Simultaneous proton resonance frequency shift thermometry and T₁ measurements using a single reference variable flip angle T₁ method

Abstract

Purpose Implement simultaneous proton resonance frequency (PRF) shift and T1 measurements with equivalent temporal resolution using a single reference variable flip angle method. This novel method allows for simultaneous thermometry in both aqueous and fatty tissue. Methods This method acquires a single reference image at the lower flip angle and all dynamic images at the higher angle. T1 is calculated using a single reference variable flip angle method, which accounts for the reference image temperature remaining constant. Monte Carlo simulations determined the optimal dynamic flip angle for combined PRF and T1 measurements. This method was evaluated in MR‐guided focused ultrasound heating experiments using a gelatin phantom and human cadaver breasts. In vivo measurement precision was demonstrated in healthy female volunteers under nonheating conditions. Results Temperature rise during MR‐guided focused ultrasound heating was measured in aqueous tissue with both PRF and T1. Both measures show good qualitative agreement in both space and time in aqueous tissue. The T1 change due to temperature increase was measured in fat, demonstrating the expected temporal response. The dynamic flip angle that produces optimal SNR for PRF measurements is lower than the optimal angle for T1 measurements, necessitating the selection of a compromise angle. Conclusion The single reference variable flip angle method provides a reliable way to simultaneously measure PRF temperature and T1 change and overcomes PRF’s inability to simultaneously monitor temperature in aqueous and adipose tissues. Future work will calibrate T1 change to temperature, enabling real‐time temperature in fat and increasing patient safety and treatment efficacy during thermal interventional treatments.

A variable flip angle golden‐angle‐ordered 3D stack‐of‐radial MRI technique for simultaneous proton resonant frequency shift and T1‐based thermometry

“…The potential variation in temporal resolution when advancing the KWIC filter across segments and flip angles in our proposed technique was largely addressed by adopting a sliding‐window pairing of different flip angles for T 1 fitting (Figure ). Performance of the proposed radial VFA sequence in nonheating in vivo subject scans was consistent with a previous report, with COV of T 1 below 5%. Future work can explore the combination of the respective advantages from our proposed technique and the technique presented in literature …”

“…Performance of the proposed radial VFA sequence in nonheating in vivo subject scans was consistent with a previous report, with COV of T 1 below 5%. Future work can explore the combination of the respective advantages from our proposed technique and the technique presented in literature …”

“…Our findings in this study add to the emerging body of work that utilizes 3D stack‐of‐radial MRI for temperature mapping. While the majority of previous studies using 3D stack‐of‐radial MRI for thermometry have only focused on PRF‐based methods, a recent paper proposed a GA‐ordered stack‐of‐radial method that acquired dynamic images of a single flip angle to calculate simultaneous PRF and T 1 ‐based 3D temperature maps in both muscle and fatty tissues. During reconstruction, this method compared dynamic images (of a single flip angle) with a nonheating baseline reference image acquired with a different flip angle to calculate T 1 changes during heating.…”

“…Since the signal model for VFA T 1 mapping requires accurate knowledge of the flip angles, additional measurement of the field is needed . To accelerate acquisition speed, two flip angles are typically used . Nonetheless, VFA PRF/T 1 methods require additional acquisition time and are typically confined to either a single 2D slice or a thin 3D slab with limited through‐plane coverage to achieve the desired temporal resolution .…”

“…Further improvements in temporal resolution were achieved by employing partial Fourier along the Cartesian‐encoded slice direction and using a k‐space‐weighted image contrast (KWIC) filter . Combined PRF and T 1 mapping using 3D stack‐of‐radial sampling may be a promising strategy to achieve dynamic 3D MR thermometry in aqueous and adipose tissues …”