Correction of breathing-induced errors in magnetic resonance thermometry of hyperthermia using multiecho field fitting techniques

Abstract

Purpose: Breathing motion can create large errors when performing magnetic resonance (MR) thermometry of the breast. Breath holds can be used to minimize these errors, but not eliminate them. Between breath holds, the referenceless method can be used to further reduce errors by relying on regions of nonheated fatty tissue surrounding the heated region. When the surrounding tissue is heated (i.e., for a hyperthermia treatment), errors can result due to phase changes of the small amounts of water in the tissue. Therefore, an extension of the referenceless method is proposed which fits for the field in fatty tissue independent of temperature change and extrapolates it to the water-rich regions.

Methods: Nonheating experiments were performed with male volunteers performing breath holds on top of a phantom mimicking a breast with a tumor. Heating experiments were also conducted with the same phantom while mechanically simulated breath holds were performed. A nonheating experiment was also performed with a healthy female breast. For each experiment, a nonlinear fitting algorithm was used to fit for temperature change and B0 field inside of the fatty tissue. The field changes were then extrapolated into water-rich (tumor) portions of the image using a least-squares fit to a fifth-order equation, to correct for field changes due to breath hold changes. Similar results were calculated using the image phase, to mimic the use of the referenceless method.

Results: Phantom results showed large reduction of mean error and standard deviation. In the non-heating experiments, the traditional referenceless method and our extended method both corrected by similar amounts. However, in the heating experiments, the average deviation of the temperature calculated with the extended method from a fiber optic probe temperature was approximately 50% less than the deviation with the referenceless method. The in vivo breast results demonstrated reduced standard deviation and mean.

Conclusions: In this paper, we have developed an extension of the referenceless method to correct for breathing errors using multiecho fitting methods to fit for the B0 field in the fatty tissue and using measured field changes as references to extrapolate field corrections into a water-only (tumor) region. This technique has been validated in a number of situations, and in all cases, the correction method has been shown to greatly reduce temperature error in water-rich regions. The method has also been shown to be an improvement over similar methods that use image phase changes instead of field changes, particularly when temperature changes are induced.

Figure 1

(a) Construction of phantom mimicking a large breast with breast cancer. (b) Magnitude image of the phantom in the applicator and the air and water bags on top for the first phantom heating experiment.

Figure 2

Picture of phantom secured inside the breast applicator. Tape was used to secure the phantom inside the applicator and prevent its movement with the volunteer’s breathing.

Figure 3

Experimental setup of the phantom in the applicator, the air bag, and the water bag. Tape was used to secure the phantom inside the applicator and prevent its movement during movement of the water bolus

Figure 4

Overlay of a typical field change map (between two breath holds) on the magnitude image of the phantom in the applicator. The colorbar is in units of Hz. Notice the complex shape of the field changes and the severity of the changes near the top of the phantom.

Figure 5

(a) Uncorrected and (b) corrected temperature errors between the first and the last breath hold of the nonheating phantom experiment with volunteer #1. Notice the increased uniformity inside the simulated tumor in the corrected image compared to the uncorrected image.

Figure 6

Uncorrected and corrected temperature change measurements in the water gelatin (simulated tumor) for experiments (a) #1, (b) #2, and (c) #3 of the nonheating phantom experiments. The values are referenced to the temperature image at the first breath hold. Since no heating was applied, the temperature change should be 0 °C across all breath holds. This is seen in the corrected measurements, while the uncorrected measurements have large temperature error. Also, note the smaller error bars for the corrected measurements. This suggests that the correction method corrects for the spatial inhomogeneities induced by the breath hold field changes.

Figure 7

Uncorrected and corrected temperatures in the water gelatin (simulated tumor) for the second phantom heating experiment. These are shown plotted against the temperature change measurements from the fiber optic temperature probes. Notice how the corrected temperature closely tracks the fiber optic temperature, while the uncorrected temperature has large errors. These errors are of greater magnitude than would most likely be seen in vivo, but can function as a worst case scenario. Also, note that the error bars are typically much smaller for the corrected data than for the uncorrected data. However, the error bars for the uncorrected data can occasionally be very small, when the spatial inhomogeneity of the field changes in the tumor happened to be small.

Figure 8

(a) Magnitude image of the volunteer’s right breast in the applicator. (b) Fat and (c) water separated images of the in vivo breast experiment using MP-IDEAL

Figure 9

(Left) ROIs of pixels used in the left side of the glandular tissue used for sampling, (right) plot of the uncorrected and corrected phase changes (expressed as equivalent temperature change in °C) across all time points for all ROIs. Temperature change (vertical scale, °C) was calculated in reference to the temperature at the first breath hold.