Highly accelerated real-time T₂-weighted imaging with through-time radial GRAPPA and low-latency GPU reconstruction

Background

T₂-weighted cardiac images are commonly used for edema detection [1–4]. However, neither black-blood TSE nor cine images can offer real-time edema monitoring, and are therefore not suitable for the guidance of cardiac ablation procedures. We proposed a radial T₂-weighted interrupted balanced SSFP (rT₂W-iSSFP), a real-time high temporal resolution sequence targeted at monitoring edema.

Methods

Sequence

rT₂W-iSSFP generates T₂-weighting with a series of 180° RF pulses. TE-effective for the radial sequence is defined as the time from the beginning of the train to the median imaging echo. rT₂W-iSSFP also incorporates through-time radial GRAPPA to achieve high temporal resolution with high degrees of acceleration (R = 8) [5], (Figure 1) which was implemented on a 48-core hybrid system with a GPU (Tesla C1060, NVIDIA), achieving 10-20 fps image acquisition with 20 ms latency reconstruction and image display [6].

Figure 1

Illustration of the acquisition scheme, flip angles and k-space sampling pattern of rT ₂ W-iSSFP. Four-fold acceleration is shown in this example. Images were reconstructed using through-time radial GRAPPA with a low-latency implementation. Typically a 192 × 192 matrix and an acceleration rate of R = 8 is used.

Simulations

Bloch equation simulations were performed to evaluate the T₂-weighting and the image quality of rT₂W-iSSFP using a variant of the Shepp-Logan phantom containing 3 ellipsoids with different T₁s and T₂s to represent cerebrospinal fluid (CSF), liver, and myocardium (Figure 2a) [7, 8]. T₂-weighted turbo spin echo (T₂W-TSE) was also simulated [3, 9]. TE-effective was 60 ms for both T₂W-TSE and rT₂W-iSSFP.

Figure 2

(a) Illustration of the Shepp-Logan phantom used for simulation. (b) T₂W-TSE and (c) r T₂W-iSSFP were simulated. Swine with acute injury was scanned by (d) breath-hold T₂W-TSE and (e) free-breathing r T₂W-iSSFP as well. The edema (the arrows in d) is depicted in both d and e. In both simulation and in-vivo imaging, T₂W-TSE was used as the reference of T₂-weighting. T_acq - acquisition time.

Animal Model

Swine with acute injury (N = 2) were imaged on a 1.5T scanner (Avanto, Siemens, Germany). Free-breathing ECG-triggered single-shot rT₂W-iSSFP was acquired (TE-effective = 80 ms; TR = 3 ms; matrix = 192 × 192; 3 slices per heartbeats). ECG-triggered, breath-held T₂W-TSE (TE = 80 ms, resolution = 1 × 1 mm², matrix = 192 × 192) was used as a reference.

Results

The results from simulation of T₂W-TSE and rT₂W-iSSFP are shown in Figures 2b and 2c. The intensity difference between CSF and liver is similar in T₂W-TSE and rT₂W-iSSFP. Streaking artifacts are seen in Figure 2c, but these are not pronounced in in vivo images. Four-chamber views of swine heart from T₂W-TSE (breath-hold) and rT₂W-iSSFP (free-breathing) are shown in Figures 2d and 2e. Edema at the anteroseptal region due to acute myocardial infarction is depicted (the arrows in d).

Conclusions

rT₂W-iSSFP offers high temporal resolution T₂-weighted imaging with image quality sufficient for visualization of edema from acute injury. rT₂W-iSSFP can be applied to real-time monitoring of edema formation during cardiac interventions.