Hydrocephalus is usually defined as ventriculomegaly with sulcal effacement and can be classified as communicating, non- communicating or Normal Pressure Hydrocephalus. Communicating Hydrocephalus is implied if the level of the obstruction is distal to the 4th ventricle while non communicating hydrocephalus is implied if the level of obstruction is cranial to the 4th ventricular outflow. Normal Pressure hydrocephalus is a special sub-class of hydrocephalus with disproportionate ventricular dilatation relative to sulcal prominence.
All causes of hydrocephalus are typically treated by shunt procedures, with external shunt procedures like Ventriculo-peritoneal (other shunts include ventriculo-pleural, ventriculo-atrial and lumbo-peritoneal shunts) and internal drainage procedures like endoscopic third ventriculostomy (ETV). External shunt procedures are usually considered the first line of surgical management, while ETV is specifically used in obstructive hydrocephalus, multiple external shunt failures and may also be used in Normal pressure hydrocephalus.
Normal pressure hydrocephalus is a subtype of chronic acquired hydrocephalus due to hyperdynamic circulation with a typical clinical triad of gait abnormalities (shuffling, wide-based or \”magnetic\” gait), dementia and urinary incontinence. Subtypes of Normal pressure hydrocephalus include primary or iNPH (idiopathic NPH) and secondary NPH (usually due to sclerosis of the arachnoid granulations), the latter has best response to shunting.
The diagnosis of iNPH is complicated by an initial incomplete clinical triad of symptoms (Gait disturbances > Dementia> Urinary disturbance ), and near normal CSF stroke volume, going on to elevated CSF pressures (at which stage shunt surgery is most beneficial) and finally after a plateau, a reduction in CSF stroke volume.
Basic techniques for evaluation are the 2D anatomic images, (T1W, T2W and FLAIR images in multiple planes) or 3D T2W anatomic images (CISS, FIESTA or DRIVE according to the vendor).
MRI of CSF flow dynamics directly involves qualitative and quantitative evaluation, most commonly using time-resolved 2D phase contrast MRI with velocity encoding. In the pre-operative setting, it helps in answering the questions of why and where the obstruction to flow occurs, as CSF Flow studies are more sensitive than conventional 2D or 3D images in visualization very low flows.
It can also be used to evaluate post-operative cases to either prove the satisfactory functioning of internal or external shunts, or for serial follow up in progressive cases (with inoperable tumors).
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.
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.
(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-
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.
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.
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
Proper evaluation of hydrocephalus on MRI needs a basic understanding of physiology of CSF flow dynamics as well the physics behind the MRI protocols. Correct imaging plan including the protocol parameters and planes of imaging is as important as the knowledge of imaging presentations to avoid misinterpretation and misdiagnosis.
The EPOS can be viewed here: http://dx.doi.org/10.26044/ecr2019/C-2799
