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

Objectives: We examined RFCA lesions within and around scar by cardiac magnetic resonance imaging (CMR) and histology. Background: Substrate modification by radiofrequency catheter ablation (RFCA) is the cornerstone therapy for ventricular arrhythmias. RFCA in scarred myocardium, however, is not well understood. Methods: We performed electroanatomical mapping and RFCA in the left-ventricles of 8 swine with myocardial infarction. Non-contrast-enhanced T1-weighted (T1w) and contrast-enhanced CMR after RFCA were compared to gross pathology and histology. Results: Of 59 lesions, 17 were in normal myocardium (NL, voltage > 1.5mV), 21 in border zone (BZ, 0.5–1.5mV), and 21 in scar (< 0.5mV). All RFCA lesions were enhanced in T1w CMR, while scar was hypo-intense, allowing discrimination between normal myocardium, scar, and RFCA lesions. With contrast-enhancement, lesions and scar were similarly enhanced and not distinguishable. Lesion width and depth in T1w CMR correlated with necrosis in pathology (both; r2=0.94, p<0.001). CMR lesion volume was significantly different in NL, BZ, and scar (397[interquartile range 301–474] mm3, 121[87–201] mm3, 66[33–123] mm3, respectively). RFCA force-time integral, impedance and voltage changes did not correlate with lesion volume in BZ or scar. Histology showed that ablation necrosis extended into fibrotic tissue in 26 lesions, and beyond in 14 lesions. In 7 lesions, necrosis expansion was blocked and redirected by fat. Conclusions: T1w CMR can selectively enhance necrotic tissue in and around scar and may allow determination of the completeness of ablation intra- and post-procedure. Lesion formation in scar is affected by tissue characteristics, with fibrosis and fat acting as thermal insulators.

BackgroundThis study demonstrates a three-dimensional (3D) free-breathing native myocardial T1 mapping sequence at 3 T.MethodsThe proposed sequence acquires three differently T1-weighted volumes. The first two volumes receive a saturation pre-pulse with different recovery time. The third volume is acquired without magnetization preparation and after a significant recovery time. Respiratory navigator gating and volume-interleaved acquisition are adopted to mitigate misregistration. The proposed sequence was validated through simulation, phantom experiments and in vivo experiments in 12 healthy adult subjects.ResultsIn phantoms, good agreement on T1 measurement was achieved between the proposed sequence and the reference inversion recovery spin echo sequence (R2 = 0.99). Homogeneous 3D T1 maps were obtained from healthy adult subjects, with a T1 value of 1476 ± 53 ms and a coefficient of variation (CV) of 6.1 ± 1.4% over the whole left-ventricular myocardium. The averaged septal T1 was 1512 ± 60 ms with a CV of 2.1 ± 0.5%.ConclusionFree-breathing 3D native T1 mapping at 3 T is feasible and may be applicable in myocardial assessment. The proposed 3D T1 mapping sequence is suitable for applications in which larger coverage is desired beyond that available with single-shot parametric mapping, or breath-holding is unfeasible.Electronic supplementary materialThe online version of this article (10.1186/s12968-018-0487-2) contains supplementary material, which is available to authorized users.