Investig Magn Reson Imaging.  2018 Dec;22(4):218-228. 10.13104/imri.2018.22.4.218.

T1-Based MR Temperature Monitoring with RF Field Change Correction at 7.0T

• Kim JM
• Lee C
• Hong SD
• Kim JH
• Sun K
• Oh CH

• KMID: 2431106
• DOI: http://doi.org/10.13104/imri.2018.22.4.218

Abstract

PURPOSE
The objective of this study is to determine the effect of physical changes on MR temperature imaging at 7.0T and to examine proton-resonance-frequency related changes of MR phase images and T1 related changes of MR magnitude images, which are obtained for MR thermometry at various magnetic field strengths.
MATERIALS AND METHODS
An MR-compatible capacitive-coupled radio-frequency hyperthermia system was implemented for heating a phantom and swine muscle tissue, which can be used for both 7.0T and 3.0T MRI. To determine the effect of flip angle correction on T1-based MR thermometry, proton resonance frequency, apparent T1, actual flip angle, and T1 images were obtained. For this purpose, three types of imaging sequences are used, namely, T1-weighted fast field echo with variable flip angle method, dual repetition time method, and variable flip angle method with radio-frequency field nonuniformity correction.
RESULTS
Signal-to-noise ratio of the proton resonance frequency shift-based temperature images obtained at 7.0T was five-fold higher than that at 3.0T. The T1 value increases with increasing temperature at both 3.0T and 7.0T. However, temperature measurement using apparent T1-based MR thermometry results in bias and error because B1 varies with temperature. After correcting for the effect of B1 changes, our experimental results confirmed that the calculated T1 increases with increasing temperature both at 3.0T and 7.0T.
CONCLUSION
This study suggests that the temperature-induced flip angle variations need to be considered for accurate temperature measurements in T1-based MR thermometry.

Keyword

MR thermometry; 7T; RF hyperthermia; Proton resonance frequency shift; T1; RF field

MeSH Terms

Bias (Epidemiology)
Fever
Heating
Hot Temperature
Magnetic Fields
Magnetic Resonance Imaging
Methods
Protons
Signal-To-Noise Ratio
Swine
Thermometry
Protons

Figure

• 

  Fig. 1 Experimental set-up of MR-compatible RF hyperthermia and phantom. An MR-compatible capacitive-coupled RF hyperthermia system (a) was used with fields of 7.0T and 3.0T for heating experimental objects. MR-compatible RF hyperthermia system and RF coil for MRI setting are shown in (b). To compensate for unwanted phase drift in PRF-based MR thermometry, oil phantoms were attached nearby the human-tissue-mimicking phantom and swine muscle tissue (c). PRF = proton resonance frequency; RF = radio-frequency.

• 

  Fig. 2 Location of the fiber-optic sensor and experimental protocols. (a) Quantitative analysis was performed within 7 × 7 ROIs to estimate changes of temperature obtained from PRF-based MR thermometry, apparent T1, actual FA, and T1. ROIs in the water region of the phantom, water region of swine muscle tissue, and fat region of swine muscle tissue are shown in (b), (c), and (d), respectively. The MR-compatible fiber-optic temperature sensor is located in the center of the ROI marked as a solid line. FA = flip angle; FFE = fast field echo; PRF = proton resonance frequency; ROI = region-of-interest; TR = repetition time; VFA = variable flip angle.

• 

  Fig. 3 Heat-induced parameter change maps of the human-tissue-mimicking phantom at 7.0T. Experimentally measured sequential changes of (a) temperature, (b) apparent T1, (c) actual FA, and (d) T1 for PRF-based MR thermometry of the human-tissue-mimicking phantom at 7.0T. With increasing temperature, apparent T1 decreased at 7.0T. The actual FA decreased with increasing temperature at 7.0T. After correcting for inaccurate FA, T1 was found to increase with increasing temperature at 7.0T. FA = flip angle; PRF = proton resonance frequency.

• 

  Fig. 4 Heat-induced parameter change maps of the human-tissue-mimicking phantom at 3.0T. Experimentally measured sequential changes of (a) temperature, (b) apparent T1, (c) actual FA, and (d) T1 for PRF-based MR thermometry of the human-tissue-mimicking phantom at 3.0T. With increasing temperature, apparent T1 and T1 increased at 3.0T. The actual FA decreased with increasing temperature at 3.0T. FA = flip angle; PRF = proton resonance frequency.

• 

  Fig. 5 Mean and standard deviation of different quantities over 7×7 ROIs of the human-tissue-mimicking phantom. (a) Temperature obtained by PRF-based MR thermometry. (b) Apparent T1 change. (c) Actual FA change. (d) T1 change. FA = flip angle; PRF = proton resonance frequency; ROI = region-of-interest.

• 

  Fig. 6 Scatter plots of tissue-mimicking phantom comparing temperature obtained by PRF-based MR thermometry and apparent T1 change, actual FA change, T1 change, and corresponding linear regressions for 3.0T and 7.0T. (a) Change in apparent T1 at 7.0T. (b) Change in actual FA at 7.0T. (c) Change in T1 at 7.0T. (d) Change in apparent T1 at 3.0T. (e) Change in actual FA at 3.0T. (f) Change in T1 at 3.0T. FA = flip angle; PRF = proton resonance frequency.

• 

  Fig. 7 Heat-induced parameter change maps of swine muscle tissue at 7.0T. (a) Temperature images obtained by PRF-based MR thermometry, (b) Changes of apparent T1, (c) Changes of actual FA, and (d) Changes of T1 for the experiment performed on swine muscle tissue at 7.0T. Mean and standard deviation of 7×7 ROIs of the swine muscle tissue for (e) temperature changes of PRF-based MR thermometry, (f) apparent T1 changes, (g) changes of actual FA, and (h) changes of T1. Mean and standard deviation of 7 × 7 ROIs of the swine muscle tissue in the fatty region for (i) temperature changes of PRF-based MR thermometry, (j) apparent T1 changes, (k) changes of actual FA, and (l) changes of T1. FA = flip angle; PRF = proton resonance frequency; ROI = region-of-interest.

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