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Radiology Essentials 101 :Localization of intrathroracic lesions


Hello Everyone!! We are happy to announce a new service for all the Residents out there who are either pursuing MD Radiodiagnosis or DNB Radiology, but mainly this will help DNB Residents as many people complain that there are no academics in their Institute. So to take care of this we will bring to you 1-2 blogs per week to understand Radiology better and to get yourself oriented with the topics better. We have tried to make these blogs simple to understand and easy to digest. Dr Aniket Agarwal who is currently Pursuing his DNB Radiodiagnosis at DCA Imaging Centre ,Delhi will be talking with you about First year topics which are important for theory and practicals. Dr Khanak Nandoliya whom you all know very well will be dealing with Second year topics. We are planning to start a similar service for Medicine and Surgery too.. So that Academics are not a problem for you . Stay tuned for more in the coming days :)

In case you find any mistake in the current blog, please contact at [email protected]



  • This blog contains few basic concepts of Xrays which I found very difficult to understand when I was very new to radiology.
  • Before reading this blog, just go through the second chapter from FELSON.



  • Centering  at  t5
  • Breasts should be compressed against the film.
  • LOW KV (60-80) = high contrast film
  • miliary shadowing and calcification seen better.
  • HIGH KV (120-170) =  low resolution
  • Hidden areas are seen better.


  • Side of interest: near to the film
  • In NORMAL person, if right diaphragm is higher than the left, then it is a right lateral view and vice versa.

Difference between AP and PA views

In AP view:

  • Heart is magnified
  • Scapulae overlie the lungs
  • Clavicles are projected more cranially over the apices
  • Disc spaces of lower cervical spine are seen more clearly

For visualisation of apices:

1.Apical view: tube is angled up to 50-60 degress

2.Lordotic view: tube is angled down with patient in lordotic position

How to detect rotation

Normally, medial ends of both clavicles are equidistant from the midline (spinous process).

  • Suppose the patient is rotated as little as 2-3 degrees away from the film, the density of the lung closer to the film will be uniformly greater than that of the other lung, i.e. the lung closer to the film will be more white as the beam will have to pass through greater thickness of thoracic wall musculature.
  •  So if patient is rotated towards right, then right lung will be slighly more black than the left lung.


Characteristics of expiratory firm

  • Heart is larger
  • Basal opacity due to crowding of normal vascular markings
  • Trachea appears more shifted to the right.
  • Shortening of trachea ( normally trachea bifurcates at the level of T4-T5): therefore, endotracheal tube is placed few centimetres above the carina, else it may occlude the bronchus on expiration.

Points to be noted

  • Left hemidiaphragm is lower than right in 97% population. Right may be lower in 3%. A difference of >3cm is significant.
  • Left hilum is always higher than the right or at the same level, but never lower than the right hilum.
  • With good inspiration, we should be able to count posterior 9-10 ribs.

§With adequate penetration, vertebrae should be just visible behind the heart shadow.

§Under-penetrated= too light, obscures the left lung base

§Over-penetrated= too dark, mimics pneumothorax.

§The medial end of clavicle should overlie the 3rd posterior rib.

We see lateral costophrenic angles (sulci) on PA/AP view and it takes about 250-300ml fluid to blunt them.

We see posterior costophrenic angles on lateral chest xray and it takes only about 75ml of fluid to blunt them. 


PA view-oblique fissure

It is usually invisible as the roentgen beam doesn’t strike it tangentially.

PA view-horizontal fissure

The horizontal fissure is seen, often incompletely, running from the hilum to the region of 6th rib in the axillary line.

May be straight or have a slightly downward curve.


The upper posterior extent of the septum lies at the level of vertebral end of 5th rib/ interspace, often slightly lower on the right than on the left. It parallels 6th rib.–FELSON.

Both oblique fissures commence posteriorly at the level of T4/5, passing through the hilum. The left is steeper and finishes 5cm behind the anterior cardiophrenic angle, whereas right ends just behind the anterior cardiophrenic angle –SUTTON.


It usually meets the oblique septum in the mid-axillary line at the level of 5th rib/interspace.

It runs anteriorly and often slightly downwards, it is gently curved with convexity upwards.


Next three slides show segments on PA and lateral views which I have marked with a glass marking pencil.

The following characteristics indicate that a lesion originates within the mediastinum:

1.Unlike lung lesions, a mediastinal mass will not contain air bronchograms.

2.The margins with the lung will be obtuse.

3.Mediastinal lines (azygoesophageal recess, anterior and posterior junction lines) will be disrupted.

There can be associated spinal, costal or sternal abnormalities.

Silhouette sign

  • An intrathoracic radiopacity, if in anatomic contact with a border of the heart or aorta, will obscure/obliterate that border.
  • If an opacity just overlaps the heart border and doesn’t obliterate it, then it must lie either anteriorly or posteriorly.



Salient Points

  • Density in the RML or Lingula can produce silhouette sign on heart border or asc. Ao, but one in the lower can not.
  • Involvement of apico-posterior segment of LUL almost always obliterates the aortic knob.
  • Density in anterior segment  of an upper lobe obliterates the heart border or ascending aorta, while a posterior segment lesion doesn’t.
  • Posterior encapsulation pf pleural fluid doesn’t produce silhouette sign on heart or acsending aorta but anteromedial encapsulation or free pleural fluid does. 

  • In previous xray pa view, we see the opacity obliterates the right diaphragm, which means that it should be present in lower lobe.
  • This is confirmed by the lateral view.
  • Normally, the vertebrae should become more blackish as we go downwards. But if they become more white, then it confirms a lower lobe lesion, that too in posterior basal segment.


The proximal segment of visible right and left pulmonary artery lies lateral to cardiac shadow or just within its outer edge in >98% people and lies slightly more than 1 cm within cardiac silhouette in rest

  • It helps in distinguishing enlarged heart from a mediastinal mass.
  • With a mediastinal mass, hilum is seen through the mass.
  • However, in case of cardiomegaly, enlarged heart will push the hilum laterally so that vessels will seem to emerge from border of enlarged heart.


  • If the pulmonary artery branches converge towards the mass rather than the heart, we are dealing with a an enlarged pulmonary artery.
  • The reverse indicates a mediastinal mass.


  • Based on the tenet that if a thoracic lesion is in anatomic contact with the soft tissues of the neck, its contiguous border will be lost.
  • Lesion visible above clavicles- lies in posterior mediastinum and entirely in the thorax.
  • Cephalic border of lesion disappears as it approaches clavicle- the lesion is cervico-thoracic i.e. it lies partly in the anterior mediastinum and partly in the neck.
  • In simple words, an anterior lesion will not seem to extend above the clavicles as its shadow will merge with the shadow of soft tissues of the neck. So if a lesion extends above the clavicle, it lies in posterior mediastinum.

In case you find any mistake, please contact at [email protected]

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