Medical Imaging Principles and Types

Study Notes

Images are a recognizable pattern carrying information.
Other meanings of "image" include an optical appearance, a mental representation, a form or semblance, and an idea or conception.

Real images: have a physical existence, like those in photographs and radiographs.
- Static: radiographs and photographs.
- Dynamic: appear on a television screen.
Mental images: pictures generated within our minds.

Medical imaging extracts anatomical and physiological information from patients for diagnosis.
Information flows from the patient to the observer through three stages: invisible image formation, conversion to a visible image, and image interpretation.

X-rays pass through the patient's body during radiographic examinations.
Different body tissues attenuate (absorb) X-rays to varying degrees, depending on tissue type, beam intensity, and tissue thickness.
The simplest image receptor is X-ray film, often coupled with intensifying screens to reduce patient dose.
X-ray film is sensitive to light and X-rays.
After exposure to X-rays, an invisible latent image forms on the film.

Primary radiation: The beam of X-ray photons that interacts with the patient's body.
Remnant radiation: The beam that exits the patient after interacting with body tissues.
Scatter radiation: Radiation that interacts with matter and changes direction, making it less useful for image production.
Attenuation: The change (partial absorption) in the intensity of primary radiation as it travels through the patient.

The invisible image is converted into a visible image using various methods:
- Photographic effect: X-rays expose a sensitive film.
- Xerographic effect: Electrically charged imaging plates are exposed to X-rays.
- Fluorescent effect: X-rays are converted into light by sensitive screens.
- Photon stimulated luminescence: An X-ray-exposed imaging plate with a phosphor coating is stimulated by laser photons to produce a visible image.

Different levels of density in an image, ranging from dark to light, show different shades of gray.
X-rays show different body features in various shades of gray.
Areas that do not absorb X-rays well (e.g., soft tissue) appear darker.
Dense areas (e.g., bones) absorb more X-rays and appear lighter.

The visible image (on film or a screen) is viewed under specific conditions to assess its quality and aid in diagnosis.

Reflected light: Light is reflected from a surface (texts, book illustrations).
Transmitted light: Light passes through a semitransparent layer (radiographs).
Emitted light: Light is emitted from a fluorescent layer (fluorescence image on a TV screen).

Noise: Random variations in the image that can obscure details.
- Real images contain a meaningful signal (pattern) and chaotic noise.
- Increased noise reduces image detail.
Contrast: The difference in density (brightness) between adjacent areas in the image.
Sharpness: The degree to which the edges of structures in the image are well-defined.
Resolution: The ability to distinguish two closely spaced objects as separate entities.