Synopsis

Idipathic Normal Pressure Hydrocephalus (iNPH) is a debilitating diesease without known cause. The symptoms are similar to those of vascular dementia, Alzheimers disease (AD) and Parkinsons (PD), making it difficult to diagnose. iNPH can be surgically treated with good results in at least a subset of patients, making a correct diagnosis crucial to the patient. Quantitative flow measurements of the cerebrospinal fluid (CSF) through the cerebral aqueduct has been suggested as a means of supporting the diagnosis of iNPH, differentiating it from AD or other forms of dementia, and for predicting outcome of ventriculoperitoneal shunting.

Measuring aqueductal flow with MRI in iNPH patients

As iNPH is a disease of the CSF system, it is natural to look for methods that can measure properties of the CSF flow and dynamics. Studies started already thirty years ago, but with improvements in MR technology and image processing, the topic is still being pursued [1,2]. Among the published studies, three main aims can be identified.

• Finding parameters that can differentiate iNPH from dementia and AD.
• Finding parameters that can predict the outcome of ventriculoperitoneal shunting, providing a way to identify the patients with a higher chance of a positive outcome.
• Giving new insight into the CSF system and its interplay with the arterial and venous systems.

MRI has the advantages of being non-invasive and repeatable. Phase contrast MR (PC-MRI) is an imaging technique that provides quantitative measures of velocity as well as the flow dynamics over the cardiac cycle.

For PC-MRI to be reproducible and accurate, a number of error sources that have to be recognized and as far as possible amended. In all modern MR-scanners, a number of built in acquisition and processing features for PC-MRI protocols are automatically included. However, some issues remain and have to be take into account when assessing the velocity measurements in the cerebral aqueduct, a situation with small velocities and structures [3]. A few of these are listed below.

• Partial volume errors. The MR signal comes from a voxel which can contain both static tissue and moving CSF, resulting in a measured velocity that will not reflect the velocity of the CSF. Such partial volume errors are extra important for small structures and low spatial resolution.
• Phase background. A number of imperfections can affect the phase image. Regardless of the cause, the signature of background phase errors is that the measured velocity in static tissue is non-zero. Several methods exist to remove the background.
• Analysis errors. Software and manual analysis can introduce systematic errors. Low velocities and small structures make the results more sensitive to manual bias or segmentation errors.
• Parameter choice. Phase contrast MRI provides a number of measurable parameters and it is not always obvious which is the most physiologically relevant to study.

Studies find that differences in stroke volume, peak flow and pulsatility can be used to reliably differentiate iNPH from AD and vascular disease [4,5]. However, newer studies find no evidence that presurgical PC-MRI has prognostic value for the outcome of ventriculoperitoneal shunting, at least not in the parameters studied so far [6,7].

Recently, more advanced methods such as 3D-flow imaging [8] and timeSLIP techniques have been applied to iNPH patients, with some promising results. New theories around CSF dynamics are also being discussed [9-11].

Summary

Quantitative flow measurements in the cerebral aqueduct can support the diagnosis of iNPH and differentiate from other forms of dementia. There is little support in recent literature to suggest that MRI flow measurements can be used to predict which patients will benefit from ventriculoperitoneal shunting. New, more advanced forms of imaging and data analysis may reveal parameters that allow for outcome prediction or give new insights into the CSF dynamic system.

Acknowledgements

References

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