PRF Thermometry for Monitoring Small Temperature Changes during Very Long Thermal Therapies: Field Drift Compensation Using FID Navigators
Tetiana Dadakova1, Axel Joachim Krafft1,2,3, Jan Gerrit Korvink4, Stephan Meckel5, and Michael Bock1

1Dept. of Radiology - Medical Physics, University Medical Center Freiburg, Freiburg, Germany, 2German Cancer Consortium (DKTK), Heidelberg, Germany, 3German Cancer Research Center (DKFZ), Heidelberg, Germany, 4Institute of Microstructure Technology, Karlsruhe Institute of Technology, Karlsruhe, Germany, 5Department of Neuroradiology, Neurocenter, University Medical Center Freiburg, Freiburg, Germany

Synopsis

Proton resonance frequency shift thermometry uses phase MR images to calculate temperature change in tissue during thermal therapies. Temporal and spatial changes of local magnetic field influence the phase images and can mimic temperature change. In order to correct for temperature errors due to magnetic field drift with time the FID navigators were used before and after imaging readout. Field-drift related phase slope during each repetition was calculated and used to correct the imaging data. This correction method is especially useful for the long thermometry procedures during which the temperature change is small and high temperature precision is needed.

Purpose

Proton resonance frequency (PRF) shift thermometry is a robust method to measure temperature changes in tissue. In PRF, phase images are used to measure the PRF change with temperature, which for water-based tissues is 0.01 ppm/°C. Unfortunately, signal phase is susceptible to spatial and temporal magnetic field changes which can mimic temperature changes. For example, magnetic field gradients can heat up over time which leads to a temporal field drift (at a typical clinical 3T-system, the field drift can be up to 0.1 ppm/h corresponding to 10 °C/h). In contrast to short ablative procedures, hypo- or hyperthermia treatments can last several hours, and the therapeutic temperature change can be on the order of several degrees only1. If large volumes are treated (e.g., cerebral hypothermia2), reference-less thermometry3, 4 cannot be used as the borders of the treated area are not precisely known, fat cannot be used as reference due to potential susceptibility changes5, and MRS of metabolites6 is time consuming.

Similar to previous work from Svedin et al7, in this work we propose to use FID navigators to correct for general field drifts to accurately detect temperature changes on the scale of 1°C.

Methods

For FID-corrected temperature measurements, a 3D spoiled gradient echo sequence (FLASH) was developed with two FID navigators before and after the imaging acquisition (Fig. 1). The navigators are applied in sections of the sequence without spatial encoding, so that the complex overall signal within the excited volume is acquired. The following MRI parameters were used: TE/TR = 10/35 ms, spatial resolution 2×2×2 mm³ and matrix 256×98×24, acquisition time 80 s, and TEnav,1 = 5 ms and TEnav,2 = 15 ms. The acquisition was repeated 60 times so that the whole measurement lasted for 80 min.

In order to reconstruct the corrected images, the phase data from two FID navigators was subtracted. Then, the remaining phase difference was divided by the time difference between navigator readouts to find the drift-induced phase slope during each TR. Finally, a correction phase was calculated for each sampling point during the imaging readout and subsequently subtracted. A more detailed description of the correction can be found in Svedin et al7.

A hypothermia experiment was conducted in a 2%-agar phantom which was cooled through a plastic tube with constant flow of ice water. Ten reference data sets were acquired while water flowing at room temperature before ice was added to the water reservoir. The experiment was continued until all ice melted. Temperature images were calculated from the phase difference of the averaged reference data and the dynamic phase values. In the temperature images, 4 different ROIs were placed near the locations of embedded fiber-optical (FO) temperature probes (FOTEMP 4, Optocon AG, Dresden, Germany).

Results

Figure 2 compares temperature changes from fiber-optical and MRI temperature measurements. Before FID correction, MRI deviates as much as 6°C for all probes, and the temperature error increases with time. After correction the maximum temperature error is 0.5°C (probes 2-4), and 0.7°C (probe 1 close to tube). The Bland-Altman plot (Fig. 3) shows a systematic but small mean deviation of 0.1°C during the cooling phase and a larger deviation of up to 0.7°C during the transient re-heating phase while the ice was melting. In total the mean difference is +0.2°C (confidence limits: -0.2°C to 0.6°C) which reflects an over-correction of MR temperature data.

Discussion and Conclusion

The results show that FID navigator correction can substantially increase the accuracy and precision of PRF temperature measurements by up to one order of magnitude. As both FID navigators integrate the signal over the entire measured volume, the phase change used for correction is influenced by both, field drift and temperature. When the temperature-related phase changes are local, the dominant phase change is caused by the field drift, whereas if temperature effects become more global both influences impact the FID phase and cannot be discriminated. This is reflected in the Bland-Altman plot where temperature errors remain small during (local) cooling, whereas systematic errors increase during the re-heating phase when temperatures start to distribute over larger portions of the phantom. To overcome this limitation, a dummy load could be added with stabilized temperature. The volume of the load could be optimized depending on the expected heated or cooled volume.

In summary, FID navigator correction can be used to improve MR thermometry in thermal therapies that apply heating or cooling over a long time (several hours) and are intended to induce small temperature differences (several °C per hour), for which a high temperature accuracy and precision (≤0.5°C) is needed.

Acknowledgements

This work was supported by the BMBF, Eurostars Project E!6620 PROFUS

References

1. Lüdemann L, et al. Non-invasive magnetic resonance thermography during regional hyperthermia. Int J Hyperthermia. 2010;26(3):273-82

2. Wu TC, Grotta JC. Hypothermia for acute ischaemic stroke. Lancet Neurol. 2013 Mar;12(3):275-84

3. Rieke V, et al. Referenceless PRF shift thermometry. Magn Reson Med. 2004 Jun;51(6):1223-31

4. Salomir R, et al. Reference-free PRFS MR-thermometry using near-harmonic 2-D reconstruction of the background phase. IEEE Trans Med Imaging. 2012 Feb;31(2):287-301

5. Sprinkhuizen SM, et al. Temperature-induced tissue susceptibility changes lead to significant temperature errors in PRFS-based MR thermometry during thermal interventions. Magn Reson Med. 2010 Nov;64(5):1360-72

6. Kuroda K. Non-invasive MR thermography using the water proton chemical shift. Int J Hyperthermia. 2005 Sep;21(6):547-60

7. Svedin BT, et al. Respiration artifact correction in three-dimensional proton resonance frequency MR thermometry using phase navigators. Magn Reson Med. 2015 Aug 13

Figures

Figure 1 Spoiled gradient echo MR pulse sequence (FLASH) includes two FID navigator readouts (light red) before and after of image acquisition readout (dark red)

Figure 2 Temperature change measured by fiber optic thermometer (FO) and calculated from MR images in 4 ROIs, where the FO probes are located. Open data points show temperature before correction. Solid data points show temperature after correction. P1-P4 correspond to locations of 4 FO probes and 4 corresponding ROIs

Figure 3 Bland-Altman plot. Difference between the temperature measured with fiber optical probes and calculated from MR images with correction is plotted as a function of average between them. Solid data points correspond to decrease of the temperature and open data points – to increase



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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