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Basic Radiology — Introduction Notes (KMTC, Exam-Oriented)

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1. DEFINITION & SCOPE OF RADIOLOGY

• Diagnostic radiology — image interpretation
• Interventional radiology — image-guided procedures (biopsies, drains, stenting)
• Therapeutic/nuclear medicine — radionuclide therapy

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2. IMAGING MODALITIES — OVERVIEW

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3. PLAIN X-RAY (CONVENTIONAL RADIOGRAPHY)

3.1 Basic Principles

• X-rays are high-energy electromagnetic waves that pass through the body
• Dense structures (bone) absorb more X-rays → appear white (radio-opaque)
• Air-filled structures (lungs) absorb very little → appear black (radiolucent)
• 5 radiographic densities (darkest to lightest): Air → Fat → Soft tissue/water → Bone → Metal

3.2 Image Receptor Technology

• Modern images stored and transmitted via PACS (Picture Archiving and Communication System)
• DR advantages: wider latitude (fewer retakes), better image quality, seamless PACS integration

3.3 Standard Projections

• PA (Postero-Anterior): Patient faces the detector; standard chest X-ray
• AP (Antero-Posterior): X-ray beam enters from front; used for sick/portable patients (enlarges cardiac shadow)
• Lateral: Right or left lateral; assesses posterior structures
• Lordotic view: Apices of lungs
• Lateral decubitus: Detects free pleural fluid (fluid layers dependently)

3.4 Portable (Bedside) Radiography

• Used for critically ill patients who cannot go to the X-ray department
• Limitations: Suboptimal technique, AP projection overestimates cardiac size, scatter radiation increases, lower resolution

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4. COMPUTED TOMOGRAPHY (CT)

4.1 Principles

• Rotating X-ray tube produces multiple cross-sectional images (slices)
• A computer reconstructs 2D and 3D images from the data
• Images displayed in Hounsfield Units (HU): water = 0 HU, air = −1000 HU, bone = +400 to +1000 HU

4.2 Evolution of CT Technology

4.3 Window Settings (EXAM FAVOURITE)

• Lung window: Wide window — shows airspaces, bronchi, vessels in lung parenchyma
• Mediastinal/Soft-tissue window: Narrow window — differentiates soft tissues, masses, vessels
• Bone window: Narrow window centred high — shows cortical and cancellous bone detail

4.4 Contrast Enhancement

• Intravenous (IV) iodinated contrast agents highlight vascular structures and hypervascular lesions
• Phases: Arterial phase, venous phase, delayed/excretory phase
• Contraindications: renal failure (risk of contrast nephropathy), iodine allergy

4.5 Dose Reduction Strategies

• Low-dose CT protocols
• Iterative reconstruction (replaces filtered back projection/FBP): reduces noise ≈50%, allows lower dose
• Automated tube current modulation

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5. ULTRASOUND (US)

5.1 Principles

• Uses high-frequency sound waves (2–15 MHz); no ionising radiation
• Transducer emits sound pulses → echoes from tissue interfaces → image formed
• High frequency → better resolution but less penetration
• Low frequency → deeper penetration but less resolution

5.2 Probe Types (EXAM TABLE)

5.3 Key Properties

• Echogenic / Hyperechoic: Bright (e.g., bone, gallstones, fat)
• Anechoic: Black/no echoes (e.g., bile, urine, blood)
• Hypoechoic: Darker than surroundings (e.g., many tumours)
• Posterior acoustic shadowing: Behind gallstones / calcified structures
• Posterior acoustic enhancement: Behind fluid-filled structures

5.4 Doppler US

• Colour Doppler: shows direction of blood flow (red = toward transducer, blue = away)
• Spectral Doppler: measures velocity of blood flow
• Used for: DVT, renal artery stenosis, carotid stenosis, portal hypertension

5.5 Clinical Uses

• First-line for: abdomen (liver, gallbladder, kidneys, spleen), pelvis, obstetrics, thyroid, breast, testes
• Guides: aspiration of pleural fluid, biopsies, vascular access
• Pneumothorax detection: absent lung sliding, absent B-lines ("comet tails"), stratosphere sign on M-mode

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6. MAGNETIC RESONANCE IMAGING (MRI)

6.1 Principles

• Uses strong magnetic field + radiofrequency (RF) pulses to align and then disturb hydrogen protons
• As protons return to equilibrium (relaxation), they emit RF signals → image formed
• No ionising radiation

6.2 Key Sequences (EXAM)

6.3 MRI Contrast

• Gadolinium (Gd)-based agents used (not iodine)
• Enhances areas with disrupted blood-brain barrier or vascularity
• Risk: Nephrogenic Systemic Fibrosis (NSF) in severe renal failure

6.4 Contraindications (EXAM FAVOURITE)

• Cardiac pacemakers / implantable defibrillators (unless MRI-conditional)
• Ferromagnetic metallic implants / aneurysm clips
• Cochlear implants
• Metallic foreign bodies in orbit
• (Relative) Claustrophobia; pregnancy (1st trimester — caution)

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7. NUCLEAR MEDICINE

7.1 Principles

• Radioactive tracers (radiopharmaceuticals) are administered (usually IV/inhaled)
• Tracer accumulates in target organs → gamma camera detects emitted radiation → functional images
• Shows physiology/metabolism, not just anatomy

7.2 Key Scans

7.3 PET-CT

• PET (Positron Emission Tomography) + CT fused in one scan
• Tracer: FDG (F-18 fluoro-deoxyglucose) — taken up by metabolically active cells
• Key uses: cancer staging, treatment response, recurrence detection
• Combines metabolic function (PET) + anatomical location (CT)

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8. FLUOROSCOPY

• Continuous X-ray imaging in real-time
• Used for: barium swallow/meal/enema (GI tract studies), arthrograms, hysterosalpingography (HSG), intraoperative guidance
• High radiation dose — 1 minute fluoroscopy ≈ 10× dose of single radiograph of same area
• Radiologist/operator must minimise screening time

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9. RADIATION PHYSICS & SAFETY (HIGH-YIELD EXAM TOPIC)

9.1 Units of Radiation

• For diagnostic X-rays: weighting factor = 1, so Gray = Sievert
• Annual background radiation (global average): ≈ 2–3 mSv/year
• US average (including medical): 6.2 mSv/year (36% from medical procedures)
• Recommended occupational limit: 50 mSv/year

9.2 Radiation Dose by Common Modality (Approximate Effective Dose)

9.3 Linear No-Threshold (LNT) Model

• Key principle: No safe dose of radiation exists; risk is directly proportional to dose
• 10 mSv effective dose → risk of malignancy in 1 in 1,000 exposed individuals
• Justifies the ALARA principle

9.4 ALARA Principle (As Low As Reasonably Achievable)

• Justification: Is the exposure clinically justified?
• Optimisation: Use the lowest dose that gives adequate diagnostic image
• Dose limitation: Apply dose limits to occupationally exposed workers

9.5 Radiation Protection for Staff — The Three Principles

1. Time — Minimise time near the radiation source
2. Distance — Maximise distance (inverse square law: doubling distance reduces dose by ¼)
3. Shielding — Lead aprons, lead glass screens, lead-lined walls

9.6 Radiation Biology

• Deterministic effects (threshold exists): Burns, cataract, radiation sickness — occur above a threshold dose
• Stochastic effects (no threshold): Cancer, genetic effects — probability increases with dose, no safe threshold

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10. CONTRAST MEDIA

10.1 Iodinated Contrast (for X-ray / CT / Fluoroscopy)

• Ionic: Higher osmolality, more side effects (older agents)
• Non-ionic: Lower osmolality, preferred in modern practice
• Routes: IV, intra-arterial, intrathecal, oral, intraluminal (GI)

10.2 Adverse Reactions

10.3 Other Contrast Agents

• Barium sulphate — oral/rectal for GI studies (insoluble, safe unless perforation suspected)
• Gadolinium — MRI contrast
• Microbubbles — ultrasound contrast (CEUS)
• Tc-99m radiopharmaceuticals — nuclear medicine

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11. RADIOLOGICAL ANATOMY — CHEST X-RAY INTERPRETATION (SYSTEMATIC APPROACH)

ABCDE Approach

Normal Cardiothoracic Ratio

• Normal: < 50% on PA film

• 50% suggests cardiomegaly (but AP film normally overestimates)

Lung Zones vs. Lobes

• Right lung: 3 lobes (upper, middle, lower)
• Left lung: 2 lobes (upper, lower) + lingula
• Right hemidiaphragm slightly higher than left (liver below)

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12. COMMON RADIOLOGICAL SIGNS (EXAM FAVOURITES)

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13. CLINICAL APPLICATIONS — WHEN TO ORDER WHICH MODALITY

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14. KEY DEFINITIONS (SHORT ANSWERS / DEFINITIONS QUESTIONS)

• Radiopaque: Appears white on X-ray (absorbs/blocks X-rays); e.g., bone, metal
• Radiolucent: Appears black on X-ray (transmits X-rays); e.g., air, fat
• Attenuation: Reduction in X-ray beam intensity as it passes through matter
• Collimation: Restricting the X-ray beam to the area of interest (reduces dose and scatter)
• PACS: Picture Archiving and Communication System — digital storage and retrieval of images
• DICOM: Digital Imaging and Communications in Medicine — standard format for medical images
• Fluoroscopy: Real-time X-ray imaging
• Echogenicity: Ability of tissue to reflect ultrasound waves
• Hounsfield Unit (HU): CT unit of tissue density (water = 0, air = −1000, bone = +400 to +1000)
• Half-life (T½): Time for radioactivity to decay to 50% of original value
• RIS: Radiology Information System — scheduling and reporting system

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15. QUICK-FIRE EXAM REVISION

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Basic Radiology — KMTC Exam Questions

MCQs | SAQs | LAQs

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SECTION A: MULTIPLE CHOICE QUESTIONS (MCQs)

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• A. Air
• B. Fat
• C. Soft tissue
• D. Cortical bone ✅
• E. Water

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• A. Less than 30%
• B. Less than 40%
• C. Less than 50% ✅
• D. Less than 60%
• E. Less than 70%

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• A. CT scan
• B. MRI without gadolinium
• C. Ultrasound ✅
• D. Plain X-ray
• E. Nuclear medicine scan

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• A. Left heart border
• B. Right heart border
• C. Aortic knuckle
• D. Right hemidiaphragm ✅
• E. Left hemidiaphragm

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• A. −1000
• B. −100
• C. 0 ✅
• D. +100
• E. +400

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• A. Cardiac pacemaker
• B. Cochlear implant
• C. Ferromagnetic aneurysm clip
• D. Titanium orthopaedic implant ✅
• E. Metallic foreign body in the orbit

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• A. Roentgen
• B. Gray
• C. Rad
• D. Sievert ✅
• E. Rem

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• A. Black (hypointense)
• B. Grey
• C. Bright white (hyperintense) ✅
• D. Isointense with brain
• E. Dark with surrounding bright rim

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• A. Air trapped in the pleural cavity
• B. Air-filled bronchi visible within opacified lung parenchyma ✅
• C. Pneumoperitoneum seen on erect CXR
• D. Hyperinflated lungs seen in emphysema
• E. Air within the mediastinum

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• A. All Levels Are Radiation Approved
• B. As Low As Reasonably Achievable ✅
• C. Allowable Levels of Acceptable Radiation Amounts
• D. Absolute Limit for All Radiation Areas
• E. Authorised Limit of Allowed Radiation Anywhere

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• A. 1–2 MHz
• B. 2–3.5 MHz
• C. 5–15 MHz ✅
• D. 20–30 MHz
• E. 40–50 MHz

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• A. Venous flow only
• B. Flow away from the transducer
• C. High-velocity flow
• D. Flow toward the transducer ✅
• E. Turbulent flow

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• A. MRI of the abdomen
• B. Ultrasound of the kidneys
• C. Plain abdominal X-ray (KUB)
• D. Non-contrast CT KUB ✅
• E. IV Urogram (IVU)

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• A. Bone scintigraphy
• B. V/Q scan
• C. PET scan ✅
• D. HIDA scan
• E. SPECT thyroid scan

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• A. Pneumothorax
• B. Pulmonary consolidation
• C. Interstitial pulmonary oedema ✅
• D. Pleural effusion
• E. Lung fibrosis (upper zone)

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• A. Radiation causes harm only above a minimum threshold dose
• B. There is a safe dose of radiation below which no harm occurs
• C. Radiation risk is inversely proportional to dose
• D. There is no safe dose; risk is directly proportional to dose ✅
• E. Low doses of radiation are protective against cancer

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• A. Underestimate cardiac size
• B. Overestimate cardiac size ✅
• C. Give better lung detail
• D. Produce lower radiation dose
• E. Show the mediastinum more clearly

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• A. Constipation
• B. Suspected bowel obstruction
• C. Suspected bowel perforation ✅
• D. Rectal bleeding
• E. Chronic diarrhoea

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• A. Narrow window / high centre
• B. Bone window
• C. Soft tissue window
• D. Wide window (lung window) ✅
• E. Liver window

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• A. 1 mSv/year
• B. 5 mSv/year
• C. 20 mSv/year
• D. 50 mSv/year ✅
• E. 100 mSv/year

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• A. Pulmonary embolism
• B. Right lower lobe collapse
• C. Perforated hollow viscus ✅
• D. Subphrenic abscess
• E. Ruptured spleen

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• A. Fluoroscopy
• B. CT scan
• C. Bone scintigraphy
• D. MRI ✅
• E. PET-CT

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• A. A pleural effusion is visible on lateral decubitus view
• B. An air bronchogram is seen in consolidation
• C. Two adjacent structures of similar radiographic density share a border and that border is lost ✅
• D. The diaphragm is elevated due to a subphrenic collection
• E. Air outlines the pericardium

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• A. Barium sulphate
• B. Iodinated contrast
• C. Gadolinium ✅
• D. Technetium-99m
• E. Fluorodeoxyglucose

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• A. Patient Assessment and Clinical Summary
• B. Picture Archiving and Communication System ✅
• C. Protocol for Acquiring Computed Scans
• D. Portable Automated CT Scanner
• E. Programmed Analogue-to-Colour System

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SECTION B: SHORT ANSWER QUESTIONS (SAQs)

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SAQ 1

• Definition (2 marks): Radiology is the medical specialty that uses various forms of energy to produce images of the body for diagnosis and to guide treatment/intervention.

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SAQ 2

1. Time (2 marks): Minimise the time spent near the radiation source. Shorter exposure = lower dose received.
2. Distance (2 marks): Maximise the distance from the radiation source. The inverse square law states that doubling the distance from the source reduces the dose to one-quarter (dose ∝ 1/distance²).
3. Shielding (2 marks): Use of physical barriers that absorb radiation. Examples include: lead aprons, lead-lined walls, lead glass screens, lead thyroid shields, and lead gloves.

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SAQ 3

• Definition (2 marks): The silhouette sign occurs when two adjacent structures of similar (water/soft tissue) radiographic density are in contact, causing their shared border/outline to become invisible (obliterated).
• Principle: Normally, borders between structures of different densities (e.g., heart and adjacent lung air) are visible. When air in the lung is replaced by fluid/pus (same density as the heart), the border disappears.

1. Right middle lobe consolidation → obliterates the right heart border
2. Lingula consolidation / left lower lobe collapse → obliterates the left heart border
3. Right lower lobe consolidation → obliterates the right hemidiaphragm

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SAQ 4

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SAQ 5

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SAQ 6

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SAQ 7

1. Cardiac pacemakers / implantable cardioverter defibrillators (unless MRI-conditional)
2. Cochlear implants
3. Ferromagnetic aneurysm clips (intracranial)
4. Metallic foreign bodies in the orbit
5. (Also acceptable): Some older prosthetic heart valves; certain drug infusion pumps

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SAQ 8

1. Air — black (most radiolucent)
2. Fat — dark grey
3. Soft tissue / Water — grey
4. Bone — white
5. Metal — bright white (most radio-opaque)

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SECTION C: LONG ANSWER QUESTIONS (LAQs)

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LAQ 1

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• Single-slice CT: Sequential acquisition; slow; older generation
• Spiral/Helical CT (1990s): Continuous volumetric data acquisition; major advancement
• Multidetector CT (MDCT, 1998): Multiple detector rows (up to 320); faster acquisition; higher spatial and temporal resolution; single breath-hold imaging; less motion artefact
• Dual-energy CT (DECT): Two X-ray energies simultaneously; tissue characterisation (e.g., gout, renal stone composition)
• Dual-source CT: Two X-ray tubes; ultra-fast; used in cardiac and paediatric imaging

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• Iodinated contrast agents are injected IV to enhance vascular structures and hypervascular lesions
• Phases:
  • Arterial phase (~25–35 sec after injection): aorta, arteries, hypervascular tumours
  • Portal venous phase (~60–70 sec): liver parenchyma, portal vein, most abdominal organs
  • Delayed/excretory phase (~3–5 min): urinary tract, fibrous/avascular lesions
• Contraindications: Significant renal impairment (risk of contrast-induced nephropathy), known iodine allergy (relative — can pre-medicate)

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• CT chest: ~7 mSv (≈ 350 chest X-rays)
• CT abdomen/pelvis: ~10–25 mSv

1. Low-dose protocols — tailored to indication (e.g., CT lung cancer screening at ~0.9 mSv)
2. Iterative reconstruction — replaces filtered back projection; reduces image noise ~50% at lower doses
3. Automated tube current modulation — adjusts current based on body thickness
4. Limiting scan extent — scan only the anatomical area of interest
5. Avoiding unnecessary phases — reduce multiphase protocols where single phase suffices

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LAQ 2

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• Occur above a threshold dose; severity increases with dose
• Examples: radiation burns (skin erythema), cataracts (lens), bone marrow suppression, radiation sickness (nausea, hair loss), gonadal damage → infertility
• Relevant to: interventional radiology, therapeutic radiation, nuclear accidents

• No threshold — any dose carries a finite risk
• Probability (not severity) increases with dose
• Examples: cancer induction (most important), heritable genetic mutations
• Basis of the Linear No-Threshold (LNT) model: risk ∝ dose
• An effective dose of 10 mSv → 1 in 1,000 risk of cancer induction

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• For diagnostic X-rays: weighting factor = 1, so Gy = Sv
• Absorbed dose used in radiotherapy; effective dose used in diagnostic radiology

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1. Time — minimise duration near source
2. Distance — inverse square law: doubling distance → dose reduced to ¼
3. Shielding — lead aprons, lead glass screens, lead-lined walls, thyroid shields

• Gonadal shielding where gonads are in/near field
• Collimation — restrict beam to area of interest
• Select appropriate modality (prefer US/MRI over CT when equivalent)
• Avoid imaging pregnant patients unless clinically essential; if essential, document and minimise

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LAQ 3

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• Ultrasound uses high-frequency sound waves (1–20 MHz) — above the range of human hearing
• A transducer (probe) contains piezoelectric crystals that:
  • Convert electrical energy → sound pulses (transmitter)
  • Convert returning echoes → electrical signals (receiver)
• Sound waves travel through tissues; at interfaces between tissues of different acoustic impedance, waves are reflected (echoes) back to the transducer
• The time taken for echoes to return determines depth; the amplitude of returned echoes determines brightness on the image
• No ionising radiation — safe for all patients including pregnant women and children

• Higher frequency → better resolution, less penetration (superficial structures)
• Lower frequency → lower resolution, greater penetration (deep structures)

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• Colour Doppler: Displays flow direction as colour overlay:
  • Red = flow toward transducer
  • Blue = flow away from transducer
  • Mnemonic: BART — Blue Away, Red Toward
• Spectral Doppler (duplex): Produces waveform showing velocity over time; assesses stenosis, resistance
• Power Doppler: More sensitive to slow/low-volume flow; no directional information

• Deep vein thrombosis (DVT)
• Carotid and renal artery stenosis
• Portal hypertension
• Testicular torsion (absent/reduced flow)
• Fetal wellbeing (umbilical artery Doppler)

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• FAST scan (Focused Assessment with Sonography in Trauma) — haemoperitoneum, haemopericardium
• Pneumothorax detection in ICU
• Cardiac function assessment

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QUICK SUMMARY TABLE — EXAM READY

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