Quantitative M Mode

Editor's Note:
 
The quantitative chapter contains some material that will be repeated in later chapters.  Since the quantiative chapter covers a wide range of topics with the common thread that something is being measured or calculated, each measurement or calculation will be repeated in the appropriate chapter.  The quantitative chapter provides you with a quick and easy method to look up calculations quickly and provides some general overview on measurements and calculations.
 
Measurement Accuracy
 

Small errors in 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 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. 


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.

  Accuracy

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

 

 
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. 

 

  Error Sources

  • Interoperator Error
  • Intraoperator Error
  • Speed of Sound Variance
  • Doppler Angle
  • Formula Assumptions
  • Pixel Variance/Interpolation
  • Patient Values
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%. 

 

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 value.  Since resolution is poorer in the far field, many of these pixel values are calculated.  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. 


Patient measurements, such as height, weight, and body surface area that are used in some formulas may be inaccurate for obvious reasons.

 
Left Ventricular Volume, Mass and Geometry
 
Introduction 
 

In response to valvular disorders, hypertension, heart failure the left ventricle undergoes remodeling. The mass, volume, and/or geometry of the left ventricle may change. Left ventricular measurements that characterize the remodeling are relative wall thickness (RWT), short axis (SAX), long axis (LAX), volume (V), mass (M), and the volume/mass (V/M) ratio. The measurements of the left ventricle can be predictive in certain disease states. For example, patients with idiopathic dilating cardiomyopathy have a poorer survival rate if their ventricle remodels to a more spherical model where the LAX and the SAX are similar.

 
M-Mode
Mass
Prolate Ellipsoid Method
A prolate ellipsis is an ellipsis with a LAX/SAX ratio of 2:1. The normal left ventricle is a prolate ellipsoid. To find the mass of the left ventricle, the volume of the left ventricular muscle must be calculated and then multiplied by its specific gravity, 1.04.  If the volume of the whole left ventricle can be calculated as the outer prolate ellipsis and the endocardial or blood volume of the left ventricle can be calculated, the difference is the left ventricular volume.  A correction factor, 0.8 is needed to calculate the final value for left ventricular volume.  The formula for left ventricular mass by the prolate ellipsoid method is:
 
LV mass = 0.8{1.04[(STd + LVIDd + PWTd)]3 - LVIDd3}} + 0.6g
Prolate Ellipsis
Prolate Ellipsis
 

The measurements required for the calculation of LV mass are the:

  • Septal Thickness in Diastole (STd)
  • Left Ventricular Internal Diameter in Diastole (LVSIDd)
  • Posterior Wall Thickness in Diastole (PWTd)
  • Specific Gravity of the Myocardium (1.04).

Since the calculation will overestimate the LV mass by 20%, a correction factor, 0.8, is used to arrive at the true LV mass. The correct beam angle for M Mode calculations can be obtained via the parasternal acoustic window LVSAX view. The view of the LVSAX is obtained just below the mitral valve leaflet tips. The beam must be perpendicular to the interventricular septum and the posterior wall. Since wall thickness is being measured, the interventricular septum and the posterior wall must not have regional wall motion abnormalities. 2D echocardiography is utilized to obtain and inspect the correct view before switching to M Mode to obtain the measurements.

M Mode
Left Ventricular Measurements
 
Volume
 
If the LVID dimension is measured, the volume of the left ventricle can be calculated from the volume formula of a sphere:

 

V = D3 or V = LVID3
Volume of a Sphere LV Volume (Spherical)
 

Overestimation of left ventricular volumes occurs in disease states such as a dilated cardiomyopathy or chronic valvular regurgitation. Teicholz et al published a formula that compensates for the remodeling of the left ventricle using only the LVSAX in a spherical heart. The formula is:


V
d = [7/{2.4+LVSAXd)] * LVSAXd3

 

This formula can be used in systole or diastole to arrive at a stroke volume that is fairly accurate when compared to other methods.

 
Relative Wall Thickness
 

While mass and volume measurements are important predictors when the left ventricle has undergone remodeling because of disease, relative wall thickness indicates the type of geometric remodeling in a hypertrophic process. Relative wall thickness can be an independent predictor of outcome rather than mass and volume of the left ventricle. Relative wall thickness is the ratio of posterior wall thickness and the LVSAX.

 
LV Remodeling States
 

Relative wall thickness increases in direct response to systolic pressure. In hypertensive disease the patients can be classified or subdivided based upon the response of the heart.

Concentric hypertrophy is an increase in LV mass and relative wall thickness.

Eccentric hypertrophy is an increase in LV mass but normal relative wall thickness.

Concentric remodeling occurs when normal LV mass is present but increased relative wall thickness.

The last state is a normal LV mass and normal relative wall thickness.

Patients with concentric remodeling or concentric hypertrophy have higher incidence of cardiovascular events when compared to patients with normal relative wall thickness (normal or eccentric hypertrophy). A relative wall thickness > 0.8 indicates an inappropriately hypertrophied ventricle compared to the systolic pressure needed to generate a stroke volume.

Image Relative Wall Thickness

LV Remodeling State LV Mass RWT
Normal Normal Normal
Concentric Remodeling Normal Increased
Concentric Hypertrophy Increased Increased
Eccentric Hypertrophy Increased Normal