THE IMAGE GRID
FOV sets the area. Matrix divides it.
Field of view, or FOV, is the anatomical area encoded in an image direction. The acquisition matrix describes how many samples divide that field in the frequency and phase directions.
A pixel is a two-dimensional picture element. A voxel is the three-dimensional volume of tissue represented by an image element. In a basic 2D acquisition, voxel dimensions combine the two in-plane pixel dimensions with slice thickness.
CALCULATE NOMINAL SIZE
Divide each FOV direction by its acquisition matrix.
240 mm FOV ÷ 256 matrix ≈ 0.94 mm
With equal dimensions in both directions and a 4 mm slice, the nominal voxel is approximately 0.94 × 0.94 × 4 mm, or 3.5 mm³.
THE VOXEL TRADEOFF
Smaller voxels improve nominal detail but collect less signal.
Less tissue contributes signal to each voxel, so SNR generally decreases.
More tissue contributes signal, but small structures can blend together through partial volume.
- Increase matrix, same FOV: smaller nominal pixels; phase-direction increases may lengthen scan time.
- Decrease FOV, same matrix: smaller nominal pixels; anatomy outside the encoded FOV may wrap.
- Decrease slice thickness: smaller voxel depth and less partial volume; generally lower SNR.
Nominal pixel or voxel dimensions describe digital sampling. True effective spatial resolution also depends on k-space coverage, filtering, reconstruction, motion, SNR, system performance, and other factors. Interpolation can display more pixels without creating newly acquired detail.
THE ICE-CUBE-TRAY ANALOGY
The tray is the FOV. The compartments are the matrix.
Keep the tray the same size but add more compartments, and each compartment becomes smaller. Make every compartment shallower, and each cube holds less water. MRI voxels behave similarly: smaller dimensions contain less tissue and usually less signal.
Remember: FOV ÷ matrix gives pixel size. Add depth to get voxel size.
CHECK YOUR UNDERSTANDING
Follow the size, signal, and resolution tradeoff.
What is the nominal pixel size for a 240 mm FOV divided by a 256 matrix?
Approximately 0.94 mm in that direction: 240 ÷ 256 = 0.9375 mm.
What usually happens to SNR when voxel size decreases?
SNR generally decreases because a smaller volume of tissue contributes signal to each voxel.
Does interpolation create newly acquired spatial detail?
No. It can make the displayed matrix look smoother or contain more pixels, but it does not replace spatial information that was never acquired.
LESSON 12 COMPLETE
You understand how FOV and matrix shape the voxel.Educational references
This lesson describes nominal acquisition dimensions. Display matrix, interpolation, reconstruction, and effective resolution may differ from the acquired matrix and nominal voxel size.