07 MRI
Dwell time and readout duration
Δt = 1 / BW_full. T_read = N Δt. Longer readouts = more distortion and T2* decay.
Listen
Listen · English
Simulation
Dwell time and readout duration — Change the numbers; the scene follows.
Where it works
MRI scanner
RF coil
At the RF coil — flip angle, SAR, SNR, receive bandwidth, and the pulse sequence.
Open this machineFormula
Variables
Results
Δt
Dwell time
20µs
T_read
Readout duration
5.12ms
Explanation
What it means
Dwell time is the interval between ADC samples. A 50 kHz full bandwidth is a 20 μs dwell; 256 samples take 5.1 ms of readout. During those 5.1 ms the T2* clock is running (blurring) and off-resonance phase is accumulating (distortion, especially in EPI where the effective echo spacing in the phase-encode direction is much longer). This is a working relation in MRI physics.
Where it is used
Clinically it sits on the MRI scanner — RF coil. At the RF coil — flip angle, SAR, SNR, receive bandwidth, and the pulse sequence. MRI physics lives in the magnet, the gradient, and the voxel: Larmor, Ernst, diffusion, and SAR. These relations decide whether a sequence is possible, safe, and worth the time.
MRI scanner · Open this machineHow to use it
Enter full BW (kHz) and N. Read dwell in μs and readout window in ms. Pair with the pixel-bandwidth calculator for chemical shift. Change one input and watch the curve and the simulation follow.
Symbols
- BWFull bandwidth50 kHz
- NSamples256
Worked example
A typical case from the default values: BW = 50 kHz (Full bandwidth); N = 256 (Samples). Substituting into the relation gives Δt = 20 µs; T_read = 5.12 ms. These are teaching numbers — align them with your machine.
Typical values give
- Δt = 20µs
- T_read = 5.12ms
Where it comes from
The displayed formula is the working relation. Δt = 1 / BW_full. T_read = N Δt. Longer readouts = more distortion and T2* decay. Usual reference: McRobbie. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: McRobbie
Assumptions & limits
Uniform sampling, no ramp sampling, no partial Fourier. EPI echo spacing is this dwell × (R_pe / echo-train undersampling) and is the number that predicts geometric distortion.
Pitfalls
γ for ¹H is not γ for ¹³C. Ernst angle needs the true T1 at that field, not a 1.5 T table used at 3 T. SAR scales with B₀² and flip² — a 3 T copy of a 1.5 T protocol is not automatically legal. Uniform sampling, no ramp sampling, no partial Fourier. EPI echo spacing is this dwell × (R_pe / echo-train undersampling) and is the number that predicts geometric distortion.
Keep this
Name the nucleus and the field before you quote a Larmor frequency. Uniform sampling, no ramp sampling, no partial Fourier. EPI echo spacing is this dwell × (R_pe / echo-train undersampling) and is the number that predicts geometric distortion.
In this specialty