Spatial Relations in Image Critique
Quinn B Carroll, MEd, RT, West Coast Educators Conference, 2017 NOTES Selecting good anatomical criteria:
When evaluating a radiograph for positioning:
1. Observe a pair of identical anatomical parts distant from each other and from the
midline of the body
2. Observe anatomical parts that have a long axis perpendicular to the expected
direction of shift
With lateral rotation, shift of the vertical lines is much more apparent than
shift of the diagonal lines (which can also be caused by tilt)
Case Study: Rotation Vs. Angle on Lateral Knee:
Perpendicularity:
To evaluate rotation, use only the posterior portions of the condyles
To evaluate CR angle (or tilt), use only the bottom portions of the condyles
3. Observe anatomical parts close to the central ray in the direction of expected shift
For lateral skull, rotation manifested by side-to-side shift:
Only the mandibular rami, close to the CR, can be expected to be projected on
top of each other - The greater wings and the orbital rims are shifted by
increasingly divergent rays
For tilt on a lateral skull, the orbital plates (roofs of orbits) are recommended
-Expected direction of shift is vertical
Rules for Shift in the Image:
General Shift Rule for Angling, Rotation, Tilt, and Flexion/Extension: Torso and Skull:
Generally, for evaluation of radiographs of torso or skull anatomy, every 1
inch of shift observed between landmarks represents about 10 degrees of
beam angulation, body part rotation, tilt, flexion, or extension
[Corollary Rule: Every ½” shift represents ab. 5 degrees of movement]
Case Study: Tilt on a Lateral-Oblique Mandible
25° CR angle and/or head tilt = 2.5” shift
35° CR angle and/or head tilt = 3.5” shift
Case Study: Angle and Rotation on a Lateral-Oblique TMJ
1. Using the posterior margins of the mandibular rami, determine which is the upside
ramus, TMJ, and EAM
Answer: The structures projected lower in the image are less sharp and more
magnified = upside; Downside EAM is more magnified and blurred
2. What was the angle and direction of the CR angle?
Answer: Shift of upside structures is ~3” almost directly below downside struc’s
Using general shift rule, this indicates a CR angle of approx. 30 degrees caudal
How Much is Repeatable?
Generally, the amount of off-angle, off-rotation, incorrect tilt, flexion or extension
that is considered to warrant a repeated exposure is more than 3 degrees
Example 1 - Angulation: AP cervical spine requiring a 15° angle is repeatable if the
angle used was >18° or <12°
Example 2 – Rotation: Oblique lumbar spine requiring 45° rotation is repeatable if
rotation used was >48° or <42°
Example 3 – Flexion/Extension: “Tunnel” projection of knee requiring 70° flexion of
femur is repeatable if the flexion used was <67 or° >73°
Example 4 – Tilt: Lateral skull is repeatable if tilted more than 3°
•3 degrees is about 1 cm shift in the image
•1/2 inch of shift is always repeatable for torso and skull (= 5° rotation)
Case Study: Rotation on PA Chest
The patient’s right SC joint is 1½” from the midline, the left SC joint is 1/2” from
the midline. (To find where they should be, average the 2 shifts observed.)
- This is 1/2” of shift; How many degrees of rotation is indicated?
Answer: For the torso, 1/2” of shift indicates 5 degrees of rotation
It is repeatable (>3°)
Specifically, how should this position be corrected? Answer: The shoulders
should be rotated back toward the patient’s left by 5 degrees
Case Study: Flexion/Extension on an Odontoid View: Incisors ½” above occiput
Is the patient’s head overflexed or overextended, and by how much?
What options are available to correct the position?
Answer: The patient’s head is overextended about 5 degrees. To Correct: 1)
Build up the upper part of the patient’s head with a sponge to flex the chin back
down 5°, or 2) Angle the CR 5° caudally
Case Study: Flexion/Extension on an Odontoid View: Incisors ½” below occiput
Correct by what amount and direction of flexion/extension?
Answer: Head is 5° hyperflexed – Correct by raising the chin 1/4”
Reposition observed surface anatomy one-half distance of shift present in image
-Here, for 1/2” shift in image, correct by raising or lowering the chin 1/4”
Shift Rule for Extremities (knee or smaller) and Spine:
Cut the general rule in half:
Every ½ inch of shift observed between landmarks represents about 10 degrees
of rotation, tilt, flexion/extension or angle
Shift Rule for Lateral Chest:
Double the general rule:
Every 2 inches of shift observed between landmarks represents about 10
degrees of rotation, tilt, flexion/extension or angle
Case Study: Rotated Lateral Chest
Posterior costophrenic angles and posterior R and L ribs shifted by ~1”
How many degrees is the chest rotated? Repeatable?
Answer: (Doubling the general rule)
Shift of 1” represents 5° rotation, repeatable
-1” shift is slightly more than the “2 fingerbreadth” rule for repeatable rotation on
a chest
Case Study: Lateral Lumbar Spine
Left view shows >1/4” shift
--Cutting general rule in half:
Left view is >5° rotated and is repeatable
Right view shows slight rotation (3°): Borderline repeatable
Case Study: Rotated Lateral Knee:
Medial femoral condyle is 1/2” posterior to lateral condyle.
How many degrees must the knee be rotated, and in which direction, to correct?
Repeatable?
Answer: (Halving the general rule) Shift of 1/2” represents 10° medial rotation,
repeatable. Rotate knee 10° downward toward table
Applications: Skull
The general shift rule can be roughly applied for skull views, but for high
accuracy, based on an average 17 cm thickness, a more specific version of the shift
rule for skull radiographs is:
Each centimeter of shift represents 6° of rotation, tilt, flexion, extension, or angle
…but to simplify (using anatomical reference):
-Each quarter-orbit (each cm of shift) presents 6° of rotation, tilt, flexion,
extension, or beam angle
To evaluate flexion/ extension on all frontal views, the starting point for
placement of the petrous ridges is the roof of the orbit (at 0° flexion with OML
perpendicular)
Examples:
Petrous ridges placed at mid-orbit = 12° of chin extension or caudal angle
At bottom rim of orbit = 24° extension or caudal angle
Practice: Petrous ridges …
A. At bottom quarter of orbit ?
B. At bottom third of orbit?
Petrous ridges placed at quarter-orbit below floor of orbits?
Answer: 30° of chin extension or caudal angle
Practice:
Petrous ridges placed at top of upper incisors, just below floor of maxillary
sinuses?
Answer: 54° of chin extension or caudal angle
53° chin extension = Waters
Practice:
Petrous ridges placed at one quarter-orbit above roof of orbits?
One quarter-orbit above roof of orbits = 6° of chin flexion or cephalic angle
Case Study: Rotation on PA Caldwell Skull:
Using distance from lateral orbital rim to side of cranium:
Foreshortened side is direction of rotation
Here, L side is ~2 cm, R side is ~4 cm (Ave = 6/2 = 3 cm, so they are 1 cm off)
= 1 cm shift to L = 5-6° rotation toward left
Practice:
For Waters view, petrous ridges placed in middle of maxillary sinuses
-Using scale, how many degrees is the chin extended?
-If chin cannot be further extended, what degree and direction of CR angle must
be used to achieve 53° equivalent total for Waters projection?
Answer: Using the scale, chin is extended 36-38 degrees
-If chin cannot be further extended, angle CR 15-17° caudal (for PA
projection) to achieve full Waters position
Applications: Rotation on Lateral Skull
Structures: Observe Greater Wings and Mandibular Rami
-ID upside structures as more magnified and blurry by comparison to downside
A: no rotation
B: >1 cm shift = 7-8° rotation
Applications: Tilt on Lateral Skull
-For tilt, use orbital roofs (frontal bone)
A: no tilt
B: 1 cm shift = 6° tilt
C: 2 cm shift = 12° rotation
C: <2 cm shift = 10-11° tilt
Reminder: For skull work:
When correcting position, move observed surface of anatomy 1/2 of shift
distance noted in image
(Movement of face corrects for ½ of shift, reverse movement of occipital bone
corrects for the other half)
Head Rotation on Frontal Views:
When cranium is included:
Foreshortened distance between outer rim of orbit and side of cranial vault
indicates head rotation toward that side
-Here: Just < 2 cm on pt’s left and just < 4 cm on pt’s right indicate 1 cm shift
(Proper location of orbital rims is 6cm / 2 = 3 cm from walls)
Rotation = 6 degrees to left (double repeatable amt)
For “coned-down” views, when walls of cranial vault are not included:
“Half-moons” between outer orbital rims and temples can be used
-Head rotation is toward widened side
Some Other Applications
Case Study: AP Sacrum/Pelvis
With 1/2” lateral shift between the symphysis pubis (arrow) and the midline of the
sacrum (line), how much rotation is present and in which direction? Correction?
Answer: There is ~5° rotation toward the patient’s left.
-Rotate the patient toward the right slightly
Predicting Angle for L5-S1 Lateral “Spot” from Routine Lateral L-Spine View:
Required angle can be from 5° cephalic to 18° caudal, “blanket rules” not helpfulTwo practical tips:
1. Use palpation of landmarks:
Three Accurate Methods for Determining CR Angle by Palpation:
1. Angle parallel to (and center to) palpated line between 2 PSIS’s
2. Angle perpendicular to palpated line between L5 spinous
process and sacral cornu
3. Angle parallel to palpated line between 2 iliac crests
2. Cue from Routine Lateral View:
Case Study: Predicting L5-S1 Angle from Lateral Lumbar Spine
A shows L5-S1 joint fully opened, B shows L5-S1 joint closed
-What are the expected CR angles for the L5-S1 spot projections?
-If joints progressively close toward L5, vertical CR indicated for “spot”
-If L5 is open on routine lat., caudal angle is indicated
Beam divergence can be quantified: At 40 inches SID:
For each inch in any direction away from the CR, beam divergence
increases by 2 degrees
Each inch of off-centering is equivalent to 2° of angle
(At 72” SID, beam divergence increases by approx. 1° per inch)
For lateral lumbar spine: At an average 3.5 inches below the CR, those rays
passing through L5-S1 are angled about 7 degrees
Thus, a 7° caudal angle is indicated for the L5-S1 “spot” lateral projection
Answers: Since a 7-8° caudally diverging beam is far from opening the joint …
A: 7-8° caudal for the L5-S1 spot
-Tendency: To under-estimate angle
B: Most likely close to vertical -Since a 7-8° caudally diverging beam is far
from opening the joint
Case Study: Lateral Elbow
Note shift of humeral condyles is vertical What is the positioning error, and how should it be corrected?
Answer: This is due to tilt of the humerus
Correction: Bring the patient’s shoulder down level with the IR or tabletop
(Alternative: longitudinal angle, not us. practical)
Case Study: Lateral Knee
Besides being rotated medially, what other error was made positioning the knee
on the left, but corrected o the right?
Answer: The larger medial femoral condyle lies below the lateral condyle,
obscuring the joint space
No 5° cephalic angle was used for the knee on the left, as recommended
by positioning atlases
Optional Case Studies: Time allowing:
Case Study: Axial Patella: “Sunrise” method
The subpatellar space is occluded in both views: With the patient supine, which
projection (A or B) demonstrates over-angulation of the CR upward, and
which demonstrates insufficient upward angulation?
A is over-angled, projecting the rounded tibia (arrow) into the joint space
B is under-angled, projecting the M-shaped femoral condyles into the joint space
Rotation on Lateral Scapula:
-With arm across shoulders, proper body rotation is just 15-20 degrees
-LEFT: Under-rotation leaves thin medial border over lower ribs
-MIDDLE: Over-rotation runs thick lateral border into rib cage