THREE DIFFERENT JOBS
NEX repeats. Bandwidth listens. Partial Fourier estimates missing data.
NEX, also called NSA or averages, repeats acquired signal and combines the measurements. Random noise varies while consistent signal remains, so averaging can improve SNR.
Receiver bandwidth is the range of frequencies sampled during readout. A wider bandwidth admits more noise but can shorten the readout and reduce frequency-related displacement and distortion.
Partial Fourier acquires a little more than half of k-space in one direction and uses symmetry plus phase information to estimate what was not collected.
THE NUMBERS THAT MATTER
The gains are not equal to their costs.
1 average = 1× SNR
2 averages ≈ 1.41× SNR
4 averages = 2× SNR
When other conditions are held constant, doubling bandwidth gives about 0.71× the SNR.
Less data can shorten time, TE, or echo train duration depending on where partial Fourier is applied.
These relationships are teaching approximations. Modern reconstruction, coil combination, sequence design, and vendor implementation can change the exact result.
CHOOSE THE TRADEOFF
Each control solves one problem while changing another.
SNR rises with the square root of averages, while the averaging portion of scan time rises approximately in direct proportion. Patient motion may also be repeated and does not necessarily average away.
Wider receiver bandwidth commonly reduces chemical-shift displacement and off-resonance distortion, and can allow shorter echo spacing or TE. The noise level increases.
Partial Fourier can reduce scan time or echo-train duration, but brings an SNR penalty and may cause smoothing or phase-related artifacts when motion, flow, or field inhomogeneity weakens the assumed symmetry.
Bandwidth may appear as total bandwidth, Hz per pixel, or a vendor-specific setting. Always confirm what the scanner displays before comparing protocols or predicting a change.
THE RECORDING STUDIO
Record again, change the microphone window, or finish from a partial recording.
NEX records another take and averages the noise. Bandwidth changes how wide a frequency window the receiver hears. Partial Fourier records enough of a patterned signal to estimate the missing portion.
More averages raise SNR slowly and time directly.
Wide is faster and less distorted, but noisier.
Acquire less, reconstruct the rest, accept limitations.
Remember: Repeat, receive, or reconstruct.
CHECK YOUR UNDERSTANDING
Match the control to its consequence.
What happens when NEX increases from 1 to 4?
SNR approximately doubles because √4 = 2. The averaging portion of scan time becomes approximately four times longer.
Why might a protocol use wider receiver bandwidth near metal?
Wider bandwidth reduces the distance that off-resonant signal is displaced in the frequency direction and can shorten readout or echo spacing. The tradeoff is lower SNR.
Does partial Fourier collect exactly half of k-space?
No. It generally collects a little more than half, including extra information needed to estimate phase and reconstruct the missing portion.
LESSON 25 COMPLETE
You understand how NEX, bandwidth, and partial Fourier trade signal for time or artifact control.Educational references
Parameter behavior varies with sequence, anatomy, hardware, reconstruction, and vendor implementation. Use approved protocols and scanner-specific guidance.