This article inaugurates a series of white papers on
radiologic patient positioning techniques that will appear
each month on AuntMinnie.com. The series will explore each of
the major modalities. If you'd like to offer your comments or
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with your thoughts and suggestions.
Ask the patient to remove clothes from the waist up, put on a
hospital gown, remove any jewelry (necklace, earrings), and,
if necessary, tie hair up on top of the head.
For posteroanterior (PA) projections, instruct the patient to
sit or stand upright. Patients are positioned to face the
film-screen cassette in order to minimize magnification of
the anteriorly positioned heart and consequent obscuration of
the lungs. Make sure the patient is standing straight and is
equally distributing the weight of the body on both feet.
The upright position is preferred for the following reasons:
It prevents engorgement (an excess of blood) of pulmonary
vessels, whereas supine or recumbent positioning tends to
increase engorgement of pulmonary vessels, which can
change the radiographic appearance of these vessels and
It allows full expansion of the lungs. In the recumbent
position, full expansion of the lungs is prevented.
The upright position is very important in order to
visualize possible air and fluid levels in the chest. In
the upright position, fluid will locate near the base of
the lung while the air will rise. In the recumbent
position, fluid will spread out over the posterior
surface of the lung, resulting in a hazy appearance of
the entire lung.
An upright chest film is preferred over an upright
abdominal film for the diagnosis of pneumoperitoneum
(free air in the abdominal cavity).
Ask the patient to move the shoulders forward and downward,
so that the chest wall and both shoulders are in contact with
the cassette. This helps to carry the clavicles below the
Adjust the height of the cassette so that its upper border is
about 2 inches above the shoulders so that the lung apices
are not cut off.
Ask the patient to extend the neck, chin, and head upward and
vertical. The neck and chin otherwise tend to superimpose the
trachea and uppermost lung regions.
The patient's arms are placed overhead or on their hips with
elbows angled anteriorly. This will rotate the scapulae off
the chest, thereby preventing their superimposition over the
In female patients with large pendulous breasts, it is very
important to minimize breast shadows. In some patients,
depending on the density and size of breasts, breast shadows
cannot be totally eliminated. Breast shadows render the
examination suboptimal by superimposing over the lower part
of the lung fields and obscuring any pathology. Ask the
patient to pull the breasts upward and laterally (outwards),
then remove her hands as she leans against the cassette
holder to keep them in position.
Place a lead shield between the x-ray tube and the patient's
pelvis. Gonadal shielding from the level of the iliac crests,
or slightly higher, to the mid-thigh area should be used on
all patients of reproductive age.
Even a small degree of rotation distorts the mediastinal
borders, and the lung nearest the film will appear less
translucent. The following points should be stressed to
obtain a true PA view (without rotation):
Ensure that the patient is standing evenly on both feet.
Both shoulders should be rolled forward and downward.
The chest radiograph should be well centered so that the
medial ends of the clavicle are equidistant from the
vertebral spinous processes at T4/5 . However, scoliosis
and other thoracic deformities negate the value of
For PA, the chest landmark that is used for locating the
center of the lung fields is the vertebra prominans
(T1). It corresponds to the apical regions of both lungs. The
vertebra prominans can be palpated at the base of the neck.
The hand-spread method can be used to locate the central ray.
To use this method, the technologist should know his or her
own hand-spread width (most hands can reach 7 inches).
For the average adult, the central ray should be directed to
the spinal column (mid-sagittal plane) approximately 7 inches
(18 cm) for a female and 8 inches (20 cm) for a male down
from the vertebra prominans. This corresponds to the level of
T7 and the inferior angle of scapula, a source to image
receptor (SID) distance of 72 inches (180 cm). Hypersthenic
body types require only 7 inches down from the vertebra
prominans, whereas athletic sthenic/hyposthenic types will
require 9 inches down from the vertebra prominans.
Film holder (image receptor) placement
The horizontal dimension of an average chest is greater than
the vertical dimension. This requires that a 14 x 17-inch
film holder or image receptor (IR) be placed crosswise.
However, depending on the body type, the most common practice
is placing the film holder or IR lengthwise. Either
lengthwise or crosswise, the goal is to adequately include
the lateral lung margins.
Collimation should be adequate to allow for some margin of
error in central ray placement and lung expansion during deep
inspiration. On each side of the posterior chest the
illuminated field margins should correspond to the outer skin
margins. The upper border of the illuminated field should be
at the level of vertebra prominans (4 cm above the apex of
lungs).This will result in a lower collimation border of 1-2
inches below the costophrenic angle, if the central ray was
Be sure to make the exposure upon a second full inspiration
by the patient. The patient should take as deep a breath as
possible, and then hold it to fully aerate the lungs. Taking
a second deep breath before holding it allows for a deeper
inspiration, as more air is inhaled during the second breath
than during the first breath.
The best way to determine the degree of inspiration is to
start at the top of the patient's rib cage, with rib number
one, and count down to the tenth or eleventh rib posteriorly.
A general rule for average adult patients is to show a
minimum of 10 ribs on a good PA chest radiograph. Older
patients have less inhalation capability, with a resulting
low lung volume, which requires a higher central ray
Evaluation criteria for a good PA projection
Entire lung fields from apices to costophrenic angles
should be clearly demonstrated.
No rotation. Rotation on a PA chest radiograph can be
determined by examining both sternal ends of the
clavicles for a symmetric appearance in relationship to
the spine. On a true PA chest without any rotation, both
the right and left sternal ends of the clavicle will be
the same distance from the center line of the spine. The
direction of rotation can be determined by which sternal
end of the clavicle is closest to the spine.
Trachea is visible in midline.
Scapula projected outside the lung fields.
Ten posterior ribs are visible above the diaphragm.
There is a sharp outline of the heart and diaphragm.
A faint shadow of the ribs and superior thoracic
vertebrae is visible through the heart shadow.
Lung markings are visible from the hilum to the periphery
of the lung.
An expiratory film may be helpful under some circumstances. A
small pneumothorax (air in the pleural cavity) may be
difficult to detect on a routine inspiratory PA film. On
expiration, the volume of the thorax and lungs is reduced but
the amount of air in the pleural space remains essentially
unchanged. The pneumothorax then occupies a larger percentage
of the area of the thorax and is more easily visible.
Another indication for an expiratory film is to demonstrate
air trapping. The bronchi increase in diameter with
inspiration, and decrease with expiration. With a foreign
body or tumor in a main bronchus, a valve action may occur,
with air bypassing the obstruction on inspiration and
becoming trapped on expiration
With expiration, the normal lung is reduced in volume and
becomes more radiopaque. The obstructed portion of the lung
retains its air, thereby retaining its radiolucency and
forcing the mediastinum to shift toward the contralateral
side. If a patient has a unilateral respiratory wheeze, air
trapping is likely, and an expiratory film may be helpful.
Film size: 14 x 17 inches (35 x 43 cm) lengthwise or
Dr. Ahmad is a second-year resident affiliated with Columbia
University's New York-Presbyterian Hospital. He is the owner
and developer of RadQuiz.com, a compendium of radiology teaching
files on the Internet.