Analysis of Magnetic Resonance Imaging
Analysis of Magnetic Resonance Imaging
Object. A pilot study was performed to assess noninvasively the change in intracranial compliance (ICC) and intracranial pressure (ICP) in patients with Chiari I malformation who undergo foramen magnum decompression. The working hypothesis was that the main effect of the decompressive surgery is a change in ICP. Noninvasive cine phase-contrast magnetic resonance (MR) imaging is a motion-sensitive dynamic MR imaging technique that allows for visualization and quantitation of tissue motion and flow. The authors' group has used dynamic phase-contrast MR imaging to visualize and quantify pulsatile blood and cerebrospinal fluid (CSF) flow in the craniospinal system.
Methods. A system approach has been used to characterize the hemodynamic-hydrodynamic coupling in the craniospinal system and to derive measures for ICC and ICP. Magnetic resonance imaging-based ICC and ICP values are derived from the ratio of the volume and pressure changes that occur naturally during each cardiac cycle. The authors conducted a prospective study of four patients, three of whom were studied before and after decompressive surgery; significant change in MR imaging-derived ICC and ICP values was documented in only one of the three surgically treated patients. A significant change in the dynamics of the intracranial volume change (ICVC) during the cardiac cycle, however, was observed in all three patients. In healthy individuals the ICVC waveform usually consists of the following sequence: monotonic increase in intracranial volume (ICV) during the systolic phase due to increased blood inflow, monotonic decrease in ICV caused by the onset of CSF outflow into the spinal canal, and increase in the venous outflow. A nonmonotonic decline in the ICVC waveform has been observed in all patients with headaches, and a relatively normal waveform was found in those without headaches or whose headaches were resolved or alleviated by the surgery. A "partial-valve" mechanism is proposed as an explanation for the abnormal ICVC dynamics. The monotonic decline in ICVC is interrupted by a "premature" reduction in the CSF outflow. This may be caused by a displacement of the hindbrain into the cervical spinal canal during the systolic phase. This obstructs the CSF flow at the later part of the systolic phase such that the ICV does not continue its gradual decline. Postsurgery, the ICVC waveforms presented a more normal-appearing ICVC dynamics profile.
Conclusions. Magnetic resonance imaging measurement of transcranial CSF and blood flow may lead to a better understanding of the pathophysiology of Chiari malformations and may prove to be an important diagnostic tool for guiding for the treatment of patients with Chiari I malformation.
Magnetic resonance imaging studies obtained in patients with Chiari I malformations have traditionally been confined to anatomical images. Further insight into the pathophysiology of the Chiari I malformation has been acquired using phase-contrast MR imaging of the cervical CSF and spinal cord. This modality allows visualization and quantification of CSF flow through the foramen magnum region. Reduced CSF flow during systole and diastole has been documented in patients with a Chiari I malformation. After decompression of the foramen magnum in these patients, the CSF flow through this region increases while the peak CSF pulse pressure decreases. In patients with Chiari I malformations and syringomyelia, this change in CSF flow across the foramen magnum correlates with a decrease in size of the syrinx.
Phase-contrast MR imaging studies were used to elucidate the progress of syringomyelia associated with Chiari I malformations by the action of the cerebellar tonsils, which partially occlude the subarachnoid space at the foramen magnum and act as a piston on the enclosed spinal subarachnoid space. This creates a pressure wave that compresses the spinal cord and propagates CSF within the syrinx caudally, with each heartbeat leading to syrinx progression. Therefore, a decompressive suboccipital craniectomy, a C-1 laminectomy, and duraplasty eliminate this mechanism, resulting in resolution of syringomyelia in most cases. It has also been suggested that a limited occipital craniectomy, C-1 laminectomy, and dural opening without subsequent duraplasty is a safe and effective treatment. Some authors have reported that a subset of patients with Chiari I malformation and syringomyelia can be treated with the removal of bone without the need for dural opening. In contrast, other authors recommend either foramen magnum decompression and placement of a syringosubarachnoid shunt or placement of a syringo-subarachnoid shunt alone as the optimal surgical treatment for this patient population. The clinical decision and the understanding of this etiology are further complicated by reports of instances of spontaneous resolution of the syrinx.
The role of increased ICP is not well established in patients with Chiari malformations. Our research group has developed an MR imaging-based method for measurement of ICC and ICP. Intracranial compliance and ICP may be important for characterization of the intracranial hydrodynamics in patients with a variety of neurological problems including Chiari I malformations. The MR imaging technique utilizes the pulsatile changes in the ICV and ICP that occur naturally during the cardiac cycle. The elastance (the inverse of compliance) is estimated by the ratio of these changes and is linearly related to absolute CSF pressure through the elastance curve. The total ICVC during the cardiac cycle is derived from net transcranial blood and CSF volumetric flow rates. The pressure change is estimated from the pulsatile CSF pressure gradient waveform that is calculated from the CSF velocities determined by phase-contrast MR imaging.
In this report, a system approach is applied in the analysis of the MR imaging studies obtained in patients with Chiari I malformation. In this approach, the craniospinal system is represented by a nearly rigid cranial compartment connected to a more compliant spinal canal through a CSF flow channel. The CSF flow between the cranium and the spinal canal is dynamically modulated by a partial obstruction (partial valve) at the level of the foramen magnum. Cerebrospinal fluid pulsation originates from pulsatile blood flow and is modulated by the mechanical compliance of the craniospinal system. The net arterial inflow into the cranium is the sum of arterial inflow through the four main arteries, the left and right ICAs and VAs. The net venous outflow is the venous flow through the jugular veins and, when present, a secondary flow channel through the epidural veins. The proposed compartmental model of the craniospinal system is shown in Figure 1.
(Enlarge Image)
. Compartmental model of the craniospinal system includes the nearly rigid cranium, the compliant spinal canal, and the channels of arterial, venous, and CSF flow.
The goal of this preliminary investigation is to apply a system approach analysis to phase-contrast MR imaging studies obtained in patients with Chiari I malformation before and after undergoing decompressive surgery. We present a correlation between our initial findings and the patients' symptoms and suggest a possible connection between the two.
Object. A pilot study was performed to assess noninvasively the change in intracranial compliance (ICC) and intracranial pressure (ICP) in patients with Chiari I malformation who undergo foramen magnum decompression. The working hypothesis was that the main effect of the decompressive surgery is a change in ICP. Noninvasive cine phase-contrast magnetic resonance (MR) imaging is a motion-sensitive dynamic MR imaging technique that allows for visualization and quantitation of tissue motion and flow. The authors' group has used dynamic phase-contrast MR imaging to visualize and quantify pulsatile blood and cerebrospinal fluid (CSF) flow in the craniospinal system.
Methods. A system approach has been used to characterize the hemodynamic-hydrodynamic coupling in the craniospinal system and to derive measures for ICC and ICP. Magnetic resonance imaging-based ICC and ICP values are derived from the ratio of the volume and pressure changes that occur naturally during each cardiac cycle. The authors conducted a prospective study of four patients, three of whom were studied before and after decompressive surgery; significant change in MR imaging-derived ICC and ICP values was documented in only one of the three surgically treated patients. A significant change in the dynamics of the intracranial volume change (ICVC) during the cardiac cycle, however, was observed in all three patients. In healthy individuals the ICVC waveform usually consists of the following sequence: monotonic increase in intracranial volume (ICV) during the systolic phase due to increased blood inflow, monotonic decrease in ICV caused by the onset of CSF outflow into the spinal canal, and increase in the venous outflow. A nonmonotonic decline in the ICVC waveform has been observed in all patients with headaches, and a relatively normal waveform was found in those without headaches or whose headaches were resolved or alleviated by the surgery. A "partial-valve" mechanism is proposed as an explanation for the abnormal ICVC dynamics. The monotonic decline in ICVC is interrupted by a "premature" reduction in the CSF outflow. This may be caused by a displacement of the hindbrain into the cervical spinal canal during the systolic phase. This obstructs the CSF flow at the later part of the systolic phase such that the ICV does not continue its gradual decline. Postsurgery, the ICVC waveforms presented a more normal-appearing ICVC dynamics profile.
Conclusions. Magnetic resonance imaging measurement of transcranial CSF and blood flow may lead to a better understanding of the pathophysiology of Chiari malformations and may prove to be an important diagnostic tool for guiding for the treatment of patients with Chiari I malformation.
Magnetic resonance imaging studies obtained in patients with Chiari I malformations have traditionally been confined to anatomical images. Further insight into the pathophysiology of the Chiari I malformation has been acquired using phase-contrast MR imaging of the cervical CSF and spinal cord. This modality allows visualization and quantification of CSF flow through the foramen magnum region. Reduced CSF flow during systole and diastole has been documented in patients with a Chiari I malformation. After decompression of the foramen magnum in these patients, the CSF flow through this region increases while the peak CSF pulse pressure decreases. In patients with Chiari I malformations and syringomyelia, this change in CSF flow across the foramen magnum correlates with a decrease in size of the syrinx.
Phase-contrast MR imaging studies were used to elucidate the progress of syringomyelia associated with Chiari I malformations by the action of the cerebellar tonsils, which partially occlude the subarachnoid space at the foramen magnum and act as a piston on the enclosed spinal subarachnoid space. This creates a pressure wave that compresses the spinal cord and propagates CSF within the syrinx caudally, with each heartbeat leading to syrinx progression. Therefore, a decompressive suboccipital craniectomy, a C-1 laminectomy, and duraplasty eliminate this mechanism, resulting in resolution of syringomyelia in most cases. It has also been suggested that a limited occipital craniectomy, C-1 laminectomy, and dural opening without subsequent duraplasty is a safe and effective treatment. Some authors have reported that a subset of patients with Chiari I malformation and syringomyelia can be treated with the removal of bone without the need for dural opening. In contrast, other authors recommend either foramen magnum decompression and placement of a syringosubarachnoid shunt or placement of a syringo-subarachnoid shunt alone as the optimal surgical treatment for this patient population. The clinical decision and the understanding of this etiology are further complicated by reports of instances of spontaneous resolution of the syrinx.
The role of increased ICP is not well established in patients with Chiari malformations. Our research group has developed an MR imaging-based method for measurement of ICC and ICP. Intracranial compliance and ICP may be important for characterization of the intracranial hydrodynamics in patients with a variety of neurological problems including Chiari I malformations. The MR imaging technique utilizes the pulsatile changes in the ICV and ICP that occur naturally during the cardiac cycle. The elastance (the inverse of compliance) is estimated by the ratio of these changes and is linearly related to absolute CSF pressure through the elastance curve. The total ICVC during the cardiac cycle is derived from net transcranial blood and CSF volumetric flow rates. The pressure change is estimated from the pulsatile CSF pressure gradient waveform that is calculated from the CSF velocities determined by phase-contrast MR imaging.
In this report, a system approach is applied in the analysis of the MR imaging studies obtained in patients with Chiari I malformation. In this approach, the craniospinal system is represented by a nearly rigid cranial compartment connected to a more compliant spinal canal through a CSF flow channel. The CSF flow between the cranium and the spinal canal is dynamically modulated by a partial obstruction (partial valve) at the level of the foramen magnum. Cerebrospinal fluid pulsation originates from pulsatile blood flow and is modulated by the mechanical compliance of the craniospinal system. The net arterial inflow into the cranium is the sum of arterial inflow through the four main arteries, the left and right ICAs and VAs. The net venous outflow is the venous flow through the jugular veins and, when present, a secondary flow channel through the epidural veins. The proposed compartmental model of the craniospinal system is shown in Figure 1.
(Enlarge Image)
. Compartmental model of the craniospinal system includes the nearly rigid cranium, the compliant spinal canal, and the channels of arterial, venous, and CSF flow.
The goal of this preliminary investigation is to apply a system approach analysis to phase-contrast MR imaging studies obtained in patients with Chiari I malformation before and after undergoing decompressive surgery. We present a correlation between our initial findings and the patients' symptoms and suggest a possible connection between the two.
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