BUILD THE ACQUIRED VOXEL
FOV sets the area. Matrix divides it. Slice thickness adds depth.
Field of view is the physical area encoded in the image. The acquisition matrix tells MRI how many samples divide that area in the frequency and phase directions. Slice thickness defines the through-plane dimension in a 2D acquisition.
ONE WORKED EXAMPLE
Calculate each dimension separately.
240 mm FOV
÷320 samples
0.75 mm240 mm FOV
÷240 samples
1.00 mmSelected thickness
＝4.00 mm
4.00 mmThis is an anisotropic voxel because its three dimensions are not equal. An isotropic voxel has equal dimensions, such as 1 × 1 × 1 mm, which can support similar spatial detail in different reformatted planes.
WHAT HAPPENS WHEN YOU CHANGE THEM?
Smaller voxels improve detail but collect less signal.
Less coverage; aliasing risk if anatomy extends beyond the encoded FOV.
Lower SNR per voxel; more phase encodes can increase scan time.
Lower SNR; more slices may be needed for the same coverage.
Interpolation or zero filling can create more displayed pixels, but it does not add newly acquired spatial information. Use the acquisition FOV and acquisition matrix when calculating nominal acquired pixel size.
Coverage comes before a pretty number.
A smaller FOV or thinner slice is not automatically better. The exam must still cover the anatomy, maintain adequate SNR, control artifacts, and answer the clinical question. Follow approved protocols and scanner-specific guidance.
THE ICE-CUBE TRAY
The tray is the FOV. The dividers are the matrix. The fill depth is slice thickness.
Keep the same tray but add more dividers, and each cube becomes narrower. Use a shallower fill, and each cube becomes thinner. Every smaller cube contains less volume, just as a smaller MRI voxel contains fewer spins and less signal.
The physical area being encoded.
How many acquired samples divide that area.
The through-plane depth of the voxel.
Remember: Area ÷ divisions, then add depth.
CHECK YOUR UNDERSTANDING
Build the voxel one dimension at a time.
A 240 mm FOV uses a 320 frequency matrix. What is the frequency pixel dimension?
240 ÷ 320 = 0.75 mm.
With FOV held constant, what happens when the acquired matrix increases?
The nominal in-plane pixel dimensions become smaller, which can improve spatial resolution. SNR per voxel falls, and added phase-encoding steps may increase scan time.
Why can a thinner slice reduce partial-volume averaging?
A thinner slice includes less tissue through the plane, so fewer different structures are averaged into one voxel. The tradeoff is less signal per voxel.
LESSON 24 COMPLETE
You can calculate voxel dimensions and explain their major tradeoffs.Educational references
The formulas describe nominal acquired dimensions. Effective spatial resolution also depends on sampling, gradients, sequence timing, filters, reconstruction, motion, contrast, SNR, and system performance.