Magn Reson Mater Phy DOI 10.1007/s10334-014-0448-1
RESEARCH ARTICLE
Transparent thin shield for radio frequency transmit coils Debra S. Rivera • Jessica Schulz • Thomas Siegert Verena Zuber • Robert Turner
•
Received: 29 October 2013 / Revised: 27 April 2014 / Accepted: 30 April 2014 Ó ESMRMB 2014
Abstract Objective To identify a shielding material compatible with optical head-motion tracking for prospective motion correction and which minimizes radio frequency (RF) radiation losses at 7 T without sacrificing line-of-sight to an imaging target. Materials and methods We evaluated a polyamide mesh coated with silver. The thickness of the coating was approximated from the composition ratio provided by the material vendor and validated by an estimate derived from electrical conductivity and light transmission measurements. The performance of the shield is compared to a split-copper shield in the context of a four-channel transmit-only loop array. Results The mesh contains less than a skin-depth of silver coating (300 MHz) and attenuates light by 15 %. Elements of the array vary less in the presence of the mesh shield as compared to the split-copper shield indicating that the array behaves more symmetrically with the mesh shield. No D. S. Rivera (&) J. Schulz T. Siegert R. Turner Department of Neurophysics, Max Planck Institute for Human Cognitive and Brain Sciences, Stephanstrasse 1a, 04103 Leipzig, Germany e-mail: [email protected] V. Zuber NORMENT, KG Jebsen Centre for Psychosis Research, Institute of Clinical Medicine, University of Oslo, Oslo, Norway V. Zuber Division of Mental Health and Addiction, Oslo University Hospital, Oslo, Norway V. Zuber Prostate Cancer Research Group, Centre for Molecular Medicine Norway, Nordic EMBL Partnership, University of Oslo and Oslo University Hospital, Oslo, Norway
degradation of transmit efficiency was observed for the mesh as compared to the split-copper shield. Conclusion We present a shield compatible with future integration of camera-based motion-tracking systems. Based on transmit performance and eddy-current evaluations the mesh shield is appropriate for use at 7 T. Keywords Instrumentation Neuroimaging Magnetic resonance imaging Optical motion tracking
Introduction There is a need for a coil shield through which optical line-ofsight to motion correction tracking markers can be maintained. In-bore optical tracking systems correct motionrelated MR artifacts [1, 2]. Open-coil designs reduce tracking error by allowing direct tracking of markers placed close to the center of the imaging targets [3]. However, in-bore electronics benefit from closed coil shields to prevent RF noise from entering the coil, and, of increasing importance at high fields, power loss from the coil. Transmit coils lose power via far-field propagation to nearby conductive structures and therefore benefit from a local shield to minimize the losses [4]. Far-field losses increase with the fourth-power of frequency [5] and can cause a power balance discrepancy between simulation and experimentation. Conventionally, shields made of copper have slits to minimize eddy-current induction, bridged by tens or hundreds of capacitors [6, 7]. Such shields are tedious to construct and introduce an additional source of variation in the coil. When space constraints necessitate shields to be within the geometric radius of a coil element, electrostatic shields, defined by a thickness less than the skin-depth of the signal
123
Magn Reson Mater Phy
Fig. 1 Coil with a conventional copper shield (a) and with novel transparent shield (b) through which the transmit and receive arrays can be seen
of interest, reduce losses due to coil-shield interactions by suppressing mirror currents [8]. Coil shields thinner than one skin depth of the Larmor frequency have been proposed for magnetic resonance (MR) applications [9] in order to minimize eddy-current artifacts from spatialencoding gradients; they have also been demonstrated as being beneficial for positron emission tomography/magnetic resonance (PET/MR) [10]. Although readily obtainable for 3 T applications and below, manufactured thin shield materials are difficult to find for use at 300 MHz. A target range of 2–4 microns is rather thick for bulk vacuum deposition manufacturing and is at the limit of foil fabrication processes including laminates. An 8 lm-thick aluminum foil was the thinnest readily available aluminum foil laminate we were able to acquire. However it is not a suitable shielding material at 7 T. Although the aluminum thickness corresponds to only two skin-depths at 300 MHz, the thin aluminum foil supports artifact-inducing eddy currents (demonstrated below) and the auditory noise in the scanner was increased perceptibly as well. We present a novel non-opaque coil shield for application at 7 T. With an emphasis on transmit performance we compared a coil array with two different shields: a novel mesh shield, and a split-copper shield (Fig. 1). We investigated properties of the shielding material, including eddycurrent artifacts and light transmission, to discern the appropriateness of the shielding material for future combination with optical motion tracking of a marker affixed to a volunteer, to enable robust real-time motion correction.
Materials and methods The shielding mesh (50 dB Aaronia-Shield, Aaronia AG, Euscheid Germany) is a polyamide weave coated with silver. For comparison a shield was built of copper (48 strips, 35-micron thick, 25-mm width, 300-mm length) with 94 nF capacitance bridging 1-mm gaps. Acrylic cylinders provided the structure for both shielding materials (400 mm diameter; 300 mm length, 5 mm thickness).
123
For all measurements, the same phantom (180 mm diameter and 420 mm long), receive array (220 mm diameter) and transmit array (300 mm inner diameter) were used. The phantom contains 7.3 L of 1.24 g/L NiSO46H2O and 2.62 g/L NaCl (T1 300 ms, 0.52 S/m, rel. permittivity 78). The receive array has eight overlapping loops with coilmounted preamplifiers and on-board cable traps (P/N: 10185702 and 10185751, Stark Contrast, MRI Coils Research, Erlangen, Germany). The transmit array is comprised of four non-overlapping rectangular loops 222.5 mm in width and 150 mm in length. The gap between adjacent elements is 13 mm. The array was made of 2-mmthick FR4 printed circuit boards (PCBs) that fit together to form the three-dimensional structure, and is hence referred to as a PCB array [11]. The 5 mm-wide copper traces are perpendicular to the surface of the optional acrylic former mounted within the array. Matching was achieved with symmetric series capacitors. A wire-wound balun/cabletrap was placed at the feed port of each element to minimize common-mode losses [12] and a second wire-wound cabletrap was incorporated within a quarter-wavelength of the feed port. The PCB array was tuned for minimum reflected power in place of decoupling [13]. Reflected power can be computed by taking the sum of squares of the matrix that results from multiplying the measured scattering-parameter matrix (complex) by the excitation matrix (complex). To minimize errors due to a discrepancy between theoretical and actual excitation phases due to cable length we instead sent the excitation sweep from the network analyzer through a splitter and through the phase shifting cables (to achieve CP mode) that we used with the array. With a directional coupler between each phase-shifting cable and the corresponding element we sampled the forward and reflected power of each channel with the network analyzer. The percent of reflected power is the ratio of the reflected and forward sum-ofsquares of the linear magnitude for the array. Symmetry was evaluated on the bench top as the variation between the elements of the array for the following metrics: loaded Q, coupling between neighboring elements (e.g., S12) and the mutual coupling coefficient k. Asymmetry on the bench may not lead to observable variations in transmit performance; therefore, we also preform B1? measurements detailed later in this section. Prior to minimum-reflected-power tuning the loaded Q of each element was measured in isolation as the ratio of the center frequency and the 7-dB bandwidth of the matched coil through the cable (Snn) [14]. We found convergence between the described method and alternative methods employing sniffer probes and opted to eliminate the possible confounding variable of probe position. To quantify the coupling factor k we measured the peak splitting caused by activating only a pair of neighboring
Magn Reson Mater Phy
coils to quantify the coupling between neighboring elements [8] fpeak1 fpeak2 k¼ ; ð1Þ f0 the distance between peaks (Snn) divided by the resonant frequency of the element in isolation f0. Equation 1 holds when peaks are prominent and k does not approach unity [8]. Coupling was also quantified as the coefficient of transmission (i.e., S12) by activating neighboring coils (tuned in isolation) using a full two-port calibrated network analyzer. Maps of the transmit array excitation profile (|B1?|) were collected by the actual flip angle imaging method [15] with the following parameters: TR2/TR1 5, TR1 75 ms, TE of 3.2 ms, adaptive combine reconstruction, bandwidth of 200 Hz, diffusion damping factor of 0.1, a 0.7-ms rectangular pulse, a three-dimensional scan of 3-mm-isotropic resolution with 60 coronal slices and with 96 9 60 voxels (60 phase-encoding-steps left/right). Measurements were performed with 88.5 Vrms, with two averages, or 79.6 Vrms, with three averages (giving flip angles of 50° and 45° respectively at the center of the phantom for a reference of 318.6 Vrms), The number of averages was adjusted to provide an approximately equivalent signal quality despite the different applied voltages. Transmit efficiency is defined as |B1?| normalized by the square-root of the power [W]. Peak voltage reported by the scanner was used to calculate the power delivered to the elements. Attenuation between the power-monitoring hardware of the scanner does not affect our comparison and was omitted. Transmit efficiency was evaluated from the average of the |B1?| and the central peak value represented in ten or more voxels to prevent spurious signals from skewing the results. The coefficient of variation, the population standard deviation normalized by the mean, indicates homogeneity of the transmit performance. The cylindrical phantom (42 cm long) was arranged with its axis along the main axial direction (z) of the scanner. One region of interest was defined as a ‘‘brain’’ region. This was a 9 cm long axial cylinder extending from the end of the phantom furthest from the shoulder position of the coil. The other region of interest corresponded to the region of the phantom experiencing a maximal transmit RF field and consisted of an 18 cm long axial cylinder extending from the same end of the phantom. As a means of evaluating variations of the transmit performance across the elements of the array we evaluated the population of voxels proximal to each element. The populations were extracted from the same regions for each shield condition. The regions were placed to circumscribe the areas of maximum excitation and all have a 20 voxel by 30 voxel cross-section (presented alongside the results).
The regions extended through the length of the 18-cm long ROI described above excluding the centimeter closest the to end of the phantom. Voxels with less than 0.015 lT/HW were omitted. The number of signal-containing voxels was observed to ensure that the average for both conditions and for each element across conditions was within the standard deviation. For each population box-plots expressing the 1.5 %, 25 %, the median (50 %), 75 % and 98.5 % quantiles were plotted. A right-sided paired t test was conducted to evaluate whether there was a statistically significant difference between the interquartile ranges (third quartile minus first quartile) for the four elements of the array for each of the shielding conditions, to test the alternative hypothesis that the mean interquartile range for the elements was greater in the presence of the copper shield (a = 0.05). The voxel populations were divided into bins 0.01 lT/HW wide and the distributions for the individual elements were superimposed on one plot for each condition. As a means of comparing the symmetry of the two conditions the upper and lower boundaries of the voxel count for each bin were compiled from the elements of the array and the area between the boundaries was calculated. Mesh properties Light transmission through the mesh was quantified by using a digital camera (D-LUX 3, Leica Camera, Solms, Germany) to obtain gray-scale images of a backlit piece of white printer paper. We observed over time that the resistance of the mesh increased. We therefore investigated optical properties of the mesh spray-coated with a clear acrylic lacquer, conventionally used to prevent oxidation of circuit board traces. The mesh and lacquered mesh were taped, without stretching the material, onto simple cardboard frames that were easily affixed to the camera. For each condition three images were averaged and the element-wise ratio of the mesh (lacquered or not) to the nomesh condition was computed. Camera position, exposure, and focus were held constant across all conditions. We observed convergence of the computed transmission between the average of the element-wise ratios and the ratio of the average of the matrices and, therefore, present the former. We estimate that the mesh has B2 microns of silver, half of the skin-depth at 300 MHz. Our estimate is based on the manufacturer-published mass ratio (Ag/Polyamide: 20 %/ 80 %) and the assumption that the manufacturer-published mesh thickness (0.1 mm) corresponds to two threads with a calculated 0.5-micron-thick conformal coat and the additional assumption that each thread contributes approximately two layers of electrically connected conformal coating at any given position. As the above estimate has multiple assumptions we used resistance per square
123
Magn Reson Mater Phy
measures and calculated values to verify the estimate. We used copper foil with a thickness of 35 lm as a reference. For the calculated resistance per square we began with a solid foil and then corrected for the sparseness of the material by using the measured light attenuation as an estimate of the thread density. Measurements were conducted with material samples 1 cm wide and 2 cm long, using an LCR meter (Agilent U1731A, Agilent Technologies, Santa Clara, California, United States of America) with alligator clips. This measurement method was also used to characterize the drift of resistance as a function of time. We evaluated the ratios of mesh resistance per square as compared to the 35 lm copper foil. In order to evaluate if the mesh presents additional eddy-current artifacts we collected echo planar images (EPIs). These measurements were done with a commercial coil (eight-channel transmit-receive loop array, RAPID Biomedical GmbH, Rimpar, Germany) that has a localized shield near the traces of the individual elements so that the mesh could be completely removed without altering tuning. The commercial coil was centered inside an acrylic former with a 40 cm diameter and a spherical oil phantom was used with a phantom holder provided by the coil vendor to sit in the center of the coil. EPIs were collected with the bare former, the former wrapped with the polyamide-mesh and the thinnest readily-available aluminum foil (8 lm thick, coated with a polymer coating on both sides, CLIMApac 2810, MetPro, Schwieberdingen, Germany). The static magnetic field was shimmed with no additional shielding in the bore and not altered for the subsequent images. The images are presented and artifacts were compared to background for the images at various 100 by 100 pixel region averages. As it is our intention to investigate the potential compatibility of the mesh with future integration of camerabased motion tracking we must also address the compatibility of the mesh with Moire´ pattern markers. Our research focuses on the use of retro-reflective sphere markers. As we do not have Moire´ markers we broke the question into two components that we could test. First of all, does the mesh interfere with all Moire´ patterns? To answer this question we created a Moire´ pattern target by doubling over a piece of the silver-polyamide mesh and imaged the target with
and without the mesh filters created for the light transmission analysis. The second concern is whether a marker pattern blurred by the mesh is still discernible. Therefore, we additionally targeted a reproduction of a square cm feature from a Moire´ pattern marker.
Results Bench top metrics indicate that for the present configuration the array elements have fewer losses and more symmetry with the silver-polyamide versus the copper shield (Table 1). The copper shield increases loading of the elements thereby decreasing the loaded Q. The novel shield reduces variation of bench-top metrics across the elements (Table 1: Q, k, and S12) indicating the mesh improved the symmetry of the set-up as compared to the evaluated splitcopper shield. There is more coupling between neighboring elements in the presence of the novel shield; however, the reported values are within the standard deviation of the copper shield. Transmit performance suggests that for the shields and array under evaluation the novel shield gives a slight reduction in shield losses (Table 2). The peak transmit efficiency was 0.69 lT/HW with the copper shield and 0.71 lT/HW for the mesh shield. Comparisons of average and peak efficiency correspond to a 0.25–0.3 dB (or 6–7 % power) loss by using the copper shield. The coefficient of variation for both regions of interest in the phantom indicates homogeneity is improved in the presence of the mesh (Table 2) reflecting a 6 % variation between the two conditions. Transmit efficiency maps for the two shields are shown in Fig. 2. The transmit data corroborate the bench-top metrics that indicated an improved symmetry provided by the novel shield. Box plots for transmit efficiency of the voxel populations proximal to each element vary less in the presence of the silver-polyamide shield (Fig. 3a) as compared to the split-copper shield (Fig. 3b). The first quartile, median, third quartile and whiskers of the boxplots vary more across the elements in the presence of the copper shield than in the presence of the silver-polyamide shield. A right-sided paired t test for the interquartile ranges rejects the null hypothesis (a = 0.05) that the mean
Table 1 Bench-top metrics measured with a vector network analyzer for array performance in the presence of novel (silver-polyamide) and conventional (copper) shields, indicating an array performance and variation across elements of the array Shield material
Q loaded Mean
k Stdevp
Coef var (%)
Mean
Neighbor coupling (dB) Stdevp
Coef var (%)
Mean
Stdevp
Coef var (%)
Silver-polyamide
42
0.8
2
0.031
0.001
3
-6.4
0.09
1
Copper
33
1.7
5
0.028
0.004
12
-6.7
0.35
5
123
Magn Reson Mater Phy Table 2 Scanner performance metrics for efficiency and homogeneity of the transmit fields for the 9-cm long ROI and 18-cm long ROI, with decimal places reflecting precision as indicated by repeated measures
Coronal
Silver-Polyamide
B1? per square root power (lT/HW) Mean
Stdevp
Silver-polyamide
Copper
Silver-polyamide
Copper
Silverpolyamide (%)
Copper (%)
9-cm ROI
0.31
0.30
0.10
0.10
32
34
18-cm ROI
0.30
0.29
0.11
0.11
35
37
for the array in the presence of the mesh (Fig. 4c) as compared to the copper (Fig. 4d) providing evidence of symmetry based on the MRI data.
Copper
0.7
Mesh properties 0.6
Sagittal
0.5
0.4
0.3
0.2
Transversal
Coef of var
0.1
Fig. 2 Transmit efficiency maps for the 18-cm ROI in Table 2. Small notches appear in the coronal and sagittal slices to indicate the 7 cm depth corresponding to the transversal slice and the end of the 9 cm region
interquartile range for the copper is less than or equivalent to the mean interquartile range of the elements in the presence of the mesh shield with a p value of 0.038, suggesting that the interquartile range is greater for the elements in the presence of the copper shield. This can be seen from the plots of the voxel distributions (Fig. 4) via the tighter clustering of distributions for the elements in the presence of the silver-polyamide shield (Fig. 4a) than in the presence of the copper shield (Fig. 4b). To quantify this feature of the data we created composite functions for the maximum and minimum values provided by the elements at each x-value (Fig. 4c, d) and calculated the area between the curves. There was 25 % less area between the curves
The novel shield maintains line-of-sight allowing transmission of light to the imaging target. Light transmission through the mesh and coated mesh are similar with the lacquer attenuating light by an additional 3 %. Figure 5 shows examples of gray-scale images of all three conditions as well as values for the transmission through the mesh (mean 84.5 %) and lacquered mesh (mean 81.3 %). Measurements of resistance per square of the mesh corroborated our metal thickness approximation and indicate resistance drift of the unlacquered mesh. The mesh resistance per square was measured as 100 times that of the 35 lm copper reference. Estimating the percent of area covered by metal as light attenuation (15 %) the estimate for a 2 lm thick mesh gives 110 times that of the calculated reference (35 micron-thick copper foil). The calculated value and measurement for the 2 lm mesh are in agreement. Over a 2-year period the unlacquered mesh increased resistivity by 270 %. The mesh did not provide measurable additional eddy current artifacts in the EPI images (Fig. 6). Differences between the mesh and the no-mesh condition were less than the standard deviation of the background and, therefore, not significant. On the other hand, the 8 lm aluminum shield, although only 20 cm in length and not electrically connected as a cylinder, produced artifacts with signal intensity consistently twice that of the position-matched 100 9 100 voxel regions in the mesh and no-shield conditions. Figure 7 shows the results of our investigations into the compatibility of the mesh with Moire´ markers with patterns discernible through the mesh. Although we cannot conclusively state that the mesh is compatible with Moire´ markers, we do not have evidence that allows us to conclude the contrary. The shield is therefore appropriate for future combination with optical motion tracking of a marker affixed to the head of a volunteer resulting in robust real-time motion correction.
123
Magn Reson Mater Phy
Transmit Efficiency [µT/√W]
(a) Silver-Polyamide Shield
1
0. 6
2
0. 5 0. 4 0. 3 0.2 0.1 0
Element 1
Element 2
Element 3
Element 4
(b) Split-Copper Shield
1
0.6
2
0.5 0.4 0.3 0.2 0.1
Element 1
Element 2
Element 3
Element 4
Fig. 3 Box plots of transmit efficiencies for the voxel populations in the vicinity of each element in the presence of the silver-polyamide mesh shield (a) and the split copper shield (b) representing the 1.5 %, 25 %, median (50 %), 75 % and 98.5 % quantiles for each
population. The boundaries of the 20 by 30 voxel cross-sections of the populations are superimposed on the 7-cm deep slices previously shown and labeled with corresponding element numbers
Silver-Polyamide Shield
Split-Copper Shield
(a)
(b)
1500
Voxel Count
Fig. 4 Distribution of the voxel populations for each of the elements in the presence of the mesh shield (a) and in the presence of the copper shield (b). The range of values at each x-position for the elements shown by plots of upper and lower boundaries for the element distributions in the presence of the mesh (c) and the copper (d) shields
4
3
0
Elem 1 Elem 2 Elem 3 Elem 4
1000
500
0
0
0.1
0.2
0.3
0.4
0.5
Voxel Count
Transmit Efficiency [µT/√W]
4
3
1500
1000
500
0
0.6
0
(c)
(d)
Voxel Count
1500
1000
500
0
0.1
0.2
0.3
0.4
0.5
Transmit Efficiency [µT/√W]
123
0.1
0.2
0.3
0.4
0.5
0.6
Transmit Efficiency [µT/√W]
Voxel Count
Transmit Efficiency [µT/√W]
0
Elem 1 Elem 2 Elem 3 Elem 4
0.6
1500
1000
500
0
0
0.1
0.2
0.3
0.4
0.5
Transmit Efficiency [µT/√W]
0.6
Magn Reson Mater Phy
(a) 100%
(b)
Fig. 7 Photographs of the improvised Moire´ pattern and a twodimensional reproduction of a square cm Moire´ feature from a commercially available marker. The patterns are discernible through the mesh and lacquered mesh Fig. 5 Gray scale images (with the same exposure and focal length) from which transparency of the mesh was quantified (a). The percent light transmission matrices for mesh (right) and lacquered mesh (left) with the scale bar doubling as the axis for the superimposed traces depicting the values (dark) and averages (white) of the matrix values across the middle of the matrix. For each condition three images were averaged and the element-wise ratio of the mesh (lacquered or not) to the no-mesh condition was computed to form the presented matrices
No Shield
Mesh
8 µm Aluminum
Fig. 6 EPIs collected with the commercial coil when introducing into the scanner bore no additional shield, the silver-polyamide mesh or an aluminum foil cylinder (8 lm thick, 20 cm long, 40-cm diameter). The line through the images indicates the position of the signal intensity traces plotted below. By comparing the relationship of artifact to background signal intensity, the degree of the eddy current artifact for each case can be inferred. Neither visual inspection nor calculations indicate that the addition of a mesh increased the eddycurrent artifact
Discussion We present a transparent RF coil shield and evidence that the shield does not degrade coil array performance for the configuration presented. The values for transmit efficiency presented are similar to values for loop coil transceiver arrays in literature at 7 T; for maximum: 0.54 lT/HW [16], 0.84 lT/HW (attenuation corrected; 0.45 S/m phantom) [13], and average values: 0.276 lT/HW [17]. Peak transmit efficiency was in the range of 0.7 lT/HW and the average over the 9-cm region was in the range of 0.3 lT/ HW for the arrays described (no attenuation correction).
The transparent mesh was first used for shielding in an in-bore motion tracking camera [1]. A transparent conductive oxide film connected to the aluminum shield did not sufficiently reduce noise. Therefore, the mesh was sourced as an alternate material. The mesh may be useful for other in-bore hardware requiring visual access such as an in-bore active displays for 7 T [18]. The display uses an indium tin oxide (ITO) on a glass substrate; however, the system still degrades coil performance [18]. The manufacturer-reported transmittance of the ITO glass is also 15 %, although the conductivity of the ITO is less than that of silver by an order of magnitude. Our motion-correction work has focused on retroreflective spheres for which we have seen no relevant degradation of tracking precision due to the mesh. We think it is because a uniform blurring of the marker still allows the edge detection algorithm to accurately determine the center of the marker. Blurring of the Moire´ pattern, however, may degrade algorithm accuracy and depends on the wavelength of the light used and the sensor-to-mesh distance, and can be estimated from single-slit diffraction as the distance between the first minima for a 0.7 mm slit. A limitation of the silver-coated polyamide is that the resistance increases over time due to oxidation. The mesh tolerates spray coating that prevents oxidation and only minimally alters the optical properties of the mesh. We minimize confounding variables in comparing the shields, including the use of the identical transmit and receive arrays and spacers to control for phantom position. For both shields the reflected power was less than 1 %. Differences in the capacitive values for tuning could give rise to different coil losses. However, high quality variable capacitors were placed in parallel with static capacitors to minimize the effect. The split copper shield contains over 600 solder joints and, therefore, many possible points of failure. By contrast, the mesh was sewn into a cylinder from a continuous piece of fabric with a 1-cm overlap. The resistance across the seam at 10 kHz was verified to be consistent (\10 % divergence) with that of other portions of the mesh.
123
Magn Reson Mater Phy
As the elements themselves are identical and spacing is precisely controlled, the observed increase in asymmetry is most likely attributed to the split copper shield. The capacitors of the split shield are discrete and hand soldered presenting the potential for asymmetries. Additionally, as mirror currents are dampened by the mesh that has an estimated resistance at 300 MHz of roughly 7 times that of the copper strips, there is less current and therefore less influence of induced magnetic fields on the underlying elements. In short, symmetry of the elements in the presence of the mesh shield is attributable to two possible features of the mesh, the symmetry and the reduced interactions between the mesh and the elements. The need for symmetric shields increases at high fields as far-field losses increase and asymmetric field patterns become more prevalent and become dangerous when not modeled properly. The use of shields to minimize far-field losses reduces uncertainty in power balance measurements for which far-field losses are not conventionally measured. Small errors in centering the shield about a multi-element array or asymmetries in the shield can lead to mismatches between in-vivo use and circuit simulations used to maintain power within safety constraints. We present a novel shield that allows light transmission with minimal attenuation (15 %) which is compatible with future integration of camera-based motion tracking systems. Based on the transmit performance and eddy current artifact evaluation the mesh shield is appropriate for use at 7 T and may be advantageous in cases in which space constraints increase shield losses. Acknowledgments Funding for the research was provided by the Max Planck Society. Thank you to Dr. Mikhail Kozlov, Carsten Koegler, Roland Mueller and Markus Streicher for their contributions.
3.
4.
5. 6.
7.
8. 9.
10.
11.
12.
13.
14.
15.
16.
References 1. Schulz J, Siegert T, Reimer E, Labadie C, Maclaren J, Herbst M, Zaitsev M, Turner R (2012) An embedded optical tracking system for motion-corrected magnetic resonance imaging at 7 T. Magn Reson Mater Phy 25(6):443–453 2. Qin L, van Gelderen P, Derbyshire JA, Jin F, Lee J, de Zwart JA, Tao Y, Duyn JH (2009) Prospective head-movement correction
123
17.
18.
for high-resolution MRI using an in-bore optical tracking system. Magn Reson Med 62(4):924–934 Sahabi H, Basu A (1996) Analysis of error in depth perception with vergence and spatially varying sensing. Comput Vis Image Underst 63(3):447–461 Liu W, Kao CP, Collins CM, Smith MB, Yang QX (2013) On consideration of radiated power in RF field simulations for MRI. Magn Reson Med 69(1):290–294 Vaughan JT (2006) Ultrahigh field MRI: high-frequency coils. Biol Magn Reson 26:127–161 Alecci M, Jezzard P (2002) Characterization and reduction of gradient-induced eddy currents in the RF shield of a TEM resonator. Magn Reson Med 48(2):404–407 Wald LL, Wiggins GC, Potthast A, Wiggins CJ, Triantafyllou C (2005) Design considerations and coil comparisons for 7 T brain imaging. Appl Magn Reson 29(1):19–37 Terman FE (1947) Radio engineering, 3rd edn. McGraw-Hill, Boston Carlson JW (1994) Apparatus and method for shielding MRI RF antennae from the effect of surrounding objects. U.S. Patent No. 5304932 Truhn D, Kiessling F, Schulz V (2011) Optimized RF shielding techniques for simultaneous PET/MR. Med Phys 38(7):3995– 4000 Rivera DS, Siegert T, Koegler C, Scha¨fer A, Streicher MN, Turner R. Transmit array with novel shield and fabrication technique for reducing losses at UHF. In: Proceedings of the 21st international society for magnetic resonance in medicine, Salt Lake City, p 2757 Rivera DS, Turner R. Baluns for UHF transmit arrays. In: Proceedings of the 21st international society for magnetic resonance in medicine, Salt Lake City, p 2755 Kozlov M, Turner R (2009) Fast MRI coil analysis based on 3-D electromagnetic and RF circuit co-simulation. J Magn Reson 200(1):147–152 Haase A, Odoj F, Von Kienlin M, Warnking J, Fidler F, Weisser A, Nittka M, Rommel E, Lanz T, Kalusche B, Griswold M (2000) NMR probeheads for in vivo applications. Concepts Magn Reson 12(6):361–388 Yarnykh VL (2007) Actual flip-angle imaging in the pulsed steady state: a method for rapid three-dimensional mapping of the transmitted radiofrequency field. Magn Reson Med 57(1): 192–200 Avdievich NI, Hetherington HP, Kuznetsov AM, Pan JW (2009) 7 T head volume coils: improvements for rostral brain imaging. J Magn Reson Imaging 29(2):461–465 Gilbert KM, Belliveau JG, Curtis AT, Gati JS, Klassen LM, Menon RS (2012) A conformal transceive array for 7 T neuroimaging. Magn Reson Med 67(5):1487–1496 Groebner J, Berger MC, Umathum R, Bock M, Rauschenberg J (2013) 7 Tesla compatible in-bore display for functional magnetic resonance imaging. Magn Reson Mater Phy 26(4):371–375
RESEARCH ARTICLE
Transparent thin shield for radio frequency transmit coils Debra S. Rivera • Jessica Schulz • Thomas Siegert Verena Zuber • Robert Turner
•
Received: 29 October 2013 / Revised: 27 April 2014 / Accepted: 30 April 2014 Ó ESMRMB 2014
Abstract Objective To identify a shielding material compatible with optical head-motion tracking for prospective motion correction and which minimizes radio frequency (RF) radiation losses at 7 T without sacrificing line-of-sight to an imaging target. Materials and methods We evaluated a polyamide mesh coated with silver. The thickness of the coating was approximated from the composition ratio provided by the material vendor and validated by an estimate derived from electrical conductivity and light transmission measurements. The performance of the shield is compared to a split-copper shield in the context of a four-channel transmit-only loop array. Results The mesh contains less than a skin-depth of silver coating (300 MHz) and attenuates light by 15 %. Elements of the array vary less in the presence of the mesh shield as compared to the split-copper shield indicating that the array behaves more symmetrically with the mesh shield. No D. S. Rivera (&) J. Schulz T. Siegert R. Turner Department of Neurophysics, Max Planck Institute for Human Cognitive and Brain Sciences, Stephanstrasse 1a, 04103 Leipzig, Germany e-mail: [email protected] V. Zuber NORMENT, KG Jebsen Centre for Psychosis Research, Institute of Clinical Medicine, University of Oslo, Oslo, Norway V. Zuber Division of Mental Health and Addiction, Oslo University Hospital, Oslo, Norway V. Zuber Prostate Cancer Research Group, Centre for Molecular Medicine Norway, Nordic EMBL Partnership, University of Oslo and Oslo University Hospital, Oslo, Norway
degradation of transmit efficiency was observed for the mesh as compared to the split-copper shield. Conclusion We present a shield compatible with future integration of camera-based motion-tracking systems. Based on transmit performance and eddy-current evaluations the mesh shield is appropriate for use at 7 T. Keywords Instrumentation Neuroimaging Magnetic resonance imaging Optical motion tracking
Introduction There is a need for a coil shield through which optical line-ofsight to motion correction tracking markers can be maintained. In-bore optical tracking systems correct motionrelated MR artifacts [1, 2]. Open-coil designs reduce tracking error by allowing direct tracking of markers placed close to the center of the imaging targets [3]. However, in-bore electronics benefit from closed coil shields to prevent RF noise from entering the coil, and, of increasing importance at high fields, power loss from the coil. Transmit coils lose power via far-field propagation to nearby conductive structures and therefore benefit from a local shield to minimize the losses [4]. Far-field losses increase with the fourth-power of frequency [5] and can cause a power balance discrepancy between simulation and experimentation. Conventionally, shields made of copper have slits to minimize eddy-current induction, bridged by tens or hundreds of capacitors [6, 7]. Such shields are tedious to construct and introduce an additional source of variation in the coil. When space constraints necessitate shields to be within the geometric radius of a coil element, electrostatic shields, defined by a thickness less than the skin-depth of the signal
123
Magn Reson Mater Phy
Fig. 1 Coil with a conventional copper shield (a) and with novel transparent shield (b) through which the transmit and receive arrays can be seen
of interest, reduce losses due to coil-shield interactions by suppressing mirror currents [8]. Coil shields thinner than one skin depth of the Larmor frequency have been proposed for magnetic resonance (MR) applications [9] in order to minimize eddy-current artifacts from spatialencoding gradients; they have also been demonstrated as being beneficial for positron emission tomography/magnetic resonance (PET/MR) [10]. Although readily obtainable for 3 T applications and below, manufactured thin shield materials are difficult to find for use at 300 MHz. A target range of 2–4 microns is rather thick for bulk vacuum deposition manufacturing and is at the limit of foil fabrication processes including laminates. An 8 lm-thick aluminum foil was the thinnest readily available aluminum foil laminate we were able to acquire. However it is not a suitable shielding material at 7 T. Although the aluminum thickness corresponds to only two skin-depths at 300 MHz, the thin aluminum foil supports artifact-inducing eddy currents (demonstrated below) and the auditory noise in the scanner was increased perceptibly as well. We present a novel non-opaque coil shield for application at 7 T. With an emphasis on transmit performance we compared a coil array with two different shields: a novel mesh shield, and a split-copper shield (Fig. 1). We investigated properties of the shielding material, including eddycurrent artifacts and light transmission, to discern the appropriateness of the shielding material for future combination with optical motion tracking of a marker affixed to a volunteer, to enable robust real-time motion correction.
Materials and methods The shielding mesh (50 dB Aaronia-Shield, Aaronia AG, Euscheid Germany) is a polyamide weave coated with silver. For comparison a shield was built of copper (48 strips, 35-micron thick, 25-mm width, 300-mm length) with 94 nF capacitance bridging 1-mm gaps. Acrylic cylinders provided the structure for both shielding materials (400 mm diameter; 300 mm length, 5 mm thickness).
123
For all measurements, the same phantom (180 mm diameter and 420 mm long), receive array (220 mm diameter) and transmit array (300 mm inner diameter) were used. The phantom contains 7.3 L of 1.24 g/L NiSO46H2O and 2.62 g/L NaCl (T1 300 ms, 0.52 S/m, rel. permittivity 78). The receive array has eight overlapping loops with coilmounted preamplifiers and on-board cable traps (P/N: 10185702 and 10185751, Stark Contrast, MRI Coils Research, Erlangen, Germany). The transmit array is comprised of four non-overlapping rectangular loops 222.5 mm in width and 150 mm in length. The gap between adjacent elements is 13 mm. The array was made of 2-mmthick FR4 printed circuit boards (PCBs) that fit together to form the three-dimensional structure, and is hence referred to as a PCB array [11]. The 5 mm-wide copper traces are perpendicular to the surface of the optional acrylic former mounted within the array. Matching was achieved with symmetric series capacitors. A wire-wound balun/cabletrap was placed at the feed port of each element to minimize common-mode losses [12] and a second wire-wound cabletrap was incorporated within a quarter-wavelength of the feed port. The PCB array was tuned for minimum reflected power in place of decoupling [13]. Reflected power can be computed by taking the sum of squares of the matrix that results from multiplying the measured scattering-parameter matrix (complex) by the excitation matrix (complex). To minimize errors due to a discrepancy between theoretical and actual excitation phases due to cable length we instead sent the excitation sweep from the network analyzer through a splitter and through the phase shifting cables (to achieve CP mode) that we used with the array. With a directional coupler between each phase-shifting cable and the corresponding element we sampled the forward and reflected power of each channel with the network analyzer. The percent of reflected power is the ratio of the reflected and forward sum-ofsquares of the linear magnitude for the array. Symmetry was evaluated on the bench top as the variation between the elements of the array for the following metrics: loaded Q, coupling between neighboring elements (e.g., S12) and the mutual coupling coefficient k. Asymmetry on the bench may not lead to observable variations in transmit performance; therefore, we also preform B1? measurements detailed later in this section. Prior to minimum-reflected-power tuning the loaded Q of each element was measured in isolation as the ratio of the center frequency and the 7-dB bandwidth of the matched coil through the cable (Snn) [14]. We found convergence between the described method and alternative methods employing sniffer probes and opted to eliminate the possible confounding variable of probe position. To quantify the coupling factor k we measured the peak splitting caused by activating only a pair of neighboring
Magn Reson Mater Phy
coils to quantify the coupling between neighboring elements [8] fpeak1 fpeak2 k¼ ; ð1Þ f0 the distance between peaks (Snn) divided by the resonant frequency of the element in isolation f0. Equation 1 holds when peaks are prominent and k does not approach unity [8]. Coupling was also quantified as the coefficient of transmission (i.e., S12) by activating neighboring coils (tuned in isolation) using a full two-port calibrated network analyzer. Maps of the transmit array excitation profile (|B1?|) were collected by the actual flip angle imaging method [15] with the following parameters: TR2/TR1 5, TR1 75 ms, TE of 3.2 ms, adaptive combine reconstruction, bandwidth of 200 Hz, diffusion damping factor of 0.1, a 0.7-ms rectangular pulse, a three-dimensional scan of 3-mm-isotropic resolution with 60 coronal slices and with 96 9 60 voxels (60 phase-encoding-steps left/right). Measurements were performed with 88.5 Vrms, with two averages, or 79.6 Vrms, with three averages (giving flip angles of 50° and 45° respectively at the center of the phantom for a reference of 318.6 Vrms), The number of averages was adjusted to provide an approximately equivalent signal quality despite the different applied voltages. Transmit efficiency is defined as |B1?| normalized by the square-root of the power [W]. Peak voltage reported by the scanner was used to calculate the power delivered to the elements. Attenuation between the power-monitoring hardware of the scanner does not affect our comparison and was omitted. Transmit efficiency was evaluated from the average of the |B1?| and the central peak value represented in ten or more voxels to prevent spurious signals from skewing the results. The coefficient of variation, the population standard deviation normalized by the mean, indicates homogeneity of the transmit performance. The cylindrical phantom (42 cm long) was arranged with its axis along the main axial direction (z) of the scanner. One region of interest was defined as a ‘‘brain’’ region. This was a 9 cm long axial cylinder extending from the end of the phantom furthest from the shoulder position of the coil. The other region of interest corresponded to the region of the phantom experiencing a maximal transmit RF field and consisted of an 18 cm long axial cylinder extending from the same end of the phantom. As a means of evaluating variations of the transmit performance across the elements of the array we evaluated the population of voxels proximal to each element. The populations were extracted from the same regions for each shield condition. The regions were placed to circumscribe the areas of maximum excitation and all have a 20 voxel by 30 voxel cross-section (presented alongside the results).
The regions extended through the length of the 18-cm long ROI described above excluding the centimeter closest the to end of the phantom. Voxels with less than 0.015 lT/HW were omitted. The number of signal-containing voxels was observed to ensure that the average for both conditions and for each element across conditions was within the standard deviation. For each population box-plots expressing the 1.5 %, 25 %, the median (50 %), 75 % and 98.5 % quantiles were plotted. A right-sided paired t test was conducted to evaluate whether there was a statistically significant difference between the interquartile ranges (third quartile minus first quartile) for the four elements of the array for each of the shielding conditions, to test the alternative hypothesis that the mean interquartile range for the elements was greater in the presence of the copper shield (a = 0.05). The voxel populations were divided into bins 0.01 lT/HW wide and the distributions for the individual elements were superimposed on one plot for each condition. As a means of comparing the symmetry of the two conditions the upper and lower boundaries of the voxel count for each bin were compiled from the elements of the array and the area between the boundaries was calculated. Mesh properties Light transmission through the mesh was quantified by using a digital camera (D-LUX 3, Leica Camera, Solms, Germany) to obtain gray-scale images of a backlit piece of white printer paper. We observed over time that the resistance of the mesh increased. We therefore investigated optical properties of the mesh spray-coated with a clear acrylic lacquer, conventionally used to prevent oxidation of circuit board traces. The mesh and lacquered mesh were taped, without stretching the material, onto simple cardboard frames that were easily affixed to the camera. For each condition three images were averaged and the element-wise ratio of the mesh (lacquered or not) to the nomesh condition was computed. Camera position, exposure, and focus were held constant across all conditions. We observed convergence of the computed transmission between the average of the element-wise ratios and the ratio of the average of the matrices and, therefore, present the former. We estimate that the mesh has B2 microns of silver, half of the skin-depth at 300 MHz. Our estimate is based on the manufacturer-published mass ratio (Ag/Polyamide: 20 %/ 80 %) and the assumption that the manufacturer-published mesh thickness (0.1 mm) corresponds to two threads with a calculated 0.5-micron-thick conformal coat and the additional assumption that each thread contributes approximately two layers of electrically connected conformal coating at any given position. As the above estimate has multiple assumptions we used resistance per square
123
Magn Reson Mater Phy
measures and calculated values to verify the estimate. We used copper foil with a thickness of 35 lm as a reference. For the calculated resistance per square we began with a solid foil and then corrected for the sparseness of the material by using the measured light attenuation as an estimate of the thread density. Measurements were conducted with material samples 1 cm wide and 2 cm long, using an LCR meter (Agilent U1731A, Agilent Technologies, Santa Clara, California, United States of America) with alligator clips. This measurement method was also used to characterize the drift of resistance as a function of time. We evaluated the ratios of mesh resistance per square as compared to the 35 lm copper foil. In order to evaluate if the mesh presents additional eddy-current artifacts we collected echo planar images (EPIs). These measurements were done with a commercial coil (eight-channel transmit-receive loop array, RAPID Biomedical GmbH, Rimpar, Germany) that has a localized shield near the traces of the individual elements so that the mesh could be completely removed without altering tuning. The commercial coil was centered inside an acrylic former with a 40 cm diameter and a spherical oil phantom was used with a phantom holder provided by the coil vendor to sit in the center of the coil. EPIs were collected with the bare former, the former wrapped with the polyamide-mesh and the thinnest readily-available aluminum foil (8 lm thick, coated with a polymer coating on both sides, CLIMApac 2810, MetPro, Schwieberdingen, Germany). The static magnetic field was shimmed with no additional shielding in the bore and not altered for the subsequent images. The images are presented and artifacts were compared to background for the images at various 100 by 100 pixel region averages. As it is our intention to investigate the potential compatibility of the mesh with future integration of camerabased motion tracking we must also address the compatibility of the mesh with Moire´ pattern markers. Our research focuses on the use of retro-reflective sphere markers. As we do not have Moire´ markers we broke the question into two components that we could test. First of all, does the mesh interfere with all Moire´ patterns? To answer this question we created a Moire´ pattern target by doubling over a piece of the silver-polyamide mesh and imaged the target with
and without the mesh filters created for the light transmission analysis. The second concern is whether a marker pattern blurred by the mesh is still discernible. Therefore, we additionally targeted a reproduction of a square cm feature from a Moire´ pattern marker.
Results Bench top metrics indicate that for the present configuration the array elements have fewer losses and more symmetry with the silver-polyamide versus the copper shield (Table 1). The copper shield increases loading of the elements thereby decreasing the loaded Q. The novel shield reduces variation of bench-top metrics across the elements (Table 1: Q, k, and S12) indicating the mesh improved the symmetry of the set-up as compared to the evaluated splitcopper shield. There is more coupling between neighboring elements in the presence of the novel shield; however, the reported values are within the standard deviation of the copper shield. Transmit performance suggests that for the shields and array under evaluation the novel shield gives a slight reduction in shield losses (Table 2). The peak transmit efficiency was 0.69 lT/HW with the copper shield and 0.71 lT/HW for the mesh shield. Comparisons of average and peak efficiency correspond to a 0.25–0.3 dB (or 6–7 % power) loss by using the copper shield. The coefficient of variation for both regions of interest in the phantom indicates homogeneity is improved in the presence of the mesh (Table 2) reflecting a 6 % variation between the two conditions. Transmit efficiency maps for the two shields are shown in Fig. 2. The transmit data corroborate the bench-top metrics that indicated an improved symmetry provided by the novel shield. Box plots for transmit efficiency of the voxel populations proximal to each element vary less in the presence of the silver-polyamide shield (Fig. 3a) as compared to the split-copper shield (Fig. 3b). The first quartile, median, third quartile and whiskers of the boxplots vary more across the elements in the presence of the copper shield than in the presence of the silver-polyamide shield. A right-sided paired t test for the interquartile ranges rejects the null hypothesis (a = 0.05) that the mean
Table 1 Bench-top metrics measured with a vector network analyzer for array performance in the presence of novel (silver-polyamide) and conventional (copper) shields, indicating an array performance and variation across elements of the array Shield material
Q loaded Mean
k Stdevp
Coef var (%)
Mean
Neighbor coupling (dB) Stdevp
Coef var (%)
Mean
Stdevp
Coef var (%)
Silver-polyamide
42
0.8
2
0.031
0.001
3
-6.4
0.09
1
Copper
33
1.7
5
0.028
0.004
12
-6.7
0.35
5
123
Magn Reson Mater Phy Table 2 Scanner performance metrics for efficiency and homogeneity of the transmit fields for the 9-cm long ROI and 18-cm long ROI, with decimal places reflecting precision as indicated by repeated measures
Coronal
Silver-Polyamide
B1? per square root power (lT/HW) Mean
Stdevp
Silver-polyamide
Copper
Silver-polyamide
Copper
Silverpolyamide (%)
Copper (%)
9-cm ROI
0.31
0.30
0.10
0.10
32
34
18-cm ROI
0.30
0.29
0.11
0.11
35
37
for the array in the presence of the mesh (Fig. 4c) as compared to the copper (Fig. 4d) providing evidence of symmetry based on the MRI data.
Copper
0.7
Mesh properties 0.6
Sagittal
0.5
0.4
0.3
0.2
Transversal
Coef of var
0.1
Fig. 2 Transmit efficiency maps for the 18-cm ROI in Table 2. Small notches appear in the coronal and sagittal slices to indicate the 7 cm depth corresponding to the transversal slice and the end of the 9 cm region
interquartile range for the copper is less than or equivalent to the mean interquartile range of the elements in the presence of the mesh shield with a p value of 0.038, suggesting that the interquartile range is greater for the elements in the presence of the copper shield. This can be seen from the plots of the voxel distributions (Fig. 4) via the tighter clustering of distributions for the elements in the presence of the silver-polyamide shield (Fig. 4a) than in the presence of the copper shield (Fig. 4b). To quantify this feature of the data we created composite functions for the maximum and minimum values provided by the elements at each x-value (Fig. 4c, d) and calculated the area between the curves. There was 25 % less area between the curves
The novel shield maintains line-of-sight allowing transmission of light to the imaging target. Light transmission through the mesh and coated mesh are similar with the lacquer attenuating light by an additional 3 %. Figure 5 shows examples of gray-scale images of all three conditions as well as values for the transmission through the mesh (mean 84.5 %) and lacquered mesh (mean 81.3 %). Measurements of resistance per square of the mesh corroborated our metal thickness approximation and indicate resistance drift of the unlacquered mesh. The mesh resistance per square was measured as 100 times that of the 35 lm copper reference. Estimating the percent of area covered by metal as light attenuation (15 %) the estimate for a 2 lm thick mesh gives 110 times that of the calculated reference (35 micron-thick copper foil). The calculated value and measurement for the 2 lm mesh are in agreement. Over a 2-year period the unlacquered mesh increased resistivity by 270 %. The mesh did not provide measurable additional eddy current artifacts in the EPI images (Fig. 6). Differences between the mesh and the no-mesh condition were less than the standard deviation of the background and, therefore, not significant. On the other hand, the 8 lm aluminum shield, although only 20 cm in length and not electrically connected as a cylinder, produced artifacts with signal intensity consistently twice that of the position-matched 100 9 100 voxel regions in the mesh and no-shield conditions. Figure 7 shows the results of our investigations into the compatibility of the mesh with Moire´ markers with patterns discernible through the mesh. Although we cannot conclusively state that the mesh is compatible with Moire´ markers, we do not have evidence that allows us to conclude the contrary. The shield is therefore appropriate for future combination with optical motion tracking of a marker affixed to the head of a volunteer resulting in robust real-time motion correction.
123
Magn Reson Mater Phy
Transmit Efficiency [µT/√W]
(a) Silver-Polyamide Shield
1
0. 6
2
0. 5 0. 4 0. 3 0.2 0.1 0
Element 1
Element 2
Element 3
Element 4
(b) Split-Copper Shield
1
0.6
2
0.5 0.4 0.3 0.2 0.1
Element 1
Element 2
Element 3
Element 4
Fig. 3 Box plots of transmit efficiencies for the voxel populations in the vicinity of each element in the presence of the silver-polyamide mesh shield (a) and the split copper shield (b) representing the 1.5 %, 25 %, median (50 %), 75 % and 98.5 % quantiles for each
population. The boundaries of the 20 by 30 voxel cross-sections of the populations are superimposed on the 7-cm deep slices previously shown and labeled with corresponding element numbers
Silver-Polyamide Shield
Split-Copper Shield
(a)
(b)
1500
Voxel Count
Fig. 4 Distribution of the voxel populations for each of the elements in the presence of the mesh shield (a) and in the presence of the copper shield (b). The range of values at each x-position for the elements shown by plots of upper and lower boundaries for the element distributions in the presence of the mesh (c) and the copper (d) shields
4
3
0
Elem 1 Elem 2 Elem 3 Elem 4
1000
500
0
0
0.1
0.2
0.3
0.4
0.5
Voxel Count
Transmit Efficiency [µT/√W]
4
3
1500
1000
500
0
0.6
0
(c)
(d)
Voxel Count
1500
1000
500
0
0.1
0.2
0.3
0.4
0.5
Transmit Efficiency [µT/√W]
123
0.1
0.2
0.3
0.4
0.5
0.6
Transmit Efficiency [µT/√W]
Voxel Count
Transmit Efficiency [µT/√W]
0
Elem 1 Elem 2 Elem 3 Elem 4
0.6
1500
1000
500
0
0
0.1
0.2
0.3
0.4
0.5
Transmit Efficiency [µT/√W]
0.6
Magn Reson Mater Phy
(a) 100%
(b)
Fig. 7 Photographs of the improvised Moire´ pattern and a twodimensional reproduction of a square cm Moire´ feature from a commercially available marker. The patterns are discernible through the mesh and lacquered mesh Fig. 5 Gray scale images (with the same exposure and focal length) from which transparency of the mesh was quantified (a). The percent light transmission matrices for mesh (right) and lacquered mesh (left) with the scale bar doubling as the axis for the superimposed traces depicting the values (dark) and averages (white) of the matrix values across the middle of the matrix. For each condition three images were averaged and the element-wise ratio of the mesh (lacquered or not) to the no-mesh condition was computed to form the presented matrices
No Shield
Mesh
8 µm Aluminum
Fig. 6 EPIs collected with the commercial coil when introducing into the scanner bore no additional shield, the silver-polyamide mesh or an aluminum foil cylinder (8 lm thick, 20 cm long, 40-cm diameter). The line through the images indicates the position of the signal intensity traces plotted below. By comparing the relationship of artifact to background signal intensity, the degree of the eddy current artifact for each case can be inferred. Neither visual inspection nor calculations indicate that the addition of a mesh increased the eddycurrent artifact
Discussion We present a transparent RF coil shield and evidence that the shield does not degrade coil array performance for the configuration presented. The values for transmit efficiency presented are similar to values for loop coil transceiver arrays in literature at 7 T; for maximum: 0.54 lT/HW [16], 0.84 lT/HW (attenuation corrected; 0.45 S/m phantom) [13], and average values: 0.276 lT/HW [17]. Peak transmit efficiency was in the range of 0.7 lT/HW and the average over the 9-cm region was in the range of 0.3 lT/ HW for the arrays described (no attenuation correction).
The transparent mesh was first used for shielding in an in-bore motion tracking camera [1]. A transparent conductive oxide film connected to the aluminum shield did not sufficiently reduce noise. Therefore, the mesh was sourced as an alternate material. The mesh may be useful for other in-bore hardware requiring visual access such as an in-bore active displays for 7 T [18]. The display uses an indium tin oxide (ITO) on a glass substrate; however, the system still degrades coil performance [18]. The manufacturer-reported transmittance of the ITO glass is also 15 %, although the conductivity of the ITO is less than that of silver by an order of magnitude. Our motion-correction work has focused on retroreflective spheres for which we have seen no relevant degradation of tracking precision due to the mesh. We think it is because a uniform blurring of the marker still allows the edge detection algorithm to accurately determine the center of the marker. Blurring of the Moire´ pattern, however, may degrade algorithm accuracy and depends on the wavelength of the light used and the sensor-to-mesh distance, and can be estimated from single-slit diffraction as the distance between the first minima for a 0.7 mm slit. A limitation of the silver-coated polyamide is that the resistance increases over time due to oxidation. The mesh tolerates spray coating that prevents oxidation and only minimally alters the optical properties of the mesh. We minimize confounding variables in comparing the shields, including the use of the identical transmit and receive arrays and spacers to control for phantom position. For both shields the reflected power was less than 1 %. Differences in the capacitive values for tuning could give rise to different coil losses. However, high quality variable capacitors were placed in parallel with static capacitors to minimize the effect. The split copper shield contains over 600 solder joints and, therefore, many possible points of failure. By contrast, the mesh was sewn into a cylinder from a continuous piece of fabric with a 1-cm overlap. The resistance across the seam at 10 kHz was verified to be consistent (\10 % divergence) with that of other portions of the mesh.
123
Magn Reson Mater Phy
As the elements themselves are identical and spacing is precisely controlled, the observed increase in asymmetry is most likely attributed to the split copper shield. The capacitors of the split shield are discrete and hand soldered presenting the potential for asymmetries. Additionally, as mirror currents are dampened by the mesh that has an estimated resistance at 300 MHz of roughly 7 times that of the copper strips, there is less current and therefore less influence of induced magnetic fields on the underlying elements. In short, symmetry of the elements in the presence of the mesh shield is attributable to two possible features of the mesh, the symmetry and the reduced interactions between the mesh and the elements. The need for symmetric shields increases at high fields as far-field losses increase and asymmetric field patterns become more prevalent and become dangerous when not modeled properly. The use of shields to minimize far-field losses reduces uncertainty in power balance measurements for which far-field losses are not conventionally measured. Small errors in centering the shield about a multi-element array or asymmetries in the shield can lead to mismatches between in-vivo use and circuit simulations used to maintain power within safety constraints. We present a novel shield that allows light transmission with minimal attenuation (15 %) which is compatible with future integration of camera-based motion tracking systems. Based on the transmit performance and eddy current artifact evaluation the mesh shield is appropriate for use at 7 T and may be advantageous in cases in which space constraints increase shield losses. Acknowledgments Funding for the research was provided by the Max Planck Society. Thank you to Dr. Mikhail Kozlov, Carsten Koegler, Roland Mueller and Markus Streicher for their contributions.
3.
4.
5. 6.
7.
8. 9.
10.
11.
12.
13.
14.
15.
16.
References 1. Schulz J, Siegert T, Reimer E, Labadie C, Maclaren J, Herbst M, Zaitsev M, Turner R (2012) An embedded optical tracking system for motion-corrected magnetic resonance imaging at 7 T. Magn Reson Mater Phy 25(6):443–453 2. Qin L, van Gelderen P, Derbyshire JA, Jin F, Lee J, de Zwart JA, Tao Y, Duyn JH (2009) Prospective head-movement correction
123
17.
18.
for high-resolution MRI using an in-bore optical tracking system. Magn Reson Med 62(4):924–934 Sahabi H, Basu A (1996) Analysis of error in depth perception with vergence and spatially varying sensing. Comput Vis Image Underst 63(3):447–461 Liu W, Kao CP, Collins CM, Smith MB, Yang QX (2013) On consideration of radiated power in RF field simulations for MRI. Magn Reson Med 69(1):290–294 Vaughan JT (2006) Ultrahigh field MRI: high-frequency coils. Biol Magn Reson 26:127–161 Alecci M, Jezzard P (2002) Characterization and reduction of gradient-induced eddy currents in the RF shield of a TEM resonator. Magn Reson Med 48(2):404–407 Wald LL, Wiggins GC, Potthast A, Wiggins CJ, Triantafyllou C (2005) Design considerations and coil comparisons for 7 T brain imaging. Appl Magn Reson 29(1):19–37 Terman FE (1947) Radio engineering, 3rd edn. McGraw-Hill, Boston Carlson JW (1994) Apparatus and method for shielding MRI RF antennae from the effect of surrounding objects. U.S. Patent No. 5304932 Truhn D, Kiessling F, Schulz V (2011) Optimized RF shielding techniques for simultaneous PET/MR. Med Phys 38(7):3995– 4000 Rivera DS, Siegert T, Koegler C, Scha¨fer A, Streicher MN, Turner R. Transmit array with novel shield and fabrication technique for reducing losses at UHF. In: Proceedings of the 21st international society for magnetic resonance in medicine, Salt Lake City, p 2757 Rivera DS, Turner R. Baluns for UHF transmit arrays. In: Proceedings of the 21st international society for magnetic resonance in medicine, Salt Lake City, p 2755 Kozlov M, Turner R (2009) Fast MRI coil analysis based on 3-D electromagnetic and RF circuit co-simulation. J Magn Reson 200(1):147–152 Haase A, Odoj F, Von Kienlin M, Warnking J, Fidler F, Weisser A, Nittka M, Rommel E, Lanz T, Kalusche B, Griswold M (2000) NMR probeheads for in vivo applications. Concepts Magn Reson 12(6):361–388 Yarnykh VL (2007) Actual flip-angle imaging in the pulsed steady state: a method for rapid three-dimensional mapping of the transmitted radiofrequency field. Magn Reson Med 57(1): 192–200 Avdievich NI, Hetherington HP, Kuznetsov AM, Pan JW (2009) 7 T head volume coils: improvements for rostral brain imaging. J Magn Reson Imaging 29(2):461–465 Gilbert KM, Belliveau JG, Curtis AT, Gati JS, Klassen LM, Menon RS (2012) A conformal transceive array for 7 T neuroimaging. Magn Reson Med 67(5):1487–1496 Groebner J, Berger MC, Umathum R, Bock M, Rauschenberg J (2013) 7 Tesla compatible in-bore display for functional magnetic resonance imaging. Magn Reson Mater Phy 26(4):371–375
