16.1 Introduction

Etiology of the dental infections can be broadly classified into mechanical including trauma, fractures, bruxism, and thermal changes; chemical including various acidic substances; and bacterial including caries, microleakage or periodontal infections [1]. Although the periapical region is an area where lesions are commonly seen, most of these lesions are of inflammatory origin [2]. Dental pulp consists of vascular connective tissue and various factors like physical, chemical, or bacterial factors can affect it resulting in the onset of the inflammatory process leading to apical lesions.

Periapical radiographs together with clinical examination as a gold standard are often sufficient without the need for CBCT imaging in the diagnosis, treatment, and follow-up of inflammatory lesions [3–5]. Inflammatory process not only describes periapical bone loss but also consists of all the dynamic host responses to the infection, and conventional radiographs are not able to provide information about these variables [6, 7]. Thus, there is a need for an extra imaging modality in the diagnosis, treatment, and follow-up stages to monitorize the content of the lesion, tissue architecture, vascularity, and mineralization.

Ultrasonography (USG) is a real-time, noninvasive, and nonionizing imaging method that is able to collect information about content, vascularity, and more by obtaining images of apical lesions as an assistive imaging procedure to conventional radiographs [8, 9]. Besides, in order for an inflammatory process to be imaged by ultrasonography, it must cause changes in periapical tissues or lead to buccal cortical resorption. Therefore, reversible or irreversible pulpitis can not be imaged by ultrasonography unless they cause any severe changes in periapical tissues, after which they become periapical infections. Eventually, USG has over 95% accuracy and very high sensitivity and specificity in diagnosing periapical lesions as compared to conventional and digital radiography [10].

Lesions associated with significant symptoms, such as pain or swelling, are referred to as acute or symptomatic, whereas those with mild or no symptoms are identified as chronic or asymptomatic [11].

Condensing osteitis is usually asymptomatic variation of apical infection, and is characterized by an increase of bone trabeculation related with persistent irritant factor. Therefore, if there is no secondary infection in soft tissue, it cannot be visualized by USG.

16.2 Acute Apical Periodontitis

Acute apical periodontitis is a painful inflammation of periodontium arising from trauma, irritation, or pulpal infection. It is an acute condition that occurs when inflammation spreads rapidly to the periradicular region for the first time. It is generally located around the apex of the root. Normally buccal cortical resorption is not expected, but periosteal reaction can be seen. Thus, changes in periapical tissues can be displayed in USG examination if cortical resorption occurs or periosteal reaction develops. Also, increased blood supply can be seen around the related region (Figs. 16.1, 16.2, and 16.3).

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