In vivo simultaneous proton resonance frequency shift thermometry and single reference variable flip angle T₁ measurements

Purpose: The single reference variable flip angle sequence with a multi-echo stack of stars acquisition (SR-VFA-SoS) simultaneously measures temperature change using proton resonance frequency (PRF) shift and T₁-based thermometry methods. This work evaluates SR-VFA-SoS thermometry in MR-guided focused ultrasound in an in vivo rabbit model.

Methods: Simultaneous PRF shift thermometry and T₁-based thermometry were obtained in a New Zealand white rabbit model (n = 7) during MR-guided focused ultrasound surgery using the SR-VFA-SoS sequence at 3 T. Distinct locations in muscle (n = 16), fat (n = 12), or the interface of both tissues (n = 23) were heated. The T₁-temperature coefficient of fat was determined using least-squares fitting of inversion recovery-based T₁ maps of untreated fat harvested from the animal and was applied to the in vivo measured heat-induced T₁ changes to create temperature maps.

Results: Using k-space weighted image contrast reconstruction, temporal resolution of 1.71 s was achieved for simultaneous thermometry at 1.5 × 1.5 × 2 mm voxel resolution. PRF shift thermometry was not sensitive to heating in fat. T₁ changes were observed in fat at the ultrasound focus. The mean T₁-temperature coefficient for fat was determined to be 1.9%/°C ± 0.2%/°C. Precision was 0.76°C ± 0.18°C for PRF shift thermometry in muscle and 1.93°C ± 0.60°C for T₁-based thermometry in fat. Sonications in muscle showed an increase in T₁ of 2.4%/°C ± 0.9%/°C.

Conclusion: The SR-VFA-SoS sequence was shown to simultaneously measure temperature change using PRF shift and T₁-based methods in an in vivo model, providing thermometry for both aqueous and fat tissues.