DOI:10.2214/AJR.05.0003
AJR 2007; 188:716-725
© American Roentgen Ray Society
Normal MRI Appearance and Motion-Related Phenomena of CSF
Christopher Lisanti1,
Carrie Carlin1,
Kevin P. Banks2 and
David Wang3
1 Department of Radiology, Wilford Hall Medical Center, Lackland AFB, TX.
2 Department of Radiology, Fort Sam Houston, MCHE-DR, 3851 Roger Brooke Dr.,
Fort Sam Houston, TX 78234.
3 Department of Radiology, University of Colorado Health Sciences Center,
Denver, CO.
Received January 4, 2005;
accepted after revision June 7, 2006.
Address correspondence to K. P. Banks
(kevin.banks{at}amedd.army.mil).
The opinions and assertions contained herein are the private views of the
authors and are not to be construed as official or as reflecting the views of
the Department of the Army, the Department of the Air Force, or the Department
of Defense.
Abstract
OBJECTIVE. The purpose of this article is to review the normal
appearance of CSF, flow physics in relation to CSF flow dynamics, and commonly
encountered appearances and artifacts of CSF due to superimposed flow
effects.
CONCLUSION. Normal CSF has inherent MRI properties of low signal
intensity on T1-weighted sequences and high signal intensity on T2-weighted
sequences. However, the normal CSF signal is frequently altered by
superimposed flow phenomena that can confound interpretation.
Keywords: CNS • CSF • MRI
Introduction
Normal CSF has long T1 and long T2 times that manifest as dark
signal on T1-weighted images and bright signal on T2-weighted images. FLAIR
imaging results in nulling and dark CSF signal. The normal CSF signal is
frequently altered by superimposed flow phenomena that can confound
interpretation [1]. This
article will review commonly encountered appearances and artifacts of CSF due
to flow effects.
CSF Physiology and Flow Dynamics
The brain and spinal cord have a total volume of about 140 mL of CSF.
Superimposed on this is an average of 500 mL per day production of CSF, with a
net CSF flow from the lateral and third ventricles, through the cerebral
aqueduct, out the fourth ventricle, and into the CSF-filled spaces of the
spinal cord and cerebral convexities
[2]. This simplification,
however, does not take into account the pulsatile to-and-fro movement of CSF
due to the expansion and contraction of the brain produced by the expansion
and contraction of the intracranial vessels associated with the cardiac cycle
[3]. This pumping can result in
CSF flow in the upper cervical spine that is approximately 40% as fast as the
flow rate seen in the internal carotid arteries
[3]. This results in
significant motion-related effects that can alter the visualized signal of the
CSF [1,
2,
4].
CSF Flow-Related Effects
CSF flow-related phenomena can be divided into two categories:
time-of-flight (TOF) effects and turbulent flow. TOF effects are further
divided into TOF loss resulting in dark CSF signal and flow-related
enhancement (FRE) producing bright CSF signal. Like TOF loss, turbulent CSF
flow results in abnormally dark signal intensity.
TOF Effects
TOF Loss
Signal formation in MRI depends on mobile protons experiencing the initial
radiofrequency pulse and the subsequent refocusing mechanism. TOF loss
typically occurs in spin-echo or fast spin-echo imaging when protons do not
experience both the initial radiofrequency pulse and the subsequent
radiofrequency refocusing pulse. TOF effects are more pronounced (darker
signal) with faster proton velocity, thinner slices, longer TE, and an imaging
plane perpendicular to flow (Figs.
1A,
1B,
1C,
1D and
2). In addition, TOF loss in
single-shot fast spin-echo (SSFSE) techniques is related to whether the pulse
sequence was acquired during systole with TOF loss or diastole without TOF
loss (Fig. 3A,
3B). Finally, gradient-recalled
echo (GRE) techniques are resistant to TOF loss because of the short TE (Fig.
4A,
4B).
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Fig. 1A —Schematic representation of factors involving occurrence and degree
of time-of-flight (TOF) losses. Portion of excited mobile protons within CSF
move out of slice volume between 90° and 180° pulse applications
(TE/2). Only those protons subjected to both pulses will yield signal.
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Fig. 1B —Schematic representation of factors involving occurrence and degree
of time-of-flight (TOF) losses. More mobile protons move out of slice volume
between 90° and 180° pulses as CSF flow velocity increases, which
results in increasing TOF losses. Same effect occurs with increasing TE
because mobile protons have more time to move out of slice volume before
application of 180° refocusing pulse.
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Fig. 1C —Schematic representation of factors involving occurrence and degree
of time-of-flight (TOF) losses. Decreasing CSF flow angle relative to imaging
plane results in lower effective velocity relative to slice volume, which
results in decreasing TOF losses. Veffective = effective velocity,
Vactual = actual velocity, COS = cosine.
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Fig. 1D —Schematic representation of factors involving occurrence and degree
of time-of-flight (TOF) losses. With increasing slice thickness, fewer mobile
protons move out of slice volume between 90° and 180° pulses, which
results in decreased TOF signal loss.
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Fig. 2 —Healthy 26-year-old female volunteer. Axial T2-weighted image
through level of lateral ventricles shows two foci of decreased CSF signal
just superior to foramen of Monro (arrows) secondary to time-of
flight losses.
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Fig. 3A —65-year-old woman with hepatitis and pancreatitis undergoing MR
cholangiopancreatography. T2-weighted single-shot fast spin-echo image
acquired during systole shows time-of-flight signal losses (arrows)
in ventral and lateral subarachnoid space.
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Fig. 3B —65-year-old woman with hepatitis and pancreatitis undergoing MR
cholangiopancreatography. Same sequence acquired during diastole shows normal
bright CSF signal surrounding thoracic spinal cord.
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Fig. 4A —71-year-old man with cervical spine pain after fall. Axial
T2-weighted image through cervical canal shows foci of signal loss in
subarachnoid space (arrows) due to to-and-fro motion of CSF and
associated time-of-flight (TOF) losses.
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Typical locations for TOF losses include the lateral ventricles just
superior to the foramen of Monro (Fig.
2), the third ventricle (Fig.
5), the fourth ventricle, and within the cervical and thoracic
spinal canal [3,
5] (Fig.
3A,
3B). Given the positive
relationship between CSF velocity and TOF losses, this effect is magnified in
individuals with an underlying abnormally hyperdynamic state such as
hydrocephalus [2,
6]. In addition, laminar flow
results in peripherally located protons moving at a slower velocity and leads
to a reduction in TOF losses (Fig.
5).
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Fig. 5 —21-month-old male infant with seizures and developmental delay.
Axial T2-weighted image through third ventricle shows darker signal centrally
(arrow) and brighter signal peripherally (arrowheads). This
is common appearance of CSF in this region due to laminar flow effects.
Laminar flow results in slower flow peripherally (less time-of-flight [TOF]
loss) and faster flow centrally (more TOF loss).
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Techniques to minimize TOF losses include using a short TE technique such
as true fast imaging with steady-state precession (true FISP) (Fig.
6A,
6B), imaging parallel to flow,
or thicker slices.
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Fig. 6A —45-year-old man with left sensorineural hearing loss. Axial
T2-weighted image at cerebellopontine angle shows signal loss anterior to
basilar artery (arrows) due to time-of-flight losses.
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FRE
FRE or "entry-slice phenomenon" results in bright signal from
unsaturated protons flowing into a slice replacing outflowing partially
saturated protons during the TR. The unsaturated protons have their full
longitudinal magnetization, which is flipped into the transverse plane by the
initial radiofrequency pulse producing a higher signal than would be expected
if they were stationary, partially saturated protons. Scanned sequentially,
the first slices of the imaging volume into which the unsaturated protons flow
show this effect most prominently, hence the term entry-slice phenomenon.
FRE can be seen on T1- and T2-weighted images and is seen on more slices
with single-slice acquisition techniques in which every slice is an entry
slice (Fig. 7). These are most
commonly seen on HASTE or SSFSE and in some GRE examinations. FRE can be
reduced by applying saturation bands or by using ultrafast techniques such as
true FISP (Fig. 8A,
8B).
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Fig. 7 —63-year-old man with vertigo. Coronal postgadolinium T1-weighted
image through third ventricle shows bright CSF signal (arrow) due to
flowrelated enhancement (FRE) on entry slice. FRE rapidly diminished on deeper
coronal slices (not shown.)
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Fig. 8A —Healthy subject. Axial T2-weighted image through upper cervical
spine shows multiple areas of abnormal bright CSF signal within periphery of
subarachnoid space (arrows) due to flow-related enhancement, No
saturation band was applied.
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Fig. 8B —Healthy subject. Axial T1-weighted image shows marked decrease in
intensity of multiple foci of bright signal (arrows) in CSF due to
application of superiorly placed saturation band. TR and TE remained
unchanged.
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FRE in FLAIR imaging represents a special case in which the FRE is not due
to unsaturated protons entering the imaging volume but due to partially
saturated protons that do not experience the 180° inversion-recovery
pulse. This results in a lack of nulling of the CSF and the subsequent
appearance of the intrinsic bright T2 signal. The same locations that are
common for TOF losses are also the most common for FLAIR FRE (Figs.
9 and
10). There are certainly some
TOF losses in this phenomenon; however, FRE usually dominates because the
inversion time (TI) is much greater than the TE (2,200 vs 165 milliseconds),
allowing more time for the protons to move during the TI than during the TE.
These effects are most profoundly seen in patients with increased CSF volumes
or flow dynamics (Figs. 11A,
11B and
12A,
12B).
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Fig. 9 —Healthy subject. Axial FLAIR image through level of lateral
ventricles illustrates high signal (arrows) seen in lateral
ventricles just superior to foramen of Monro due to flow-related
enhancement.
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Fig. 10 —19-year-old woman with headaches. Midline sagittal FLAIR image shows
flow-related enhancement (arrow) at foramen magnum initially thought
to be Chiari I malformation. Sagittal T1-weighted images (not shown) showed
normal dark CSF signal at foramen magnum.
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Fig. 11B —57-year-old man with remote history of left cerebellar astrocytoma
resection. Axial T2-weighted image shows time-of-flight (TOF) loss
(arrow) in exact location of FRE in A. This example nicely
shows importance of background signal and sequence type on whether moving CSF
protons will result in bright signal (FRE) or dark signal (TOF loss).
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Fig. 12A —8-year-old girl with history of right frontal lobe glioma resection.
Axial FLAIR image at level of lateral ventricles shows marked bright signal
(arrows) secondary to flow-related enhancement (FRE) in enlarged
right lateral ventricle and surgical defect with minimal FRE in normal-sized
left lateral ventricle.
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Troubleshooting Techniques
Either TOF loss or FRE can result in interpretive difficulties, and
although most effects are seen in the axial plane, coronal
(Fig. 7) and sagittal
(Fig. 10) flow effects are
also seen. Cross-referencing other imaging planes is helpful and comparing
with faster imaging sequences such as GRE or true FISP imaging that reduce
superimposed flow phenomena (Figs.
4A,
4B and
6A,
6B). In addition, ghosting
artifacts in the phase-encoding direction often accompany TOF- and FRE-related
signal changes and can help point out the true nature of those changes.
Turbulent Flow
Turbulent flow results in a broader spectrum of proton velocities and a
wide range of flow directions that are not seen in typical laminar flow. The
varied flow velocities and directions result in more rapid dephasing and
signal loss termed "intravoxel dephasing."
A commonly encountered CSF flow artifact is the signal void in the dorsal
subarachnoid space on sagittal T2-weighted images of the thoracic spine
(Fig. 13). This artifact is
due to a combination of the respiratory and cardiacrelated pulsatile CSF flow
superimposed on cranially directed bulk CSF flow and turbulent flow from CSF
moving from the ventral subarachnoid space to the dorsal subarachnoid space.
This complex CSF motion results in phase incoherence leading to signal
loss.
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Fig. 13 —25-year-old man with thoracic spine pain. Sagittal T2-weighted image
through thoracic spine shows globular signal loss in dorsal subarachnoid space
(arrows) resulting from turbulence and time-of-flight effects
associated with complex CSF flow.
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Turbulent flow is often seen in the aqueduct of Sylvius
(Fig. 14), the fourth
ventricle, and some disease states (Fig.
15A,
15B) and is sometimes seen
from transmitted motion from arteries
(Fig. 16). Turbulence
dephasing can be reduced by shorter TE sequences and smaller voxel volume
(decreased slice thickness or higher matrix).
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Fig. 14 —70-year-old man with normal pressure hydrocephalus. Midline sagittal
3D fast spin-echo T2-weighted image shows significant loss of signal
associated with increased velocities and turbulent flow in superior aspect of
fourth ventricle (arrows). Corresponding sagittal T1-weighted image
(not shown) shows normal dark CSF signal.
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Fig. 15A —32-year-old woman with butterfly vertebral body and scoliosis. Axial
T2-weighted image at level of thoracic cord shows significant asymmetric
time-of-flight signal losses in CSF (arrows).
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Fig. 16 —Healthy 32-year-old man. Axial T2-weighted image through cerebellum
shows focus of near-complete signal loss adjacent to basilar artery
(arrow) from transmitted turbulence from artery and some
time-of-flight (TOF) losses resulting in appearance of artifactual basilar
aneurysm. Notice also mild TOF losses in CSF (arrowheads).
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CSF Motion Artifacts
The most common source of artifact in MRI is motion of the subject. Whereas
random movement leads to blurring, periodic motion, such as with CSF
pulsation, cardiac motion, and respiratory motion, leads to ghosting artifact.
This occurs along the phase-encoding direction because phase information is
acquired over an entire scan (minutes), whereas frequency information is
acquired over a single frequency readout (milliseconds). Ghosting artifacts
become more conspicuous as the source of the ghost becomes brighter relative
to the surrounding tissues (Fig.
17A,
17B). Ghosts can either be
bright if they are in phase or dark if they are out of phase with the
background signal [7] (Fig.
18A,
18B). The number of ghosts
varies inversely to the pulsation rate of the source.
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Fig. 18A —Healthy 2-year-old boy. As TE increases, ghosting artifact becomes
more conspicuous due to increased brightness of ghosting source. Axial
proton-density-weighted image (TR/TE, 3,000/30) shows dark and bright ghosting
(arrows) due to CSF flow within fourth ventricle.
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Fig. 18B —Healthy 2-year-old boy. As TE increases, ghosting artifact becomes
more conspicuous due to increased brightness of ghosting source. By increasing
TE to 120 milliseconds, image shows dark and bright ghosting (arrows)
become more evident.
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Troubleshooting
Strategies to reduce CSF ghosting artifact include all the techniques for
reducing TOF and FRE effects and increasing the number of excitations (but
this results in longer scanning times). Identifying an identical ghost outside
the body is useful in confirming ghosting artifact
(Fig. 19), and swapping the
phase and frequency encoding directions will change the axis of the artifact,
rendering it less troublesome.
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Fig. 19 —48-year-old woman. Axial fast spin-echo T2 image shows bright
ringshaped lesion (black arrows) in left lobe of liver due to
ghosting of CSF motion. Notice faint ring ghost more posteriorly and more
conspicuous ghost posterior to patient (white arrow). Ghosts and
spinal canal are all in anteroposterior phase-encoding direction, which
confirms identity of this lesion as ghosting artifact.
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Conclusion
The interpretation of CSF and related abnormalities on MR images can be
difficult because of the ever-present motion that is intrinsic to CSF. An
understanding of the common flow-related appearances and locations, coupled
with troubleshooting techniques, is critical to discerning flow effects from
true abnormalities. Additional ghosting artifacts from the CSF can also mimic
abnormality outside the CSF.
References
- Enzmann DR, Rubin JB, DeLaPaz R, Wright A. Cerebrospinal fluid
pulsation: benefits and pitfalls in MR imaging.
Radiology 1986;161
: 773-778[Abstract/Free Full Text]
- Bradley WG, Kortman KE, Burgoyne B. Flowing cerebrospinal fluid in
normal and hydrocephalic states: appearance on MR images.
Radiology 1986;159
: 611-616[Abstract/Free Full Text]
- Baledent O, Henry-Feugeas MC, Idy-Peretti I. Cerebrospinal fluid
dynamics and relation with blood flow: a magnetic resonance study with
semiautomated cerebrospinal fluid segmentation. Invest
Radiol 2001; 36:368
-377[CrossRef][Medline]
- Kallmes DF, Hui FK, Mugler JP. Suppression of cerebrospinal fluid
and blood flow artifacts in FLAIR MR imaging with a single-slab
three-dimensional pulse sequence: initial experience.
Radiology 2001;221
: 251-255[Abstract/Free Full Text]
- Bakshi R, Caruthers SD, Janardhan V, Wasay M. Intraventricular CSF
pulsation artifact on fast fluid-attenuated inversion-recovery MR images:
analysis of 100 consecutive normal studies. Am J
Neuroradiol 2000; 21:503
-508[Abstract/Free Full Text]
- Parkkola RK, Komu ME, Aarimaa TM, Alanen MS, Thomsen C.
Cerebrospinal fluid flow in children with normal and dilated ventricles
studied by MR imaging. Acta Radiol 2001;42
: 33-38[CrossRef][Medline]
- Wood ML, Runge VM, Henkelman RM. Overcoming motion in abdominal MR
imaging. AJR 1988;150
: 513-522[Abstract/Free Full Text]
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Abstract
Introduction
