Development of a dedicated solenoidal ring-based RF Coil for MRI of the Larynx
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 QO/QL 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

[1] Towler C.R. et al Magnetic Resonance Quarterly Vol.5 No. 3, pp. 228-241, Raven Press, Ltd., 1989 [2] Zinreich, S, J., Otolaryngnol Clin N Am 35, 971-991, 2002 [3] Abraham J. Surg Oncol Clin N Am 24 455-471, 2015 [4] C. Leussler Proc. SMRM Vol 3 p. 1349, 1993 [5] C. Leussler Proc. SMR p. 179, 1995 [6] Lufkin RB et al, AJR Am J Roentgenol. 1986 Feb; 146 (2):409-12 [7] C. Leussler Proc. SMRM et al p. 724, 1991 [8] CONCEPT EM Simulator Univ. Hamburg Harburg

Figures

Fig.1: Positioning of the patient is realized using a dedicated head-neck rest. The B1 orientation of the larynx coil is orthogonally positioned relative to the static magnetic field of the cylindrical magnet. The coil setup is less claustrophobic than conventional volume head neck coils. For efficient and robust workflow, the coil can easily be positioned by means of a mechanical opening.

Fig.2: Simulation results (circular polarized B1 in µT) for feeding three different coils with 1W at 1.5T (63,86MHz): Left 180mm loop around 80° rotated cylinder, which represents a realistic patient positioning, center: close fitting (180mm) quadrature birdcage, right: typical quadrature birdcage (270mm).

Fig.3: Volunteer TSE images using the dedicated laryngeal coil. (a, b) T2 weighted TSE, (c, d) T1 weighted TSE.

Fig.4: Volunteer ZTE images to visualize short T2 components. Reformats are given showing the anatomy of the larynx in the center and demonstrating the coil coverage. Native ZTE shows poor contrast, because all species contribute to the signal and there is no time for other contrast mechanisms to evolve substantially.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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