Initial results of cardiac imaging at 7 Tesla

Abstract

This work reports preliminary results from the first human cardiac imaging at 7 Tesla (T). Images were acquired using an eight-channel transmission line (TEM) array together with local B(1) shimming. The TEM array consisted of anterior and posterior plates closely positioned to the subjects' thorax. The currents in the independent elements of these arrays were phased to promote constructive interference of the complex, short wavelength radio frequency field over the entire heart. Anatomic and functional images were acquired within a single breath hold to reduce respiratory motion artifacts while a vector cardiogram (VCG) was used to mitigate cardiac motion artifacts and gating. SAR exposure was modeled, monitored, and was limited to FDA guidelines for the human torso in subject studies. Preliminary results including short-axis and four-chamber VCG-retrogated FLASH cines, as well as, short-axis TSE images demonstrate the feasibility of safe and accurate human cardiac imaging at 7T.

Figure 1

(a) Half of the eight channel TEM array used for cardiac imaging. (b) Axial FLASH image showing the approximate placement of the eight channels.

Figure 2

Vector cardiogram traces of the aV_F channel acquired (a) outside (approximately 0.5 T) and (b) inside the 7 T whole body magnet. The red arrows show the accuracy of the gating in spite of the severe magneto-hydrodynamic effect.

Figure 3

Available B₁⁺ field are predicted for (a) equal phased current elements and and (c) optimally phased current elements for the heart region, given the same RF transmit pulse. (b) and (d) show the low resolution GRE images for cases (a) and (c) respectively. Available B₁⁺ is normalized to 1 if in a voxel all the individual B₁⁺ vectors generated by each coil array element are in phase add constructively.

Figure 4

SAR is calculated for (a) B₁⁺ shimmed elements, and (b) when all elements have equal transmit phase. The shimming method used for this simulation is described by (19,20,22). For a given average flip angle over the heart, a significant increase in RF transmit power and consequential SAR is required when B₁ shimming is not used.

Figure 5

Gradient echo images of the heart with plots of the SNR along the indicated oblique line for (a) array elements equally phased, (b) array elements opposite phased, and (c) B₁⁺ shimmed. (d) is a line plot through the chest showing the changes in SNR in the heart for the three different cases (a–c).

Figure 6

Four time points from FLASH cine along the (a) short axis and (b) four–chambered view. Both sets of cines were collected in one breath hold, approximately 20 sec, with spatial resolution of 1.8mm × 1.8mm × 5.5 mm.

Figure 7

(a) FLASH cine and (b) TSE images of the short axis from the same subject.