Patent Application: US-201214005804-A

Abstract:
a device with at least one channel for measuring high dynamic range , radio frequency power levels over broad - ranging duty cycles includes a power sensor circuit comprising at least one logarithmic amplifier ; at least one directional rf coupler electrically connected to the at least one power sensor ; at least one rf attenuator electrically connected to the at least one rf coupler ; and at least one sampling circuit electrically connected to the at least one rf attenuator and the at least one rf coupler . the at least one sampling circuit performs analog - to - digital conversion of electrical signals received to provide digitals signals for measuring the rf power level in the at least one channel .

Description:
some embodiments of the current invention are discussed in detail below . in describing embodiments , specific terminology is employed for the sake of clarity . however , the invention is not intended to be limited to the specific terminology so selected . a person skilled in the relevant art will recognize that other equivalent components can be employed and other methods developed without departing from the broad concepts of the current invention . all references cited anywhere in this specification , including the background and detailed description sections , are incorporated by reference as if each had been individually incorporated . some embodiments of the current invention provide a high - dr , mri - compatible , power profiling system for measuring and recording rf power over a wide range of mri scan conditions . the system is broadband up to 400 mhz , can be used to sample power for both local and whole - body power flow and , unlike commercial meters , has six channels and a buffer size suitable for monitoring power at multiple locations over extended time periods . we provide some examples of its application to real - time rf power monitoring in human whole - body mri studies of volunteers performed in commercial philips medical systems &# 39 ; ( best , the netherlands ) and siemens medical solutions &# 39 ; ( malvern , pa .) 3 t mri scanners . we show that the power deposited and the body - average sar , 1 , 2 often vary considerably from the scanners &# 39 ; own estimates . in an example , the losses in the rf power chain of a philips 3 t achieva 3 t scanner 26 were first characterized using a 4395a agilent technologies ( santa clara , calif .) network analyzer by measuring the attenuation in each stage in accordance with the schematic in fig1 . measured losses in these components show that the power output at the q - hybrid ( points d , e , fig1 ) is only about 59 % of the power out of the rf amplifier ( point a ). ( see , for example , u . s . pat . pub . no . 2011 / 0148411 ; u . s . application ser . no . 12 / 677 , 097 assigned to the same assignee as the current application , the entire content of which is incorporated herein by reference .) to measure the pulse power during mri , we first tried commercial inline power meters . bird 5014 and bird 5010b ( bird technologies , solon , ohio ) did not work correctly for peak / average power ratios greater than ten . even when operating the scanner at minimum trs and low rf field intensity ( b 1 ), measurements were unstable and irreproducible . we next used a ladybug technologies llc ( santa rosa , calif . ), lb480a power profiling meter in combination with 50 db dual directional couplers to measure forward and reverse power at the outputs of the power amplifier and the q - hybrid during mri . the ladybug meter sampled the pulse profile at 10 μs intervals and stored results for power calculations . while this yielded accurate measurements on four volunteers , 27 the use of usb cables from ladybug to the computer necessitated a person inside the scanner room . moreover , the ladybug meter did not have sufficient channels for monitoring the forward and reverse power at the three locations of interest simultaneously ( a , d , and e in fig1 , location a ). in addition , its small buffer size (≦ 1 sec ) was inadequate for providing continuous measurements of power for many mri sequences with long trs over the several cycles needed for accurately measuring time - averaged power , thus rendering real - time measurements impractical . we therefore built a 6 - channel scanner - independent power monitoring system according to an embodiment of the current invention . an embodiment has six power sensor circuits ( psc ), ( fig2 a ) assembled from ad8310 logarithmic amplifier ic &# 39 ; s ( analog devices , norwood mass .). at each of three locations ( rf amplifier output , two q - hybrid outputs ), a power profiling measurement module ( ppmm ) having a 50 db directional coupler ( werlatone inc ., patterson n . y .) connected to two pscs , one to its forward channel and one to its reverse channel ( fig2 b ), was deployed . a 10 db attenuator was added to the forward channels to allow measurements of up to 50 kw of peak power . the design dr was from 17 dbm ( nearly the maximum input power of the ad8310 ) to − 80 dbm over the desired frequency range 1 - 440 mhz . each psc is powered by a rechargeable lithium ion , non - magnetic 4v battery ( powerstream , orem utah ) and can operate continuously for at least 10 hours before recharging . the video bandwidth of the ics was set to 112 khz using a 470 pf capacitor ( fig2 a ). although this embodiment describes three circuits , such as the one illustrated in the embodiment of fig2 a and 2b , one , two , three or more such circuits could be used in various embodiments of the current invention . the outputs of each of the six pscs are simultaneously sampled in differential input mode at 200 khz by a 16 bit usb - 6251 national instruments ( austin , tex .) data acquisition system controlled by a laptop computer that also stores the power measurement data . the 5 μs sampling resolution accurately captures the mri rf pulse modulation whose time resolution in the philips scanner was about 6 . 4 μs . a matlab ( the mathworks , natick , mass .) program was written to read the saved voltage files , convert them to power profiles using the linear calibration curves for each channel , and to calculate average power values for all experiments . a schematic of the system configured to monitor rf power flow is shown in fig3 . the ( low frequency ) power profiling lines from the ppmms attached to the quad hybrid outputs are fed through the scanner room &# 39 ; s connection panel . the lines from the ppmm connected to the rf power amplifier were wound around ferrite cores to prevent rf interference . each ppmm was bench calibrated for the operational frequencies of the philips 3 t achieva scanner and a siemens 3 t trio scanner ( 127 . 8 mhz and 123 mhz , respectively ) using the setup shown in fig4 a . the calibration was performed against the lb480a meter using a 10 dbm frequency synthesizer whose output was connected to a 0 - 100 db variable attenuator to vary the input power level . the psc voltage - to - logarithmic power was measured over a 70 db range ( limited by the lb480a unit &# 39 ; s operational dynamic range ) and was highly linear as shown in fig4 . fig4 b . the slopes of the calibration curves were about 0 . 24 v / 10 dbm . the net sampling resolution of the a / d was set to 0 . 004 dbm . after calibration the full dr was tested over a range of 90 db as shown in fig4 c and exhibited a maximum deviation of 0 . 8 dbm from linearity at − 80 dbm . the total insertion loss of the monitoring system ppmms at 128 mhz was 0 . 1 db or about 2 %. fig5 shows a schematic resonant circuit for an mri coil producing a transmit rf field , b1 , proportional to the current , i , in the coil . the power loss in the circuit is the sum of the coil and subject losses in resistive loads rc and rs , respectively . the pickup loops , are fixed by the manufacturer inside the rf body coil . they are used by the scanner to monitor and set the initial value of the rf field produced by the coil during set - up . the power loss in the coil p coil , is measured as the net power flow at the output of the q - hybrid with a lossless sample placed in the coil . the lossless sample is a 1 liter bottle of mineral oil whose rf dielectric constant , conductivity and size are orders - of - magnitude lower than those of the body . 28 , 29 this was verified by measuring p coil with additional mineral oil sample volumes of 2 liters and 3 liters ; no significant change in power absorption was observed . the desired b1 , and therefore the current i required to produce it , is approximately constant , independent of the subject being imaged . 30 , 31 therefore the coil power dissipation , p coil , is constant for a given pulse sequence , independent of the subject . the power deposited in the subject is then p subject = p total − p coil , where p total is the total power dissipated in the coil plus the subject measured at the q - hybrid . note that larger subjects have greater p total but the same p coil . to measure the rf power deposited in human subjects during mri , the power monitoring system was connected to the output of the rf power amplifier and the two outputs of the q - hybrid before the scan . eleven healthy volunteers ( 9 men , 2 women ; age 22 - 65 yrs ) were recruited and provided informed consent for this study approved by the johns hopkins institutional review board on human investigation . subjects were positioned in the philips 3t scanner and the scanner &# 39 ; s automated scan preparation sequence initiated . volunteers were landmarked at the xiphoid , placed at the isocenter of the scanner and a transverse slice containing both the heart and liver was targeted . a reference b1 rf field is first set based on pickup coil sensors , followed by the scanner &# 39 ; s mri - based b1 optimization algorithm which sets the final flip - angle . the b1 optimization algorithm is based on a stimulated echo sequence similar to the one described by akoka et al . 32 where an average signal projection is used , thus rendering the result stable against local field variations . two field - echo ( fe ) mri sequences with tr = 50 ms and two different rf pulse shapes ( a short 1 ms asymmetric two lobe pulse and a long 7 ms “ spredrex ” pulse 33 ) were used . the total scanner time per subject — including entry , positioning , and egress from the scanner — was 10 - 15 min . the delivered rf power reported by the scanner , as well as the measured power output , was recorded . the subject was replaced by the mineral oil phantom and the pulse sequence repeated to produce the same b1 detected by the pickup coil . scanner sar and power were again recorded , along with the power measured by our power monitoring system . body - average sar was taken as the power deposited divided by the subject &# 39 ; s weight , in accordance with the standard definition . 1 , 2 the same protocol was repeated on six of the volunteers ( men , age 23 - 66 yrs ) in a siemens 3t trio scanner . the fe sequence used the scanner &# 39 ; s default 2 ms rf sinc pulse with one side lobe . the scanner &# 39 ; s console sar differed from the value reported in its log file , so both values were recorded . all power values measured by our power monitoring system were calculated by averaging instantaneous power over a 0 . 5 s time window ( 10 pulses for fe pulse sequence ). mri experiments showed no noticeable interference or image degradation with the ppmms connected . connecting the ppmms did not increase noise , as was confirmed by noise scans acquired with the rf and gradients turned off . fig6 exemplifies the 6 - channel real - time recordings of an asymmetric , multi - lobe , slice selective rf pulse on the philips scanner with a subject present . the detailed instantaneous recording of rf power is shown for spredrex pulses on a logarithmic scale . 33 the results for the forward power delivered to the philips body mri coil and body average sar for all subjects in the philips scanner are plotted in fig7 as a function of the power reported by the scanner , the patient weight , and the body mass index ( bmi ). fig7 a shows that the scanner - reported power at the rf amplifier &# 39 ; s output agrees with our ppmm system results to within 6 % for short pulses (˜ 1 ms ). this is not true for longer pulses (˜ 7 ms ), where the scanner &# 39 ; s rf power monitoring fails when compared to the ppmm system that had been calibrated over the full dr and duty - cycles used for mri . for all volunteers , the power delivered at the output of the philips quadrature hybrid ( q - hybrid ) is 56 . 5 ± 2 . 5 % of the output of the amplifier . this figure is consistent with the 58 . 5 % predicted from the measured losses in the philips rf chain plus the measured insertion losses in the power monitoring modules . for the 50 ms tr , the average power dissipated in the coil is 8 . 8 ± 0 . 6 w for the short pulses and 11 . 1 ± 0 . 8 w for the long pulses , independent of the size of the mineral oil bottle ( 1 - 3 liter ). the philips achieva scanner initially establishes a b1 that is the same for all samples using pickup loops . it is worth noting that the final mri optimization yielded a b1 that was , on average , within 5 % of the initial pickup loop b1 in all samples , from small to large human subjects as well as in the mineral oil bottles . this result supports the assumption that the current i required to produce a desired mri flip angle across the slice projection is essentially independent of sample size , and that the power dissipation in the rf coil always equals the power dissipation with the mineral oil sample to a good approximation . fig7 b shows that the measured deposited power varies linearly with bmi with a correlation coefficient r 2 = 0 . 8 ( 0 . 7 ) for short ( long ) rf pulses . fig7 c and fig7 d show that the scanner almost always overestimates body - average sar . the scanner overestimated sar by up to 78 % for short pulses and 123 % for long pulses when compared to values obtained from our ppmm direct power determination and subject weights . fig8 shows the calculated sar values from the real - time power monitor versus the scanner reported values for 3t siemens scanner . the power delivered at the output of the q - hybrid is 90 ± 2 % of the power measured at the rf amplifier output . sar values listed in the siemens log file differ from those reported at the console : siemens does not state which values they use . in any case , as with the philips scanner , the siemens scanner overestimated sar . the siemens scanner log overestimated sar by up to 103 % while the console values were up to 71 % above the actual measured sar . some embodiments of the current invention address the problem of providing accurate real - time measurements of the rf power delivered to the body , which is inadequately served by existing technology . specifically , we found that two commonly available commercial rf power meters are unsuitable for the full range of drs , duty cycles and pulse types encountered in mri . this was further underscored by differences and errors in power monitoring for short and long rf pulses in the philips scanner . we therefore developed a real - time , multi - channel power monitoring system according to an embodiment of the current invention that is suitable for a full range of mri rf pulses and sequences operating over a frequency range that will accommodate scanners with fields up to 10 t . 34 the accuracy of measurements provided by our power monitoring system was independently validated three ways : 1 ) on the bench using the ladybug power meter ( fig4 ); 2 ) using the 3t scanner &# 39 ; s power monitoring unit at the output of the amplifier for high - power , short rf pulses ( fig7 a ); and 3 ) by measuring the losses in the 3t scanner &# 39 ; s rf chain using our power monitor and comparing the results with independent measurements made with a network analyzer . our new power monitoring system was used to determine the true power deposited and the body - average sar delivered to adult volunteers in two clinical 3 t mri systems . the results showed that the scanners almost always overestimate the body - average sar as compared to the actual power deposited . the overestimates were as much as 120 % and 100 %, respectively , in the philips and siemens 3t systems studied here ( fig7 c , 7 d ; fig8 ). unfortunately , the exact details of the manufacturer &# 39 ; s sar modeling are proprietary , precluding the identification of specific causes for the differences . nevertheless , the data in fig7 suggest philips &# 39 ; use of a worst - case estimate that is independent of the subject loading , while siemens &# 39 ; model evidently depends on the subject &# 39 ; s weight ( fig8 ). although the evaluations were performed on the scanner &# 39 ; s whole - body coils with sample - dominant losses , application of the power monitoring system is not limited by coil geometry , and similar measurements could be performed on other vendors &# 39 ; scanners and other coil sets , including multi - transmit systems at various field strengths . 34 the power monitoring system and protocol presented here provide measures of the total power deposited in the body during mri , or the body average sar defined as the total power divided by the subject &# 39 ; s weight . 1 , 2 local sar exposure , such as peak 1 - g or 10 - g averages , are also important for safety compliance . 1 , 2 at present , these must be obtained by numerical electromagnetic modeling , 11 - 14 , 35 - 37 from which ratios of the peak local sar to the total power can be derived . these are , however , anatomy dependent . in practice , the total deposited power may be used in conjunction with numerical electromagnetic models to provide estimated local sar values . 17 , 35 - 40 in the philips scanner , because of losses in the cables , rf coil and other transmit chain components , the power reaching the imaging subject in the philips scanner was less than half the power supplied by the rf transmitter . the smaller power loss for the siemens scanner indicates the use of lower loss components . moreover , both scanners &# 39 ; whole - body sar estimates reported to the scanner operator seem conservatively overstated . while this may provide an extra safety margin for rf exposure , it nevertheless means that scanner sar values are not reliable for specifying rf exposure when testing the mri safety of peripheral , implanted and interventional devices . 3 , 20 the overestimate may also limit high - sar pulse sequences , forcing unnecessary reductions in duty cycle or pulse power that increase scan time and / or compromise efficiency . some embodiments of the current invention can provide a versatile approach to accurately measure , in real - time , the total rf power deposition during mri , independent of the scanner . we have used our real - time power monitoring system to demonstrate deficiencies in commercial scanner reported rf sar values in some examples . some embodiments can be used to monitor regulatory compliance , sar dosimetry , evaluation of scanner function following burn injuries and for setting rf exposure levels during device safety testing . in addition , applications of the current invention are not limited to mri , but can be used for measuring rf power in other applications including radar , medical rf diathermy , rf ablation systems , and rf telecommunications systems . 1 . guidance for industry and fda : “ staff criteria for significant risk investigations of magnetic resonance diagnostic devices ,” united states food and drug administration ( fda ), 2003 . 2 . medical electrical equipment — part 2 - 33 : “ particular requirements for the safety of magnetic resonance equipment for medical diagnosis ,” european committee for electrotechnical standardization central secretariat , iec report no . 60601 - 2 - 33 : 2002 . 3 . p . a . bottomley , a . kumar , w . a . edelstein , j . m . allen and p . v . karmarkar , “ designing passive mri - safe implantable conducting leads with electrodes ,” med phys 37 , 3828 - 3843 ( 2010 ). 4 . e . mattei , g . calcagnini , m . triventi , f . censi , p . bartolini , w . kainz and h . bassen , “ mri induced heating of pacemaker leads : effect of temperature probe positioning and pacemaker placement on lead tip heating and local sar ,” conf proc ieee eng med biol soc 1 , 1889 - 1892 ( 2006 ). 5 . h . muranaka , t . horiguchi , s . usui , y . ueda , o . nakamura and f . ikeda , “ dependence of rf heating on sar and implant position in a 1 . 5t mr system ,” magn reson med sci 6 , 199 - 209 ( 2007 ). 6 . w . r . nitz , g . brinker , d . diehl and g . frese , “ specific absorption rate as a poor indicator of magnetic resonance - related implant heating ,” invest radiol 40 , 773 - 776 ( 2005 ). 7 . p . a . bottomley and e . r . andrew , “ rf magnetic field penetration , phase - shift and power dissipation in biological tissue : implications for nmr imaging ,” physics in medicine and biology 23 , 630 - 643 ( 1978 ). 8 . p . a . bottomley , r . w . redington , w . a . edelstein and j . f . schenck , “ estimating radiofrequency power deposition in body nmr imaging ,” magn reson med 2 , 336 - 349 ( 1985 ). 9 . p . a . bottomley and w . a . edelstein , “ power deposition in whole - body nmr imaging ,” med phys 8 , 510 - 512 ( 1981 ). 10 . c . wang , g . x . shen , j . yuan , p . qu and b . wu , “ theoretical and experimental investigation of the relationship among sar , tissues and radio frequencies in mri ,” physica medica 21 , 61 - 64 ( 2005 ). 11 . c . m . collins and z . wang , “ calculation of radiofrequency electromagnetic fields and their effects in mri of human subjects ,” magn reson med 65 , 1470 - 1482 ( 2011 ). 12 . h . homann , p . bornert , h . eggers , k . nehrke , o . dössel and i . graesslin , “ toward individualized sar models and in vivo validation ,” magn reson med 66 , 1767 - 1776 ( 2011 ). 13 . z . wang , j . c . lin , w . mao , w . liu , m . b . smith and c . m . collins , “ sar and temperature : simulations and comparison to regulatory limits for mri ,” j magn reson imaging 26 , 437 - 441 ( 2007 ). 14 . s . oh , a . g . webb , t . neuberger , b . park and c . m . collins , “ experimental and numerical assessment of mri - induced temperature change and sar distributions in phantoms and in vivo ,” magn reson med 63 , 218 - 223 ( 2010 ). 15 . p . ehses , f . fidler , p . nordbeck , e . d . pracht , m . warmuth , p . m . jakob and w . r . bauer , “ mri thermometry : fast mapping of rf - induced heating along conductive wires ,” magn reson med 60 , 457 - 461 ( 2008 ). 16 . “ u . s food and drug administration , center for devices and radiological health , maude data base reports of adverse events involving medical devices . 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