1. Noninvasive Measurement of Intracranial Pressure by MRI (MR-ICP) Overview
Noam Alperin, PhD
Physiologic Imaging and Modeling Lab Department of Radiology
University of Illinois at Chicago

2. Importance
Normal brain function requires regulation of cerebral blood flow (CBF) and intracranial pressure (ICP). In many neurological problems this regulation is disrupted.
Our lab is developing noninvasive method to quantify these important physiological parameters by MRI. The method could potentially become an important diagnostic test for over million patients annually in the US alone, who suffer from related neurological problems (e.g., strokes, intracranial hemorrhages head injuries, hydrocephalus, Chiari malformations, etc…)

3. ICP Measurement by MRI
Monitoring
Diagnostic test
The MRI-based method (MR-ICP) integrates knowledge from human neuro-physiology, principles of fluid dynamics, and dynamic MRI to non-invasively measure ICP. These are briefly described in the following slides.

4. Blood and CSF Flows through the Craniospinal Compartments
The pulsatile cerebral blood flow drives the cerebrospinal fluid (CSF) flow. During systole, arterial inflow exceeds venous outflow, which result in CSF outflow1. Intracranial compliance and pressure are calculated from measurements of these flows.
1. Alperin et al, Magn Reson, in Med. 1996

5. The Neurophysiology Basis
Intracranial Pressure-Volume Curve
Intracranial pressure and volume are exponentially related.
Thus, at low ICP, a small increase in volume (dV) will cause a small increase in pressure (dP).
At high ICP, the same small volume increase (dV) will cause a large increase in pressure (dP).
The ratio of dP/dV (elastance) is a linear function of ICP.
MR-ICP measures dP and dV using CSF and blood volumetric flow rate measurements.
Elastance = dP/dV

6. The Principles of MR-ICP
MR-ICP is the noninvasive analogous of the bolus pressure-volume infusion test (Marmarou 1978)
Pulsatile cerebral blood flow causes a momentary increase in intracranial volume (dV) during systole
dV causes the pulse pressure (dP)
dV/dP is intracranial compliance
The inverse of compliance is a linear function of mean ICP.
dV and dP are measured by MR imaging of blood and CSF flow2
2. Alperin et. al. Radiology. 2000; 217 (3); 877–885.

7. How is dV Measured?
The systolic increase in intracranial volume (ICV) during each cardiac cycle is derived from momentary difference between volumes of blood, and CSF that entering and leaving the cranium during the cardiac cycle
Arterial
blood in
Venous
blood out
ICV
CSF
out and in

8. How is Volumetric Blood Flow Measured?
Phase contrast MRI scan with high velocity encoding provides velocity images from which blood flow to and from the cranium is calculated.
Velocity image

9. Blood Flow Dynamics

10. Volumetric Blood Flow Measurement
Vertebral art.
Jugular v.
Int. Carotid art.
Volumetric Flow = Velocity X Lumen area
Lumen area is identified automatically using the Pulsatility Based Segmentation (PUBS) Method 3
3. Alperin N, Lee S. Magn. Reson. in Med. 49:934–944 (2003)

11. Volumetric Arterial Inflow and Venous Outflow (in mL/min)
One cardiac cycle
Total CBF = is the cycle-averaged arterial inflow

12. How is CSF Flow Measured?
Phase contrast MRI scan with low velocity encoding provides images of CSF velocities from which CSF flow to and from the cranium is calculated.
Velocity images
Cord
CSF
Systole
Diastole

13. Cervical CSF and Epidural Venous Flow Dynamics

14. CSF Volumetric Flow
One cardiac cycle

15. Calculating Systolic ICV Change (DICV)
A - V – CSF
DICV waveform
DICV= 0.5 mL
∫dt

16. The MR-ICP Provides a Patient’s Specific Cranio-Spinal Flow Dynamics

17. How is dP Measured?  dv/dt + v v-v -  p
The pressure change during the cardiac cycle is derived from CSF pressure gradient ( p), i.e., the pressure difference that causes the CSF to flow out from and back into the cranium.4
The Navier-Stokes equation is used to calculate CSF pressure gradients from the CSF velocities.
 dv/dt + v v-v -  p
inertial force
viscous losses
4 Loth FM, Yardimici MA, Alperin N. Jour. of Biomechanical Engineering. 2001, Vol. 123, pp

18. CSF Pressure Gradient Waveform
%SD of
PTP
is <7%

19. Invasive ICP Recording and Corresponding MRI Derived Pressure Gradients in Humans
Elevated ICP
Low ICP
red = viscous term

20. Validation of dP Measurement
Validations were done with large nonhuman primates because fluid dynamics principles are scale-dependent.
0.012
0.01
0.008
0.006
Pressure gradient (mmHg/cm)
0.004
0.002 2 4 6 8 10
PTP Pressure (mmHg) )
Baboon
Human

21. Initial Validation of MR-ICP in Humans
The MR-ICP method was validated in patients with SAH who had an EVD at the time of their MRI scan.
Alperin et. al. Radiology. 2000; 217 (3); 877–885.

22. What is Normal MR-ICP ? MR-ICP in Healthy Subjects Invasive
(71 measurements in 23 subjects)
Invasive
Mean: 9.6 mmHg ± 31%
Range: 3.5 to 17.1 mmHg

23. Reduction to Practice
Total MRI scan time with the current technique is approximately 3 minutes.
A software tool is being developed to facilitate quantitation of the blood and CSF volumetric flow rates, from which total CBF, compliance, and ICP are determined within few minutes.

24. The automated lumen segmentation tool is based on the PUBS technique.3
3. Alperin N, Lee S. Magn. Reson. in Med. (2003)

25. Physiologic Imaging and Modeling Lab, Dept. of Radiology
University of Illinois at Chicago
Current and previous lab members who contributed to this work
Noam Alperin, PhD
Aaron Lee, MS
Naresh Yallapragada, MS
Hasan Dhoondia, MS
Anusha Sivaramakrishnana
RSNA 2003
Collaborators:
Terry Lichtor, MD, PhD - Neurosurgery, Cook County
Roberta Glick, MD - Neurosurgery, Cook County
Francis Loth, PhD - Mechanical Eng., Uni. of Illinois