Increased use of ultrasonography of the head and neck by clinicians has resulted from more compact, higher resolution ultrasound machines that can be more readily used in the office setting. Palm-sized machines are already used for vascular access and bladder assessment. As the resolution of these machines becomes adequate for head and neck assessment, ultrasonography is likely to become a routine adjunct to the office physical examination. Further techniques to reduce artifact beyond spatial compounding, second harmonics, and broadband inversion techniques are likely to be developed to improve ultrasound images. Manual palpation using the ultrasound transducer or “sound palpation,” using sound to recreate vibration provides information on tissue “stiffness,” which has been successfully used to distinguish between benign and malignant lesions in the head and neck (particularly thyroid nodules). Microbubble contrast-enhanced ultrasound provides improved resolution of ultrasound images. Three- and four-dimensional ultrasonography provides for more accurate diagnosis. The ability of microbubbles with ligands affixed to their outer surface to target specific tissue makes them excellent delivery vehicles. DNA plasmids, chemotherapy agents, and therapeutic drugs can be released at a specific anatomic site. The motion of microbubbles stimulated by ultrasound can be used to increase drug penetration through tissues and has been shown to be effective in breaking up clots in stroke patients (without increased risk). High-intensity focused ultrasound can be used to create coagulation necrosis without significant damage to adjacent tissue. It has been effectively used in neurosurgery and urology, but its effectiveness in the head and neck is still being determined. A prototype for surgical navigation with ultrasound has been developed for the head and neck, which allows real-time imaging of anatomic surgical changes.
Ultrasound: current and future use
Ultrasonography of the head and neck is currently a cost-effective imaging tool allowing assessment beyond the clinician’s physical examination. Ultrasonography is uniquely portable when compared with other imaging modalities, making it ideal for use in the clinical setting. It provides the clinician immediate feedback, allowing the provider to make accurate assessments in a timely manner. Ultrasonography does not carry the risk of irradiation and has been the tool of choice for diagnostic and therapeutic interventions, such as fine-needle aspiration and line placement.
Ultrasound technology has distinct advantages in these areas, with continued advances and new technology likely to emerge. To allow increased ease of use in the clinical settling, palm-sized ultrasound machines are being produced, which are likely to be miniaturized even further. The ability to palpate with the ultrasound probe provides unique information on a lesion’s compressibility or stiffness not available with computerized tomography (CT) and magnetic resonance imaging (MRI). This “objective” palpation is proving useful in determining whether a lesion is benign or malignant. Although attempts to use ultrasonography for therapeutic intervention in the head and neck date back to its use in Ménière disease in 1960, it is in therapeutic intervention that future applications show the most promise. Two promising therapeutic ultrasound interventions are the use of (1) drug-containing microbubbles that, using ultrasound, can release an antitumor agent, deliver gene therapy, or release other therapeutic substances at the target tissue and (2) focused ultrasound to coagulate bleeding vessels or destroy inaccessible tumors.
Discussions on emerging technology in ultrasonography in this article are limited to those in which there is clinical evidence to support the benefit, although in some cases that evidence is not specific to the head and neck.
Machine size
Laptop ultrasound systems are increasingly common. Several manufacturers are introducing palm-sized ultrasound machines, including the Acuson P10 (Siemens Medical Solutions Inc, Malvern, PA, USA) introduced in 2007, the Signos (Signostics Inc, Palo Alto, CA, USA) introduced in 2009, and the Vscan (GE Healthcare, Piscataway, NJ, USA) in 2010. Although the palm-sized machines are currently designed for vascular access, bladder examination, and trauma settings, it is likely that they will eventually be improved to provide the image quality necessary for use in head and neck examination ( Fig. 1 ).
Machine size
Laptop ultrasound systems are increasingly common. Several manufacturers are introducing palm-sized ultrasound machines, including the Acuson P10 (Siemens Medical Solutions Inc, Malvern, PA, USA) introduced in 2007, the Signos (Signostics Inc, Palo Alto, CA, USA) introduced in 2009, and the Vscan (GE Healthcare, Piscataway, NJ, USA) in 2010. Although the palm-sized machines are currently designed for vascular access, bladder examination, and trauma settings, it is likely that they will eventually be improved to provide the image quality necessary for use in head and neck examination ( Fig. 1 ).
Palpation with elastography
Ultrasound examiners have long used palpation of masses with the ultrasound probe as an adjunct to their visual examination. Ultrasound elastography provides a more objective measurement of stiffness or, in more precise physics terms, strain. Changes in returning echoes are measured at the transducer before being converted to B-mode ultrasound before and after compression with the ultrasound transducer. The difference is depicted on an elastogram as lighter for less dense and darker for more dense tissue or masses ( Figs. 2 and 3 ). Since its approval by US Food and Drug Administration in 2006, ultrasound elastography has been used to discriminate between malignant and benign breast masses based on its objective measure of the stiffness of those masses. There is initial evidence that ultrasound elastography may be useful in differentiating between benign and malignant thyroid nodules. Scoring tissue stiffness on ultrasound elastography from 1 (low stiffness) to 6 (high stiffness), Hong and colleagues in 2009 demonstrated in 90 consecutive surgical patients that 86 of 96 benign thyroid nodules (90%) had a score of 1 to 3, whereas 43 of 49 malignant thyroid nodules (88%) had a score of 4 to 6. Similarly, Rago and colleagues in 2007 used a scoring from 1 to 5 based on elastography in 92 consecutive patients undergoing thyroidectomy for compressive symptoms or suspicion of malignancy. Low stiffness scores of 1 and 2 were found in 49 cases, all benign thyroid nodules; scores of 3, in 13 cases, one malignant and 12 benign thyroid lesions; and high stiffness scores of 4 and 5, in 30 cases, all carcinomas. Sensitivity in these 2 studies ranged from 88% to 90% and specificity, from 90% to 100%. Asteria and colleagues in 2008 in a third study of 67 patients with 86 thyroid nodules used stiffness scores of 1 through 4 and found slightly higher specificity for malignancy of 94%, but a decreased sensitivity of 81%. There remains some doubt over interobserver reliability of ultrasound elastography for the thyroid nodule.
Replacing palpation with sound using sonoelastography and acoustic radiation force imaging
In place of manual palpation (displacement) with the transducer, sonoelastography uses Doppler ultrasound to detect movement in a neck mass created by an external vibration. Lyshchik and colleagues in 2007 examined 141 cervical lymph nodes in 43 patients with suspected hypopharyngeal or thyroid cancer using ultrasound sonoelastography. Stiffness or strain of the lymph node and surrounding muscle was measured. When the ratio of muscle/lymph node strain was greater than 1.5, they found a 98% specificity, 85% sensitivity, and 92% overall accuracy of identifying metastatic lymph nodes. Dighe and colleagues used compression generated by the carotid artery on thyroid masses along with Doppler ultrasound to determine stiffness or strain in 53 patients with thyroid lesions. This obviated a vibration source external to the neck, and they were able to distinguish 10 papillary carcinomas from 43 other thyroid lesions based on the stiffness.
Acoustic radiation force impulse (AFRI) imaging uses short-duration ultrasound pulses (0.03 to 0.4 ms) to create tissue movement/displacement and recovery, which is recorded with ultrasound correlation or Doppler ultrasound. There are no clinical studies of the head and neck, but AFRI has been used to better identify isoechoic lesions within the liver, stiffness of the heart myocardium, solid versus cystic lesions in the pancreas, and luminal intestinal lesions. Certainly, there is a potential value within the head and neck. One of the early concerns of ARFI is heat produced at the transducer and tissue being examined. Advances in AFRI beam sequencing and parallel imaging have shortened acquisition time and reduced transducer heating significantly, reducing this concern.
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