A Fast 0.5 T Prepolarizer Module for Preclinical Magnetic Resonance Imaging

Abstract

We present a magnet and high power electronics for Prepolarized Magnetic Resonance Imaging (PMRI) in a homemade, special-purpose preclinical system designed for simultaneous visualization of hard and soft biological tissues. PMRI boosts the signal-to-noise ratio (SNR) by means of a long and strong magnetic pulse which must be rapidly switched off prior to the imaging pulse sequence, in timescales shorter than the spin relaxation (or 𝑇 1 ) time of the sample. We have operated the prepolarizer at up to 0.5 T and demonstrated enhanced magnetization, image SNR and tissue contrast with PMRI of tap water, an ex vivo mouse brain and food samples. These have 𝑇 1 times ranging from hundreds of milli-seconds to single seconds, while the preliminary high-power electronics setup employed in this work can switch off the prepolarization field in tens of milliseconds. In order to make this system suitable for solid-state matter and hard tissues, which feature 𝑇 1 times as short as 10 ms, we are developing new electronics which can cut switching times to ∼ 300 μs. This does not require changes in the prepolarizer module, opening the door to the first experimental demonstration of PMRI on hard biological tissues.

Prepolarized MRI of Hard Tissues and Solid-State Matter

“…The latter can ramp currents from 0 to ±260 A in ≈ 200 µs in our ≈ 600 µH load (see Fig. 3), where we were previously limited to ≈ 35 ms [10]. Figure 3 also shows a smoother transition corresponding to the case where we low-pass filter the digital output with an RC circuit of characteristic time constant ≈ 350 µs.…”

“…The "DentMRI -Gen I" 0.26 T scanner and prepolarization modules employed for this work (see Fig. 2) are described in detail elsewhere [9], [10]. Essentially, our group has designed, built and characterized a prepolarizer coil whose main parameters of inductance, resistance and efficiency are 𝐿 ≈ 600 µH, 𝑅 ≈ 75 mΩ and 𝜂 ≈ 1.9 mT/A.…”

“…The gap between the planar gradient stacks is ≈ 210 mm, placing a hard boundary on the prepolarizer module size and, consequently, to the maximum achievable coil inductance. Due to geometric limitations and to ease accessibility, we placed the prepolarizer module so that ì 𝐵 p is perpendicular to ì 𝐵 0 [10]. This reduces the maximum achievable 𝐵 t from | ì…”

“…In order to cope with the short 𝑇 1 of hard biological tissues, the high power electronics setup for the prepolarizer module has been substantially upgraded with respect to the system introduced in Ref. [10]. In the current apparatus, a digital output from the RadioProcessor-G board (SpinCore Electronics LLC) is amplified in two stages, first in a homemade variable-gain low-voltage amplifier, and then in a high power (400 A and 750 V) gradient amplifier from International Electric Co. (GPA 400-750).…”

“…This is easily met for liquids and soft biological tissues, where spin-lattice interactions are averaged out by the molecular tumbling of water, leading to relaxation times above 100 ms [11]. Indeed, PMRI has already demonstrated its potential for ex vivo and in vivo imaging of soft samples at field strengths ranging from hundreds of milli-tesla to hundreds of microtesla [10], [12], [13], [14], [15], [16]. For solid-state matter or hard biological tissues (e.g.…”