3D/4D ultrasound

As is happening in the other imaging modalities, many ultrasound systems now have the ability to generate a 3D display. 

To create a 3D ultrasound image, data is collected from three planes of section (the X, Y, and Z axes). This is done by acquiring a series of 2D slices, with each slice being obtained from a different position along the Z axis.  A nice analogy is the creation of a loaf of bread (a 3D structure) from the combination of a series of 2D slices (the individual bread slices).  The smallest distinguishable part of a 3D image is called a VOXEL  Voxel is short for volume pixel.  It is the 3D equivalent of the 2D pixel.  The smaller the voxel, the better the 3D image quality.

The 3D data acquisition is done in one of two ways. 

1. using a standard 2D array transducer, the sonographer can manually move the transducer through the Z axis while it scans the X and Y axes.

2. a dedicated automatic 3D transducer can be used.  This transducer automatically moves the beam through the Z axis while acquiring the X and Y data.  These transducers tend to be bulkier than standard 2D transducers and are commonly used in obstetrics. 

Obtaining and processing data from multiple slices to create a 3D image takes longer than producing a 2D image. However, with increasing computer processing speeds, many manufacturers can now achieve real-time 3D imaging. This is called 4D scanning: 3D scanning in real-time.

There are also two major ways in which the 3D data is displayed.

1. Multiplanar reformatting.  In this technique, the X, Y, & Z axis images are displayed as separate images on the screen.  Multiplanar reformatting is commonly used in gynecological scanning. For example, an endovaginal coronal section of the uterus is often difficult to obtain using 2D ultrasound. However, using 3D, data from the three axes can be used to reconstruct the coronal plane of section through the uterus.  This technique is very helpful when assessing uterine anomalies.

2. Surface rendering.  Surface rendering does what the name suggests.  It takes the 3D data set and manipulates it to render (display) the surface features of structures.  A common use of surface rendering is to display the facial features of the fetus. When this is done in real-time it is called 4D surface rendering.