Rationale and Objectives
Magnetic resonance imaging (MRI) temperature imaging systems need to be routinely calibrated to guarantee accurate temperature results and qualified MRI. No independent physical temperature calibration phantom (TCP) set is currently available. An economical TCP set was developed to routinely ensure the quality of MRI temperature imaging system.
Materials and Methods
The novel TCP was constructed using a heating unit, temperature sensor, and MRI phantom liquid. A specialized heating unit was developed using carbon fibers. The TCP set design was an integration of the TCP, temperature measurement unit, display unit, and control unit. The proposed MRI calibration kit, which is a combination of the TCP set and standard MRI phantom, was used in the MRI thermometry calibration and MRI quality calibration.
Results
The TCP set provided an efficient, accurate, and homogeneous temperature map as the reference standard temperature for calibration. Accuracy and heating efficiency of the TCP set were 1°C and 1°C/minute, respectively. Calibration of the MRI thermometry and MRI quality were implemented successfully.
Conclusion
The proposed TCP set is completely compatible with the MRI system and can be used to calibrate MRI thermometry and MRI quality to ensure the quality performance of the MRI temperature imaging system.
Thermal ablation of a tumor inside the body through minimal invasive or noninvasive procedure has attracted the interest of the tumor therapy community. During thermal ablation, the temperature distribution of the target organ has to be identified to control the deposited heat energy. Magnetic resonance imaging (MRI) is an appropriate thermal surgery guide and monitor because it is temperature-sensitive and noninvasive. Furthermore, MRI has the advantages of nonionizing radiation and good spatial and temporal resolutions in any scan orientation . Applicable MRI-guided thermal ablation surgery systems are already installed in hospitals. For example, the Exablate 2000 system (InSightec Ltd., Dallas, TX), an MRI-guided focused ultrasound surgery system, has been approved by the US Food and Drug Administration for fibroid ablation . Recently, relevant research for vertical-field MRI-guided focused ultrasound surgery system has been reported . Moreover, MRI-guided laser induced thermotherapy was reported by Ding et al , and MRI-monitored radiofrequency ablation for breast cancer was reviewed by Postma et al . Developing the MRI temperature imaging system is an attractive topic in the medical physics field.
Calibration of the MRI temperature imaging system is crucial for qualified application in clinics. Previously, Wu and Felmlee reported an MRI system–based quality control program to assess focused ultrasound beam positioning and power delivery accuracy, MRI quality, and system safety for MRI-guided focused ultrasound ablation system . They constructed a special phantom suitable for focused ultrasound power absorption and published a report that such phantom can be stable for months . However, relative errors measured from the MRI to calibrate the latter MRI temperature measurement results were used. The calibration was based on the calculation results from the system software, not the results from independent real physical temperature measurement. McDannold and Hynynen reported on the use of temperature maps acquired during heating into an ultrasound phantom to retrospectively evaluate the deviations in the performance of the system for quality assurance . This method also relied on the proper function of the system software. When the errors of the temperature results calculated from the system software were beyond the permitted scope, the reported two methods were no longer appropriate. Thus, MRI temperature results had to be calibrated based on independent physical measurement results. However, no independent calibration phantom set with physical temperature measurement as reference standard for routine calibration of MRI temperature imaging system is available.
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Materials and methods
Configuration of the Temperature Calibration Phantom Set
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Developing the Carbon Fiber Heating Unit
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Integration of the Temperature Calibration Phantom Set
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Experiments Using the Calibration Phantom Kit
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ΔT=ϕ(T)−ϕ(T0)γαB0TE, Δ
T
=
ϕ
(
T
)
−
ϕ
(
T
0
)
γ
α
B
0
T
E
,
where ϕ(T) ϕ
(
T
) is the testing image phase, ϕ(T0) ϕ
(
T
0
) is the reference image phase, γ is the gyromagnetic ratio, α is the proton resonance frequency shift coefficient ( α=−0.01 α
=
−
0.01 ppm/°C), B 0 is the main magnetic field strength, and TE is the echo time. The MR temperature measurement errors were obtained by comparing the real physical temperature result and the MRI temperature measurement result.
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Results
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Discussion and conclusion
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