Imaging of cerebrospinal fluid flow: fundamentals, techniques, and clinical applications of phase-contrast magnetic resonance imaging

Abstract

Cerebrospinal fluid (CSF) is a dynamic compartment of the brain, constantly circulating through the ventricles and subarachnoid space. In recent years knowledge about CSF has expended due to numerous applications of phase-contrast magnetic resonance imaging (PC-MRI) in CSF flow evaluation, leading to the revision of former theories and new concepts about pathophysiology of CSF disorders, which are caused either by alterations in CSF production, absorption, or its hydrodynamics. Although alternative non-invasive techniques have emerged in recent years, PC-MRI is still a fundamental sequence that provides both qualitative and quantitative CSF assessment. PC-MRI is widely used to evaluate CSF hydrodynamics in normal pressure hydrocephalus (NPH), Chiari type I malformations (CMI), syringomyelia, and after neurosurgical procedures. In NPH precisely performed PC-MRI provides reliable clinical information useful for differential diagnosis and selection of patients benefiting from surgical operation. Patients with CMI show abnormalities in CSF dynamics within the subarachnoid space, which are pronounced even further if syringomyelia coexists. Another indication for PC-MRI may be assessment of post-surgical CSF flow normalisation. The aim of this review is to highlight the significance of CSF as a multifunctional entity, to outline both the physical and technical background of PC-MRI, and to state current applications of this technique, not only in the diagnosis of central nervous system disorders, but also in the further clinical monitoring and prognosis after treatment.

Keywords: Chari malformation; cerebrospinal fluid; normal pressure hydrocephalus; phase-contrast MRI; syringomyelia.

Figure 1

Phase shift during the application of bipolar gradient. 1. Stationary spins (blue line) acquire the same amount of negative phase shift and positive phase shift as the bipolar gradient is applied. 2. Spins moving in the opposite direction (yellow line) acquire negative phase shift because they are more prone to negative lobe of the bipolar gradient – they are more exposed to a higher magnetic field in the opposite direction. 3. Spins moving in the same direction (pink line) acquire higher positive phase shift as the positive lobe of the bipolar gradient is applied – they are exposed to the increased magnetic field with the direction of the flow

Figure 2

Reliance between phase shift and velocity encoding value. In the first example VENC equal to 20 cm/s will influence flow of –10 cm/s and +10 cm/s, causing phase shift –90o and +90°, respectively. In the second example analogous changes are visible; however, because of a faster flow, the acquired phase shift is more pronounced. In the third example, the velocity of 25 cm/s exceeds the VENC value, thus exceeding a 180° phase shift (red arrow) – it causes an aliasing artefact. This flow would be indistinguishable from a flow of 15 cm/s in the opposite direction

Figure 3

Magnitude (A) and phase (B) images. The magnitude image – flow is represented as bright signal, while stationary tissues are shown as a black background (signal is suppressed). The phase image – direction of the flow is encoded due to phase shift of flowing spins. The forward flow is represented as a bright signal, contrary to the reverse flow represented as a dark signal. Stationary background tissues are mid-grey

Figure 4

Flux curve and parameters of flow derived from the phase-contrast images. Region of interest (ROI) was placed in the perpendicular intersection of the aqueduct (red arrow). Images were acquired with velocity encoding value (VENC) = 12 cm/s. The flux curve represents the flow plotted against cardiac cycle. The values were obtained and calculated from the ROI. Several parameters can be calculated from the selected ROI. Stroke volume, mean velocity, and peak velocity are mostly used for quantitative comparison, with the stroke volume being the most comprehensive

Figure 5

Aliasing artefact in the phase image (yellow arrow). Flow velocity in the centre of the aqueduct is higher in comparison to selected velocity encoding values. Underestimation of velocity encoding leads to incorrect mapping of the flow in the centre, as a flow in the opposite direction (black point)

Figure 6

Aliasing artefact in the flow curve. 1. Red line represents data after post-processing, which is why aliasing artefact is no longer present and cerebrospinal fluid flow curve is corrected. 2. Light blue line represents unprocessed data. Flow velocity exceeds velocity encoding value and +/–π rephasing; as a result, it is mapped as flow in the opposite direction

Figure 7

Morphological changes in normal pressure hydrocephalus. Ventriculomegaly with pronounced dilatation of the frontal and temporal horns (left image) and a narrow callosal angle (right image)

Figure 8

Periventricular high T2 signal on T2-weighted imaging. Example of magnetic resonance imaging findings in patients with normal pressure hydrocephalus

Figure 9

McRae line (yellow line). Line connecting the basion and the opisthion on a sagittal brain image serves as a reference for assessment of tonsillar ectopia in Chiari malformation