PARENT SESSION
1:30 PM to 3:30 PM
Saturday, April 20, 2002
Poster Session 1 Noninvasive Treatment Monitoring and Treatment Planning
Room: Nevada 1-2

(MP01-1) Improvement of MRI-monitoring during RF-hyperthermia by using ferrofluids.

Hagmann, M.^*,1, Reinl, H.^2,3, Peller, M.², Issels, R.³, Reiser, M.², Turner, P.¹, ¹ BSD Medical Corporation, Salt Lake City, Utah² Dept. of Clinical Radiology³ Clinical Corporation Group Hyperthermia (Medical Clinic III, University of Munich), Munich

ABSTRACT-
A water-filled bolus is required to couple RF power to the patient and to cool the skin during hyperthermia, and the signals from this water interfere with the MRI measurements. Generally the water in the bolus gives the strongest signal, causing compression of the signals from the regions of interest. In addition, parts of the bolus outside the maximum field of view cause artifacts that perturb the image. Water-soluble paramagnetic compounds (causing appreciable RF loss) and heavy water have been used to mitigate the signal compression and artifacts that are caused by the water in the bolus. We have found that superparamagnetic ferrofluids may be added to the water to suppress the water signal without creating susceptibility artifacts. The ferrofluid MSG-W11 (from Ferrofluidics Corporation, U.S. or FerroTec GmbH, Germany) is an aqueous colloidal suspension of magnetite particles that have an average size of about 10 nm. This ferrofluid contains 2.8-3.5 vol% of magnetite. All of the imaging experiments were done in a hybrid MRI-hyperthermia system consisting of a 0.2 Tesla Magnetom Open Viva (Siemens, Germany) with a DODEK and a Sigma Eye applicator having an integrated MR receive coil (BSD Medical, Utah). Measurements were made using samples having different concentrations, 0.01-3.5 vol% of ferrofluid (3 -1200 ppm of magnetite) added to the water: first in 50 ml tubes, and then filling the water-bolus itself. In the concentrations at which it is used the ferrofluid causes no significant loss of RF power. The mixtures of ferrofluid in water are stable, with no noticeable reaction, segregation, or precipitation over a period of six months. It appears that the optimum concentration of ferrofluid is approximately 0.25 vol% (70-90 ppm of magnetite). Due to signal extinction a significant improvement of image quality and of the spatial and temporal resolution is achieved. The T1-maps taken during a hyperthermia heating protocol using a gel phantom clearly show the focused RF heating in the phantom when the bolus contains a ferrofluid/water mixture, but only artifacts are seen with a water-filled bolus. This method of signal extinction may also be useful for other procedures where suppression of the water signal is necessary.

KEYWORDS: MRI-monitoring, Noninvasive thermometry, hyperthermia