Signs of internal structures

1. air bronchogram (sign)
2. angiogram sign
3. CT halo sign
4. reversed halo sign

Air bronchogram –chest radiograph- Fig. 1

  Phenomenon of air-filled bronchi being made visible by the opacification of surrounding alveoli. It is almost always caused by a pathologic airspace/alveolar processes, in which something other than air fills the alveoli¹.

 Air bronchogram on chest radiograph is an important sign to describe a feature of airspace consolidation, but it is rarely used for nodular lesions. 

Fig. 1: Pneumonia. Chest radiograph shows consolidation with air bronchogram.

Air bronchogram -CT- Fig. 2  Fig. 4

  Meaning of the air bronchogram on CT is slightly different from that on chest radiograph. The CT air bronchogram sign is not only seen in air space consolidation, but also in solitary pulmonary nodules, more common in malignant than in benign ones².

  In small lung adenocarcinomas, AIS (adenocarinoma in site) and MIA (minimally invasive adenocarcinoma) are　characterized by air containing structures, which are differentiating features from invasive adenocarcinoma³. However, focal organizing pneumonia also shows air bronchogram or small bubble-like lucency in majority of the lesions⁴.

  MALT lymphoma can also present as a nodule or consolidation with air bronchogram, which often simulates adenocarcinoma.

Fig. 2: Adenocarcinoma. Thin-section CT (TSCT) shows an “air bronchogram” running into the lesion.

Fig. 3: Adenocarcinoma. Pathological microphotograh in low magnification view shows dilated bronchiole (arrow) entering into the tumor.

Fig. 4: MALT lymphoma. TSCT shows an “air bronchogram” in the lesion.

Angiogram sign  Fig. 5  Fig. 8

  CT angiogram sign consists of enhancing branching pulmonary vessels in a homogeneous low attenuating consolidation of lung parenchyma⁵. The sign can be observed in pulmonary consolidation of varying etiologies⁶.

Fig. 5: Mucinous adenocarcinoma. Contrast-enhanced CT scan reveals CT angiogram sign.

Fig. 6: Invasive mucinous adenocarcinoma. Pathological microphotograh in low magnification view shows patent pulmonary arteries (arrow) inside the lesion.

Fig. 7: Invasive mucinous adenocarcinoma. Pathological microphotograh in higher magnification view shows alveolar space filled with atypical goblet cells and mucus.

Fig. 8: MALT lymphoma. Contrast-enhanced CT scan reveals CT angiogram sign.

CT halo sign  Fig. 9 Fig. 10 Fig. 12

  CT halo sign is characterized by ground glass opacity surrounding a pulmonary nodule or mass. This finding was first described in patients with angioinvasive aspergillosis⁷; however, it can be seen in many other pathological conditions such as infection, neoplastic and inflammatory diseases⁸.

Fig. 9: Angioinvasive aspergillosis. TSCT shows ill-defined ground-glass opacity surrounding the pulmonary nodules.

Fig. 10: Adenocarcinoma. TSCT shows zone of intermediate attenuation surrounding the nodule.

Fig. 11: Adenocarcinoma. Pathological microphotograh in low magnification view shows fibrotic scarring at the center (arrows) and lepidic growth pattern (arrow heads) at the periphery.

Fig. 12: MALT lymphoma. TSCT shows ill-defined ground-glass opacity surrounding the pulmonary nodule.

Differential Diagnosis of CT halo sign⁸

• Infection

1. Fungal infection (aspergillosis,mucormycosis, etc)
2. Septic embolism
3. Mycobacterial infection
4. Viral infection(herpes simplex, varicella zoster, cytomegalovirus, etc)

• Neoplasm

1. Primary tumor (adenocarcinoma, Kaposi sarcoma, squamous cell carcinoma)
2. Metastasis (angiosarcoma,choriocarcinoma, osteosarcoma,etc)

• Miscellaneous

1. GPA (granulomatosis with polyangitis)
2. Eosinophic lung disease
3. Organizing pneumonia
4. Pulmonary endometriosis

 Reversed halo sign  Fig. 13

  A reversed halo sign represents a nodule, which has central ground-glass opacity surrounded by consolidation. This sign has been described as a finding in organizing pneumonia and other infectious or　inflammatory etiologies. The nodules with this pattern are typically multiple⁹.

Fig. 13: Cryptogenic organizing pneumonia. TSCT shows a ring of consolidation surrounding central ground-glass opacity.

Signs of marginal characteristics

1. spicula
2. notch
3. pleural indentation
4. pit-fall sign

Spicula  Fig. 14 Fig. 16

  Spicula in nodular lesions are defined as linear strands extending from the margin of nodules into the lung parenchyma but not extending to the pleura¹⁰. Spicula is one of the chracteristic findings in malignant nodules.

 This finding, however, can also be seen in benign nodules in the presence of emphysema¹¹. Spicula, therefore, cannot be used to reliably discriminate between malignant and benign nodules associated with severe emphysema.

Fig. 14: Adenocarcinoma. TSCT shows a pulmonary nodule with irregular margin and spicula.

Fig. 15: Adenocarcinoma. Pathological microphotograph shows irregular margin of the tumor with a spicula (arrow).

Fig. 16: Inflammatory nodule. TSCT shows a pulmonary nodule associated with pulmonary emphysema. The nodule shows irregular margin with spiculas simulating a malignant lesion. This nodule disappeared one month later.

Notch  Fig. 17 Fig. 18

 Notch or lobulation is defined as an abrupt bulging of the lesion contour. Malignant nodules generally have irregular spiculated margin with notch or lobulation¹⁰. The Rigler notch sign refers to an indentation in the border of a solid lung mass at a feeding vessel, thus suggesting bronchial carcinoma. However, this sign is also observed in other conditions including granulomatous infections, and its value in differential diagnosis is therefore limited.

 Notch can also be seen at the portion in contact with vessels in rapidly growing tumors, such as peripheral squamous or large cell carcinoma.

Fig. 17: Squamous cell carcinoma. TSCT shows a lobulated nodule.

Fig. 18: Vessels are seen at the notch of the tumor margin (arrow) in contrast-enhanced CT.

Fig. 19: Pathological microphotograph in low magnification view shows the vessels (arrows) at edge of the squamous cell carcinoma.

Pleural indentation  Fig. 20

  Pleural indentation (or pleural tag) consists of a linear opacity that extends from a peripheral nodule or mass to the visceral pleura. It can represent a strand of fibrous tissue that extends from the nodule to the visceral pleura or can result from inward retraction and apposition of a thickened visceral pleura¹⁰.

  Although they are associated most commonly with adenocarcinoma, they may be seen with other histologic subtypes; they also may be identified in pulmonary metastases and granulomas¹².

Fig. 20: Adenocarcinoma. TSCT shows a pulmonary nodule with irregular margin, spicula and pleural indentation.

Pit-fall sign  Fig. 21

  Pit-fall signs refer to multiple linear strands between the nodule and chest wall and/or interlobar fissure. Adjacent normal lung expands to fill the dead space between the retracted visceral pleura that corresponds to multiple indentations on CT.

    The pit-fall sign on preoperative CT suggests a possible pleural involvement correlated with a poor prognosis^13,14.

Fig. 21: Adenocarcinoma. TSCT shows a pulmonary nodule present far from chest wall. Adjacent normal lung expands to fill the dead space between the retracted visceral pleura that corresponds to multiple indentations on CT.

Fig. 22: Pathological microphotograph in low magnification shows the adenocarcinoma beneath the pleura. The pleura is invaginated into the lung.

Various CT signs

Sq. ca.; squamous cell carcinoma

FOP; focal organizing pneumonia 

Signs of specific lesions

1. air crescent sign
2. comet tail sign
3. feeding vessel sign
4. sarcoid galaxy sign 

Air-crescent sign  Fig. 23

  “Air-crescent sign” refers to the crescent of air seen in invasive aspergillosis, semi-invasive aspergillosis or other processes with necrosis¹⁵.

  However, the air around the fungus ball is also crescent shaped and the term ”air-crescent sign” is often used in that instance¹⁶.

Fig. 23: Pulmonary aspergillosis. TSCT shows a mass with “air-crescent sign” in the right middle lobe.

Comet tail sign   Fig. 24

  Rounded atelectasis is an unusual type of lung atelectasis where there is infolding of redundant pleura. It is a radiological diagnosis and must be differentiated from other mass-like lesions in the basal part of the lung, including malignant pleural and pulmonary tumors. Specific radiological features such as irregular pleural thickening and the “comet-tail” sign help to make the correct diagnosis and to avoid unnecessary surgery¹⁷.

Fig. 24: Rounded atelectasis. A “comet tail sign” is produced by the distortion of vessels and bronchi that lead to an adjacent area of rounded atelectasis on chest CT.

Feeding vessel sign  Fig. 25

  Feeding vessel sign consists of a distinct vessel leading directly to a nodule or a mass. This sign indicates either that the lesion has a hematogenous origin or that the disease process occurs near small pulmonary vessels¹⁸.

  A number of hematogenous non-neoplastic disorders of the lung can show this sign, for example:

• Pulmonary vasculitis
• Pulmonary infarction
• Septic embolism
• Angioinvasive pulmonary aspergillosis
• 

  Fig. 25: GPA (Granulomatosis with polyangitis). TSCT shows a cavitating nodule with distinct central vessel leading into it.

Sarcoid galaxy sign  Fig. 26

  Parenchymal nodules in pulmonary sarcoidosis shows a characteristic pattern resembling a galaxy, which corresponded to coalescent　granulomas¹⁹.

  This appearance is thought to result from aggregation of large numbers of interstitial granulomas rather than representing a true alveolar process. Some authors have therefore applied a more appropriate term "pseudoalveolar sarcoidosis".

Fig. 26: Pulmonary sarcoidosis. TSCT shows “sarcoid galaxies” that are composed of numerous small granulomas.