No abstract

information without the use of ionizing radiation, rendering itself an advanced diagnostic, research tool. When a subject is placed within a strong magnetic field (B 0 ), the spins of MRI-active nuclei (e.g., 1 H) have the tendency to align with B 0 and precess at a frequency that scales with the field strength. [2] MRI is based on the observation of the relaxation of these nuclei after their excitation by a radiofrequency (RF) pulse using a transmit (T x ) coil. The excitation results in the deviation of these spins from their original orientation with respect to B 0 . Upon relaxation, the spins will return to their original orientation, inducing a voltage in a receive (R x ) coil (Figure 1A,B). An R x coil circuit consists of the principal metal inductor in which the signal is induced; two capacitors, one for impedance matching (C M ) and the other for tuning the resonance frequency (C T ), and an additional inductor in series with a diode making up the active detuning circuit to decouple between the transmit and receive phases of the RF coil (Figure 1C). [3] MR image quality is determined by its resolution, the contrast, the absence of artifacts, and the signal-to-noise ratio High signal-to-noise ratio (SNR) is crucial to obtaining high-quality magnetic resonance (MR) images. However, a poor fit of fixed-size radiofrequency (RF) coils to the subject often limits the SNR both in research and clinical magnetic resonance imaging (MRI) practice. Therefore, there is an urgent need to fabricate RF coils that exhibit a close geometrical fit (or are subject conformal) to the to-be-imaged region. A range of 3D printing methods are proposed for producing such conformal coils and overcoming constraints in geometrical complexity, production time, and cost. Laser powder bed fusion and stereolithography-based methods are explored. The fully digital workflow allows for the seamless integration of electromagnetic simulations of geometrically complex coils, resulting in rapid design iterations. SNR gains up to 68% are observed for single 3D-printed subject-conformal coils compared to a state-of-the-art commercially available (nonconformal) coil array. In addition to tests on phantoms, a conformal 3D-printed coil is used to image the metacarpophalangeal joint of the thumb from a volunteer on an MRI scanner to demonstrate the improved image quality.

Additive manufacturing methods are cost and time efficient methods, in particular, for the fabrication of subject-specific MRI hardware components. A novel powder-binder jetting method was developed to deposit variable concentrations of magnetic ink at precise pre-calculated positions to sculpt B0 towards a target distribution via the passive response of the 3D printed shims. Discrete spherical harmonic terms were printed as a proof-of-concept towards the manufacturing of subject-specific passive shims. Stereolithography (SLA) and Laser Powder Bed Fusion (L-PBF) manufacturing methods were developed for the fabrication of subject-specific RF coils. Significant gains in image quality, scan time and subject comfort were observed.