B₁ artifact reduction in abdominal DCE‐MRI using k_T‐points: First clinical assessment of dynamic RF shimming at 3T

Abstract

1 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2018;47:1562-1571.

SmartPulse, a machine learning approach for calibration‐free dynamic RF shimming: Preliminary study in a clinical environment

“…Data acquired for a previous study on 50 consecutive subjects were reused here as the clustering set: it corresponds to subjects 1 through N S,0 = 50. For the present work, measurements were performed over a 3‐week period on N S,1 = 30 additional subjects, thus forming a training population of N train = 80 subjects.…”

“…Nine k T ‐point tailored pulses (p TKT ) were designed, under SAR and hardware constraints, as done in Tomi‐Tricot et al With the goal of using them for DCE‐MRI, a FA of = 11° was targeted, for a TR of 6 ms, and a 1300‐µs pulse duration limit was set. For local SAR prediction, virtual observation points provided by the scanner for each of the subjects were used.…”

“…Additional sequences were run on subjects from the imaging population to compare images obtained with p TF , p TSS , p TKT , and the predicted p SP . To avoid disturbing the diagnostic DCE‐MRI protocol, those sequences were only inserted before contrast agent injection and in late phase (Supporting Information Figure ), similarly to what was done in Tomi‐Tricot et al…”

“…Both passive and active RF shimming methods have been implemented to counteract artifacts, at high and ultra‐high field (UHF: 7T and above) for various imaging regions and contrast weightings. Active RF shimming makes use of parallel transmission (pTx), which is widely used on research scanners at UHF with 8 transmission channels or more, and has been made available on some clinical 3T scanners, with 2 channels.…”

“…In that case, only static RF shimming is currently proposed commercially: the user can choose between a “standard” shimming with fixed RF complex weights, and a “tailored” one exploiting the measurement of the fields from each transmit channel to adapt the coefficients to the patient in the magnet. In a previous study, the superiority of k T ‐points dynamic RF shimming was demonstrated in clinical routine for nonselective excitation of the abdomen at 3T. Indeed, although tailored static RF shimming is satisfactory for most patients, it fails to offer sufficiently homogeneous excitation in approximately 10 to 20% of the population.…”