Patient-Image Optimization and Image Quality Factors

Discuss patient factors related to body habitus and analyze how these factors influence radiographic exposure technique factors.
Describe the explicit relationships among spatial resolution, contrast resolution, and image detail.
Identify and define the distinct types of radiographic distortion.
Define image artifact and summarize its primary categories.
Describe magnification radiography and its clinical uses.

Diagram showing body habitus classifications: Sthenic, Hyposthenic, Hypersthenic, and Asthenic

Radiographic technique charts embedded in operative consoles are standardized based on sthenic patients.
Sthenic:
- Meaning: "strong, active"
- Describes standard, average-build patients.
Hyposthenic:
- Thin, but healthy-appearing patients.
- Generally require less exposure technique.
Hypersthenic:
- Patients who are big in frame and usually overweight.
- Require higher exposure technique to penetrate larger tissue volume.
Asthenic:
- Patients who are small, frail, emaciated, and often elderly.
- Require lower exposure technique factors.

Patient thickness should never be guessed; accurate physical measurement is essential for selecting proper technical parameters.
Calipers:
- Definition: An instrument with two bent or curved legs used for measuring the thickness of a solid object or body part.
Relationship to penetration:
- The thicker the patient or body part, the more x-radiation is required to penetrate the tissue and properly expose the IR.

Rule of Thumb: For every $4\,\text{cm}$ of additional body part thickness, the attenuation of the x-ray beam is doubled.
Because attenuation doubles with every $4\,\text{cm}$ increase, the penetration of x-rays reaching the IR is cut in half ( $\frac{1}{2}$ ).
Practical Example: A body part that is $4\,\text{cm}$ thicker than another area absorbs about twice as much x-radiation, resulting in an IR exposure under that region that is approximately one-half ( $\frac{1}{2}$ ) as much.
Alterations in body part thickness cause subject contrast to either decrease or increase, depending specifically on which anatomical part has changed.

Radiologic technologists must estimate the mass density of the target anatomical part and evaluate the full range of mass densities involved.
Subject Contrast Variation across Anatomy:
- Chest: High subject contrast due to substantial density differences between air-filled lung tissue, heart muscle, and bony structures.
- Abdomen: Low subject contrast due to uniform tissue densities.
Technical Factor Adjustments Based on Composition:
- Soft Tissue Imaging: When imaging soft tissue alone, low $\text{kVp}$ and high $\text{mAs}$ are utilized.
- Extremities: Consist of both soft tissue and bone; low $\text{kVp}$ is utilized because the anatomical part is physically thin.

Pathological type, physical size, and chemical composition directly influence radiographic technique selection.
Pathological processes present radiographically with either increased radiolucency or increased radiopacity.
Destructive Pathology:
- Pathology that is destructive decreases anatomical thickness, mass density, or average atomic number.
- Decreases attenuation, requiring a reduction in technique.
Constructive (Additive) Pathology:
- Pathology that is constructive (additive) causes tissue to become more radiopaque by increasing thickness, mass density, or atomic number.
- Increases attenuation, requiring an increase in technique.

Chest radiograph demonstrating radiolucent and radiopaque anatomical regions

Radiolucent:
- Definition: Attenuates few x-rays and appears black or dark on a radiograph.
- Penetrating x-rays pass through the patient to produce the "black" (radiolucent) and darker areas on the radiograph.
Radiopaque:
- Definition: Absorbs x-rays and appears white or light on radiographs.
- Photoelectric interactions (complete absorption) produce the "whites" (radiopaque) or lighter areas on the image.

Chest radiographs showing the impact of scatter radiation and fogging

Compton scatter lays down an unwanted "blanket" of fog across the image receptor, degrading overall image contrast.

Image quality factors refer to distinct characteristics of the radiographic image, including spatial resolution, contrast resolution, distortion, and image artifacts.
These quality factors form the fundamental "language" of radiography and are closely interrelated.

Spatial Resolution:
- Definition: The ability of an imaging system to resolve and render on the image a small high-contrast object.
Contrast Resolution:
- Definition: The ability to distinguish between and to image similar tissues.

Image detail historically described the visual appearance of anatomical structures on the image.
Sharpness of Image Detail:
- Best measured by spatial resolution.
Visibility of Image Detail:
- Best measured by contrast resolution.
- Describes the overall ability to see the detail on the radiograph.

Scapular radiographic distortion view A

Distortion: The misrepresentation of object size and shape on the image.
Method of Reduction:
- Distortion is minimized by positioning the anatomic part of interest in a plane parallel to that of the image receptor (IR) and aligning the central ray perpendicular to both the part and the IR.

Elongation:
- Anatomic part of interest appears bigger (longer) than normal.
- Results directly from poor alignment of either the image receptor or the x-ray tube.
Foreshortening:
- Anatomic part appears smaller (shorter) than normal.
- Results directly from improper angling/alignment of the anatomical body part relative to the IR.

Artifact: Unintended optical density on a radiograph or another film-type image receptor.