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Use of a radio frequency shield during 1.5 and 3.0 Tesla magnetic resonance imaging: experimental evaluation.

Favazza CP, King DM, Edmonson HA, Felmlee JP, Rossman PJ, Hangiandreou NJ, Watson RE, Gorny KR - Med Devices (Auckl) (2014)

Bottom Line: Attenuation, by as much as 35 dB, of RF field power was found inside the RF shield.These results were supported by temperature measurements of metallic leads placed inside the shield, in which no measurable RF heating was found.These results suggest that the RF shield could be a valuable tool for clinical MRI practices.

View Article: PubMed Central - PubMed

Affiliation: Department of Radiology, Mayo Clinic, Rochester, MN, USA.

ABSTRACT
Radiofrequency (RF) shields have been recently developed for the purpose of shielding portions of the patient's body during magnetic resonance imaging (MRI) examinations. We present an experimental evaluation of a commercially available RF shield in the MRI environment. All tests were performed on 1.5 T and 3.0 T clinical MRI scanners. The tests were repeated with and without the RF shield present in the bore, for comparison. Effects of the shield, placed within the scanner bore, on the RF fields generated by the scanner were measured directly using tuned pick-up coils. Attenuation, by as much as 35 dB, of RF field power was found inside the RF shield. These results were supported by temperature measurements of metallic leads placed inside the shield, in which no measurable RF heating was found. In addition, there was a small, simultaneous detectable increase (∼1 dB) of RF power just outside the edges of the shield. For these particular scanners, the autocalibrated RF power levels were reduced for scan locations prescribed just outside the edges of the shield, which corresponded with estimations based on the pick-up coil measurements. Additionally, no significant heating during MRI scanning was observed on the shield surface. The impact of the RF shield on the RF fields inside the magnet bore is likely to be dependent on the particular model of the RF shield or the MRI scanner. These results suggest that the RF shield could be a valuable tool for clinical MRI practices.

No MeSH data available.


Experimental setup for direct measurements of RF attenuation.Notes: (A) Photograph of the set-up. The RF shield (wrapped around acrylic cylinder) was positioned within the bore of the 1.5 T and 3.0 T scanners. (B) Experimental configuration utilized to measure the RF attenuation of the shield. (C) Experiment configuration utilized to measure the RF field outside of the shield. In both (B and C), the dashed green lines represent the lines along which measurements were acquired.Abbreviation: RF, radiofrequency.
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f1-mder-7-363: Experimental setup for direct measurements of RF attenuation.Notes: (A) Photograph of the set-up. The RF shield (wrapped around acrylic cylinder) was positioned within the bore of the 1.5 T and 3.0 T scanners. (B) Experimental configuration utilized to measure the RF attenuation of the shield. (C) Experiment configuration utilized to measure the RF field outside of the shield. In both (B and C), the dashed green lines represent the lines along which measurements were acquired.Abbreviation: RF, radiofrequency.

Mentions: To measure RF field perturbations caused by placement of the RF shield within the scanner bore, two single-loop RF pick-up coils were constructed and tuned to 64 MHz (1.5 T) and 128 MHz (3.0 T). The scanner RF power output, also called the transmit gain (TG), were set to clinically relevant levels observed on each scanner (based on our clinical experience). The TG was set to 150 and 95 (in units of 0.1 dB, relative to the calibrated power supply of the scanner) at 1.5 T and 3.0 T, respectively. One millisecond RF pulses at a constant power output were continually executed at one-second intervals, and the induced voltages in the pick-up coils were measured as peak-to-peak voltages on an oscilloscope. A set of baseline measurements was first acquired without the RF shield present inside the bore. In a second set of measurements, the RF shield was wrapped around a 28 cm diameter acrylic tube for stability and centered at isocenter inside the bore (Figure 1). Measurements were acquired at 2 cm increments along the z-axis of the bore. At each position, one measurement was obtained with the pick-up coil oriented in a horizontal position, and a second after a 90° rotation of the coil. At each position, the voltage in the bore was determined as: , where Vx and Vy are orthogonal voltage measurements.


Use of a radio frequency shield during 1.5 and 3.0 Tesla magnetic resonance imaging: experimental evaluation.

Favazza CP, King DM, Edmonson HA, Felmlee JP, Rossman PJ, Hangiandreou NJ, Watson RE, Gorny KR - Med Devices (Auckl) (2014)

Experimental setup for direct measurements of RF attenuation.Notes: (A) Photograph of the set-up. The RF shield (wrapped around acrylic cylinder) was positioned within the bore of the 1.5 T and 3.0 T scanners. (B) Experimental configuration utilized to measure the RF attenuation of the shield. (C) Experiment configuration utilized to measure the RF field outside of the shield. In both (B and C), the dashed green lines represent the lines along which measurements were acquired.Abbreviation: RF, radiofrequency.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4219642&req=5

f1-mder-7-363: Experimental setup for direct measurements of RF attenuation.Notes: (A) Photograph of the set-up. The RF shield (wrapped around acrylic cylinder) was positioned within the bore of the 1.5 T and 3.0 T scanners. (B) Experimental configuration utilized to measure the RF attenuation of the shield. (C) Experiment configuration utilized to measure the RF field outside of the shield. In both (B and C), the dashed green lines represent the lines along which measurements were acquired.Abbreviation: RF, radiofrequency.
Mentions: To measure RF field perturbations caused by placement of the RF shield within the scanner bore, two single-loop RF pick-up coils were constructed and tuned to 64 MHz (1.5 T) and 128 MHz (3.0 T). The scanner RF power output, also called the transmit gain (TG), were set to clinically relevant levels observed on each scanner (based on our clinical experience). The TG was set to 150 and 95 (in units of 0.1 dB, relative to the calibrated power supply of the scanner) at 1.5 T and 3.0 T, respectively. One millisecond RF pulses at a constant power output were continually executed at one-second intervals, and the induced voltages in the pick-up coils were measured as peak-to-peak voltages on an oscilloscope. A set of baseline measurements was first acquired without the RF shield present inside the bore. In a second set of measurements, the RF shield was wrapped around a 28 cm diameter acrylic tube for stability and centered at isocenter inside the bore (Figure 1). Measurements were acquired at 2 cm increments along the z-axis of the bore. At each position, one measurement was obtained with the pick-up coil oriented in a horizontal position, and a second after a 90° rotation of the coil. At each position, the voltage in the bore was determined as: , where Vx and Vy are orthogonal voltage measurements.

Bottom Line: Attenuation, by as much as 35 dB, of RF field power was found inside the RF shield.These results were supported by temperature measurements of metallic leads placed inside the shield, in which no measurable RF heating was found.These results suggest that the RF shield could be a valuable tool for clinical MRI practices.

View Article: PubMed Central - PubMed

Affiliation: Department of Radiology, Mayo Clinic, Rochester, MN, USA.

ABSTRACT
Radiofrequency (RF) shields have been recently developed for the purpose of shielding portions of the patient's body during magnetic resonance imaging (MRI) examinations. We present an experimental evaluation of a commercially available RF shield in the MRI environment. All tests were performed on 1.5 T and 3.0 T clinical MRI scanners. The tests were repeated with and without the RF shield present in the bore, for comparison. Effects of the shield, placed within the scanner bore, on the RF fields generated by the scanner were measured directly using tuned pick-up coils. Attenuation, by as much as 35 dB, of RF field power was found inside the RF shield. These results were supported by temperature measurements of metallic leads placed inside the shield, in which no measurable RF heating was found. In addition, there was a small, simultaneous detectable increase (∼1 dB) of RF power just outside the edges of the shield. For these particular scanners, the autocalibrated RF power levels were reduced for scan locations prescribed just outside the edges of the shield, which corresponded with estimations based on the pick-up coil measurements. Additionally, no significant heating during MRI scanning was observed on the shield surface. The impact of the RF shield on the RF fields inside the magnet bore is likely to be dependent on the particular model of the RF shield or the MRI scanner. These results suggest that the RF shield could be a valuable tool for clinical MRI practices.

No MeSH data available.