Your ARDMS SPI exam is days away. This SPI last-minute review is not a textbook. It is the short list of formulas, relationships and traps that decide most physics questions, plus a 7-day plan and exam-day tips.
| SPI at a glance | |
|---|---|
| Questions | About 110 multiple-choice questions |
| Time | 2 hours (about 65 seconds per question) |
| Fee | US$275 |
| Passing score | 555 on a 300–700 scale (not a percentage) |
| Rule | Pass SPI and a specialty within 5 years of each other |
A 7-day final plan
| Day | Focus |
|---|---|
| 7–6 days | Formulas and direct/inverse relationships; 50 timed questions a day |
| 5 days | Transducers, beam and resolution (axial, lateral, elevational, temporal) |
| 4 days | Doppler, haemodynamics and aliasing |
| 3 days | Artifacts, bioeffects, safety and quality assurance |
| 2 days | One full 110-question mock in 2 hours; review every mistake |
| 1 day | Light review of this page only, then rest |
The formulas you must know cold
| Formula | Remember |
|---|---|
| Wavelength = propagation speed ÷ frequency | Soft tissue speed 1,540 m/s, so λ (mm) = 1.54 ÷ f (MHz) |
| 13 µs rule | Every 1 cm of depth adds 13 µs of go-return time |
| Spatial pulse length = cycles per pulse × wavelength | Axial resolution = SPL ÷ 2; shorter pulses give better axial resolution |
| Duty factor = pulse duration ÷ PRP | Continuous wave = 100%; imaging is usually under 1% |
| PRF and depth | Deeper imaging means a lower maximum PRF (inverse) |
| Doppler shift = 2 × f₀ × v × cos θ ÷ c | At 90° there is no shift; at 0° it is maximal |
| Nyquist limit = PRF ÷ 2 | Doppler shifts above it alias |
| Impedance Z = density × speed | Reflection needs an impedance mismatch |
| Snell's law | Refraction needs oblique incidence and different speeds |
| Decibels | +3 dB doubles intensity, +10 dB is ×10; −3 dB halves it |
| Attenuation in soft tissue | About 0.5 dB per cm per MHz |
| Reynolds number | Above about 2,000 flow becomes turbulent |
Relationships that win easy marks
- Frequency up: wavelength down, axial resolution better, penetration worse, attenuation higher.
- Depth up: PRF down, frame rate down, Nyquist limit down.
- Focusing: narrows the beam at the focus and improves lateral resolution there; the near zone length increases with larger diameter and higher frequency.
- Things the sonographer can change: output power, gain, TGC, depth, PRF (via depth or scale), focus and sector width. Things set by the source and medium: frequency, period and propagation speed.
- Output power changes patient exposure; receiver gain does not.
Artifacts in one table
| Artifact | Cause | Looks like |
|---|---|---|
| Reverberation / comet tail | Two strong reflectors | Equally spaced parallel lines |
| Mirror image | Strong curved reflector (diaphragm) | Duplicate deep to the reflector |
| Acoustic shadowing | High attenuation (stone, bone) | Dark band behind |
| Posterior enhancement | Low attenuation (fluid) | Bright band behind |
| Refraction / ghosting | Speed change at an oblique interface | Side-by-side duplication |
| Side and grating lobes | Off-axis energy | Echoes in anechoic structures |
| Range ambiguity | PRF too high for the depth | Deep echoes placed too shallow |
| Speed error | Medium slower or faster than 1,540 m/s | Structures placed too deep or too shallow |
| Aliasing | Doppler shift above the Nyquist limit | Wrap-around on spectral and colour Doppler |
Fixing aliasing: raise the scale (PRF), shift the baseline, use a lower-frequency transducer, image shallower, increase the Doppler angle or switch to continuous wave.
Bioeffects and safety
- Thermal index (TI) estimates heating; mechanical index (MI) estimates cavitation risk and rises with higher pressure and lower frequency.
- ALARA: reduce output power first and keep scanning time short, especially in obstetrics.
- Quality assurance phantoms test depth calibration, axial and lateral resolution, dead zone and grey-scale.
1,000 SPI practice questions
The ARDMS Q Bank has 1,000 explained SPI questions with calculations, artifacts and Doppler, plus Abdomen, OB/GYN, Vascular, MSK and Breast pools.
Open the ARDMS Q BankThe day before and exam day
- Stop new topics 24 hours before. Skim your own notes and the tables on this page, then rest.
- Check your Pearson VUE confirmation: test centre address, start time and the exact name on your ID, which must match your registration.
- Arrive 30 minutes early with your valid, government-issued photo ID.
- Use the flag button: answer every question on the first pass, flag the slow ones, then come back. There is no penalty for guessing, so never leave a blank.
- Hotspot questions: read exactly what is asked, then click precisely on the structure or finding.
- If you don't pass: you can reapply after 3 days but must wait 60 days to re-sit, so plan a focused 8-week review of your weak domains.
For the full study plan, see our complete ARDMS SPI exam guide.
Frequently asked questions
What should I study the night before the SPI?
Only quick review: the formulas, the direct and inverse relationships, the artifact table and the aliasing fixes. Avoid new topics and get a full night's sleep.
How many questions are on the SPI and how long is it?
About 110 multiple-choice questions in 2 hours, so roughly 65 seconds per question.
What score do I need to pass the ARDMS SPI?
555 on a 300–700 scale. It is not a percentage and is not curved.
Can I use a calculator on the SPI?
Inteleos provides an on-screen calculator in the exam. Check the current candidate rules from Inteleos and Pearson VUE before your test day.
What are the most tested SPI topics?
Resolution, transducers and beam formation, Doppler and aliasing, artifacts, haemodynamics, bioeffects and quality assurance.
Exam facts from Inteleos (ARDMS) exam pages, checked October 2026. Always confirm current rules on inteleos.org. Radiology Boards is not affiliated with ARDMS or Inteleos.