Measurements

Editor's Note: Much of the information in this chapter is to provide you an understand on how the measure, calculate and assess in general terms. In the following chapters, this information will be repeated and tailored to the specific chapter. This chapter will provide you with a good foundation so when you see the information presented in a specific chapter your understanding will be more clear.

Introduction

Echocardiography calculations give the echocardiographer a value that can indicate mild, moderate, or severe diease or the absence of disease (normal range). Echocardigraphic derrived calculations involves a measurement of a length, area, or volume. Many of the measurements are indexed to the body surface area (BSA). Since the heart is in constant motion, some of the measurements involve taking the measurements at the correct time in the cardiac cycle.

Assessments by measurements and calculations can be qualitative, semi-qualitative, or quantitative. Qualitative assessment are made by visualizing the abnormality but do not involve any measurement. A floppy valve that is prolapse or flail valve would be a qualitative assessment where the amount of prolapse or flail is typically not measured. A semi-qualitative assessment is a measurement that indicates severity but may not be entirely accurate. For example, the size of the mitral valve regugitant jet area can be measured. The jet area can correlate with the severity of the mitral regurgitation but is not a direct measurement of the hole or defect in the mitral valve. Many factors can affect the size of the jet area (color Doppler Nyquist limit, color gain, parallel ultrasound beam angle, ejection fraction, left atrial pressure, systemic vascular resistance or blood pressure, etc). Quantitative measurements and calculations based upon quantitative measurements give direct evidence of the severity of disease, assuming the measurements are correct. The vena contracta (VC) or the regurgitant orifice area (ROA) are more quantitative. However, please realize that the vena contracta and regurgitant orifice area are affected by the cardiac cycle and/or the type of disease causing the mitral regurgitation. Functional (ischemic) mitral regurgitation has the largest ROA at the beginning and the end of systole but it's nadir is mid-systole. Organic (degenerative) mitral regurgitation has the largest ROA at mid-systole. Even with quantitative measurements, other factors can affect the measurement. When utilizing qualitative, semi-qualitative, and quantitative measurements and calculations, an overall assessment can be made (even if some measurements and/or calculations don't fit the assessment). However, watch out for assumptions and errors in measurements!

Measurement Accuracy

Small errors in small measurements can lead to large errors in calculated results. Measurement accuracy is dependent upon the resolution of the image and following standard measurement techniques. The transducer that will provide the best 2D resolution for the measurement should be selected. Use the highest frequency possible that will allow visualization of the structure to be measured. The focal point should be located at the structure to be measured. The display should be maximized (magnified using zoom, depth, width) so only the area of interest is visualized.

Accuracy

Resolution
Focal Point
Display Size
Sweep Speed
Consistency
Leading Edge Technique
Slope Points
ECG Utilization

Timing measurements should be done under higher sweep speeds. Consistent settings and probes should be used for similar measurements between patients and between studies of the same patients. Do not change settings or change probes and expect that the measurements will be equally accurate.

Measurements of length or area of a cardiac structure should be made on the leading edge (inside edge) to leading edge technique. When measuring slopes, use two points as far apart as possible. Use the ECG for locating mechanical end-diastolic and end-systolic phases.

Some measurements can be made by M-Mode, 2D mode, or 3D mode. M-Mode measures along a single line of sight and may miss abnormalities that are not present in it's line of sight. 2D provides a better view than M-Mode when making measurements. However, on some measurements in 2D mode, some assumptions may need to be used that may make the measurement inaccurate. 3D mode will not include the assumptions and may provide more accurate measurements than 2D in some cases. Therefore, as a general rule, the measurment accuracy is 3D > 2D > M-Mode.

Sources of Error

Interoperator and intraoperator variability is a source of error. Different sonographers will image and measure the same structure differently and the same operator will measure the same structure differently as they gain experience and knowledge of measurement techniques. Training and consistency are key methods of decreasing interoperator and intraoperator errors.

The speed of sound will vary from the assumed 1540 m/sec used in calculations. The speed of sound can vary as much as 5% if adipose tissue is present between the transducer and the interrogated structures, but it typically has a variance of 2%.

Error Sources

Interoperator Error

Intraoperator Error

Speed of Sound Variance

Doppler Angle

Formula Assumptions

Pixel Variance/Interpolation

Patient Values

The Doppler angle may be off more than 20 degrees which will induce an error greater than 5%. While the Doppler in one plane may appear to be correct, in another plane it can be off more than 20 degrees.

Formulas may include assumptions that increase the error. Volume formulas assume a certain shape of the structure to be measured. Therefore, the volume calculated may not be the actual volume of the structure being measured if it doesn't match the formula's shape assumption.

The pixels that are displayed may not match the actual structure. Pixel values can be the actual value returned by the ultrasound signal or it can be a calculated (interpolated) value. Since resolution is poorer in the far field, many of these pixel values are calculated (interpolated). Interpolation of pixel values when the lines of pixels must be added in order to display the image in an analog system also can induce errors in pixel values.

Some measurements and/or calculations are indexed to Body Surface Area (BSA) to provide comparison between different sizes of patients.