Exploiting multicompartment effects in triple‐echo steady‐state T2 mapping for fat fraction quantification

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Nonbalanced steady‐state free‐precession (SSFP) sequences have proven useful for fast morphological and quantitative MRI 1. Using appropriate gradient waveforms, individual transverse states of the magnetization can be selected, which allows the acquisition of MR images with different contrasts. Dual‐echo steady‐state (DESS) imaging 2 is an important variant of nonbalanced SSFP sequences, and relies on the acquisition of the two lowest‐order transverse states. The dependence of these states on quantitative MR parameters enables rapid parameter mapping in conjunction with water‐selective pulses 4. In particular, the measurement of transverse relaxation times (T2) in articular cartilage has been of major interest 6. More recently, the triple‐echo steady‐state (TESS) sequence was introduced 8, which additionally acquires a third transverse state. Applications for T2 quantification in cartilage have been demonstrated 9.
Articular cartilage consists of 65 to 80% water, and the remainder of other molecules include 10 to 15% of glycoproteins 10 and up to 4% of lipids 11. It can be described as a mixture of different tissue compartments, which may exhibit different T1 and T2 values as a result of water exchange 12 or different off‐resonant frequencies from susceptibility differences, partial volume effects, or chemical shift 14. In the human body, another prevalent example of multicompartment tissue is found in adipose tissue and bone marrow, in which water and fat are the two main compartments 15. The chemical shifts between the compartments could induce phase offsets, which modulate the total signal of the transverse states. Consequently, the assumption of a single‐compartment model may confound the accuracy of T2 quantification. Therefore, off‐resonance signal modulations in the transverse states of nonbalanced SSFP sequences need to be considered.
The purpose of the present work was to investigate the effect of multicompartments on the MR signals in the TESS sequence without water‐selective pulses, using simulations and in vitro measurements. The accuracy of TESS for T2 quantification in multicompartment tissue was assessed using in vitro phantom samples and in vivo experiments in the knee. A novel approach to exploit the multicompartment effect for fat fraction quantification is demonstrated for monitoring fat digestion in the stomach.

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