Calwarden

Ultrasound Doppler Calculator

Calculate Resistive Index (RI) and S/D Ratio.

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Inputs

cm/s
cm/s

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Results

Resistive Index (RI)
S/D Ratio3.33

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Assumptions

  • The formula assumes the Doppler sample and waveform tracing were acquired correctly and consistently.
  • Interpretive cutoffs vary by organ system, gestational age, transplant status, and institutional protocol.
  • These indices support waveform review but do not establish a diagnosis on their own.

Sensitivity preview

Peak Systolic (PSV) -10%: Resistive Index (RI) 0.67

Peak Systolic (PSV) Current: Resistive Index (RI) 0.7

Peak Systolic (PSV) +10%: Resistive Index (RI) 0.73

Formula: RI = (PSV - EDV) / PSV

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About this Doppler Indices

In Doppler ultrasound, the waveform of blood flow provides critical information about downstream vascular resistance. Two key metrics derived from this waveform are the Resistive Index (RI) and the Systolic/Diastolic (S/D) Ratio.

Who this calculator is for

  • Sonographers, radiology trainees, and clinicians reviewing measured Doppler velocities.
  • People who already have peak systolic and end-diastolic velocities from an ultrasound trace.
  • Learners studying how resistance-related Doppler indices are derived from waveform measurements.

When to use it

  • Use this calculator after sampling a waveform and recording PSV and EDV from the vessel of interest.
  • Use it to double-check RI and S/D calculations in renal, transplant, obstetric, or vascular teaching cases.
  • Use it as an educational aid when discussing how low or absent diastolic flow changes derived indices.

How to use this calculator

  1. Measure the Peak Systolic Velocity (PSV).
  2. Measure the End Diastolic Velocity (EDV).
  3. The tool calculates RI and S/D Ratio.

Key assumptions

  • The formula assumes the Doppler sample and waveform tracing were acquired correctly and consistently.
  • Interpretive cutoffs vary by organ system, gestational age, transplant status, and institutional protocol.
  • These indices support waveform review but do not establish a diagnosis on their own.

Accuracy and testing

Review formula sources, default test cases, assumptions, unit handling, rounding rules, and changelog for this calculator.

View accuracy page

How this calculator works

* Resistive Index (RI) = (PSV - EDV) / PSV * S/D Ratio = PSV / EDV High resistance flow (like in resting muscle) has a low EDV, leading to a high RI. Low resistance flow (like in the brain or kidneys) has a high EDV, leading to a low RI.
RI = (PSV - EDV) / PSV

Inputs explained

Peak Systolic (PSV)

The maximum velocity during the heart's contraction (systole).

End Diastolic (EDV)

The velocity at the end of the heart's relaxation phase (diastole).

Understanding your results

* Renal Example: RI values under about 0.70 are often described as lower resistance, while higher values can prompt closer review. * Obstetric Example: Umbilical artery S/D and RI are interpreted against gestational-age-specific reference ranges and the full clinical context.

Example scenarios

Normal Kidney

PSV 100, EDV 40. RI = 0.60.

High Resistance

PSV 100, EDV 10. RI = 0.90. (Possible rejection or stenosis).

Key Features

  • Calculates RI & S/D
  • Vascular Assessment
  • Simple Interface

Glossary of Terms

Doppler Effect
Change in frequency of a wave due to motion of the source (blood cells).
Stenosis
Narrowing of a blood vessel.