6.3 Slice Thickness, Scan Planes & Field of View

Why This Topic Matters

The last three items in ARRT's "Imaging Parameters" list - acquisition slice thickness, the x, y, z planes, and scan field of view (SFOV) - describe how a CT dataset is spatially organized before any postprocessing happens. These concepts also set up two topics tested later in the guide: image reconstruction (Chapter 7) and image display, including display field of view or DFOV (Chapter 8). Getting the acquisition-stage definitions right here prevents a common source of missed points: confusing what happens during scanning with what happens afterward during reconstruction or display.

Acquisition Slice Thickness

Acquisition slice thickness is the thickness of the raw-data slice as determined by the detector row width actually used during the scan (in combination with post-patient/detector collimation). On a scanner with 0.625 mm individual detector rows, the thinnest possible acquisition slice thickness is 0.625 mm; the raw data can also be combined across adjacent rows to acquire thicker slices (for example, combining four 0.625 mm rows to acquire a 2.5 mm slice) when fine z-axis detail is not clinically needed.

The trade-off is fundamental and appears repeatedly on the exam:

• Thinner slices: better z-axis (longitudinal) spatial resolution, less partial volume averaging (blurring caused by different tissue types occupying the same voxel), better detection of small lesions, fine fractures, or subtle nodules - but each thin slice contains fewer photons, so image noise increases unless mAs is raised to compensate.
• Thicker slices: lower noise per slice (more photons averaged into each voxel), but more partial volume averaging blurs small structures and can obscure or misrepresent the true size/shape of small lesions.

Example: A radiologist requests 1 mm reconstructions to characterize a 5 mm ground-glass lung nodule instead of the routine 5 mm slices. The thinner slices reduce partial volume averaging so the nodule's true margins and internal density are visible, at the cost of a noisier-looking individual image - an acceptable trade-off given the diagnostic goal.

The x, y, and z Planes

CT data is organized in three spatial axes:

• x-axis: horizontal, side-to-side (patient's left-right)
• y-axis: vertical, front-to-back (patient's anterior-posterior)
• z-axis: the long axis of the patient and the table, i.e., the direction of table travel through the gantry (head-to-foot, craniocaudal)

Each individual axial slice is a snapshot in the x-y plane at one position along the z-axis. Because CT scans a volume by moving the table along z while the gantry rotates in the x-y plane, "z-axis resolution" is really just another way of describing how thin and how well-sampled the slices are along the length of the patient - directly tied to acquisition slice thickness and pitch (previous section). Understanding this coordinate system also underlies multiplanar reformation (MPR), covered later in the postprocessing section, where sagittal and coronal images are reconstructed by resampling the same volumetric raw data along different plane combinations of x, y, and z.

Scan Field of View (SFOV)

The scan field of view (SFOV) is the physical diameter, measured in the x-y plane, over which the scanner actually collects raw data during acquisition. It is selected by the technologist before scanning starts (for example, a small/head SFOV around 25 cm versus a standard body SFOV around 50 cm) and is distinct from the gantry's physical bore opening, which is simply the maximum aperture the patient fits through.

SFOV must be chosen correctly at acquisition because of a key limitation: raw data is only collected within the selected SFOV circle. Anatomy positioned outside the SFOV is not represented in the raw dataset at all, and this cannot be recovered afterward - unlike the display field of view (DFOV), discussed in the image display section of this guide, which can be adjusted after the scan (by "zooming" the image reconstruction) as long as it stays within the boundaries of the SFOV that was actually acquired.

Choosing a smaller SFOV for smaller anatomy (such as the head, orbits, or temporal bones) concentrates the same reconstruction matrix (commonly 512 x 512 pixels) over a smaller physical area, which produces smaller individual pixels and higher spatial resolution for that anatomy. Choosing an unnecessarily large SFOV for small anatomy spreads the same matrix over more physical area, producing larger pixels and coarser spatial resolution - a preventable image-quality mistake.

Quick Reference

Exam Scenario

A technologist scans a patient's head using the scanner's default 50 cm body SFOV instead of switching to a 25 cm head SFOV. The images demonstrate poorer spatial resolution of fine skull-base and temporal bone detail than expected. What went wrong, and can it be fixed after the fact by adjusting the display? The SFOV was set too large for the anatomy, spreading the fixed reconstruction matrix over unnecessarily large physical area and coarsening pixel size; because SFOV determines what raw data is collected, this cannot be corrected by changing the display field of view afterward - the study would need to be repeated with the correct, smaller SFOV.

Key Traps to Avoid

• Do not confuse acquisition slice thickness (set by detector row combination during scanning) with reconstruction slice thickness (selected afterward from already-acquired raw data, covered in the image reconstruction section).
• Do not confuse SFOV (fixed at acquisition, determines what raw data exists) with DFOV (adjustable afterward, determines what part of the existing data is displayed/magnified).
• Remember the z-axis is the table-travel/long-axis direction, not a synonym for "slice thickness" itself - slice thickness is a measurement along the z-axis, not the axis name.