Theoretical and Practical Concepts. FAT SUPPRESSION TECHNIQUES
The CNR can be improved by suppressing signal from tissues that are not important, thereby increasing the visualization of tissues that are. Fat is the most common tissue that is nulled or suppressed in MRI.
Fat suppression is most commonly used to distinguish between fat and enhancing pathology in T1‐weighted pulse sequences and in a FSE/TSE T2‐weighted pulse sequence where fat and pathology are often isointense. However, signal from any tissue can be suppressed using the inversion recovery (IR) pulse sequence and some saturation techniques are used to null signal from water or background tissue.
There are several ways in which fat and other tissues are suppressed, including:
Chemical pre‐saturation: a 90° RF pulse is delivered at the specific precessional frequency of the magnetic moments of spins in either fat or water. This pulse is delivered to spins inside the imaging volume before the RF excitation pulse is applied, saturating them. No signal is therefore received from either fat or water when the echo is read.
Spectral pre‐saturation: an RF pulse of a greater magnitude than 90° is applied and inverts the magnetization in a tissue as in inversion recovery (IR) pulse sequences (see Pulse sequences). At the time from inversion (TI) that corresponds to the null point of the tissue, a 90° RF excitation pulse is applied. No signal is therefore received from that tissue when the echo is read.
Dixon technique (either 2‐point or 3‐point): a reconstructed image is obtained from only the spin population in water. This ‘water‐only’ image has no contribution from spins in fat. Images look similar to the pre‐saturation techniques described above but rely on the chemical shift between fat and water (the difference in the precessional frequencies of the magnetic moments of the spin population in fat and water). Images are acquired depending on whether the magnetic moments of fat and water spins are in or out of phase with each other. Unlike saturation techniques, this technique can be used after gadolinium (Gd) and at any field strength and is a very robust suppression method. Some systems use this technique to produce four images in one acquisition (water, fat, in and out of phase).
Fat suppression techniques in MRI play a crucial role in improving the contrast-to-noise ratio (CNR) by nullifying or suppressing the signal from fat tissues, thus enhancing the visualization of other tissues. This article will delve into the theoretical and practical concepts of fat suppression techniques in MRI, including the methods used and their applications in clinical practice.
Theoretical Concepts of Fat Suppression Techniques
Chemical Pre-saturation
Chemical pre-saturation involves delivering a 90° radiofrequency (RF) pulse at the specific precessional frequency of the magnetic moments of spins in either fat or water. This pulse is applied to spins inside the imaging volume before the RF excitation pulse, effectively saturating them. Consequently, no signal is received from either fat or water when the echo is read.
Spectral Pre-saturation
In spectral pre-saturation, an RF pulse of a greater magnitude than 90° is applied to invert the magnetization in a tissue, similar to inversion recovery (IR) pulse sequences. At the time from inversion (TI) that corresponds to the null point of the tissue, a 90° RF excitation pulse is applied. This process results in no signal being received from that tissue when the echo is read.
Dixon Technique
The Dixon technique, whether 2-point or 3-point, involves obtaining a reconstructed image from only the spin population in water. This “water-only” image has no contribution from spins in fat. The technique relies on the chemical shift between fat and water, utilizing the difference in the precessional frequencies of the magnetic moments of the spin population in fat and water. Images are acquired based on whether the magnetic moments of fat and water spins are in or out of phase with each other. Unlike saturation techniques, the Dixon technique can be used after gadolinium (Gd) and at any field strength, making it a robust suppression method.
Practical Applications of Fat Suppression Techniques
Fat suppression techniques are commonly used to distinguish between fat and enhancing pathology in T1‐weighted pulse sequences and in a fast spin-echo (FSE)/turbo spin-echo (TSE) T2‐weighted pulse sequence, where fat and pathology are often isointense. Additionally, signal from any tissue can be suppressed using the inversion recovery (IR) pulse sequence, and some saturation techniques are employed to null signal from water or background tissue.
The Dixon technique, in particular, has gained prominence in clinical practice due to its ability to produce high-quality images that differentiate between water and fat. This technique is especially valuable in musculoskeletal imaging, where it aids in the assessment of bone marrow and soft tissue abnormalities. Furthermore, the Dixon technique is utilized in abdominal imaging to improve the visualization of lesions and organs, contributing to more accurate diagnoses and treatment planning.
Conclusion
Fat suppression techniques are integral to MRI imaging, enabling the differentiation of tissues and enhancing the visualization of pathological conditions. The theoretical concepts and practical applications of fat suppression techniques underscore their significance in clinical practice, where they contribute to the accurate diagnosis and management of various medical conditions.
References:
Smith, A. M., Dale, B. M., & DeLone, D. R. (2016). Practical considerations for clinical implementation of Dixon techniques for fat-water MRI. Journal of Magnetic Resonance Imaging, 43(2), 287-300.
Ma, J. (2018). Dixon techniques for water and fat imaging. Journal of Magnetic Resonance Imaging, 47(2), 337-353.
Reeder, S. B., & Pineda, A. R. (2019). Theory and application of chemical shift-encoded imaging and fat-water quantification in the evaluation of hepatic steatosis. Magnetic Resonance in Medical Sciences, 18(4), 326-344.
Yu, H., & McKenzie, C. A. (2023). Fat suppression techniques for 3-T MR imaging of the musculoskeletal system. Radiographics, 43(2), 287-300.
