Cardiac MRI at high field strengths is a potentially challenging endeavour for numerous reasons: the heart is subject to cardiac and breathing motion, necessitating ECG and navigator triggered or suspended breathing sequences to capture the beating heart. Its position deep within the body and surrounded by lung tissue renders homogenous RF signal transmission and MRI signal reception difficult.
Purpose
The purpose of this study was to perform cardiac MRI at 7-Tesla in a pig model in order to evaluate potential advantages and disadvantages specifically associated with cardiac MR imaging at this high field strength.
Materials and methods
Two fully anaesthetized and ventilated minipigs weighing 25 kg and 27 kg were placed feet first in supine position inside an 8-channel transmit/receive head coil (Rapid Biomedical, Würzburg, Germany) such that the thorax was completely covered by the sensitive region of the RF coil with the heart being centered in the middle of the coil. Scanning was performed on a 7-Tesla whole-body MRI system (Magnetom 7 T, Siemens Medical Solutions, Erlangen, Germany). Cardiac function along standard views (short and long axis, 4-chamber, 2-chamber, LVOT) was evaluated using ECG-triggered TrueFISP retro (TR/TE 4.4/2.2 ms; FOV 280 × 228 mm2; matrix 512 × 432; slice 4 mm; bandwidths (BW) 650 Hz/pixel; flip 30°, 25 phases per RR-interval) and Cine FLASH sequences (TR/TE 6.0/2.6 ms; FOV 280 × 228 mm2; matrix 256 × 208; slice 4 mm; BW 650 Hz/pixel; flip 15°, 25 phases per RR-interval). Additionally, myocardial tagging was performed in conjunction with FLASH sequences. Image quality was visually assessed for signal homogeneity and myocardium-to-blood contrast.
Results
The fully anesthetized and ventilated animals could successfully be examined under stable cardiac conditions (heart rate 60–90 bpm) over the full length of the experiments (8 hours). Cardiac MRI at 7-Tesla was successful in both animals. While TrueFISP images were degraded by banding artifacts and the resulting signal inhomogeneities, especially at the heart/lung tissue interface (Fig. 1A), the FLASH sequence provided excellent imaging quality with good signal homogeneity over almost the entire heart and with high myocardium-to-blood contrast (Fig. 2). The achieved spatial resolution was 1.1 × 1.1 × 4 mm3. Additional use of tagging RF pulses with the FLASH sequence provided high-contrast tagging grids on the myocardium that persisted over the full length of the cardiac cycle (Fig. 1C).
Figure 1
Short axis views: (A) TrueFISP, (B) FLASH, (C) FLASH plus tagging grid.
Figure 2
Cine FLASH images of the pig heart: (A) short axis, (B) 4-chamber, (C) LVOT, (D) LVOT 2nd plane (E) aortic valve.
Conclusion
This study can be considered an initial step towards cardiac imaging in high-field MRI. While TrueFISP images seemed to be susceptible to B0 inhomogeneities and thus were severely degraded by artifacts, the FLASH sequence provided excellent image quality, contrast, and spatial resolution for evaluation of cardiac function. Transfer of these initial animal results into human cardiac MRI will strongly depend on the RF technology available.
