A handheld MRI, from first principles.
Place a one-sided magnetic resonance probe against a wrist. Watch its useful field volume shift with distance, tune the RF resonance, encode a shallow tissue slice, then reconstruct the signal from sampled k-space. Hardware concepts and the limitations are part of the experiment.
Single-sided MR has experimental precedents; these rendered images are synthetic, never measurements of a person.
Position the sensing head
The coil and magnets remain outside the tissue. Move the probe laterally, vary the gap, and select a resonant depth in front of it.
Engineer the field and pulse
A front-facing sensor uses an inhomogeneous static field. Changing its depth shifts the proton resonance frequency. An RF pulse excites a thin sensitive region only when its bandwidth and tuning allow it.
f₀(z) = γ̅·B₀(z)
B₀(z) ≈ Bface / (1 + z/L)²
Δz ≈ BW / (γ̅ |∂B/∂z|)
The probe's physical stack
Select any subsystem to highlight it in 3D and inspect what an experimental platform would need.
Magnetic assembly
Geometry-specific mapping and validation required.
What would it actually detect?
Near-surface MR relaxometry is more achievable than a deep, fast diagnostic 2D MRI. Imaging multiple pixels requires spatial encoding, stable timing and substantially more signal.
Field decay and sensitivity are conceptual analytic curves. The vertical cursor is the selected tissue depth. A tighter RF bandwidth narrows the ideal resonant region but increases pulse complexity.
Spin-echo timing is drawn schematically. The receiver detects the RF voltage induced by transverse proton magnetization; the real experiment would also require T/R switching, amplifiers and synchronized digitization.
Polarize a shallow volume
Permanent magnets generate B₀ outside their front face. Proton spins establish a tiny net magnetization; B₀ varies strongly with distance.
Tune to a depth
RF at f = 42.577 MHz/T × B₀ excites resonant spins. With a depth gradient, RF bandwidth sets the approximate selected layer.
Apply additional gradients
For 2D localization, controlled X/Y gradients assign phase and frequency by position. The built-in depth gradient alone is not enough.
Detect and reconstruct
A sensitive RF coil picks up a tiny spin echo. Repeated encodings fill complex k-space; a 2D inverse FFT reconstructs a synthetic map.
From concept to a meaningful experiment
What the hand-sized form factor costs
The front-facing single-sided architecture is most credible as a near-surface **NMR measurement instrument** first. A 2D MR imaging mode is a harder research milestone, especially for living tissue and motion. Conventional hospital-quality imaging is not implied.
k(kx,ky) = ∫ S(x,y) · e^(−i2π[kx·x+ky·y]) dx dy
Image(x,y) = |FFT₂⁻¹(k)|
| Subsystem | Candidate hardware / material | Function | Main validation need |
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