High-intensity focused ultrasound (HIFU) is a promising, noninvasive technique to ablate bone tumors and palliate painful bone metastases.^1,2 During HIFU treatment, temperature mapping is desirable for proper heat deposition to targeted bone regions. However, conventional PRF-based thermometry cannot be applied for cortical bone due to its short T₂ relaxation time. Recently, a linear increase in cortical bone T₁ with temperature was demonstrated using 3D UTE imaging combined with a variable flip angle method.³ However, this previously used UTE T₁ mapping method with 3D radial acquisitions requires a long scan time. In this work, we evaluated acceleration of this technique by combining parallel imaging and compressed sensing to depict T₁ changes due to heating with ultrasound.

An ex vivo study was performed with a diaphysis segment of bovine femur on a Discovery MR 750w 3T scanner (GE Healthcare, Waukesha, WI) using an eight-channel phased-array wrist coil (Invivo, Gainesville, FL). A 7.7 MHz catheter-cooled interstitial ultrasound applicator with two-sectored cylindrical transducers⁴ was inserted into the fatty yellow bone marrow. Heating of cortical bone was conducted by delivering high intensity ultrasound energy with a 180° directional heating pattern towards the bone. Before heating began, 3D UTE imaging combining non-selective excitation and 3D radial acquisitions (no undersampling) was conducted using a 11 ms TR, 175 μs TE (center of the RF to the start of acquisition), 1 x 1 x 1.5 mm³ spatial resolution, 9 x 9 x 9.6 cm³ FOV, and RF spoiling. Data acquisition started with the rising slope of the readout gradient (ramp sampling), and 256 samples were acquired for each spoke with a sampling interval of 4 μs. Imaging with two flip angles of 8° and 44° (each with 4 min scan time) was conducted to calculate T₁ using a variable flip angle scheme.⁵ Temperature was assumed to reach steady state at 12 min after heating began, and UTE imaging with the two flip angles was performed again.

The acquired data was retrospectively undersampled by a factor of 8. Reconstruction was performed using an autocalibrated parallel imaging method, ESPIRiT, which can incorporate compressed sensing as well.⁶ The central, fully-sampled, k-space data was used for coil calibration. The l₁ norm of the wavelet coefficients was added as a sparsity regularization⁷ during iterative reconstruction of the images (l₁-ESPIRiT). T₁ maps were calculated using reconstructed images from fully-sampled data by 3D gridding and those from undersampled data by 3D gridding and l₁-ESPiRIT, before and after heating, respectively. The l₁-ESPiRIT and gridding reconstructions were performed with Berkeley Advanced Reconstruction Toolbox.⁸

Figure 1a-c shows reconstructed UTE images applying the 44° flip angle before heating from full k-space data and from undersampled data. We can see significant reduction of streak artifacts and noise by l₁-ESPIRiT reconstruction on undersampled data. Figure 2 demonstrate Maps of T₁ changes between before and after heating, measured using the three different sets of reconstructed UTE images. T₁ increase can be seen in the heated region of cortical bone. L₁-ESPIRiT reconstruction of the undersampled data provided T1 values similar to those from fully-sampled data. Acceleration techniques such as parallel imaging and compressed sensing are well suited for 3D radial acquisitions due to their autocalibration capability from a central over-sampled k-space data and incoherent aliasing artifact patterns in the image domain. Combination with compressed sensing can allow for a higher acceleration factor than just using parallel imaging itself due to its denoising effects. In this work, we accelerated UTE imaging by a factor of 8 by using l₁-ESPIRiT reconstruction, which provided T₁ quantification similar to that from fully-sampled data. Further acceleration might be achievable by incorporating similarity between two flip angle images or those over the temporal domain. These acceleration methods might allow for applying 3D T₁ mapping in cortical bone to monitor temperature during bone HIFU treatment. We have shown that l₁-ESPIRiT can accelerate 3D UTE imaging with a high acceleration factor without significant artifacts, and allows for measuring T₁ changes due to heating similar to those from fully-sampled data.