The human body is mostly made up of the elements hydrogen, carbon and oxygen, including 60 % as water and variable amounts of fat (lipids). The nucleus of each hydrogen atom is a proton, which is positively charged, has the quantum property of spin and has a magnetic moment. Placing protons in the static magnetic field of the MRI unit aligns the protons so they precess, or “wobble”, about the main axis of the magnetic field. The precession frequency is directly proportional to the strength of the magnetic field according to the gyromagnetic ratio .² In magnetic resonance imaging, radio-frequency (RF) pulses, delivered at this specific precession frequency (the Larmor frequency ), are absorbed by protons. After the RF pulse stops, the protons relax to their resting alignment by emitting the absorbed RF energy. The emitted RF signal varies according to the local tissue environment and various other physical parameters (e.g. T1 weighted, T2 weighted, diffusivity and proton density).

Three orthogonal magnetic field gradients (magnetic fields that change in intensity linearly over the body), known as the slice-select, phase-encoding and frequency-encoding gradients, are used to localise the received RF signal in space. These gradients alter the strength of the magnetic field over the body, and consequently, the Larmor frequency will vary across these regions. The bandwidth of RF delivered to the protons, in the imaged structure, must therefore be sufficient to span these frequencies. After applying the RF pulse, the relaxation of protons in the varying magnetic fields results in the emission of different frequencies of released RF energy from the imaged structure. These are used to encode the RF signal in three planes (X, Y and Z). The released RF signal is received by an antenna (coil), converted from analogue to digital form and then changed mathematically, using Fourier transformation, from an intensity-and-time function to an intensity-and-frequency function. A powerful computer analyses the digital signal and calculates intensity values for each spatial location in the imaging volume. It then creates and displays grey-scale images that represent the magnetic resonance characteristics of normal and abnormal anatomical structures within the imaging volume (Hashemi et al. 2010).

Phase-contrast cine MRI of cerebrospinal fluid velocity is often referred to as a CSF flow study . This phase-contrast MRI technique delivers an RF pulse followed by sequential positive and negative magnetic gradients. These produce a net positive or negative phase change in the RF emission from moving protons but no net phase change from stationary protons (Fig. 9.2). The sign of the phase change (positive or negative) indicates the direction of fluid (proton) movement. The magnitude of the phase change indicates the velocity of moving fluid (Fig. 9.2b). Between 16 and 32 images are produced, displaying CSF velocity throughout the cardiac cycle , with each image being labelled with its temporal relation (ms) to the R wave of the electrocardiogram. When this set of images is viewed as a cine loop, the CSF oscillation, which repeats every cardiac cycle, is visualised, with CSF moving inferiorly during systole and superiorly during diastole (Enzmann and Pelc 1991) (Fig. 9.3c, d). Phase-contrast cine MRI can be used to identify regions of reduced CSF movement at the level of the foramen magnum or within the spine which may be associated with syrinx formation (Heiss et al. 1999; Oldfield et al. 1994).

T1-weighted images are anatomically accurate and clearly identify fluid within a syrinx cavity and CSF pathways (Fig. 9.3a). Measurements of the width of the CSF channels are more accurate using T1-weighted, as compared with T2-weighted, imaging (Fig. 9.3b). Although uncommon, communications between the fourth ventricle and syrinx and between the subarachnoid space and syrinx may be detected with T1-weighted imaging (Beuls et al. 1996; Bogdanov et al. 2000, 2006). Postoperative T1-weighted MRI can also reveal the location of a shunt tip within a syrinx (Schwartz et al. 1999a).

The presence of an intramedullary tumour associated with a syrinx is detected by giving intravenous gadolinium contrast, which escapes from permeable tumour capillaries and produces high signal within the lesion.

T2-weighted imaging is more sensitive to the presence of excessive fluid within CNS tissues than is T1-weighted imaging. Fluid can form and expand both the extracellular and the intracellular spaces of the CNS (Table 9.1). T2-weighted imaging may detect, in particular, a “pre-syringomyelia” state, which is spinal cord oedema from a chronic condition. If left untreated, this may eventually result in syrinx formation (Fischbein et al. 1999; Jinkins et al. 1998; Levy et al. 2000).

Fast imaging employing steady state acquisition (FIESTA )³ MRI produces high contrast between CSF and structures contained within the subarachnoid space, such as nerve roots. It uses short acquisition times to reduce motion artefacts (Chavez et al. 2005). It is particularly useful in studying syringomyelia, outlining arachnoid webs and cysts lying within the subarachnoid space.

Diffusion-weighted MRI (DWI) identifies diffusion of water molecules, which is less restricted within the extracellular than the intracellular space. This technique and its related sequences, diffusion tensor imaging (DTI) and fractional anisotropy (FA), have been used primarily as research tools in syringomyelia, to evaluate the integrity of white matter tracts (Roser et al. 2010a; Ries et al. 2000) and to identify injury in the spinal cord earlier than is possible with T1-weighted and T2-weighted images (Schwartz et al. 1999b). DWI of the spinal cord has lower resolution than brain DWI because of the smaller size of the spinal cord and the artefacts created by CSF flow and cardiac and respiratory motion around the spinal cord (Clark et al. 2000). In a patient with multiple sclerosis , a large cervical syrinx and a stable neurological deficit, DTI confirmed preservation of the white matter tracts around the syrinx (Agosta et al. 2004). In a study comparing 28 patients, with cervical syringomyelia and sensory dysfunction, to 19 normal volunteers, the patients had lower fractional anisotropy, which correlated with loss of pain and temperature sensation in their hands and prolongation of their spinothalamic conduction time on electrophysiological studies (Hatem et al. 2009).

Other MRI studies are mostly of research interest. These include volume imaging, using extremely thin (1 mm thickness) MRI slices, to improve anatomical detail and appreciation of communications between the fourth ventricle and syrinx and between the subarachnoid space and syrinx.

Phase-contrast cine MRI measures CSF motion that arises from the brain pulsations that occur during the cardiac cycle (Fig. 9.5). Brain expansion during cardiac systole results in CSF being driven caudally, from the intracranial subarachnoid cisterns into the more compliant spinal subarachnoid compartment. Relaxation of the brain then results in the cephalad movement of CSF, back across the foramen magnum, during diastole. Syrinx fluid also moves during the cardiac cycle, in response to spinal CSF pressure waves and spinal cord motion.

Cine imaging can measure both CSF and syrinx fluid velocities in a defined region of interest. In patients with hindbrain-related syringomyelia, preoperative cine phase-contrast MRI demonstrates a reduction in CSF flow at the level of foramen magnum, in both Chiari 0 and Chiari I malformations. This improves after posterior fossa decompressive procedures (Iskandar et al. 1998) (Fig. 9.5). Progressive neurological symptoms are more likely when cine MRI demonstrates movement of syrinx fluid. Stable or absent symptoms are associated with lack of fluid flow within the syrinx (Tobimatsu et al. 1991) (Fig. 9.3). Cine imaging can also be helpful in diagnosing spinal CSF flow obstructions in patients with primary spinal syringomyelia (Mauer et al. 2008) (Fig. 9.4).