Quantitative Temperature Imaging in Chemically Designed Phantoms
Scott D. Swanson1, Dariya I. Malyarenko1, and Thomas L. Chenevert1

1Department of Radiology, University of Michigan, Ann Arbor, MI, United States


Quantitative temperature mapping


Absolute temperature measurements are desired for MR guided HIFU (1) or calibration of quantitative diffusion phantoms (2). The chemical shift between hydroxyl and aliphatic protons in methanol or ethylene glycol is often used to calibrate the thermocouple temperature measurements in high-resolution NMR spectrometers (3). Previous work has shown that gradient echo imaging of the beat frequency between these resonances can be used to obtain quantitative and absolute images of temperature (4). The precision of this method is limited by the T2 of the resonance, about 100 ms at room temperature for ethylene glycol (5). Water can be added to ethylene glycol (or alcohols) to create equimolar amounts of CH2 (or CH3) and hydroxyl protons. However, hydroxyl proton exchange is slow at neutral pH and exchange broadening between water and ethylene glycol further reduces T2. This exchange is proton or hydroxyl catalyzed and creating high or low pH samples will place the kinetics in the fast motion limit and significantly increase T2. We present here data of a chemically balanced, pH adjusted, aqueous ethylene glycol solution with equimolar amounts of hydroxyl and methylene protons.


Phantoms were made with water, and either ethylene glycol, methanol, or t-butanol to create equimolar amounts of hydroxyl and aliphatic protons. pH was adjusted by adding sodium hydroxide or hydrochloric acid. The ethylene glycol solution studied here (pH 12), was placed in a 25 ml vial and studied at 2T. Gradient echo images were obtained typically with 100 ms TR and variable TE. In plane resolution was 1 mm and the slice thickness was 2 mm. Temperature was varied by warming house compressed air in a nichrome wire heating element and passing the warmed air over the sample. Temperature at one location was measured by a fiber-optic probe inserted into the sample.


The signal of the water resonance is given by $$$M_w = e^{i 2 \pi \nu_w TE} e^{-TE/T{2w}} $$$ and similarly for the methylene resonance. The splitting between water and methylene changes with temperature according to $$$\Delta \nu = 144.4 – 0.807 T(°C)$$$ at 2T in the ethylene glycol phantom. T2 in neat ethylene glycol is 100 ms at 25°C (4) whereas in the pH 12 solution, T2 is 675 ms. T2 of methylene protons increases due to reduced viscosity created by adding water. The spectrum of the sample (Fig.1) shows two sharp resonance of equal amplitude. Gradient echo imaging as a function of TE (Fig. 2) shows minimal T2 decay and near complete cosine modulation due to the equimolar proton populations. Heating of the sample from 18.4 °C to 29.6 °C is shown in Fig. 3. The above equations can be used to calculate image intensity as a function of temperature. This calculation is shown to correspond to measured signal intensity (Fig. 3 bottom panel).


Figure 2 shows that a particular image amplitude can represent a number of TE times (or a number of temperatures at a fixed TE). As presented here, the method is accurate over the 18 to 30 degree range of temperatures studied when TE is set to 43.5 ms. The sinusoidal nature of the splitting means that a broader absolute temperature range measurement is achieved by imaging at multiple TE times. The improved temperature-sensitive materials allow for quantitative, absolute temperature mapping without phase unwraping.


No acknowledgement found.


(1) Rieke et al. (2008) JMRI 27 376-390. (2) Malyarenko et al. 2013 JMRI 37 1238-1246. (3) Ammann et al. 1982 JMR 46 319-321. (4) Sprinkhuizen et al (2010) (2010)64 239 (5). Spees et al. (2012) MRM 68 319-324.


Fig. 1. Spectrum of equimolar hydroxyl proton - methylene proton sample at pH = 12. $$$k_{ex}$$$ is fast and T2 of each resonance is about 650 ms.

Fig. 2. Images (top), and calculated and measured signal intensity in a sample of equimolar water and ethylene glycol protons (bottom) as a function of TE at pH = 12. Intensity oscillates as a function of TE. At a constant TE, intensity will change periodically with temperature.

Fig. 3. Temperature study at fixed TE (43.5 ms). Temperature was measured at the tip of the fiber-optic probe (artifact in images). Top panel shows magnitude images and bottom panel shows the signal intensity calculated from known temperature (black line) and measured from an ROI near the fiber-optic probe (circles).

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