Anteroposterior (AP) thumb

Traditionally the AP thumb projection has been described with the patient seated,¹ but these positions create difficulties when trying to clear the hypothenar eminence from the field. Method 1 described here uses a position considered to be significantly more comfortable and achievable than others and may be at variance with the most commonly performed methods (methods 2 and 3). The idea for method 1 was originally researched with the patient in an erect position,² with the later suggestion that radiation protection and immobilisation might be more effective if the patient is supine.³

It is clear that the patient’s thyroid and the lenses of the eyes are close to the primary beam and edge of collimation in method 1, but if the head is turned away efficiently, the image receptor (IR) is placed as far away as possible from the trunk and lead rubber is used effectively, risks can be minimised.

For all projections of the thumb the IR is placed horizontal unless otherwise specified.

Positioning

Method 1: Patient supine (Fig. 5.1A,B)

• The patient is supine with the affected arm flexed at the elbow and the dorsum of the hand initially in contact with the IR. Lead rubber is applied to the trunk

• The fingers are extended and separated from the thumb

• The anterior aspect of the thumb is placed in contact with the IR and adjusted until the long axis of the thumb is parallel to it; the hypothenar eminence is cleared from the thumb and thenar eminence

• As the dorsum of the hand is now not in contact with the IR, a radiolucent pad is used under the dorsum to aid immobilisation

• The head is turned away from the primary beam

Figure 5.1 (A) AP thumb with patient supine; (B) AP thumb.

Method 2: Patient seated alongside table (Fig. 5.2)

• The patient is seated with the affected side next to the table; lead rubber is applied to the waist

• The affected hand is externally rotated and the thumb cleared from the fingers

• The anterior aspect of the thumb is placed in contact with the IR; it may be necessary for the patient to lean towards the table in order to facilitate this

• A radiolucent pad is used under the dorsum of the hand to aid immobilisation

• Care must be taken to clear the hypothenar eminence from the first metacarpal

• The head is turned away from the primary beam

Figure 5.2 AP thumb with patient seated next to the table.

Method 3: Patient seated with back to table (Fig. 5.3)

• The patient is seated with their back to the table, with a lead rubber apron fastened behind the waist

• The affected arm is abducted posteriorly and medially rotated

• The anterior aspect of the thumb is placed in contact with the IR; the hypothenar eminence is cleared from the thumb and thenar eminence

• A radiolucent pad is used under the dorsum of the hand to aid immobilisation

• Care must be taken to clear the hypothenar eminence from the first metacarpal

Figure 5.3 AP thumb with patient’s back to the table.

For patients who are unable to achieve any of these positions, the posteroanterior (PA) projection should be used. Principles of radiographic imaging indicate that there will be some magnification of the thumb with this projection, thereby increasing unsharpness. However, an increase in the focus receptor distance (FRD) will compensate for and reduce the effects of this. An increase in mAs will also be necessary to account for reduction in radiographic density due to the inverse square law. However, this is likely to be minimal and the balance of benefit versus risk should be considered.

Popular opinion would suggest that the creation of an air gap between the thumb and the IR also requires an increase in mAs, in order to effect further film blackening as compensation for the reduction in scatter. For denser body areas requiring higher exposure factors than the thumb, this would be a relevant consideration. However, as this projection is performed with the selection of a relatively low kVp, the dominant interaction process is one of absorption rather than production of scatter. Therefore this negates the requirement for an increase in mAs (see Ch. 3). Possible other disadvantages of using the PA projection are the possibility of poor maintenance of position and immobilisation; use of immobilisation aids therefore becomes of paramount importance.

PA thumb (Fig. 5.4)

Positioning

• The patient is seated with the affected side next to the table; lead rubber is applied to the waist

• From a dorsipalmar (DP) position, the hand is externally rotated through 90° and the lateral border of the wrist placed in contact with the table

• The fingers are extended and superimposed vertically; the thumb is extended and cleared away from the fingers

• The long axis of the thumb is supported in a horizontal position by a radiolucent pad

• The thumb and thenar eminence are cleared from the hypothenar eminence and palm of the hand

Figure 5.4 PA thumb. The immobilisation pad is removed to show position more clearly.

Lateral thumb (Fig. 5.5A–C)

Positioning

• The patient is seated with the affected side next to the table; lead rubber is applied to the waist

• In the DP position the thumb is cleared from the fingers and the hand is medially rotated until the thumb lies laterally, with its phalangeal condyles superimposed

• Because the medial aspect of the hand will be raised to achieve the correct position, a radiolucent pad is used under the palmar aspect of the hand to aid immobilisation

• An alternative method for immobilisation is to flex the fingers into the palm while maintaining separation of the thumb from the rest of the hand, and use the fist to support the dorsum in the required position (Fig. 5.5B)

Figure 5.5 (A, B) Lateral thumb positions; (C) lateral thumb.

DP fingers (Fig. 5.6A,B)

For all projections of the fingers the IR is horizontal.

Figure 5.6 (A) DP finger; (B) DP middle finger.

Lateral fingers

Lateral projections of some fingers can prove difficult to achieve and maintain in position, especially when attempting to separate and immobilise middle, ring and little fingers. The injured or arthritic patient may be even less cooperative. Small wedge-shaped radiolucent pads are efficient aids in separating fingers for radiographic examination.

Positioning

Index (first) finger (Fig. 5.7A,B)

• From the DP position the hand is internally rotated through 90° and the third and fourth fingers are flexed and held in position by the thumb

• The index finger is extended and positioned with its lateral aspect in contact with the IR

• The long axis of the index finger is separated from the palmar-flexed middle finger with a radiolucent pad

Figure 5.7 Lateral index finger.

Middle finger (Fig. 5.8)

• From the DP position, the hand is internally rotated 90° and positioned as for the lateral index finger projection

• The middle finger is extended and separated from the index finger with a radiolucent pad

• The middle finger is supported in a horizontal position by a radiolucent pad

Figure 5.8 Lateral middle finger.

Ring and little finger: method 1 (Fig. 5.9)

• From the DP position the hand is externally rotated through 90°

• The index and middle fingers are flexed and held by the thumb; the little finger remains extended, as does the ring finger

• The medial aspect of the fifth metacarpal is in contact with the IR

• The ring finger is slightly dorsiflexed to clear it from the little finger

• If under examination, the ring finger is supported in a horizontal position; in any event it is separated from the little finger by a radiolucent pad

Figure 5.9 Ring and little finger – method 1.

Ring and little finger: method 2 (Fig. 5.10)

• From the DP position the hand is externally rotated through 90°

• The index finger is flexed and held by the thumb; the remaining fingers are slightly dorsiflexed and fanned out; their long axes remain horizontal

• If under examination, the ring finger is supported in a horizontal position; in any event it is separated from the other fingers by radiolucent pads

Figure 5.10 Ring and little finger – method 2.

For all the fingers and positions

DP hand (Fig. 5.11A,B)

For all projections of the hand the IR is placed on the table-top.

Figure 5.11 DP hand.

Criteria for assessing image quality

• All phalanges, the wrist joint and the soft tissue outline of the hand are demonstrated

• The fingers are separated, and the interphalangeal and metacarpophalangeal joints are clear

• Symmetrical appearance of the heads of metacarpals 2–4

• Obliquity of thumb and the heads of metacarpals 1 and 5

• Sharp image demonstrating the soft tissue margins of the hand, bony cortex and trabeculae Adequate penetration to demonstrate the hook of hamate whilst showing distal phalanges

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Tags: Medical Imaging Techniques, Reflection and Evaluation

Mar 3, 2016 | Posted by admin in GENERAL RADIOLOGY | Comments Off on Fingers, hand and wrist

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Common errors	Possible reasons
Superimposition of soft tissue outlines of fingers	Fingers are not separated adequately
Poor demonstration of joint spaces