Referenceless PRF shift thermometry

Abstract

The proton resonance frequency (PRF) shift provides a means of measuring temperature changes during minimally invasive thermotherapy. However, conventional PRF thermometry relies on the subtraction of baseline images, which makes it sensitive to tissue motion and frequency drift during the course of treatment. In this study, a new method is presented that eliminates these problems by estimating the background phase from each acquired image phase. In this referenceless method, a polynomial is fit to the background phase outside the heated region in a weighted least-squares fit. Extrapolation of the polynomial to the heated region serves as the background phase estimate, which is then subtracted from the actual phase. Minimally invasive thermal therapy is a promising treatment for a variety of cancers. It is desirable to monitor temperature during such a procedure by magnetic resonance proton resonance frequency (PRF) shift thermometry (1,2) because it provides quantitative temperature information in near real time. This method uses changes in the phase of gradient-recalled echo (GRE) images to estimate the relative temperature change ⌬T, as given bywhere ␣ ϭ -0.01 ppm/°C is the PRF change coefficient for aqueous tissue, ␥ is the gyromagnetic ratio, B 0 is the main magnetic field, TE is the echo time, and baseline is the initial phase before heating. In conventional PRF shift thermometry, phase images acquired prior to heating (i.e., baseline or reference images) are subtracted from phase images acquired during heating. However, when tissue motion is present, images acquired during heating are not registered to the baseline images, and the background magnetic field changes nonuniformly, resulting in erroneous baseline phase elimination and inaccurate temperature measurements. Many of the target areas for thermotherapy are in the abdomen, where motion is ubiquitous. For example, motion of the liver has an average amplitude of 1.3 cm during normal breathing (3). Because thermal therapy treatments require several minutes to perform, they cannot be performed in a single breath-hold. Furthermore, it is difficult to use multiple breath-holds, because reproducible breathholding is hard to achieve. Even without respiratory motion, displacement between images can occur. Thermal coagulation leads to structural changes and deformation of the tissue (4,5), which can even be observed ex vivo without any other contribution to motion. This heating-induced tissue motion is frequently not a simple displacement, and, unlike respiration, it cannot be corrected by a reregistration scheme. Swelling caused by the formation of edema can also contribute to tissue displacement (6), as can changes in bowel-filling and the state of muscles. For example, we have measured shifts in the canine prostate of Ͼ5 mm over the course of a 48-min ablation experiment.In recent studies, investigators have used conventional respiratory gating in animals under general anesthesia and mechanical respiration (7,8) to overcome the problem of motion in thermal...

Evaluation of MR thermometry with proton resonance frequency method at 7T

“…Some schemes called referenceless PRF thermometry, or self-reference methods have been devised and optimized to overcome this problem (17,29), in which the baseline phase of the heated ROI is estimated by extrapolating the phase in the surrounding tissue. Subtracting the estimated baseline phase from the actual phase under heated condition, the temperature distribution in the heated region can be obtained.…”

Current status and future potential of MRI‐guided focused ultrasound surgery

“…Ultimately, it would be desirable to be able to treat free-breathing awake patients. This may be possible using real-time motion tracking, perhaps using the ultrasound probe itself to track the motion (30), a phased array with a large steering range, and motioninsensitive MR thermometry schemes (31)(32)(33).…”

“…Such phased array devices will develop further to their full potential through the implementation of treatment plans that include beam aberration correction (80) and the optimization of thermal dose delivery (81). Fully automatic closed-loop feedback to control heating (40,57,81,82) combined with robust motion detection and correction schemes for the MR thermometry (31)(32)(33)83) will also likely reach the clinic for MRgFUS.…”

Monitoring and correcting spatio-temporal variations of the MR scanner’s static magnetic field

“…where SNR is the image signal-to-noise ratio [17]. The first experiment commenced with the scanner thermally unperturbed and proceeded for 40 min.…”