CT Imaging and Hounsfield Units Quiz

Computed Tomography (CT)

• CT, or computed tomography, is a medical imaging technique using X-rays to create detailed images of the body.
• A heated cathode releases high-energy electrons, producing X-ray radiation.
• X-rays pass through tissues, absorbing more in denser tissues.
• Spiral CT uses a single radiation source and multiple detectors rotating around the patient.
• This gathers data as patient moves through, providing cross-sectional images.
• Tissue density is measured in Hounsfield units (HU).
  • Air = -1000 HU, Water = 0 HU, Bone = +1000 HU.
  • Air < Fat < Fluid < Soft Tissue < Bone < Metal
• Safety: Radiation dose measured in millisieverts (mSv).

How CT Works

• X-ray source rotates around the patient.
• Multiple detectors capture X-rays after passing through the body.
• Data is processed to create 2D cross-sectional images (slices).
• These slices can be stacked to create 3D images for better visualization of internal structures.

CT Scanner Components

• Gantry: Large circular part of the machine where the patient lies. X-ray tube and detectors rotate inside the gantry.
• Table: Motorized table that moves the patient in and out of the gantry during the scan.
• Computer System: Processes raw data from detectors and reconstructs it into images.

Applications of CT

• Diagnosis: Diagnoses conditions involving bones, organs, soft tissues, and blood vessels (tumors, fractures, infections, bleeding).
• Guided Procedures: Guides biopsy procedures and surgical planning.
• Monitoring: Monitors diseases like cancer and effectiveness of treatments.

Types of CT Scans

• Standard CT: Captures images of the body using a rotating X-ray beam.
• Spiral (or Helical) CT: Scanner continuously rotates while patient moves through the gantry. Faster and higher-resolution images.
• CT Angiography: Visualizes blood vessels after injecting contrast material.

CT vs. X-ray

• CT provides more detail than standard X-rays.
• CT scans are faster (often taking a few minutes).
• CT exposes patients to more radiation than standard X-rays, but is generally considered safe when used appropriately.

Contrast for CT

• Iodine-based IV contrast medium is used for most CTs.
• Oral Contrast: Liquid contrast material (usually barium) is ingested before abdominal or pelvic scans to visualize the gastrointestinal tract.
• Intravenous (IV) Contrast: Contrast dye injected into a vein for enhancing blood vessels, organs, or tissues.
• Exceptions: Contrast may not be used in some CT studies, such as kidney stone scans or head scans unless a mass is suspected. Poor renal function may require alternative imaging techniques instead of contrast material.

Risk of Contrast Administration

• Contrast-induced Acute Kidney Injury (AKI): (Within 3 days)
• Incidence: ~2% in patients without risk factors (e.g., diabetes)
• Anaphylaxis: (Immediate, within 1 hour)
• Allergy: (Delayed, up to 7 days)

CT of the Brain

• Technique: Non-contrast scans are usually the first step to evaluate the brain in emergency situations (trauma, stroke). Contrast-enhanced CT may be performed for specific conditions.
• Applications (Non-Contrast): Detecting hemorrhage, fractures, ischemic strokes, and brain edema.
• Applications (Contrast-enhanced): Visualizes tumors, infections (like abscesses), and blood vessel abnormalities.
• Preparation: Usually no special preparation is required, although fasting may be necessary if IV contrast is used.

CT of Other Body Parts

• CT Skull: Evaluation of the skull bones and contents.
• CT Chest: Evaluation of blood vessels, lung structures, and mediastinal anatomy. Used in diagnosing pulmonary embolism (PE). High-resolution CT (HRCT) assesses interstitial lung diseases, fibrosis, emphysema, or bronchiectasis; often used in cancer evaluations.
• CT Abdomen and Pelvis: Evaluate liver, kidneys, pancreas, appendix for abnormalities like masses, cirrhosis, tumors, or infections.
• CT of the Spine: Evaluate bony structures, fractures or degenerative changes, herniated discs.
• CT of the Musculoskeletal System: Detects bone fractures, complex joint structures, and soft tissue injuries. Useful in evaluating bone tumors, metastases, osteomyelitis, and arthritis.
• Shoulder, Knee, Foot and Hand CTs: Evaluate specific anatomy for fractures, injuries and/or conditions.

CT Angiography (CTA)

• Technique: Uses IV contrast to highlight blood vessels throughout the body.
• Applications: Assessing coronary artery disease, blockages, stenosis; aortic aneurysms or dissection; peripheral artery disease (PAD), aneurysms, or blockages in legs/arms; cerebral arteries.
• Preparation: Fasting 4-6 hours before, and potential beta blocker use for better imaging.

CT Windows and Filtering

• Filtering: Bone and soft tissue filtering help highlight specific structures.
• Windows: Adjust window settings (for instance in lungs) to provide optimal view of the body part. Three main windows include:
  • Mediastinal Window: Focuses on the mediastinum (area between lungs).
  • Lung Window: Helps visualize lung structures.
  • Bone Window: Highlights bone structures.

Clinical Goals (CT Chest Appearance in Different Diagnoses)

• Multiple images with various clinical presentations shown.

Test your knowledge on the advantages of CT imaging over plain film and the specifics of tissue density measurement. This quiz covers how X-rays are produced in CT machines and data collection methods in Spiral CT, along with an understanding of Hounsfield unit values. Perfect for students and professionals in radiology!