Christoph Leussler1, Christian Findeklee1, Peter Mazurkewitz1, Jürgen Gieseke2, and Peter Börnert1
1Philips GmbH Innovative Technologies, Hamburg, Germany, 2Philips Deutschland GmbH, Hamburg, Germany
Synopsis
A dedicated solenoidal RF coil for imaging of the larynx
was developed for cylindrical MRI systems using a static magnetic field in
axial direction. The coil consists of two flexible solenoidal windings
entangling the cervix anatomy. Numerical and experimental evaluation
demonstrates higher SNR for the region of the larynx compared with conventional
neck coil designs.Introduction
A thorough understanding of the basic
neck anatomy is essential for the accurate diagnosis and successful treatment
of malignant laryngeal tumors [1,2,3]. Head neck imaging is typically performed
in today’s clinical systems positioning the patient horizontally using combined
head neck coil arrays [4,5]. Modern multi-element coils combine high head
homogeneity and neck coverage from the top of the head to the aortic arch.
General purpose coils cover a large region and are designed to allow MRI for a
broad range of different patient sizes. However, the anatomy of the neck region
is challenging for efficient coil designs due to the large patient variety and
due to the different potential applications like imaging the carotids or the
larynx. Recently wide bore MRI systems were getting into clinical use. Among
other advantages, they allow the bedding of the patient in an almost or more
upright position. This allows the application of RF coils with a solenoidal
design for the (head) neck region. Such solenoid coils are typically used in
vertical field MRI systems [6, 7] and do have a high sensitivity and excellent
homogeneity. Signal drop off, arising in deep anatomical regions with planar
surface coils is eliminated.
Methods
A dedicated solenoidal RF coil for imaging of the larynx was
developed (Fig.1). The larynx is located in the anterior
compartment of the neck, suspended from the hyoid bone, between the levels
of the third and sixth vertebral bodies. The coil consists of two flexible
solenoidal windings entangling the cervix anatomy, thus B1 orientation is almost
perpendicular to the static axial magnetic field (Fig.1). To confirm operation
and to allow predicting the coil performance in the presence of realistic loads,
electro-magnetic simulations were performed (using CONCEPT [8]). For efficient
and robust workflow, mechanical contacts allow for an easy opening and closing
of the loop. The flexible and lightweight coil was tuned to 63.86 MHz (for
1.5T) and has a diameter of 180 mm. Integrated preamplifier and detuning
electronics were directly connected to a fully digital interface
(dStream). We measured Q
O/Q
L ratio of (4-5) for
different patient neck sizes. Patient positioning was performed using a
dedicated head neck rest. We obtained MRI images using a 1.5T Ingenia (Philips,
Best, the Netherlands) and the dedicated laryngeal coil (T2 weighted TSE,
slice thickness 4mm, FOV 250mm, TE 100ms, TR 2,5s, flip angle 90°, 4 avg. ; T1
weighted TSE, slice thickness 4mm, FOV (250mm)
2, TE 14ms, TR 400ms,
flip angle 90°, 4 averages. Silent MRI images using a ZTE sequence, visualizing
short T2 components, were also measured (FOV (220mm)³, voxel (1.38mm)³, TE/TR
0.05/4ms, flip angle 3°).
Results
Figure 2 illustrates results of
numerical simulations, comparing the potential SNR of the larynx coil to
quadrature volume birdcage coils (close fitting [180mm diameter] and volume
birdcage [270mm diameter]) at equal overall transmit power. Due to reciprocity,
these fields (at a certain frequency) are a direct measure for the SNR in
receive mode. As shown in Fig. 2, the proposed loop performs about as good as a
close fitting birdcage coil with the same diameter (92.2% at isocenter). Such a
birdcage cannot be used in practical clinical routine due to anatomical
constraints. The larger birdcage just reaches 78.2% of the close fitting
birdcage SNR at the central position. Experimental measurements of the SNR were
performed using phantoms of different sizes, showing an almost two-fold SNR
gain compared with a head birdcage and 6 element coil array with a diameter of
290 mm and 260 mm, respectively. The coverage in the z-direction (patient head
feet axis) turned out to be very similar for birdcage and loop coil. This is
confirmed by in-vivo experiments. The T1 and T2 weighed TSE volunteer images
show good signal homogeneity in the central region of the coil (Fig.3). The
volume coverage of approx. 150mm can be appreciated also from the ZTE reformats
shown in Fig.4. Furthermore, this coil and the corresponding patient setup (see
Fig.1) was found more convenient for claustrophobic volunteers / patients,
compared with conventional volume head neck coils.
Conclusion
We show a new option for MR imaging of
the neck region using a ring solenoid coil design, which is orthogonally
oriented to the B0 field of a cylindrical MRI magnet. The proposed coil and the
workflow concept are suited for imaging of the larynx region. The coil might be
combined with additional loop coils orthogonally oriented to the axial B0 field
to provide further options for SNR and parallel imaging.
Acknowledgements
The authors thank Peter Koken and Kay
Nehrke for helpful support and discussion.References
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