is the field offset in ppm.

Figure 5.1 shows typical brain magnitude, phase, and B0 field map images of a brain in a 3 T scanner.

Source:Jupyter Notebook

Figure 5.1:Magnitude image of the reconstructed MRI signal acquired at 3 T (A), the phase in radians (B), and a computed field map in Hz (C), 𝜇T (D) and ppm (E). The dropdown can be used to select between the different images.

References¶

Jezzard, P., & Balaban, R. S. (1995). Correction for geometric distortion in echo planar images from B0 field variations. Magn. Reson. Med., 34(1), 65–73.
Anzai, Y., Lufkin, R. B., Jabour, B. A., & Hanafee, W. N. (1992). Fat-suppression failure artifacts simulating pathology on frequency-selective fat-suppression MR images of the head and neck. AJNR Am. J. Neuroradiol., 13(3), 879–884.