Post-contrast non-selective double inversion recovery imaging of the coronary arteries in patients with coronary allograft vasculopathy

The uptake of gadolinium contrast agent in coronary walls may indicate metabolically-active atherosclerosis (Maintz et al, 2006 ) and therefore be useful in the setting of coronary allograft vasculopathy (CAV). The interpretation of inversion recovery (IR) images can be hampered by signal from tissues with longer T1 times (particularly the myocardium) as tissue suppression is T1 dependent and only optimal for one specific T1 species (e.g. blood). We sought to improve contrast-enhanced coronary vessel wall imaging using a novel non-selective double inversion recovery (NS-DIR) prepulse that provides signal suppression over a wide user-defined T1-range.


Introduction
The uptake of gadolinium contrast agent in coronary walls may indicate metabolically-active atherosclerosis (Maintz et al, 2006 ) and therefore be useful in the setting of coronary allograft vasculopathy (CAV). The interpretation of inversion recovery (IR) images can be hampered by signal from tissues with longer T1 times (particularly the myocardium) as tissue suppression is T1 dependent and only optimal for one specific T1 species (e.g. blood). We sought to improve contrastenhanced coronary vessel wall imaging using a novel non-selective double inversion recovery (NS-DIR) prepulse that provides signal suppression over a wide userdefined T 1 -range.

Methods
The NS-DIR prepulse with two time delays, TI 1 and TI 2, was implemented on a 1.5T MR scanner. TI 1 and TI 2 were optimized in MATLAB simulations by minimizing M Z NS-DIR over a user-defined T 1 -range for a given heart rate.
A T 1 -phantom containing 11 T 1 -samples (T 1 -range=120ms-1730ms) was imaged with the IR and NS-DIR pre-pulses for simulated heart rates between 45 and 105bpm. For each prepulse, the signal-to-noise ratio (SNR) was calculated for each sample.
Nine patients who had undergone heart transplantation (ages=12-17y) were imaged~20minutes after injection of 0.2ml/kg Gadobutrol using a 32-channel coil on a 1.5T 1 Division of Imaging Sciences, King's College London, London, UK Full list of author information is available at the end of the article Figure 1 Simulated M 2 values (solid lines) and phantom SNR values (data points) for a) the IR sequence (T1 set to null T1 species 340ms for different heart rates) and b) the NS-DIR sequence (TI 1 and TI 2 values optimized to minimize M 2 for a range between 200 and 1400ms for different heart rates.) N.B. The theoretical Mz values have been scaled in order to display the data on the same graph.
Peel et al. Journal of Cardiovascular Magnetic Resonance 2011, 13(Suppl 1):P241 http://jcmr-online.com/content/13/S1/P241 MR Scanner. Firstly a coronary MRA was performed followed by a targeted, free-breathing, ECG-triggered, 3D-IR segmented gradient-echo (TFE) sequence along the right and left coronary arteries. Imaging parameters included spatial-resolution=1.25x1.25x3mm, TR/TE=3.5/ 1.4ms, FA=30°and the TI was chosen to null blood from a Look-Locker sequence. Subsequently, identical planes were repeated with the IR replaced by the NS-DIR prepulse with imaging parameters maintained. Inversion times TI 1 and TI 2 were set to suppress tissues with T1 values between 200-1400ms according to the patient's heart rate. Imaging was performed every heartbeat at the mid-diastolic rest period.

Results
Simulations and phantom studies show that the IR sequence ( fig.1a) only nulls one T1 species whereas the NS-DIR sequence ( fig.1b) achieves excellent signal suppression over the desired T 1 -range.
Patient studies showed that the NS-DIR sequence ( fig.2a) achieved simultaneous suppression of the blood and myocardium. Only the areas of contrast uptake are visible, which correspond to the path of the LCA ( fig.2b). In contrast, interpretation of the IR images ( fig.2c and fig.2d) was hampered by the bright signal in the myocardium.

Conclusion
Simulations and phantom studies demonstrate that the NS-DIR sequence exhibits excellent tissue suppression over a wide T 1 -range. Preliminary patient data show improvement in contrast agent visualization in the coronary vessel walls in patients with CAV.