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.
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.
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.
* 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.
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.
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 Io/It).
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).
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. |
