Abstract
Magnetic Resonance Imaging (MRI) of hard biological tissues is challenging due to the fleeting lifetime and low strength of their response to resonant stimuli, especially at low magnetic fields. Consequently, the impact of MRI on some medical applications, such as dentistry, continues to be limited. Here, we present three-dimensional reconstructions of ex-vivo human teeth, as well as a rabbit head and part of a cow femur, all obtained at a field strength of 260 mT. These images are the first featuring soft and hard tissues simultaneously at sub-Tesla fields, and they have been acquired in a home-made, special-purpose, pre-medical MRI scanner designed with the goal of demonstrating dental imaging at low field settings. We encode spatial information with two pulse sequences: Pointwise-Encoding Time reduction with Radial Acquisition and a new sequence we have called Double Radial Non-Stop Spin Echo, which we find to perform better than the former. For image reconstruction we employ Algebraic Reconstruction Techniques (ART) as well as standard Fourier methods. An analysis of the resulting images shows that ART reconstructions exhibit a higher signal-to-noise ratio with a more homogeneous noise distribution.
Highlights
Magnetic Resonance Imaging (MRI) of hard biological tissues is challenging due to the fleeting lifetime and low strength of their response to resonant stimuli, especially at low magnetic fields
We find that Double Radial Non-Stop Spin Echo (DRaNSSE) can perform significantly better than PETRA and that Algebraic Reconstruction Techniques (ART) results in higher quality reconstructions than Fourier spectral analysis with our settings
In summary: we have demonstrated the capability of our new low-cost “DentMRI - Gen I” scanner to simultaneously image hard and soft biological tissues; we have devised a new pulse sequence (DRaNSSE) that, compared to standard short T2∗ sequences such as PETRA, yields higher signal-to-noise ratio (SNR) images and enhanced tissue contrast; and we have shown that iterative techniques (ART) outperform traditional Fourier methods in all quantified metrics, except for the computational time required for the reconstruction
Summary
“DentMRI - Gen I” (Fig. 1a) is our first-generation MRI dental scanner, designed and built to demonstrate hard tissue imaging techniques in low magnetic field settings. The right (left) column reconstructions show soft (and hard) tissues Common to both sequences are: a total scan time of ≈ 30 min, a flip angle of ≈ 90 degrees, a repetition time of 50 ms, a field of view of 44 × 52 × 42 mm[3 ], an isotropic voxel resolution of 1 mm, a sampling rate of 26 kHz (5.2 kHz for long td PETRA), and a total of 1426 radial spokes in k-space, corresponding to an undersampling factor of 5. Since the results with ART depend on the reconstruction parameters and nit , and there is room for potential improvement with respect to the results we present here, we conclude that ART is better suited for image reconstruction than Fourier analysis with our settings
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