TECHNICAL DEVELOPMENT
MRI Measurements of Intracranial Pressure in the Upright Posture: The Effect of the Hydrostatic Pressure Gradient Noam Alperin, PhD,* Sang H. Lee, MS, and Ahmet M. Bagci, PhD Purpose: To add the hydrostatic component of the cerebrospinal fluid (CSF) pressure to magnetic resonance imaging (MRI)-derived intracranial pressure (ICP) measurements in the upright posture for derivation of pressure value in a central cranial location often used in invasive ICP measurements. Materials and Methods: Additional analyses were performed using data previously collected from 10 healthy subjects scanned in supine and sitting positions with a 0.5T vertical gap MRI scanner (GE Medical). Pulsatile blood and CSF flows to and from the brain were quantified using cine phase-contrast. Intracranial compliance and pressure were calculated using a previously described method. The vertical distance between the location of the CSF flow measurement and a central cranial location was measured manually in the mid-sagittal T1-weighted image obtained in the upright posture. The hydrostatic pressure gradient of a CSF column with similar height was then added to the MR-ICP value. Results: After adjustment for the hydrostatic component, the mean ICP value was reduced by 7.6 mmHg. Mean ICP referenced to the central cranial level was 23.4 6 1.7 mmHg compared to the unadjusted value of 14.3 6 1.8 mmHg. Conclusion: In the upright posture, the hydrostatic pressure component needs to be added to the MRI-derived ICP values for compatibility with invasive ICP at a central cranial location. J. MAGN. RESON. IMAGING 2015;42:1158–1163.
D
ue to the invasive nature of commonly used techniques for measurements of intracranial pressure (ICP), only limited data are available on normative values in the upright posture.1 Furthermore, as ICP is not commonly measured in the upright posture, no standard exists regarding reference location and how to account for the cerebrospinal fluid (CSF) hydrostatic pressure gradient. In the supine posture, ICP is measured at the level of the foramen of Monro, although, in practical terms, the external auditory meatus/ tragus is used as a marker of this reference2. This location is central on the vertical axis of the cranium and therefore represents the pressure in the center of the cranium. In the upright posture, the level of the tragus is not central to the cranium and therefore ICP measurements were made with respect to different hydrostatic references, making comparison measurements challenging. For example, Chapman et al3 and Fox et al4 referenced their invasive ICP measurements in the sitting position to a point near the foramen of Monro. Chapman et al used a telemetric pressure sensor to measure ventricular CSF pressures in nonshunted and
shunted patients of various etiologies at multiple body positions, including upright posture.3 ICP values recorded in five nonshunted patients with normal size ventricles ranged from 15 to 25 cm H2O. ICP in the shunted patients were considerably lower, 215 to 235 cmH2O, likely due to the shunt siphoning effect. Fox et al measured ventricular pressures from 17 to 214 cmH2O in 12 hydrocephalic patients prior to CSF shunting in the sitting position.4 In a more recent study, Farahmand et al5 assessed the impact of shunt valve adjustment and posture on ICP in hydrocephalic patients using the level of the shunt valve as a hydrostatic reference. Regardless of the relatively high reference point, mean ICP with ligated shunts was positive at 2 mmHg. Establishing normative ICP values in the upright posture is important for improving CSF shunting technology to better match the normal CSF physiology in the upright posture. In addition, better understanding of posture-related changes in ICP will help elucidate the ill effects of long duration exposure to microgravity and the loss of the
View this article online at wileyonlinelibrary.com. DOI: 10.1002/jmri.24882 Received Aug 21, 2014, and in revised form Feb 19, 2015. Accepted for publication Feb 20, 2015. *Address reprint requests to: N.A., Department of Radiology, University of Miami, Professional Arts Center, Suite 713, 1150 N.W. 14th St., Miami, FL 33136. E-mail: [email protected] From the and Department of Radiology, University of Miami, Miami, Florida, USA
C 2015 Wiley Periodicals, Inc. 1158 V
Alperin et al.: MRI Measurement of ICP in Upright Posture
hydrostatic pressure gradient in space, such as structural ocular and visual changes in astronauts.6 One of the few noninvasive investigations of the effect of posture on the intracranial CSF physiology and ICP in healthy individuals used an open magnetic resonance imaging (MRI) system and cine phase contrast measurements of blood and CSF flows to and from the cranium.1 ICP was estimated based on the inverse relationship between compliance and pressure.7,8 While measurements of the intracranial compliance do not require hydrostatic reference, intracranial pressure varies across the intracranial compartment because of the hydrostatic component of the CSF pressure. This work aims to add the hydrostatic CSF pressure component to MR-ICP measurements in the upright posture for derivation of the pressure at a central cranial location often used in invasive ICP measurements.
waveform calculated using the Navier-Stokes relationship between derivatives of flow velocities and pressure gradient.9
Derivation of the Intracranial Volume and Pressure Changes The volume change during the cardiac cycle is calculated using Eq. (1) and the condition described by Eq. (2), which states that in steady state and over several cardiac cycles, the total volume of blood and CSF remains unchanged as noted by the Kellie and Monro principle of a constant intracranial volume. The intracranial volume change (ICVC) waveform represents the temporary increase in intracranial volume during systole and the return to baseline during diastole. The ICVC waveform is obtained by time integration of Eq. 1:
D ICVCðtiÞ 5 ½Q A ðtiÞ – Q v ðtiÞ – Q CSF ðtiÞ Dt (1) X ½Q A ðtiÞ–Q v ðtiÞ–Q CSF ðtiÞDt50 (2) ICVCðTÞ5
0
MRI Measurements of Intracranial Pressure in the Upright Posture: The Effect of the Hydrostatic Pressure Gradient Noam Alperin, PhD,* Sang H. Lee, MS, and Ahmet M. Bagci, PhD Purpose: To add the hydrostatic component of the cerebrospinal fluid (CSF) pressure to magnetic resonance imaging (MRI)-derived intracranial pressure (ICP) measurements in the upright posture for derivation of pressure value in a central cranial location often used in invasive ICP measurements. Materials and Methods: Additional analyses were performed using data previously collected from 10 healthy subjects scanned in supine and sitting positions with a 0.5T vertical gap MRI scanner (GE Medical). Pulsatile blood and CSF flows to and from the brain were quantified using cine phase-contrast. Intracranial compliance and pressure were calculated using a previously described method. The vertical distance between the location of the CSF flow measurement and a central cranial location was measured manually in the mid-sagittal T1-weighted image obtained in the upright posture. The hydrostatic pressure gradient of a CSF column with similar height was then added to the MR-ICP value. Results: After adjustment for the hydrostatic component, the mean ICP value was reduced by 7.6 mmHg. Mean ICP referenced to the central cranial level was 23.4 6 1.7 mmHg compared to the unadjusted value of 14.3 6 1.8 mmHg. Conclusion: In the upright posture, the hydrostatic pressure component needs to be added to the MRI-derived ICP values for compatibility with invasive ICP at a central cranial location. J. MAGN. RESON. IMAGING 2015;42:1158–1163.
D
ue to the invasive nature of commonly used techniques for measurements of intracranial pressure (ICP), only limited data are available on normative values in the upright posture.1 Furthermore, as ICP is not commonly measured in the upright posture, no standard exists regarding reference location and how to account for the cerebrospinal fluid (CSF) hydrostatic pressure gradient. In the supine posture, ICP is measured at the level of the foramen of Monro, although, in practical terms, the external auditory meatus/ tragus is used as a marker of this reference2. This location is central on the vertical axis of the cranium and therefore represents the pressure in the center of the cranium. In the upright posture, the level of the tragus is not central to the cranium and therefore ICP measurements were made with respect to different hydrostatic references, making comparison measurements challenging. For example, Chapman et al3 and Fox et al4 referenced their invasive ICP measurements in the sitting position to a point near the foramen of Monro. Chapman et al used a telemetric pressure sensor to measure ventricular CSF pressures in nonshunted and
shunted patients of various etiologies at multiple body positions, including upright posture.3 ICP values recorded in five nonshunted patients with normal size ventricles ranged from 15 to 25 cm H2O. ICP in the shunted patients were considerably lower, 215 to 235 cmH2O, likely due to the shunt siphoning effect. Fox et al measured ventricular pressures from 17 to 214 cmH2O in 12 hydrocephalic patients prior to CSF shunting in the sitting position.4 In a more recent study, Farahmand et al5 assessed the impact of shunt valve adjustment and posture on ICP in hydrocephalic patients using the level of the shunt valve as a hydrostatic reference. Regardless of the relatively high reference point, mean ICP with ligated shunts was positive at 2 mmHg. Establishing normative ICP values in the upright posture is important for improving CSF shunting technology to better match the normal CSF physiology in the upright posture. In addition, better understanding of posture-related changes in ICP will help elucidate the ill effects of long duration exposure to microgravity and the loss of the
View this article online at wileyonlinelibrary.com. DOI: 10.1002/jmri.24882 Received Aug 21, 2014, and in revised form Feb 19, 2015. Accepted for publication Feb 20, 2015. *Address reprint requests to: N.A., Department of Radiology, University of Miami, Professional Arts Center, Suite 713, 1150 N.W. 14th St., Miami, FL 33136. E-mail: [email protected] From the and Department of Radiology, University of Miami, Miami, Florida, USA
C 2015 Wiley Periodicals, Inc. 1158 V
Alperin et al.: MRI Measurement of ICP in Upright Posture
hydrostatic pressure gradient in space, such as structural ocular and visual changes in astronauts.6 One of the few noninvasive investigations of the effect of posture on the intracranial CSF physiology and ICP in healthy individuals used an open magnetic resonance imaging (MRI) system and cine phase contrast measurements of blood and CSF flows to and from the cranium.1 ICP was estimated based on the inverse relationship between compliance and pressure.7,8 While measurements of the intracranial compliance do not require hydrostatic reference, intracranial pressure varies across the intracranial compartment because of the hydrostatic component of the CSF pressure. This work aims to add the hydrostatic CSF pressure component to MR-ICP measurements in the upright posture for derivation of the pressure at a central cranial location often used in invasive ICP measurements.
waveform calculated using the Navier-Stokes relationship between derivatives of flow velocities and pressure gradient.9
Derivation of the Intracranial Volume and Pressure Changes The volume change during the cardiac cycle is calculated using Eq. (1) and the condition described by Eq. (2), which states that in steady state and over several cardiac cycles, the total volume of blood and CSF remains unchanged as noted by the Kellie and Monro principle of a constant intracranial volume. The intracranial volume change (ICVC) waveform represents the temporary increase in intracranial volume during systole and the return to baseline during diastole. The ICVC waveform is obtained by time integration of Eq. 1:
D ICVCðtiÞ 5 ½Q A ðtiÞ – Q v ðtiÞ – Q CSF ðtiÞ Dt (1) X ½Q A ðtiÞ–Q v ðtiÞ–Q CSF ðtiÞDt50 (2) ICVCðTÞ5
0
