Barbara Cervantes1, Houchun Harry Hu2, Amber Pokorney2, Dominik Weidlich1, Hendrik Kooijman3, Ernst Rummeny1, Axel Haase4, Jan S Kirschke5, and Dimitrios C Karampinos1
1Diagnostic and Interventional Radiology, Technische Universität München, Munich, Germany, 2Radiology, Phoenix Children’s Hospital, Phoenix, AZ, United States, 3Philips Healthcare, Hamburg, Germany, 4Zentralinstitut für Medizintechnik, Garching, Germany, 5Neuroradiology, Technische Universität München, Munich, Germany
Synopsis
High-resolution MRI with 3D turbo spin echo (TSE)
is arising as an accurate, non-invasive method for detecting disease and injury
in the nerves of the lumbar plexus. Imaging of the lumbar plexus with 3D TSE frequently
faces signal contamination of the cerebrospinal fluid (CSF) within the spine. Increasing
spatial resolution in 3D TSE can affect flowing signal. The present study describes
numerically the effects of the imaging gradients in 3D TSE on flowing CSF and
demonstrates in vivo that CSF can be
completely suppressed without modifications to refocusing angle modulation when
sub-millimeter voxel sizes are used with 3D TSE.Purpose
Lumbar
nerves are susceptible to traumatic and inflammatory disorders, sometimes only
reliably detected with high-resolution MRI [1]. T2-weighted 3D turbo
spin echo (TSE) is frequently used to visualize lumbar nerve roots but faces signal
contamination from high cerebrospinal fluid (CSF) within the spine [2].
Previous studies have shown that refocusing flip angle (FA) modulation in 3D
TSE can reduce CSF signal in the cervical spine [3]. In 3D TSE imaging of the
lumbar plexus, given that frequency encoding is commonly aligned to the
direction of flow of CSF, increasing resolution can reduce CSF signal without
altering FA modulation. The present work shows, for 3D TSE, 1) numerical
reductions of CSF signal with increasing readout resolution and 2) complete CSF
suppression
in vivo in adult and
pediatric subjects.
Theory
Extended
phase graphs (EPG) allows numerical assessment of signal in pulse sequences
with long refocusing trains [4] considering FA modulation and relaxation.
Effects related to the imaging gradients can be characterized by EPG by
incorporating additional information into the algorithm [5]. Motion
sensitization in 3D TSE can become significant when large readout gradients are
used, which occurs when the readout voxel dimension is reduced. The signal of a
tissue moving with velocity v can be computed with EPG by
including the phase shift implied by each readout gradient in the gradient
duration $$$\Delta t$$$, $$$\Delta \phi = \gamma (\text{v} \times
\text{m}_{\text{0}}) \Delta t$$$, where $$$\text{m}_{\text{0}}$$$ is in the gradient
direction and $$$|\text{m}_{\text{0}}|$$$ is the gradient area. FA modulation
gives different $$$\Delta \phi$$$ for different magnetization pathways,
resulting in signal losses that scale with gradient area and velocity (Fig.1).
Methods
Simulations: Signal of CSF was
simulated using EPG considering dephasing from coherent motion for voxel sizes from
0.3 to 1.3 mm and for CSF velocities from 0 to 4 cm/s, in agreement with
observed values [6]. FA modulation optimized to yield high signal of small
nerves [7] was used. CSF signal was computed with $$$\text{T}_{1}=3120$$$ms
and $$$\text{T}_{2}=160$$$ms.
In vivo measurements: The lumbar plexus of
all subjects was imaged using a 3T Philips scanner (Philips Ingenia, Best, the
Netherlands) with the 12-channel posterior coil and a 16-channel torso coil
only in the study of the healthy adult volunteer. $$$\text{T}_{2}$$$-weighted 3D TSE was
used with echo spacing=4ms, TR/TE=2000/330ms, TSE factor=150. First, a
resolution comparison in a healthy 29-year-old adult volunteer was performed
with FOV=488×400×80$$$\text{mm}^{3}$$$, acquisition voxel =
a) 1.4, b) 0.7, c) 0.4 ×1.25×1.4 $$$\text{mm}^{3}$$$, scan duration =
4m22s. Second, pediatric subjects a) 8 months, b) 2 years and c) 8 years of age
were scanned with FOV = a) 180×208×50, b) 230×230×50, c) 200×231×50 $$$\text{mm}^{3}$$$, acquisition voxel =
0.7×0.7×1.4$$$\text{mm}^{3}$$$, respectively, scan
duration = 4min18s. Scans a) and b) of pediatric subjects used a 32-channel
head coil in combination with the posterior coil.
Results
Simulated
CSF signal is reduced by over 90% compared to the maximum simulated signal for
flow velocities above 2cm/s and readout voxel sizes below 0.7mm (Fig.2). In the
healthy adult volunteer, using a voxel size of 1.4mm in the readout direction
yields high signal intensities of CSF within the spine (Fig3a). Using a voxel
size of 0.7mm results in reduced CSF signal (Fig. 3b); further reducing the
voxel size to 0.4mm results in the complete CSF suppression (Fig.3c). Imaging
of the lumbar plexus of pediatric subjects using a readout voxel size of 0.7mm
results in complete CSF suppression (Fig.4).
Discussion & Conclusion
Simulation
results show complete CSF suppression, without FA modulation modifications,
only with sub-millimeter readout voxel sizes. Although it has been shown that FA
modulation alone can reduce the CSF signal [3], doing so can affect the signal
of small nerves. Here it is shown that the signal of CSF can be completely
suppressed while using FA modulation optimized for nerve signal. A particular
readout voxel size results in different CSF signal reductions in subjects with
different ages. CSF velocities in the lumbar spine in adults have been observed
to reach 2cm/s [8], while those in children have been observed to be higher [9].
CSF suppression using sub-millimeter readout resolutions is therefore shown to
work optimally in pediatric subjects. Complete CSF suppression is possible in
adult subjects by further reducing the readout voxel size, with the obvious
disadvantage of reducing SNR. The present work therefore 1) shows that
complete suppression of CSF within the spine is possible in adult and pediatric
subjects using sub-millimeter-resolution 3D TSE and 2) demonstrates
in
vivo that CSF suppression allows the visualization of nerve structures
within the spine.
Acknowledgements
The present work was supported by Philips Healthcare.References
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