Medical Imaging Systems

1. Chapter Overview

Medical imaging is how clinicians see inside the body without surgery. For the BME exit exam, Biomedical Imaging carries 9 blueprint items (9% of the exam) — smaller than instrumentation or signal processing, but highly fact-dense: one confused CT generation or T1/T2 pairing costs a full mark.

This chapter covers high-yield imaging topics from the exit exam blueprint.md`, It follows the standard biomedical imaging systems curriculum.

Why This Chapter Matters

Exit imaging items test four layers:

1. Physics — How is signal generated? (X-ray photons, RF spin echo, ultrasound reflection, gamma annihilation)
2. Hardware — What components produce and detect that signal?
3. Image formation — How is raw data reconstructed into a diagnostic image?
4. Clinical judgment — Which modality for which indication? What are the risks?

Students who memorize modality names without understanding photoelectric ∝ Z³ (often taught Z⁴ for contrast) or who swap T1 bright fat / T2 bright water miss high-yield items repeatedly.

Sources Integrated

Healthcare Relevance

2. Learning Outcomes

After mastering this chapter, you should be able to:

1. Classify imaging modalities as ionizing (X-ray, CT, nuclear) or non-ionizing (MRI, ultrasound) and state their primary contrast mechanism.
2. Explain X-ray production and interaction — Bremsstrahlung, characteristic X-rays, photoelectric effect, Compton scatter — and relate atomic number Z to contrast.
3. Describe CT hardware generations, helical acquisition (slip rings), reconstruction, and Hounsfield unit scaling.
4. Explain MRI spin physics — B₀ alignment, Larmor frequency, RF excitation, relaxation (T1, T2, T2*), and spatial encoding with gradients.
5. Compare T1-weighted vs T2-weighted MRI and predict tissue appearance (fat vs water).
6. Detail ultrasound transducer design, imaging modes (A/B/M/Doppler), resolution trade-offs, and common artifacts.
7. Distinguish PET, SPECT, and planar scintigraphy and explain why PET-CT is the standard hybrid system.
8. Apply MRI safety zones, cryogen management, and projectile ferromagnetic risks.
9. Select appropriate first-line imaging for common clinical scenarios (e.g., suspected IBD, trauma).
10. Troubleshoot image quality problems — heel effect, scatter, beam hardening, motion, aliasing.

3. Core Concepts

3.1 What Is a Medical Image?

A medical image is a 2D or 3D spatial map of a physical property inside the body:

Digital images are M × N pixel matrices. Each pixel stores intensity (8-bit grayscale 0–255 for display; CT may use signed Hounsfield units beyond 8-bit). Color images use RGB planes. Medical images often exceed 256 gray levels — CT HU range roughly −1000 (air) to +3000 (dense bone).

3.2 Electromagnetic Spectrum and Radiation Types

Diagnostic X-rays occupy ~10–150 keV — higher energy than visible light, able to penetrate tissue.

Ionizing radiation removes electrons from atoms (biological damage risk): X-rays, gamma rays, beta particles from PET tracers.

Non-ionizing radiation does not ionize at diagnostic levels: ultrasound (2–18 MHz sound waves) and MRI (static B₀ + RF pulses at Larmor frequency, ~42.58 MHz/T for ¹H).

3.3 Contrast Mechanisms — The Exam Backbone

Photoelectric effect (dominant at low keV, high Z):

• Photon absorbed; inner-shell electron ejected.
• Probability $\propto Z^3/E^3$ (often simplified to $Z^4$ for relative contrast between materials at fixed energy).
• High-yield: Iodine (Z = 53) and barium are excellent contrast agents because photoelectric absorption is dramatically higher than soft tissue (Z ≈ 7.4 effective).

Compton scatter (dominant at diagnostic energies in soft tissue):

• Photon strikes outer-shell electron; photon loses energy and changes direction.
• Demonstrates photons have momentum (exit-0604).
• Scattered photons degrade image contrast (scatter radiation) — anti-scatter grids in radiography reduce this.

Bremsstrahlung ("braking radiation"):

• Accelerated electrons decelerate near target nucleus → continuous X-ray spectrum.
• Dominant production mechanism in X-ray tubes (exit-0444, exit-0639).

Characteristic X-rays:

• Inner-shell vacancy filled by outer electron → discrete energy peaks (e.g., tungsten K-lines).

3.4 Modality Taxonomy (Lecture Ch 01)

Anatomical (structure)          Functional (physiology)
─────────────────────          ───────────────────────
X-ray, CT, MRI, US               PET, SPECT, fMRI, Doppler US

Projection vs tomography:

• Projection (radiography, fluoroscopy): 3D anatomy collapsed onto 2D — superimposition of structures.
• Tomography (CT, MRI, PET): Cross-sectional slices; can reconstruct 3D volumes.

3.5 Image Quality Metrics (Exit-Level)

Every imaging system trades four parameters:

Modality-specific resolution drivers:

• X-ray: Focal spot size, detector pixel, SID, motion.
• CT: Slice thickness, detector row width, reconstruction kernel (sharp vs smooth).
• MRI: Matrix size, FOV, slice thickness, bandwidth (exit-0392 — matrix → resolution).
• Ultrasound: Frequency, pulse length, beam width, focusing.

Windowing (CT/MRI display): Window width (contrast) and window level (brightness) map HU or signal intensity to gray levels visible on monitor — same raw data, different diagnostic emphasis.

3.6 Radon Transform and Tomographic Reconstruction (Conceptual)

CT reconstruction rests on the idea that each projection measures the sum of attenuation along a ray path. The Radon transform mathematically describes all such line integrals. Filtered back-projection inverts this transform to recover μ(x,y) per slice. Modern iterative reconstruction (ASIR, MBIR) reduces noise at lower dose — increasingly tested as dose optimization moves clinical.

Understanding this separates CT from "just many X-rays" — it is computed tomography requiring projection geometry, detector calibration, and reconstruction mathematics (2D FFT in MRI; FBP/iterative in CT).

4. Technical Deep Dive

Each major modality is organized by: Physics → Components → Image Formation → Advantages → Disadvantages → Artifacts → Safety.

4.1 X-Ray and Radiography

Physics

• Electrons accelerated from cathode to anode (tungsten target) by kilovoltage (kVp).
• Bremsstrahlung produces continuous spectrum; characteristic peaks superimposed.
• Beam filtered (aluminum) to remove low-energy photons that would only dose skin.
• Patient attenuation follows Beer–Lambert law: $I = I_0 e^{-\int \mu\,dl}$.
• Photoelectric → contrast for bone (high Z) and iodinated contrast.
• Compton → scatter; reduces contrast.

Anode heel effect: X-ray intensity is lower on the anode side because photons emitted toward the anode traverse more target material (the "heel"). Intensity gradient along cathode–anode axis.

Factors aggravating heel effect (exit-0021): shorter SID (source-to-image distance) — beam diverges more, using extreme angles where gradient is worst. Larger anode angle actually reduces heel effect by allowing more photons to exit.

Components

Digital radiography:

• CR (computed radiography): Photostimulable phosphor plate → laser readout → digitized (exit Q&A #10).
• DR (direct digital radiography): Direct digital detector — faster, higher DQE, no plate readout step.

Image Formation

1. X-rays pass through patient.
2. Transmitted intensity pattern hits detector.
3. Negative image logic: Dense structures (bone) attenuate more → fewer photons → darker on film (historically) or appropriately windowed in digital display.
4. mAs controls quantity (photon fluence); kVp controls penetration and contrast.

Advantages

• Fast, widely available, low cost per study.
• Excellent bone and lung detail.
• Portable units for bedside and trauma.
• Mammography: optimized low-dose system for high-resolution soft tissue (exit Q&A #9 — not because non-ionizing). Dedicated molybdenum/rhodium anode spectra and compression paddle maximize lesion conspicuity at acceptable breast dose.

Disadvantages

• 2D superimposition — may hide lesions behind bone.
• Poor soft-tissue contrast without contrast agents.
• Ionizing radiation — cumulative dose concern.
• Heel effect and scatter degrade uniformity.

Artifacts

Safety

• ALARA — As Low As Reasonably Achievable.
• Lead apron, thyroid shield, gonadal shielding when appropriate.
• Collimation to ROI.
• Pregnancy: justify exam; shield fetus when possible.
• Staff: distance, shielding, dosimetry badges.

4.2 Fluoroscopy and C-Arm

Physics

Same X-ray tube physics as radiography, but continuous or pulsed low-dose exposure over time → real-time images.

Components

• X-ray tube + collimator
• Image intensifier (II) OR flat-panel detector
• C-arm gantry (mobile OR angiography suite)
• Monitor and recording system

Image intensifier chain (exit-0442, exit-0637):

1. Input phosphor (CsI) converts X-rays to light.
2. Photocathode emits electrons.
3. Electron optics amplify (~10³–10⁴ gain).
4. Output phosphor → visible image for camera/TV chain.

Key operational difference (exit-0443, exit-0638): Fluoroscopy provides real-time dynamic imaging; static radiography is a single snapshot. Both use X-rays (not gamma rays). Fluoroscopy guides interventions (catheter placement, orthopedic reduction).

Image Formation

Continuous detection → video frame rate (15–30 fps pulsed fluoro). Last-image-hold stores frame for review.

Advantages

• Real-time guidance for surgery and cardiology.
• Contrast swallow studies, voiding cystourethrography.
• Adjustable magnification modes on II.

Disadvantages

• High cumulative dose vs single radiograph (minutes of fluoro).
• Lower spatial resolution than static radiography.
• II distortion and vignetting (legacy systems).

Artifacts

• Motion blur during breathing/cardiac cycle.
• Blooming/saturation if exposure not adjusted.
• Pincushion distortion (II systems).

Safety

• Pulsed fluoroscopy; minimize fluoro time.
• Collimation; use lowest acceptable frame rate and dose rate.
• Radiation protection for staff (lead, distance, mobile shields).

4.3 Computed Tomography (CT)

Physics

• Same X-ray attenuation as radiography, but tomographic reconstruction from many projection angles.
• Hounsfield Unit (HU): $\mathrm{HU} = 1000 \times (\mu_{\mathrm{tissue}} - \mu_{\mathrm{water}}) / (\mu_{\mathrm{water}} - \mu_{\mathrm{air}})$. Water = 0 HU; air ≈ −1000 HU; bone +400 to +1000 HU.
• HU primarily reflects tissue composition and electron density (exit Q&A #14).
• Beam hardening: Lower-energy photons preferentially absorbed → mean energy rises through patient → cupping artifact in reconstruction.

Components

CT Generations — HIGH-YIELD

EXAM CALLOUT — Q696 (exit-0696): "Rotating x-ray tube and a fixed ring of detectors" → Fourth generation.

Do not confuse with 3rd generation, where both tube and detectors rotate as a coupled pair (rotate–rotate). Your blueprint note may abbreviate this — the bank answer is 4th gen for fixed detector ring.

Helical (spiral) CT (exit-0640): Enabled by slip-ring technology — allows continuous gantry rotation while table moves → volume acquisition without stop-and-shoot between slices.

Spatial resolution influenced by slice thickness, detector size, focal spot, reconstruction kernel (exit Q&A #12). Automatic exposure control (AEC) modulates mA based on patient size — dose optimization (exit Q&A #13).

CT vs conventional X-ray detectors (exit Q&A #11): CT detector arrays are more sensitive and acquire multiple slices per rotation — enabling tomographic reconstruction. Conventional radiography uses a single large-area detector (film or flat panel) for one projection only.

Slice thickness is a primary determinant of spatial resolution in the z-axis (exit Q&A #12). Thinner slices improve detail but increase noise and dose per volume covered; reconstruction kernel (sharp vs smooth) trades edge definition against noise.

Image Formation

1. Acquire projections over 180°–360° rotation.
2. Filtered back-projection or iterative reconstruction builds cross-sectional slice.
3. Stacking slices → 3D volume → MPR, MIP, 3D rendering.

Radon transform concept: each projection is a line integral of μ; inverse Radon reconstructs μ(x,y).

Advantages

• Eliminates superimposition — excellent cross-sectional anatomy.
• Fast (modern MDCT: sub-second whole chest).
• Sensitive to acute hemorrhage, lung nodules, fractures.
• CT angiography with iodinated contrast.

Disadvantages

• Ionizing radiation — higher dose than single X-ray (especially multiphase contrast).
• Poor soft-tissue contrast vs MRI for many neurologic/MSK indications.
• Iodine contrast: allergy, nephrotoxicity.
• Metal streak artifacts.

Artifacts

Safety

• Dose modulation (kV/mA adjustment).
• Pregnancy: justify; consider US/MRI alternatives.
• Contrast: eGFR check, hydration, emergency anaphylaxis kit.
• Pediatric protocols — size-based dosing.

4.4 Magnetic Resonance Imaging (MRI)

Physics

1. B₀ (main magnet, 0.5–3 T clinical; 1.5 T and 3 T common) aligns hydrogen proton spins (partial polarization ~few ppm excess parallel).
2. Protons precess at Larmor frequency: ω = γ B₀; f = γ B₀ / 2π. For ¹H: γ/2π ≈ 42.58 MHz/T → 1.5 T ≈ 63.9 MHz.
3. RF pulse (B₁) at resonance tips net magnetization into transverse plane.
4. Precessing transverse magnetization induces signal in receive coil.
5. Relaxation returns spins to equilibrium — source of contrast.

Relaxation:

High-yield T1 vs T2:

Pulse sequences: Spin echo (SE), fast spin echo (FSE/TSE), gradient echo (GRE) — trade speed, SNR, and contrast weighting via TR and TE.

How to weight an image (exam drill):

Spin-echo mechanism: 90° RF pulse tips M to transverse plane; spins dephase (T2*); 180° refocusing pulse reverses dephasing → echo at TE with T2 (not T2*) contrast. This is the workhorse sequence for clinical MRI.

Gradient echo (GRE): No 180° pulse; faster but T2* weighted; sensitive to susceptibility (hemorrhage, metal). Used in SWI and fast cardiac imaging.

Gadolinium contrast (T1 shortening): Paramagnetic Gd shortens T1 of adjacent water → bright enhancement on T1W in areas of blood-brain barrier breakdown or vascularity. Not iodine — MRI uses Gd-based agents; screen for nephrogenic systemic fibrosis in renal impairment.

fMRI: BOLD (Blood Oxygen Level Dependent) — deoxyhemoglobin is paramagnetic → local T2* changes with activation (exit Q&A #17). Measures indirect neural activity via blood oxygenation, not electrical spikes.

Diffusion-weighted imaging (DWI): Sensitive to random Brownian motion of water — restricted diffusion in acute stroke (cytotoxic edema) appears bright on DWI within minutes, before T2 changes — critical in stroke protocols alongside CT hemorrhage exclusion.

Components

Cooling agent (exit-0602): Liquid helium — maintains superconductivity. Argon/neon/xenon are not standard MRI cryogens.

Image Formation

1. Slice selection — gradient + RF bandwidth selects one plane.
2. Phase encoding — gradient phase-encodes rows.
3. Frequency encoding — readout gradient during echo sampling.
4. k-space filled → 2D FFT → image.
5. Matrix size (e.g., 256×256) sets number of encoding steps → spatial resolution when FOV fixed (exit-0392). Resolution ≈ FOV / matrix.

Advantages

• No ionizing radiation.
• Superior soft-tissue contrast — brain, spine, joints, liver lesions.
• Multiplanar imaging (axial, sagittal, coronal) without repositioning.
• Spectroscopy, diffusion, perfusion, fMRI extensions.

Disadvantages

• Expensive purchase and maintenance (helium costs).
• Long scan times; motion-sensitive.
• Contraindications: pacemakers (many legacy), cochlear implants, some aneurysm clips, ferromagnetic foreign bodies.
• Loud acoustic noise; claustrophobia.
• Cannot image patients with certain metal — projectile risk.

Artifacts

Safety — HIGH-YIELD

MRI zones:

• Zone I — public areas
• Zone II — screening; changing
• Zone III — control room access
• Zone IV — magnet room — always on

Hazards:

1. Projectile effect — ferromagnetic objects accelerated into bore (oxygen tanks, wheelchairs, scissors).
2. RF burns — loops (crossed legs, ECG leads, tattoos with metal ink).
3. Quench — sudden loss of superconductivity; helium boil-off; evacuate room.
4. Acoustic — hearing protection required.
5. Implant screening — every patient; MR Conditional vs MR Safe vs MR Unsafe.

Cryogen: Helium boil-off monitored; quench vent to exterior. Never enter magnet room during quench without training.

4.5 Ultrasound

Physics

• Longitudinal sound waves — NOT electromagnetic (lecture emphasis).
• Diagnostic frequency: 2–18 MHz (audible limit ~20 kHz).
• Speed in soft tissue ≈ 1540 m/s (exit Q&A #1).
• Wavelength $\lambda = c/f$ — frequency ↑ → wavelength ↓ → resolution ↑, penetration ↓.
• Acoustic impedance $Z = \rho c$ determines reflection at boundaries.
• Primary imaging interaction: reflection at impedance mismatches (exit Q&A #2).

Doppler: Frequency shift Δf related to reflector velocity toward/away from transducer — measures blood velocity (exit Q&A #6). Maximum shift at 0°/180°; zero at 90° insonation.

Components

Transducer types:

• Linear array — superficial structures; high frequency 10–12 MHz (exit Q&A #4, lecture).
• Curvilinear (convex) — abdominal; wider field; lower frequency for depth.
• Phased array — cardiac; steered beam without moving probe (exit Q&A #5).
• Arrays: 128–512 elements typical.

Frequency selection (lecture):

• Superficial (thyroid, breast, MSK): high frequency (10–12 MHz).
• Obese patients / deep structures: low frequency (2–5 MHz).

Image Formation

• A-mode: Amplitude vs depth — one line.
• B-mode: 2D gray-scale map — most common; fetal biometry (exit Q&A #3).
• M-mode: Motion along one line — cardiac valve timing.
• Doppler / color flow: Overlay velocity on B-mode.

Time-of-flight ranging: depth = (c × Δt) / 2.

Advantages

• Real-time, portable, no ionizing radiation.
• Inexpensive vs CT/MRI; bedside, ambulance, rural use (3rd-gen portable US).
• Excellent for OB, gallstones, DVT screening, pleural effusion.
• Doppler for vascular stenosis, cardiac valves.

Disadvantages

• Operator-dependent — skill and probe pressure matter.
• Cannot penetrate bone or air-filled lung (acoustic shadow).
• Limited field of view in obese patients at low frequency.
• Tissue harmonic and contrast agents add cost/complexity.

Artifacts — HIGH-YIELD

Safety

• Diagnostic US: no confirmed biological harm at diagnostic intensities (FDA MI/TI limits).
• Mechanical index (MI) and thermal index (TI) displayed.
• Avoid unnecessary fetal exposure in first trimester unless indicated (ALARA).
• Endocavitary probes — high-level disinfection between patients.

Axial vs lateral resolution: Axial (along beam) improved by shorter pulse (backing block damping). Lateral (across beam) improved by focusing and higher frequency. Near field resolution best at focal zone.

Harmonic imaging: Uses second harmonic echoes generated in tissue — reduces clutter from superficial structures; improves endocardial border definition in cardiac US.

4.6 PET, SPECT, and Nuclear Medicine

Physics

Radiopharmaceutical emits radiation detected externally:

• PET: Positron (β⁺) emitter (e.g., ¹⁸F-FDG). Positron annihilates with electron → two 511 keV gamma photons at ~180° (exit Q&A #19).
• SPECT: Single photon emitter (e.g., ⁹⁹ᵐTc). Gamma camera with collimators detects photon direction.
• Planar scintigraphy: 2D gamma camera image — whole-body bone scan (exit Q&A #18).

Hybrid imaging (exit-0643): PET-CT is standard — metabolic PET fused with anatomical CT for localization. SPECT-CT also exists. CT alone is not "combined with" PET in the sense of hybrid — PET-CT pairs them.

Components

Image Formation

• PET: Coincidence lines of response (LOR) → tomographic reconstruction.
• SPECT: Multiple angular projections → reconstruction (lower sensitivity than PET).
• Attenuation correction — CT map in hybrid systems.

Advantages

• Functional/metabolic information (glucose uptake, perfusion, receptor binding).
• Oncology staging, treatment response (FDG-PET).
• Cardiac perfusion (SPECT).

Disadvantages

• Radiation from tracer + CT component.
• PET resolution ~4–5 mm; cost; cyclotron dependency for short half-life isotopes.
• SPECT lower sensitivity than PET.

Artifacts

• Attenuation artifacts (photon loss in dense tissue).
• Motion between PET and CT components in hybrid scan.
• Partial volume effect in small lesions.

Safety

• Radiopharmaceutical handling — hot lab, dose calibrator, ALARA.
• Pregnancy/breastfeeding restrictions.
• Time–distance–shielding for staff.
• Patient hydration and voiding after FDG to reduce bladder dose.

5. Equipment and Device Focus

5.1 X-Ray Room

Daily QA: kVp/mAs accuracy, beam alignment, collimator light field congruence.

Maintenance: Tube heat unit tracking; anode crack inspection; DR panel calibration.

BME role: Accept commissioning; establish exposure charts; train technologists on pediatric protocols.

5.2 CT Scanner

Slip ring wear — helical capability depends on continuous rotation.

Detector calibration — ring artifact if failed.

Tube replacement — major capital planning (links to exit-0028 recurrent vs capital budget).

5.3 MRI Scanner

Helium level monitored weekly; refill schedules; quench drill.

Cold head maintenance on superconducting systems.

SAR limits — RF heating; sequence adjustment for patient size.

Quench pipe — must vent outdoors; never block.

5.4 Ultrasound

Probe damage — dead elements cause dropouts.

Disinfection — high-level for endocavitary probes.

Battery-powered portable — 3rd generation smartphone-linked systems (lecture).

5.5 Gamma Camera / PET-CT

Collimator handling — fragile, expensive.

Daily uniformity and sensitivity checks.

Radiopharmacy QC — dose assay, expiry.

5.6 Microscope (exit-0049 — imaging-adjacent)

Electron microscope differs from phase contrast, fluorescence, and dark field — all optical microscopes use photons; EM uses electron beam in vacuum → nanoscale resolution.

6. Practical Biomedical Engineering Perspective

Modality Selection Logic

Perfusion (exit-0428): Blood flow through tissues (mL/min/100 g) — not air movement, nerve conduction, or muscle contraction. Perfusion MRI, CT perfusion, and nuclear medicine quantify this.

Dose Management

• Justify every ionizing study.
• Track cumulative dose (especially pediatrics).
• Use AEC on CT; pulsed fluoro in cardiology.

Ethiopian Context

Imaging infrastructure is concentrated in referral hospitals (CT, MRI limited). BME graduates may be the only technical staff maintaining X-ray and ultrasound — mastery of basic physics + safety + procurement (recurrent vs capital budget, exit-0028) is career-critical.

Recurrent fund covers: consumables (US gel, CR plates, contrast), spare parts, training — NOT replacing entire imaging systems (capital expenditure).

Acceptance Testing and QA Workflow

When a new imaging system arrives:

1. Site planning — floor load (MRI magnet tons), RF shielding (MRI), power conditioning, cooling ventilation.
2. Installation and commissioning — vendor performs geometric calibration, dose output (CT/X-ray), uniformity, SNR (MRI), phantom images.
3. Baseline acceptance — hospital BME documents as-found metrics; signs acceptance only when contract specifications met.
4. Routine QA — daily/weekly/monthly tests per manufacturer and AAPM/IAEA protocols: CT water phantom HU, US phantom resolution, X-ray kVp accuracy.
5. Preventive maintenance — tube heat history, helium level, detector cleaning.

Failure at any step → service ticket before clinical release. This workflow links imaging to HTM (Chapter 10) and hospital engineering shielding requirements (Chapter 9).

PACS and DICOM (Brief)

Modern departments store images in PACS (Picture Archiving and Communication System). DICOM is the standard file format and network protocol. BME staff troubleshoot connectivity (modality → PACS server → workstation), not just hardware. Lossless compression preserves diagnosis; never confuse display JPEG export with primary DICOM storage.

7. Frequently Tested Concepts

The following callouts cover frequently tested imaging concepts.

Rapid Review — High-Yield Items

X-Ray Physics and Fluoroscopy

EXAM CALLOUT — Q21 (exit-0021): Factor that **aggravates anode heel effect?

Answer: Shorter SID (source-to-image distance).

Key insight: Heel effect = intensity falloff toward anode side. Short SID increases divergence → worse gradient. Increase SID; center patient in beam; larger anode angle reduces heel effect.

EXAM CALLOUT — Q444 / Q639 (exit-0444, exit-0639): Bremsstrahlung occurs when?

Answer: High-speed electrons slowed/deflected by target nucleus.

Key insight: Not inner-shell replacement (characteristic X-ray). Not Compton in patient tissue.

EXAM CALLOUT — Q442 / Q637 (exit-0442, exit-0637): Image intensifier primary function?

Answer: Convert X-rays to visible light and amplify brightness.

Key insight: Does not increase photon energy, collimate beam, or cool tube.

EXAM CALLOUT — Q443 / Q638 (exit-0443, exit-0638): Static radiography vs fluoroscopy?

Answer: Fluoroscopy provides real-time dynamic imaging.

Key insight: Both use X-rays. Fluoro often has higher cumulative dose; it does guide interventions.

CT

EXAM CALLOUT — Q696 (exit-0696): CT generation with rotating tube + fixed ring of detectors?

Answer: Fourth generation.

Key insight: Third generation = rotate–rotate (tube AND detectors rotate together). Fourth = rotate–stationary (fixed detector ring). Slip rings enabled helical CT (exit-0640).

EXAM CALLOUT — Q640 (exit-0640): Innovation for axial 3rd-gen → helical CT?

Answer: Slip-ring technology.

EXAM CALLOUT — Q643 (exit-0643): CT often combined with?

Answer: PET (PET-CT hybrid).

MRI

EXAM CALLOUT — Q392 (exit-0392): MRI matrix size determines?

Answer: Spatial resolution (when FOV is fixed).

Key insight: FOV set separately. More matrix points → smaller pixels → finer detail → longer scan.

EXAM CALLOUT — Q602 (exit-0602): MRI magnet cooling agent?

Answer: Liquid helium.

Radiation Physics

EXAM CALLOUT — Q604 (exit-0604): Compton effect shows?

Answer: Photons have momentum.

Key insight: Photon–electron collision; energy and direction change. Dominant scatter in soft tissue at diagnostic energies.

Clinical Selection and Cross-Cutting

EXAM CALLOUT — Q504 (exit-0504): 20-year-old bloody diarrhea — first for IBD suspicion?

Answer: Colonoscopy (direct visualization + biopsy).

Key insight: Not abdominal US, PET, or MRI colon as first-line.

EXAM CALLOUT — Q428 (exit-0428): Perfusion refers to?

Answer: Blood flow through tissues.

EXAM CALLOUT — Q28 (exit-0028): Recurrent fund covers all EXCEPT?

Answer: Expenditure for replacing equipment (capital, not recurrent).

EXAM CALLOUT — Q49 (exit-0049): Microscope type different from others?

Answer: Electron microscope (vs optical: phase contrast, fluorescence, dark field).

EXAM CALLOUT (imaging-modalities-exit-2024): Speed of sound in soft tissue ≈ 1540 m/s. Primary ultrasound imaging interaction = reflection at impedance boundaries.

EXAM CALLOUT (imaging-modalities-exit-2024): Higher transducer frequency → better spatial resolution, lower penetration depth. Phased array for cardiac imaging — electronic beam steering without probe movement.

EXAM CALLOUT (imaging-modalities-exit-2024): Doppler ultrasound measures velocity of moving scatterers (blood cells). Reverberation artifact = multiple reflections between strong parallel interfaces.

EXAM CALLOUT (imaging-modalities-exit-2024): Radiography grid reduces scatter radiation reaching detector. Mammography chosen for screening because high soft-tissue resolution at acceptable dose.

Photoelectric / Z Dependence — HIGH-YIELD

EXAM CALLOUT — Contrast agents: Photoelectric absorption probability increases sharply with atomic number — taught as Z³/E³ or simplified Z⁴ for exam contrast between iodine (Z=53) and soft tissue.

Key insight: This is why iodinated IV contrast and barium sulfate opacify vessels and GI lumen on CT and X-ray.

8. Comparison Tables

8.1 Modality Master Comparison

8.2 T1 vs T2 Weighting

8.3 CT Generations

8.4 Ultrasound Imaging Modes

8.5 Nuclear Medicine

8.6 X-Ray Interaction Summary

9. Exam-Oriented Memory Aids

"BRET CP" — X-ray production: Bremsstrahlung (continuous), Characteristic (discrete peaks).

"3rd rotates twice, 4th tube only": 3rd gen = tube + detectors rotate; 4th gen = tube rotates, detectors fixed (Q696).

"Slip ring = spiral": Slip rings → helical CT → volume imaging.

"T1 fat, T2 water": T1W fat bright; T2W water/CSF bright.

"Helium keeps MRI cold": Liquid helium at 4 K for superconducting magnet.

"US speed 1540": Soft tissue sound speed ≈ 1540 m/s — use for depth calculations.

"High freq = high res, low pen": 10 MHz superficial; 2 MHz deep abdomen.

"PET-CT not PET-MRI first": Standard hybrid for oncology = PET-CT (Q643).

"Fluoro = movie": Real-time dynamic X-ray; II converts and amplifies.

"Z four for contrast store": Photoelectric ∝ Z⁴ (simplified) — iodine shines on CT.

"Compton = momentum": Photon–electron scatter proves particle nature.

"Heel hurts when close": Short SID worsens anode heel effect.

Mnemonic — US artifacts: "RC SAM" — Reverberation, Comet tail, Shadow, Acoustic mirror, Motion (edge).

MRI safety zones: "IV is deadly" — Zone IV is magnet room always on.

9.1 Pulse Sequence Quick Reference (MRI)

Spin echo vs gradient echo: SE uses 180° refocusing pulse to reverse dephasing → true T2 weighting. GRE uses gradient reversal — faster but more T2* and susceptibility artifact. Functional MRI and SWI exploit GRE T2* sensitivity.

9.2 Image Quality Metrics (Cross-Modality)

Exit exams reference these terms from lecture assignments on image quality:

Sampling and reconstruction: CT and MRI both rely on sampling k-space or projection space adequately — undersampling causes aliasing (MRI wrap-around when FOV too small).

9.3 Non-Ionizing Radiation Context (Lecture Ch 02)

Ultrasound and MRI are classified as non-ionizing because they do not carry enough energy per photon/phonon to ionize atoms at diagnostic levels. This does not mean zero risk:

• Ultrasound: Mechanical and thermal indices (MI, TI) bound diagnostic output; FDA/CDRH limits apply. Therapeutic US (HIFU) operates at higher intensities for ablation — distinct from diagnostic imaging.
• MRI: Static B₀ and RF fields interact with tissue; SAR limits restrict RF heating; peripheral nerve stimulation at high gradient switching rates in fast sequences.

Clinical implication: Pregnancy imaging defaults to ultrasound for obstetric questions; MRI without contrast is used when US insufficient; CT/X-ray only when benefit clearly outweighs fetal dose.

9.4 Digital Image Representation (Lecture Ch 01)

Pixels store quantized intensity:

• 8-bit grayscale: 256 levels (display standard).
• 12–16 bit acquisition: Preserves dynamic range for windowing (CT HU, MRI ADC).
• RGB color: 24 bits (8 per channel) — used in Doppler overlay and PET fusion displays.
• DICOM: Standard storage format linking pixel data to patient metadata, window/level, slice position.

Windowing (CT): Narrow window (e.g., W=80, L=40) emphasizes soft tissue; wide window (W=2000, L=500) shows bone and lung simultaneously.

10. Chapter Summary

Medical imaging transforms physical signals into diagnostic images. X-ray and CT measure attenuation of ionizing photons — Bremsstrahlung produces the beam; photoelectric effect (Z-dependent) creates contrast; Compton scatter degrades images and proves photon momentum. Fluoroscopy adds real-time capability via image intensifiers or flat panels. CT reconstructs cross-sections from projections; know generations (3rd = rotate–rotate; 4th = fixed detector ring per Q696), slip rings for helical acquisition, and HU scale. MRI uses hydrogen spin in B₀, RF excitation at Larmor frequency, and T1/T2 relaxation for contrast — matrix size sets resolution; liquid helium cools superconducting magnets; safety focuses on projectiles, quench, and implants. Ultrasound uses MHz sound reflections — frequency trades resolution for penetration; Doppler measures velocity; reverberation, shadow, and comet-tail artifacts are exam favorites. PET/SPECT detect radiotracer emissions — PET-CT is the standard hybrid. Clinical selection matters: colonoscopy for suspected IBD, not PET. HTM crosses imaging: recurrent budgets exclude equipment replacement. Master Section 7 EXAM CALLOUTs and practice below until automatic.

11. Exam Practice Section

Sources: Medical Imaging Modalities Exit Q&A (Sept 2024), Pre-tutorial Assessment 2026, Mock Exam 2025/26, Previous Exit Exam AAU.

Basic Questions (10 MCQs)

B1. Typical speed of sound in soft tissue?

A) 1540 m/s
B) 340 m/s
C) 500 m/s
D) 1000 m/s

Answer: A — 1540 m/s is the standard assumption (c ≈ 1540 m/s in soft tissue).

B2. Primary ultrasound–tissue interaction used for imaging?

A) Reflection
B) Absorption
C) Refraction
D) Transmission

Answer: A — Echo imaging depends on reflected energy at impedance boundaries.

B3. Mode used for fetal biometry and gestational age?

A) A-mode
B) B-mode
C) M-mode
D) Doppler

Answer: B — B-mode 2D gray-scale is standard for OB measurements.

B4. Increased transducer frequency effect on spatial resolution?

A) Decreases
B) Increases
C) Unchanged
D) Variable only

Answer: B — Higher frequency → shorter wavelength → better axial/lateral resolution (less penetration).

B5. Cooling agent for superconducting MRI magnet?

A) Argon
B) Neon
C) Helium
D) Nitrogen only

Answer: C — Liquid helium maintains coils below critical temperature (~4 K).

B6. Bremsstrahlung X-rays produced when?

A) Inner-shell electron replaced
B) Patient tissue Compton scatter
C) Electrons decelerate near target nucleus
D) Target melts thermally

Answer: C — "Braking radiation" from tube electron–nucleus interaction.

B7. Purpose of anti-scatter grid in radiography?

A) Increase patient dose
B) Reduce scatter reaching detector
C) Increase kVp
D) Magnify image

Answer: B — Grid absorbs oblique scatter; improves contrast (increases required dose slightly).

B8. MRI phenomenon responsible for detected signal?

A) Nuclear fusion
B) Nuclear fission
C) Nuclear spin relaxation
D) Electron excitation in lens

Answer: C — RF tips spins; relaxation induces detectable RF in coil.

B9. Which uses annihilation photons (511 keV)?

A) SPECT only
B) PET
C) B-mode ultrasound
D) Static radiography

Answer: B — PET detects coincidence 511 keV photons from β⁺ annihilation.

B10. Photoelectric effect probability increases most with?

A) Decreasing atomic number
B) Increasing atomic number (Z)
C) Increasing patient motion
D) Decreasing mAs

Answer: B — ∝ Z³/E³ (≈ Z⁴ for contrast comparisons) — basis of iodine contrast.

Intermediate Questions (10 MCQs)

I1. Image intensifier in C-arm fluoroscopy?

A) Increases X-ray photon energy
B) Converts X-rays to light and amplifies
C) Collimates beam
D) Cools anode

Answer: B

I2. Key difference: fluoroscopy vs static radiography?

A) Fluoro uses gamma rays
B) Fluoro provides real-time dynamic imaging
C) Static always higher total dose
D) Fluoro cannot guide procedures

Answer: B

I3. Compton effect demonstrates?

A) X-rays are waves only
B) Photons have momentum
C) MRI safety limits
D) Ultrasound harmonics

Answer: B

I4. CT Hounsfield units primarily reflect?

A) Patient age
B) Tissue composition and electron density
C) Room temperature
D) Matrix size only

Answer: B

I5. MRI matrix size (256×256) with fixed FOV primarily affects?

A) Field of view
B) Spatial resolution
C) Larmor frequency
D) Cryogen type

Answer: B — More pixels in same FOV → smaller pixel size → higher resolution.

I6. T2-weighted MRI: CSF appears?

A) Dark
B) Bright
C) Isointense to fat
D) Black always

Answer: B — Long T2 of water → bright on T2W.

I7. Phased array transducer advantage in cardiac imaging?

A) Highest frequency always
B) Steered beam without physically moving probe
C) No gel required
D) Uses ionizing radiation

Answer: B

I8. Innovation enabling helical CT?

A) Rotating anode
B) Slip-ring technology
C) MRI gradient coils
D) Image intensifier

Answer: B

I9. CT scan most often combined with which modality in hybrid scanner?

A) Fluoroscopy
B) MRI only
C) PET
D) M-mode ultrasound

Answer: C — PET-CT standard in oncology.

I10. Ultrasound artifact: equally spaced echoes between strong reflectors?

A) Comet tail
B) Reverberation
C) Aliasing
D) Chemical shift

Answer: B

Advanced Questions (10 MCQs)

A1. Fourth-generation CT geometry?

A) Translate–rotate pencil beam
B) Rotating tube, fixed detector ring
C) Stationary tube and detector
D) No detectors

Answer: B — exit-0696.

A2. Third-generation CT geometry?

A) Pencil beam single detector
B) Rotating tube AND rotating detector array together
C) Fixed tube only
D) Ultrasound phased array

Answer: B — Rotate–rotate; distinguish from 4th gen.

A3. Factor aggravating anode heel effect?

A) Longer SID
B) Shorter SID
C) Smaller anode angle
D) Higher grid ratio only

Answer: B — exit-0021.

A4. 20-year-old with bloody diarrhea — first study for suspected IBD?

A) Abdominal ultrasound
B) PET
C) MRI colonography
D) Colonoscopy

Answer: D — exit-0504; mucosal view + biopsy.

A5. BOLD fMRI detects?

A) Bone density
B) Blood oxygenation changes
C) Gallstones
D) Radiofrequency cryogen level

Answer: B — Deoxyhemoglobin paramagnetic → T2* changes.

A6. Doppler ultrasound at 90° insonation angle to flow?

A) Maximum velocity measured
B) No Doppler shift
C) Doubled frequency
D) PET coincidence loss

Answer: B — cos(90°) = 0 → no component of flow toward transducer.

A7. CR vs DR radiography — correct distinction?

A) CR uses film; DR uses film
B) CR uses photostimulable phosphor plate read by laser; DR uses direct digital detector
C) CR has no digital output
D) DR cannot do chest X-ray

Answer: B — Exit Q&A #10.

A8. Automatic exposure control on CT primarily?

A) Replaces radiologist
B) Modulates tube current based on patient size/attenuation
C) Eliminates all radiation
D) Converts MRI to CT

Answer: B — Exit Q&A #13.

A9. Microscope different from phase contrast, fluorescence, dark field?

A) Electron microscope
B) Light microscope
C) All use electrons
D) All identical

Answer: A — exit-0049; EM uses electron beam, not photons.

A10. Recurrent equipment fund does NOT cover?

A) Consumables
B) Spare parts
C) Ongoing training
D) Replacing entire imaging system

Answer: D — exit-0028; capital expenditure.

Short Answer Questions (10)

SA1. State the Larmor equation and give ¹H frequency at 1.5 T.

Answer: f = (γ/2π) B₀. For ¹H, γ/2π ≈ 42.58 MHz/T → f ≈ 63.9 MHz at 1.5 T.

SA2. Define Hounsfield unit and give HU for air, water, and cortical bone.

Answer: HU = 1000 × (μ − μ_water)/(μ_water − μ_air). Air ≈ −1000; water = 0; cortical bone ≈ +400 to +1000.

SA3. List three MRI scanner main components and one function each.

Answer: Main magnet (B₀); gradient coils (spatial encoding); RF coil (transmit/receive); any one correct trio accepted.

SA4. Explain anode heel effect and one method to reduce it.

Answer: Lower intensity on anode side due to self-attenuation in target; reduce by increasing SID, centering field, or larger anode angle.

SA5. Name four ultrasound artifacts and one cause each.

Answer: Reverberation (multiple reflections); acoustic shadow (bone/gas); comet tail (ring-down at calcifications); mirror artifact (strong reflector).

SA6. Distinguish photoelectric effect and Compton scatter at diagnostic energies.

Answer: Photoelectric: photon absorbed, Z-dependent, dominant for high-Z at low keV. Compton: photon scatters from electron, energy and direction change, dominant in soft tissue; demonstrates photon momentum.

SA7. What is perfusion?

Answer: Blood flow delivery to tissue, typically mL/min/100 g tissue (exit-0428).

SA8. Why is iodine an effective CT/radiography contrast agent?

Answer: High atomic number (Z = 53) → high photoelectric absorption → increased attenuation vs soft tissue.

SA9. State three MRI safety hazards.

Answer: Projectile effect (ferromagnetic objects); RF burns (conducting loops); quench (helium release); acoustic noise; any three.

SA10. Compare PET and SPECT detection principle.

Answer: PET: coincidence detection of 511 keV annihilation photon pairs from β⁺ emitters. SPECT: single gamma photons through collimated rotating camera from gamma emitters.

Scenario-Based Questions (10)

SC1. Trauma patient, suspected internal bleeding, hemodynamically unstable. Fastest imaging?

Answer: CT (whole-body trauma protocol) — speed, detects hemorrhage and fractures. MRI too slow; US limited for retroperitoneal hemorrhage.

SC2. 32-week pregnancy, assess fetal growth. Modality?

Answer: Ultrasound B-mode — no ionizing radiation; biometry standard.

SC3. Knee injury, suspected ACL tear. Best modality?

Answer: MRI — superior soft-tissue contrast for ligaments and menisci.

SC4. Technologist notes darker image on anode side of chest radiograph. Cause and fix?

Answer: Anode heel effect — increase SID, ensure patient centered, verify anode–cathode orientation (heart under cathode side traditionally).

SC5. Oncology follow-up after treatment — assess metabolic response of known lung mass.

Answer: FDG PET-CT — functional uptake + anatomical localization.

SC6. Patient with pacemaker (non-MR-conditional) needs brain imaging for stroke.

Answer: CT typically — MRI contraindicated unless specialized MR-conditional device and protocol. US not useful for brain parenchyma.

SC7. Gallstone suspected in right upper quadrant pain.

Answer: Ultrasound — first-line; stones cast acoustic shadow; no radiation.

SC8. MRI technologist hears loud venting and alarm — magnet room visibility foggy.

Answer: Quench — evacuate; do not enter; ensure quench pipe function; call vendor; no ferromagnetic rescue attempt.

SC9. Hospital finance: annual ultrasound gel, probe repairs, technologist training vs new CT purchase.

Answer: Gel, repairs, training = recurrent/operational. New CT = capital (exit-0028).

SC10. Cardiologist needs real-time valve motion during cardiac cycle.

Answer: M-mode ultrasound (or 2D B-mode with cine) — M-mode plots motion along one line vs time.

Calculation Problems

C1. Ultrasound pulse returns echo after 65 μs round-trip in soft tissue. Depth to reflector?

Solution: depth = (c × t) / 2 = (1540 m/s × 65 × 10⁻⁶ s) / 2 = 0.050 m = 5.0 cm.

C2. ¹H MRI at B₀ = 3.0 T. Larmor frequency? (γ/2π = 42.58 MHz/T)

Solution: f = 42.58 × 3.0 = 127.74 MHz.

C3. CT voxel size: FOV = 320 mm, matrix = 512. Pixel size?

Solution: 320/512 = 0.625 mm.

C4. If ultrasound frequency doubles from 3 MHz to 6 MHz, how does wavelength change in same tissue?

Solution: λ = c/f → halving λ when f doubles. Wavelength reduced by factor of 2 (better resolution, worse penetration).

C5. N₁₅ at B₀ = 7 T, |γ| = 4.316 MHz/T (exit-0603 pattern). Larmor frequency?

Solution: f = 4.316 × 7 = 30.212 MHz.

C6. CT tube outputs 200 mA for 2 s rotation at 120 kVp. mAs for that exposure?

Solution: mAs = mA × time = 200 × 2 = 400 mAs. (mAs with kVp sets dose; exam tests mAs = current × time.)

C7. Ultrasound at 5 MHz in tissue (c = 1540 m/s). Wavelength?

Solution: λ = c/f = 1540 / (5 × 10⁶) = 0.308 mm. Element thickness ≈ λ/2 for resonance.

C8. CT FOV = 40 cm, matrix = 512. Pixel size?

Solution: 400 mm / 512 = 0.78 mm.

Additional Rapid-Fire Review (Exit Q&A Bank)

C9. Beer-Lambert: I₀ = 5000 photons, μ = 0.3 cm⁻¹, path x = 3 cm. Find transmitted intensity I.

Solution: I = I₀e^(-μx) = 5000 × e^(-0.9) ≈ 5000 × 0.407 = 2035 photons.

C10. If bone photoelectric attenuation is 81× soft tissue due to Z ratio alone, and Z_bone/Z_soft = 3, verify Z⁴ scaling: 3⁴ = 81.

Solution: Photoelectric probability ∝ Z⁴ at fixed energy — explains why cortical bone and iodine contrast dominate radiographic appearance relative to equal-thickness soft tissue.

Appendix A — Digital Radiography and PACS

CR vs DR

Computed radiography (CR) uses photostimulable phosphor plates read by a separate scanner after exposure. Direct digital radiography (DR) uses flat-panel detectors (amorphous selenium or cesium iodide on TFT array) with immediate digital readout. Both eliminate film processing; DR offers higher detective quantum efficiency and faster workflow.

PACS and DICOM

PACS (Picture Archiving and Communication System) stores and distributes images across the hospital. DICOM is the standard file format and network protocol. BME responsibilities include network uptime, display calibration for diagnostic monitors (consistent grayscale), and integration with RIS/HIS worklists.

Image Quality Metrics

Noise in X-ray imaging follows quantum mottle — fewer photons → grainier image → requires higher mAs (dose trade-off).

Appendix B — CT Dose and Protocol Optimization

CTDIvol (mGy) standardizes dose per slice thickness. DLP (dose-length product, mGy·cm) = CTDIvol × scan length — estimates effective dose when multiplied by k-factor.

Dose reduction strategies:

• Automatic exposure control (modulate mA with patient size)
• Iterative reconstruction (allows lower mA without excessive noise)
• Limit multiphase studies when single phase suffices
• Pediatric protocols with reduced kVp/mA
• Pregnancy: justify CT vs US/MRI alternatives

Iodinated contrast precautions: Screen eGFR for nephropathy risk; premedicate known allergy per protocol; have emergency kit available.

Appendix C — MRI Pulse Sequence Quick Reference

Gadolinium (Gd): Shortens T1 → enhances tissues with leaky vasculature (tumor, inflammation). Contraindicated in severe renal failure (NSF risk).

Appendix D — Ultrasound Artifact Drill (Exit Favorite)

Work through each artifact: name → appearance → cause → fix.

1. Shadowing — dark distal to stone — high attenuation — angle probe, harmonics
2. Enhancement — bright behind cyst — fluid low attenuation — recognize simple cyst
3. Reverberation — parallel lines — multiple reflections — change angle, pressure
4. Mirror — duplicate structure — specular reflector — recognize artifact pattern
5. Speckle — grainy texture — coherent scatter — compounding, spatial filtering
6. Aliasing (Doppler) — color wrap — velocity > Nyquist — lower scale, adjust angle
7. Side lobe — false echoes — off-axis energy — adjust gain, frequency

Frequency rule: High MHz = superficial (thyroid 12 MHz). Low MHz = deep/obese abdomen (3–5 MHz).

Appendix E — Nuclear Medicine Tracer Table

PET-CT fusion rationale: PET shows where metabolism is abnormal; CT shows anatomy and provides attenuation correction map. Without CT, PET alone has poor anatomic localization and inaccurate quantification in dense tissue.

Appendix — High-Yield Topic Study Guide

Write one-sentence answers without looking, then verify Section 7:

1. Q21 — Shorter SID worsens anode heel effect.
2. Q28 — Recurrent fund excludes capital equipment replacement.
3. Q49 — Electron microscope uses electrons; others are optical.
4. Q392 — Matrix size sets resolution at fixed FOV.
5. Q428 — Perfusion = tissue blood flow delivery.
6. Q442/Q637 — Image intensifier converts X-rays to amplified visible light.
7. Q443/Q638 — Fluoroscopy = real-time dynamic X-ray imaging.
8. Q444/Q639 — Bremsstrahlung from electron deceleration at nucleus.
9. Q504 — Suspected IBD → colonoscopy with biopsy first.
10. Q602 — Superconducting MRI cooled by liquid helium.
11. Q604 — Compton scattering demonstrates photon momentum.
12. Q640 — Slip rings enabled helical/spiral CT.
13. Q643 — PET-CT is standard hybrid (CT + PET).
14. Q696 — Fourth-gen CT: rotating tube, fixed detector ring.
15. Q661 — Duplicate heel effect / SID question.
16. Q623 — Perfusion duplicate (blood flow, not ventilation).

Appendix G — Study Workflow

Week 1: X-ray physics (Bremsstrahlung, photoelectric Z⁴, Compton, heel effect, II function). Drill B1–B10.

Week 2: CT generations, slip rings, HU scale, metal artifacts. MRI Larmor, T1/T2, matrix, helium, zones. Drill I1–I10.

Week 3: Ultrasound artifacts and frequency selection. PET/SPECT/PET-CT. Drill A1–A10 and SAQs.

Week 4: the high-yield appendix daily. Scenarios SC1–SC10 timed. Night-before: review Section 7 EXAM CALLOUT table.

Day-before checklist:

• Third vs fourth CT generation geometry?
• T1 fat bright vs T2 water bright?
• Image intensifier function?
• Larmor f at 1.5 T and 3 T for ¹H?
• Three ultrasound artifacts?
• Why no metal in MRI Zone IV?
• Recurrent vs capital budget?
• Slip-ring role in helical CT?