Computed tomography Study Guide

📖 Core Concepts Computed Tomography (CT) – X‑ray based imaging that creates cross‑sectional “slices” by rotating an X‑ray tube around the patient and reconstructing attenuation data. Hounsfield Unit (HU) – Linear scale of X‑ray attenuation; water = 0 HU, air = –1000 HU, cancellous bone ≈ +400 HU, cortical bone > +2000 HU. Sinogram – Intermediate 2‑D representation of raw projection data (detector position × rotation angle) used for reconstruction. Reconstruction Algorithms – Filtered Back‑Projection (fast, noise‑sensitive) and Iterative Reconstruction (noise‑reduced, dose‑saving). Dual‑Energy (Spectral) CT – Acquires two energy spectra (low & high) to differentiate materials (e.g., iodine vs calcium). Radiation Dose Metrics – CTDI (computed tomography dose index) & DLP (dose‑length product) describe scanner output; effective dose (Sv) estimates whole‑body cancer risk. --- 📌 Must Remember HU Ranges: Air –1000 HU, Water 0 HU, Fat ≈ –100 HU, Soft tissue 30–60 HU, Cancellous bone ≈ +400 HU, Cortical bone > +2000 HU. Window Settings: Brain window ≈ 0 HU–80 HU; adjust width/level to highlight desired tissue. Radiation Risk: Typical CT = 10–20 mGy organ dose; a single abdominal CT (≈ 8 mSv) adds 0.05 % lifetime cancer risk. Contrast‑Induced Nephropathy (CIN): Occurs in 2–7 % of patients; risk ↑ with pre‑existing kidney disease, diabetes, dehydration. Hydration ↓ risk. Artifact Types: Beam‑hardening/streak (high‑density contrast, metal), motion (patient movement), partial‑volume (thick slices), ring (detector mis‑calibration). --- 🔄 Key Processes Data Acquisition Rotate X‑ray tube → fan/cone beam → detectors record attenuated intensity → generate sinogram. Reconstruction FBP: Apply high‑pass filter → back‑project filtered projections → form image. Iterative: Guess image → simulate projections → compare to measured → update guess → repeat until convergence. Windowing Select window width (WW) & level (WL). Pixels < (WL – WW/2) → black; > (WL + WW/2) → white; within range → grayscale mapping. Multiplanar Reconstruction (MPR) Re‑slice 3‑D voxel volume into coronal, sagittal, or oblique planes. Dual‑Energy Material Decomposition Acquire low‑ and high‑energy datasets → compute material‑specific attenuation → generate virtual monochromatic images or iodine maps. --- 🔍 Key Comparisons Sequential (Step‑and‑Shoot) vs Spiral (Helical) CT Sequential: Table stops per slice → slower, less motion blur. Spiral: Continuous table motion + rotating tube → faster, helical data set, prone to motion artifacts if patient moves. Single‑Source Dual‑Energy vs Dual‑Source Dual‑Energy Single‑Source: Rapid kV switching during one rotation. Dual‑Source: Two offset tubes/detectors → simultaneous acquisition, higher temporal resolution. Filtered Back‑Projection vs Iterative Reconstruction FBP: Quick, high‑frequency noise, streaks with low dose. Iterative: Slower, reduces noise, permits lower radiation dose. --- ⚠️ Common Misunderstandings “Higher HU always means bone.” – Very dense metal can exceed +3000 HU and produce artifacts; not true bone. “Dual‑Energy eliminates all metal artifacts.” – It reduces but does not fully remove streaking; metal‑artifact reduction software may still be needed. “Low‑dose CT is always non‑diagnostic.” – Modern iterative reconstruction can maintain diagnostic quality at substantially lower dose. --- 🧠 Mental Models / Intuition CT as a “CT‑scan puzzle” – Each projection is a slice of the puzzle; reconstruction pieces them together like a jigsaw to reveal the hidden 3‑D picture. HU as “density dial” – Turn the window knob to zoom in on the density range you care about, just like adjusting a radio frequency to hear a specific station. Dual‑Energy as “color separation” – Think of low‑energy photons as “red” and high‑energy as “blue”; combining them lets you isolate “green” (iodine) from bone. --- 🚩 Exceptions & Edge Cases Metal Implants: May completely extinguish the X‑ray beam → severe streaks; use dual‑energy, metal‑artifact reduction, or adjust tube voltage. Partial‑Volume Effect: Thick slices (> 3 mm) can blur small structures; thin slices (≤ 1 mm) required for fine detail (e.g., pulmonary nodules). Pediatric Patients: Higher radiosensitivity → mandatory low‑kV protocols, aggressive dose tailoring, and use of “Image Gently” guidelines. --- 📍 When to Use Which Spiral CT – Preferred for most routine body imaging (fast, whole‑organ coverage). Sequential CT – When motion must be minimized (e.g., high‑resolution brain imaging). Dual‑Energy CT – To characterize material composition (iodine vs calcium, gout urate crystals) or reduce metal artifacts. Iterative Reconstruction – For low‑dose protocols, pediatric scans, or any exam where noise is a concern. CT Angiography (CTA) – When vascular anatomy is needed; use iodine contrast + rapid acquisition. Non‑contrast CT – Kidney stones, calcium scoring, or when contrast is contraindicated. --- 👀 Patterns to Recognize Hyperdense (bright) on non‑contrast CT → hemorrhage, calcification, bone. Hypodense (dark) area with surrounding edema → acute infarction or tumor necrosis. Streaks radiating from a high‑density object → beam‑hardening or metal artifact. “Ring” of high attenuation around a low‑attenuation core → possible abscess (capsule) or cyst with contrast rim. --- 🗂️ Exam Traps Confusing HU values – Mistaking +400 HU (cancellous bone) for soft tissue; remember bone is markedly brighter. Assuming all contrast‑enhanced studies are high‑dose – Dual‑energy or low‑kV protocols can keep dose low while preserving contrast. Selecting “Sequential” for speed – Sequential is slower; many students pick it for fast scans incorrectly. Mixing up CTDI vs Effective Dose – CTDI measures scanner output; effective dose (Sv) estimates patient risk. ---