Theme 01 · Foundations
Explore the complete MRI exam, an interactive 3D scanner cutaway, proton signal sources, and physical versus logical coordinates.
Interactive MRI physics00 — 09
Start with RF excitation and relaxation, then follow gradients as they turn position into frequency and phase—from body and local coils to an oblique trajectory through k-space, quantitative single-shot EPI artifacts, and an image reconstructed line by line.
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The complete simplified patient—head through feet—rests on the table. Table motion changes which anatomy is near isocenter; it does not move through k-space and does not electronically select a slice.
SOLID physical hardware GLOW / ARROWS invisible field cue PLANE / CLOCKS / BOXES selected region or calculated data cue
At 0%, the coil layers sit where they are installed: concentrically around the bore.
S(k) = ∫ ρeff(r) e−i2π k·r dr
In plain words: treat the excited body as many tiny, equal-size regions. Each region contributes some receive-coil voltage. Gradients give those contributions a calculable phase—where they sit around one repeating cycle. At address k, the receiver adds every region and stores one pair of signed numbers, I and Q, called S(k). That pair describes the whole excited region; it is not one image pixel. Tap the equation for every symbol, unit, zero case, and a live numerical example.SEARCHABLE MRI DICTIONARY · 143 / 143 TERMS HAVE THE COMPLETE BEGINNER EXPLANATION
Every result starts without assumed MRI knowledge, then lets you go deeper into a labeled visual, scanner behavior, clinical consequence, numerical example, and connected terms.
BEGINNER-FIRST LEARNING · CHOOSE HOW FAR TO ZOOM
The same three-level control appears as a short entry card at every major topic. It changes the explanation layer—not the MRI physics or any simulated parameter.
START SIMPLEEach chapter now begins with one plain-language mental model. Use “Zoom in” on any card when you are ready.
THE WHOLE MRI EXAM · NOT ONLY RF, GRADIENTS, OR K-SPACE
An MRI scanner is a coordinated system: safety screening, a continuously energized main magnet, a moving patient table, shim and gradient coils, RF transmit/receive, monitoring, sequence control, digitization, corrections, reconstruction, storage, and display.
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WHAT THIS VIEW CURRENTLY MEANSThe patient, table, local coil, and selected landmark are one moving setup. The magnet housing and yellow isocenter marker stay fixed.
SOLID SHAPES physical patient or hardwareGLOW / ARROWS / PLANE field, projected positioning-laser light, or selected-region cue—read its labelMOVING SQUARES stored-data flow—not particles traveling through the patient
Technologists screen the patient and every entering object, position anatomy and coils, choose a landmark, provide hearing protection and an alarm device, and use MR-conditional monitoring or gating when required.
The main magnet supplies B₀ continuously. The cryostat thermally supports the superconducting system; site infrastructure, shielding, and emergency procedures manage fields and rare abnormal events.
Fast survey images establish geometry. Field mapping and shim adjustments improve uniformity; reference scans can estimate coil sensitivity, center frequency, transmit behavior, and reconstruction corrections.
The controller schedules RF synthesizers/amplifiers, gradient amplifiers, transmit/receive protection, receiver bandwidth, and ADC sampling. Timing—not a single component—defines the acquisition.
Raw multi-channel I/Q data may be corrected for sampling and hardware behavior, calibrated, reconstructed, combined across coils, filtered, scaled, and packaged with geometry and protocol metadata.
The console and downstream systems show reconstructed images with orientation, scaling, annotations, and metadata. A displayed gray level is the end of a long weighted chain—not a direct photograph or universal tissue unit.
ρ EXPLAINED WITHOUT HIDDEN FACTORS · SOURCE → ECHO → COIL → IMAGE
Textbooks often reuse ρ for both proton density and the already-weighted signal distribution. This lab keeps those quantities separate, names every factor in the teaching product, and states the reference behind every “relative” number.
Most clinical proton MRI signal comes from hydrogen nuclei in mobile water and fat. Hydrogen locked in very rigid material can lose transverse coherence before the receiver can sample it; air has very few hydrogen nuclei. “Visible” never means visible light.
Here, an equal-size reference voxel with ρH = 1.00, complete recovery, no T₂* loss, a 90° excitation, and receive sensitivity 1.00 has ρeff = 1.00. Scanner gain and display windowing can rescale all image numbers, so there is no universal brightness or volt value.
One k-space measurement is not a pixel. After all complex samples are reconstructed, the ideal local complex value is proportional to ρeff. A magnitude image displays its size, usually after coil combination, scaling, filtering, and window/level.
Local mobile-¹H source, one T₁ recovery term, one T₂* survival term, ideal sin α excitation, and one relative receive-sensitivity number.
Flow/inflow, diffusion gradients, magnetization transfer, chemical exchange, contrast agents, fat/water phase, B₀ and B₁ nonuniformity, motion, multi-echo history, receive-channel combination, filters, noise, gain, and display window/level. These are named here rather than hidden inside “other factors,” but they are deliberately held out of this five-factor lab.
Brighter does not automatically mean “more protons.” First ask whether sequence timing, excitation, coil position, pathology, reconstruction, or display scaling also changed.
COORDINATE PRIMER · FIX THE NAMES BEFORE ENCODING
An axial example often pairs read with Gx, phase with Gy, and slice with Gz—but that pairing is not a law. Rotate the prescribed image plane and the scanner synthesizes each logical job by firing two or three physical gradient coils together.
WHY CARE · Confusing these naming systems can make an oblique image plane, artifact direction, or amplifier limit look wrong.[Gx Gy Gz]ᵀ = [+1.000 0.000 0.000]ᵀ × +40.0 mT/m
Each coefficient is a unitless direction cosine. Superscript ᵀ means “write this row as a column vector” (transpose)—it does not mean tesla here. Multiplying by +40.0 mT/m gives a real physical-coil command.STEP 1
The scanner’s main field aligns proton magnetization. Gradient coils add controlled spatial slopes.