Clot-blood contrast in fast gradient-echo magnetic resonance imaging

RATIONALE AND OBJECTIVES. Contrast between clot and blood in magnetic resonance imaging (MRI) at 1.5T using fast gradient-echo pulse sequences (fast GRE), with 8 ms < TR < 20 mseconds was studied both in vitro and in clinical human deep venous thrombosis (DVT) to assess whether good contrast could be obtained at such short repetition times and at clinically relevant flow rates. METHODS. In vitro studies used an apparatus that contained flowing MnCl(2[aq]) (water adjusted with manganese chloride to have T1, T2 similar to blood) and an immobilized clot (T1, T2 similar to those in DVT) for flow velocities between 0 and 16.5 cm/sec. Seven patients with DVT were imaged with the fast GRE sequences to observe the clot-blood contrast in vivo. RESULTS. Peak contrast-to-noise ratio (CNR) was achieved using flip angles between 20° and 40° (increasing with flow velocity) with or without radiofrequency 'spoiling,' consistent with a natural spoiling effect of flow. The CNR between MnCl(2[aq]) and clot decreased less than 10% as TR was reduced 56% from 18 mseconds to 8 mseconds (30° flip angle). In four patients with nonocclusive DVT, fast GRE imaging provided good contrast while in occlusive cases (three patients) the contrast was not as good as conventional GRE sequences with longer TR values (TR = 33 mseconds). CONCLUSION. A fast GRE sequence with TR = 8 mseconds, TE = 3 mseconds, and a flip angle = 40° is a promising approach to speeding up the detection of nonocclusive clinical DVT.