MRI 001—Week 2 Lecture Notes (Instrumentation & Magnetism)

Class Logistics and Housekeeping

• Meeting opened with informal check-in (hurricane damage, power outages in FL & Carolinas).
• Lecture time: scheduled to end at 12 PM ET; usually finishes early unless syllabus items.
• Recording started and shared later on Moodle.
• Instructor habit: begins each session with an open Q&A on quizzes, due-dates, technology.
• Grading status
  • Discussion 1 already partially graded; remaining posts graded after class.
  • Feedback is written in Moodle gradebook; if major gaps → invitation to office hours.
• Upload problems: if Moodle “upload” tab fails, email the PDF directly.
• Quiz review restrictions
  • System shows points only (no full re-view) to protect the item bank.
  • Individual quiz review possible during office hours or at end of lecture (most-missed items).
• Calendar vs. Moodle homepage discrepancies
  • Trust the calendar embedded in the course site (Assignment 2, Quiz 2, Discussion 2 all due 10 Oct @ 23:59).
• Proctorio troubleshooting: stay after class; instructor can read back-end data; lenient on week-1/2 technical hiccups.
• Office-hour link posted on Moodle (Calendly style); cancel slots if no longer needed.

Course Schedule Snapshot

• Week 2 deliverables (all due 10 Oct):
  • Assignment 2 (short-answer worksheet).
  • Quiz 2 (content from Ch. 9 + lecture slides).
  • Discussion 2 (RF topic).
• Forthcoming reading prompts (examples):
  • MRI in Practice Ch. 1 (pp 11-20) & Ch. 2 (pp 20-22).
  • Today’s focus: Ch. 9 Instrumentation & Equipment (4th ed. pp 307-322; page numbers vary in 5th ed.).

Teaching Style & Learning Resources

• Multiple modalities—reading, handwriting, live demos, videos, hands-on drawings.
• Students encouraged to pre-read to guide note-taking.
• Ebook access: some embedded animations prompt for a 5th-ed code; instructor will inquire with IT/Bob.

MRI Hardware Overview

• To create an MR image, five major subsystems must work together:
  1. Main Magnet (B_0)
  2. Radio-Frequency (RF) Source & Coils
  3. Gradient System
  4. Computer / Host Console
  5. Image Processor

1. Main Magnet (B_0)

• Synonyms written in class:
  • Main magnet = giant machine in Zone 4 = $B_0$ = field strength $0.2\,\text{T} - 7\,\text{T}$ (clinical)
• Sole job: align nuclear magnetic moments.
• Rule of thumb: stronger magnet → higher signal-to-noise ratio (SNR) → better image quality.
• Quick note for quiz: $\text{Higher }B_0 \Rightarrow \uparrow SNR$.

2. Radio-Frequency (RF) Source / Coils

• Mnemonic written in notes: RF Source = Coils.
• Dual functions:
  1. Excite nuclei (perturbation).
  2. Receive emitted signal (listen).
• Physical examples: head “helmet”, knee coil, abdomen array, extremity cylindrical coil.

3. Gradient System

• Write-up in notes: Gradient System = spatial encoding.
  • Locates voxels in 3-D space (frequency, phase, slice-select).
• Additional function: de-phase & re-phase nuclei (crucial for contrast).

4. Computer / Host

• “User interface” for tech; enters demographics, protocols, sequence parameters.
• Sends commands to gradient/RF amplifiers.

5. Image Processor / Array Processor

• Converts raw k-space signals → final images (FFT, filtering, reconstruction).

Shims (brief intro)

• Purpose: maintain $B_0$ homogeneity.
  • Passive shims = small ferromagnetic blocks fixed during install.
  • Active shims = secondary coils that fine-tune the field dynamically.
• Quiz hint: “Homogeneity is affected by shimming” → True.

Atomic & Magnetic Fundamentals

Structure Reminder

• Nucleus: protons (+) & neutrons (0).
• Electrons (−) orbit in shells; exhibit:
  1. Orbital motion.
  2. Spin about own axis.
• According to Faraday’s Law of Electromagnetic Induction: moving charge + motion ⇒ magnetic field.
• Therefore all atoms are tiny magnets; degree varies = magnetic susceptibility.

Categories of Magnetic Susceptibility

Types of Clinical MRI Magnets

Permanent Magnets

• Built from stacked ferrite or rare-earth ceramic “bricks”.
• Very heavy (≈ 15 000 kg ≈ 33 000 lb ≈ 16 tons).
• No electricity / cryogen; always ON.
• Open C-shaped design; field strengths $0.2\text{–}1.0\,\text{T}$.
• Pros: low operating cost, claustrophobia-friendly, reduced fringe.
• Cons: limited SNR; installation floor-loading issues.

Electromagnets

a) Solenoid / Resistive

• Copper wire wrapped around iron core; requires continuous current.
• Generates heat (Ohmic resistance) & high power bill.
• Field strength up to $\sim1.5\,\text{T}$; can be powered OFF (field collapses gradually).

b) Superconductive (dominant in modern imaging)

• Coils of niobium–titanium kept below $4.2\,\text{K}$ in liquid helium bath.
• Zero resistance → high $B_0$ (clinical 1.5 T, 3 T; research 7 T–14 T).
• Requires initial ramp-up power; then minimal energy to maintain.
• Characteristic “bird-chirp” sound = helium cryo-cooler.

Hybrid Concepts

• Combines permanent + actively shielded superconductive elements for specialized installs.

Magnetic Field Containment

• Passive shielding: thick steel plates & copper RF cage in walls/ceiling/floor.
• Active shielding: secondary coils energized to return flux inward, shrinking fringe.
• Flux lines: visual map of field strength emanating from isocenter.
• Fringe field: portion of $B_0$ extending into room; decreases with distance.
• 5 Gauss (0.5 mT) line: regulatory safety boundary where pacemaker interference risk falls to that of Earth’s field. Instructor caveat: do not treat line as “safe to park metal”; screen all objects.
• Doors should remain closed to preserve shielding integrity.

Safety & Zones (preview)

• Zone 3/4 access controlled; wheelchair, O_2 tank, tools must bear green “MR Safe” label before entry.
• Large ferromagnetic items accelerate toward isocenter → projectile hazard (“missile”).

Coil Examples & Imaging Set-ups

• Head transmit-receive birdcage.
• 8-ch knee/foot hybrid.
• Extremity “barrel” scanner (low-field).
• Coils act like camera lenses; choose closest fit for max SNR.

Study & Assessment Tips

• Quizzes pull directly from slide bullets marked in yellow; one or two items may test slide details not stated verbally.
• Read corresponding chapter for context; highlight instructor-flagged paragraphs.
• Example missed Q’s from Quiz 1:
  1. “MRI is best for lung density/volume.” → False (CT/X-ray superior; lungs low in H and full of O₂ – weak MR signal).
  2. Larmor-equation pioneer → Joseph Larmor (slide reference).
  3. Faraday’s law MCQ: electron in field will move (induce current).
• Equation worth memorizing for future weeks: $\omega<em>0 = \gamma B</em>0$ (Larmor).

Common Technical/Administrative Issues & Instructor Solutions

• Proctorio “extension required” or Chrome install failures → email instructor; alternate browsers, version update, loaner PC.
• Ebook animation password prompt: linked to 5th-ed access code; IT investigating.
• Older MacBook (2012) may not meet Proctorio hardware cut-off; consider OS 10.15+ or affordable replacement (open-box Best Buy, Costco return policy).
• Cancelling Calendly slots: student must manually delete if not attending.

Ethical & Practical Implications Discussed

• Helium scarcity: vendors exploring closed-loop cryocoolers, nitrogen, high-Tc superconductors.
• MRI predicted to gain market share over CT as AI-accelerated sequences cut scan times (stroke protocol ↓ to 5 min).

Breaks & Timing

• 09:55–10:05 AM: first 10-min break.
• 11:00–11:10 AM: second break before slide deck.

Next Steps for Students

• Finish reading Ch. 9 and slides before Quiz 2.
• Work on Assignment 2 worksheet; keep answers concise (can be bullet list).
• Post on Discussion 2 (RF topic) by 10 Oct; respond to two peers.
• Book office hours for quiz item review or tech help.
• Optional: share useful YouTube explanations in General Forum (instructor will vet).