THE BASIC CARTESIAN IDEA
Scan time counts how long each repetition takes and how many repetitions are needed.
In a conventional 2D Cartesian spin-echo acquisition, one phase-encoding line is collected during each TR. The basic estimate is:
Fast spin echo collects several phase-encoding lines after one excitation. For a simple estimate, divide by echo train length:
WORK IT IN SECONDS FIRST
TR 500 ms · 256 phase steps · 2 NEX.
Convert TR to seconds.
Multiply TR, phase steps, and NEX.
4 minutes 16 seconds.
If the same simplified example used FSE with ETL 8, the estimated time would be 256 ÷ 8 = 32 seconds. The scanner may show additional time because real sequences include preparation, calibration, oversampling, dummy cycles, slice packages, acceleration details, and hardware timing.
WHAT CHANGES THE CLOCK?
Separate direct multipliers from acquisition-specific modifiers.
Longer cycles generally lengthen a conventional acquisition, though multislice packing and sequence design matter.
More acquired phase lines generally require more repetitions and improve phase-direction sampling.
Twice the averages usually means roughly twice the averaging time, while SNR rises only by √2.
More lines are acquired each TR in FSE, but long trains can change blurring, contrast, SAR, and available slices.
Partial Fourier · parallel imaging · compressed sensing · phase oversampling · number of 3D slice encodes · concatenations or packages · cardiac or respiratory gating · averages by direction · preparation pulses · sequence-specific delays
The displayed scan time is the practical authority.
EPI, radial and spiral imaging, 3D acquisitions, simultaneous multislice, gated exams, dynamic imaging, and advanced reconstructions do not always follow the simple 2D formula. Use it to understand direction of change, not to override the scanner or an approved protocol.
THE BUS ROUTE
TR is the trip, phase lines are the passengers, and ETL is the number of seats filled per trip.
NEX sends the bus through the route again. A longer trip, more passengers, or more repeats takes longer. Carrying more passengers per trip finishes sooner.
TR sets the basic repetition interval.
Phase encodes and NEX add repetitions.
ETL collects more lines each TR.
Remember: Time per trip × trips needed ÷ work done per trip.
CHECK YOUR UNDERSTANDING
Predict the direction before calculating.
A basic spin-echo scan uses TR 600 ms, 200 phase steps, and 1 NEX. What is the estimate?
0.6 seconds × 200 × 1 = 120 seconds, or 2 minutes.
If NEX changes from 1 to 2 and everything else stays fixed, what happens?
The averaging portion of scan time approximately doubles. SNR increases by about √2, or 1.41 times.
Why may the scanner time differ from the hand calculation?
The scanner includes the sequence's actual timing, calibration, oversampling, acceleration, slice organization, preparation pulses, gating, and other implementation details that the basic equation leaves out.
LESSON 27 COMPLETE
You can estimate basic scan time and recognize when a fuller model is needed.Educational references
These formulas are simplified teaching models for basic Cartesian acquisitions. Always use the scanner's calculated time and approved protocol for clinical work.