Chen, Albert P.

Hansen, Rie Beck

Malloy, Craig R.

Park, Jae Mo

Reed, Galen D.

Schulte, Rolf F.

We present a protocol for measurement of SNR profile of 13C RF coils for clinical imaging systems. This protocol makes use of standard, vendor-provided pulse sequences as well as the natural abundance 13CH3 resonance of the dimethyl silicone (DMS) phantoms which are widely distributed. We also provide anopen source code for processing and analysis

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Downloaded from orbit.dtu.dk on: Jun 01, 2019Protocol for multi-site quantitative evaluation of 13C radio frequency coilsReed, Galen D.; Schulte, Rolf F.; Park, Jae Mo; Malloy, Craig R.; Hansen, Rie Beck; Chen, Albert P.Publication date:2019Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Reed, G. D., Schulte, R. F., Park, J. M., Malloy, C. R., Hansen, R. B., & Chen, A. P. (2019). Protocol for multi-site quantitative evaluation of 13C radio frequency coils. Abstract from ISMRM 27th Annual Meeting &Exhibition, Montréal, Canada.2510Protocol for multi-site quantitative evaluation of 13C radiofrequency coilsGalen D Reed , Rolf F Schulte , Jae Mo Park , Craig R Malloy , Rie B Hansen ,and Albert P ChenGE Healthcare, Dallas, TX, United States, GE Healthcare, Munich, Germany, UT SouthwesternMedical Center, Dallas, TX, United States, Technical University of Denmark, Kongens Lyngby,Denmark, GE Healthcare, Toronto, ON, CanadaSynopsisWe present a protocol for measurement of SNR profile of C RF coils for clinical imaging systems.This protocol makes use of standard, vendor-provided pulse sequences as well as the naturalabundance CH  resonance of the dimethyl silicone (DMS)phantoms which are widely distributed. We also provide anopen source code for processing and analysis.IntroductionQuantitative comparison of the performance of MRI transmitters and receivers between sites iscritically important in establishing standards for performance, selecting appropriate coil geometry fora given application, and verifying hardware reliability for clinical trials. Comparing C coils isinherently diﬃcult due to the lack of availability of standardized C phantoms, pulse sequences, anddata reconstruction and analysis tools. Furthermore, material costs for C – enriched compounds forlarge volume phantoms is prohibitively expensive, and a consensus has not been reached for theselection of the appropriate compound. Developing multi-site studies will be important in the evolutionof hyperpolarized C technology. We propose here a solution using standard pulse sequences andMRI phantoms, that are widely available. The proposed QA protocol utilizes the natural abundanceCH  resonance of the dimethyl silicone (DMS) phantoms available as a QA phantom from somevendors or easily prepared [1] (Figure 1). This phantom has a measured C T  of 899 ms at 3T bysaturation recovery, and C - H coupling constant is measured to be J=118Hz (Figure 1). Comparedto the commonly used natural abundance ethylene glycol CH  resonance for phantom calibration,DMS resonates at 62 PPM lower frequency with approximately 50% less sensitivity. Pulse sequenceand parameters are discussed, and an open source data reconstruction framework is provided.MethodsPulse Sequence: The following list gives the relevant parameters of this acquisition protocol. Flipangle calibration was performed manually by finding the null of the resonance using a 3 second TR. Aseparate protocol was developed for head and torso array testing; this was to ensure that the pixelvolume compensated the reduced sensitivity of the larger coils, and to ensure that the FOV suﬃcientlyencompassed a noise region. A 2D phase-encoded, slice-selective spectroscopy sequence wasselected since the protocol resolved the spectral splitting without generating image blurring, and noiseand signal separation could be achieved by simple band selection in post processing.FOV = 32 cm (head), 48 cm (torso)acquisition size = 16 by 16slice thickness = 2 cm (head), 3 cm (torso)averages = 2TR = 500 msflip angle = 22.5bandwidth, samples = 5 kHz, 256Data processing: Figure 2 shows a schematic of the processing pipeline. Since the A/D period isspectroscopically resolved, signal and noise contributions may be diﬀerentiated via band selection. Aconvenient method is peak selection using the Dietrich method, which relies on computing anumerical derivative, converting to power spectrum, then iterative thresholding based on mean andstandard deviations. References [2,3] provide a detailed description of this method, and 1D erosion /dilation morphology operations may be applied to clean up noise spikes. The integration of peaksselected with this method provides maximally eﬃcient signal usage while minimizing noise bias. Sincethe noise statistics are calculated after performing a sum-of-squares operation, the standard deviationmust be corrected for the non-central chi distribution statistics that describe summing multiplechannels of Rician-distributed magnitude noise [4,5]. Source code, written in Octave, is openlydistributed at the following link [6].Experiments: This protocol was applied at 3 diﬀerent sites inthe US and Canada using GE 3T Signa systems (GE1 2 3 3 451 2 3451313 31313131313 313 113 113 213FiguresFigure 1: a) the DMS phantom,with ring loader, inside a H headcoil. b) the DMS molecularstructure. c) DMS naturalabundance C spectrum (J =118 Hz). d) C saturationrecovery curve (T  = 899 ms)Figure 2: the data processingpipeline for SNR determination.Data are split into separate signaland noise processing pathsdepending on their location in thefrequency band. The latter maybe selected manually or via peakdetection as given in thisexample. Since sum-of squaresprocessing is used, the noisedistribution must be corrected forchannel count NFigure 3: SNR maps generated at3 diﬀerent MRI facilities with theprotocol. The coil geometriesshown are quadrature transmit, 8channel receive array (left),quadrature birdcage transmit /receive (center), and clamshellHelmholtz transmit, 8 channelpaddle array receive (right).113131Healthcare, Waukesha WI) using 3 diﬀerent C arraygeometries: 8 channel receive array, quadrature birdcagetransmit / receive, and clamshell Helmholtz transmit, 8 channelpaddle array receive.ResultsFigure 3 shows 3 sample images acquired with this protocol on the head-sized phantom. In eachcase, a clear profile of the in-plane SNR behavior of the coil could be achieved, with regions ofSNR>50, and scan times under 5 minutes. Figure 3a and 3b show data from an 8 channel receivearray and birdcage, respectively. The brightening of the posterior array elements indicates thephantom was more proximal to the posterior side, while the birdcage has smaller inner diameter andthus more even SNR throughout the volume.Discussion / ConclusionWe present a standardized calibration procedure for receive arrays using readily available phantomsand pulse sequences and using open source data processing. Limitations of this protocol include theinherent diﬃculty in transmit gain determination and lack of robust automated prescan, whichobviously may lead to systematic bias of SNR measurements. The eﬀect of coil loading on SNRmeasurements must be explored further. Although the DMS phantoms are impervious to standingwave eﬀects at high field [1], they are not, by default, loaded and are typically used in conjunction witha cylindrical loading ring (Figure 1). This ring is compatible with most 13C torso arrays, but the smallerhead-sized loader is not mechanically compatible with many head arrays. Further study is required totest the adequacy of saline bags for this purpose.AcknowledgementsNo acknowledgement found.References1. Skloss T. Phantom Fluids for High Field MR Imaging. Proc. Intl. Soc. Mag. Reson. Med. 11(2004)2. Cobas JC, Bernstein MA, Martín-Pastor M, Tahoces PG. A new general-purpose fullyautomatic baseline-correction procedure for 1D and 2D NMR data. J Magn Reson. 2006Nov;183(1):145-513. Dietrich W, Rudel CH, and Neumann M. Fast and precise automatic baseline correction ofone- and two-dimensional NMR spectra, J. Magn. Reson. 91 (1991) 1–11.4. Gilbert G. Measurement of Signal-to-Noise Ratios in Sum-of-Squares MR Images. J MagnReson Imaging. 2007 Dec;26(6):1678; author reply 1679.5. Constantinides CD, Atalar E, McVeigh ER. Signal-to-noise measurements in magnitudeimages from NMR phased arrays. Magn Reson Med 1997:38:852–857.6. https://github.com/galenreed/C13PhantomSNRProtocol.13Proc. Intl. Soc. Mag. Reson. Med. 27 (2019) 2510

Protocol for multi-site quantitative evaluation of <sup>13</sup>C radio frequency coils

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Protocol for multi-site quantitative evaluation of <sup>13</sup>C radio frequency coils

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