Opportunities in Interventional and Diagnostic Imaging by Using High-Performance Low-Field-Strength MRI

Abstract

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

Assessment of cardiac function, blood flow and myocardial tissue relaxation parameters at 0.35 T

“…Myocardial T2 and T2* measured at 1.5 and 3 T agreed with previous studies, 31,53,56,58–60 and were the longest at 0.35 T. Myocardial T2 was higher by 20% and 47% at 0.35 T with respect to 1.5 and 3 T, while T2* at 0.35 T was higher by 27% and 75% compared with 1.5 and 3 T, respectively. Longer myocardial T2 and T2* at lower B0 also agrees with the recent findings reported by Campbell‐Washburn et al at 0.55 T 21 …”

“…Based on this relationship, T1 at 0.35 T would be 41% and 53% lower than at 1.5 and 3 T, respectively; our results agreed with these predicted values. An earlier study at 0.15 T determined normal myocardial T1 to be 352 ms, 55 while Campbell‐Washburn et al recently demonstrated an ~ 30% decrease in myocardial T1 at 0.55 T compared with 1.5 T (701 vs. 990 ms); 21 our findings at 0.35 T agree with this trend. T1 values measured at 1.5 and 3 T were similar to previous reports using the same MOLLI T1 mapping technique 56,57 …”

“…Techniques requiring contrast agent injection, including MR angiography, first‐pass perfusion, postcontrast T1 mapping and late gadolinium enhancement, were not evaluated in this study. While contrast agent relaxivity is expected to be higher at lower field, 21,63 the significantly shorter native T1 may reduce the relative enhancement that these techniques rely on. The effect on postcontrast blood‐myocardium signal intensities and sensitivity to pathological tissue changes such as myocardial fibrosis, edema and inflammation at lower field strengths needs to be evaluated further 23,64–67 .…”

“…A combined radiation therapy‐MRI system such as this could facilitate serial cardiotoxicity assessment along with radiation therapy, 68 and electrophysiology and image‐guided cardiac radiation for ablation of ventricular tachycardia 69 . High‐performance, low field strength systems could also benefit MRI‐guided cardiovascular catheterization procedures with fewer device‐related susceptibility artifacts and safer use of metallic interventional devices 21 …”

“…These positive attributes of low field would be advantageous for certain patient groups such as children with congenital heart defects who require cardiac MR evaluation as part of surgical planning and ongoing post‐interventional monitoring 20 . Interventional MR procedures are also gaining popularity, and conventional interventional devices may be safer at lower field 21 . There is a growing number of adult patients with larger body size, and patients with cardiovascular disease having implanted passive devices such as stents, and active devices such as pacemakers, defibrillators and ventricular assist devices.…”