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

Abstract

Background Commercial low-field-strength MRI systems are generally not equipped with state-of-the-art MRI hardware, and are not suitable for demanding imaging techniques. An MRI system was developed that combines low field strength (0.55 T) with high-performance imaging technology. Purpose To evaluate applications of a high-performance low-field-strength MRI system, specifically MRI-guided cardiovascular catheterizations with metallic devices, diagnostic imaging in high-susceptibility regions, and efficient image acquisition strategies. Materials and Methods A commercial 1.5-T MRI system was modified to operate at 0.55 T while maintaining high-performance hardware, shielded gradients (45 mT/m; 200 T/m/sec), and advanced imaging methods. MRI was performed between January 2018 and April 2019. T1, T2, and T2* were measured at 0.55 T; relaxivity of exogenous contrast agents was measured; and clinical applications advantageous at low field were evaluated. Results There were 83 0.55-T MRI examinations performed in study participants (45 women; mean age, 34 years ± 13). On average, T1 was 32% shorter, T2 was 26% longer, and T2* was 40% longer at 0.55 T compared with 1.5 T. Nine metallic interventional devices were found to be intrinsically safe at 0.55 T (<1°C heating) and MRI-guided right heart catheterization was performed in seven study participants with commercial metallic guidewires. Compared with 1.5 T, reduced image distortion was shown in lungs, upper airway, cranial sinuses, and intestines because of improved field homogeneity. Oxygen inhalation generated lung signal enhancement of 19% ± 11 (standard deviation) at 0.55 T compared with 7.6% ± 6.3 at 1.5 T (P = .02; five participants) because of the increased T1 relaxivity of oxygen (4.7e-4 mmHg^-1sec^-1). Efficient spiral image acquisitions were amenable to low field strength and generated increased signal-to-noise ratio compared with Cartesian acquisitions (P < .02). Representative imaging of the brain, spine, abdomen, and heart generated good image quality with this system. Conclusion This initial study suggests that high-performance low-field-strength MRI offers advantages for MRI-guided catheterizations with metal devices, MRI in high-susceptibility regions, and efficient imaging. © RSNA, 2019 Online supplemental material is available for this article. See also the editorial by Grist in this issue.

Graphical abstract

Figure 1:

A, Reduced heating during real-time MRI is illustrated for a metallic commercial guidewire (180 cm × 0.035 inches with angled tip; Glidewire, Terumo, Tokyo, Japan). B, A balloon filled with gadolinium chelate is shown (arrowheads) during navigation of a metallic guidewire for MRI-guided right heart catheterization in a 64-year-old woman. Standard balanced steady-state free precession imaging with a partial saturation preparation was used for imaging at 0.55 T (repetition time msec/echo time msec, 4/2; flip angle, 45°; field of view, 400 mm; section thickness, 8–24 mm; 160 × 144 matrix; acceleration factor of two; partial saturation prepulse [8] flip angle, 60°). ΔT = change in temperature following 2 minutes of continuous real-time balanced steady-state free precession imaging, LPA = left pulmonary artery, MPA = main pulmonary artery, RA = right atrium, RV = right ventricle.

Figure 2:

A, Lungs show increased signal intensity at 0.55 T compared with 1.5 T because of improved field homogeneity demonstrated in a healthy 26-year-old woman and a 54-year-old woman with lymphangioleiomyomatosis (T2-weighted fast spin echo; repetition time msec/echo time msec, 4403/47; field of view, 270 × 360 mm; 480 × 640; 32 sections; section thickness, 6 mm; bandwidth, 260 Hz per pixel; respiratory triggered). B, Reduced image distortion around air-tissue interfaces is shown in two men (ages 24 years and 28 years) for real-time speech imaging, wherein the velum is not visible at 1.5 T because of off-resonance blurring (arrow; pseudo–golden angle spiral gradient echo; section thickness, 10 cm; field of view, 280 mm × 280 mm; spatial resolution, 2.2 × 2.2 mm; 11.3/0.86; sampling duration, 9.4 msec; acceleration factor of six). C, Single-shot echo-planar diffusion-weighted imaging near the bowel in a 32-year-old man shows improved boundary delineation at 0.55 T (left), and apparent diffusion coefficient map near the sinuses in a 30-year-old man shows improved delineation of the optic nerve at 0.55 T (right) (abdomen imaging at 0.55 T [left]: field of view, 313 × 380 mm; 132 × 160 matrix; b value, 50; 30 sections; section thickness, 6 mm; 2500/91; receiver bandwidth, 1200 Hz per pixel; acceleration factor of four; echo train length, 57; acquisition time, 2:19 minutes; abdomen imaging at 1.5 T [left]: field of view, 274 × 340 mm; 108 × 134 matrix; b value, 0; 31 sections; section thickness, 6 mm; 5700/59; receiver bandwidth, 2300 Hz per pixel; acceleration factor of two; echo train length, 53; acquisition time, 3:32 minutes; brain imaging at 0.55 T [right]: field of view, 229 × 229 mm; 176 × 176 matrix; b values, 0 and 1000; 20 sections; section thickness, 5 mm; 4000/80; receiver bandwidth, 860 Hz per pixel; acceleration factor of two; echo train length, 53; acquisition time, 2:38 minutes; and brain imaging at 1.5 T [right]: field of view, 229 × 229 mm; 176 × 176 matrix; b values, 0 and 1000; three averages; 21 sections; section thickness, 5 mm; 6300/89; receiver bandwidth, 1130 Hz per pixel; acceleration factor of two; echo train length, 71; acquisition time, 1:36 minutes). D, T1-weighted ultrashort echo time lung images during inhalation of 100% oxygen in a 19-year-old man show higher signal intensity, normalized to skeletal muscle, at 0.55 T (three-dimensional stack of spirals; gradient echo; 8.54/0.15 [at 0.55 T] and 6.21/0.17 [at 1.5 T]; flip angle, 20°; section thickness, 10 mm; field of view, 450 mm; 128 × 128 matrix; 16-shot spiral design; readout length, 7 msec [at 0.55 T] and 5 msec [at 1.5 T]).

Figure 3:

Signal-to-noise ratio–efficient spiral imaging applied for, A, T1-weighted neuroimaging in a 23-year-old woman (axial sections) and a 28-year-old woman (sagittal sections), and, B, balanced steady-state free precession cardiac imaging demonstrated in a 23-year-old woman. Total acquisition time was fixed, but sampling efficiency was increased with spiral acquisitions. Good image quality and signal-to-noise ratio improvements at 0.55T are observed with the spiral sampling scheme. Images were acquired with Cartesian sampling at 1.5 T and 0.55 T, and spiral-out at 0.55 T (1.5-T Cartesian spin echo: field of view [FOV], 216 × 230 mm; 240 × 320 matrix; section thickness, 5 mm; repetition time msec/echo time msec, 550/8.9; receiver bandwidth, 150 Hz per pixel; acquisition time, 1:33 minutes; 0.55-T Cartesian spin echo: FOV, 215 × 230 mm; 240 × 320 matrix; section thickness, 5 mm; 450/9.3; receiver bandwidth, 110 Hz per pixel; acquisition time, 2:20 minutes; 0.55-T spiral-out spin echo: FOV, 230 × 230 mm; 320 × 320 matrix; 500/15; section thickness, 5 mm; 24-shot spiral design; readout length, 21 msec; 1.5-T Cartesian balanced steady-state free precession: FOV, 270 × 360 mm; 140 × 256 matrix; section thickness, 8 mm; 2.79/1.2; receiver bandwidth, 1085 Hz per pixel; acquisition time, 9 seconds; 0.55-T Cartesian balanced steady-state free precession: FOV, 270 × 360 mm; 192 × 256 matrix; section thickness, 8 mm; 4.1/1.67; receiver bandwidth, 350 Hz per pixel; acquisition time, 9 seconds; 0.55-T spiral-out balanced steady-state free precession: FOV, 360 × 360 mm; 256 × 256 matrix; 8/0.86; section thickness, 8 mm; 66-shot spiral design; readout length, 6 msec; zeroth and first moment balancing of gradients).

Figure 4:

Images show common pulse sequences that were applied to study participants to illustrate routine image quality with a 0.55-T system configuration. A, T2-weighted (T2w)–turbo spin echo (TSE; left side) in a 42-year-old woman with meningioma and postcontrast T2w fluid-attenuated inversion recovery (FLAIR; right side) images in 22-year-old woman with a low-grade tumor (T2w-FSE: field of view [FOV], 229 × 229 mm; 176 × 176 matrix; eight signal averages; 20 sections; section thickness, 5 mm; repetition time msec/echo time msec, 4000/80; receiver bandwidth, 860 Hz per pixel; acceleration factor of two; echo train length, 65; acquisition time, 2:38 minutes; T2-FLAIR: FOV, 201 × 230 mm; 210 × 320 matrix; two averages; 20 sections; section thickness, 5 mm; 8000/95; inversion time, 2371 msec; receiver bandwidth, 120 Hz per pixel; echo train length, 15; acquisition time, 3:36 minutes). B, T1-weighted (T1w) and T2w-TSE images (left and right side, respectively) of the spine in a healthy 24-year-old woman (T1w-TSE: FOV, 260 × 260 mm; 240 × 320 matrix; five averages; 15 sections; section thickness, 4 mm; 396/13; receiver bandwidth, 110 Hz per pixel; acceleration factor of two; echo train length, 3; acquisition time, 5:16 minutes; T2w-TSE: FOV, 200 × 200 mm; 256 × 320 matrix; five averages; 15 sections; section thickness, 4 mm; 3200/89; receiver bandwidth, 120 Hz per pixel; echo train length, 15; acquisition time, 5:32 minutes). C, Successful fat suppression with T2w-TSE in the abdomen of a 64-year-old woman with a hepatic lesion (FOV, 308 × 380 mm; 231 × 320 matrix; four averages; 30 sections; section thickness, 6 mm; 4886/47; receiver bandwidth, 260 Hz per pixel; echo train length, 7; spectral selective fat suppression; acquisition time, 15:15 minutes). D, Cardiac balanced steady-state free precession cine (bSSFP cine) in a heathy 19-year-old man (left side), phase contrast flow measurement in a healthy 43-year-old woman (middle), and late gadolinium-chelate enhancement (LGE) images in a 42-year-old man with chronic myocardial infarction (right side) (bSSFP cine: FOV, 350 × 380 mm; 192 × 256 matrix; one signal average; section thickness, 8 mm; flip angle, 78°; 4.1/1.67; receiver bandwidth, 350 Hz per pixel; acceleration factor of two; breath-hold length, 9 seconds; phase contrast flow: FOV, 270 × 360 mm; 144 × 192 matrix; three averages; section thickness, 6 mm; flip angle, 25°; 7.6/3.71; receiver bandwidth, 200 Hz per pixel; acquisition time, 1:37 minutes; LGE: gradient echo; FOV, 300 × 400 mm; 144 × 256 matrix; one average; section thickness, 8 mm; flip angle, 25°; 8.3/3.16; receiver bandwidth, 140 Hz per pixel; breath-hold length, 12 seconds).