Deep Vein Thrombosis Demonstrated by Magnetic Resonance Direct Thrombus Imaging
Magnetic resonance direct thrombus imaging (MRDTI) is a new application of magnetic resonance technology that capitalizes on the fact that thrombus is associated with a substantial reduction in T1 (spin-lattice relaxation time). A T1-weighted magnetization-prepared 3-dimensional gradient-echo sequence is used with a selective water-excitation radiofrequency pulse to abolish fat signal. The effective inversion time is chosen to null the blood signal, so that thrombus shows up as a high signal on T1-weighted images against a background of suppressed blood and fat without requiring intravenous contrast. The technique is highly accurate for deep vein thrombosis (DVT; Figure 1) and pulmonary embolism (PE) and can be performed using a standard scanner with a lower limb acquisition time of only 7 minutes.
MRDTI has 2 major advantages over conventional imaging modalities. Firstly, direct visualization of thrombus not only overcomes many of the pitfalls of alternative techniques that have either identified thrombus as a filling defect or in terms of surrogate markers, but it also provides additional information about thrombus characteristics. Secondly, simultaneous imaging of the legs and chest allows a comprehensive assessment of thrombus load, minimizing the importance of overlooked subsegmental PE (where the prognosis is that of the source), and potentially facilitating a more titrated approach to treatment.
The high T1 signal is thought to be due to formation of methemoglobin within red cells in thrombus, which has strong paramagnetic properties. Initially, this occurs subjacent to the endothelium before propagating centripetally so that recent thrombus viewed axially exhibits a characteristic target appearance (Figure 2).
The editor of Images in Cardiovascular Medicine is Hugh A. McAllister, Jr, MD, Chief, Department of Pathology, St Luke’s Episcopal Hospital and Texas Heart Institute, and Clinical Professor of Pathology, University of Texas Medical School and Baylor College of Medicine.
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