7.1 X-ray and Computed Tomography (CT)

X-ray and CT principles

X-rays and CT scans are foundational imaging tools in medicine. Both use ionizing radiation to visualize internal anatomy, but they do so in fundamentally different ways: X-rays produce 2D projection images, while CT reconstructs detailed 3D cross-sectional views from many X-ray projections taken at different angles.

Choosing between them involves trade-offs. X-rays are fast, inexpensive, and use lower radiation doses, but they struggle with soft tissue contrast. CT provides far more anatomical detail and eliminates the problem of overlapping structures, but at the cost of higher radiation exposure and expense. Understanding the physics behind both helps you reason about when each is appropriate.

X-ray generation and properties

X-rays are electromagnetic radiation with wavelengths much shorter than visible light, typically in the range of 0.01 to 10 nanometers. They're generated by accelerating electrons to high kinetic energies and then rapidly decelerating them against a metal target (usually tungsten or molybdenum). This sudden deceleration converts kinetic energy into X-ray photons.

When X-rays pass through matter, they interact with it in three main ways, depending on photon energy and the atomic composition of the material:

• Photoelectric absorption: The X-ray photon is completely absorbed by an atom, ejecting an inner-shell electron. This dominates at lower photon energies and in materials with high atomic numbers. It's the primary source of contrast in diagnostic imaging.
• Compton scattering: The photon interacts with a loosely bound outer-shell electron, transferring some of its energy. The photon continues in a new direction with lower energy. This is the dominant interaction at typical diagnostic energies in soft tissue, and it degrades image quality by adding noise.
• Pair production: A high-energy photon interacts with an atomic nucleus and converts into an electron-positron pair. This requires photon energies above 1.022 MeV, so it's not relevant in diagnostic imaging but matters in radiation therapy.

X-ray attenuation and contrast

As X-rays travel through the body, they are progressively absorbed and scattered. This reduction in intensity is called attenuation, and it depends on three properties of the material:

• Thickness: Thicker objects attenuate more X-rays.
• Density: Denser materials (like bone) attenuate more than less dense materials (like soft tissue or air).
• Atomic number: Higher atomic number materials (like lead, $Z = 82$) attenuate far more than lower atomic number materials (like carbon, $Z = 6$).

Attenuation follows the Beer-Lambert law. For a monoenergetic beam passing through a uniform material:

$I = I_0 \, e^{-\mu x}$

where $I_0$ is the initial intensity, $\mu$ is the linear attenuation coefficient, and $x$ is the material thickness.

Image contrast comes from differences in attenuation between adjacent tissues:

• Bone appears white (high attenuation, few photons reach the detector).
• Air appears black (very low attenuation, most photons pass through).
• Soft tissues appear as shades of gray, with relatively small differences between them. This is why plain X-rays are poor at distinguishing, say, liver from kidney.

CT imaging principles

Computed Tomography solves the superimposition problem of plain X-rays. Instead of a single projection, the X-ray tube and detector rotate around the patient, acquiring hundreds of projections from different angles. These projections are then mathematically combined to reconstruct cross-sectional (axial) images.

The two main reconstruction approaches are:

1. Filtered back-projection (FBP): Each projection is filtered with a high-pass (ramp) filter to remove blurring, then "back-projected" across the image space. Where back-projected data from many angles overlap, the true anatomy is reconstructed. FBP is computationally fast and has been the standard method for decades.
2. Iterative reconstruction: Techniques like the algebraic reconstruction technique (ART) and maximum likelihood expectation maximization (MLEM) start with an initial image estimate, simulate what projections that estimate would produce, compare those to the actual measured projections, and iteratively update the estimate to minimize the difference. These methods produce images with less noise at the same radiation dose, but they're more computationally demanding.

By stacking sequential cross-sectional slices, CT provides full 3D visualization of anatomy. Each pixel in a CT image is assigned a value in Hounsfield Units (HU), where water = 0 HU, air = −1000 HU, and dense bone can exceed +1000 HU. This standardized scale allows quantitative comparison of tissue densities.

X-ray and CT systems

X-ray imaging system components

A basic X-ray system has four main components:

• X-ray tube: Contains a cathode that emits electrons (via thermionic emission) and an anode that serves as the target. When electrons strike the anode, X-rays are produced. The anode also dissipates significant heat, which is why rotating anodes are common.
• Collimator: Shapes and restricts the X-ray beam to the region of interest. This reduces patient dose and limits scatter radiation that would degrade image quality.
• Patient table: Supports and positions the patient during the exam.
• Detector: Captures the transmitted X-rays and converts them into an image. Detector technology has evolved significantly:
  • Film: The original method. X-rays expose a film that requires chemical processing. Offers good spatial resolution but no post-processing flexibility.
  • Computed radiography (CR): Uses photostimulable phosphor plates that store the X-ray energy pattern. A laser scanner reads the plate and produces a digital image. CR fits into existing film-based systems.
  • Digital radiography (DR): Flat-panel detectors directly convert X-rays to digital signals using either amorphous silicon (indirect conversion, X-rays → light → electrical signal) or amorphous selenium (direct conversion, X-rays → electrical signal). DR provides immediate image display and better dose efficiency than CR.

CT scanner components

• Gantry: The large ring-shaped structure that houses the X-ray tube and detector array. It rotates continuously around the patient during scanning.
• Patient table: Translates the patient through the gantry opening. In helical (spiral) CT, the table moves continuously during rotation, tracing a helical path through the patient.
• Computer system: Controls scanner operation, performs image reconstruction, and displays images for interpretation. Modern CT reconstruction is computationally intensive, especially with iterative methods.
• Slip ring technology: Provides electrical connections to the rotating gantry without cables, enabling continuous rotation. Before slip rings, the gantry had to stop and reverse direction after each rotation, making scans much slower.

Advancements in CT technology

• Multi-detector row CT (MDCT): Uses multiple rows of detectors (16, 64, 128, or 256 rows are common) to acquire many slices per rotation simultaneously. This dramatically improves scan speed and allows isotropic spatial resolution, meaning the voxel dimensions are equal in all three directions. A 256-slice scanner can image the entire heart in a single rotation.
• Dual-energy CT (DECT): Acquires data at two different X-ray tube voltages (e.g., 80 kVp and 140 kVp). Because different materials attenuate low- and high-energy photons differently, DECT can distinguish materials that look identical on conventional CT. For example, it can differentiate iodine contrast agent from calcium in a coronary artery, or characterize kidney stones by composition.
• Photon-counting detectors: Unlike conventional energy-integrating detectors that sum all photon energies together, photon-counting detectors register each individual photon and measure its energy. This provides improved spatial resolution, better contrast resolution, reduced electronic noise, and the ability to do multi-energy imaging from a single scan. These detectors are beginning to enter clinical use.

X-ray and CT applications

Advantages and limitations of X-ray imaging

Advantages:

• High spatial resolution for visualizing fine bony detail (fractures, joint spaces, lung nodules)
• Relatively inexpensive and widely available in nearly all clinical settings
• Low radiation dose per exam (a chest X-ray delivers roughly 0.02 mSv, comparable to a few hours of natural background radiation)
• Fast acquisition, often under a second

Limitations:

• Poor soft tissue contrast, making it difficult to distinguish organs or detect subtle lesions
• 2D projection means structures overlap. A lesion behind the heart, for instance, can be hidden on a frontal chest X-ray
• Uses ionizing radiation, which carries a small but nonzero risk, particularly concerning for pediatric and pregnant patients

Advantages and limitations of CT imaging

Advantages:

• Excellent spatial and contrast resolution. CT can detect differences of just a few Hounsfield Units between tissues
• Cross-sectional images eliminate superimposition, and 3D reconstructions aid surgical planning
• Versatile: effective for nearly every body region and a wide range of pathologies (tumors, infections, vascular disease, trauma)

Limitations:

• Higher radiation dose than plain X-ray. A typical abdominal CT delivers roughly 8–10 mSv
• More expensive and less widely available than X-ray, especially in resource-limited settings
• Susceptible to artifacts from patient motion, metallic implants, and beam hardening

Clinical applications of X-ray imaging

• Diagnosis of fractures, dislocations, and bone lesions
• Detection of pneumonia, pneumothorax, and lung nodules on chest X-ray
• Evaluation of dental pathologies (cavities, periodontal disease)
• Interventional procedures using fluoroscopy (real-time X-ray), such as angiography and biopsy guidance

Clinical applications of CT imaging

• Diagnosis and staging of cancers (lung, liver, pancreatic, and many others)
• Evaluation of traumatic injuries to the head, spine, and abdomen
• Assessment of cardiovascular diseases, including coronary artery disease and aortic dissection
• Image-guided interventions such as biopsy, tumor ablation, and abscess drainage
• Virtual colonoscopy (CT colonography) for colorectal cancer screening
• Dental implant planning and maxillofacial surgical planning

Image quality in X-ray and CT

Factors affecting image quality

Three core metrics define image quality:

Spatial resolution is the ability to distinguish small, closely spaced objects. It's influenced by:

• Focal spot size (smaller focal spot → sharper image)
• Detector element size (smaller elements → finer detail)
• Geometric magnification (greater magnification can improve apparent resolution but also increases dose and blur from a finite focal spot)

Contrast resolution is the ability to distinguish between objects with similar attenuation. It depends on:

• X-ray spectrum, controlled by tube voltage (kVp) and beam filtration
• Object thickness and composition
• Detector efficiency

CT has much better contrast resolution than plain X-ray because it eliminates scatter and superimposition.

Noise refers to random variations in image intensity that can obscure fine details. Sources include:

• Quantum mottle: Statistical fluctuations in the number of X-ray photons detected. This is the dominant noise source and decreases with higher dose (more photons = less relative noise).
• Electronic noise: From the detector and readout electronics.
• Scattered radiation: Photons that change direction in the patient and reach the detector at incorrect positions.

There's a fundamental trade-off: reducing noise generally requires increasing radiation dose. Iterative reconstruction algorithms help by reducing noise without additional dose.

X-ray imaging artifacts

• Geometric distortion: If the anatomy isn't parallel to the detector, structures appear magnified or foreshortened. Proper patient positioning and alignment minimize this.
• Motion artifacts: Patient movement during exposure causes blurring or ghosting. Short exposure times, immobilization devices, and breath-hold instructions help reduce motion blur.
• Scatter artifacts: Scattered photons that reach the detector reduce contrast and add a uniform haze to the image. Anti-scatter grids (placed between the patient and detector), increased air gaps, and post-processing correction algorithms all help mitigate scatter.

CT imaging artifacts

• Beam hardening: As a polychromatic X-ray beam passes through tissue, lower-energy photons are preferentially absorbed, shifting the beam's average energy higher ("harder"). This causes cupping artifacts (the center of a uniform object appears darker than its edges) and dark streaks between dense structures like bones. Beam hardening correction algorithms and dual-energy CT techniques can reduce these artifacts.
• Partial volume effects: When a single voxel contains a mix of different tissues, their attenuation values are averaged together. This blurs boundaries and can obscure small structures. Using thinner slices reduces partial volume averaging.
• Metal artifacts: High-density metallic objects (joint replacements, surgical clips, dental fillings) cause severe beam hardening and photon starvation, producing bright and dark streaks radiating from the metal. Metal artifact reduction (MAR) algorithms, dual-energy CT, and iterative reconstruction can all reduce these artifacts, though they rarely eliminate them completely.

Image quality assessment

Quantitative metrics:

• Signal-to-noise ratio (SNR): The ratio of average signal intensity to the standard deviation of noise. Higher SNR means a cleaner image. $SNR = \frac{\bar{S}}{\sigma_{noise}}$
• Contrast-to-noise ratio (CNR): Measures how well you can distinguish two regions. $CNR = \frac{|S_A - S_B|}{\sigma_{noise}}$ where $S_A$ and $S_B$ are the mean signal intensities of two regions. A higher CNR means better ability to detect a lesion against its background.
• Modulation transfer function (MTF): Describes how well the imaging system preserves contrast at different spatial frequencies. An MTF of 1.0 at a given frequency means perfect contrast transfer; as MTF drops toward 0, fine details at that frequency are lost.

Qualitative evaluation: Radiologists also assess images subjectively, evaluating diagnostic accuracy, artifact severity, and overall image quality. The goal is always to ensure images are diagnostically sufficient while keeping radiation exposure as low as reasonably achievable (the ALARA principle).