Radiologic Physics
Core concepts
Understand X-ray production: the tube, kVp (penetration and contrast), mAs (quantity), and the heel effect. Know the interactions of X-rays with matter: photoelectric absorption (increases with atomic number and lower energy), Compton scatter (decreases contrast), and pair production. Master image quality: spatial resolution, contrast resolution, noise, and the modulation transfer function. Understand CT physics: Hounsfield units, windowing, reconstruction algorithms, and the trade-off between dose and noise. Know MRI physics: T1 and T2 relaxation, sequences, and artifacts. Recognize the principles of ultrasound and nuclear medicine physics.
Traps
Do not confuse kVp with mAs: kVp affects contrast and penetration; mAs affects quantity and noise. Photoelectric effect dominates at low energy and high atomic number (bone, contrast). Compton scatter degrades contrast. Windowing changes display, not acquired data.
Rule of thumb
Increasing kVp lowers contrast and increases penetration; increasing mAs reduces noise but raises dose.
Worked example
A radiograph of the abdomen is underexposed: increasing mAs improves density but also increases patient dose; optimizing kVp may improve penetration without as much dose penalty.
Radiation Safety
Core concepts
Understand radiation quantities: absorbed dose (gray), equivalent dose (sievert), and effective dose. Know the ALARA principle and the three pillars: time, distance, and shielding. Master the principles of radiation protection for patients and staff: collimation, gonadal shielding, and dose monitoring (dosimeters). Recognize the deterministic effects (threshold dose, e.g., skin erythema) versus stochastic effects (no threshold, e.g., cancer risk). Understand the regulations and dose limits for occupational exposure. Know the principles of shielding design and the inverse square law.
Traps
Do not confuse deterministic and stochastic effects; deterministic effects have thresholds. Effective dose is not the same as absorbed dose. Beware assuming that a low dose carries no risk-stochastic effects have no threshold. Dose limits apply to occupational exposure, not patients.
Rule of thumb
ALARA: minimize time, maximize distance, and use appropriate shielding.
Worked example
A fluoroscopy procedure: reducing fluoroscopy time and using pulsed fluoroscopy lowers patient and staff dose without compromising the study.
Quality Assurance
Core concepts
Understand the goals of a quality assurance program: consistent image quality, dose optimization, and equipment performance. Know the components of QA: acceptance testing, routine performance monitoring, and corrective action. Master the metrics: image quality phantoms, dose indices (CTDIvol, DLP), and reject analysis. Recognize the role of accreditation and regulatory requirements. Understand the principles of quality improvement: PDSA cycles, root cause analysis, and error reporting. Know the signs of equipment malfunction and the importance of documentation.
Traps
Do not confuse QA with quality control; QA is the broader program. Beware ignoring reject analysis as a source of improvement. Recognize that dose indices are not the same as patient dose. Documentation is essential for accreditation.
Rule of thumb
A robust QA program includes acceptance testing, routine monitoring, and corrective action with documentation.
Worked example
A CT scanner shows increasing noise on phantom images: routine QA testing identifies tube degradation, prompting service and recalibration before clinical impact.