Table 2.
Proposed image acquisition standards for neuroimaging of small vessel disease
| Purpose* | Orientation | Target-slice thickness and in-plane resolution | Comment | |
|---|---|---|---|---|
| Minimum essential sequences—eg, for clinical or large-scale epidemiological studies, available on most MRI scanners | ||||
| T1-weighted | Important for discriminating lacunes from dilated perivascular spaces; for discriminating grey from white matter, and for studying brain atrophy | 2D axial, sagittal, or coronal | 3–5 mm, and 1 mm × 1 mm | At least one sequence in sagittal or coronal plane is helpful to visualise full extent and orientation of lesions |
| DWI | The most sensitive sequences for acute ischaemic lesions; positive for up to several weeks after cerebrovascular event | 2D axial | 3–5 mm, and 2 mm × 2 mm | Reduced signal on apparent diffusion coefficient map helps to discriminate recent lesions from old lesions |
| T2-weighted | To characterise brain structure; to differentiate lacunes from white matter hyperintensities and perivascular spaces; to identify old infarcts | 2D axial | 3–5 mm, and 1 mm × 1 mm | .. |
| FLAIR | To identify white matter hyperintensities and established cortical or large subcortical infarcts; to differentiate white matter lesions from perivascular spaces and lacunes | 2D axial | 3–5 mm, and 1 mm × 1 mm | .. |
| T2*-weighted GRE | To detect haemorrhage, cerebral microbleeds, siderosis; for measurement of intracranial volume | 2D axial | 3–5 mm, and 1 mm × 1 mm | Only reliable routine sequence for detection of haemorrhage |
| Other routine sequences, available on most MR scanners | ||||
| Proton density-weighted | To detect white matter hyperintensities, infarcts, perivascular spaces (with T2-weighted dual echo), or other pathologies | 2D axial | 3–5 mm, and 2 mm × 2 mm | Mostly replaced by FLAIR |
| MRA | To detect stenosis of vertebral, basilar, internal carotid, middle cerebral, anterior cerebral, or posterior cerebral artery, or other pathologies | Post-contrast or 3D time-of-flight for intracranial arteries | 3D, axial, coronal, sagittal reconstruction; 1 mm isotropic voxels | Only large vessels visible at 1·5 T or 3·0 T; see below for perforating arterioles |
| Sequences commonly available on commercial clinical MR scanners; at present, used more for research studies, but some techniques are increasingly used in clinical protocols | ||||
| DTI with six-gradient direction diffusion encoding | To diagnose recent infarct; measurement of mean diffusivity and fractional anisotropy | 2D axial | 3–5 mm, and 2 mm × 2 mm | More detailed characterisation than with DWI; acquisition time is double that for DWI |
| SWI or equivalent | Very sensitive to haemosiderin, measurement of intracranial volume | 2D or 3D axial | 2D: 3–5 mm, and 2 mm × 2 mm; 3D: 1 mm isotropic voxels | Enables visualisation of more cerebral microbleeds than T2*-weighted GRE imaging and is more sensitive to artifacts including motion |
| Research-only sequences; require research expertise | ||||
| Isotropic volumetric T2-weighted | To display fine detail of perivascular spaces | 3D axial | 1 mm isotropic voxels | Allows post-acquisition reformatting; could potentially replace 2D T2-weighted imaging if signal-to-noise ratio is adequate |
| Isotropic volumetric 3D T1-weighted (eg, MP-RAGE) | Provides improved global and regional volumetric brain measurements | 3D axial | 1 mm isotropic voxels | Allows post-acquisition reformatting; could potentially replace 2D T1-weighted imaging if signal-to-noise ratio is adequate |
| Isotropic volumetric FLAIR | Enables identification of white matter hyperintensities; used for imaging cortical or subcortical infarcts | 3D axial | 1 mm isotropic voxels | Allows post-acquisition reformatting; could potentially replace 2D FLAIR imaging if signal-to-noise ratio is adequate; more homogeneous CSF suppression |
| Advanced DTI with more than six-direction diffusion encoding (eg, 32 or more diffusion-encoding directions) | Provides refined and superior quantitative measurements of microscopic tissue changes | 2D axial | 3–5 mm, and 2 mm × 2 mm | Allows for tractography, connectome mapping, and more accurate measurements of mean diffusivity and fractional anisotropy |
| MTR | To detect demyelination and axonal loss | 2D axial | 3–5 mm, and 1 mm × 1 mm | Experience in acquisition and interpretation needed; involves two measurements (with and without magnetisation transfer-pulse) |
| T1 mapping | To measure water content of tissue | Axial | 3–5 mm, and 2 mm × 2 mm | Experience in acquisition and interpretation needed |
| Permeability imaging | To estimate permeability of the blood–brain barrier | Axial; sequential before and after contrast | 3–5 mm, and 2 mm × 2 mm | Intravenous contrast injection needed; involves complex image processing; methods improving rapidly |
| ASL perfusion imaging | To measure tissue perfusion; quantitative, with assumptions | 2D axial | 3–5 mm, and 2 mm × 2 mm | Complex to set up and run accurately; needs post-processing; optimum processing strategies not yet confirmed; contrast injection not needed |
| Perfusion imaging (DCE or DSC) | To semiquantitatively measure blood perfusion in tissue | 2D axial | 3–5 mm, and 2 mm × 2 mm | Needs intravenous injection of contrast agent and post-processing; optimum acquisition and processing not yet confirmed for T1 (DCE) or T2*-weighted (DSC) approaches |
| fMRI | To measure brain function in response to tasks or stimuli, or at rest for default mode networks | 2D axial | 3–5 mm, and 2 mm × 2 mm | Complex set-up, acquisition, and processing |
| QSM | To provide quantitative measures of susceptibility changes, independent of scanner or acquisition variables | 2D or 3D axial | 2D: 3–5 mm, and 2 mm × 2 mm; 3D: 1 mm isotropic voxels | Uses an SWI-like acquisition, but needs very complex post-processing methods; post-processing strategies currently under investigation |
| Microatheroma and arteriolar imaging | To visualise perforating arteriolar anatomy and atheroma | Uncertain, emerging method | Uncertain, emerging method | Promising experimental approach that needs a scanner that is more than 3·0 T |
DWI=diffusion-weighted imaging. FLAIR=fluid-attenuated inversion recovery. GRE= gradient-recalled echo. MRA=magnetic resonance angiography. DTI=diffusion tensor imaging. SWI=susceptibility-weighted imaging. MP-RAGE=magnetisation-prepared rapid acquisition with gradient echo. MTR=magnetisation transfer ratio. ASL=arterial spin labelling. DCE=dynamic contrast-enhancement. DSC=dynamic susceptibility contrast. fMRI=functional MRI. QSM=quantitative susceptibility mapping.
*
MRI at 3·0 T is preferred to 1·5 T. However, these standards are listed as minimum and essential to research-only applications. These categories are not absolute; purposes are variable, and will vary with investigators' interest, expertise, and available technology.