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Technical Briefs

# Reduction of Noise From MR Thermometry Measurements During HIFU Characterization Procedures

[+] Author and Article Information
Subhashish Dasgupta, Prasenjeet Das

Department of Mechanical Engineering, University of Cincinnati, Cincinnati, OH 45220

Janaka Wansapura, Ron Pratt

Department of X-Ray/Radiology, Cincinnati Children’s Hospital Medical Center, Cincinnati, OH 45220

Prasanna Hariharan, Matthew R. Myers

Division of Solid and Fluid Mechanics, Center for Devices and Radiological Health, U.S. Food and Drug Administration, Silver Spring, MD 20993

David Witte

Department of Histopathology, Cincinnati Children’s Hospital Medical Center, Cincinnati, OH 45220

Rupak K. Banerjee1

Department of Mechanical Engineering, and Department of Biomedical Engineering, University of Cincinnati, Cincinnati, OH 45220rupak.banerjee@uc.edu

1

Corresponding author.

J. Nanotechnol. Eng. Med 2(2), 024501 (May 19, 2011) (4 pages) doi:10.1115/1.4003861 History: Received December 27, 2010; Revised January 19, 2011; Published May 19, 2011; Online May 19, 2011

## Abstract

Magnetic resonance (MR) thermometry is a valuable method for characterizing thermal fields generated by high intensity focused ultrasound (HIFU) transducers in tissue phantoms and excised tissues. However, infiltration of noise signals generated by external rf sources into the scanner orifice limits the ability of the scanner to measure temperature rise during the heating or ablation phase. In this study, magnetic resonance interferometry (MRI) monitored HIFU ablations are performed on freshly excised porcine liver samples, at varying sonication times, 20 s, 30 s, and 40 s at a constant acoustic intensity level of $1244 W/cm2$. Temperature throughout the procedure was measured using proton resonant frequency MR thermometry. Without filtering, reliable temperature measurements during the heating phase could not be obtained since temperature maps appeared blurred and analysis was impossible. Also, measurements acquired during the cooling phase decayed manifested an unrealistically slow rate of temperature decay. This abnormally slow rate was confirmed with computational results. A low-pass $RC$ filter circuit was subsequently incorporated into the experimental setup to prevent infiltration of noise signals in the MRI orifice. This modified $RC$ filter circuit allowed noninvasive measurement of the HIFU induced temperature rise during the heating phase followed by temperature decay during cooling. The measured data were within 13% agreement with the temperature rise computed by solving the acoustic and heat equations.

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## Figures

Figure 1

(a) Schematic of HIFU experimental setup. (b) Low-pass filter circuit.

Figure 2

MRI calibration curve showing linear relationship between magnetic phase angle (Δø) and temperature rise (ΔT). (R2=0.975).

Figure 3

Experimental and computational HIFU induced transient temperature profiles without low-pass RC filter circuit

Figure 4

Experimental and computational HIFU induced transient temperature profiles at (a) 20 s, (b) 30 s, and (c) 40 s sonication times at a power level of 70 W. Low-pass RC filter circuit is used.

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