Axial vs. Helical Scanning & Pitch Mechanics

The table motion distinguishes the acquisition modes

In axial or sequential CT, the table is stationary during an exposure and moves between acquisitions when another location is needed. In helical or spiral CT, rotation and longitudinal table movement occur together, producing a helical source trajectory relative to the patient. Slip-ring technology supports continuous rotation without repeated cable unwinding. The exposure and reconstruction options depend on the scanner.

Axial does not mean that only one thin slice is acquired: a multidetector array can acquire a slab, and a wide-detector system can cover a substantial organ volume without table movement. Helical likewise does not mean that every possible reconstructed image has unlimited spatial resolution. Both modes are constrained by acquired sampling, beam geometry, exposure and reconstruction.

Axial acquisition: strengths and limitations

Sequential imaging can be appropriate for routine head studies, selected cardiac applications and image-guided procedures. A stationary table avoids the helical trajectory during that exposure, but wide axial arrays still have cone-beam geometry. Do not call an axial image inherently free of every interpolation, cone-beam or partial-volume effect.

Between axial acquisitions, the system may need table movement and settling. If repeated breath holds place the anatomy differently, adjacent slabs can be misregistered. The actual interscan delay depends on the hardware and protocol, not one universal 0.75–1.5-second interval. Contrast timing also matters when coverage requires several sequential acquisitions.

Prospective ECG-synchronized axial imaging can expose selected cardiac phases at lower dose than some continuously acquired retrospective protocols. Supported windows may be systolic or diastolic, and padding may be used. An axial wide-volume scanner can also perform selected angiographic acquisitions; CTA is not universally restricted to helical mode.

Helical acquisition and longitudinal reconstruction

Helical scanning provides efficient coverage and supports reconstruction at selected positions within the acquired volume. One breath hold can reduce misregistration compared with multiple separately positioned slabs. Appropriate thin data support multiplanar and 3D views. None of these features guarantees that a small lesion cannot be missed; contrast, resolution, noise and artifacts still matter.

Because measurements are acquired at changing longitudinal positions, the reconstruction must account for the helical trajectory. Legacy single-slice 360-degree linear interpolation uses same-angle measurements one rotation apart. 180-degree interpolation uses complementary-ray information with the applicable fan geometry. Modern multidetector systems use more complex methods. Do not attach one universal percentage of slice broadening to every algorithm or assume that every planar image requires a full 360-degree set collected at one physical z-coordinate.

An overlapping reconstruction interval can improve reformation continuity from the same acquired data. It does not add radiation by itself or create new independently sampled thin detail. Distinguish reconstructed image spacing from the table movement during acquisition.

Beam pitch and legacy detector pitch

For multidetector helical CT, beam pitch is table travel per rotation divided by total nominal acquisition collimation:

$$P=\frac{I}{N\times T}$$

Here I is table movement per rotation, N is the longitudinal acquisition-channel count and T is nominal channel width at isocenter. Pitch is dimensionless because both numerator and denominator are lengths. In a single-slice system the denominator is the nominal slice collimation.

Older terminology sometimes used detector pitch, with travel divided by one detector-row width. If a question explicitly specifies that convention, use it; otherwise identify whether total collimation is given. For four 2.5 mm channels and 10 mm table travel, beam pitch is 10/(4 × 2.5) = 1, while the legacy row-based value is 10/2.5 = 4. Confusing the denominators produces a fourfold numerical error.

Worked speed and coverage examples

A configuration of 64 × 0.625 mm has nominal width 40 mm. At 100 mm/s table speed and 0.5 second rotation time, movement per rotation is 50 mm, and pitch is 50/40 = 1.25. A 300 mm planned range takes approximately 300/100 = 3 seconds of steady table motion, excluding start/end behavior and other workflow time.

For a second example, 128 × 0.6 mm gives 76.8 mm width. At pitch 1.15, travel per rotation is 88.32 mm. With a 0.33-second rotation, speed is 88.32/0.33 = 267.64 mm/s. The arithmetic is hypothetical; the scanner's supported configuration and limits determine whether such a program is available.

If an examination instead asks for table travel, multiply pitch by total collimation. If it asks for table speed, divide that travel by rotation time. Keep mm/rotation and mm/s distinct. A rotation time alone is not a table speed, and a high slice count alone does not state the coverage rate.

Pitch and dose: state what is held constant

Under the applicable helical dose-index definition, CTDIvol = CTDIw/pitch. If mAs per rotation and other relevant factors stay fixed, increasing pitch from 1 to 1.25 reduces the index to 0.80 of its original value. At pitch 0.20, the index is five times the pitch-1 value under those same assumptions.

Automatic output control or an effective-mAs setting can increase tube output when pitch increases, so the actual dose result need not follow that simple comparison. Cardiac modulation can also vary output across the ECG cycle. Reconstructed slice width can remain largely independent of pitch within a modern scanner's validated range. Do not use inverse pitch as an unconditional promise of patient-dose reduction or improved spatial resolution.

Select the mode for the task

Use the approved acquisition that provides the needed coverage, temporal behavior and image quality. A dyspneic patient's short breath hold may favor rapid coverage, while a procedural check may favor a limited stationary acquisition. Inspect for motion, missing coverage and phase timing before deciding whether additional images are needed. If a problem can be solved by a supported reconstruction from available data, that may avoid another exposure.

Reference: AAPM CT terminology lexicon.