X-ray Imaging (Radiography)

X-rays are a form of electromagnetic radiation with wavelengths from 0.01 to 10 nanometres. X-ray images provide detailed visual representations of internal body structures, where different tissues absorb radiation to varying degrees. Bone absorbs X-rays the most and appears white, while fat and softer tissues appear grey. Air absorbs the least and therefore appears as black areas, as seen in the lungs on chest radiographs.

In diagnostic radiology, X-ray imaging has long been one of the most widely used methods to visualise body structures and assist in diagnosing disease.

Clinical Uses of X-ray Imaging

• Assessment of the bony skeleton: X-rays are commonly used to evaluate fractures and determine the type and severity of injury.

• Chest examinations: Can reveal different types of pneumonia, tumours, and other pathologies.

• Contrast studies: With contrast media, X-rays can provide insight into soft-tissue organs such as the gastrointestinal tract and the uterus.

• Guidance during procedures: X-rays are used in several medical procedures, including:

  • Catheter angiography

  • Stereotactic breast biopsies

  • Intra-articular steroid injections

• Evaluation of pathologies: X-rays are useful for identifying:

  • Fractures

  • Pneumonias

  • Malignancies

  • Congenital anatomical abnormalities

Interpretation of X-rays

X-rays are analysed by assessing opacities (dense areas) on the image. These opacities provide information about the density and structure of different tissues and help clinicians reach an accurate diagnosis.

X-ray imaging remains a cornerstone of medical diagnosis and procedural guidance, particularly due to its availability, rapid results, and broad range of applications¹.

Other Uses of X-rays in Imaging

X-ray technology is not limited to traditional diagnostic studies. Modern technologies have expanded X-ray applications, including fluoroscopy, angiography, and DEXA scanning, each offering specific benefits across medical fields.

Fluoroscopy

Fluoroscopy produces real-time images of internal structures using a continuous, low dose of X-rays. This technology yields moving projection radiographs that are useful to:

• Visualise motion of organs or contrast agents: e.g., the gastrointestinal tract with contrast agents such as barium or Gastrografin.

• Guide medical procedures: Used during interventions such as angioplasty, pacemaker placement, and joint repair/replacement.

• Intraoperative procedures: Provides continuous feedback during surgery or catheter insertion.

Example of use:Gastrointestinal tract: Using barium as a positive contrast agent (white on X-ray) and air as a negative contrast agent (black on X-ray) clearly outlines the contours of the digestive tract.

Angiography

Angiography is a specialised form of fluoroscopy focusing on the cardiovascular system. The procedure involves:

• Injection of iodinated contrast: To visualise blood vessels and flow under X-ray.

• Detection of pathologies: Such as aneurysms, leaks, occlusions (thromboses), neovascularisation, and the placement of catheters or stents.

• Balloon angioplasty: Often performed in combination with angiography to treat vascular stenoses.

Dual-Energy X-ray Absorptiometry (DEXA or Bone Densitometry)

DEXA scans are primarily used to evaluate bone mineral density and diagnose osteoporosis. This method differs from conventional X-ray in that it:

• Uses two narrow X-ray beams: Scanning the body from two different angles (90°) to measure calcium content in bone.

• Common measurement sites: Hip (femoral head), lower back (lumbar spine), and heel (calcaneus).

• T-score: Provides a quantitative assessment of bone density.

• Low radiation dose: Radiation from DEXA is much lower than in standard radiographic exams.

Clinical applications:

• Diagnosis of osteoporosis and monitoring of bone density.

• Assessment of total body fat (less common).

Advanced applications such as fluoroscopy, angiography, and DEXA provide healthcare with valuable tools for diagnosis and treatment, offering specific, precise data ranging from real-time visualisation of organ motion to assessment of bone density and cardiovascular health².

Physiotherapy Use of X-ray Imaging

X-rays play an important role in physiotherapy, particularly in diagnosing and monitoring pathologies of the musculoskeletal and respiratory systems. These images provide valuable information that helps therapists assess and plan treatment pathways.

Musculoskeletal System

X-rays are highly useful for assessing conditions involving bony structures. Because bone reflects X-rays and appears white, clinicians can readily identify:

• Fractures: Location, type, and severity can be identified rapidly.

• Malalignment: For example, skeletal deformities that can affect joint function.

• Monitoring progression: X-rays can evaluate fracture healing, including stages of:

  • Callus formation (bridging): When callus begins to develop.

  • Union: When the fracture is fully consolidated.

Incorporating X-rays into a treatment plan helps the physiotherapist determine whether additional interventions, such as strengthening or rehabilitation exercises, are needed to support recovery.

Use in the Respiratory System

Chest radiographs (thoracic radiography) are crucial for identifying and monitoring cardiopulmonary pathologies, including:

• Pneumothorax: Collapse of part of a lung due to air in the pleural space.

• Haemothorax: Accumulation of blood in the pleural space.

• Atelectasis: Localised lung collapse.

For physiotherapists, chest X-rays support:

• Localisation:X-rays can help identify areas of atelectasis or other pulmonary disease, allowing therapists to target treatment techniques—such as breathing exercises—to the affected region.Example: With atelectasis in the right lobe, the therapist can focus on specific breathing strategies to promote re-expansion of the involved area.

• Monitoring treatment effectiveness:Secretion accumulation and atelectasis should theoretically decrease with effective therapy, which can be confirmed on follow-up radiographs.

Combining with Other Objective Measures

While X-rays provide valuable information, they should be complemented with other objective measures to ensure comprehensive evaluation, including:

• Chest expansion measurements: To assess changes in lung capacity.

• Auscultation: Listening for lung sounds to detect secretions or ventilation issues.

• Endurance tests: To measure aerobic capacity and ability to perform activities of daily living.

Using X-rays alongside these methods enables a more holistic and effective treatment approach.

Formation of X-ray Images

X-ray images are formed as X-ray photons pass through tissues and are partially absorbed, while the remainder passes through to expose radiographic film or a digital detector. Absorption depends on a tissue’s atomic number, thickness, and density. Tissues with a higher atomic number and greater thickness absorb more radiation, resulting in a whiter appearance on the radiograph. This creates a density gradient from white (radiopaque tissues) to black (radiolucent), with greys in between.

Radiopacity – Radiographic Density

Radiopacity refers to the degree to which different tissues and objects absorb X-rays, determining how they appear on the image. Differences in radiopacity allow structures and tissues to be distinguished.

• Radiopaque (white on X-ray)

  • Definition: Structures with high radiopacity absorb most X-rays and appear white.

  • Examples:

    • Bone: High atomic number and thickness make bone appear very white.

    • Metal (e.g., implants): Extremely high atomic number yields near-total absorption.

  Radiolucent (black on X-ray)

  • Definition: Structures with low radiopacity transmit more radiation and appear black.

  • Examples:

    • Air: Low density and specific gravity (~0.001) make air appear completely black.

    • Lungs: Appear dark due to air content.

  Intermediate Grey Tones

  Soft tissue and fluid:

  • Have roughly similar atomic numbers but different densities compared with air and bone, producing grey appearances on radiographs.

  • Examples: Muscles, fat, and organs.

  
  

  Factors Affecting X-ray Images

  Atomic Number

  • Higher atomic numbers (e.g., calcium in bone) → more absorption → lighter appearance.

  • Lower atomic numbers (e.g., carbon, hydrogen in fat and muscle) → less absorption → darker appearance.

  Density

  • Air (low density) → black regions.

  • Soft tissue and fluid → greys that vary with specific gravity and density.

  Thickness

  • Thicker structures absorb more X-rays and appear lighter.

  • Example: A thick layer of muscle appears lighter than a thinner layer.

  
  

  Interpretation of X-ray Images

  X-rays are analysed based on the opacities seen:

  • White: High absorption (bone, metal).

  • Greys: Moderate absorption (soft tissue, fluid).

  • Black: Minimal absorption (air).

  This gradient enables clinicians to identify pathological changes such as fractures, fluid accumulation, or soft-tissue abnormalities. Understanding radiopacity is therefore essential for accurate interpretation and diagnosis.

Five Basic Radiographic Opacities

When interpreting radiographs, five basic opacities represent different tissue/material types. These opacities provide valuable information about structural properties and potential pathology. Below are the five main opacities and their features:

1. Mineral (Bone)

Composition: Primarily calcium and phosphorus, giving bone high radiopacity.

Opacity variations:

Cortical vs. cancellous bone: Cortical bone is more radiopaque than cancellous due to higher density.

Trabecular bone vs. intertrabecular spaces: Trabecular bone appears lighter than adjacent air- or fluid-filled spaces.

Cortex vs. medullary canal: The cortical shell is denser than the marrow-containing medullary canal.

Pathological variations:

Sclerotic bone: Increased radiopacity, often in chronic conditions (e.g., osteosclerosis).

Porous bone: Reduced radiopacity, as in osteoporosis or pathological fractures.

2. Soft Tissue/Fluid

Radiopacity: Soft tissue and fluid share similar radiopacity, making them difficult to distinguish from each other on plain films.

Examples:

Normal opacity of organs such as the heart, liver, spleen, and urinary bladder.

Fluid collections such as blood or inflammatory exudate.

Opacity variations:

Depend on volume, thickness, and compactness, producing gradations of grey useful for identifying structures and pathology.

3. Fat

Radiopacity: More radiolucent than bone and soft tissue, but more radiopaque than gas.

Role:

Provides contrast between organs and structures; periorgan fat can delineate borders.

Clinical considerations:

Lack of fat in thin or young individuals reduces contrast on radiographs.

Abnormal fat accumulation can occur in conditions such as lipomas.

4. Gas

Radiopacity: Most radiolucent; appears black.

Examples:

• Pulmonary air provides strong contrast to the heart and vasculature.

• Free intraperitoneal gas may indicate organ perforation.

Role in diagnosis:

Gas offers natural contrast that helps visualise otherwise indistinct structures.

5. Metal

Radiopacity: Most radiopaque; appears completely white.

Examples:

• Contrast agents such as barium or water-soluble iodine.

• Orthopaedic implants (e.g., screws, plates).

• Metallic foreign bodies.

Uses:

• Clear visualisation of fractures, joint prostheses, and catheter placement.

Although there are only five basic opacities, variations within each category provide crucial information. These differences underpin radiographic interpretation and help clinicians identify pathology, assess injury extent, and monitor healing.

Potential Sources of Error in X-ray Image Acquisition and Processing

Errors in producing or processing radiographs can lead to distortion, misinterpretation, or inaccuracy. This can significantly affect diagnosis and treatment planning. Below is an overview of potential errors and their sources:

Acquisition-Related Error Sources

• Heel effect:

  • Description: Arises because X-ray photons are not emitted uniformly; the cathode end emits more photons than the anode end.

  • Consequence: Overexposure at the cathode end and underexposure at the anode end.

  • Mitigation: Position the thickest part of the anatomy near the cathode and the thinnest near the anode.

• Artifact:

  • Description: Visual errors or disturbances, often seen as abnormal findings or foreign objects.

  • Cause: Fingerprints or particles on cassettes housing imaging plates.

  • Mitigation: Keep cassettes and plates clean and free of debris.

• Exposure:

  • Description: Amount of ionising radiation determined by time, X-ray energy, and photon quantity.

  • Consequence: Over-penetration accentuates bony structures; under-penetration emphasises soft tissue.

  • Mitigation: Adjust exposure to highlight the desired structures.

• Motion:

  • Description: Blur due to patient movement during exposure.

  • Mitigation: Instruct the patient to remain still and use stabilising aids if needed.

• Film/processing:

  • Description: Errors during development can affect contrast, detail, or density.

  • Mitigation: Use correct chemistry and development times; follow standard

    procedures.

Radiodensity and Visual Interpretation

Radiodensity refers to tissue density as depicted in shades on the radiograph:

• Air: Black (e.g., lungs, bowel, trachea).

• Fat: Dark grey (e.g., thicker fat layers).

• Soft tissue/Fluid: Neutral or mid-grey (muscles, tendons, organs).

• Bone:

  • Cancellous bone: Light grey.

  • Cortical bone: White.

• Contrast media: White.

• Metal: White (e.g., jewellery, dental fillings, orthopaedic implants).

Four Main Sources of Radiographic Error

• Magnification:

  • Cause: Divergent (conical) X-ray beam makes objects closer to the source appear larger.

  • Mitigation: Position the object correctly relative to the source.

• Elongation:

  • Cause: Increased beam angulation in the periphery makes objects appear stretched.

  • Mitigation: Place the object in the centre of the beam cone.

• Foreshortening:

  • Cause: Occurs when the area of interest is angled relative to the beam, making it appear shortened.

  • Mitigation: Align the object parallel to the beam.

• Superimposition:

  • Cause: Multiple anatomical structures overlap, creating apparent increased density or pseudo-lesions.

  • Mitigation: Use multiple projections (e.g., AP, lateral, oblique) to minimise misinterpretation.

Understanding and minimising acquisition and processing errors is crucial for accurate diagnosis and treatment planning. Awareness of common pitfalls helps technologists and clinicians improve image quality and patient care.

References

1. Tafti D, Maani CV. Radiation X-ray Production 24.9.2019.Available from:https://www.ncbi.nlm.nih.gov/books/NBK537046/

2. Swain J, Bush K. Diagnostic Imaging for Physical Therapists. St. Louis: Saunders Elsevier; 2009

3.  Biederman, R. E., Wilmarth, M. A., & Editor, C. M. D. T. (n.d.). Diagnostic Imaging in Physical Therapy Avoiding the Pitfalls. Diagnostic Imaging.