Right Middle lobe (RML) is a relatively common site for consolidation. Its wedge-shape must be understood in three dimensions to fully understand its appearance on plain film

One of the unfortunate aspects of the term consolidation is that its meaning can be different depending on who is using the term. When a clinician uses the term consolidation he/she is usually referring to a consolidation associated with acute pneumonia. Thus, the term consolidation and pneumonia have very similar meanings and are almost used interchangeably.

Strictly speaking, the term consolidation does not imply any particular aetiology or pathology. Acute pneumonia is the commonest cause but not the only cause of consolidation. (other causes include chronic pneumonia, pulmonary oedema and neoplasm). Thus when a radiologist has reported a chest X-ray examination and notes the presence of consolidation he/she is simply stating that some of the long airspace has been replaced by a fluid.

• Refers to fluid in the airspaces of the lung

• Consolidation may be complete or incomplete

• The distribution of the consolidation can vary widely. A consolidation could be described as “patchy”, “homogenous”, or generalised”.

• A consolidation may be described as focal or by the lobe or segment of lobe affected

adapted from By Fred W. Wright Radiology of the Chest and Related Conditions: Together with an Extensive Illustrative Collection of Radiographs CRC Press, 2002

The right middle lobe has two pulmonary segments which are situated side by side; the more lateral segment, approximates the size of its adjacent neighbour ( medial segment). The medial segment abuts the right heart border medially , while lateral segment extends to and comprises a portion of the lateral border of the right lung. http://lib.cpums.edu.cn/jiepou/tupu/atlas/www.vh.org/adult/provider/radiology/LungAnatomy/RightLung/RtLungSegAnat.html. When viewing chest radiographs with pathology involving the right middle lobe, it is important to think about the shape and position of the RML in three dimensions. This may not be easy at first.

adapted from By Fred W. Wright Radiology of the Chest and Related Conditions: Together with an Extensive Illustrative Collection of Radiographs CRC Press, 2002

Further information on lung anatomy here

Radiological appearances common to all lobes are:

1.Abnormal lung opacity

2.Increase in the size and number of lung markings

3.Loss of clarity of the diaphragm on the AP and/or lateral views

4.Loss of clarity of the heart border on the AP and/or lateral views

5.Air bronchogram lines

6.Loss of the normal darkening inferiorly of the thoracic vertebral bodies on the lateral view

7.Opacification of the lung behind the heart shadow or below the diaphragms

It is widely considered axiomatic that the loss of visualisation the right hemidiaphtragm is a sign of RLL disease. Felson (Chest Roentgenology, W.B. Saunders, 1973, p38) has challenged this contention on the grounds that the right hemidiaphragm is widely variable in its shape and could be obliterated by middle lobe disease in some patients

It is also widely considered axiomatic that the loss of visualisation of the right heart border is a sign of RML disease. Felson (Chest Roentgenology, W.B. Saunders, 1973, p38) notes that in about 5% of normal individuals, the right border of the heart and aorta do not project into the right hemithorax. The silhouette sign cannot be applied to these individuals in respect of any loss of visualisation of the right heart border. It is also true that is some individuals with RML disease, silhouette sign will provide the only plain film evidence.

•Seen as an area of increased opacity in the shaded area •Loss of the definition of the right heart border is often seen	RML opacification Loss of adjacent right heart border
•RML consolidation is characteristically seen as a wedge opacity in the lateral view •May be sharply bordered by the horizontal and oblique fissures •(collapse of the lingula segment of the LUL has a similar appearance)	Wedge shaped opacity characteristic of RML consolidation(black arrow) lingula segment consolidation can have a similar appearance on the lateral view

K Ashizawa, MD, K Hayashi, MD, N Aso, MD and K Minami, MD Lobar atelectasis: diagnostic pitfalls on chest radiography British Journal of Radiology 74 (2001),89-97 © 2001	The right cardiac border is not clearly seen suggesting silhouette sign associated with collapse and/or consolidation (white arrow). The appearance is a little inconclusive- the same appearance can be seen in patients with pectus excavatus
K Ashizawa, MD, K Hayashi, MD, N Aso, MD and K Minami, MD Lobar atelectasis: diagnostic pitfalls on chest radiography British Journal of Radiology 74 (2001),89-97 © 2001	the lateral image demonstrates a prominent oblique fissure (arrowed)
K Ashizawa, MD, K Hayashi, MD, N Aso, MD and K Minami, MD Lobar atelectasis: diagnostic pitfalls on chest radiography British Journal of Radiology 74 (2001),89-97 © 2001	The lordotic view image clearly demonstrates complete collapse of the right middle lobe (arrowed)

This patient presented for chest radiography with a history of recent facial nerve palsy. There is loss of the right cardiac border.

There is no abnormal RML opacity. There is evidence of pectus excavatum (depression of the sternum). Pectus excavatum is a known cause of false RML disease and is likely to be a contributing cause of the pseudo-silhouette sign on the PA image. In these patients the heart tends to be displaced towards the left as a result of the limited space between the depressed sternum and the spine.Other contributing factors could be pericardial fat (although none is seen in this patient) and overlying pulmonary vessel. Pectus Excavatum http://en.wikipedia.org/wiki/File:Pectus1.jpg

Pericardial Fat

Validation of cardiovascular magnetic resonance assessment of pericardial adipose tissue volumeAdam J Nelson , Matthew I Worthley , Peter J Psaltis , Angelo Carbone , Benjamin K Dundon , Rae F Duncan , Cynthia Piantadosi , Dennis H Lau , Prashanthan Sanders , Gary A Wittert and Stephen G WorthleyCardiovascular Research Centre, Royal Adelaide Hospital & Disciplines of Medicine and Physiology, University of Adelaide, Adelaide, SA, Australia

This is a sheep's heart with the pericardium partly dissected off. Note the pericardial fat. Felson (Chest Roentgenology, W.B. Saunders, 1973, p55) notes that "For reasons that escape me, fat in the thorax is often impossible to differentiate from water density. Perhaps the adjacent pulmonary gas density has something to do with this illusion." This is an interesting point given the clear differentiaton between fat and water density structures seenin the abdominal plain film.

Case 1

This 49 year old female patient presented for chest radiography with "recent onset of chest pain". There is loss of clarity of the right heart border. In addition, there is loss of clarity of the left heart border. There are good reasons to consider that these silhouette signs are not caused by lung consolidation. The patients history and clinical signs do not suggest a diagnosis of infection/consolidation there is no evidence of alveolar opacity the lateral view does not demonstrate alveolar opacity that you would expect with a lung consolidation The likely cause of the loss of clarity of the cardiac silhouette is the presence of pericardial fat.

There is no abnormal RML or left upper lobe (lingula segment) opacity. Where there is a significant amount of pericardial fat visible on the PA/AP chest image, you would expect to see evidence of this on the lateral image (white arrow)

patient with minimal pericardial fat for comparison (ignore arrow)

The triangular density demonstrated in the right upper thorax is not RML disease. Rather, it is a normal paediatric thymus.