Positioning Techniques and Terminology
ROBERT L. BARON, CASIMIR F. STRUGIELSKI, AND ROBERT A. CHRISTMAN
Radiography has played, and continues to play, an integral role in assessing foot and ankle disorders. Initially, views must be selected that will best image the part in question (see Chapter 10, View Selection for the Radiographic Study). The objective is to select positioning techniques that will yield the most information while minimizing radiation exposure. A firm knowledge of normal radiographic anatomy is a prerequisite.
A diversity of positioning techniques are available for foot and ankle radiography. The limited x-ray machine operator (LXMO) must clearly understand specific terminology used to describe positioning techniques before attempting to perform them. Unfortunately, discrepancy exists regarding use of the terms view, position, and projection. Even the word lateral can have two meanings. The following discussion of these terms, we hope, will clarify their application, especially as to the naming of positioning techniques.
USE OF THE TERMS VIEWS, PROJECTIONS, AND POSITIONS
Numerous techniques have been described for radiography of the foot and ankle. They range from non–weight bearing to weight bearing and have every possible aspect of the extremity positioned against the image receptor. Oblique foot techniques can be especially confounding; they may be performed non–weight bearing, with the plantomedial, plantolateral, dorsomedial, or dorsolateral aspect of the foot positioned against the image receptor (IR), or weight bearing, with the central x-ray beam directed at 45° toward either the dorsomedial or dorsolateral aspect of the foot. The result is confusion not only over to which positioning techniques to choose for a particular study but also over what to name them.
The Predicament
Several factors contribute to the current disarray regarding terminology for positioning techniques and radiographs. They include whether positioning techniques are named as positions or projections and how people use the terms projection, position, and view.
Positioning techniques have historically been named by two methods: (1) according to the path or projection of the x-ray beam through the body part and (2) by that surface of the body positioned closer to the IR. The former classically pertains to examination of the coronal anatomic plane (e.g., anteroposterior and posteroanterior projections). Position, in contrast, is used to describe oblique and lateral techniques.1,2 Initially, positional terms alone were sufficient to specify oblique and lateral techniques. However, as specialized positioning techniques evolved, projectional terms were being used in conjunction with, and even in place of, positional terms to describe oblique and lateral techniques. Use of the term “view” further confounds the predicament.
The term “view” should pertain only to the final image or radiograph. It should not be used to describe a positioning technique or projection.3 Doctors usually attach the term view to whatever positioning technique was performed when naming the radiographic image; that is, the term view is used synonymously with projection and position.4 For example, an anteroposterior projection is an anteroposterior view. However, one publication states that the term view means exactly the opposite of projection.5 By this definition, the image produced by an anteroposterior projection should be called a posteroanterior view. Taken a step further, a view was named either by that part of the body closest to the IR or by where the x-ray beam exits.6 The authors who distinguish between the terms view and projection by definition, however, fail to address the issue in practice. Figures illustrating positioning techniques in those texts are named as projections, but the radiographic images accompanying the technical descriptions are either named as a projection or not named at all! The term view is omitted intentionally. Another positioning textbook does not define how the term view is applied except that it is reserved for discussing the radiograph, not the technique.3
Discrepancies abound in the medical and technical literature regarding the names of positioning techniques and resultant images. An issue of Clinics in Podiatric Medicine and Surgery dedicated to radiology of the foot and ankle demonstrates this inconsistency. One article in this issue stresses routine use of the term projection for describing both technique and radiograph7; the projection is based on the body surface that the x-ray beam first enters. In contrast, another article consistently uses the term view when describing positioning techniques8; oblique positions are named by the surface nearest to the IR, yet the dorsoplantar view is named by the direction of the x-ray beam.
This confusion appears to be rooted in the separation of a radiographic study into technical and diagnostic components. The technical component is three-dimensional, and, therefore, appropriate terminology is needed to adequately describe the procedure. The radiograph, however, is only two-dimensional. It is not necessary to consider that the body part is being visualized from the film side, as suggested by Merrill.4 The radiograph looks the same no matter how the film is placed on the viewbox. In fact, anteroposterior and posteroanterior chest radiographs are customarily viewed as if the doctor were facing the patient (e.g., the patient’s right is on the viewer’s left).9 This practice has no regard for the patient’s position relative to the film.
A final source of confusion pertains to usage of the term lateral. The term lateral can be defined in two ways. (1) It can, generally speaking, refer to any side of the body; in this case, both sides of the foot are considered lateral. Lateral positions of the foot simply correspond to side positions of the body relative to the IR. The term does not differentiate between true lateral and medial sides. Therefore, medial refers to the center or midline of the foot or ankle.10 (2) The second definition of lateral corresponds to true lateral, that is, that aspect of the foot farther from the body’s midline, the latter being located between both extremities and separating them into right and left.11 In this case, medial corresponds to that side of an extremity that is closer to the midline of the torso. In radiography, the designation “lateral position” is not specific and corresponds to the first definition. The modifier “lateromedial” or “mediolateral projection” must be added to specify the precise positioning technique. The second definition (true medial and lateral) is applied when describing projections.
The lateral foot and ankle positions were originally performed with the patient not bearing weight and with the true lateral aspect of the extremity positioned against the IR. More specific terminology was not necessary to further identify the technique performed. The weight-bearing lateral foot and ankle techniques were developed later. The true medial aspect of the foot or ankle was positioned against the IR, not the lateral. However, instead of naming it a lateral position/lateromedial projection, technicians named it by that aspect of the extremity closer to the x-ray tube (i.e., by the surface the x-ray beam enters first). The two techniques were differentiated as being either weight bearing or recumbent.12
The most recent editions of two radiographic positioning textbooks have made some changes (again!) to terms and tube head angles used for standard foot- and ankle-positioning techniques.5,13 These newer terms will be referenced in the descriptions of relevant positioning techniques as “other names.”
The Solution
One goal of this textbook is to standardize the terminology applied to positioning techniques and radiographic images of the foot and ankle. This can be easily accomplished without grossly changing the terminology or routines already being practiced. Definitions for the terms projection, position, view, and positioning technique are followed by specific applications.
Position: Pertains to that aspect of the body closest to the IR. It is used to name the oblique and lateral (not the true lateral but a side) positioning techniques. A directional term (projection) and other adjectives (e.g., weight bearing or non–weight bearing) should be used with the oblique and lateral positions to further define the positioning technique.
Projection: The direction that the x-ray beam travels through the body. This direction is described as being anteroposterior or posteroanterior, dorsoplantar or plantodorsal, or lateromedial or mediolateral. (The latter terms refer to true lateral and medial.) The projection is used in addition to a position term to further describe a particular oblique or lateral position. The term projection is used to describe a positioning technique; it does not refer to the radiographic image.
View: Pertains to the radiographic image only. The terminology used to describe the positioning technique will simply be applied to the image, but the word view will replace the term position or projection. For example, the technique for a dorsoplantar projection produces a dorsoplantar view, the technique for a lateral position (lateromedial projection) produces a lateral (or lateromedial) view, and the technique for a medial oblique position produces a medial oblique view. View is not the opposite of projection. Additional terms should be included when appropriate, such as weight bearing or non–weight bearing.
Positioning Technique: The actual method of performing the study, including the position of the patient, tube head, and IR and the projection of the x-ray beam.
Application
As noted earlier, numerous positioning techniques are possible for imaging the foot and ankle. Each is discussed in detail in the following sections. The terminology will adhere to the definitions just noted. Some positioning techniques have been named after the authors who first described them. Their names are included with the description of the positioning technique.
Positioning techniques that produce images of the ankle and foot in the coronal and transverse planes, respectively, should be named by projections alone. Ankle techniques are anteroposterior projections, and foot techniques are typically dorsoplantar projections. The descriptor weight bearing or non–weight bearing should precede the name. The radiographic image has the same name except the word view replaces the term projection. An example of a positioning technique and corresponding view is the weight-bearing dorsoplantar projection and weight-bearing dorsoplantar view.
Descriptions of lateral positioning techniques, in contrast, require two designations: a position term accompanied by a projection term. The term describing the projection of the x-ray beam follows the position term. The designation weight bearing or non–weight bearing precedes the term lateral position; for example, “weight-bearing lateral position (lateromedial projection).”
The four lateral positioning techniques, including the weight-bearing mediolateral and lateromedial projections and the non–weight-bearing mediolateral and lateromedial projections, produce views that look similar to one another. The resultant images or views can all be named lateral views; the projection terms need not be included (especially if the full positioning technique has been previously described).
Oblique positioning techniques may or may not require additional position and projection terms. Non–weight-bearing oblique positioning techniques should include a term designating that aspect of the extremity closest to the IR. An example is the non–weight-bearing medial oblique position. Weight-bearing oblique positions (which are not advocated for the reasons listed in the section Positioning Considerations) do not need the position term plantar added to the title. The term weight bearing implies that the plantar surface of the foot is closest to the IR; including it would be redundant. However, a projection term is required to designate the direction of the x-ray beam through the extremity. The two weight-bearing oblique positioning techniques are either mediolateral or lateromedial oblique projections. Radiographic images are named the same except the term view replaces the words position or projection. An example of an oblique positioning technique and corresponding view is the non–weight-bearing medial oblique position and the non–weight-bearing medial oblique view.
POSITIONING CONSIDERATIONS
Weight-bearing oblique foot-positioning techniques with the tube head angled at 45° have been advocated by some doctors of podiatric medicine. One reason for this practice has been to standardize performance of the technique. Weight-bearing techniques alone, however, do not necessarily standardize the resultant view. It has been shown that foot positioning (supination and pronation) influences the radiographic positional relationships of the bones during weight bearing.14 Another attempt to standardize positioning techniques is by positioning the extremity in its angle and base of gait.15 This technique is used so that biomechanical measurements of the foot are standardized and reproducible.16 Biomechanical measurements, however, are only performed on dorsoplantar and lateral radiographs, not on oblique views.
Magnification (size distortion) and shape distortion of the image result from weight-bearing oblique positioning techniques. If true magnification is desired, a magniposer can be used to increase the object-to-image distance (OID); the geometric distortion is much less than that obtained by the weight-bearing oblique study. Occasionally, distortion of the object may be desirable in an attempt to better visualize a particular pathology. If so, the distorted oblique projection should be performed adjunctively as a special technique. A non–weight-bearing oblique position, with the foot tilted 45° and the tube head directed perpendicular to the IR, should be performed initially. Non–weight-bearing oblique foot positions can be accurately performed with a radiolucent foam wedge positioning aid. The foot should be positioned perpendicular to the leg as the sole of the foot rests against the wedge. The only other time that the distorted weight-bearing oblique positioning technique should be performed is if the patient cannot position his or her extremity properly for the non–weight-bearing oblique study. On a final note, the non–weight-bearing oblique requires a lesser exposure technique and can be performed on half of a 10 × 12-in image receptor. The weight-bearing oblique may require the use of a separate image receptor for each extremity if performing a bilateral study.
Positioning techniques for imaging the foot and ankle should be relatively simple to perform by both technician and patient. The type of x-ray unit being used, however, may already impose limitations. Many techniques can be performed with lower extremity–specific x-ray units. Orthoposer-mounted units offer convenience (the source-to-image distance [SID] is fixed) and safety (the patient can hold on to an attached railing, and the weight of the tube head holds the orthoposer unit steady); they are best for performing weight-bearing studies. However, it is difficult, if not impossible, to perform non–weight-bearing ankle studies with these particular units. A mobile x-ray unit, in contrast, can serve two purposes: (1) non–weight-bearing studies can be performed with the patient lying on an examination chair or table and (2) weight-bearing studies can be performed on an orthoposer platform placed adjacent to the chair or table. Greater technical expertise, however, is necessary to position the tube head properly with a mobile unit. Extremity-positioning aids are useful for performing certain techniques, such as positioning blocks and the weight-bearing sesamoid axial projection.
Positioning blocks or wedges are useful aids to position the foot or ankle for non–weight-bearing oblique techniques. They are made of a radiolucent material and are resilient, firm, durable, and washable. Positioning blocks will not slip when positioned properly under the patient. Wedges are available in a variety of shapes and sizes; a 45° triangular wedge accommodates most oblique positioning techniques.
A device called the axial poser is valuable for performing the weight-bearing axial sesamoid-positioning technique. It is sold in pairs, for the left and right foot, and is made of radiolucent plastic. The front and back of the device are angled superiorly to elevate the digits and rearfoot. This positioning causes the metatarsal heads to be positioned inferiorly relative to the remainder of the foot.
After obtaining standard views (or based on the clinical location of a finding), additional, nonstandard positioning techniques may be chosen to more clearly view an area of concern. For example, Osher et al.17 have shown the value of oblique forefoot axial modifications. (Other articles proposing specialized techniques are listed in the section Suggested Readings.)
OVERVIEW OF POSITIONING TECHNIQUES AND TERMINIOLOGY
The following is an outline of the positioning techniques that can be performed on the foot and ankle. They are categorized as foot-, toe-, sesamoid-, tarsal-, and ankle-positioning techniques. Each category is further subdivided into dorsoplantar (plantodorsal) or anteroposterior (posteroanterior) projections, oblique positions, and lateral positions, when applicable.
I. Foot-Positioning Techniques
A. Dorsoplantar (DP) projections
1. Weight bearing
2. Non–weight bearing
a. Foot flat
b. Forefoot angled 15°
B. Oblique positions
1. Non–weight bearing
a. Medial
b. Lateral
2. Weight bearing
a. Lateromedial oblique projection
b. Mediolateral oblique projection
C. Lateral positions (may be performed weight bearing or non–weight bearing)
1. Lateromedial projection
2. Mediolateral projection
II. Toe-Positioning Techniques
A. Dorsoplantar projection (weight bearing or non–weight bearing)
B. Oblique positions
1. Non–weight bearing
a. Medial oblique
b. Lateral oblique
2. Weight bearing
a. Lateromedial oblique projection
b. Mediolateral oblique projection
C. Lateral positions (may be performed weight bearing or non–weight bearing)
1. Lateromedial projection
2. Mediolateral projection
III. Sesamoid-Positioning Techniques
A. Posteroanterior sesamoid axial projections
1. Weight-bearing axialposer
2. Lewis method
B. Anteroposterior sesamoid axial projection (Holly method)
C. Lateromedial tangential projection (Causton method)
IV. Tarsal-Positioning Techniques
A. Dorsoplantar calcaneal axial projection
B. Plantodorsal calcaneal axial projection
C. Harris–Beath
D. Broden
E. Isherwood
V. Ankle-Positioning Techniques (may be performed weight bearing or non–weight bearing)
A. Anteroposterior (AP) projection
B. Mortise position
C. Oblique positions
1. Medial (or internal) oblique
2. Lateral (or external) oblique
D. Lateral positions
1. Lateromedial projection
2. Mediolateral projection
Terminology
1. Angle of gait: The angle formed between the feet and line of progression while walking (approximately 10°–15° of abduction in the normal individual).
2. Base of gait: The distance between both medial malleoli while walking (approximately 2 in).
3. Midline of foot: The imaginary line that enters the center of the heel and exits through the second digit, thereby dividing the foot into two halves.
4. Central ray (CR): The most direct beam of radiation from the tube.
5. Dorsum (dorsal): In reference to the top of the foot.
6. Plantar: Bottom (sole) of foot.
7. Lateral (two definitions):
a. Away from the torso’s midline (outer side of the extremity).
b. Away from the midline of the extremity (inner or outer side of the extremity).
8. Medial: Inner side of the extremity (toward the torso’s midline).
9. Extension: The process of straightening the joint.
10. Flexion: The process of bending the joint in an angle.
11. Supine: Lying face up.
12. Prone: Lying face down.
13. Specific body position: Describes the placement of the foot or ankle relative to the image receptor. Non–weight-bearing oblique positions are named by that aspect of the extremity nearest the image receptor.
14. Positioning: Manner in which the tube head, IR, patient, and central ray are placed to obtain a radiographic image of a particular body part.
15. Projection: The direction that the primary x-ray beam travels through the body part. Used to describe anteroposterior (dorsoplantar), lateral, and weight-bearing foot oblique positioning techniques. For example, the x-ray beam enters the dorsal aspect of the foot and exits the plantar aspect in the dorsoplantar projection.
16. Tube head angulation: The number of degrees the tube head is set from vertical (0°).
17. View: This term refers to the image in the radiograph. It is not a positioning term.3
18. Oblique: The condition when the plane of a body part is neither perpendicular nor parallel to the IR.
19. Axial: The long axis of a structure or body part.
20. Image receptor (IR): A device that receives the remnant x-rays after passing through the paient and stores the latent image. In plain film radiography it houses the film and intensifying screens. Also referred to as the imaging plate (IP).
21. Orthoposer: Platform used to perform weight-bearing studies.
GENERAL CONSIDERATIONS
Most views of the foot and ankle can be obtained with either a weight-bearing or a non–weight-bearing positioning technique. The patient being considered for weight-bearing positioning techniques should be ambulatory, able to walk without assistance. If not, partial weight bearing or non–weight bearing may be preferable. The added weight of a lead apron or shield to a patient whose mobility is questionable poses a considerable risk for injury if an unsteady patient is positioned on an orthoposer for a weight-bearing study. Partial or non–weight bearing should be considered for the patient who is not surefooted or uses an ambulation assistance device (e.g., walker, cane, or crutches). Patients who have recently had a cast put on or who have undergone a surgical procedure may not be stable enough to allow full weight bearing. Non–weight-bearing positioning techniques are necessary for individuals who are confined to a wheelchair, have recently undergone an extensive surgical procedure, or have acute pain (e.g., secondary to recent trauma).
Positioning techniques can be modified as needed. This is especially true for oblique positions if you are trying to view a specific anatomic landmark or location.
If a lesion marker is placed on the skin, only dorsoplantar and lateral positioning techniques should be performed. Marker position cannot be accurately assessed with oblique views. Furthermore, the tube head must be positioned vertically (0°) for the dorsoplantar projection. Any other tube head angulation will alter the position of the marker relative to the internal foot structures. This also applies to the location of radiopaque foreign objects.
PREPARATION FOR THE RADIOGRAPHIC STUDY
Preparation of the radiography room expedites patient turnaround and minimizes the chance for mistakes. Several aspects of the study should be considered and performed before the patient is brought into the room. Initially, the desired views must be selected; positioning techniques are chosen that will result in the desired images. The LXMO can then gather all applicable items pertaining to the study: image receptor, positioning aids, and identification markers, for example. Standard technical factors (mA, kVp, time, and SID) should be posted by each unit’s control panel for all positioning techniques. Any modifications should be determined in advance. Tube head and image receptor placement, identification markers, and control panel adjustments can be ready for the first positioning technique before the patient enters the radiography room.
Once the patient is in the radiography room, he or she must be given precise instructions as to what to do and what is expected for each of the positioning techniques. The patient must clearly understand these instructions so that the necessity for repeat studies is minimized. Image blur caused by patient movement is probably the most common reason for repeating any particular study. Clear instructions must also be given to the parent or guardian who remains in the room and is helping to maintain an infant’s or young child’s position. Occasionally, a patient, particularly a trauma or geriatric patient, may express discomfort during positioning of the extremity. If it becomes unreasonable to perform a certain positioning technique, an alternative position or projection or even a non–weight-bearing technique should be considered.
A final check should be made by the LXMO to make sure that all technical parameters are correct: patient positioning, identification markers, and technical factors. This should be done for each positioning technique. Error regarding any of these parameters can result in having to repeat a study.
Lead aprons must be provided for and used by each patient who is being examined. This also applies to the infant or child patient. An apron and lead gloves must be provided to any individual who remains in the room during the exposure to assist in positioning.
ANGLE AND BASE OF GAIT POSITIONING
Weight-bearing dorsoplantar and lateral foot radiographs have been advocated for decades, as early as 1943.18 The weight-bearing attitude is felt to create an anatomic image that is most feasible for assessing normal versus pathological biomechanical conditions under the stresses and strain of body weight.19 Shortly after Sgarlato15 described the kinesiologic and structural relationships of the angle of gait, Hlavac14 demonstrated that foot position has a profound influence on the radiographic relationships and forms of osseous structures and angular biomechanical measurements. Weight-bearing foot radiography with the foot positioned in angle and base of gait is considered standard technique in podiatric practice for dorsoplantar and lateral foot radiographs and has since been advocated to the orthopedic community.20
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