Normal Anatomy on chest X-ray

NORMAL  ANATOMY ON CHEST X-RAY

 The normal roentgen anatomy of the as seen on chest radiographs can be described in following headings.

Trachea

   Trachea is straight tube, midline in the upper part and deviates slightly to the right around the aortic knuckle. It shortens and deviates more to right on expiration. Its caliber is even with decreasing translucency as it is traced caudally. On plain chest radiograph the upper limits of coronal diameters in adults are 21 mm ( in females) and 25 mm (in males). The right tracheal margin ( Right paratracheal stripe ) can be traced down to the right main bronchus. It is 4 mm or less in thickness and measured above the azygos vein. The left paratracheal line is rarely visualized. After the age of 40 years, calcification of the cartilage rings of the trachea is a common finding. The enlarged azygos vein, which lies in the angle between the right main bronchus and trachea, may be normally seen as a round opacity in the tracheobronchial angle in the supine chest film.

Tracheobronchial Division

   The trachea divides into right and left main bronchus usually at D5 or D6 level in adults. The left main bronchus is longer and has more acute angle with trachea as compared to right main bronchus.

     The right main bronchus divides into upper lobe bronchus and bronchus intermedius. The upper lobe bronchus divides into apical, posterior and anterior segment bronchi. The bronchus intermedius divides into middle and lower lobe bronchi. Middle lobe bronchus has medial and lateral branches. The lower lobe bronchus has five branches; each for superior, anterior, lateral, posterior and medial basal segments of lower lobe. Absence of middle lobe on left side modifies the bronchial division on left side. The left main bronchus divides into upper and lower lobe bronchi. The upper lobe bronchus has two divisions; the upper division divides into apico-posterior and anterior branches to supply upper lobe, The lower division supplies the lingula with superior and inferior branches. The lower lobe bronchus on left side divides similar to the right side except the absence of separate medial basal branch. Major tracheobronchial divisions 

major Tracheobronchial division

                                          

Figures 1.6 A and B: Diagrammatic representation major tracheobronchial division as seen on frontal (A) and lateral (B) Orientation: (1-apical, 2-posterior and 3-anterior segments of upper lobe; 4-lateral segment of middle lobe/superior lingula, 5-medial segment of middle lobe/inferior lingula, 6-superior, 7-medial basal, 8-anterior basal, 9-lateral basal and 10- posterior basal segments of lower lobe)

Lungs

The lungs are divided into three lobes on the right side and two lobes on the left side by the interlobar. The major (oblique) fissures on both sides are similar. It runs obliquely forwards and downwards (upper portion facing forward and laterally and the lower portion facing backward and medially), passing through the hilum. On a lateral view, it starts at the level of fourth or fifth thoracic vertebra to reach the diaphragm 5 cm behind the costophrenic angle on the left and just behind the angle on the right side. 

major fissure on lateral chest

                                 1. Minor fissure

                                 2. Major fissure

Line diagram showing the position of major fissure on lateral chest radiograph (Reproduced with permission)

     The right lung has an additional fissure, the minor (horizontal) fissure. It can be drawn on chest PA film from right hilum to the sixth rib in axillary line 

minor fissure on PA chest

 Line diagram showing the positing of minor fissure on PA chest radiograph ( Reproduced with permission)

It separates the middle lobe from right upper lobe. There are some accessory fissure. which are occasionally seen. The azygos lobe fissure, so called because it contains the azygos vein on right and hemiazygos vein on left within its lower margin, is commonly seen on the right side with an incidence on 0.4 parcent.²⁸ It appears as a hairline with slight lateral convexity running across the right upper zone to end in a comma like expansion (azygos vein) near the hilum. The azygos lobe is the area of the ling medial to the azygos fissure. The left sided horizontal fissure, similar to the minor fissure  on the right, separated the lingular from the other upper lobe segment. The superior accessory fissure separated the apical from the basal segment of the lower lobes. The inferior accessory fissure separates the medial from the other basal segment.

Bronchopulmonary Segment     

        Beonchopulmonary segment of individual lobes are basal on the subdivisions of the lung, Which is supplied by an integral and relatively constant segmental bronchus and blood vessels. The boundaries between various segments are complex and with the rare exception of accessory fissure, the segments are not divided by septae. Although many pathological process may predominate in one segment or another, these usually never confirms precisely to whole of just one segment since collateral air drift occur across segmental boundaries. However, information of segmental involvement in disease process is particulary important to surgeons since these segments can be removed separately. These bronchopulmonary segments are designated as per the divisions of segmental bronchi. There is lot of overlap of bronchopulmpnary segments on a PA view of chest but they project separately on a lateral view. Their approximate location as seen on frontal and lateral radiographs is illustrated

  

Upper and middle lobe/lingula on PA projection

Lower lobe on PA projection

Right lung on lateral projection

Left lung on lateral projection

Line diagram showing approximate locations of various bronchopulmonary segments. A. upper and middle lobe/lingula on PA projection, B. lower lobe on PA projection, C. Right lung on lateral projection, D. Left lung on lateral projection (key same as figure 1.6)

   The radiographic density of the two lungs is symmetrical on a well-taken PA film. If the patient is rotated, the hemithorax closer to the film appear more radiodense. Both PA and lateral views are necessary to localise in one or more of the pulmonary segment. Since the normal bronchi are not visualised in the peripheral lung fields, it is difficult to make out the boundary of different pulmonary segment on plain radiograph of the chest.

Hilum and Pulmonary Vasculature   

The structures contributing to the formation of the hilum are the pulmonary arteries and their main branches, upper lobe pulmonary veins, the major bronchi and lymph glands. Of all the structures in the hilum, only the pulmonary arteries and upper lobe veins significantly contribute to the hilar shadows on a plain radiograph. Normal lymph nodes are not seen. The left hilum is usually 0.5 to 2 cm higher than the right . Both hila are of equal density and size with a concave lateral border on PA film.

    The diameter of the normal descending branch of right pulmonary artery is between 10-16 mm in males and 9-15 mm in female. The course of the pulmonary vessels can be described by dividing them into three zones depending upon their positions in the lunges, i.e. hilar, mid lung and peripheral. Mid lung vessels extend from hilum apto 2 cm from the chest wall. Peripheral vessels are present in other 2 cm of the lung fields and these are rarely seen on a normal chest radiograph. The pulmonary veins have fever branches and are straighter. The distinction between intrapulmonary arteries and veins is difficult and seldom useful so that they are collectively referred to us pulmonary vasculature. The pulmonary vessels taper radiographs; the upper zone vessels are comparatively narrower than lower zone vessels because of the effect of gravity. The bronchial vessels are normally not seen on chest radiograph.

Pleura  

Normal pleura is not visible on chest radiograph . The mediastinal surface of the pleura can occasionally be demonstrated near midline in a well-penetrated chest radiograph.

Mediastinum

It is a space lying between two lungs. It is bounded by sternum anteriorly, dorsal spine posteriorly and pleural sacs on both sides. The borders of the hearts and mediastinum are clearly defined except where the heart is in contact with the left hemidiaphragm. The bracheocephalic (innominate) vessels superior vena cava and right atrium from the right madiastinal border. Rarely a dilated aorta may also contribute. The left border is formed by left subclavian artery, aortic knuckle, left atrial appendage and left ventricle.

    The radiological division of the mediastinum can be ascetrained on a lateral chest radiograph by two imaginary lines 

Radiological divisions of the mediastinum

 Line diagram showing radiological divisions of the mediastinum (Reproduced with permission)

The first line is drawn from the diaphragm upward along the posterior border of heart and anterior border of the trachea into the neck. A second line is drawn connecting a point on each thoracic vertebra, 1 cm behind their anterior border. The anterior mediastinum is in front of the first line, the middle mediastinum is between the two lines and the posterior mediastinum is behind the second line.

  The anterior mediastinum contains thymus, heart with pericardium, great vessels and occasionally, aberrant thyroid. Middle mediastinum contains trachea and oesophagun. Nerve roots and descending thoracic aorta are the main contents of posterior mediastinum. Normal lymph nodes and adipose tissue is seen in all divisions of mediastinum. Conventional PA and lateral views of the chest are the first radiological investigation in any suspected mediastinal abnormality. However, a lesion may not be detected if it is not large enough to cause contour abnormality in the lung-mediastinum intrephase.

     In neonates and young children the normal thymus is seen as a triangular sail shaped structure with well-defined borders, sometimes wavy in outline. Its borders project from one or both sides of the mediastinum.

Mediastinal Lines and Interfaces

     As the two lungs approximate anteriorly, four layers of pleura and anterior mediastinum separate them forming a septum called as anterior junctional line. On PA film this line is oriented from upper right to lower left of the sternum. Similarly, posterior junctional line is produced by the posterior approximation of the lungs behind the oesophagus and anterior to spine. On PA film, the postrior junctional line usually projects through the air columm of trachea. Adjacent to the vertebral bodies runs the para spinal lines. Azygoesophageal recess is formed by contact of right lower lobe with esophagus and azygos vein. The recess is frequently identified on a well-penetrated PA film as an interface that extends from the diaphragm below to the azygos arch above. Typically, it is seen as a continuous arch concave to the right. It may be straight in young adults.²⁸ The paraspinal lines are usually 1 to 2 mm wide on PA film.

Heart

   Normally two thirds of the cardiac shadow lies to the left of the midline and one-third to the right. In normal individuals the transverse diameter of the heart on PA film is usually in the range of 11.5 to 15.5 cm. It is less than 11.5 cm in about 5 percent of people and only rarely exceeds 15.5 cm in heavy, stocky individuals. Assessment of cardiac size by determining cardiothoracic ratio is more usefil. Cardiothoracic ratio of 50 percent is accepted widely as the upper limit of normal, however, it exceeds 50 percent in at last 10 percent of normal individuals.²⁹ The cardiothoracic ratio may be upto 60 percent in neonates.³⁰

Diaphragm

   In most individuals it has a smooth dome shape. The peak or the hoghest point of the dome is medial. Flattening of the dome can be measured on PA view by dropping a perpendicular from mid point of the dome to the line connecting costophrenic and cardiophrenic angles of the same side. The distance is normally greater than 1.5 cm. In approximately 90 percent of normal individuals, the right hemidiaphragm is 1.5 to 2.5 cm higher than the left. In rest, either the domes are at the same level. The discrepancy in the level of the diaphragms is related to the position of the cardiac apex and not to the position of the liver. A  difference greater than 3 cm in the levels of the two hemidiaphragms is significant.

CONCLUSION

   Chest radiography still remains the first investigation in the diagnosis of various chest diseases. Knowledge of normal anatomy has utmost importance in proper diagnosis of disease process on chest X-ray. Conventional radiobraph may have technical limitation in some situations like critically ill patients in ICU; however, recent advances in electronics and computer technology have resulted in development of digital imaging which improves diagnostic quality of chest imaging.  

Chest X-ray Techniques and anatomy

                     Chest X-ray: Techniques and Anatomy

INTRODUCTION

Chest X-ray is the most commonly performed radiological investigation around the world and it forms an integral part of the routine study of individual case along with and as important as physical examination and laboratory investigation. The Chest radiograph nearly constitutes 50 to 60 percent of the total work load of the radiology department of any large or small general hospital. The cornerstone of the radiological diagnosis of the chest disease is chest radiograph. All other radiological procedures including bronchography, computed tomography (CT) and magnetic resonance imaging (MRI) are strictly ancillary.¹

        The techniques, various radiographic projections and normal anatomy of lung, mediastinum and diaphragm as demonstrated on plain chest radiographs have been discussed herewith.

CONVENTIONAL CHEST RADIOGRAPHY
Conventional film screen radiography using kV range of 50-85 depending on patient^,s build is the standard and most commonly used technique for chest evaluation. the benefits of this technique include low cost, high spatial resolution, operation simplicity and dependability. The important factors that influence the contrast in the radiograph include kilovoltage, shape of sensitometric curve of film, exposure parameter and conditions of film processing. At low kV, the difference in attenuation by soft tissue and bone or air and bone is large, resulting in high contrast. Calcified lesions, pleural plaque, pulmonary nodules are well delineated in low kV radiograph. However, some of the limitations of conventional chest radiograph are given here.

 * There is poor visibility of mediastinum, retro cardiac and subphrenic areas when

    lungs are well seen.

 * Lungs may be obscured by high contrast of bones.

 * Inadequate detail of airway and lung apices.

TECHNICAL ADVANCES

Following technical advance have been developed over the year to overcome limitation of the conventional chest radiograph.

 * High kV technique.

 * New film screen combinations.

 * Beam equalization radiography.

 *  Digital chest radiography.

HIGH kV TECHNIQUE

In this technique we use more than 120 kV. The coefficient of X-ray absorption of bone and soft tissue approach each other at high kV and thus the lungs are not obscured by bones. It has better penetration of the mediastinum which provides more details of airway. Short exposure time with high kV allows less scatter radiation to reach intensifying screens and results in sharp details of structures within the lang. However, high kV results in greater scatter radiation as compared to conventional radiography. Use of an air gap of 6 inches is required to reduce scatter radiation.²

NEW SCREEN FILM COMBINATION

Fine datails on radiography is principally determined by screen film system. Generally, medium speed system is preferred which provided better visualization of small vessels, fissures and depiction of abnormalities. The major advance in screen film system has been the introduction of faster rare earth phosphor screen and development of wide latitude film. The improved light emission from rare earth phosphor over traditional calciun tungastate crystal screen results in short exposure time and thus sharp image.

 Another important development is the introduction of asymmetric screen film system, the asymmetric zero cross over screen film system. It was introduced by eastman 

Kodak in 1990 called insight thoracic imaging system.³This uses different emulsion on either side of film base different front and back intensifying screens. In addition layers of absorbing dye in the film base prevent crossover of light between two emulsions so that both screen film combinations operate independently. Mediastinum without over

penetration of lung. Patient dose reduction up to 30 percent has been reported.⁴

Dupont in 1993 introduced an ultravision screen film system. In this system, screens use a high density rare earth phosphor (yattrium tantalate) which emits ultraviolet light that diffuses substantially less than the lower energy wave length visible light. The film emulsion used is symmetric.

 These combinations of film screen system have provided increased information that can be recorded and displayed. The asymmetric system is slightly superior particularly for visualization of mediastinal and retro diaphragmatic structures. The improved image sharpness achieved with these systems potentially can improve visualization of subtle parenchymal abnormalities.

BEAM EQUALIZATION RADIOGRAPHY

Screen film system provides acceptable image-contrast of chest radiograph in most situations. However, the relatively narrow range of film sensitivity limits image contrast in poorly penetrated areas of chest. The technique of beam equalization radiography refers to various parts of chest so as to produse a chest radiograph with uniform density of areas with extremely variable attenuation differences on the same film. This can be achieved by two methods:

A. Interposing a customized filter unique to the patient that would    attenuate the beam over the lungs and allow increased radiation exposure over the mediastinum.

B.  Modulation of exposure for each part of the chest by electronic feed back system.

  The first one lacks practicality, the latter one is the principle used in technique of beam equalization radiography that utilizes screen film receptors by increasing X-ray exposure in the thicker, denser part of chest while keeping the lung exposure unchanged, thereby reducing the dynamic range of intensities that ultimately reach the image recorder.^5.6

     Oldelft from Netherlands introduced in 1986 the Advanced Multiple Beam Equalization Radiography (AMBER) which is the only commercially available system for chest radiography. This system has horizontal X-ray fan beam which is divided into 20 adjacent beam segment,each of which is independently controlled by its own intensity modulator located in front of X-ray tube and corresponding exposure detector between patient and image recorder. As the fan beam scans the patient, the detector array measure local X-ray intensity passing through the patient and an electronic feed back mechanism dynamically adjust each of beam modulators such that dense areas are imaged at higher exposure levels. This increases signal to noise ratio in the denser areas of chest and shift the background film optical density in these areas on to higher contrast portion of H and D curve.

     The advantage of this technique are:

 *  Better delineation of mediastinum, restrocardiac and restrodiaphragmatic areas.

 *  Improved visualization of lung apices in lateral view.

The reported disadvantage of AMBER are:

 *  Decreased contrast between consolidation and normal lung.

 *  Edge artifacts occur where there are abrupt changes in radiolucency, e.g. lung heart       interface , lung diaphragm interface.

 *  Dark halo around the heart may simulate pneumo-mediastinum.

 *  Active imaging areas is limited to upright 14X17 orientation so it is not possible to        acquire transverse image of chest.

 *   Exposure parameter to be set manually.

 *  Difficulty in comparing the radiograph of patient with previous one using                       conventional technique. 
 * This system can not be used on bed side and for patient on stretcher.

 * Radiation does is about 50 percent more than conventional chest radiograph.

   The experience till date is not clearly indicative of the justification of additional expense even though images are more informative and this seems to have limited its popularity in clinical use.

 DIGITAL RADIOGRAPHY

Advances in electronics and computer technology over the past decades, have led to development of digital radiography or computed radiography system. This is different from conventional film based analogue system where the film is in direct contact with intensifying screen and there is no storage of information as digits in computer. In digital radiography, image detection can be completely separated from image display. The data of image is stored in the computer and can be retrieved , displayed, quantified, manipulated and hard copied whenever required.⁶

    Digital system using phosphor technique in which the entire receptor is exposed by conventional radiography equipment was introduced by Fuji in 1980 and is the most widely used technique for general digital radiography. This technique is based on reusable imaging plate coated with photostimulable phosphor material. When exposed to X-ray, a portion of X-ray is absorbed as to release stored energy as light and intensity of light measured and digitized. The resultant digital image is then preprocessed for contrast and spatial resolution before display. Imaging plate is ready for reuse after exposure to room light.

  Introduction of selenium detector system is an important development in digital chest radiography. Unlike storage phosphor detector which requires laser stimulation for image acquisition, selenium based detector capture image information as charge pattern and thus image can be read directly, eliminating image noise.^7,8 Also selenium is more efficient in detection of X-rays.
   Flat panel detectors are relatively new development in the technology. Depending on the material, there are two type of flat panel detectors, indirect type use a phosphor screen like cesium iodide to convert the X-ray to light photons. Direct flat panel detectors use instead a photoconductive layer, most commonly amorphous selenium that converts X-ray energy directly to charge. By using flat panel detectors, patient dose can be reduced without degradation of image quality and multiple images can be acquired in short-time.^9,10
        Dual energy imaging is s new technique which utilizes a receptor with two layers, each of which records different energy components of X-ray beam and is possible for a computer to analyze and separate the components of dual energy in order to display both soft tissue and bone of the few areas in which digital radiography has proved of diagnostic advantage over conventional chest radiography.
     Temporal subtraction imaging is used to improve the visual assessment of chest radiograph. This technique aim to selectively enhance areas of internal change by subtracting the patient^,s previous radiograph from the current one. Studies have shown that temporal subtraction improves the visual perception of subtle abnormalities such as pulmonary nodules, infiltrative opacities and diffuse lung disease.^11,12

  Digital tomosynthesis is a technique that has evolved from conventional tomography and solves many of the problems associated with conventional tomography. Digital Tomosynthesis can produce an unlimited number of section images at arbitrary depths from single set of acquisition images. This technique is another method for improving detection of subtle lesions such as pulmonary nodules.^13,14

DIGITAL RADIOGRAPHY AND CHEST

Major advantage of digital radiography lies in the control of display of optical density of radiographs in portable chest X-ray examination with dynamic range and control processing. It improves visibility of tubes and lines superimposed on the mediastinum. Although it may not offer any significant advantage over conventional film screen system, Digital radiography improves visibility of normal lung structures, thus one has to be careful in distinguishing prominent blood vessels from interstitial disease. To avoid this misinterpretation, mild to moderate edge enhancement is required for better visualization of interstitial disease. Due to smaller size of digital radiograph there is a definite learning curve to adjust to digital radiograph and one may have to interpret the film from a closer distance.

   Numerous observe performance studies have shown that digital radiography can equal conventional radiography in virtually any specific task. However, for this, post processing of the digital image is required to match the digital radiograph to the task. A problem inherent in all forms of digital manipulation is that enhancement of the image for one purpose, degrades it for another.

  There have been conflicting reports about whether digital. Chest radiography can be satisfactorily interpreted on high resolution television monitors, as distinct from laser printed films. Recent studies suggest that 2 K X 2 K monitors may be adequate for making primary diagnosis on digital chest radiograph.

RADIOGRAPHIC PROJECTIONS

Posteroanterior View (PA VIEW)

   The most satisfactory and standard radiographic view for evaluation of the chest is posteroanterior view with patient standing (fig. x-ray chest pa). Visualization of lung is excellent because of inherent contrast of the tissues of the thorax.

  The diagnostic accuracy of the chest disease is partly related to the quality of radiographic images. It is incumbent on all radiologists to ensure that images on which their diagnostic impression is based are of the highest quality. Careful attention to several variables is necessary to ensure such quality.
  Patient positioning

 Positioning must be such that the X-ray beam is properly centered, the patient^,s body is not rotated, and the scapulas are rotated sufficiently anteriorly  so that they are projected away from the lung. On properly centered radiographs, the medial ends of the clavicles are projected equidistant from the margins of the vertebral column.    

                                     Normal x-ray chest pa                                   Patient Respiration

Respiration must be fully suspended, preferably at total lung capacity (TLC). It has been shown that in erect chest radiographs, normal subjects routinely inhale to approximately 95 present of TLC without coaxing.¹⁵ thus, such radiographs can be of value in estimating lung volume and, by comparison with subsequent radiographs in appreciating an increase or decrease in volume as a result of disease.

Film Exposure

Exposure factors should be such that the resultant radiograph permits faint visualization of the thoracic spine and the intervertebral disks on the PA radiograph so that lung markings behind the heart are clearly visible. Exposure should be as short as possible, consistent with the production of adequate contrast. Unfortunately, all too frequently technical factors are such that optimal radiographic density is achieved over the lung generally but without adequate exposure of the mediastinum or the left side if the heart, a tendency that seriously limits radiological interpretation, moderate overexposure can be easily compensated for by bright illumination; underexposure on the other hand cannot be compensated for by any viewing technique and since it prevents visualization of vital areas of the thorax, should not be tolerated in any circumstances. With perseverance, it is always possible to overcome problems of underexposure.

   For a PA chest radiograph, the mean radiation dose at skin entrance should not exceed 03 mGy per exposure and the exposure time should not exceed 40 msec.¹⁶ An optimally exposed radiograph present the lung at a mid gray level (average optical density, 1.6 to 1.9). (Optical density is a measurement of the ability of the film to stop light (film blackness), and it is equal to the logarithm of light incident on the film over light transmitted by the film (D= log I_o/I_t). The focal film distance should be at least 180 cm ( 72 inches) to minimize magnification¹⁶ (Focal film distance is the distance between the focal spot of the X-ray tube and the radiograph).

     Kilovaltage
  A high kilovaltage technique appropriate to the film speed should be used;¹⁰ for PA and lateral chest radiographs, the recommended kvp is 115 to 150 kvp. Since the coefficients of X-ray absorption of bone and soft tissue approximate each other in the higher kilo-voltage ranges, radiographic visibility of the bony thorax is reduced with only slight changes in the overall visibility of lung structures. Furthermore, the mediastinum is better penetrated, thereby permitting visibility of lung behind the heart and many mediastinum lines and interfaces whose identification is so important to the overall assessment of both the mediastinum and lung. This technique can produce chest radiographs superior in all respects to those obtained with other techniques in addition to better penetration of the mediastinum. High kilo-voltage also results in lower radiation exposure than does lower kilo-voltage. The only drawback of the high kilo-voltage techniques is the diminished visibility of calcium that results from the lower coefficient of X-ray absorption; however this shortcoming has not proved troublesome in practice.
  Grids and filters
When using a grid, at least a 10:1 aluminum interspace grid with a minimum of 103 lines per inch recommended by the American college of Radiology.¹⁶ An alternative option uses an air gap technique in which a space of 15 cm (6 inches) is interposed between the patient and the X-ray.¹⁷ Since the air gap reduces radiation scatter by distance dispersion, no grid is required. When this technique is used a constant focal film distance of 10 feet is recommended. In a comparative study of air gap and grid technique, it was shows that the former can provide contrast equal to those obtained with grids;¹⁸ of the various combinations of distances possible. A focal distance of 10 feet with an air gap of 6 inches provides a good compromise. Patient exposure with an air gap technique was comparable to a no-grid, no-air gap technique and was less than that obtained with a grid.