Phase contrast magnetic resonance imaging

Phase contrast can quantitatively measure stroke volume in selected regions, notably the aqueduct of Sylvius, synchronized to the heartbeat. Judicious fine-tuning of the technique is needed to achieve maximal temporal resolution, and it has limited visualization of CSF motion in many CNS regions. Phase-contrast is frequently used to evaluate those patients with suspected normal pressure hydrocephalus and a Chiari I malformation. Correlation with successful treatment outcome has been problematic. Time-spatial labeling inversion pulse, with a high signal-to-noise ratio, assesses linear and turbulent motion of CSF anywhere in the CNS. Time-spatial labeling inversion pulse can qualitatively visualize whether CSF flows between 2 compartments and determine whether there is flow through the aqueduct of Sylvius or a new surgically created stoma. Cine images reveal CSF linear and turbulent flow patterns 1)

Phase contrast MRI (PC-MRI) can be used to quantify cerebrospinal fluid flow at the level of the aqueduct of Sylvius by synchronizing the acquisition of the images with the cardiac cycle 2).

The cerebrospinal fluid flow ed and its direction can be measured non-invasively via Phase contrast magnetic resonance imaging (Cine-Contrast MR). When CSF flow is obstructed at any level, hydrocephaly occurs 3) 4).

Phase contrast imaging is an MRI technique that can be used to visualise moving fluid. It is typically used for MR venography as a non-IV-contrast requiring technique.

Spins that are moving in the same direction as a magnetic field gradient develop a phase shift that is proportional to the velocity of the spins. This is the basis of phase-contrast angiography. In the simplest phase-contrast pulse sequence, bipolar gradients (two gradients with equal magnitude but opposite direction) are used to encode the velocity of the spins. Stationary spins undergo no net change in phase after the two gradients are applied. Moving spins will experience a different magnitude of the second gradient compared to the first, because of its different spatial position. This results in a net phase shift. This information can be used directly to determine the velocity of the spins. Alternatively, the image can be subtracted from one acquired without the velocity encoding gradients to obtain an angiogram.

The PC MRI generates signal contrast between flowing and stationary nuclei by sensitising the phase of the transverse magnetisation to the velocity of motion.

Two data sets are acquired with opposite sensitisation, yielding opposite phase for moving nuclei and identical phases for stationary nuclei.

For stationary nuclei, the net phase is zero, and their signal is eliminated in the final image. However, flowing nuclei move from one position in the field gradient to another between the time of the first sensitisation and that of the second sensitisation. Because phase varies with position in the field, the net phase after subtraction of the two data sets is non-zero, and there is residual signal from flowing CSF.

When the two data sets are subtracted, the signal contribution from stationary nuclei is eliminated and only flowing nuclei are seen.

Before PC MRI data are acquired, the anticipated maximum CSF flow velocity must be entered into the pulse sequence protocol (velocity encoding (VENC)).

To obtain the optimal signal, the CSF flow velocity should be the same as, or slightly less than, the selected VENC. CSF flow velocities greater than VENC can produce aliasing artefacts, whereas velocities much smaller than VENC result in a weak signal.

The mean VENC value is 5–8 cm s−1 for standard CSF flow imaging. Low VENC values (2–4 cm s−1) can be helpful in the discrimination of communicating and non-communicating arachnoid cysts, and in the assessment of the ventriculoperitoneal shunt patency. In normal pressure hydrocephalus, significantly higher VENC values (20–25 cm s−1) should be chosen owing to hyperdynamic CSF flow within the cerebral aqueduct.

The signal initially contains phase and magnitude information. Magnitude and phase images can be generated for anatomy and velocity information, respectively. The result is that the greyscale intensity of each pixel is directly related to the velocity of CSF. Caudal flow of CSF is conventionally represented as shades of white on phase images, whereas cranial flow is by shades of black. Since it reflects the phase shifts, PC velocity image is far more sensitive to CSF flow than is the magnitude image. Two series of PC imaging techniques are applied in the evaluation of CSF flow, one in the axial plane, with through-plane velocity encoding in the craniocaudal direction for flow quantification, and one in the sagittal plane, with in-plane velocity encoding in the craniocaudal direction for qualitative assessment. Through-plane evaluation is performed in the axial oblique plane perpendicular to the aqueduct and is more accurate for quantitative analysis because the partial volume effects are minimised.

Quantitative CSF velocity and qualitative flow information can be obtained in 8–10 additional minutes in connection with routine MRI.

CSF flow is pulsatile and synchronous with the cardiac cycle, therefore cardiac gating can be used to provide increased sensitivity.

Cardiac gating can be provided with two different methods: prospective gating and retrospective gating. In retrospective gating, the computer follows the R wave and the data are acquired throughout the cardiac cycle. While the entire cardiac cycle can be sampled in retrospective gating, the prospectively gated acquisitions must be completed 100–200 ms before the next anticipated R wave. Thus, there appears to be large net flow of CSF in the systolic direction owing to partially sampled cardiac cycle in prospective gating. More accurate results can be obtained with retrospective gating when compared with prospective gating 5).

see Phase contrast magnetic resonance angiography

see Phase contrast magnetic resonance imaging for idiopathic normal pressure hydrocephalus

Yamada S, Tsuchiya K, Bradley WG, Law M, Winkler ML, Borzage MT, Miyazaki M, Kelly EJ, McComb JG. Current and emerging MR imaging techniques for the diagnosis and management of CSF flow disorders: a review of phase-contrast and time-spatial labeling inversion pulse. AJNR Am J Neuroradiol. 2015 Apr;36(4):623-30. doi: 10.3174/ajnr.A4030. Epub 2014 Jul 10. Review. PubMed PMID: 25012672.
Nitz WR, Bradley WG Jr, Watanabe AS, Lee RR, Burgoyne B, O'Sullivan RM, Herbst MD. Flow dynamics of cerebrospinal fluid: assessment with phase-contrast velocity MR imaging performed with retrospective cardiac gating. Radiology. 1992 May;183(2):395-405. PubMed PMID: 1561340.
Weerakkody RA, Czosnyka M, Schuhmann MU, Schmidt E, Keong N, Santarius T, Pickard JD, Czosnyka Z: Clinical assessment of cerebrospinal fluid dynamics in hydrocephalus. Guide to interpretation based on observational study. Acta Neurolojica Scandinavia 124(2):85-98, 2011
Yi KC, Kim HS, Hong SR, Chi JG: Absence of the septum pellucidum associated with a midline fornical nodule and ventriculomegaly: A report of two cases. J Korean Med Sci 25: 970–973, 2010
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