Computed Tomography Dose Index Estimate Calculator

• Peripheral average: mean of provided peripheral readings.
• Weighted dose index: CTDI_w = (1/3)·CTDI_center + (2/3)·CTDI_{periphery,avg}.
• Volume dose index: CTDI_vol = CTDI_w / pitch (helical). For axial, pitch = 1.
• Dose-length product: DLP = CTDI_vol × scan length.
• Effective dose estimate: E ≈ DLP × k.

Indices depend on phantom size (commonly 16 cm head or 32 cm body) and do not directly represent patient absorbed dose.

1. Choose Helical or Axial mode.
2. Enter the center reading from the phantom measurement.
3. Enter one to four peripheral readings. One value is accepted as an average.
4. For helical scans, enter pitch. Axial scans use pitch 1 automatically.
5. Enter the scan length in centimeters for DLP.
6. Select a region preset or type your own k factor.
7. Click Calculate to view results above the form.
8. Use Download CSV or Download PDF for reports.

Example values illustrate calculations only and are not protocol recommendations.

1) Why CTDI matters for protocol review

Computed Tomography Dose Index (CTDI) is a standardized way to summarize scanner output using test phantoms. It supports protocol comparison, quality assurance, and trend monitoring when techniques change. CTDI values are reported in milligray (mGy) and should be interpreted as indices, not patient absorbed dose.

2) Center and peripheral readings: what they represent

A 100 mm ionization chamber is placed in a cylindrical phantom at the center and at peripheral holes. Center values typically reflect beam penetration and scatter conditions, while peripheral values reflect higher dose near the surface. This calculator accepts one to four peripheral values and uses their mean for the weighted estimate.

3) CTDI_w: combining center and periphery

Weighted CTDI (CTDI_w) summarizes the radial dose distribution with a conventional weighting: one-third from the center and two-thirds from the periphery average. If your periphery readings differ widely, it can indicate nonuniform output, bow-tie filter effects, or measurement setup issues.

4) Helical pitch and CTDI_vol

For helical scanning, pitch describes table travel per rotation relative to beam width. Increasing pitch (greater table travel) generally reduces dose index per slice because the same output is spread over a longer path. CTDI_vol is computed as CTDI_w divided by pitch; axial mode uses pitch = 1 by definition.

5) DLP: adding scan length

Dose Length Product (DLP) extends CTDI_vol by incorporating scan length in centimeters. DLP (mGy·cm) is useful for comparing complete examinations, because two protocols with similar CTDI_vol can produce different total output if scan lengths differ (for example, 20 cm versus 35 cm).

6) Effective dose estimate and region factors

Effective dose is sometimes approximated as E ≈ DLP × k, where k depends on the scanned region and reference population. Typical adult k values are on the order of 0.002–0.016 mSv per mGy·cm, with head generally lower than chest or abdomen. This page provides region presets and allows custom entry when your facility uses different references.

7) Phantom size: 16 cm versus 32 cm

Many scanners report CTDI relative to either a 16 cm (head) or 32 cm (body) phantom. Mixing phantom references can mislead comparisons, because the same technique can produce different displayed indices. When documenting results, record the phantom reference along with k factors and scan length assumptions.

8) Practical checks for safer comparisons

Use consistent units, confirm the scan mode, and verify that pitch matches the protocol sheet. For helical studies, small pitch changes can shift CTDI_vol noticeably. If you have patient size information, consider size-specific dose estimates (SSDE) rather than CTDI alone. For audits, compare medians and interquartile ranges across time, not single outliers.

1) Is CTDI the same as patient dose?

No. CTDI is a standardized phantom-based index of scanner output. Patient absorbed dose varies with body size, anatomy, and technique. Use SSDE or patient-specific tools for closer dose estimates.

2) Why does the calculator weight the periphery more than the center?

CTDI_w uses a conventional weighting: two-thirds peripheral and one-third center. It reflects typical higher dose near the phantom surface and provides a single summary number for comparisons.

3) What happens to CTDI_vol when pitch increases?

For helical mode, CTDI_vol = CTDI_w / pitch. If pitch increases while CTDI_w stays constant, CTDI_vol decreases proportionally.

4) Why do you ask for scan length in centimeters?

DLP uses scan length in cm because DLP = CTDI_vol × length. Keep units consistent so the resulting DLP is in mGy·cm and matches common reporting formats.

5) Can I enter only one peripheral reading?

Yes. The calculator accepts one to four peripheral values and uses the mean of the values provided. Using more readings generally improves representativeness if measurements are available.

6) Are the k-factor presets universal?

No. k factors depend on region definitions, scanner output reference, and population assumptions. Presets are typical adult approximations for quick comparisons. Use your institution’s published factors when available.

7) Which mode should I choose: axial or helical?

Select the mode that matches your protocol. Helical mode uses your pitch value. Axial mode forces pitch = 1 for CTDI_vol, aligning with the standard definition for step-and-shoot acquisitions.