Arjen Lindenholz

50 CHAPTER 3 Our 3T protocol The 3T MRI systems at our center (Achieva 3.0T, Philips Healthcare, Best, The Netherlands) were recently equipped with a 32-channel phased array SENSE head coil. Previously, we used an 8-channel head coil. We clinically use a 3D T 1 - weighted Volumetric Isotropically Reconstructed Turbo Spin-Echo Acquisition (VIRTA) sequence (adapted from Qiao et al.) 17,42 that is acquired after contrast administration only (gadobutrol, Gadovist 1.0mmol/mL, single dose, adjusted to patient weight; Bayer Schering Pharma, Newbury, England). The images are acquired at an anisotropic spatial resolution of 0.5 x 0.5 x 1.0 mm 3 that is subsequently reconstructed to 0.5 x 0.5 x 0.5 mm 3 isotropic resolution. We have chosen this particular spatial resolution, which is considerably below the ‘ideal’ spatial resolution of 0.18 x 0.18 x 0.18 mm 3 mentioned above, because of the limitations regarding acquisition time and SNR. Also, in a recent small study, we found that sequences with a lower spatial resolution (0.5 mm x 0.5 mm x 1.0 mm 3 ) and a short scan duration (4 minutes 39 seconds) have a good subjective quality score and good performance with respect to lesion detection. 39 An anisotropic sequence was chosen for two main reasons: (a) In our experience, assessment of the in-plane slices, planned in a transverse oblique plane to image the circle of Willis in the classic anatomical way ( Figure 1 ), most often will suffice for detection of larger lesions; and (b) we mainly use this sequence to detect vessel wall enhancement, which is less dependent on partial volume effects. In this regard, an anisotropic voxel size can in our opinion be a good compromise with an increase in SNR and shorter acquisition times. However, the above-mentioned drawbacks of anisotropic sequences always need to be taken into account when assessing the images. In the VIRTA sequence, black blood is obtained by means of low flip-angle refocusing pulses. To optimize CSF suppression, we use the anti-driven-equilibrium (ADE) method. 17,33,39 Because CSF suppression is not optimal using this method, we have currently started implementing DANTE for better CSF suppression. Because the FOV of the sequence is restricted in the through-plane direction – measuring 45 mm thickness – care must be taken to plan the FOV correctly ( Figure 1 ). Key parameters of our 3T protocol are shown in Table 1 .