MR Thermography–Guided Head and Neck Lesion Laser Ablation

Head and Neck Laser Ablation Indications and Complications

Laser ablation has been successfully performed for treating various lesions in the head and neck, brain, breast, liver, prostate, and colon among other sites.^{2,7⇓⇓⇓⇓–12,15⇓⇓⇓⇓⇓⇓–22} Regarding the head and neck in particular, percutaneous laser ablation has been implemented for treating benign solid thyroid nodules, lymphatic malformations, and various malignant neoplasms.^{8,9,16⇓⇓⇓⇓⇓–22} Laser ablation performed to shrink symptomatic benign solid thyroid nodules has been reported to yield a 47%–82% reduction in volume, a minor complication rate of 38%, and a major complication rate of 3%.²² Major complications related to laser ablation of benign solid thyroid nodules include dysphonia, nodule rupture, hypothyroidism, and brachial plexus injury, while minor complications include hematoma, vomiting, skin burn, pain, edema, fever, and coughing,²² though many of the procedures were not performed under MR thermography guidance.

In a small study of lymphangiomas in the tongue and neck, MR imaging–guided laser ablation performed via a percutaneous approach using a multiapplicator technique resulted in a considerable decrease in the size of all lesions by 3 months, with improvement in functions such as speech and swallowing in most patients.²⁰ In particular, the cystic components showed the most change as the treated lymphatic channels become surrounded by fibrous tissue. Recurrence of the lymphatic malformations is nevertheless a possibility, but laser ablation of these lesions can be performed to facilitate subsequent debulking surgery.

Recurrent head and neck cancers after major surgery and/or radiation therapy are difficult to manage via conventional treatment methods. Laser ablation is a relatively safe treatment option for recurrent head and neck cancers for which surgery is limited by the proximity of vital neurovascular structures and the aggressive nature of these tumors.¹⁹ A 78% partial or complete response and a median survival of 19 months was achieved for recurrent head and neck cancers treated with laser ablation, which surpasses the results obtained via a standard regimen of cisplatin/fluorouracil (5-FU).⁸ In particular, oral cavity squamous cell carcinomas showed the greatest response to the procedure.⁸ Furthermore, laser ablation reduces clinical symptoms in most patients. Infusion of chemotherapy into the ablated tumor can potentially augment the effects of laser thermal therapy while mitigating systemic toxicity and is the subject of investigation.⁸

MR Imaging Guidance for Head and Neck Laser Ablation

Various navigation techniques and devices have been developed for MR imaging–guided interventions.^23⇓–25 The advantages and disadvantages mainly depend on the type of the MR imaging scanner and the interventional environment.²³ For example, open MR imaging scanners require minimal patient transfer during the procedure, which facilitates the interventional workflow. On the other hand, while most high-field-strength closed-bore MR imaging scanners provide better image quality, they do not enable instruments to be inserted within the bore of the magnet. Consequently, interventions in closed-bore scanners require the patient being moved out of the bore, though multimodal overlays, augmented reality displays, and robotic assistance devices can facilitate the process.²³

Insertion of the laser probe can be accomplished under MR imaging using freehand or stereotactic approaches. In either case, the device can be advanced into the target lesion under MR imaging “fluoroscopic” guidance, in which the choice of tissue contrast weighting can be tailored to maximize lesion conspicuity on the basis of preprocedural imaging. Furthermore, MR imaging provides the ability to continuously visualize vessels throughout a procedure without using intravenous contrast.

Lesion targeting has traditionally been performed using “in-and-out” techniques in closed-bore MR imaging scanners with control imaging performed inside the bore, and needle adjustments, outside the bore. Alternatively, the process of targeting the lesion can be expedited using guidance and navigation systems, such as virtual 3D-MR imaging with real-time overlay of an optically tracked biopsy needle using reference markers or the interventional MR imaging suite (iSuite) image guidance and mapping system (Philips Research Labs, Hamburg, Germany), which facilitates triangulation of the laser probe insertion trajectory (Fig 1).²⁶ The biopsy needles and laser probe used for the procedure can be secured in position using commercially available skin-adhesive holder devices, such as the Simplify needle holder (NeoRad, Oslo, Norway) or Cradles Needle Localization Wire Protectors (Beekley Medical, Norcross, Georgia) (Fig 2).

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Fig 1.

MR imaging guidance for laser applicator insertion. 3D T1-weighted fast-field echo image shows saturation bands (arrowheads) that are used to triangulate the target “lesion” (arrow) in the neck phantom.

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Fig 2.

Photograph shows the laser applicator (Medtronic, Minneapolis, Minnesota) (arrow) inserted into the neck phantom (CIRS, Norfolk, Virginia) and supported by a skin-adhesive device (NeoRad).

MR Thermography

In general, the success of laser interstitial thermal ablation depends on accurate insertion of the laser fiber into the target lesion, real-time monitoring of the effects of the treatment, and subsequent evaluation of the extent of thermal tissue damage.¹⁹ MR thermography is a validated noninvasive imaging technique that can provide accurate monitoring during laser ablation.^19,27 Several MR thermography methods have been developed, including sequences based on the proton resonance frequency, the diffusion coefficient, T1 and T2 relaxation times, magnetization transfer, proton density, and temperature-sensitive contrast agents.^4,24,27 In particular, proton resonance frequency techniques use temperature-induced effects of chemical shift, in which the temperature difference is directly proportional to the phase difference. This method provides accurate temperature measurements in the temperature range of interest for thermal ablation.²⁴

A thermal dose can be calculated from a map of temperature as a function of time and is used for the prediction of tissue destruction (Fig 3).⁷ Ultimately, continuous MR thermometry during thermal ablation procedures can help optimize safe thermal therapy delivery. Although the adequacy of ablation is estimated on the basis of the laser ablation system–generated damage model, it is important to obtain an immediate postablation MR imaging scan to ensure adequate ablation of the lesion.

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Fig 3.

MR thermography during laser ablation. Temperature map acquired using the proton resonance frequency shift (A) and heat-damage model in a neck phantom (B) shows the ablation zone surrounding the tip of the probe (arrows).

Important technical considerations for performing MR imaging–guided laser ablation include coil selection and assessing the quality of MR imaging sequences in the head and neck region. For example, the use of whole-body coils contained within the closed bore of a high-field-strength MR imaging scanner is an ergonomic option with fewer obstacles between the interventionalist and the patient. Although these types of coils yield lower signal-to-noise compared with head and neck neurovascular coils, for example, the image quality may be generally adequate for clinical use.²⁸

Furthermore, the quality of MR thermography can be limited by the presence of susceptibility artifacts from hemorrhage, calcifications, or underlying surgical hardware; the excessive presence of fat surrounding the lesion; and misregistration artifacts related to motion.³ Ultimately, interstitial laser ablation should be performed only with MR thermography if a lesion can be adequately discerned on MR thermography images obtained as part of the pretreatment MR imaging.