ARRT Radiography Exam Quiz: Position Spine Imaging Studies
17 questions · exam conditions
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Position Spine Imaging StudiesQuestion 1 of 17

A radiography student asks why the lateral cervical spine protocol specifies 72-inch SID rather than the 40-inch SID used for most other projections. Which of the following MOST accurately explains the rationale?

The 72-inch SID is used to reduce patient dose; increasing the distance from the tube to the patient reduces radiation dose to the skin in proportion to the increased distance
The 72-inch SID minimizes magnification and improves sharpness by compensating for the increased object-to-image distance (OID) inherent in lateral cervical spine positioning.
The 72-inch SID is required to magnify the small cervical vertebral bodies; a longer SID increases magnification, making the small cervical vertebrae easier to visualize and measure
The 72-inch SID improves beam penetration through the shoulder soft tissue; greater SID allows higher-energy photons to predominate at the IR
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ARRT Radiography Exam Quiz

ARRT Radiography Exam Quiz: Position Spine Imaging Studies

Practice Position Spine Imaging Studies in ARRT Radiography Exam with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Position Spine Imaging Studies, giving you a quick way to practice the rules, question types, and explanations that matter most for ARRT Radiography Exam.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

A radiography student asks why the lateral cervical spine protocol specifies 72-inch SID rather than the 40-inch SID used for most other projections. Which of the following MOST accurately explains the rationale?

  1. The 72-inch SID is used to reduce patient dose; increasing the distance from the tube to the patient reduces radiation dose to the skin in proportion to the increased distance
  2. The 72-inch SID minimizes magnification and improves sharpness by compensating for the increased object-to-image distance (OID) inherent in lateral cervical spine positioning. (correct answer)
  3. The 72-inch SID is required to magnify the small cervical vertebral bodies; a longer SID increases magnification, making the small cervical vertebrae easier to visualize and measure
  4. The 72-inch SID improves beam penetration through the shoulder soft tissue; greater SID allows higher-energy photons to predominate at the IR
Explanation: How to get the right answer: In the lateral cervical position, the patient's posterior is against the receptor; the cervical spine is not adjacent to the IR; it is positioned in the center of the neck with approximately 8 to 12 cm of tissue between the vertebrae and the lateral skin surface. This inherent OID, which cannot be reduced by positioning, produces magnification and geometric unsharpness from beam divergence at that distance. The standard compensation for inherent OID that cannot be eliminated is to increase the SID; a longer SID makes the beam more parallel at the level of the anatomy, reducing the magnification and geometric unsharpness caused by the OID. This is the same principle that explains 72-inch SID for PA chest radiography, where the heart has inherent OID from the posterior thoracic wall. Why the other answers are wrong: Choice A claims the primary rationale is dose reduction; while increased SID does reduce beam intensity at the patient surface via the inverse square law, dose reduction is not the primary clinical rationale for 72-inch lateral cervical SID; OID compensation and geometric image quality are the drivers. Choice C claims 72-inch SID increases magnification; increasing SID reduces magnification by making the beam more parallel, not increases it; the purpose is to counteract the magnification produced by the inherent OID. Choice D invokes SID effects on photon energy; photon energy is determined by kVp at the generator and SID has no effect on the energy spectrum of the primary beam. Big idea to remember: Lateral cervical 72-inch SID compensates for the inherent OID of the cervical spine (approximately 8 to 12 cm from the lateral skin surface); at 40-inch SID this OID causes significant magnification and geometric unsharpness; 72-inch SID reduces beam divergence at the anatomy and minimizes these effects; the same principle applies to the PA chest.

Question 2

When performing a lateral thoracic spine on a patient with a large chest, the radiographer notes significant tissue thickness variation between the upper and lower thoracic regions. Using a 72-inch SID, what technique modification will best ensure uniform density throughout the thoracic spine?

  1. Use a compensating filter with the thick portion positioned over the lower thoracic region
  2. Apply a wedge filter with the thin edge positioned superiorly to balance tissue density differences (correct answer)
  3. Increase overall kVp significantly and reduce mAs to improve penetration of thicker lower regions
  4. Use breathing technique with extended exposure time to average out tissue density variations
Explanation: In lateral thoracic spine, upper thoracic regions have less tissue thickness than lower regions due to chest anatomy. A wedge filter with thin edge superior compensates for this difference. Option A positions the filter incorrectly, Option C changes technique factors inappropriately, and Option D breathing technique doesn't address density uniformity issues.

Question 3

A patient with suspected lumbar spondylolisthesis requires lateral lumbar spine imaging. The standard lateral view shows possible L5-S1 anterior displacement, but the lumbosacral junction is not optimally demonstrated due to overlying iliac crests. What supplemental positioning technique will best visualize this region?

  1. Perform a lateral view with the patient standing to accentuate any instability present
  2. Obtain a 30-degree oblique view to project the lumbosacral junction away from the pelvis
  3. Use a lateral position with the patient's hips and knees flexed to reduce lumbar lordosis
  4. Perform a lateral L5-S1 spot view with 5-10 degrees caudal angulation to clear the iliac crests (correct answer)
Explanation: When imaging suspected spondylolisthesis at L5-S1, the primary challenge is that the iliac crests frequently obscure this critical junction on standard lateral views. This is a classic positioning problem in lumbar spine radiography that requires a specific technical solution. The correct approach is answer D - a lateral L5-S1 spot view with 5-10 degrees caudal angulation. This technique works by angling the central ray downward, which projects the lumbosacral junction above the iliac crest shadows. The caudal angulation essentially "lifts" the area of interest out from behind the bony pelvis, providing clear visualization of any anterior displacement of L5 on S1. Answer A (standing lateral) can demonstrate instability but doesn't solve the fundamental problem of iliac crest overlap. The anatomical obstruction remains regardless of patient position. Answer B (30-degree oblique) is incorrect because oblique views are used to visualize pars interarticularis defects (spondylolysis), not to clear iliac crest overlap or optimally show anterior displacement. Answer C (flexed hips and knees) reduces lumbar lordosis but actually worsens iliac crest overlap by bringing the pelvis higher relative to the lumbosacral junction. Remember this key principle: when anatomical structures overlap the area of interest, tube angulation is often your best solution. For L5-S1 visualization specifically, always think "caudal angle" to clear the iliac crests. This is a high-yield concept that appears frequently on registry exams.

Question 4

During oblique lumbar spine positioning (LPO), a radiographer achieves good demonstration of the right zygapophyseal joints but notes the spinous processes are not properly positioned relative to the vertebral bodies. What positioning error most likely occurred?

  1. Insufficient rotation - patient needs additional 15 degrees of obliquity to achieve proper joint space opening
  2. Excessive rotation - patient was rotated beyond 45 degrees causing vertebral body distortion (correct answer)
  3. Incorrect central ray centering - beam should be directed to L3 rather than L4 for proper anatomy demonstration
  4. Patient positioning is correct - spinous process position varies with normal anatomical variation
Explanation: In oblique lumbar positioning, excessive rotation beyond 45 degrees causes the spinous processes to project away from their normal relationship to vertebral bodies, indicating over-rotation. Option A would result in closed joint spaces rather than spinous process malposition, Option C addresses centering rather than rotation issues, and Option D incorrectly dismisses a positioning error.

Question 5

A patient requires cervical spine imaging following a motor vehicle accident. The trauma team has applied a rigid cervical collar and the patient cannot be moved from supine position. For the lateral cervical view using a horizontal beam technique, what is the critical positioning consideration to ensure diagnostic quality?

  1. Center the beam at C4 level and ensure the cassette is parallel to the midsagittal plane
  2. Position the cassette perpendicular to the long axis of the cervical spine with beam centered at C3-C4
  3. Place the cassette against the patient's side with the beam horizontal and perpendicular to the cassette (correct answer)
  4. Angle the cassette 15 degrees to match cervical lordosis and center beam at external auditory meatus
Explanation: For cross-table lateral cervical spine, the cassette must be placed against the patient's side with horizontal beam perpendicular to the cassette to maintain proper geometric relationships. Option A describes incorrect cassette orientation, Option B has wrong anatomical centering, and Option D introduces unnecessary angulation that would distort anatomy.

Question 6

For an AP open-mouth (odontoid) view, the patient's mouth is properly opened and centered, but the resulting image shows the base of the skull superimposed over the upper portion of the dens. The mandible is properly positioned below the occiput. What adjustment is needed?

  1. Increase cephalic angulation of the central ray to project the skull base above the odontoid process
  2. Flex the patient's neck slightly to bring the occiput and mandible into proper alignment
  3. Decrease caudal angulation or position the head in slight extension to clear skull base from dens (correct answer)
  4. The positioning is acceptable since skull base superimposition is normal in patients with short necks
Explanation: Skull base superimposition over the dens indicates excessive neck flexion or too much caudal angulation. Slight extension or decreased caudal angle will project the skull base above the odontoid. Option A would worsen the superimposition, Option B would increase unwanted flexion, and Option D incorrectly accepts suboptimal positioning.

Question 7

During lumbar spine flexion and extension views, a 68-year-old patient with degenerative disc disease experiences significant pain during the flexion portion. The extension view was completed successfully showing L4-L5 instability. What is the most appropriate course of action for the flexion view?

  1. Complete the flexion view with maximum patient effort since extension showed pathology requiring bilateral comparison
  2. Substitute a neutral lateral view and document patient's inability to flex due to pain and limited range of motion
  3. Perform flexion to patient's tolerance level and clearly document the degree of flexion achieved on the image (correct answer)
  4. Reschedule the examination after pain management consultation since incomplete flexion invalidates the study
Explanation: Functional studies should be performed within patient tolerance, with documentation of achieved range of motion. Limited flexion still provides diagnostic information about segmental motion. Option A risks patient injury, Option B loses valuable diagnostic information about forward flexion stability, and Option D is unnecessarily restrictive since limited flexion views retain clinical value.

Question 8

A patient presents for cervical spine imaging with a history of rheumatoid arthritis and suspected atlantoaxial instability. During the lateral cervical spine positioning, the radiographer notes the patient cannot achieve full extension due to muscle spasms. What is the most appropriate modification to ensure diagnostic quality while maintaining patient safety?

  1. Proceed with maximum achievable extension and increase exposure factors to compensate for suboptimal positioning
  2. Maintain neutral positioning and ensure C1-C2 relationship is clearly demonstrated without forcing patient movement (correct answer)
  3. Apply gentle manual traction to achieve proper extension while monitoring patient comfort levels
  4. Position patient in slight flexion to relax muscles, then gradually move to extension for the exposure
Explanation: In patients with suspected atlantoaxial instability, forcing cervical extension can cause neurological compromise or subluxation. Neutral positioning with clear demonstration of the C1-C2 relationship is safest and most diagnostic. Option A compromises image quality, Option C risks injury through forced movement, and Option D involves potentially dangerous flexion-extension motion in an unstable spine.

Question 9

For a swimmer's lateral cervical spine view on a patient with broad shoulders, the radiographer achieves good visualization of C1-C6 but C7-T1 remains obscured despite proper arm positioning. The patient cannot achieve further shoulder depression. What technical modification will best demonstrate the cervicothoracic junction?

  1. Increase the caudal angulation of the central ray to project below the shoulder level while maintaining lateral positioning (correct answer)
  2. Perform a separate lateral view with 5-10 degree cephalic angulation to open the C7-T1 disc space
  3. Switch to an oblique projection at 45 degrees to move shoulders away from the cervicothoracic junction
  4. Obtain a lateral view with breathing technique to blur overlying soft tissue shadows from the shoulders
Explanation: Increased caudal angulation projects the central ray below shoulder level, improving visualization of C7-T1 in swimmer's position. Option B describes cephalic angulation which would worsen shoulder superimposition, Option C changes to oblique positioning losing lateral anatomy, and Option D breathing technique doesn't address the fundamental shoulder superimposition problem.

Question 10

When positioning for an AP sacrum view, a radiographer notes the patient has a prominent lumbar lordosis and thick pelvis. After proper centering and collimation, what central ray angulation will best demonstrate the sacral segments without foreshortening?

  1. 15 degrees cephalic to compensate for the sacral inclination and lumbar lordosis curvature (correct answer)
  2. 10-15 degrees caudal to align perpendicular to the long axis of the sacrum
  3. 20-25 degrees cephalic to project through the sacral foramina and avoid pelvic rim overlap
  4. 5 degrees caudal with increased penetration to account for increased tissue thickness
Explanation: The sacrum is angled posteriorly, requiring cephalic angulation (typically 15 degrees) to align perpendicular to its long axis and prevent foreshortening. Increased lordosis may require additional cephalic angle. Option B uses caudal angle which would increase foreshortening, Option C uses excessive cephalic angle, and Option D uses minimal caudal angle with wrong reasoning.

Question 11

When positioning for an AP thoracic spine on a patient with severe kyphoscoliosis, the radiographer observes significant vertebral rotation and lateral curvature. To optimize visualization of the vertebral bodies and minimize superimposition, what positioning adjustment should be implemented?

  1. Angle the central ray perpendicular to the curve apex and rotate patient until spinous processes align with table midline (correct answer)
  2. Maintain standard AP positioning but increase kVp significantly to penetrate the increased tissue density from rotation
  3. Rotate the patient toward the convex side of the curve and angle the tube to match the spinal curvature
  4. Position patient lateral recumbent on the convex side to straighten the curve and use standard AP technique
Explanation: In severe scoliosis, angling the central ray perpendicular to the curve apex and aligning spinous processes with the midline helps reduce vertebral rotation artifacts and superimposition. Option B doesn't address the geometric distortion, Option C would worsen the rotation, and Option D describes lateral positioning rather than AP modification for scoliosis.

Question 12

A radiographer is performing a lateral L5–S1 spot projection as a supplement to the standard lateral lumbar spine. The patient is in the left lateral recumbent position. Which of the following MOST accurately describes the correct CR direction and the rationale?

  1. The CR is directed 5 to 8 degrees caudad to align with the inclined L5–S1 disc plane, optimizing visualization by opening the disc space. (correct answer)
  2. The CR is always directed perpendicular to the IR for the lateral L5–S1 spot; no angulation is used because the L5–S1 disc space is horizontal in all patients
  3. The CR is directed 15 degrees cephalad for the lateral L5–S1 spot to compensate for pelvic tilt in the lateral recumbent position
  4. Caudad CR angulation is only used for the lateral L5–S1 spot when the patient has clinically confirmed spondylolisthesis; perpendicular CR is used for all routine cases regardless of anatomy
Explanation: How to get the right answer: The L5–S1 disc space does not lie in a horizontal plane; it is inclined anteroinferiorly (the anterior aspect is lower than the posterior aspect) due to the natural lumbosacral angulation present in all patients. In the lateral recumbent position, a perpendicular CR passes obliquely through this angled disc, producing foreshortening that narrows the apparent disc space on the image. A 5 to 8 degree caudad angulation redirects the CR to be more perpendicular to the inclined L5–S1 disc surface, opening the disc space for accurate demonstration of disc height and any pathological narrowing. The specific angle is adjusted for the individual patient's lumbosacral curvature. Why the other answers are wrong: Choice B applies a perpendicular CR for all patients; a perpendicular beam foreshortens the L5–S1 disc in most patients because of the lumbosacral angle and caudad angulation is specifically required to compensate. Choice C uses a cephalad angle; a cephalad angle would direct the beam further from perpendicular relative to the anteroinferiorly inclined disc plane, worsening foreshortening rather than correcting it. Choice D limits angulation to spondylolisthesis; the lumbosacral angle is a normal anatomical feature present in all patients, and the caudad angulation for the L5–S1 spot is applied based on universal anatomy rather than a specific diagnosis. Big idea to remember: Lateral L5–S1 spot: CR 5 to 8 degrees caudad, centered at the L5–S1 junction; this compensates for the anteroinferiorly inclined lumbosacral disc plane that causes foreshortening with a perpendicular beam; the caudad angle opens the disc space for accurate height assessment.

Question 13

A radiographer is obtaining a trauma cervical spine series on a supine immobilized patient. The cross-table lateral has been completed. The next projection is the AP axial cervical spine. Which of the following MOST accurately describes the correct technique?

  1. The CR is directed 15 to 20 degrees cephalad, centered at C4, with the patient supine and immobilized, ensuring no movement of the head or neck during the exposure. (correct answer)
  2. The AP axial cervical spine cannot be performed on a trauma patient; the patient must first be transferred to an upright position before this projection can be obtained
  3. The CR is directed perpendicular to the IR to avoid any cervical manipulation; CR angulation is only used for the non-trauma AP axial cervical spine
  4. The CR is directed 15 to 20 degrees caudad to project the cervical vertebrae inferiorly, free from mandibular superimposition, in the trauma patient
Explanation: How to get the right answer: The trauma AP axial cervical spine uses the identical CR direction as the routine AP axial technique: 15 to 20 degrees cephalad centered at C4. The critical modification for trauma is that the patient remains supine and immobilized throughout with no head or neck movement. Since trauma patients are already supine (which is the standard position for AP cervical technique), the trauma version requires minimal change from the routine approach. The cephalad angulation projects the mandible and occiput superiorly without requiring patient movement, making it fully compatible with cervical immobilization. All spinal precautions remain in place during the exposure. Why the other answers are wrong: Choice B claims the AP axial cannot be performed on a trauma patient; the AP axial cervical is routinely and safely performed with the patient supine, and transferring a potentially unstable cervical injury patient to the upright position before spinal clearance is unnecessary and dangerous. Choice C uses a perpendicular CR; a perpendicular beam directed at a supine patient would superimpose the mandible and occiput over the upper cervical vertebrae, which is the specific problem the cephalad angulation is designed to correct in both trauma and non-trauma patients. Choice D uses caudad angulation; caudad angulation projects the mandible further down over the cervical vertebrae, worsening superimposition rather than correcting it. Big idea to remember: Trauma AP axial cervical: patient remains supine and immobilized, no head or neck movement, CR 15 to 20 degrees cephalad at C4; the technique is identical to the routine AP axial cervical; the cephalad angle clears the mandible and occiput without requiring patient movement and all spinal precautions remain in effect.

Question 14

A radiographer is performing a four-projection thoracolumbar spine series: AP thoracic, lateral thoracic, AP lumbar, and lateral lumbar. Which of the following correctly pairs each projection with its respiration instruction?

  1. AP thoracic — suspended expiration; lateral thoracic — breathing technique (2 to 3 seconds); AP lumbar — suspended inspiration; lateral lumbar — breathing technique
  2. AP thoracic — suspended inspiration; lateral thoracic — breathing technique (2 to 3 seconds); AP lumbar — suspended expiration; lateral lumbar — suspended expiration (correct answer)
  3. AP thoracic — breathing technique; lateral thoracic — suspended inspiration; AP lumbar — suspended expiration; lateral lumbar — breathing technique
  4. AP thoracic — suspended expiration; lateral thoracic — suspended expiration; AP lumbar — suspended inspiration; lateral lumbar — breathing technique
Explanation: How to get the right answer: Each projection has a specific physiological rationale. AP thoracic uses full inspiration suspended: inhalation depresses the diaphragm away from T11–T12, fills the lungs with air that provides natural contrast, and reduces diaphragm superimposition over the lower thoracic vertebrae. Lateral thoracic uses the breathing technique (2 to 3 seconds of continuous quiet breathing): rib motion during the exposure blurs the ribs and pulmonary vasculature while the relatively fixed thoracic vertebrae remain comparatively sharp, improving vertebral body visibility. AP lumbar uses suspended expiration: expiration relaxes the abdominal musculature, reduces bowel gas distension, and decreases scatter from abdominal contents overlying the lumbar vertebrae. Lateral lumbar also uses suspended expiration for the same muscular relaxation and scatter reduction reasons, with suspended (not breathing) technique to prevent motion blur. Why the other answers are wrong: Choice A pairs AP thoracic with expiration; expiration elevates the diaphragm toward the lower thoracic vertebrae and reduces natural lung contrast, which is opposite to the goal of AP thoracic positioning, and it also incorrectly assigns inspiration to the AP lumbar. Choice C uses the breathing technique for AP thoracic; the breathing technique is specific to the lateral thoracic and is not used for the AP thoracic, which requires suspended respiration. Choice D pairs lateral lumbar with the breathing technique; the breathing technique is used only for the lateral thoracic projection to blur the ribs, while the lateral lumbar requires suspended expiration. Big idea to remember: AP thoracic = suspended inspiration; lateral thoracic = breathing technique (2 to 3 seconds); AP lumbar = suspended expiration; lateral lumbar = suspended expiration; only the lateral thoracic uses the breathing technique, and all other routine spine projections use suspended respiration.

Question 15

A myelography procedure using non-ionic water-soluble contrast injected into the lumbar subarachnoid space has been completed. The radiologist indicates the procedure is finished and the patient is ready to be transferred. Which of the following MOST accurately describes the required post-procedure positioning and its clinical rationale?

  1. The patient should be kept in the lateral recumbent position for 4 hours post-procedure to prevent headache from positional CSF pressure changes following the dural puncture
  2. The patient must be maintained flat (supine) for 4 to 8 hours following myelography to prevent post-lumbar puncture headache by maintaining CSF pressure at the injection site
  3. The patient should be positioned with the head elevated 30 to 45 degrees for 4 to 8 hours to prevent contrast migration into the intracranial space. (correct answer)
  4. No specific positioning restrictions are required following modern water-soluble myelography; the patient may ambulate immediately because water-soluble contrast is absorbed and eliminated rapidly without meaningful migration risk
Explanation: How to get the right answer: Water-soluble non-ionic contrast is well-tolerated at spinal concentrations but neurotoxic if it reaches the intracranial compartment in sufficient concentration, where it interacts with brain tissue and can cause seizures, encephalopathy, and severe headache. Maintaining the head and thorax elevated at 30 to 45 degrees for several hours uses gravity to retain the contrast in the lower spinal region while it is cleared by CSF reabsorption and renal excretion. Oral hydration accelerates contrast elimination. Flat positioning and especially Trendelenburg positioning actively promote superior contrast migration and are strictly contraindicated. Why the other answers are wrong: Choice A prescribes lateral recumbent positioning; the lateral position without head elevation does not prevent superior contrast migration because gravity still acts in a plane parallel to the spine; head elevation above horizontal is the essential requirement. Choice B prescribes flat supine positioning; flat positioning allows contrast to migrate superiorly toward the intracranial space and directly increases the risk of the neurotoxic complications the restriction is designed to prevent. Choice D claims no positioning restriction is needed; post-procedure head elevation remains an important safety requirement for water-soluble myelography and immediate ambulation without elevation is not appropriate. Big idea to remember: Post-myelography (water-soluble contrast): head and thorax elevated 30 to 45 degrees for 4 to 8 hours; gravity prevents intracranial contrast migration and reduces the risk of seizures and encephalopathy; flat and Trendelenburg positions are strictly contraindicated because they actively promote superior contrast flow toward the intracranial cisterns.

Question 16

A radiographer is performing posterior oblique projections of the sacroiliac joints. The RPO position is being used with the patient elevated 25 to 30 degrees from the table on the right side. Which of the following MOST accurately describes which sacroiliac joint is demonstrated on the RPO oblique and the correct CR direction?

  1. The RPO demonstrates the RIGHT sacroiliac joint; the right side is dependent and the right SI joint is closest to the IR, following the same convention as lumbar zygapophyseal joint obliques where the dependent side's joints are demonstrated
  2. The RPO demonstrates both sacroiliac joints simultaneously; posterior oblique positioning always demonstrates bilateral SI joint anatomy
  3. The RPO demonstrates the LEFT sacroiliac joint; the CR is directed perpendicular to the IR, centered 1 inch medial to the elevated left ASIS. (correct answer)
  4. The RPO demonstrates the right sacroiliac joint; the CR is directed 30 to 35 degrees cephalad toward the left iliac crest for the RPO posterior oblique SI joint projection
Explanation: How to get the right answer: The sacroiliac joint surfaces are oriented obliquely in the pelvis, facing posterolaterally. To project an SI joint en face, the beam must approach from the posterolateral direction of that joint's side. In the RPO, the patient's right posterior surface is against the IR; this geometry positions the LEFT SI joint's posterolateral surface to face the incoming beam directly from the tube. Therefore the RPO demonstrates the LEFT SI joint. This rule is the opposite of the lumbar oblique convention, where the dependent side's facets are demonstrated. The CR is perpendicular to the IR, centered 1 inch medial to the elevated ASIS (the left ASIS in the RPO position). Why the other answers are wrong: Choice A applies the lumbar facet oblique rule to the SI joints; the lumbar oblique convention (dependent side's joints demonstrated) does not apply to SI joint obliques; the SI joint rule is opposite and the elevated side's joint is demonstrated. Choice B claims bilateral SI joint demonstration; each posterior oblique SI projection demonstrates only one joint and bilateral evaluation requires both RPO and LPO. Choice D both misidentifies the joint as the right SI joint and prescribes 30 to 35 degrees cephalad angulation; the cephalad angulation is used for the AP axial SI joint projection with the patient supine, not for the posterior oblique SI joint projections, which use a perpendicular CR. Big idea to remember: Posterior oblique SI joints: RPO demonstrates the LEFT SI joint; LPO demonstrates the RIGHT SI joint; CR is perpendicular to the IR; this is the OPPOSITE of lumbar facet obliques where the dependent side's joints are shown; bilateral evaluation requires both RPO and LPO.

Question 17

A radiographer reviews a completed AP open-mouth projection and finds that the dens is not visible: the lower incisors are projected over the dens and the occiput appears to superimpose the C1–C2 region. Which of the following MOST accurately identifies the positioning error and the correction?

  1. The occiput superimposing C1–C2 indicates the patient rotated their head; rotating the head back to midline will clear the occipital shadow from the dens
  2. The CR was not directed perpendicular to the IR; directing the CR 10 degrees cephalad will project the occiput superiorly away from the dens
  3. The mouth is not opened wide enough; instructing the patient to open their mouth as wide as possible will lower the dental arch below the dens
  4. The chin is over-extended, causing the occiput and incisors to overlap the dens; slightly flexing the chin forward will align the dens between the incisors. (correct answer)
Explanation: How to get the right answer: The APOM correction algorithm is based on which dental arch overlies the dens; the dens should appear between the upper and lower incisors on a correctly positioned image. When the chin is over-extended (tilted too far posteriorly), the posterior cranial base drops inferiorly toward C1–C2 and the lower dental arch also descends over the dens. Flexing the chin forward raises the occiput away from C1–C2 and repositions the dental arches so the dens appears between them. The complete algorithm: upper incisors over dens means the chin is not extended enough, extend more; lower incisors and occiput over dens means the chin is over-extended, flex forward. Why the other answers are wrong: Choice A diagnoses head rotation; rotation produces lateral asymmetry of the C1 lateral masses and atlantoaxial joint spaces but does not cause the posterior cranium to descend over the dens in the superior-inferior direction as described. Choice B prescribes cephalad CR angulation; the APOM uses a perpendicular CR throughout and CR angulation is not the standard correction for any APOM superimposition error; head flexion and extension do all the corrective work. Choice C prescribes wider mouth opening; mouth opening affects only the vertical gap between the dental arches but does not change the position of the occiput relative to the C1–C2 region; the positional correction of chin flexion is required. Big idea to remember: APOM correction algorithm: upper incisors over dens = chin under-extended, extend more; lower incisors and occiput over dens = chin over-extended, flex forward; the correct position shows the dens between the upper and lower incisors with the CR perpendicular.