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CSF flow quantification through aqueduct

Application Tip
Springorum, Rudolf Philips Healthcare

Introduction

Performing aqueductal flow measurements can be difficult as the results sometimes seem to differ from the values mentioned in the literature. Schroeder [1] evaluates the aqueductal flow after endoscopic aqueductoplasty and presents an overview of different measurements such as end-systolic and end-diastolic peak and mean velocities and timing parameters. He didn't however capture the whole cardiac cycle and therefore only presents the systolic stroke volume. Bradley [2] evaluates CSF flow in normal-pressure hydrocephalus and explains the term stroke volume as the mean of absolute systolic and diastolic stroke volumes.

Aqueductal flow measurement

Usually the term "stroke volume" represents the net result of blood flow after one cardiac cycle. It is the amount of blood that flows through a vessel minus the amount of regurgant flow. In the evaluation of CSF flow through the aqueduct this net result is approaching zero. The CSF travels forward and backward through the aqueduct and the production of CSF is so small that it is almost impossible to measure during the small time window of a single cardiac cycle. Bradley [2] is actually correcting his measurements for any net flow results assuming that the CSF production over one cardiac cycle indeed is zero.

 

Measurement of aqueductal flow is mainly performed to evaluate the patency of the aqueduct. The term stroke volume as it is used in both articles is therefore different compared to blood flow analysis. In the context of aqueductal CSF flow the term "stroke volume" represents the amount of CSF flowing through the aqueduct in one direction averaged with the amount of CSF flowing through the aqueduct in the opposite direction. It is thus the absolute flow volume independent of its direction.

Acquisition technique

The standard protocol follows a straightforward retrospective cardiac triggered FFE. The required high spatial resolution is established by combining a small FOV with high matrix size. The voxel size approaches 0.3 mm x 0.3 mm and the velocity encoding is set at 20 cm/s. To compensate for the low signal the water-fat shift is fixed at two pixels. The high matrix size results in lengthy scan times. It is attempting to apply halfscan as proposed by Bradley but for quantitative flow measurements - where the accuracy depends on phase information - it is better not to apply half-Fourier techniques in K-space such as halfscan or partial echo.

 

The high spatial resolution and the low velocity encoding result in long TR's of approximately 35 ms or more. Taking into account that flow encoding is interleaved per TR the duration of a single cardiac phase equals at least 70 ms. At a heart rate of 60 beats per minute this allows for 14 cardiac phases only. Therefore the use of TFE to speed up image acquisition is not an option since it would only further decrease the temporal resolution.

 

With the introduction of concomitant gradient correction in release 1 and 11 there is no real need anymore for selecting regular gradient mode. Switching to default gradient mode reduces the scan time to five or six minutes.

Data analysis

Both the quantitative flow analysis packages on the Achieva and the ViewForum can be used for  data analysis. A region of interest is drawn around the aqueduct and copied to all cardiac phases. In the setup panel the units of measurement are adjusted to suit the need for slow flow applications. Velocity is expressed in mm/s, Flux in mm3/s (1 mm3 = 1 ul) and Area in mm2.
QFlow analysis package Region of interest Flow curve and results
QFlow analysis package
Region of interest
Flow curve and results

Systolic stroke volume (cranio-caudal) is presented as backward flow as it appears as black on the PCA/P images. Diastolic stroke volume (caudo-cranial) is presented as forward flow as it appears as white on the PCA/P images. The stroke volume as described by Bradley [2] (the mean of the absolute value of systolic and diastolic stroke volumes) is not directly available from the results page. The parameter absolute stroke volume is adding forward flow and backward flow together, not averaging them. So to obtain the stroke volume as described in most literature the absolute stroke volume must be divided by two.

References

[1] Schroeder HWS, Schweim C, Schweim H, Gaab MR. Analysis of aqueductal cerebrospinal fluid flow after endoscopic aqueductoplasty by using cine phase-contrast magnetic resonance imaging. J Neurosurg 2000;93:237-244

 

[2] Bradely WG Jr, Scalzo D, Queralt J, et al. Normal-pressure hydrocephalus: evaluation with cerebrospinal fluid flow measurements at MR imaging. Radiology 1996;198:523-529



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Brain, Neuro, Vascular
 

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