Chest Radiography

CHEST RADIOGRAPHY

CHEST RADIOGRAPHIC ANATOMY

The lungs lie within the thoracic cavity on either side of the mediastinum, separated from the abdomen by the diaphragm.

The right lung is larger than the left due to the inclination of the heart to the left side.

In normal radiographs of the thorax, some lung tissue is obscured by the ribs, clavicles and, to a certain extent, the heart, and also by the diaphragm and upper abdominal organs in the postero-anterior projection.

Chest Radiography

The right lung is divided into upper, middle and lower lobes, and the left lung is divided into upper and lower lobes. The fissures that separate the lobes can be demonstrated in various projections of the thorax, when the plane of each fissure is parallel to the beam.

Chest Radiography

On a postero-anterior radiograph, however, the main lobes overlap, so that for descriptive purposes the lungs are divided into three zones separated by imaginary horizontal lines.

Chest Radiography

The upper zone is above the anterior end of the second ribs, the mid-zone is between the second and fourth ribs anteriorly, and the lower zone is below the level of the fourth ribs.

Chest Radiography

On a lateral radiograph, where horizontal and oblique fissures are visible, upper, middle and lower lobes can be defined separately.

Chest Radiography

On a postero-anterior radiograph, a horizontal fissure separating the upper and middle lobes may be seen extending from the right hilum to the level of the right sixth rib laterally.

Chest Radiography

An accessory lobe called the azygos lobe is sometimes seen in the right upper zone as a result of aberrant embryological migration of the azygos vein to the normal medial position.

Chest Radiography

The trachea is seen centrally as a radiolucent air-filled structure in the upper thorax, which divides at the level of the fourth thoracic vertebra into the right and left main bronchi. The right main bronchus is wider, shorter and more vertical than the left, and as a result inhaled foreign bodies are more likely to pass into the right bronchial tree.

Chest Radiography

The main bronchi enter the hila, beyond which they divide into bronchi, bronchioles and, finally, alveolar air spaces, each getting progressively smaller.

Chest Radiography

As these passages are filled with air, they do not appear on a normal radiograph of the thorax, since the surrounding lung is also air-filled. If the parenchyma is consolidated, however, an air-filled bronchogram is shown.

The hila regions appear as regions.

Chest Radiography

The hilar regions appear as regions of increased radio-opacity and are formed mainly by the main branches of the pulmonary arteries. The lung markings that spread out from the hilar regions are branches of these pulmonary arteries, and are seen diminishing in size as they pass distally from the hilar regions.

Chest Radiography

The right dome of the diaphragm lies higher than the left, due mainly to the presence of the liver on the right. The cost-phrenic angles and lateral chest walls should be defined clearly.

HEART

a, superior vena cava

• b, ascending thoracic aorta
• c, right atrium
• d, inferior vena cava
• e, left subclavian vein
• f, aortic knuckle
• g, main pulmonary artery

h, left ventricle.

Cardiothoracic ratio

The size of the heart is estimated from the postero-anterior radiograph of the chest by calculating the CRT.

This is the ratio between the maximum transverse diameter of the heart and the maximum width of the thorax above the costophrenic angles, measured from the inner edges of the ribs.

In adults, the normal CRT is maximally 0.5. In children, however, the CRT is usually greater.

Chest Radiography

The aortic knuckle is shown as a rounded protrusion slightly to the left of the vertebrae and above the heart shadow.

The prominence of the aortic knuckle depends upon the degree of dilation or unfolding of the aorta and the presence (or absence) of cardiac disease.

It also alters shape as a result of deformities in the thorax, intrinsic abnormalities and with old age. Calcification in the arch, when present, is demonstrated as curvilinear opacities.

PATIENT POSITION

• Basic Postero-anterior – erect
• Alternative
• Antero-posterior – erect
• Antero-posterior – supine
• Antero-posterior – semi-erect
• Supplementary
• Lateral
• Postero-anterior – expiration
• Apices
• Lateral – upper anterior region
• Decubitus with horizontal beam

Tomography

PATIENT POSITION

The choice of erect or decubitus technique is governed primarily by the condition of the patient, with the majority of patients positioned erect. Very ill patients and patients who are immobile are X-rayed in the supine or semi-erect position

WHY ERECT POSITION

control of respiration is more satisfactory, Due to gravity effect on the abdominal organs allows for the disclosure of the maximum area of lung tissue, fluid levels are defined more easily with the use of a horizontal central ray.

WHY PA VIEW

Heart size : In a PA projection, the heart is closer to the digital detector/ image receptor so magnification is reduced Scapula: Its easier to move the scapula out of lung field Radiation dose: A PA projection reduces the amount of radiation that reaches the radiosensitive organs in the front of the body eg breast.

Respiration: standing up for PA projection makes respiration more effective and anterior ribs are better defined.

Patient comfort: patients can lean on the detector for support, which makes them more comfortable Fluid levels : A horizontal central beam make it easier to see fluid levels

WHY RESPIRATION

Images are normally acquired on arrested deep inspiration, which ensures maximum visualization of the air-filled lungs.

The adequacy of inspiration of an exposed radiograph can be assessed by the position of the ribs above the diaphragm.

In the correctly exposed image, it should be possible to visualize either six ribs anteriorly or ten ribs posteriorly.

Postero-anterior – erect

Required equipment

A 35 x 43-cm or 35 x 35-cm cassette is selected, depending on the size of the patient. Orientation of the larger cassette will depend on the width of the thorax.

Position of patient and cassette

• The patient is positioned facing the cassette, with the chin extended and centered to the middle of the top of the cassette.
• The feet are paced slightly apart so that the patient is able to remain steady.
• The median sagittal plane is adjusted at right-angles to the middle of the cassette. The shoulders are rotated forward and pressed downward in contact with the cassette.
• This is achieved by placing the dorsal aspect of the hands behind and below the hips, with the elbows brought forward, or by allowing the arms to encircle the cassette.

Direction and centering of the X-ray beam

The horizontal central beam is directed at right-angles to the cassette at the level of the eighth thoracic vertebrae (i.e. spinous process of T7), which is coincident with the lung midpoint (Unett and Carver 2001).

• The surface marking of T7 spinous process can be assessed by using the inferior angle of the scapula before the shoulders are pushed forward.
• Exposure is made in full normal arrested inspiration.
• In a number of automatic chest film-changer devices, the central beam is centred automatically to the middle of the film.

Essential image characteristics

• The ideal postero-anterior chest radiograph should demonstrate
• the following:
• Full lung fields with the scapulae projected laterally away

from the lung fields.

• The clavicles symmetrical and equidistant from the spinous processes and not obscuring the lung apices.
• The lungs well inflated, i.e. it should be possible to visualize either six ribs anteriorly or ten ribs posteriorly.
• The costophrenic angles and diaphragm outlined clearly.
• The mediastinum and heart central and defined sharply.
• The fine demarcation of the lung tissues shown from the hilum to the periphery.

Expiration technique

A radiograph may be taken on full expiration to confirm the presence of a pneumothorax. This has the effect of increasing intrapleural pressure, which results in the compression of the lung, making a pneumothorax bigger.

Chest Radiography

The technique is useful in demonstrating a small pneumothorax and is also used to demonstrate the effects of air-trapping associated with an inhaled foreign body obstructing the passage of air into a segment of lung, and the extent of diaphragmatic movement.

CHEST LATERAL

A supplementary lateral projection may be useful in certain clinical circumstances for localizing the position of a lesion and demonstrating anterior mediastinal masses not shown on the postero-anterior projection. Lateral radiographs, however, are not taken as part of a routine examination of the lung fields, because of the additional radiation patient dose

Position of patient and cassette

• The patient is turned to bring the side under investigation in contact with the cassette.
• The median sagittal plane is adjusted parallel to the cassette.
• The arms are folded over the head or raised above the head to rest on a horizontal bar.
• The mid-axillary line is coincident with the middle of the film, and the cassette is adjusted to include the apices and the lower lobes to the level of the first lumbar vertebra.

Direction and centring of the X-ray beam

• Direct the horizontal central ray at right-angles to the middle of the cassette at the mid-axillary line.

Radiological considerations

• Insufficient elevation of the arms will cause the soft tissues of the upper arms to obscure the lung apices and thoracic inlet, and even the retrosternal window, leading to masses or other lesions in these areas being missed.

Position of patient and cassette – AP

The patient may be standing or sitting with their back against the cassette, which is supported vertically with the upper edge of the cassette above the lung apices.

• The median sagittal plane is adjusted at right-angles to the middle of the cassette

Chest Radiography

The shoulders are brought downward and forward, with the backs of the hands below the hips and the elbows well forward, which has the effect of projecting the scapulae clear of the lung fields.

Direction and centring of the X-ray beam

The horizontal ray is directed first at right-angles to the cassette and towards the sternal notch.

• The central ray is then angled until it is coincident with the middle of the cassette.

This has the effect of confining the radiation field to the film, thus avoiding unnecessary exposure of the eyes.

• The exposure is taken on normal full inspiration

RADIOLOGICAL CONSIDERATION

• This projection moves the heart away from the film plane, increasing magnification and reducing the accuracy of assessment of heart size (in this projection, a cardiothoracic ratio (CRT) of greater than 50% does not necessarily indicate cardiomegaly).

Antero-posterior – supine

This projection is selected when patients are unable to either stand or sit for the projections described previously. The patient is usually lying supine on a trolley or bed.

Position of patient and cassette

• With assistance, a cassette is carefully positioned under the patient’s chest with the upper edge of the cassette above the lung apices. • The median sagittal plane is adjusted at right-angles to the middle of the cassette, and the patient’s pelvis is checked to ensure that it is not rotated.
• The arms are rotated laterally and supported by the side of the trunk. The head is supported on a pillow, with the chin slightly raised.

Direction and centring of the X-ray beam

• The central ray is directed first at right-angles and towards the sternal notch.
• The central ray is then angled until it is coincident with the middle of the film, thus avoiding unnecessary exposure to the eyes.

NOTE

The exposure is taken on full normal inspiration.

• An FFD of at least 120 cm is essential to reduce unequal magnification of intra-thoracic structures.
• In this projection, maximum lung demonstration is lost due to the absence of the gravity effect of the abdominal organs, which is present in the erect position.
• Images of heavy breasts are not readily diffused.

Radiological considerations

Compared with the postero-anterior projection, this projection moves the heart away from the image receptor plane, increasing magnification and reducing the accuracy of assessment of heart size (in this projection, a CTR of greater than 50% does not necessarily indicate cardiomegaly)

Chest Radiography

• The normal biomechanics of blood flow are different from those in the erect position, producing relative prominence of upper-lobe vessels and mimicking the signs of heart failure.

Chest Radiography

Pleural fluid will layer against the posterior chest wall, producing an ill-defined increase attenuation of the affected hemithorax rather than the usual blunting of the costophrenic angle; fluid levels are not seen

Chest Radiography

A pneumothorax, if present, will be located at the front of the chest in the supine position. Unless it is large, it will be more difficult to detect if a lateral horizontal beam image is not employed

AIR GAP TECHNIQUE

This technique employs the displacement of the subject from the film by a distance of 15 cm. To reduce any geometric unsharpness, the FFD is increased to 300 cm.

A high proportion of oblique scattered radiation from the subject will no longer fall on the film because of the increased distance between subject and the film.

A patient support, which is 15 cm in front of the cassette, is used to steady the patient and provide the subject-to-film distance.