Why is the cardiothoracic (CT) ratio larger in the anteroposterior (AP) view X-ray?

Why the Cardiothoracic Ratio is Larger on AP View X-rays

The cardiothoracic ratio (CTR) is larger on anteroposterior (AP) chest radiographs compared to posteroanterior (PA) views because the heart is positioned closer to the X-ray detector in AP positioning, causing magnification of the cardiac silhouette due to increased beam divergence.

Physical Principles of Magnification

The fundamental issue is geometric magnification based on the heart's position relative to the X-ray source and detector:

• In PA radiographs (standard positioning), the X-ray beam travels from posterior to anterior, placing the heart closer to the detector and farther from the X-ray source, which minimizes cardiac magnification 1
• In AP radiographs (portable positioning), the beam travels from anterior to posterior, positioning the heart farther from the detector and closer to the X-ray source, resulting in greater beam divergence and cardiac magnification 2, 3
• The mathematical relationship shows that cardiac diameter increases proportionally with radiation distance, with the formula: CD(PA)/CD(AP) ratio = {0.00099 × (radiation distance [cm])} + 0.79 2

Quantitative Differences Between AP and PA Views

The magnitude of CTR difference is clinically significant:

• AP chest radiographs consistently show CTR values approximately 0.075 higher than axial CT measurements, with measured heart diameters approximately 3 cm larger and thoracic diameters approximately 2 cm larger compared to true dimensions 3
• The traditional CTR cutoff of 0.50 for cardiomegaly (established for PA views) has poor specificity (only 32%) when applied to AP radiographs 3
• For AP radiographs, a CTR cutoff of 0.60 is more appropriate, providing 34% sensitivity but 92% specificity for detecting cardiac chamber enlargement, while a cutoff of 0.55 offers balanced performance with 61% sensitivity and 66% specificity 3

Clinical Implications and Pitfalls

Critical Recognition Points

• The CTR should only be reliably measured and interpreted on PA radiographs, as the American College of Radiology emphasizes that attempts to use CTR in other projections face significant limitations 4, 1
• When only AP radiographs are available (common in critically ill or immobile patients), physicians must adjust their interpretation threshold upward to avoid false-positive diagnoses of cardiomegaly 2, 3

Correction Methods

• A "corrected" CTR can be calculated from AP radiographs if a previous PA radiograph is available by applying the radiation distance correction formula and substituting the most recent thoracic diameter from PA imaging 2
• This corrected CTR shows high correlation with conventional PA-based CTR (r = 0.92, difference: 0.00016 ± 0.029) and can help detect congestive cardiomegaly in patients undergoing portable AP imaging 2

Additional Technical Factors

• Bedside AP radiographs are commonly limited in quality due to motion artifacts, overlying devices and garments, and suboptimal patient positioning 4
• The evaluation of the mediastinum is inherently limited with only an AP projection, and AP radiographs have lower diagnostic yield for thoracic pathology compared to CT, missing 80% of hemothorax cases and 50% of vertebral and rib fractures 4

When AP Radiographs Are Necessary

Despite limitations, AP chest radiographs remain valuable as first-line imaging:

• They are the standard initial modality in hemodynamically unstable trauma patients and critically ill patients who cannot stand for PA imaging 4
• They can identify indirect findings suggesting cardiac injury including hemothorax, widened mediastinum, enlarged cardiomediastinal silhouette, and abnormal cardiac contour 4
• However, clinicians must remember that an apparently enlarged cardiac silhouette on AP view may be artifactual rather than pathological, requiring correlation with clinical findings and potentially advanced imaging 1, 3