Physics & Technique
CSF Flow is pulsatile & is synchronized to cardiac pulsations,
with a phase difference between cardiac pulsations and CSF pulsations,
resulting in caudad CSF flow during cardiac systole and cephalad CSF flow during cardiac diastole.
Typical CSF flow velocities are in the range of 5 to 8 cm/s.
In hyperdynamic circulations,
velocities can go up to 25 cm/s.
Time-resolved 2D phase contrast imaging with velocity encoding uses Phase encoding pulses in opposing directions,
sensitized to a velocity (called VENC- Velocity Encoding).
In all cases,
the VENC used should be equal or slightly more than the CSF flow to ensure accurate readings.
If no signal is obtained,
the VENC should be reduced in steps of 5cm/sec till signal is obtained.
If on the other hand aliasing (indicated by speckled dark and white areas centrally) is seen,
the VENC should be increased by 5cm/sec steps.
In a clinical setting,
the default oblique axial acquisitions are taken with VENC of 10,20 and 30cm/sec and modified up or down depending on the presence /absence of flow or aliasing. The direction of flow encoding is caudocranial,
with cranial flow conventionally black,
and caudal flow displayed as white.
Protocol
Pre-operative protocol
A.Conventional imaging includes
1.
Axial T2W,
coronal FLAIR,
sagittal T2W with and without flow compensation (to rule out prominent aqueductal CSF flow void,
reaching until the roof of third ventricle or posterior wall of the 4th ventricle indicating hyperdynamic CSF circulation).
2.
3D T1W axial whole brain (1.2mm)
3.
3D sagittal balanced gradient recalled images (CISS/ FIESTA or DRIVE according to vendor)
The Cine mode Phase-contrast images are taken according to the indication,
with pulse gating in two planes- in-plane & through-plane.
The two types of protocols are
A.
Aqueduct centered protocols - Communicating / Obstructive hydrocephalus/ NPH.
Mid Sagittal PC images are the in-plane images,
and oblique transverse images perpendicular to the proximal 1/3rd aqueduct are the through-plane images.
For the aqueduct of Sylvius,
the in-plane is the sagittal plane acquisition,
used only for qualitative evaluation,
while the through-plane is the oblique transverse plane that is used for quantitative evaluation.
B.
Special protocol for post-op cases
- (ETV) Centered protocol - Sag PC images and the 3D Sag T2 images are used to look for flow in cine flow loops.
Oblique axial PC images are used to quantify stroke volume.
- Ventriculoperitoneal (VP) shunt. Oblique Axial in plane images are used to view cine flow in loops.
Oblique sagittal / coronal through-plane images are used to quantify flow just distal to the tip of the VP shunt.
Alternately oblique transverse through-plane,
perpendicular to the aqueduct is used to evaluate residual flow through the aqueduct in case of an obstructed shunt.
3 sets of images are obtained-
- Re-phased image- background is visible
- Magnitude image- background is suppressed
- Phase image- these are the important images for analysis.
Here background is seen as mid-grey.
Cephalad flow is dark,
while caudad flow is bright.
Qualitative analysis
The in-plane Phase images are viewed in a cine loop for presence or absence of flow at the region of interest and compared with adjacent vessels (e.g.
the basilar artery) for synchronization of CSF flow with the arterial flow.
Quantitative analysis
The through-plane Phase images are loaded on a workstation and using a zoomed image,
an ROI is carefully drawn taking care that only voxels that show flow are included in the analysis.
The surrounding brain parenchyma is then marked around the target for assessing CSF flow velocity and detecting the CSF stroke volume.
Graph analysis
During acquisition,
the study first needs to be loaded and assessed for an equal area of representation of CSF systole and diastole.
CSF stroke volume = Positive flow + {Negative flow} (ignore negative symbol)/2