Spatial Resolution & Distortion Review

Spatial Resolution & Distortion Review

Chapter 28 - Spatial Resolution

Objectives (1 of 2)
  • Definition of Spatial Resolution:
    • Refers to the ability to distinguish small details in an image.
    • Synonymous terms used: sharpness, recorded detail.
    • Derived Units:
    • Typically expressed in line pairs per millimeter (lp/mm).
Objectives (2 of 2)
  • Impact of Distance on Spatial Resolution:
    • Explanation of the relationship between different distances:
    • Source-to-image distance (SID)
    • Object-to-image distance (OID)
  • Preventing Patient Motion:
    • Techniques include patient instruction, supporting devices.
  • Geometrical Factors:
    • Integrating these factors into clinical protocols for enhanced resolution.
    • Recommendations for immobilization techniques to reduce motion artifacts.

Assessing Spatial Resolution

Geometric Properties
  • Two critical properties:
    • Spatial Resolution
    • Distortion
  • Definition of Distortion:
    • Degree of geometric unsharpness in an image.
Related Terms for Distortion
  • Refers to:
    • Definition
    • Sharpness of the image
    • Recorded detail

Assessing Recorded Detail (1 of 4)

  • Digital Imaging Systems:
    • Spatial resolution depends on:
    • Matrix size
    • Pixel size
    • Grayscale bit depth
    • Commonly referred to as Spatial Frequency:
    • High Spatial Frequency: Short wavelength signal.
    • Low Spatial Frequency: Long wavelength signal.

Assessing Recorded Detail (2 of 4)

  • Unit of Resolution:
    • Measured in lp/mm.
  • Clinical Evaluation Tools:
    • Assessment conducted through:
    • Examination of trabecular patterns in bone,
    • Assessment of bone cortex,
    • Evaluation of bronchopulmonary markings within lungs.

Assessing Recorded Detail (3 of 4)

  • Resolution Test Tools:
    • Used for measuring recorded detail on imaging systems:
    • Results displayed as lp/mm.
    • High spatial resolution correlates with high frequencies.
  • Unsharpness:
    • Also known as penumbra, indicating image blur due to various reasons.

Spatial Resolution

  • Defined as the ability of an imaging system to accurately display objects in two dimensions.
  • Penumbra vs. Umbra:
    • Umbra is the fully shaded area, while penumbra refers to the edges of that shadow where it slowly fades to light.

Noise in Imaging (1 of 2)

  • Definition of Noise:
    • Background information collected by the image receptor.
    • Types of noise include:
    • System noise from electronic components.
    • Ambient noise from the environment.
    • Quantum Noise: Refers to random variations in the number of x-ray photons detected, also termed as quantum mottle.

Noise in Imaging (2 of 2)

  • Quantification of Noise:
    • Expressed in terms of Signal-to-Noise Ratio (SNR):
    • Higher values of SNR are desirable for optimal image quality.
  • Contrast-to-Noise Ratio (CNR):
    • Defined as the ratio of region of interest (ROI) signal differences to noise.

Factors Affecting Recorded Detail

  • Strategies to minimize motion and increase detail:
    • Use the shortest exposure time possible.
    • Synchronize exposure with patient motion to avoid blur.
    • Reduce Object-to-Image Distance (OID).
    • Utilize a smaller focal spot size.
    • Opt for the detector with the smallest Detector Element (DEL).
    • Reduce intensifying screen phosphor size and concentration, particularly in film systems.
    • Increase Source-to-Image Distance (SID).

Focal Spot Size

  • Line Focus Principle:
    • Affects the geometry of penumbra and causes inherent softening of sharpness due to focal spot-size and distances involved.
  • Significance of Focal Spot Size:
    • Smaller focal spots yield improved spatial resolution compared to larger focal spots.

Attenuation/Absorption Unsharpness

  • Unsharpness arises due to:
    • The shape and size of patient anatomy causing divergence of the x-ray beam.
    • It reflects the incongruence of the imaging beam with anatomical structures.

Motion Considerations

  • Voluntary Motion:
    • Controlled through efficient communication with the patient.
  • Involuntary Motion:
    • Managed by reducing exposure time and using immobilization tools.
  • Consideration of equipment motion that can affect image quality.
Voluntary Motion Reduction Strategies
  • Effective communication creates a professional and competent atmosphere conducive to patient cooperation.
Involuntary Motion Reduction Strategies
  • Best managed by short exposure times or devices that immobilize the patient.

Chapter 29 - Distortion

Objectives (1 of 3)
  • Define Types of Distortion:
    • Distinction between size and shape distortion.
  • Effects of SID and OID on Distortion:
    • Discuss the significance of variations in these distances on imaging outcomes.
  • Methods for Minimizing Distortion:
    • Use of variations in SID and OID to reduce distortion in images.
Objectives (2 of 3)
  • Anatomical Relationships:
    • Routine alignment of central ray, anatomical part, and image receptor.
  • Angulation Terms:
    • Properly describing the direction and degree of angulation in imaging.
  • Differentiating Distorted Images:
    • Contrast with standard projections.
  • Calculating Magnification Factor:
    • Using parameters SID and SOD (Source-to-Object Distance).
Objectives (3 of 3)
  • Actual Size Calculations:
    • Ability to calculate an object's actual size using projected size, SID, and OID data.
  • Minimizing Magnification:
    • Adjusting SID and OID as necessary.
  • Shape Distortion Reduction:
    • Modifications to the central ray, anatomical part, and image receptor can minimize shape distortion.

Assessing Distortion

  • Key geometric property influencing image quality.
  • Definitions and types of distortion:
    • Size Distortion: Refers to changes in image size relative to object size.
    • Shape Distortion: Refers to changes in the shape of the object as represented in the image.
  • Distortion is always present but can be minimized.

Factors Affecting Size Distortion

  • Key distances influencing size distortion: SID and OID.
  • Magnification is primarily controlled through SID and OID, with digital systems allowing for post-processing resizing as well.
  • Electronic Magnification/Minification: Can alter image size after capture.

Source-to-Image Distance (SID) and Object-to-Image Distance (OID)

  • SID as a Factor:
    • Longer SID results in less magnification, therefore preferred.
    • Extended SID reduces entrance skin exposure (ESE).
  • OID should be minimized to enhance image resolution and limit magnification effects.

Calculating Size Distortion

  • Magnification Factor:
    • Formula: Magnification = image size / object size.
  • Object Size Calculations:
    • Formula: I = M * O (where I represents image size, M is the magnification factor, and O is the object size).
    • In medical radiography, image size is conventionally larger than object size.

Factors Affecting Shape Distortion

  • Role of Central Ray:
    • The alignment of the central ray with the anatomical part and image receptor is crucial to avoid distortion.
    • Angulation Degree and Direction: Should maintain a perpendicular relationship between the focal spot and image receptor to minimize distortion.
  • Shape Distortion Defined:
    • Describes the unequal magnification across dimensions of an object, leading to foreshortening and elongation effects.

Effect on Image Appearance

  • Radiographic Positioning:
    • Deliberately utilizes distortion to separate anatomy in view or to achieve the correct orientation of structures.
    • Size and shape adjustments improve spatial resolution by angling the anatomical part instead of angling the central ray.

Evaluating Shape Distortion

  • Evaluation Process:
    • The effect of shape distortion isn't quantifiable and instead relies on subjective analysis.
    • Requires comprehensive knowledge of typical anatomy and standard imaging projections to assess distortions effectively.