Ultrasound Measurement Techniques

Ultrasound Scanning Techniques

Measurement Techniques in Ultrasound

Principles of Caliper Placement, Standard Imaging Planes, Organ Measurements, Obstetric Biometry, Doppler Measurements, Volume Calculation, and Measurement Accuracy

8.5 Measurement Techniques General Sonography USG Technique Educational Reference

Importance of ultrasound measurement → Accurate measurement is an essential component of diagnostic ultrasound examination. Correct measurements allow the sonographer and interpreting clinician to document the size, thickness, length, diameter, area, volume, and growth of anatomical structures. Measurement should always be performed using an appropriate imaging plane, anatomical landmark, caliper position, magnification, and standardized technique.

Measurements should be reproducible and obtained in a manner that allows comparison with previous and future examinations.

General principles of accurate measurement → Before measuring any structure, optimize the image first. The target anatomy should be clearly visualized with appropriate depth, gain, focus, dynamic range, frequency, and image orientation.

The measurement should then be performed in the appropriate anatomical plane. Calipers should be positioned precisely at the intended anatomical boundaries. Avoid measuring structures that are poorly visualized, obliquely displayed, distorted by compression, or incompletely included within the imaging field.
SonoAcademy hierarchy for accurate measurement → → Patient Preparation
→ Patient Positioning
→ Equipment & Transducer
→ Image Optimization
→ Anatomical Plane
→ Anatomical Landmarks
→ Caliper Placement
→ Measurement Convention
→ Doppler Measurements
→ Error Avoidance
→ Standardization
→ Repeatability/Reproducibility
→ Documentation
→ Quality Control
→ Clinical Interpretation

Caliper placement → Calipers are electronic markers used to define the endpoints or boundaries of a measurement.

The caliper should be placed according to the specific measurement convention being used. Depending on the structure, measurements may be taken outer-to-outer, inner-to-inner, or edge-to-edge. The calipers should not be placed arbitrarily on the screen. They should correspond to clearly identifiable anatomical landmarks.

Distance measurement → Distance measurement determines the linear distance between two selected points.

It is commonly used for:
Organ length
Lesion diameter
Wall thickness
Fetal biometric dimensions
Cervical length
Vessel diameter

The measurement line should generally represent the intended anatomical dimension rather than an oblique dimension caused by incorrect probe orientation.

Length, width, and thickness → Many anatomical structures require measurement in more than one dimension.

Length represents the longest or specified longitudinal dimension.
Width represents the transverse dimension.
Thickness represents the anterior-posterior or short-axis dimension when applicable.

Measurements should be obtained in standardized planes so that the same technique can be repeated during follow-up examinations.

Area measurement → Area measurement is used when the size of an anatomical structure or lesion cannot be adequately represented by a single linear dimension.

An electronic tracing or ellipse can be used to outline the outer boundary of the structure or lesion.

The tracing should follow the visible anatomical margin as accurately as possible and should avoid including surrounding normal tissue.

Volume measurement → Volume is useful for structures that have measurable dimensions in three planes.

A commonly used ellipsoid approximation is based on length × width × height.

Volume ≈ Length × Width × Height × 0.523

This approximation assumes that the measured structure has an approximately ellipsoid shape.

Volume measurements are used in selected examinations of organs, cysts, masses, ovaries, prostate, bladder, and other structures.

Organ measurement technique → Organ measurements should be performed using a standardized anatomical plane. The organ should be completely visualized before placing the calipers.

The measurement should follow the intended long axis rather than an oblique plane.

Representative measurements should be documented with the corresponding image plane and anatomical landmarks.

Liver measurement → Liver size is assessed using an appropriate longitudinal or oblique plane according to the selected standardized technique.

The measurement should include the appropriate hepatic extent and avoid an oblique measurement that falsely increases the apparent size.

The image should clearly demonstrate the anatomical landmarks used for the measurement.

Gallbladder measurement → Gallbladder measurements may include length, transverse dimensions, and wall thickness when clinically indicated.

Wall thickness should be measured in a standardized plane, preferably where the wall is clearly visualized and not distorted by oblique insonation or excessive probe pressure.

Measurements should distinguish the gallbladder wall from adjacent liver or surrounding structures.

Spleen measurement → Splenic length is generally assessed in a plane that demonstrates the spleen along its appropriate long axis.

The spleen should be visualized from one end to the other before placing the measurement calipers.

Avoid oblique measurements that do not represent the intended anatomical dimension.

Kidney measurement → Renal measurements commonly include longitudinal length and transverse dimensions.

The kidney should be demonstrated in its longest appropriate longitudinal plane. The calipers should follow the renal poles rather than an artificially oblique axis.

Renal cortical thickness and other measurements may be obtained when clinically indicated using a standardized technique.

Pancreatic measurement → Pancreatic dimensions may be assessed in appropriate transverse and longitudinal planes.

The measurement should be made perpendicular to the relevant anatomical axis and should avoid inclusion of adjacent vessels, bowel, or surrounding tissue.

Thyroid measurement → Each thyroid lobe may be measured in three dimensions:

Length × width × thickness.

Measurements should be obtained in appropriate longitudinal and transverse planes.

If thyroid volume is calculated, the dimensions can be used with the ellipsoid approximation.

Thyroid lobe volume ≈ Length × Width × Thickness × 0.523

The right and left lobes can be measured separately when required.

Breast lesion measurement → Breast lesions should be measured in the appropriate imaging planes.

Typically, the lesion is documented in two perpendicular planes, with a third dimension obtained when clinically appropriate.

The measurement should include the visible lesion margins without unnecessarily including surrounding tissue.

Lesion orientation, margins, shape, and location should be documented together with the measurements when relevant.

Uterine measurement → Uterine dimensions are commonly assessed in longitudinal and transverse planes.

Depending on the examination, measurements may include:
Uterine length
Anterior-posterior dimension
Transverse width

The uterus should be aligned appropriately before caliper placement.

Endometrial thickness measurement → Endometrial thickness is measured in the mid-sagittal plane when the endometrial stripe is optimally demonstrated.

The measurement should represent the maximum appropriate thickness of the endometrial complex.

The measurement technique should be consistent between examinations to allow reliable follow-up comparison.

Ovarian measurement → Ovarian dimensions are commonly obtained in three orthogonal planes.

The measurements may be used to estimate ovarian volume.

Ovarian volume ≈ Length × Width × Height × 0.523

The calipers should be placed along the visible ovarian boundaries, excluding adjacent structures.

Prostate measurement → Prostate volume can be estimated using three orthogonal dimensions.

The dimensions are obtained in appropriate transverse and longitudinal planes.

Prostate volume ≈ Length × Width × Height × 0.523

Consistent plane selection and caliper placement are important for reliable serial measurements.

Testicular measurement → Each testis should be measured independently. Measurements generally include:

Length
Width
Height / thickness

These dimensions can be used to estimate testicular volume when required.

Testicular volume ≈ Length × Width × Height × 0.523

Both testes should be measured using comparable technique when comparison is clinically relevant.

Obstetric biometric measurements → Fetal biometry requires highly standardized image acquisition and caliper placement.

Common biometric parameters include:
CRL – Crown-rump length
BPD – Biparietal diameter
HC – Head circumference
AC – Abdominal circumference
FL – Femur length

The measurement plane should satisfy the relevant anatomical landmarks before the calipers are placed.

Crown-rump length (CRL) → CRL is obtained in an appropriate fetal sagittal plane. The fetus should be visualized in a suitable position, with the body adequately demonstrated.

The calipers are placed at the appropriate crown and rump endpoints according to the standardized CRL technique.

The measurement should not include the yolk sac, limbs, or other structures outside the intended endpoints.

Biparietal diameter (BPD) → BPD is obtained from an appropriate transverse fetal head plane.

The plane should demonstrate the required anatomical landmarks before the measurement is made.

Caliper placement must follow the selected standardized measurement convention.

Avoid measuring an oblique head plane because it may introduce significant measurement error.

Head circumference (HC) → HC is obtained around the fetal head in the appropriate standardized plane.

The ellipse should follow the outer contour of the skull according to the measurement convention used by the system.

The head should be appropriately centered and not obliquely sectioned.

Abdominal circumference (AC) → AC is obtained in a standardized transverse abdominal plane.

The required anatomical landmarks should be clearly demonstrated before placing the ellipse.

The ellipse should follow the outer abdominal skin contour according to the selected biometric standard.

Incorrect fetal rotation or an oblique abdominal plane can significantly affect the measurement.

Femur length (FL) → Femur length is measured in an appropriate longitudinal plane showing the femoral shaft clearly.

The calipers should be positioned according to the selected biometric convention and should correspond to the intended ossified femoral segment.

The femur should not be measured in an oblique plane.

Estimated fetal weight (EFW) → Estimated fetal weight is calculated using combinations of standardized biometric measurements.

Commonly used parameters include HC, AC, BPD, and FL, depending on the selected formula.

The calculated value is an estimate and depends on the accuracy of the underlying biometric measurements.

Doppler measurements → Doppler measurements require appropriate vessel identification, sample placement, scale, wall filter, gain, and Doppler angle.

Common Doppler measurements include:
PSV – Peak systolic velocity
EDV – End-diastolic velocity
RI – Resistive index
PI – Pulsatility index
S/D ratio – Systolic-to-diastolic ratio

Correct sample-volume placement and appropriate Doppler technique are essential for reliable results.

Doppler index calculations → Doppler indices are derived from measured velocity waveforms.

RI = (PSV − EDV) / PSV

S/D ratio = PSV / EDV

PI is calculated from the pulsatile waveform using the appropriate standardized definition.

Measurements should be obtained from technically adequate waveforms with appropriate insonation conditions.

Amniotic fluid measurement → Amniotic fluid assessment may use standardized methods such as the Amniotic Fluid Index (AFI) or Single Deepest Pocket (SDP).

The measurement should be obtained in an appropriate vertical pocket without including fetal parts or umbilical cord when applying the selected technique.

The same standardized method should be used consistently when serial assessment is required.

Cervical length measurement → Cervical length is measured along the cervical canal using an appropriate imaging plane.

The internal and external os should be identified, and the measurement should follow the appropriate cervical axis.

Probe pressure should be minimized because excessive pressure can alter cervical appearance and potentially affect the measurement.

Lesion and mass measurement → A focal lesion should generally be measured in the planes that best demonstrate its maximum dimensions.

When appropriate, document:
Length
Width
Height

Measurements should be taken perpendicular to one another when three-dimensional assessment is required.

The same technique should be used during follow-up examinations whenever possible.

Wall thickness measurement → Wall thickness measurements should be obtained where the wall is clearly visualized.

The measurement line should be perpendicular to the wall rather than oblique.

Excessive transducer pressure should be avoided when compression could alter the measured thickness.

Common measurement errors → Common technical errors include:

(1) Measuring in an oblique plane.
(2) Incorrect caliper placement.
(3) Including surrounding tissue.
(4) Excluding part of the target structure.
(5) Excessive probe pressure.
(6) Poor image optimization.
(7) Incorrect anatomical landmark selection.
(8) Measuring before the structure is fully visualized.
(9) Using inconsistent measurement planes during follow-up.
(10) Recording measurements without representative images.

Standardization and reproducibility → A good ultrasound measurement should be accurate, reproducible, anatomically appropriate, and technically documented.

During follow-up examinations, use the same or equivalent standardized technique whenever possible.

Differences in patient position, imaging plane, caliper placement, machine settings, and operator technique may produce apparent changes that are not true anatomical changes.

Measurement documentation → Every clinically relevant measurement should be associated with an appropriate representative image whenever required.

Documentation should include the structure measured, measurement value, units, imaging plane, and relevant anatomical context.

Serial measurements should be recorded in a consistent format to facilitate comparison.

Systematic measurement sequence → A systematic measurement workflow can be remembered as:

Identify → Optimize → Orient → Visualize → Select Plane → Place Calipers → Measure → Verify → Document

First identify the target structure. Optimize the image and select the appropriate imaging plane. Visualize the complete anatomy and identify the correct landmarks. Place the calipers precisely according to the measurement convention. Verify that the measurement represents the intended anatomical dimension. Finally, document the measurement with an appropriate representative image.

Quick academic classification → Basic measurements
1. Distance
2. Length
3. Width
4. Thickness
5. Area
6. Volume

Common measurement techniques
7. Two-point caliper measurement
8. Ellipse measurement
9. Freehand tracing
10. Three-dimensional measurement

Major applications
11. Abdominal organs
12. Pelvic organs
13. Thyroid
14. Breast
15. Scrotum
16. Obstetric biometry
17. Doppler
18. Amniotic fluid
19. Lesions and masses

Essential measurement principle

Always obtain the measurement in the correct anatomical plane, using the correct landmarks and caliper placement.

If the anatomy is poorly visualized, optimize the image or reposition the patient before measuring.

Core rule: Identify → Optimize → Select the correct plane → Place calipers correctly → Verify → Measure → Document.

Educational note: Measurement conventions can differ between clinical guidelines, examination protocols, equipment manufacturers, and reporting systems. For clinical use, measurements should follow the applicable standardized protocol and local practice requirements.