Errors in laterality identification on radiographic images can lead to unnecessary repeat radiographic exposures or even errors in therapeutic or surgical procedures. The author reviews radiographic exposure and labeling protocols to help to reduce the chance of such errors.
is a Clinical Professor, Emeritus, University of Miami School of
Medicine, Miami, FL.
Images of the right and left sides of the body are not
infrequently mistaken for one another during the interpretation of
radiographs. The two sides of the body are essentially identical in
appearance except that one is the mirror image of the other. When
an image is made of a body part that can be viewed from either
surface, such as a transparent radiograph, or a digitized image
that can be electronically inverted, it is possible for laterality
to be mistaken. As an example, the frontal view of the foot,
whether on film or digitized, is normally taken with the sole of
the foot against the cassette, resulting in an anteroposterior (AP)
image. This view of the foot is both the easiest for the patient
and the technologist and results in the best image and has
consequently become the standard projection. The resulting image,
however, can be viewed from either the front or the back. If viewed
from the front or exposure side, the side from which the X-ray beam
originally entered the body, it will appear to be a left foot. But
if the same image is looked upon from the back-the opposite surface
or nonexposure side, the viewer sees the mirror image, which
appears to be a right foot.
Therefore, whenever a radiograph is made, it is important for
the technologist to identify which side of the body appears on the
image. This is usually achieved by placing a lead marker on the
front of the cassette, corresponding to the side examined.
Alternatively, a label is attached directly to the film after it is
developed, indicating which side is represented thereon. Since
humans are occasionally fallible, however, this marking may be
incorrect or omitted altogether.
Hence, when the image is interpreted, there may be a question
regarding which side of the body-right or left-actually appears on
the image. This often occurs in busy radiology departments,
especially in emergency departments (EDs). When an error is
suspected in marking which side of the body has been examined there
are two main possible outcomes.
The first possibility is that the mistake creates such an
obvious discrepancy that the error is promptly discovered and
rectified. The following is a typical example of how this may
A patient injures the right hand but the busy ED physician
erroneously writes "left" on the requisition. The alert
technologist, after seeing the patient, realizes the physician's
error and X-rays the right hand instead of the requested left hand.
The technologist also properly identifies the image with a "RIGHT"
marker, but then fails to make a note that a correction was made.
When the film arrives to be interpreted, the obvious discrepancy
between the requisition and the film marking must first be resolved
and the usual question is "which side is actually on the
radiograph?" This question arises every day and is usually dealt
with by questioning the technologist. If it cannot be resolved
onsite in this manner, the examination may be repeated, even if it
means bringing the patient back from the ED or possibly even from
In the first case, in which an obvious discrepancy is apparent,
the question of laterality is usually resolved onsite and merely
results in confusion, wasted time and effort, and perhaps
unnecessary radiation. If the question cannot be resolved onsite
and the patient is recalled for a repeat examination, there is a
further loss of time and expense, and administration of unnecessary
radiation, but, more importantly, at least the question is
In the second and more serious circumstance, however, no
discrepancy is apparent and the mistake goes undetected. This can
result in medical mismanagement-either the absence of care where it
is needed or inappropriate treatment to the wrong body part.
Occasionally, the wrong side of the patient is casted, pinned, or,
rarely, even amputated. The author has knowledge of one such case
in which the error had a fatal outcome. The phrase "wrong site"
surgery often actually refers to "wrong side" surgery. From time to
time, such right/left mix-ups, among other major medical errors,
attract the attention of the news media, and-as has recently been
the case-have led to calls for efforts to adopt measures to avoid
The advent of digitized radiography has not altered the basic
situation. A radiodense marker is still applied to the outside of
the cassette by the technologist to identify the side of the body
displayed; and the marking can be erroneous. Either way, marking
the side of the part examined is operator-dependent and, therefore,
subject to human error.
A proposed solution
A different way to approach to this potentially serious problem
is to positively identify the side of the image that received the
entering X-ray beam. The major portion of a radiographic image is
actually generated by the visible light fluorescing from the
intensifying screens rather than from the radiation passing through
the cassette. X-rays cause the screens to emanate light, and film
is much more sensitive to this light than to the radiation itself.
Therefore, simply blocking the light from the screen(s) with a
marking (such as a decal) that is opaque to light but not to X-rays
leaves the underlying portion of the film essentially
Radiologists have long realized that if they knew the side of
the image that received the exposure they were usually able to
differentiate right from left, and they used the position of the
identification blocker on the radiograph to make this
determination. However, at present, the location of the "blocker"
may vary from one manufacturer to another, and this method has
largely fallen into disuse.
An approach exists that is automatic and not subject to the
human error of mislabeling-positive identification of the side of
the image that received the exposure. The words "EXPOSURE SIDE" are
applied to the surface of the screen facing the cassette front-the
side facing the X-ray tube. Since the film cassette contains two
intensifying screens, optimal results are obtained when a second,
mirror image of this marker is placed on the other screen, directly
across from the first, so that when the cassette is closed the two
markers align with one another and block light from both screens.
This notation is placed on one or more locations along the margins
of the screen, in letters just large enough to be readable (Figure
Strips of tape are placed on the front of the cassette
corresponding to the positions of these internal markers to
indicate that this cassette bears such markers and to alert the
technologist not to cone off the marker or place important anatomic
structures over the site (Figure 2). While this marking can be
installed on existing casettes in the radiology department, ideally
it would be furnished by the manufacturer with new cassettes. The
exposure side of the image could then be identified as the side
from which the words "EXPOSURE SIDE" read correctly or in a forward
direction. Even if the image is turned upside down or sideways
during viewing, the words will always read in a forward direction
(Figure 3). Only if the image is turned over to the other side and
looked at from the back will the words appear backward (or as
Simply knowing which is the exposure side of the film will
generally allow the viewer to establish right from left in almost
every case, presupposing the viewer has a basic understanding of
how patients are positioned for radiographic exposures. While it is
expected that this is common knowledge among radiologists, it will
be summarized below.
Lower extremity-Including foot, ankle, tibia and fibula,
knee, femur, and hip
Frontal views of all the bones and joints of the lower extremity
are generally exposed in the AP projection (front to back) (Figures
4, 5, and 6). Lateral views are normally exposed mediolaterally
(ML), with the X-ray beam directed at the inner aspect of these
parts and exiting laterally (Figure 7).
Upper extremity-Hand, wrist, forearm, elbow, humerus, and
Frontal views of the hand and wrist are exceptions in that they
are normally exposed in the prone position, posteroanteriorly (PA),
while frontal views of the remaining structures of the arm are
exposed anteroposteriorly or (AP). Frontal views of the hand and
wrist are normally taken in pronation, palm down, as this position
is easiest for the patient and technologist and generally results
in the best images (Figure 8). Views of the forearm, however, are
taken supine (AP), since the radius and ulna would overlie one
another on a prone view. All other frontal views of the upper
extremity are normally exposed in the AP projection, front to back,
as is the lower extremity. Frontal views of the shoulder are
likewise exposed AP, and if the image includes part of the thorax,
lateralization becomes even easier.
Lateral views of the upper extremity are generally taken in the
radioulnar direction, with the thumb directed upward. Remembering
that the anatomic position of the upper extremity has the palm of
the hand facing forward, lateral views are actually taken in the
lateromedial direction, opposite of the lower extremity. Therefore,
on lateral views of the arm, just as of the leg, one can determine
from the direction that the ventral and dorsal aspects of the body
part examined face whether it is a right or left extremity.
The pelvis is usually radiographed AP, from the patient's front,
with the patient lying flat on the X-ray table.
Flat and upright views of the abdomen are usually taken in the
AP projection as well, with the patient's back flat against the
Almost all frontal views of the lumbar spine are taken in the
same projection as the abdomen-AP, with the patient's back against
the table. However, as noted below, as an exception to the standard
projection, occasionally some physicians will request that frontal
views of the lumbar spine be exposed PA, to better visualize the
Frontal views of the cervical spine are generally taken in the
AP projection, front to back, as are the obliques. Lateral views
can be identified as to whether they are right or left lateral by
the direction in which the elements of the spine are oriented.
Frontal views of the skull may be exposed in either the AP or PA
projections. One can generally determine which view was taken by
whether the facial structures are magnified or not, these
structures being less magnified on a PA view. Lateral views can be
identified as to whether they are right or left lateral by the
direction in which the facial structures are oriented (Figure
Because the heart lies asymmetrically to the left in the thorax,
determining right from left in the thorax is not a problem except
in the rare case of situs inversus. Conversely, when viewing a
chest radiograph, knowing which side of the body is the right side
and also knowing which surface is the exposure side tells the
reader whether it is an AP or PA view. This fact can have
diagnostic significance at times. For example, knowing it is a PA
view virtually assures that it was also taken in the upright
position and any pleural fluid present will tend to gravitate
inferiorly (Figure 10).
The film mammogram is a special case. While, in practice, it is
rather rare for the side to be mismarked, at times it is still
helpful for the viewer to be able to verify which is right and
left. The breasts, as other paired body parts, are mirror images of
one another. Therefore, by viewing the film from the exposure side,
with the apex of the breast oriented away from the reader, it is
usually possible to tell from the contour of the breast and the
distribution of ductal tissue which is the right breast and which
is the left.
With mamography, one generally does not need a marking to
identify the exposure side of the film, since the two surfaces
differ from one another. The emulsion is on only one side, the
exposure side, and this gives it a dull, matte finish. The
opposite, nonexposure side is smooth and shiny.
The essence of this simple system is based on the fact that the
vast majority of radiographic examinations are exposed in standard
projections. There are standard positions for all body parts,
generally established because they are the easiest positions for
both the technologist and the patient and also because they tend to
display the anatomy to optimal advantage. A prime example of this,
as noted above, is the AP view of the foot. The frontal view of the
foot is exposed with the sole downward, against the X-ray cassette.
For the practical reasons stated above, this projection is used
whenever a frontal view of the foot is requested; so if the viewer
looks at the image from the exposure side, it will be almost
intuitively obvious whether it is a right or left foot.
However, for various reasons, images are occasionally taken in
nonstandard projections (see examples below); in such cases, this
system cannot be relied upon to give proper lateralization. The
radiologist should be aware of this possibility and be alert to the
various circumstances in which this may occur.
Bedside or portable examinations
When the radiographic examination is made at the bedside, the
technologist may vary from the standard technique, usually for
practical reasons (eg, to minimize inconvenience to the patient, or
to decrease the need to move the equipment around the room).
Typically, for example, when portable views of the knees are taken,
both lateral views may be exposed with the X-ray tube on the same
side of the bed, using a horizontal beam and elevating the knee to
be examined. One lateral knee view then will be exposed in the
usual mediolateral projection, while the other lateral may be taken
lateromedially. The corresponding frontal views of both knees will
still be exposed AP.
Particular views of interest may be requested to view one area
to advantage. Such views may be customized to the area in question
and often may not be standard. One such nonstandard view is the
frontal view of the lumbar spine, requested by some physicians to
be done in the PA projection rather than in the conventional AP
view. This is requested because the normal lordotic curvature of
the lumbar spine allows the intervertebral interspaces to be shown
more optimally in this projection.
In spite of such exceptions, however, the vast majority of
radiographs are exposed in the standard projections summarized
above. When nonstandard projections are used, there is usually a
reason that is known to the radiologist.
When the viewer knows the side of the image from which the
exposure was made-the side from which the words "EXPOSURE SIDE"
read correctly in a forward direction-and looks at the image from
that side, he or she can generally tell which side of the body is
shown on the image. Using the frontal view of the foot, for
example, one should look at the image from the exposure side and
then orient the foot so that the toes are pointed away, as one
looks down at one's own foot; if the great toe is to the right, it
is a left foot image. This same simple principle applies to any
body part examined.
The traditional methods of radiographic marking (eg, using a
lead marker on the cassette exterior or affixing a side label to
the film after it is developed) will, of course, still be used. The
additional information obtained by marking the exposure side of the
image, however, can serve as a valuable backup or crosscheck to the
conventional, operator-dependent method. When the information
provided by the two systems is in conflict, this discrepancy should
alert the viewer to a possible error in film marking. Knowing the
exposure side should help to resolve questions of laterality
on-site, usually without the need to repeat the examination and,
ultimately, it is hoped, will prevent erroneous interpretation.