I n North America most clinical MRI is performed at 1.5 T or 3.0 T, and some research applications have moved to 7.0 T. High field is motivated by higher polarization, promising increased signal-to-noise ratio (SNR), and resolution. However, this causes image distortion, constrained imaging efficiency, increased specific absorption rate, and higher cost. For some applications, low field strength may offer intrinsic advantages (1,2). At low field strength, short T1 and long T2* allow more efficient pulse sequence design; imaging near air-tissue interfaces is improved by virtue of reduced susceptibility gradients; and specific absorption rate is reduced, which can diminish heating of conductive devices and implants, and can eliminate pulse sequence parameter constraints (3). Commercial lower field systems have been largely overlooked as hardware and software have improved over the last 2 decades, and therefore are not well-suited for technically demanding imaging. We developed and evaluated a custom 0.55-T MRI system equipped with contemporary

Sequences with ultrashort echo times enable new applications of MRI, including bone, tendon, ligament, and dental imaging. In this article, a sequence is presented that achieves the shortest possible encoding time for each k-space point, limited by pulse length, hardware switching times, and gradient performance of the scanner. In pointwise encoding time reduction with radial acquisition (PETRA), outer k-space is filled with radial half-projections, whereas the centre is measured single point- In MR experiments, the transversal magnetization decays with T * 2 . Standard MR sequences offer echo times (TEs) in the range of a few milliseconds for spin-echo sequences and down to 1 ms for gradient-echo sequences. Signals arising from tissues with a very short T 2 , well below 1 ms, are therefore not visible using standard sequences, as the signal has already decayed by the time of acquisition. In the image, these tissues appear dark, similar to air cavities or noise. A short T 2 can usually be found in tissue with strong couplings of solid materials like teeth, ligaments, tendons, and bones in the human body.Many regions of the human body have already been investigated with ultrashort echo time sequences. Clinical applications of sequences with ultrashort TE are used in orthopedics, dental imaging, and many other special applications. Studies not only of the knee (1), Achilles

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A physical sound pressure reduction of 81% (TSE) and 86% (GRE) for MRI patients was achieved. The results can be used to render MRI scans more patient-friendly in clinical practice, particularly for patients who are young, scared, or elderly.

PETRA enables silent, free-breathing, isotropic, and submillimeter imaging of the bronchi and lung parenchyma with high CNR and SNR and may be an alternative to CT for patients with cystic fibrosis.