Investigation of temperature dependent changes in signal intensity, T1 and T2* in cortical bone
Henrik Odéen1, Bradley Bolster2, Eun Kee Jeong1, and Dennis L Parker1

1Utah Center for Advanced Imaging Research, Department of Radiology, University of Utah, Salt Lake City, UT, United States, 2Siemens Healthcare, Salt Lake City, UT, United States


Measurements of changes in signal intensity and T1 relaxation time with temperature has been suggested for temperature monitoring in cortical bone during MR guided focused ultrasound treatments. In this study we compare changes in signal intensity, T1, and T2* with temperature using a 3D ultrashort echo time pulse sequence and a 2D gradient recalled echo pulse sequence with short TE. The effects of T1 and T2* change with temperature counteract each other making the change in signal intensity small, and therefore T1 and T2* appears to have the greatest sensitivity to changes in temperature.


For MR guided focused ultrasound treatments in or close to bone, such as for transcranial applications focusing through the intact skull bone and treatments of bone metastases, significant heating can occur in the bone. MRI of bone is in general challenging due to the short T2 of bone. For MR temperature imaging the short T2 also severely decreases the accuracy of the standard proton resonance frequency shift method. Instead researchers have investigated the temperature dependence of the MR signal intensity (SI) and T1 relaxation time for temperature monitoring1–4. Miller1 and Fielden3 et al showed that the SI from cortical bone decreases with increasing temperature using ultrashort echo time (UTE) pulse sequences, and Ramsay2 et al found that, contrary to what Miller and Fielden observed, the SI increases with increasing temperature using a short TE gradient recalled echo (GRE) pulse sequence. Han4 et al further showed that T1 increases with temperature, also using UTE.

In this work we investigate the temperature dependence of the SI (dSI/dT) and the T1 and T2* relaxation times (dT1/dT and dT2*/dT, respectively) using 3D UTE and short TE 2D GRE to investigate which parameter has the highest sensitivity to temperature change.


All imaging was performed on a 3T MRI scanner (MAGNETOM PrismaFit, Siemens Healthcare, Erlangen, DE) using a 3D UTE and a 2D GRE pulse sequence. The UTE sequence utilizes radial, ramped sampling of k-space in 3D starting at the k-space center after a 80 μs hard RF pulse, allowing TEs down to 50 μs. T2* was measured by an exponential fit to data acquired at TE=50, 90, 130, 170, and 250 μs (other scan parameters are listed in Table 1). The 50 μs TE was also used for dSI/dT calculations. T1 was measured using the variable flip angle (VFA) method5 with FA 8 and 36° (Table 1).

The spoiled GRE sequence utilizes an asymmetric echo to allow TE down to 1.20 ms. For T2* measurements 6 contrasts with TEs between 1.20 and 7.50 ms were acquired, and then repeated 4 more times with TEs starting at 1.30, 1.40, 1.50, and 1.60 ms, Table 1. The 1.20 ms echo was used for dSI/dT calculations. T1 measurements were performed using the VFA method with FA 8 and 36°.

An approximately 4-cm long bovine femur bone (marrow and connective tissue removed) was placed in a phantom holder that allowed heated water to circulate around the bone, Figure 1. 1 fiber optic probe measured the water temperature, and 3 probes were inserted in 1-mm diameter, 2-cm deep, drill holes in the bone to measure the temperature of the bone. The water was warmed to 4 temperatures (~22, 35, 50, and 65 °C) and the data was collected when all 4 probes measured within 1 °C. The whole setup was places in a 20-channel RF head coil.


Figure 2 shows 2D maps of T1 and T2* relaxation times for the 4 different temperatures for the UTE and GRE scans. Mean and standard error values from a 9x9 ROI close to each probe is shown in Figure 3, together with calculated changes in %/°C. From the UTE data a decrease in SI of 0.3-0.5 %/°C, and increases in T1 and T2* of 0.5-0.9 %/°C and 0.6-0.9 %/°C, respectively, was observed. From the GRE data a slight increase in SI of 0.06 %/°C, and decreases in T1 and T2* of 0.4-0.5 %/°C and 0.3-0.4 %/°C, respectively, was observed.

Using the temperature dependent spoiled GRE signal equation6 and the observed values for dT1/dT and dT2*/dT, dSI/dT can be closely predicted for the UTE case, whereas the GRE case predicts a small decrease in dSI/dT.

Discussion and Conclusions

The decrease in SI for UTE, and increase in SI for GRE, is in accordance with previously published results. However, we observed a lower change in the GRE SI (0.06%/°C) than the 0.9 %/°C reported by Ramsay. The measured change in T1 using UTE agrees well with the 0.6 %/°C reported by Han.

The lower dSI/dT compared to Ramsay may be partly explained by our slightly longer TE (1.20 versus 1.05 ms) which will detect more long T2-component protons. For both UTE and GRE the effect of T1 and T2* on SI are counter-acting each other (both increasing for UTE, and both decreasing for GRE), which reduces the sensitivity of dSI/dT. This may suggest that dT1/dT and dT2*/dT are more suitable candidates for bone MR thermometry, and Figure 3 also shows higher sensitivity for relaxation times that for SI.


This work was supported by The Focused Ultrasound Surgery Foundation, Siemens Healthcare, The Ben B. and Iris M. Margolis Foundation, and NIH grants R01s EB013433 and CA134599


1. Miller W. Toward T1-Based Thermometry in Cortical Bone Using Ultrashort Echo-Time MRI. In: International Symposium on Focused Ultrasound. Bethesda, Maryland; 2012. p. 65–BN.

2. Ramsay E, Mougenot C, Kazem M, Laetsch TW, Chopra R. Temperature-Dependent MR Signals in Cortical Bone?: Potential for Monitoring Temperature Changes during High-Intensity Focused Ultrasound Treatment in Bone. Magn. Reson. Med. 2014:ePub ahead of print. doi: 10.1002/mrm.25492.

3. Fielden S, Mugler III J, Miller W, Butts Pauly K, Meyer C. Detecting signal changes in heated bone with a 3D spiral ultra-short echo time sequence. In: Proc. Intl. Soc. Mag. Reson. Med. Vol. 23. ; 2015. p. 1632.

4. Han M, Rieke V, Scott SJ, Ozhinsky E, Salgaonkar VA, Jones PD, Larson PEZ, Diederich CJ, Krug R. Quantifying temperature-dependent T1 changes in cortical bone using ultrashort echo-time MRI. Magn. Reson. Med. 2015;ePub ahead of print. doi: 10.1002/mrm.25994.

5. Deoni SCL, Rutt BK, Peters TM. Rapid combined T1 and T2 mapping using gradient recalled acquisition in the steady state. Magn. Reson. Med. 2003;49:515–26.

6. Cline HE, Hynynen K, Hardy CJ, Watkins RD, Schenck JF, Jolesz FA. MR temperature mapping of focused ultrasound surgery. Magn. Reson. Med. 1994;31:628–636.


Figure 1. Scan setup. A ~4 cm long bone sample was placed in a phantom holder that allowed water circulation to slowly and homogenously heat the bone. 4 fiber optic probes were used; 1 in the water and 3 in the bone sample.

Figure 2. 2D maps of relaxation times for the 4 different temperatures (~22, 35, 50, and 65 °C), a) UTE T1, b) GRE T1, c) UTE T2*, and d) GRE T2*.

Figure 3. Changes versus temperature for a) UTE SI, b) UTE T1, c) UTE T2*, d) GRE SI, e) GRE T1, and f) GRE T2*. For all but d) the mean and standard error value from a 9x9 voxel ROI close to each probe is shown. For d) the mean and standard error for the whole 2D slice is shown.

Table 1. Scan parameters. FOV – Field on view, Res – Resolution, TR – Repetition time, TE- Echo time, FA – Flip angle, BW – Bandwidth (read out).

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