Chest & thorax

Chest & thorax

Exam category: Procedures → Thorax and Abdomen
Why it matters: Nearly every positioning choice in this module exists to solve one specific diagnostic problem — magnification, superimposition, or gravity-dependent fluid/air — not habit or convention.

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The big picture (plain English)

Chest and rib imaging choices are all about getting gravity and geometry to work for you: which side is up, how far the tube is from the patient, and whether the patient inhales or exhales, all change what becomes visible.

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1. The anatomy you need to read a chest image

A handful of anatomical facts do real work when you evaluate a chest radiograph:

• Lobes: the right lung has three (superior, middle, inferior), the left lung has two (superior and inferior). The right is divided by two fissures, the left by a single oblique fissure.
• Carina: the ridge of the lowest tracheal cartilage marking where the trachea bifurcates into the right and left primary bronchi, at about the level of T4–T5. It is a landmark, not a cartilage of the larynx and not the apex of the lung.
• The right hemidiaphragm sits higher than the left. The liver occupies the right upper abdomen and pushes it up, making the right lung roughly 1 inch (2.5 cm) shorter than the left — on the order of half a rib space to a full rib space, not several. On a normal PA the right dome is the more superior of the two.

2. Why PA, not AP, for routine chest

The heart sits anteriorly in the mediastinum. On a PA chest, the heart is closer to the IR, minimizing magnification. On an AP chest, the heart is farther from the IR (closer to the diverging x-ray source), magnifying it — which can look like cardiac enlargement that isn't really there. Routine chest imaging also uses a 72-inch (180 cm) SID specifically to further reduce beam divergence and magnification.

3. Centering, positioning, and what a good chest image looks like

Central ray. Both routine chest projections center at T7:

Why the shoulders get rolled forward. On a PA chest the shoulders are rotated forward against the IR so the scapulae swing laterally, clear of the lung fields — otherwise they superimpose on the lungs. This is a separate manoeuvre from depressing the shoulders downward, which moves the clavicles below the apices. Forward rotation is about the scapulae; downward depression is about the clavicles.

Why a lateral is taken alongside the PA. The lateral is a 90° perspective, and what it adds is the anatomy the PA can't show: structures lying posterior to the heart, the great vessels, and the sternum. It is normally a left lateral — the heart lies primarily in the left thoracic cavity, so putting the left side against the IR keeps the heart close to the receptor and demonstrates it with less magnification. A right lateral is used when the complaint involves the right side.

Evaluation criteria. Two checks come up constantly:

• Adequate inspiration — a minimum of 10 posterior ribs visible above the diaphragm, and 11 in many healthy patients. Put the other way round, the dome of the right hemidiaphragm should sit at or below the level of the 10th posterior rib. Expiration typically shows only 8.
• No rotation — the sternal ends of the clavicles are equidistant from the vertebral column (equivalently, both sternoclavicular joints sit the same distance from the midline of the spine). Note that counting ribs tests inspiration, not rotation, and scapulae over the lung fields indicates insufficient shoulder rotation — neither one is the rotation check.

4. Why erect whenever the patient can manage it

There are three standard reasons for imaging the chest erect, and questions often ask which one applies to a particular clinical problem:

1. The diaphragm can move farther down. Erect, the liver and abdominal organs drop, so the diaphragm descends further on full inspiration and the lungs aerate more completely.
2. Air and fluid levels become visible. Fluid gravitates to the lowest point while air rises. Erect, a pleural effusion collects near the base of the lung where it can be seen as a discrete collection; recumbent, that same fluid spreads over the posterior surface and just makes the whole lung look hazy. This is the reason that specifically matters for a suspected pleural effusion.
3. Engorgement and hyperemia of the pulmonary vessels are avoided.

5. The lordotic position

A dedicated AP axial projection that rotates the clavicles up and out of the way, so the lung apices — normally obscured by the clavicles on a standard PA — become visible. The patient leans back against the IR with shoulders, neck, and the back of the head touching it; that backward lean angles the midcoronal plane relative to the central ray, throwing the clavicles superior to the apices so they no longer overlie them. On a correct image the clavicles appear nearly horizontal and above the apices. If the patient can't stand for a true lordotic position, an AP semiaxial with the CR angled 15-20° cephalad from supine achieves the same goal.

Don't confuse this with the AP semierect chest, where the CR is angled roughly 5° caudad. That small caudad angle keeps it perpendicular to the long axis of the sternum and projects the clavicles inferiorly, again to stop them obscuring the apices — the same goal as the lordotic, reached from the opposite direction.

6. Lateral decubitus — which side up depends on what you're looking for

7. Ribs

How the 12 pairs are classified. The first seven pairs are true ribs — they connect to the sternum directly by their own costal cartilage. The last five pairs (8–12) are false ribs: pairs 8, 9 and 10 have cartilage that merges into the cartilage of rib 7 rather than reaching the sternum themselves, and pairs 11 and 12 have no costal cartilage at all, so they reach the sternum not at all and are called floating ribs.

Above vs. below the diaphragm changes the breathing instruction, the body position, and the technique:

The recumbent part of the below-diaphragm rule has its own reason: lying down lets the diaphragm rise to its highest position and flattens the abdomen so it is less thick — especially in hypersthenic patients — so the lower ribs are seen better through the abdominal structures. Injuries below the diaphragm are generally to the posterior ribs, which is why an AP projection is used there.

Which oblique. A straight AP or PA foreshortens the axillary (lateral) portion of the ribs; an oblique is what demonstrates it. The rule for which oblique is affected side toward the IR:

• Posterior rib injury → posterior oblique, affected side down. Right posterior ribs → RPO (paired with a straight AP). Left posterior ribs → LPO.
• Anterior rib injury → anterior oblique, affected side toward the IR.

Rotating this way both moves the spinous processes away from the side of interest and puts the affected ribs closest and parallel to the IR.

8. Sternum — why oblique, not frontal

A direct AP or PA superimposes the thin sternum over the dense thoracic spine, making it nearly invisible. The RAO position rotates the sternum away from the spine into the more homogeneous soft tissue of the heart/lung shadow, where it actually becomes visible.

How much rotation is not a fixed number. You rotate until the sternum is projected off the vertebrae and over the cardiac shadow, and how far that takes depends on body habitus: a large, deep-chested (hypersthenic) patient needs less rotation — around 15–20° — while a thin-chested (asthenic) patient needs more.

The breathing technique is deliberate. An orthostatic (shallow-breathing) technique may be used instead of suspended respiration: a long exposure (3+ seconds) at low mA lets gentle breathing blur the overlying lung markings and rib shadows while the sternum, which doesn't move, stays sharply defined. On a correct image the bony margins of the sternum are sharp but the lung markings are blurred.

9. Sternoclavicular joints

The sternoclavicular (SC) joints — where the clavicles meet the manubrium — sit directly over the vertebral column, so a straight PA buries them in the spine. They're imaged with a PA (centered at the jugular notch) plus a slight anterior oblique (RAO/LAO): a small rotation of 10–15° swings the joint of interest just off the spine, where it can actually be seen. Both sides are taken for comparison, since a subtle dislocation only shows up against the normal side.

Why PA rather than AP, even though the joints are easier to locate with the patient AP: the PA gives the least magnification distortion and reduces the radiation dose reaching the thyroid.

Which oblique shows which joint — the anterior oblique demonstrates the DOWNSIDE joint, the one nearer the IR, projecting it across the spine into the opposite lung field. So an RAO demonstrates the RIGHT SC joint, and an LAO demonstrates the left. (This is the reverse of the posterior-oblique adaptation used for patients who can't lie prone, where the upside joint is the one best seen.)

10. Soft-tissue neck (upper airway)

The lateral soft-tissue neck (lateral upper airway) images the air-filled pharynx, larynx, and trachea in profile instead of the bones — the go-to for epiglottitis, croup, and airway foreign bodies. The trick is to expose on full inspiration: filling the airway with air provides natural negative contrast that outlines the epiglottis and the whole air column. A lower-kVp soft-tissue technique keeps those soft tissues visible instead of burning through them.

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Registry must-know facts

• Right lung 3 lobes, left lung 2. The carina is the ridge where the trachea bifurcates into the primary bronchi.
• The right hemidiaphragm is higher than the left (liver pushes it up) — about 1 inch / 2.5 cm, i.e. roughly half to one rib space.
• CR at T7 for both PA (18–20 cm below vertebra prominens, or the inferior scapular angles) and lateral (midthorax, 7.5–10 cm below the jugular notch).
• Shoulders forward = scapulae off the lung fields. Shoulders down = clavicles below the apices. Different manoeuvres, different targets.
• The lateral adds what lies behind the heart, great vessels, and sternum; it's a left lateral because the heart is a left-sided structure.
• Adequate inspiration = 10 posterior ribs above the diaphragm (11 is common). No rotation = sternal ends of the clavicles equidistant from the spine.
• Three reasons to image erect: diaphragm descends farther, air/fluid levels become visible, vessel engorgement avoided. The fluid-level one is why a suspected pleural effusion is imaged erect.
• AP semierect CR is ~5° caudad to project the clavicles off the apices — not to be confused with the lordotic, which leans the patient back to throw the clavicles above the apices.
• Ribs: 1–7 true, 8–12 false, 11–12 floating (no costal cartilage).
• Below-diaphragm ribs: recumbent, expiration, AP, medium kVp (75–85) — recumbent because the diaphragm rises and the abdomen flattens.
• Obliques show the axillary ribs; affected side toward the IR (right posterior ribs → RPO).
• Sternum RAO rotation varies by habitus: hypersthenic needs less (~15–20°), asthenic needs more. Orthostatic breathing blurs lung markings while the sternum stays sharp.
• SC joints are done PA for less magnification and less thyroid dose; the anterior oblique shows the downside joint, so RAO → right SC joint.
• PA chest minimizes heart magnification (heart is anterior, closer to IR); 72-inch SID further reduces magnification.
• Lordotic position exists specifically to project the clavicles above the lung apices.
• Pneumothorax → affected side up on lateral decubitus. Pleural effusion → affected side down.
• Ribs above diaphragm: suspend on inspiration. Ribs below diaphragm: suspend on expiration.
• Sternum RAO exists to move the sternum off the dense thoracic spine and into the more uniform heart/lung shadow.
• SC joints overlie the spine, so they're imaged PA (centered at the jugular notch) plus a slight anterior oblique to swing the joint off the vertebrae, bilateral for comparison.
• Lateral soft-tissue neck (upper airway) shows the air-filled pharynx/larynx/trachea for epiglottitis, croup, and airway foreign bodies; expose on full inspiration to distend the airway (negative contrast), with a lower-kVp soft-tissue technique.

Common trap

The decubitus positioning rule (pneumothorax up, effusion down) is the single most commonly flipped fact in this module — both are "lateral decubitus chest," so it's easy to apply the wrong side without a clear memory anchor for why they're opposite.

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Source: Bontrager, Textbook of Radiographic Positioning and Related Anatomy, 10th ed., Ch. 2 (chest anatomy and lobes, carina, hemidiaphragm levels, erect-position rationale, PA and lateral centering and evaluation criteria, AP semierect and lordotic, soft-tissue neck), Ch. 10 (bony thorax — rib classification, above/below-diaphragm positioning, rib obliques, sternum RAO and orthostatic technique, sternoclavicular joints); upper-airway indications (epiglottitis, croup) per Ch. 16.