Core Concepts
X-Ray
Electromagnetic radiation that passes through soft tissue but is absorbed by dense structures (bone). First imaging modality (1895).
CT (Computed Tomography)
Combines multiple X-ray images taken from different angles to create cross-sectional images of the body.
MRI (Magnetic Resonance Imaging)
Uses strong magnetic fields and radio waves to create detailed images of organs and soft tissues.
Ultrasound
High-frequency sound waves used to image soft tissues, particularly useful for obstetrics and cardiology.
Hounsfield Units
Quantitative scale used in CT scans: Air = −1000, Water = 0, Bone = +400 to +1000, Metal = +3000.
Worked Examples & Applications
Hounsfield Units in CT: Air: −1000 HU. Fat: −100 to −50 HU. Water: 0 HU. Brain: 35-45 HU. Liver: 40-60 HU. Bone: 400-1000 HU. Acute blood: 50-75 HU. Calcification: >130 HU.
MRI T1 vs T2: T1-weighted: Fat bright, water dark. Good for anatomy. T2-weighted: Water bright, fat bright. Good for pathology (edema, inflammation). FLAIR: Water dark, lesions bright.
Practice Questions
Q: What are Hounsfield Units? A: A quantitative scale used in CT imaging to measure tissue density. Air = −1000, Water = 0, Bone = +400 to +1000.
Q: When would you use MRI over CT? A: MRI is preferred for soft tissue detail (brain, spinal cord, joints, muscles). CT is faster and better for bone, acute trauma, and hemorrhage.