Lisette van Dam

Chapter 6 100 Since it can be difficult to distinguish an acute thrombus from other tissues with a short T1 relaxation time, e.g. fat-tissue which is present in the liver, adding a fat-suppression technique that suppresses the fat signal may help depicting thrombosis. With this technique, selective pulses cause signal from fat to be nulled (saturated) while the water signal is relatively unaffected. The Dixon technique is an often-used fat suppression technique that can be used with several sequences such as T1- and T2-weighted imaging, and gradient echo MRI 16,17 . An alternative to fat suppression methods is water excitation by means of a spectral spatial pulse. With this technique, only water is excited by using section-selective composite pulses, while lipid spins are left in equilibrium, thereby producing no signal. 18 ProSET, a water excitation technique, was successfully used in a pregnant patient with proximal iliac vein thrombosis 19 and may therefore be applicable in portal vein imaging as well. Sequence testing and optimization All three selected imaging sequences (3D T1 TFE, 3D TSE SPAIR and THRIVE) and black-blood, Dixon and ProSET techniques were tested in a healthy volunteer. Table 1. 3D T1 TFE and T1 Dixon FFE scan parameters 3D T1 TFE 3D T1 Dixon FFE Technique T1 TFE T1 FFE Orientation Coronal Coronal/Transversal Respiratory motion suppression Respiratory gating Breath hold Slices 80 160 Slice thickness (mm) 4.0 3.0 Slice distance (mm) 2.0 1.5 FOV 400 400 Voxel size (mm) 1.56 x 2.24 x 4 1.7 x 1.7 x 3.5 Scan time (ms) 02:36 00:19 Echo time (ms) 3.73 - Repetition time (ms) 7.41 3.75 Flip angle 10 15 3D T1 Dixon FFE, three-dimensional T1 fast field echo sequence; 3D T1 TFE, three-dimensional T1 turbo field echo; FOV, field of view.