Ch. 17 Carlton

Attenuation

• Attenuation is the reduction in X-ray photons remaining in the beam after passing through a thickness of material.

• Increased part thickness results in increased attenuation.

• Attenuation is a result of:

  • Photoelectric absorption: Provides radiologically significant information

  • Compton scattering: Provides no useful information, contributes to personnel exposure

The Human Body as an Attenuator

• The patient is the radiographer's greatest variable because the composition of the human body determines its radiographic appearance.

• Four major substances account for variable attenuation:

  • Air:

    • Effective atomic number: 7.78, greater than fat or muscle

    • low tissue density

    • Absorbs few photons

    • Results in increased area of exposure on the image receptor.

  • Fat:

    • Soft tissue

    • Effective atomic number and tissue density similar to water & slightly less than muscle.

    • Tissue density less than muscle

  • Muscle:

    • Soft tissue

    • Slightly higher atomic number and tissue density than fat.

  • Bone:

    • Calcium among the highest atomic number of elements found in the body

    • Greatest tissue density.

    • Absorbs many photons

    • Decreased area of exposure on the image receptor.

Patient's Relationship to Image Quality

• Visibility of image is determined by overall blackness (density) and differences between the blackness (contrast).

• Subject Density: Image receptor exposure will be altered by changes in the amount or type of tissue being irradiated.

• Subject Contrast: Degree of differential absorption resulting from differing absorption characteristics of tissues in the body.

• Subject Detail: Recorded detail of structures is dependent on position within the body and the body's placement in relation to the image receptor.

• Subject Distortion: Unless the patient is positioned specifically to demonstrate a particular structure, it may not be accurately represented on the image receptor.

Pathology and Radiation Absorption

• Patients are radiographers greatest variables

• Beam attenuation is dependent on the thickness and composition of patient's tissues.

• Pathology can alter the thickness and composition of patient's tissue density.

• Small, localized pathology does not require a change in technical factors.

Additive Conditions (Increased Attenuation)

• Increase tissue thickness, effective atomic number, and/or tissue density.

• Increase attenuation and are inversely related to image receptor exposure.

• Require an increase in technical factors to properly expose the image receptor.

• Thicker, denser part requires more penetration (higher kvp)

• General compensation: Increase in kVp.

• Examples:

  • Multiple systems: Abscess, edema, tumors

  • Chest: Atelectasis, bronchiectasis, cardiomegaly, congestive heart failure (CHF), empyema, pleural effusions, hemothorax and hydrothorax, pneumoconiosis, pneumonia (pneumonitis), pneumonectomy, pulmonary edema, tuberculosis

  • Abdomen: Aortic aneurysm, ascites, cirrhosis, calcified stones

  • Extremities and skull: Acromegaly, chronic osteomyelitis, hydrocephalus, osteoblastic metastases, osteochondroma, Paget's disease, sclerosis

Destructive Conditions (Decreased Attenuation)

• Decrease tissue thickness, effective atomic number, and/or tissue density.

• Decrease attenuation and are directly related to image receptor exposure.

• Require a decrease in technical factors to properly expose the image receptor.

• General compensation: Decrease mAs.

• Examples:

  • Multiple systems: Anorexia nervosa, atrophy, emaciation

  • Chest: Emphysema, pneumothorax

  • Abdomen: Aerophagia, bowel obstruction

  • Extremities and skull: Active osteomyelitis, aseptic necrosis, carcinoma, degenerative arthritis, fibrosarcoma, gout, hyperparathyroidism, multiple myeloma, osteolytic metastases, osteomalacia, osteoporosis