What happens if the X-ray (X-radiation) beam is at the wrong angle during a tangential X-ray?

Impact of Incorrect X-Ray Beam Angle on Tangential X-Ray Images

When the X-ray beam is at the wrong angle during a sunrise, Merchant, or tangential X-ray, it results in distorted images that can lead to inaccurate measurements, misdiagnosis, and potentially inappropriate treatment decisions, ultimately affecting patient outcomes.

Effects of Incorrect Beam Angle

Image Distortion and Quality Degradation

• Anatomical Distortion: Incorrect angles cause anatomical structures to appear distorted, with altered spatial relationships and proportions 1
• Decreased Contrast: Improper angles may require increased kVp to achieve adequate detector dose, which decreases image contrast and degrades image quality 1
• Measurement Errors: Studies show significant measurement errors with oblique radiographs - up to ±6° error in measurements that increases with greater obliquity 2
• Visualization Problems: Critical articular surfaces may not be properly visualized when the beam angle is incorrect 3

Diagnostic Implications

• Missed Pathology: Subtle fractures, subluxations, or degenerative changes may not be apparent with improper beam angles 3
• Inaccurate Measurements: For example, Bohler's angle measurements (used in calcaneus fractures) become increasingly inaccurate as X-ray obliquity increases 2
• False Negative/Positive Results: Incorrect angles can hide true pathology or create the appearance of abnormalities that don't exist

Radiation Exposure Concerns

• Increased Patient Dose: Improper positioning may require:

  • Repeat imaging, doubling radiation exposure
  • Increased X-ray output to compensate for poor positioning 1
  • Concentrated radiation on smaller skin areas if positioned too close to the X-ray source 1

• Increased Staff Exposure: Incorrect positioning can increase scattered radiation exposure to medical personnel 1

Technical Factors Affected by Beam Angle

Beam Entrance and Exit

• If the patient is positioned too close to the X-ray source, radiation is concentrated on a smaller skin area, increasing entrance port exposure rate and skin injury risk 1
• If positioned too far, increased kVp is required, decreasing image contrast 1

Parallel Surface Problems

• When surfaces are parallel to the X-ray beam, they are not properly penetrated, leading to image artifacts and lack of detail 1
• This is particularly problematic in tangential views where specific angles are required to visualize joint spaces

Collimation and Field Size Issues

• Incorrect angles may require larger field sizes to capture the intended anatomy, increasing total radiation dose 1
• Improper positioning can negate the benefits of collimation in reducing scattered radiation 1

Specific Impacts by View Type

Sunrise View (Patella)

• Incorrect angle fails to demonstrate articular surfaces of the patellofemoral joint 3
• May miss patellar subluxation, fractures, or degenerative joint disease 3

Merchant View (Patellofemoral Joint)

• Improper angles alter the apparent congruence angle and lateral patellofemoral angle
• Can lead to misdiagnosis of patellar tracking disorders

Tangential Views

• Incorrect angles may completely miss the intended anatomical relationship
• Critical joint spaces may not be visualized at all

Best Practices to Avoid Incorrect Angles

1. Proper Patient Positioning: Load samples at a slight angle to ensure parallel surfaces to the X-ray beam are minimized 1

2. Optimal Distance: Maintain approximately 70 cm between patient's skin surface and X-ray source 1

3. Detector Positioning: Position the detector close to the patient to:

   • Reduce entrance port skin dose
   • Intercept scattered radiation
   • Improve image quality 1

4. Regular Equipment Calibration: Ensure X-ray equipment is well-maintained and calibrated periodically 1

5. Awareness of Angle-Specific Requirements: Different views require specific angles - adhere to standardized protocols for each view type

Common Pitfalls to Avoid

• Assuming the Inverse Square Law Fully Applies: Studies show that stepping back from the table doesn't reduce scatter radiation as much as the inverse square law predicts 4

• Overlooking Patient Size: Image quality deteriorates as patient size increases due to more scattered radiation and higher kVp requirements 1

• Neglecting Beam Collimation: Proper collimation reduces scattered radiation and improves image contrast 1

• Failing to Consider Repeat Imaging: When angles are incorrect, repeat imaging is often necessary, doubling patient radiation exposure

By understanding these effects and following proper positioning protocols, clinicians can avoid the diagnostic errors and unnecessary radiation exposure that result from incorrect X-ray beam angles in tangential radiography.