Arjen Lindenholz

48 CHAPTER 3 Isotropic versus anisotropic voxels A related question is whether and/or when to use isotropic or anisotropic voxels. Most experts in the field of vessel wall MRI sequence development prefer sequences with isotropic voxels, which render multiplanar assessment more feasible. Compared with, for example, the carotid artery, intracranial arteries are often tortuous and have varying orientations, making multiplanar reconstruction an important asset in assessment of these arteries. In anisotropic sequences, a very high in-plane spatial resolution can be achieved within reasonable acquisition times, enabling detailed assessment of vessel wall lesions and atherosclerotic plaque characterization when the FOV is placed perpendicular to the lesion. However, due to the larger through-plane voxel size, small lesions are subject to partial volume effects. Therefore, it may well be dependent on the specific clinical question on a single-patient base which type of sequence (isotropic versus anisotropic voxels) to use. For instance, a radiologist may consider isotropic sequences in patients with no previous imaging or known vessel wall lesion as a method of screening the intracranial arteries, while an anisotropic sequence could be used to assess a known lesion. The advantage of interpreting vessel wall MR images in multiple planes is currently not supported by much research data. The results of recent studies suggest that the use of transverse images alone may be sufficient for the interpretation of vessel wall MR images. 42,43 In one of these studies, 3D sequences with a high in- plane spatial resolution and a larger slice thickness were either performed in the transverse plane or perpendicular to the MCA. Vessel wall lesions of the MCA were also detected on the images that were acquired non-perpendicular to the artery. 2D versus 3D To increase in-plane spatial resolution, 2D spin-echo sequences can be used as an alternative to 3D methods. 1,28,31,44 The 2D in-plane spatial resolution is equivalent to 3D, and volume averaging errors due to thicker sections (2 mm, no gap) are alleviated by repeating the sequences in sagittal and/or axial planes targeted to a lesion in question. Sequence acquisition times are short (approximately 3 minutes) with good SNR. The drawbacks of these 2D spin-echo methods are the need to target a specific lesion due to limited coverage in order to keep imaging times short, and the increased dependence on correct positioning of the FOV. Two-dimensional methods can therefore best be applied in cases in which there is a known intracranial stenosis on which the FOV can be focused, and has highest value in characterizing the underlying pathological process (e.g., atherosclerotic plaque characterization).