Referenceless MR thermometry for monitoring thermal ablation in the prostate

Abstract

Referenceless proton resonance frequency (PRF) shift thermometry provides a means to measure temperature changes during minimally invasive thermotherapy that is inherently robust to motion and tissue displacement. However, if the referenceless method is used to determine temperature changes during prostate ablation, phase gaps between water and fat in image regions used to determine the background phase can confound the phase estimation. We demonstrate an extension to referenceless thermometry which eliminates this problem by allowing background phase estimation in the presence of phase discontinuities between aqueous and fatty tissue. In this method, images are acquired with a multiecho sequence and binary water and fat maps are generated from a Dixon reconstruction. For the background phase estimation, water and fat regions are treated separately and the phase offset between the two tissue types is determined. The method is demonstrated feasibile in phantoms and during in vivo thermal ablation of canine prostate.

Fig. 1

Magnitude image (upper left) showing the frame region around the prostate and unwrapped phase image (upper right). Note the small phase discontinuities between the water and fat regions in the unwrapped phase image (arrows) to which the proposed modified referenceless method is more robust. Binary maps of water (lower left) and fat (lower right) regions.

Fig. 2

Water image (left), fat image (middle), and combined image (right) of the simulated water/fat phantom when there is no temperature rise.

Fig. 3

Dixon reconstructed water image (left) and fat image (middle) of the simulated phantom. Error in the water/fat decomposition are caused by the temperature change. The error map (right) shows the error of the real compared to the ideal water decomposition.

Fig. 4

Plot of the ideal (dotted line) and actual (solid line) water decomposition as a function of temperature for different water fractions (as labeled).

Fig. 4

Plot of the ideal (dotted line) and actual (solid line) water decomposition as a function of temperature for different water fractions (as labeled).

Fig. 4

Plot of the ideal (dotted line) and actual (solid line) water decomposition as a function of temperature for different water fractions (as labeled).

Fig. 5

Temperature maps measured in a meat phantom during heating with an ultrasound applicator (color images in the online edition). The images on the left are reconstructed with baseline subtraction, images on the right are reconstructed with the modified referenceless method. The temperature maps show the same time frame, but are reconstructed from the first, second, and third echo; the frame ROIs used for modified referenceless reconstruction are shown on the right. The images show that subtraction and modified referenceless method provide very similar results in a stationary phantom. Note that no temperature change is detected in the fat layer (dark band running horizontally), because the PRF in lipids does not change with temperature.

Fig. 6

Temperature maps reconstructed from the first (left column) and third echo (right column). The unmodified referenceless method (middle row) suffers from phase gaps between water and fat in the third echo, whereas the modified referenceless method is not affected. Comparing baseline subtraction and modified referenceless reconstruction shows that both methods measure very similar temperatures and heating patterns in this experiment where no tissue motion was observed. Both methods display some artifacts from cooling water flow in the transurethral US applicator.

Fig. 7

Temperature maps acquired during canine prostate ablation with a transurethral ultrasound applicator. No tissue motion was observed in this experiment. The images are reconstructed from three consecutive time frames using baseline subtraction (left column) and modified referenceless reconstruction (right column). The first and third time frame use the in-phase echoes, the second time frame uses the out-of-phase echo for temperature reconstruction. The modified referenceless reconstruction provides accurate temperature measurements in both in-phase and out-of-phase images, very similar to those measured with baseline subtraction.

Fig. 8

Temperature time course of a pixel near the center of the heating area (shown by the white arrow in Fig. 6) comparing modified referenceless reconstruction and baseline subtraction. Heating started after ten minutes and was applied for ten minutes. Measurements in in-phase and out-of-phase echoes and with both methods correlate well.

Fig. 9

Temperature images acquired during preheating comparing modified referenceless reconstruction and baseline subtraction. Before motion, the temperature maps are similar (upper row). When tissue motion occurs (middle row), severe errors render baseline subtraction useless. After motion (bottom row), artifacts remain with baseline subtraction but are not as prevalent in the modified referenceless method.