Diagnostic Imaging
Diagnostic Imaging
This domain tests radiographic technique, positioning, contrast studies, ultrasound, and radiation safety. Expect questions about which factor controls density or contrast, and about which view is needed.
Core rules
- mAs controls the number of x-rays reaching the detector and therefore density (blackening). kVp controls penetration and contrast.
- Sante's rule estimates kVp: kVp ≈ (2 × tissue thickness in cm) + 40.
- Take at least two orthogonal views; a single view cannot reliably localize a lesion.
- Thoracic films are best taken at peak inspiration; abdominal films are taken on expiration for better organ contrast.
- Radiation safety: ALARA, collimate to the area of interest, wear lead apron and gloves, wear a dosimeter badge, and never hold a patient or cassette in the beam.
- Ultrasound is real-time and non-ionizing; it is excellent for soft tissue and fluid but poor through bone and gas.
Worked example
A lateral thorax of a dog measuring 10 cm thick: kVp = (2 × 10) + 40 = 60 kVp. mAs is then adjusted for the system and the patient's motion, not used to compensate for an incorrect kVp.
Traps
- Using kVp to "fix" a film that is too light when the real problem is mAs.
- Calling a lesion on one view only.
- Standing in the primary beam while restraining a patient.
- Forgetting to collimate, which increases scatter and reduces contrast.