Applications of Ultrasonography in Dentomaxillofacial Region

INTRODUCTION

Use of ultrasonography (US) in the dentomaxillofacial region became popular in recent years owing to increasing radiation dose concerns and economic limitations. Sound frequencies above those of audible limits (30 Hz to 20 KHz) are known as ultrasound.¹ In diagnostic US, high frequency sound waves are transmitted into the body by use of a transducer and echos from tissue interface are detected and displayed on a screen.^2,3 Sound waves are emitted via piezoelectric crystals from the ultrasound transducer. Piezoelectric crystals are constructed from material that changes electric signals to mechanical vibrations and vice versa.^4,5 The piezoelectric system is based on the principle that quartz is subject to a change in shape when placed within an electrical field.^1,6. The main component of the transducer is a thin piezoelectric crystal or material made up of a great number of dipoles - distorted molecules that have a positive charge on one end and a negative charge on the other - arranged in a geometric pattern. Currently the most widely used piezoelectric material is lead zirconatetitanate.⁷Acoustic impedance is the term used to define the resistance of a material to propagation of ultrasound waves. It depends on the density of the material. If the material is solid, the particles are denser and sonographic waves are reflected more. Therefore, solid material transmits fewer sound waves than fluids and less ultrasound waves are reflected back from fluids. As a result, an echogenic ‘‘black’’ image is produced. Stones and bones reflect more sound waves than fluid and they produce ‘‘white’’ bright images. Since ultrasound waves cannot transmit through stones, a black acoustic shadow is present behind them. Air is a robust ultrasound beam reflector which makes it difficult to visualize structures.⁴

ADVANTAGES OF ULTRASONOGRAPHY (US)

ultrasonography provides several advantages for dento-maxillofacial imaging compared to other advanced imaging techniques such as: Absence of ionizing radiation, portability, possibility of dynamic and repeated examinations and low cost. US is well recognized in inflammatory soft tissue conditions of the head and neck region along with superficial tissue disorders.^2,8 US is also an alternative diagnostic method for the imaging of temporomandibular disorders owing to satisfactory and promising results obtained from high resolution US in the assessment of temporomandibular joint (TMJ). It is harmless, fast, comfortable, economic and available in most centers and those make US a good candidate for TMJ evaluation.⁹US is the only available imaging technique that can be used for frequent routine follow-up of cervical lenf node metastases.^1,12 US guided core needle biopsy is recommended as a safe and reliable technique in the diagnosis of cervico-facial masses with a high diagnostic yield.^13,14 The application of US in midfacial injuries is most useful for the visualization of the zygomatic arch when immediate imaging is performed after closed reduction. US can be considered as the imaging of choice when there is a contraindication to CT or plain films (for example, in pregnant women and patients with cervical spine injuries).^15,16 When a trauma occurs, US can be used to investigate potential fracture lines of the injured bone through a real time examination.¹⁷US images can be used to assess the size, content and vascular supply, and provide a provisional diagnosis that may differentiate cysts and granulomas. An X-ray image may show a lesion more accurate than ultrasound. However, it is not possible to figure out the pathological nature of the lesion with the X-Ray whereas an ultrasound image can provide accurate information about the pathological nature of the lesion.¹⁷ Using a non-invasive and non-ionizing radiation technique makes it possible for the clinician to evaluate soft and hard tissue healing after periodontal surgery. Also, US may be used for the clinical assessment and treatment planning prior to implant placement.¹⁸

DISADVANTAGES OF ULTRASONOGRAPHY(US)

US waves can damage tissues at high exposure levels, in addition to having teratogenic effects, due to heat, and acoustic cavitation. However, within the diagnostic range at low intensities and pressure levels, occurence of heating beyond the normal physiological range has very low probability.¹⁹ In addition, metallic implants, dental fillings and restorations may cause blurring of the image due to artefacts generated by the metal.²⁰It is difficult to visualize the articular disk with US when it is placed between two hard tissue structures. Therefore, imaging disk position by using US is difficult and may be problematic.²¹ US is also unable to detect minimal and/or non-displaced fractures. In addition, it is not possible to delineate complex multiple facial fractures and to distinguish new fractures from old ones. Identification of intracapsular fracture of mandibular condyle due to overlapping of zygomatic arch may be impossible. Finally, in the case of acute conditions with facial edema and empyema bone visualization may be complicated.^16,22

Application of ultrasonography(US) in dentomaxillofacial region

In general terms, US can be utilizied for the assessment of maxillofacial region fractures, temporomandibular joint disorders, cervical lymphadenopathy, swelling in the orofacial region, and salivary gland pathology.¹⁴ Besides, periodontal US is a non-invasive diagnostic method for measuring pocket depth which is an indicator of periodontal health.⁶

INFLAMMATION

Acute inflammation causes swelling with loss of the normal glandular homogeneous bright echotexture. US is able to show hyperreflectivemicrobubbles of gas in supurativesialadenitis with adjacent reactive nodesAstudy⁴², found that most of the inflammatory swellings had relatively clear boundaries, hypoechoic intensity and homogeneous ultrasound architecture of lesions. Considering inflammatory swellings, US had a sensitivity of 97% and specificity of 100%, whereas; clinical diagnosis had a sensitivity and specificity of 85.7%⁴². As can be seen, US was found to have high sensitivity in the diagnosis of inflammatory swellings of the head and neck region.

MUSCLE DISORDERS

On US imaging, temporalis muscle is seen as a thin hypoechogenic band lying adjacent to the medial part of the temporalis fossa. The bony landmark is identified as a hyperdense line, whereas the course of the temporalis muscle is best visualized by having the patient clench. The masseter muscle is seen as a homogeneous structure lying adjacent to the echogenic band of the mandible. The anterior digastric muscle corresponds to round hypoechogenic zones located lateral to the respective mylohyoid muscles. The posterior digastric muscle is seen as a hypoechogenic band located under the homogeneous ultrasonographic pattern of the parotid gland. Sternocleidomastoid muscle is easily visualized due to its large size and typical band shape which shows a solid hypoechogenicultrasonographic pattern. The medial boundary of the sternocleidomastoid muscle is identified as a very dense hyperechogenicline³⁹. US was found to be useful for the measurement of masseter muscle thickness⁴⁰. In the inflammatory muscle, the echogenic bands, which correspond to the internal fascia or tendon of the muscle, are frequently diminished or disappeared. Muscle with histologically verified edema shows less echogenity compared to that of muscle without edema⁴. In view of the importance of preoperative assessment of the hyomental distance ratio in predicting difficult incubation and laryngoscopy procedures, authors assessed the feasibility of measuring the hyomental distance ratio as well as volumes of the tongue and muscles of the floor of the mouth in obese patients using submandibular sonography⁴³.

TEMPOROMANDIBULAR DISORDERS

US, an alternative technique to magnetic resonance imaging (MRI), was utilized for assessing TMJ in the beginning of 1990´s. The imaging procedure includes transverse and longitudinal scans, thereby; the antereriosuperior joint compartment can be examined in axial, coronal, and oblique views. The condyle and glenoid fossa are generally hyperechoic, whereas; connective and muscular tissues are isoechoic and they appear heterogeneously grey. However, surface of the joint capsule, highly reflects the sound waves, thus generating a hyperechoic line. Superior and inferior joint spaces are seen as hypoechoic²¹. When the condyle translates from closed mouth position to open mouth position operator should constantly adjust the position of the transducer during imaging, for better visibility of the disc³⁷. Emshoff et al³⁸ found high specificity values at the closed mouth (1.00), half mouth opening (0.94), and maximum mouth opening (0.95) positions. Thus, they concluded that US was a reliable diagnostic tool in diagnosing normal disc position at the various mouth opening positions. A meta-analysis of US for the detection of TMJ anterior disc displacement revealed that high resolution US was superior in the diagnosis of anterior disc displacement without reduction[39]. On the other hand, utilization of US for detecting lateral and posterior displacements was not suggested. Overall, the diagnostic efficacy of US in TMJ evaluation is acceptable and can be used as a rapid preliminary diagnostic method⁴⁰.

MIDFACIAL FRACTURES

There are several studies which were conducted in order to assess the versatality of US for midfacial fracture diagnosis in trauma cases. Authors of a study used ultrasound in diagnosing zygomatico-orbital complex fractures and found an accuracy of 94%²³. McCann et al²⁴ found lower accuracy (85%) in diagnosing fractures of the zygomatico-orbital complex when compared to aforementioned study. Another study, reported 86% accuracy in diagnosing fractures of the orbital floor²⁰. Gülicher et al²⁵ showed that ultrasonographic control of fracture repair led to excellent results in almost all patients. Visualization of different types of midfacial fractures was assessed by Friedrich et al¹⁶ using ultrasound imaging. Types of fractures evaluated were orbito-zygomatical complex fractures, isolated fractures of the zygomatic arch, orbital floor, nasal bone, frontal sinus, along with complex Le-Fort fractures. The application of ultrasound in midfacial fractures was found to be most useful for the visualization of the anterior wall of the frontal sinüs and zygomatic arch. However, it was difficult to detect non dislocated fracture²⁶. According to Blessmann et al²⁷ by using US, zygomatic arch could be visualized quite reliably whereas assessment of the orbital floor proved to be rather difficult. Soft tissue covering of the tissues impairs imaging of fractures in several planes. Therefore, the application of US is not a substitute for accurately taken X-ray imaging for detecting fractures of the mandibular ramus and condyle¹⁶.

We may conclude that most reliable diagnosis with the use of US in traumatic cases is achieved in zgomatico-orbital complex and anterior frontal sinus wall fractures.

SALIVARY GLAND

The first use of ultrasound to identify and locate a parotid calculus was reported by Pickrell in 1978. Transcutaneous extra-oral ultrasound has been introduced as a simple and safe imaging technique for the detection of calculi in the salivary glands. It was found to be as effective as sialography in identifying calculi of 2 mm in diameter. Contemporary innovative small high frequency ultrasound probes allow access to the ducts both in the submandibular and parotid glands via an intraoral approach.

The most common benign salivary gland tumors are adenolymphoma, pleomorphic adenoma, basal cell adenoma, myoepithelioma and papillary cystadenoma. The most common malignant salivary gland tumors are mucoepidermoid carcinoma, adenoid cystic carcinoma, acinic cell carsinoma and adenocarcinoma³³. The intraglandular mass lesions are hypoechoic when compared with surrounding homogeneous echogenicity of the normal gland parenchyma. Benign lesions tend to be small, well defined and not associated with enlarged cervical nodes, whereas malignant lesions are usually irregular and have heterogeneous internal structure³⁴. Malignant nodes are recognized by round shape, heterogeneity, loss of hilar architecture, abnormal, disorganized vascularity, cystic change and extracapsularspread35. Liu et al³³ compared US, computed tomography and magnetic resonance imaging for the clinical differential diagnosis of patients with salivary gland tumors. The specificity of US is generally good due to the fact that the majority of salivary gland tumors are benign.

LYMPH NODES

In lymphadenitis, the lymph nodes are enlarged (axial diameter measures more than 10 mm) with an ovoid to round shape. Nonspecific inflammatory lymph nodes are usually sharply bordered and the hilum is rarely visible. The ultrasonographic features of metastatic lymph nodes that can be depicted are increased size, a rounder shape, and heterogeneity caused by tumor necrosis, keratinization or cystic degeneration inside the tumor. Generally, round shape is considered to be more suspicious than an oval or flat shape. The size criteria may vary between 5 and 30 mm³¹. The lymph node status is one of the most important predictors of poor prognosis in head and neck tumors and it is important for the treatment plan. De Bondt et al³¹ performed a meta-analysis of the detection of lymph node metastases by comparing US, US guided fine needle aspiration cytology, computed tomography and magnetic resonance imagining in patients with head and neck cancer. In conclusion, US guided fine needle aspiration cytology was found to be the most reliable imaging technique to assess the presence of metastases in cervical lymph nodes in patients with head and neck cancer. Authors of another study conducted US on 18 patients with stage 1 and stage 2 squamous cell carcinoma of the tongue. They evaluated the histolopathological metastatic nodes. The sensitivity in the detection of smaller metastatic nodes for US (58%) was lower than that of CT (83%)³¹

IMPLANTOLOGY

US may play an important role in locating submerged implants. The authors reported that a new ultrasonic device including a soft tissue matched transducer with a customized transreceiver and signal processing was capable of measuring soft tissue thickness over bone and implants placed in porcine models. The authors also suggested that this new ultrasound device was efficient as a diagnostic tool for intraoral measurements of the inferior alveolar canal and floor of the maxillary sinus before dental implant placement¹⁹. Authors measured the distance from the bottom of the osteotome to the inferior canal and maxillary sinus floor using a novel ultrasonic device and conventional radiographs. A significant positive correlation was observed between the radiographic and US measurements. US has the potential to be an alternative diagnostic tool for implant dentistry owing to its nonionizing nature³².

DOPPLER ULTRASONOGRAPHY(US)

Doppler US has found wide spread use in the assessment of peripheral vascular disease. Authors evaluated Doppler US in the assessment of congenital vascular lesions of the maxillofacial region. Doppler US can be used to characterize the flow of head and neck vascular anomalies and to differentiate hemangiomas from other vascular malformations which is crucial in treatment planning⁶. It was also shown that ultrasound with color Doppler US was an effective tool in monitoring the healing of periapical lesions after surgery³⁷. Baladiutlizied US to identify factors associated with alterations of mental artery flow. Intraoral B-mode Doppler US was used to assess mental artery flow and mental artery pulse strength³⁸. Martins et al³⁹ by use of a Doppler US examined 65 patients who had submucosal and subcutaneous nodules. They found that US was an effective tool in the definitive diagnosis of nonspecific nodular lesions of the soft tissues located in the oral and maxillofacial region. Another study evaluated the efficacy of MRI and color doppler US in the diagnosis and differentiation of benign and malignant salivary gland tumors. Accuracy of Color doppler US was found to be 95% in determining tumor site.

CONCLUSION

As a conclusion, US is an innovative and evolving imaging technology with plenty of research continuing to be done in medical field. It is safe, rapid, portable and economic. Further studies towards clinical applications of the US in the dento-maxillofacial region are essential in order to obtain information regarding accurate and appropriate clinical usage of the system in dentistry.

REFERENCES