How is fat signal removed in magnetic resonance mammography (breast MRI)?

Updated: Feb 25, 2019
  • Author: Preeti Gupta, MD, FRCR; Chief Editor: Eugene C Lin, MD  more...
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In general, T1W sequences of any type that are sensitive to contrast enhancement are also highly sensitive to other intrinsic short-T1 substances, the most common of which in the breast is fat. Four major approaches have been used to reduce or remove fat signal to show enhancement more clearly: GRE technique, digital image subtraction, spectral fat saturation, and magnetization transfer suppression (MTS). [46]

GRE imaging can be performed with a carefully chosen repetition time (TR), echo time (TE), and flip angle to minimize the (in-phase) fat signal while remaining sensitive to the presence of gadolinium enhancement. This approach was initially developed with field strengths of less than 1T, and it tends to be less suitable with high field strengths, where the T1 of fat is longer. Because this method is usually a 2D technique with relatively thick sections, the risk of missing a small lesion is significant.

Pixel-by-pixel digital image subtraction with precontrast and postcontrast images is the only reliable method of fat suppression with low field strengths. This method permits the short imaging times required for breast MRI. Inversion recovery and Dixon techniques tend to be slow at low field strengths, largely because of the poor SNR.

Subtraction is the best means of canceling signal inhomogeneity across the breast at any field strength. However, it is time-consuming and susceptible to motion artifacts. Worse, misregistration due to patient motion may cause a lesion to become less visible. Therefore, if subtraction is used, the source images must be carefully reviewed.

Spectral fat saturation using frequency-selective pulses is another technique. At less than 1T, the spectral separation of fat and water resonances is too narrow for this technique to be reliable. Successful application requires careful shimming; a good coil design; and, frequently, manual preimaging tuning. Even so, homogeneous fat suppression may not be possible with large breasts. Nevertheless, this technique remains the best method of obtaining high-spatial-resolution 3D scans without resorting to subtraction.

The MTS method reduces the glandular tissue signal by using detuned saturation pulses before the imaging sequence; this shortens the water relaxation by coupling it to macromolecular motion. This technique, as either an adjunct to or a substitution for spectral fat suppression, achieves high sensitivity in terms of contrast-enhancement sensitivity. [47, 48, 49] However, the technique has not been formally studied to determine whether it is consistently superior to other methods.

Of all of these approaches, image subtraction and spectral fat suppression are the 2 most commonly used strategies for improving enhancement detection.

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