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Abstract
BackgroundMaintaining the quality of magnetic resonance images acquired with the current implantable coil technology is challenging in longitudinal studies. To overcome this challenge, the principle of 'inductive overcoupling' is introduced as a method to tune and match a dual coil system. This system consists of an imaging coil built with fixed electrical elements and a matching coil equipped with tuning and matching capabilities. Overcoupling here refers to the condition beyond which the peak of the current in the imaging coil splits.MethodsThe combined coils are coupled inductively to operate like a transformer. Each coil circuit is electrically represented by equivalent lumped-elements. A theoretical analysis is given to identify the frequency response characteristics of the currents in each coil. The predictions from this analysis are translated into experiments and applied to locally image rat spinal cord at 9.4 T using an implantable coil as the imaging coil and an external volume coil as the matching coil.ResultsThe theoretical analysis indicated that strong coupling between the coils divides the resonance peaks on the response curves of the currents. Once these newly generated peaks were tuned and matched to the desired frequency and impedance of operation, in vivo images were acquired from the rat spinal cord at high quality and high resolution.ConclusionAfter proper implementation, inductive overcoupling provides a unique opportunity for tuning and matching the coil system, and allows reliable and repeatable acquisitions of magnetic resonance data. This feature is likely to be useful in experimental studies, such as those aimed at longitudinally imaging the rat following spinal cord injury.
Highlights
Maintaining the quality of magnetic resonance images acquired with the current implantable coil technology is challenging in longitudinal studies
We provide the theoretical basis for such an approach and demonstrate how it can successfully be utilized in practice using an implantable coil as the SC and a standard capacitively-tuned birdcage volume coil serving as the PC
Iau1mFnns2imgidv8niuvg(4×ocr5t)eh2s4smpe51ai6nmg1pm,i-atesmtr×lcaichlm8fepo5elrtaitmhnetihercmasekgneTafaoecsRxrqs/ioTsautflEhi,=rs=e3ep15di2snam5omag0lnmimt0ctoahma×lrendsvd8d/i1ei5anN0wym(EsmoaXm)fsca,=foxiomi2ral a,litmgF(hebOep)mlaVccnoao=trrarooti4xnino5aa=nll In vivo spin-echo images of spinal cord in (a) axial (b) coronal and (c) sagittal planes acquired on the day of coil implantation using the parameters TR/TE = 2500 ms/10 ms, image matrix = 128 × 256, slice thickness = 1 mm and NEX = 2, FOV = 45 mm × 45 mm for the axial, 35 mm × 85 mm for the coronal and 45 mm × 85 mm for the sagittal views
Summary
Gradient echo (GE) and SE sequences were employed to demonstrate the excitation field of the combined volume and implanted coils in large field-of-view (FOV) selected in axial, coronal and sagittal planes. High-resolution SE images were acquired in the same planes but in smaller FOV using TR/TE = 2500 ms/10 ms, image matrix = 128 × 256, slice thickness = 1 mm and NEX = 2, FOV = 15 mm × 20 mm for the axial, 24 mm × 33 mm for the coronal and 15 mm × 33 mm for the sagittal views These scans were repeated on days 7 and 14 of the coil implantation to show the repeatability of the scans at the same level of imaging quality. Additional scans were performed on a uniform phantom, consisting of a 15 cc plastic tube filled with 0.9% NaCl water solution, by placing the SC on the surface of the tube
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