Synopsis

Keywords: Gradients, High-Field MRI, Ambiguity

Gradients with a conventional, bipolar design generally face a trade-off between performance, encoding ambiguity, and circumventing the latter by means of RF selectivity. This problem is particularly limiting in cutting-edge brain imaging performed at field strengths ≥ 7T and using high-performance head gradients. To address this issue, the present work proposes to fundamentally eliminate the encoding ambiguity in head gradients by using a unipolar z-gradient design that takes advantage of the signal-free range on one side of the imaging volume. This concept is demonstrated by design of a unipolar high-performance head gradient and simulated 7T imaging with such a system.

Introduction

Gradient coils for the z-axis traditionally follow the principle of a Maxwell pair, comprising two sections that generate field of the same spatial structure but opposite polarity¹. The two field lobes superimpose to form a bipolar field with a zero in the iso-centre and a surrounding linear range. Outside this range the field reaches maximum excursions, beyond which it gradually drops back to zero, causing ambiguity of gradient encoding. To prevent the related backfolding, the unambiguous range must be made sufficiently long¹. However, unambiguous range comes at great expense in terms of gradient performance for given amplifier and PNS constraints². Therefore, the gradient range required to prevent backfolding is commonly contained by limiting the spatial coverage of RF transmission and detection.
While long-established and successful for clinical whole-body systems, this approach to gradient ambiguity is less favourable for cutting-edge brain imaging, which often relies on field strengths of 7T and beyond where RF fields (for 1H) are less contained. At the same time, it demands ever-higher gradient performance, which is increasingly implemented through head-only gradients. These, in turn have intrinsically smaller unambiguous range^3-14 and high-field imaging with head gradients has indeed been reported to suffer from backfolding^15-18. Maximizing their unambiguous range of head gradients again comes at the expense of performance.
To address this issue, the present work proposes an alternative approach to gradient ambiguity. It takes advantage of the fact that, when imaging the head, ambiguity is of concern only in the trunk and body and thus only on one side of the imaging volume. Gradient encoding without any ambiguity can thus be performed with a unipolar rather than bipolar field, effectively reducing the Maxwell pair to one of its halves. This concept is demonstrated by design of a unipolar head gradient and simulated 7T imaging with such a system. Magnetic and electrical measurements on coils built according to this design confirm feasibility and competitive specifications.

Methods

The ambiguity issue and the proposed concept for its solution for a head gradient are illustrated in Figure 1. The ambiguity of a conventional, bipolar design is eliminated by generating a unipolar z-gradient field. This leads to an additional field offset which requires appropriate modulation and demodulation as in off-centre imaging with conventional gradients.
To demonstrate the ambiguity effect and support the design procedure, MRI simulations were performed using full 3D signal encoding and image reconstruction with gradient non-linearity correction, based on calculated gradient fields and an experimentally determined signal source (see Figure 1).
Using the proposed concept, a head gradient was designed for seamless integration into a Philips 7T Achieva system after removal of the body gradient and for operation with a standard environment including a dual-mode amplifier (max. 720 A, 1300 V). The target specifications were: free bore diameter 390 mm, linearity volume (LV) 220x220x200 mm³, nonlinearity £ 20%, strength up to 200 mT/m, slew rate up to 560 mT/m/ms, and duty cycle 100 % (i.e. continuous operation at maximum strength as e.g. required for zero-TE sequences¹⁹). The design employs a conical opening to achieve appropriate patient access¹³. To contain current density, a system of double layers of conductors was chosen and optimised to achieve force, torque, and impedance balancing. Cooling is based on a combination of hollow and solid conductors and monitored with 32 fibre-optic temperature sensors. In addition, a full set of shims up to 3^rd order was integrated.

Results

The imaging simulations in Figure 2 demonstrate that with a conventional, bipolar design, signals from the trunk are aliased into the LV with considerable amplitude, in particular in the centre. Using a unipolar gradient, this artefact is eliminated.
Figure 3 shows an illustration of geometrical design and optimised layer structure of the unipolar gradient design pursued in this work. The gradient coils according to this design were manufactured (Figure 4).
Figure 5 shows results of measurements of electromagnetic properties of the gradient, confirming the targeted unipolar field characteristics and its efficiency.

Discussion

Simulation based on real RF characteristics at 7T has confirmed both the backfolding problem and its solution by a unipolar gradient design. One consequence is greater maximum field strength. That is offset, however, by the fact that the maximum occurs only on one side and well outside the subject where PNS is not an issue. For the neck and shoulders, the unipolar approach is expected to reduce PNS relative to conventional designs (c.f. Figure 1). However, potentially increased PNS at the top of the head needs to be evaluated. As evident from the gradient specifications reached here, the higher field maximum does not prevent clearly competitive performance.
Notably, gradient systems with offsets have also been proposed for other purposes such as displacing the slice position²⁰, shifting the LV ^6,21, or in fringe field MRI ²².
The reported gradient design has been confirmed by magnetic and electrical measurements on assembled coils. The prospect of full gradient systems based on this sort of design holds promise for advanced neuroimaging that demands high gradient performance. It will make the greatest difference at 7T and beyond where the absence of ambiguity removes a key concern in terms of RF behaviour and instrumentation.

Acknowledgements

No acknowledgement found.

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