Ultrasound may locate moderately sized adrenal tumours, but it can be technically challenging and is operator dependent. The majority of lesions are completely solid, round and well-defined with a homogeneous echotexture which may be iso- or hyperechoic to liver and therefore similar to that of adjacent renal parenchyma [25]. However, approximately half of the solid tumours will appear complex, demonstrating a heterogeneous appearance with interspersed hyper- and hypoechoic regions. This correlates with macroscopic findings of intratumoural haemorrhage and necrosis respectively [25]. Cystic tumours appear as primarily anechoic lesions with posterior acoustic enhancement confirming their liquid property, correlating with old evolved haemorrhage and necrotic debris at pathology.

An adrenal protocol scan (unenhanced, 60 s portal-venous phase and 15 min delayed post-contrast phase) is usually performed in order to quantify the intracellular lipid content of the lesion and to measure enhancement and washout characteristics (Fig. 8.2). Pheochromocytomas vary widely in appearance on non-contrast CT, with a range of densities from low to soft tissue. Although the majority will have an attenuation value of around 40–50 HU rarely HU may be much lower [17, 26]. Conversely higher-density lesions are found in cases of intratumoural haemorrhage. Approximately 10 % of lesions are reported to contain calcification, with nearly a quarter of symptomatic lesions being calcified. Following contrast administration, these tumours typically show avid enhancement of their solid components and may show homogeneous or heterogeneous enhancement depending on how complex their composition and relative amounts of internal haemorrhage and necrosis. Lesions show an absolute washout of less than 60 % and a relative percentage washout of less than 40 % at 15 min, or an absolute washout of less than 50 % and a relative percentage washout of less than 40 % at 10 min [27, 28]. However, variations can occur, and one study has shown pheochromocytomas to have similar washout patterns to adenomas [29].

Pheochromocytomas have been described as having a classical ‘light-bulb’ bright appearance on T2W, being brighter than fat and comparable with CSF [30]. They are typically isointense to muscle and hypointense to liver on T1W imaging. However, variations in appearance occur due to the often complex haemorrhagic and necrotic components. Haemorrhage of varying ages can have different signal characteristic on T1W images, with methaemoglobin in subacute haemorrhage resulting in high T1 and low T2 signal intensities. The rare cases that contain intracellular lipid will show loss of signal intensity on CSI (dual echo in and out of phase T1-weighted MRI) [31]. Pheochromocytomas typically demonstrate avid contrast enhancement post gadolinium administration, with a heterogeneous appearance when there are haemorrhagic, necrotic or cystic components [32].

The carotid body is the commonest site, followed by jugular foramen, middle ear and along the distribution of the vagus nerve. Less common sites include the orbit, nasal cavity, nasopharynx, thyroid gland, pineal gland and cheek [35]. The carotid body, within the medial aspect of each carotid bifurcation, is the largest compact collection of paraganglia in the head and neck [33]. Carotid body tumours appear as a well-defined soft tissue mass within the carotid space of the infrahyoid neck, splaying the internal and external carotid arteries [36]. On CT, these lesions demonstrate avid homogeneous enhancement due to their marked hypervascularity (Fig. 8.3a–d) [33]. Occasionally, particularly in larger tumours, haemorrhage and necrosis will result in heterogeneous non-enhancing regions. On MRI they are typically of intermediate signal intensity on T1W and high on T2W imaging. A classical feature is the so-called ‘salt-and-pepper’ appearance, characterised by multiple punctate and serpiginous signal voids due to fast-flowing internal vessels, interspersed with high-signal regions due to slow flow or haemorrhage [37].

These constitute 1–2 % of all paragangliomas and occur predominantly in mediastinal compartments, with less common sites being the trachea, lung, heart and oesophagus. Aorticopulmonary paragangliomas, which are located within the anterior mediastinum, tend to present late as incidental lesions on chest radiographs, whereas the sympathetic posterior mediastinal lesions present earlier with symptoms related to catecholamine secretion [38]. On CT, lesions appear as well-enhancing typically homogeneous masses; however, in cases where there has been haemorrhage or cystic degeneration, patchy areas of low attenuation within the lesion may be seen [33]. Angiography demonstrates marked hypervascularity with multiple feeding vessels; preoperative embolisation may be considered in cases of bulky or surgically challenging tumours [39]. On MRI, lesions are homogeneous or heterogeneous intermediate signal intensity similar to liver parenchyma on T1W imaging, and are hyperintense compared to liver parenchyma on T2W images (Fig. 8.3e) [40].

Retroperitoneal paragangliomas are most commonly seen in the tissues surrounding the aorta and inferior mesenteric artery. CT demonstrates a soft tissue mass with either homogeneous enhancement or central areas of low attenuation. Focal areas of high-density or punctate calcification due to haemorrhage may be seen [41]. On MRI lesions appear isointense or hypointense compared to liver on T1W imaging and are markedly hyperintense on T2W imaging [42].

A wide variety of imaging methods may be used for localising small functioning primary tumours, reflecting the difficulties encountered in detection. Local expertise also dictates the type of imaging used.

Bronchopulmonary NETs arise from Kulchitsky cells, neuroepithelial bodies or pluripotential bronchial epithelial stem cells in the bronchial epithelium [88, 89]. Tumours are graded by the World Health Organization according to their malignant potential, from low grade typical carcinoid to small-cell carcinoma [90].

Imaging features vary depending on whether the tumour is located in the airways of the central/middle third of the lung (80 % of cases) or in the peripheral airways [91]. Plain radiography may demonstrate a well-demarcated round or ovoid mass, which is often notched [92]. Centrally located tumours will appear as a central mediastinal or hilar mass, usually measuring 2–5 cm in diameter. These lesions may result in airways obstruction, with recurrent infection and/or lobar collapse [92]. However, as they are often small, CT scanning is more sensitive for detection. Lesions within the bronchial lumen typically brightly enhance (which can mimic a vessel), usually with both an intra- and extraluminal component being visible. Distal collapse or air trapping may be seen due to ball-valve obstruction of the bronchial lumen. Peripheral bronchial NETs are seen in 20 % of cases, appearing as solitary pulmonary nodules, typically round or ovoid with a smooth or lobulated border, commonly with punctate or diffuse calcification. Cavitation and hilar adenopathy and mediastinal invasion may be seen in the more aggressive histological subtypes [92]. In patients with occult ectopic ACTH secretion, a bronchial NET is the commonest source; however, these can be difficult to identify. When a pulmonary lesion is suspected but not seen on CT, MR may aid localisation. Bronchial carcinoids have high signal intensity on T2W and short tau inversion recovery (STIR) images, allowing differentiation between a small mass and the pulmonary vasculature [93].

NETs of the thymus are extremely rare, typically presenting with an anterior mediastinal mass. Thought to arise from thymic cells of neural crest origin (Kulchitsky cells), these tumours have similarities to bronchial NETs with a range of tumour differentiation and behaviour. Approximately 50 % of tumours are functioning, with about 40 % presenting with Cushing’s syndrome, in which case the mass is usually smaller than those patients presenting with non-functioning tumours [94]. Patients with non-functioning tumours (Fig. 8.8) usually present with symptoms related either to mass effect and/or local invasion. Lesions may be heterogeneous on CT (Fig. 8.9) and may show calcification. Assessment of the SVC for obstruction is vital [95, 96]. Invasive disease as evidenced by extension into the pleura, pericardium, great vessels or regional lymph nodes is also frequently seen in this group of patients [97]. Bilateral adrenal hyperplasia may be present in those with functioning ACTH-secreting tumours [95]. Metastatic disease in lungs, liver and bone, which may be sclerotic, may also be present at the time of diagnosis [96–99].

Gastric NETs account for 0.3 % of gastric neoplasms but 11–41 % of all GI NETs [100]. Gastric, duodenal and colorectal neuroendocrine tumours are usually diagnosed by endoscopy with EUS to determine depth of invasion and regional nodal status and for fine-needle aspiration [62]. CT is predominantly done to detect regional and distant metastatic disease. Administration of an antiperistaltic agent and a negative oral contrast agent (water) may also improve detection of the gastric NETs. CT should include an arterial phase acquisition at 25–30 s following the injection of contrast agent at a rate of 4–5 ml/s, followed by a delayed portal-venous phase at 60 s.

Type I gastric NETs, associated with atrophic gastritis, are poorly seen on CT and MR, being multicentric and typically <1 cm in size. The disease is almost always benign, with metastases present in only 2 % of cases. Type II gastric NETs are associated with MEN-1. There are typically multiple lesions within the gastric wall, which is thickened due to the association with the ZES. There is an increased tendency to metastasise to regional lymph nodes, although the prognosis is generally good [101]. Type I and type II gastric NETs may appear similar to other gastric polyps as numerous enhancing submucosal lesions [102]. Type III sporadic gastric NETs are usually solitary, large lesions with an irregular contour and diffuse margins that may ulcerate. Local invasion with extension into the surrounding gastric fat and metastases are common at presentation.

Midgut NETs arise beyond the ligament of Treitz to the level of the mid-transverse colon. They are the commonest primary malignant tumour of the small bowel, as well as being the commonest location of all GI NETs [103]. Patients present with abdominal pain, usually with associated colic and diarrhoea, or with symptoms of obstruction that may be due to the primary tumour or due to intussusception (Fig. 8.10). Tumour-associated desmoplastic mesenteric fibrosis due to the local production of fibrogenic agents results in tethering and kinking of the small bowel mesentery that can also result in obstruction. Metastases are often present at diagnosis, most frequently to the liver, bone and lung [104–106]. The incidence of metastases is dependent on tumour size [104, 106], with tumours <1 cm having metastases in 15–25 % of cases, 1–2 cm in size 58–80 % of cases and >2 cm in over 70 % of cases.