Improvements in High Resolution Laryngeal Magnetic Resonance Imaging for Preoperative Transoral Laser Microsurgery and Radiotherapy Considerations in Early Lesions

Abstract

As the benefits, limitations, and contraindications of transoral laser microsurgery (TLM) in glottic carcinoma treatments become better defined, pretreatment imaging has become more important to assess the case-specific suitability of TLM and to predict functional outcomes both for treatment consideration and patient counseling. Magnetic resonance imaging (MRI) is the preferred modality to image such laryngeal tumors, even though imaging the larynx using MRI can be difficult. The first challenge is that there are no commercial radiofrequency (RF) coils that are specifically designed for imaging the larynx, and performance in terms of coverage and signal-to-noise ratio is compromised using general-purpose RF coils. Second, motion in the neck region induced by breathing, swallowing, and vessel pulsation can induce severe image artifacts, sometimes rendering the images unusable. In this paper, we design a dedicated RF coil array, which allows high quality high-resolution imaging of the larynx. In addition, we show that introducing respiratory-triggered acquisition improves the diagnostic quality of the images by minimizing breathing and swallowing artifacts. Together, these developments enable robust, essentially artifact-free images of the full larynx with an isotropic resolution of 1 mm to be acquired within a few minutes.

Keywords: glottic carcinoma; imaging protocols; laryngeal imaging; magnetic resonance imaging; radiofrequency coil arrays; transoral laser microsurgery.

Figure 1

Illustrations of the challenges associated with current approaches to laryngeal magnetic resonance imaging. (A) Two individual, commercial circular receive coils, each with a diameter of 4 cm, are placed around the larynx. Positioning in this particular setup is critical for imaging the larynx and can be experienced as uncomfortable by the subject since the loops have to be held firmly in place with tape and/or velcro straps. Written informed consent was obtained from the subject for publication of this photo. (B) Images acquired with a limited penetration depth due to incorrect coil positioning, (C) images showing signal voids in the anterior part of the larynx due to incorrect coil positioning, and (D) images showing movement artifacts due to subject breathing.

Figure 2

(A) The dedicated larynx coil with the electronics visible (an encased version is used for in vivo scans). The coil is flexible and, therefore, adjustable to different neck sizes. The white boxes are floating cable traps, which prevent current from flowing on the outer shield of the connecting cables. Written informed consent was obtained from the subject for publication of this photo. (B) A schematic of the electrical circuit of each of the four loops constituting the coil. The individual loops are decoupled from the neighboring elements using an induced current elimination (ICE) circuit. Radiofrequency chokes (RFCs) are used to allow a DC current path to turn on the PIN diodes to isolate the receive coils from the transmit body coil.

Figure 3

Respiratory triggered T1 TSE sequence (1 mm isotropic voxel size) with parameters as in Table 1. (A) Shows an image at the level of the vocal cords made with the two loops setup (as in Figure 1A) and at the maximum acceleration factor of 2.0: total scan time 234 s. (B) Shows an image at the same level made with the dedicated coil and an acceleration factor of 3.0: total scan time 189 s.

Figure 4

T1 TSE multi-slice sequence (transversal) at 1.0 mm isotropic resolution with SPIR fat suppression (scan parameters in Table 1) (A) without respiratory trigger, duration 57 s, (B) with respiratory trigger, duration 216 s. Triggering mitigates breathing and swallowing motion artifacts and increases image quality.

Figure 5

T1 TSE 1.0 mm isotropic scan with parameters as in Table 1. A selection of eight out of 40 slices ranging from aryepiglottic fold to subglottis on a healthy volunteer showing depth of penetration far beyond the larynx.

Figure 6

Magnetic resonance imaging scans 8 months after radiation therapy for a T1b glottis carcinoma. Increased signal intensity on T2-weighted scans without (A) and with (B) fat suppression in both vocal cords, together with the absence of diffusion restriction on DWI/ADC (C,D) is compatible with posttreatment edema. A biopsy in the superficial area corresponding to intermediate signal on T2 and diffusion restriction on the left vocal cord revealed tumor recurrence. Contrast-enhanced T1-weighted scans at two different time points show gradual enhancement (E,F).

Figure 7

Laryngeal magnetic resonance imaging of a patient with superficial recurrence after radiotherapy of an early glottis carcinoma: high signal intensity in the right vocal cord and posterior paraglottic space on T2 (A) combined with a high signal intensity on both DWI (C) and ADC (D) is compatible with post irradiation edema and rules out deep tumor extension. There is diffuse enhancement on contrast enhanced T1 with fat suppression (B).