A multichannel, real‐time MRI RF power monitor for independent SAR determination

El-Sharkawy, Abdel Monem M.; Qian, Di; Bottomley, Paul A.; Edelstein, William A.

doi:10.1118/1.3700169

Cited by 32 publications

(48 citation statements)

References 33 publications

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“…These are both below the FDA guidelines for average and local SAR, 10,11 reflecting conservatism in the scanners' in-built SAR computations. 16,17 The maximum temperature increase seen at these levels was 1.4°C or less in both scanners (Table 1, 2) in all but the 2 Arrow catheters at 3 T. These temperature increases are below levels that which would be considered thermally injurious. The temperature increases are in general lower at 1.5 T, and it appears that all of the catheters tested would be safe for use at 1.5 T. The B. Braun and the Smith/Portex devices could be used at 3 T under the MRI conditions tested as well and may not need to be removed during MRI out of concern for RF heating.…”

Section: Discussionmentioning

confidence: 84%

“…9 Although SARs can be computed numerically from EM simulations, these still require empirical measures of the total power delivered to the body by the scanner in order to scale them to an actual MRI study. 16,17 For catheters and electrically conducting implants, local SAR depends on a variety of factors including implant length, insulation, shape, and the RF field transmitted by the MRI coil. 12,13,18,19 Validating the computations and/or measuring the local SAR is not straightforward, other than by measuring the direct consequence of SAR (ie, heating) as was done here and elsewhere.…”

Section: Discussionmentioning

confidence: 99%

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Evaluation of Epidural and Peripheral Nerve Catheter Heating During Magnetic Resonance Imaging

Owens¹,

Erturk

Ouanes³

et al. 2014

Regional Anesthesia and Pain Medicine

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Background Many epidural and peripheral nerve catheters contain conducting wire that could heat during magnetic resonance imaging (MRI), requiring removal for scanning. Methods We tested 2 each of 6 brands of regional analgesia catheters (from Arrow International, B. Braun Medical, and Smiths Medical/Portex) for exposure to clinical 1.5 and 3 Tesla (T) MRI. Catheters testing as non-magnetic were placed in an epidural configuration in a standard human torso-sized phantom, and an MRI pulse sequence applied at the maximum scanner-allowed radio frequency (RF) specific absorption rate (SAR) for 15 minutes Temperature and SAR exposure were sampled during MRI using multiple fiber-optic temperature sensors. Results Two catheters (the Arrow StimuCath Peripheral Nerve, and Braun Medical Perifix FX Epidural) were found to be magnetic and not tested further. At 3T, exposure of the remaining 3 epidural and 1 peripheral nerve catheter to the scanner’s maximum RF exposure, elicited anomalous heating of 4 to 7°C in 2 Arrow Epidural (MultiPort and Flex-Tip Plus) catheters at the entry points. Temperature increases for the other catheters at 3T and all catheters at 1.5T were ≤1.4°C. When normalized to the body-average FDA guideline SAR of 4W/kg, maximum projected temperature increases were 0.1 to 2.5°C at 1.5T and 0.7 to 2.7°C at 3T, except for the Arrow MultiPort Flex-Tip Plus catheter at 3T whose increase was 14°C. Conclusions Most but not all catheters can be left in place during 1.5T MRI scans. Heating of <3°C during MRI for most catheters is not expected to be injurious. While heating was lower at 1.5T vs 3T, performance differences between products underscore the need for safety testing before performing MRI.

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Section: Discussionmentioning

confidence: 84%

Section: Discussionmentioning

confidence: 99%

Evaluation of Epidural and Peripheral Nerve Catheter Heating During Magnetic Resonance Imaging

Owens¹,

Erturk

Ouanes³

et al. 2014

Regional Anesthesia and Pain Medicine

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“…The whole body average SAR for the “standard” subject can be calculated from P deposited measured by the dosimeter divided by the subject’s weight. As has been shown previously, P deposited is approximately a linear function of BMI [2], and can be used to estimate SAR for different-sized subjects, based on a single dosimeter measurement.…”

Section: Resultsmentioning

confidence: 99%

“…Forward and reflected powers were measured at the inputs of the body coil using a high dynamic range multi-channel power monitor [2] to determine deposited power. The power monitor was connected across a 50 Ω series resistor in each loop via directional couplers, attenuators and baluns for protection (Fig 1a) to measure induced currents.…”

Section: Methodsmentioning

confidence: 99%

An RF dosimeter for independent SAR measurement in MRI scanners

Qian

El-Sharkawy²,

Bottomley

et al. 2013

Medical Physics

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Purpose:The monitoring and management of radio frequency (RF) exposure is critical for ensuring magnetic resonance imaging (MRI) safety. Commercial MRI scanners can overestimate specific absorption rates (SAR) and improperly restrict clinical MRI scans or the application of new MRI sequences, while underestimation of SAR can lead to tissue heating and thermal injury. Accurate scanner-independent RF dosimetry is essential for measuring actual exposure when SAR is critical for ensuring regulatory compliance and MRI safety, for establishing RF exposure while evaluating interventional leads and devices, and for routine MRI quality assessment by medical physicists. However, at present there are no scanner-independent SAR dosimeters. Methods: An SAR dosimeter with an RF transducer comprises two orthogonal, rectangular copper loops and a spherical MRI phantom. The transducer is placed in the magnet bore and calibrated to approximate the resistive loading of the scanner's whole-body birdcage RF coil for human subjects in Philips, GE and Siemens 3 tesla (3T) MRI scanners. The transducer loop reactances are adjusted to minimize interference with the transmit RF field (B 1 ) at the MRI frequency. Power from the RF transducer is sampled with a high dynamic range power monitor and recorded on a computer. The deposited power is calibrated and tested on eight different MRI scanners. Whole-body absorbed power vs weight and body mass index (BMI) is measured directly on 26 subjects. Results: A single linear calibration curve sufficed for RF dosimetry at 127.8 MHz on three different Philips and three GE 3T MRI scanners. An RF dosimeter operating at 123.2 MHz on two Siemens 3T scanners required a separate transducer and a slightly different calibration curve. Measurement accuracy was ∼3%. With the torso landmarked at the xiphoid, human adult whole-body absorbed power varied approximately linearly with patient weight and BMI. This indicates that whole-body torso SAR is on average independent of the imaging subject, albeit with fluctuations. Conclusions: Our 3T RF dosimeter and transducers accurately measure RF exposure in bodyequivalent loads and provide scanner-independent assessments of whole-body RF power deposition for establishing safety compliance useful for MRI sequence and device testing.

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“…The effects of temperature reduction due to natural and forced convection are also unknown [41]. In addition, SAR values may not be compared across similar scanners at the same field strength to assess radiofrequency power deposition, even as a linear relationship between change in temperature and SAR is maintained [54], and scanner-estimated SAR values may be more than twice the actual values [55].…”

Section: Specific Absorption Rate Andmentioning

confidence: 99%