The sonography registry exam tests your knowledge of normal values, clinical interpretation, and scanning technique under time pressure. Measurement tables are some of the most high-yield topics — they appear across multiple question domains and reward memorization with reliable points.

This study guide covers the most frequently tested ultrasound measurements on the ARDMS SPI and specialty examinations, organized by system so you can study efficiently.

How to Use This Guide

This is a quick-reference study tool, not a replacement for your registry prep books or ultrasound physics course
Focus on the clinical context, not just the numbers — registry questions often ask why a value is abnormal, not just what it is
Use SonoBuddy's measurement tables during clinical shifts to reinforce what you're studying

Vascular Measurements

Abdominal Aorta

Finding	Value
Normal diameter	< 3.0 cm (outer-to-outer)
AAA definition	≥ 3.0 cm
Surgical threshold (men)	≥ 5.5 cm
Surgical threshold (women)	≥ 5.0 cm
Normal waveform	Triphasic

Registry tip: Questions on the aorta commonly test the measurement method (outer-to-outer, not inner lumen) and the distinction between men and women surgical thresholds.

Carotid Artery (SRU 2003 Criteria)

Grade	ICA PSV	ICA/CCA Ratio
Normal / < 50%	< 125 cm/s	< 2.0
50–69%	125–230 cm/s	2.0–4.0
≥ 70%	> 230 cm/s	> 4.0

Registry tip: Know that near-occlusion can have lower-than-expected velocities. This is a classic misleading scenario in vascular questions.

ABI (Ankle-Brachial Index)

ABI Value	Interpretation
> 1.40	Non-compressible vessels (calcified)
1.00 – 1.40	Normal
0.91 – 0.99	Borderline
0.70 – 0.90	Mild PAD
0.40 – 0.69	Moderate PAD
< 0.40	Severe PAD

Registry tip: ABI > 1.40 does not mean normal — it means the vessels are non-compressible (Mönckeberg's arteriosclerosis or severe DM). Toe-brachial index (TBI) is used instead.

Renal Artery Doppler

Parameter	Normal	Abnormal (RAS)
Renal artery PSV	< 180 cm/s	> 180–200 cm/s
Renal-to-aortic ratio (RAR)	< 3.5	> 3.5
Resistive Index (RI)	0.58 – 0.70	> 0.70–0.80
Acceleration time (AT)	< 70 ms	> 100 ms

Abdominal Measurements

Liver

Parameter	Normal Value
Length (MCL)	≤ 15 cm
Portal vein diameter	≤ 13 mm
CBD diameter (< 60 yrs)	≤ 6 mm
CBD diameter (≥ 60 yrs)	≤ 8 mm
CBD (post-cholecystectomy)	≤ 10 mm
Echogenicity	Isoechoic or mildly hyperechoic to right kidney

Gallbladder

Parameter	Normal Value
Length	≤ 10 cm
Width	≤ 4 cm
Wall thickness (fasting)	≤ 3 mm

Spleen

Parameter	Normal Value
Length	≤ 12 cm (some use 13 cm)
Echogenicity	Isoechoic or slightly hyperechoic to liver

Kidneys

Parameter	Normal Value
Length	9 – 12 cm
Cortical thickness	≥ 10 mm
Echogenicity (R)	Hypoechoic compared to liver
Echogenicity (L)	Hypoechoic compared to spleen

Pancreatic Duct

Parameter	Normal Value
Main pancreatic duct	≤ 3 mm
Dilated	> 3 mm
Significantly dilated	> 5 mm

OB/Gynecology Measurements

Uterus (Adult)

Parameter	Normal Range
Length (nulliparous)	6 – 8.5 cm
Length (multiparous)	8 – 10.5 cm
AP diameter	≤ 4 cm
Endometrial thickness (premenopausal, proliferative)	4 – 8 mm
Endometrial thickness (postmenopausal)	≤ 5 mm
Endometrial thickness (postmenopausal + bleeding)	> 4–5 mm = biopsy threshold

Registry tip: Postmenopausal endometrial thickness > 5 mm in a bleeding patient requires biopsy — this is one of the most commonly tested OB/GYN facts.

Ovaries

Parameter	Normal Range
Volume	≤ 10 mL (premenopausal)
Volume (postmenopausal)	≤ 6 mL
Formula	Length × Width × Depth × 0.523

First Trimester

Parameter	Normal Value / Threshold
GS visible (TVUS)	≥ 4.5 weeks
Yolk sac visible	≥ 5.5 weeks
FHR at 7–10 weeks	120 – 175 bpm
CRL accuracy	± 5–7 days
NT window	CRL 45–84 mm
Anembryonic gestation	MSD ≥ 25 mm, no embryo

Amniotic Fluid Index (AFI)

Value	Interpretation
5 – 24 cm	Normal
< 5 cm	Oligohydramnios
> 24 cm	Polyhydramnios
Single deepest pocket < 2 cm	Oligohydramnios (single pocket method)

Thyroid Measurements

Thyroid Lobe Dimensions

Dimension	Normal (Adult)
Length	4 – 6 cm
AP diameter	1.3 – 1.8 cm
Depth	1.5 – 2.0 cm
Isthmus (AP)	≤ 3 mm

Thyroid Volume (Ellipsoid Formula)

Volume = L × W × D × 0.479

Population	Normal Volume
Adult female	6 – 16 mL
Adult male	8 – 20 mL

ACR TI-RADS (Simplified)

TR Level	Points	Risk	FNA Threshold
TR1	0	Benign	No FNA
TR2	2	< 2%	No FNA
TR3	3	~5%	≥ 2.5 cm
TR4	4–6	5–20%	≥ 1.5 cm
TR5	≥ 7	> 20%	≥ 1.0 cm

Physics Concepts Tested Alongside Measurements

Don't neglect physics — it's a large portion of the SPI exam. Key concepts that relate to Doppler measurements:

Concept	Key Fact
Nyquist limit	Maximum unambiguous Doppler frequency = PRF/2
Aliasing	Occurs when true Doppler shift > Nyquist limit
Mirror artifact	High-amplitude reflector causes duplicate image on opposite side
Angle of insonation	Keep < 60° for accurate Doppler velocity measurement
Wall filter	Removes low-frequency, high-amplitude noise (slow-moving vessel walls)

Registry tip: Doppler angle should be ≤ 60°. At 90°, the cos(θ) = 0 and velocity cannot be calculated. This is a frequent physics question.

Registry Exam Tips

Memorize the thresholds, understand the context. Knowing that AAA surgical threshold differs by sex — and why — is more useful than just memorizing 5.5 cm.
Learn the measurement methods, not just the values. Outer-to-outer vs. inner lumen for the aorta, the ellipsoid formula for volumes — these appear as technique questions.
Know what changes interpretation. Post-cholecystectomy CBD, fasting status for gallbladder wall, contralateral disease in carotids — these nuances appear as "which of the following best explains" questions.
Practice with real scans. Registry exam questions are written by practicing sonographers. The scenarios match what you see in the scan room.

Use SonoBuddy as Your Clinical Reference

During clinical rotations, having SonoBuddy on your phone reinforces what you're studying. Every time you look up a measurement during a scan, you're building the same memory you'll use on the exam.

Open SonoBuddy → Measurements for the complete reference tables. Calculators → Resistive Index, Carotid Stenosis, ABI, and Thyroid Volume are all available instantly.

Note: Registry exam content is updated periodically by ARDMS and ARRT. Always confirm current content outlines at ardms.org and arrt.org.

SonoBuddy is a reference tool, not a diagnostic authority. Clinical decisions must involve the ordering provider and interpreting physician.

Sonography Registry Exam Study Guide: Top Measurement Tables You Must Know