Triggered, navigated, multi‐baseline method for proton resonance frequency temperature mapping with respiratory motion

Abstract

A technique is presented for the acquisition of temperature maps in the presence of variable respiratory motion using the proton resonance frequency (PRF) shift. The technique uses respiratory triggering, diaphragm position determination with a navigator echo, and the collection of multiple baseline images to generate temperature maps. Laser ablations were performed in an ex vivo liver phantom undergoing variable simulated respiratory motion and in vivo in four porcine livers, demonstrating a reduction of artifacts in the computed temperature maps compared with conventional single baseline techniques, both uncorrected and corrected for motion. Interstitial thermal therapy techniques can be effective methods for treatment of malignancies. Techniques for monitoring therapy under MR guidance include diffusionweighted MR imaging, measurement of temperature-dependent T 1 changes, and use of the temperature-dependent proton resonance frequency (PRF) shift. Of these, the PRF method is desirable in temperature monitoring for its ability to quantify temperature changes (1). Unfortunately, using the PRF method in the liver or other organs that experience respiratory motion is challenging, since the phase of a pretreatment baseline image is normally subtracted from that of the images acquired during heating. Motion between the acquisition of these images produces artifacts in the temperature maps. Because treatment times for interstitial therapies are several minutes long and reproducible breathholding by patients is often unreliable and difficult, imaging must occur during free-breathing if general anesthesia is not used. Artifacts resulting from motion can be reduced with rapid imaging and triggering, but the baseline and treatment images must still be registered for proper subtraction. Another challenge for MR thermometry is that the background magnetic field is altered nonuniformly during breathing, with spatially nonlinear frequency shifts. In addition, respiratory motion can be variable between respiratory cycles during free-breathing.Recent studies involving thermal therapies in the liver have used several approaches for monitoring tissue damage during heating in regions that undergo respiratory motion. Vogl and colleagues (2,3) performed a significant number of ablations with internally cooled laser applicators under MR guidance with T 1 -weighted images. Other investigators have used T 1 -based temperature mapping strategies (4 -6), and while qualitatively depicting the heated tissue, this technique can be quantitatively inaccurate due to effects caused by tissue changes during coagulation (7,8). Ablations in the kidney have also been performed with T 1 -based thermal mapping (9) and T 2 -weighted imaging (10).Previously, investigators have combined motion detection with navigator echoes to obtain PRF temperature maps in phantoms for both brief displacements of ex vivo tissue (11) and simulated, computer-controlled nonvariable motion in phantoms (12,13). Others have described methods for PRF temperature mapping of...

MR thermometry

“…However, the use of navigator echoes is restricted to rigid body motion and may not be optimal for complex organ displacements or deformations. A triggered, navigated, multi-baseline method was demonstrated in the liver in vivo with variable respiratory motion (95). This technique uses respiratory triggering, diaphragm position determination with a navigator echo, and the collection of multiple baseline images to generate the temperature maps.…”

Spectrally selective pencil‐beam navigator for motion compensation of MR‐guided high‐intensity focused ultrasound therapy of abdominal organs

“…Several approaches for the removal of thermometry artifacts have been proposed in the past, such as multibaseline corrections to sample periodic changes (11,12) and referenceless phase corrections (13). A recent approach combined multibaseline corrections with rapid two dimensional (2D) image registration to provide both a removal of thermometry artifacts and a correction of in-plane displacement in real-time (14).…”

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.…”