GE Healthcare eXplore Locus SP, Service Manual

Page: 22 / 266

GE H
EALTHCARE
D
IRECTION
5158821-100, R
EVISION
3
E
X
PLORE
L
OCUS
SP S
ERVICE
G
UIDE
Page 22
ADUs, CT NUMBER AND HOUNSFIELD UNITS
When the first projections are created, the values of the pixels are given in ADUs. ADU stands for
Arbitrary Digital Units, and it is a scale that is particular to the eXplore Micro CT system. The same
material may be given a different ADU value in a different scan, because the scale is particular to
each scan and changes depending on the materials scanned and on the strength and intensity of
the x-ray beam.
When a volume is first reconstructed, the ADUs are converted to CT values. The CT value is a
number that represents the attenuation coefficient of a particular voxel in the reconstruction. CT
values should be given in Hounsfield Units (HU), but the Hounsfield scale has specific known values
for air and water, and the scale must be calibrated properly, using actual air and water (or
equivalents) in a specific scan as reference points.
THE HOUNSFIELD SCALE
• Air is -1000 HU
• Water is 0 HU
• Fat is approximately -150 HU
• Compact bone is approximately 3000-4000 HU
• Ethyl alcohol is -700 HU
Until the scale is properly calibrated, the CT numbers are not given in HU.
PIXELS AND VOXELS
A pixel is a pic ture el ement. It is a single element in a two-dimensional image. Images are made
up of an array of individual pixels. Each pixel has a numerical value. In MicroView, each pixel is
represented by a shade of gray.
A voxel is a vo lume el ement. It is a single element in a three-dimensional image. As with pixels,
each voxel has a numerical value. In MicroView, each voxel is represented by a shade of gray on
the screen.
RESOLUTION
The resolution of an image refers to the smallest structure that can be discretely represented. If the
resolution of an image is 25 microns (µm), then any structure smaller than 25 µm will not be
distinguishable.
It is important to note that resolution and pixel/voxel size are separate. Pixel/voxel size is defined
as the amount of space represented by each pixel or voxel. Resolution is limited by pixel size, but
many factors will cause resolution to be larger than pixel/voxel size.
Factors that affect resolution include the pixel/voxel size, the signal-to-noise ratio, the immobility of
the subject during scanning, and the x-ray tube properties.
SIGNAL-TO-NOISE RATIO (SNR)
Signal is useful information, and noise is non-useful information. In general, the SNR can be
positively affected by choosing a higher binning setting, using a filter, and performing some frame
averaging.
With digital imaging, considerable control is given to the user to affect the SNR.
To understand how the SNR in the micro CT system is affected by scanning parameters, let the
number of x-ray photons collected by each detector pixel for an image be
n . Signal strength in the
image increases linearly with n , while noise in the image generally increases with the square root
of n . Therefore, in order to increase the SNR by a factor of two, n must be increased by a factor of
four. This can be accomplished by increasing the binning by a factor of two in each direction (e.g.,