UNIVERSITY OF THESSALY MEDICAL DEGREE ENGLISH PROGRAM MEDICAL PHYSICS

Introduction to Radiology – Medical Imaging: Mammography and Fluoroscopy

Learning Objectives

This section outlines the core learning objectives associated with the study of X-rays, mammography, and fluoroscopy.

Understand X-rays:

• Production and Interaction: Explain how X-rays are produced and how they interact with tissues.
• Applications in Medicine: Identify key applications of X-rays in various medical contexts.

Learn the Basics of Radiography:

• 2D Image Formation: Understand how two-dimensional images are formed using X-rays.
• Image Quality Components: Recognize the role of components like anti-scatter grids and collimators in improving image quality.

Grasp the Principles of Mammography:

• Early Detection of Breast Cancer: Explain how mammography aids in the early detection of breast cancer.
• Importance of Compression and Low-Energy X-rays: Understand the significance of using compression and low-energy X-rays for producing high-quality imaging.

Comprehend Fluoroscopy:

• Real-Time Imaging: Describe how fluoroscopy provides real-time imaging for dynamic processes within the body.
• Dose Reduction Techniques: Understand the techniques for reducing radiation exposure, such as pulsed fluoroscopy.

Mammography

What is Mammography?

• Definition: Mammography is a specialized radiographic imaging technique designed to visualize the internal structure of the breast using X-rays. Its primary objective is the early detection and diagnosis of breast diseases, especially breast cancer, by identifying abnormalities such as masses, architectural distortions, and microcalcifications.

Significance:

• Incidence of Breast Cancer: Approximately 1 in 8 women will develop breast cancer in their lifetime. Early detection through mammography significantly improves treatment outcomes and survival rates.

Types of Mammography:

1. Screening Mammography:
   • Targets asymptomatic women to detect early-stage cancers.
2. Diagnostic Mammography:
   • Focuses on symptomatic women or further evaluates suspicious findings.

Historical Evolution

• Early Techniques: Initial mammography employed direct exposure film, requiring high radiation doses and yielding poor diagnostic quality.
• Xeroradiography (1970s-80s):
  • Featured high spatial resolution but poor sensitivity for detecting breast masses. Eventually replaced by screen-film technology.
• Digital Mammography (2000s):
  • Improved image contrast, reduced radiation doses, and enhanced diagnostic accuracy.
• Digital Tomosynthesis:
  • Provides three-dimensional imaging, reducing tissue superposition and improving lesion detection.

X-ray Absorption, Attenuation, and the Linear Attenuation Coefficient (μ)

Understanding Attenuation:

• When an X-ray beam passes through tissue, its intensity diminishes due to absorption and scattering, collectively termed attenuation.
• **Key Interactions:
  • Absorption:** Photons are completely removed from the beam, depositing their energy in the tissue.
  • Scattering: Photons are deflected from their original pathway but may retain some energy.

Photoelectric Effect:

• Occurs primarily at lower photon energies within materials possessing higher atomic numbers (Z). In this interaction:
  • An X-ray photon interacts with an inner-shell electron, transferring all its energy to that electron, resulting in complete absorption.
• Contrast in Mammography: The photoelectric effect catalyzes the high contrast observed between tissues with slightly different compositions, particularly when using low-energy X-rays to accentuate these differences.
• Dependence on Atomic Number and Energy: The probability of photoelectric absorption is given by the expression $rac{Z^3}{E^3}$ indicating its strong dependence on the atomic number of the tissue and the energy of the incident photons.

Exponential Decay Law:

• Attenuation describes the total loss of primary photons due to absorption and scattering as the beam penetrates the matter.
• The transmitted intensity $I$ after passing through a material of thickness $x$ follows: I = I_0 e^{-C imes x}
  • where $I_0$ is the initial intensity and C is the linear attenuation coefficient ( ext{cm}^{-1}).
  • This coefficient quantifies how effectively a material attenuates X-rays, varying according to photon energy and the material’s composition (atomic number and density).

Characteristic Attenuation Coefficients:

• Different breast tissues have distinct linear attenuation coefficients:
  • Adipose Tissue: Composed primarily of low-Z elements, exhibits a small C value; attenuates X-rays weakly, appearing darker on mammograms.
  • Glandular Tissue: Denser, with higher-Z elements like oxygen and nitrogen; displays a higher C value, absorbing more photons, and appears less dark than adipose.
  • Cancerous/Fibrotic Tissues: High attenuation coefficients due to increased cellular density and calcifications, making them brighter against surrounding fatty tissues.

Principles of X-ray Interaction in Mammography

Differential Attenuation:

• Breast tissues (fat, glandular, cancerous) exhibit subtle differences in X-ray absorption, essential for diagnostic imaging.
• Low-energy X-rays (20-35 keV):
  • These energies maximize contrast between glandular and cancerous tissues, with careful selection required to balance dose and image quality.
• Trade-offs Involved:
  • Low-energy X-rays raise patient radiation dose; hence, careful optimization is necessary for effective imaging.

Image Formation by Differential Attenuation:

• The mammographic image relies on differential attenuation of X-rays traversing the diverse breast tissues, with each tissue exhibiting slight variances in attenuation coefficients.
• Enhanced Soft-Tissue Contrast:
  • Low-energy X-rays are essential for accentuating this contrast, particularly when set in the range of 20-35 keV where the photoelectric effect dominantly influences the results.
• Selection around this energy range ensures the visualization of microcalcifications and small tumors, significant for early cancer detection.

Optimization Factors in X-ray Use:

• Utilizing low-energy X-rays necessitates a balance to optimize imaging quality while minimizing radiation exposure. The following elements are considered:
  • Tube High Voltage (kVp): Need careful selection to reach adequate penetration through compressed breast tissue while maximizing contrast.
  • Anode/Filter Combinations: These are fundamental to producing an energetically suitable beam spectrum.
  • Automatic Exposure Control (AEC): Ensures consistent image quality and safety concerning radiation exposure.

Key Components of a Mammography Unit

1. X-ray Tube:
   • Designed with a molybdenum or rhodium target/anode to produce low-energy X-rays.
2. Compression Paddle:
   • Reduces tissue thickness for enhanced image quality.
3. Anti-Scatter Grids:
   • Improve contrast by minimizing scatter radiation.
4. Dedicated X-ray Detectors:
   • Incorporate either screen-film or digital technologies for image capturing.

X-ray Tube Design in Mammography

• Differentiates from traditional radiography in operational parameters:
  • Tube Voltage: Typically operates between 20 and 40 kV.
  • Materials Used:
  • The anode can be molybdenum (Z=42) or rhodium (Z=45), providing characteristic X-ray spectra suited for varying breast densities.
  • Target Angling: Aimed to ensure consistent intensity distribution across the image receptor mechanism.
  • Filtering Processes: Further refine X-ray beams by eliminating low and high-energy photons to produce a balanced energy spectrum.

Compression in Mammography

Compression Paddle:

• Utilized to apply gentle pressure to the breast, enhanced by the following functions:
  • Reduced Thickness: Leads to lowered radiation dose and enhanced image contrast.
  • Minimization of Motion Blur: Immobilizes the breast to stabilize imaging during exposure.
  • Spread of Overlapping Structures: Allows better visibility of internal pathologies.
• Despite some discomfort, compression is critical for maintaining diagnostic accuracy across varied imaging positions.

Scattered Radiation and Anti-Scatter Grids

Anti-Scatter Grid:

• A crucial feature in mammography systems aimed at enhancing image quality by absorbing scattered photons.
• Structure:
  • Comprises lead strips that absorb scatter radiation and radiolucent materials permitting primary radiation to pass.
• Trade-offs: While grid use improves contrast, it also slightly increases radiation dose, necessitating careful optimization based on breast thickness and imaging technique.

X-ray Detectors for Mammography

• The image receptor performs crucial roles in capturing transmitted X-rays and converting them into either visible or digital forms.
• Evolution of Detection Systems:
  • Earlier systems utilized screen-film configurations, while modern systems utilize digital detectors, such as cesium iodide with amorphous silicon photodiodes.
• Advantages of Digital Detectors Include:
  • Wider dynamic range, improved performance under lower dose conditions, and enhanced post-processing capabilities.

Automatic Exposure Control Detector (AEC)

• Operates beneath the X-ray detector to monitor radiation amounts reaching the detector in real time, terminating the exposure upon achieving the pre-set signal intensity.
• This automation guarantees consistent quality in imaging and adaptive responses to varying breast sizes anding compositions.

Breast Density and Its Impact

• Definition: Breast density refers to the ratio of fibroglandular tissue to fatty tissue in the breast.
• Clinical Importance: Dense breasts can obscure lesions on mammography and represent heightened breast cancer risk.
• Imaging Challenges: Reduced contrast can necessitate supplementary imaging techniques like tomosynthesis and ultrasound for clarity.

Digital Mammography

• Represents a significant advancement over traditional screen-film technologies:
  • Image Quality: Enhancement leads to noticeable enhancement in quality and diagnostic capabilities.
  • Workflow Efficiency and Flexibility: Digital systems afford radiologists adjustments for contrast and brightness, facilitating optimization of visual information.
  • Post-Acquisition Flexibility: Utilizes computer-aided detection (CAD) algorithms for improved identification of subtle lesions and microcalcifications that may escape detection by traditional methods.

Advantages of Mammography

• Early Detection and Mortality Reduction: Proven to lower breast-cancer mortality rates in screening programs by approximately 30-40%.
• Visual Accuracy: High spatial resolution facilitates clear visualization, including microcalcifications.
• Accessibility: Widely available, rapid examination process; cost-effective for large-scale population screening.
  • Recommendations for screening typically involve annual or biennial sessions for ages 40-74, with adjustments for individual risk factors.
• Quality Control and Standardization: Reliable QC programs are established to ensure consistency in results.

Limitations of Mammography

1. Sensitivity Issues with Dense Breasts: Higher fibroglandular densities can mask cancers, leading to false negatives, while tissue overlaps risk false positives.
2. Overdiagnosis Dilemmas: Risks of detecting non-threatening lesions with potential psychological impacts and economic costs.
3. Functional Data Limitations: Mammography primarily provides anatomical data; often necessitates ultrasound or MRI utilization for further evaluation in dense or high-risk populations.
4. Patient Experience: The discomfort associated with compression may hinder patient compliance with screening recommendations.
5. Radiation Exposure: Although exposure is minimal, it is not negligible, and adherence to quality control is vital.

Fluoroscopy

What is Fluoroscopy?

• Definition: Fluoroscopy is an advanced projection imaging technique that delivers real-time visualization of internal structures within the body.
• Key Feature: High temporal resolution exists to observe dynamic processes such as blood flow and organ movement.
• Applications: Utilized widely in interventional procedures (angioplasty, catheter placements) and diagnostic imaging (GI studies, cardiac imaging).

Basic Principles of Fluoroscopy

• Operates as an X-ray-based technique for real-time visualization:
  • Continuous generation of images unlike conventional radiography, allowing for the observation of dynamics like swallowing and blood flow.
• Fundamental principle relies on X-ray attenuation as they traverse tissues varying in density:
  • Photon Interactions: As X-rays pass through a patient, some photons are absorbed or scattered while others reach the detector, with detection dependent on the tissue density, thickness, and atomic number.
  • Dense tissues appear bright; softer tissues allow greater photon transmission, appearing darker.

Components of Fluoroscopic Devices

Main Components:

1. X-ray Tube: Generates a continuous or pulsed X-ray beam, crucial for enabling the imaging process.
2. Beam Filtration and Collimation Systems: Improve image quality while minimizing patient exposure.
3. Image Receptor (Flat-Panel Detector): Converts transmitted X-rays into visible or electronic images.
4. Camera and Display System: Completes the imaging setup by visually presenting the outputs.

Key Components of a C-arm

X-ray Tube:

• Functions similarly to conventional radiography systems but operates with lower tube currents (mA) to facilitate longer exposure durations, essential for continuous imaging.
• Typical Voltages: Range from 70-110 kVp, customized for specific patient size and examination type.
• Design: Often incorporates rotating anodes and advanced cooling systems for efficient operation during prolonged procedures.

Beam Filtration and Collimation Systems:

• Shape and filter the X-ray beam before patient exposure to optimize image quality and reduce unnecessary radiation.
• Beam Filtration: Removes low-energy photons that could unnecessarily increase patient exposure without contributing to image quality.
• Collimation: Utilizes lead shutters to restrict the beam to the clinical area of focus, enhancing image contrast.
  • Modern systems often feature motorized collimators adjustable to match desired magnification or region of interest.

Image Receptor:

• At the core of the fluoroscopy setup, the image receptor transforms transmitted X-rays into visible images.
• Traditional: Image intensifiers convert X-rays to visible light (optical images that can be captured).
• Modern Systems: Often deploy flat-panel detectors that offer superior performance, are more streamlined, and facilitate quicker image acquisition.

Image Intensifier Function:

1. Input Phosphor: X-rays are converted to light photons.
2. Photocathode Process: Light photons are transformed into electrons.
3. Acceleration by Focusing Electrodes: Accelerated and focused electrons produce an intensified optical image.
4. Output Conversion: The bright image can be captured and displayed in real-time.

Fluoroscopy Modes of Operation

Common Modes:

1. Continuous Fluoroscopy:
   • Maintains continuous X-ray exposure; ideal for dynamic studies but results in higher radiation exposure.
2. Pulsed Fluoroscopy:
   • Consists of pulsed X-ray emissions synchronized with frame acquisition; significantly reduces radiation dose while preserving image quality.

Advantages of Fluoroscopy

1. Real-Time Imaging: Offers dynamic visualization of processes (e.g., blood flow, organ movement) for guiding interventional activities.
2. Versatility in Applications: Widely employed in diverse clinical disciplines like cardiology, orthopedics, gastroenterology, and urology.
3. Compatibility with Other Modalities: Often combined with modalities like CT (CT fluoroscopy) to enhance procedural accuracy.
4. Immediate Results: Supports rapid decision-making based on real-time observations during procedures.

Limitations of Fluoroscopy

1. Radiation Exposure Concerns: Extended usage results in increased radiation doses for both patients and healthcare workers.
2. Limited Contrast Resolution: Inferior to modalities like CT or MRI, hindering the distinction of subtle soft tissue disparities.
3. Dependence on Operator Skill: Requires trained personnel to ensure optimal patient positioning and effective interpretation.
4. Motion and Scatter Artifacts: Patient or equipment movement can blur images, while scatter radiation degrades image quality.