Introduction

The T₂ relaxation properties of MRI signals are important parameters in assessment of pathology. Many musculoskeletal (MSK) tissues (cortical bone, tendon, ligaments, etc.) have very short transverse relaxation times and require specialized pulse sequences such as ultrashort echo time (UTE) for optimal signal acquisition and quantification. These scans can be performed either with or without Fat Saturation (FS). Here we analyze the effects that FS pulses of various bandwidths (BW) have on the values of T₂ quantification.

Theory

For a given single T₂ component, application of FS results in a reduction of the available SNR of that tissue, but has little effect on the subsequent decay curves. Often however, the MSK tissues of interest contain structures with multiple short T₂ components [1,2], which simultaneously contribute to signals in the region of interest (ROI). Since components with different T₂ values get attenuated by varying amounts (e.g. shorter T₂ tissues that have a broad linewidth get attenuated more), the overall signal decay is altered and simple single component T₂ fitting will yield corresponding altered results. Fig.1A shows theoretical signals (at TE = 0) of two different T₂s (10ms and 0.3ms) after application of FS pulses with various BWs using Bloch simulations (non-FS corresponding to FS pulse BW=0). As shown, the short T₂ signals get attenuated more than the longer T₂ signals as expected. The corresponding effects on single component T₂ fitting is shown in Fig.1B which results in overall T₂ values lying between the two components. For higher FS pulse BWs, the shorter T₂ components are attenuated more and the overall measured T₂ therefore increases.

Methods

The experimental phantom setup shown in Fig.1D (insert) was used to study the effects of Fat Saturation on T₂ quantification. Test tubes containing water doped with different amounts of MnCl₂ (resulting in T₂s of ~10ms and ~0.3ms) were imaged using coronal multi-echo UTE with TE=[0:0.8:8.8ms]. ROI measurements were obtained in the individual tubes and combined to emulate multi-component signals. In vivo scans were performed on a healthy male volunteer (age 72). Sagittal multi-echo images (see Fig.2) were obtained without FS, as well as with two separate FS pulse BWs. T₂ values were obtained using a single component fit in the patella tendon, meniscus, PCL, and cartilage.

Results

Shown Fig.1C are signal decay curves for the two individual phantoms. The measured values of T₂ of the combined signals are shown in Fig.1D as a function of the FS pulse BW and show good agreement with the theoretical simulations. Fig.3 shows the measured T₂ values of the in-vivo slices (from Fig.2). For all anatomies a similar systematic increase of the measured T₂ can be observed for larger FS pulse BW’s.

Discussion

Fat Saturation can have a notable effect on the measured single component T₂ values of multi-component tissues. Since the dispersion is less pronounced for smaller FS bandwidths, longer FS pulses can help to minimize the effects. Alternatively, multi-component fitting is another possible solution. The effect of the FS pulse BW on bi-component signal decay fitting will be investigated in future studies.

Acknowledgements

No acknowledgement found.