Biomedical Imaging - Principles of Biomedical Engineering - page 325

Medical image quality can better be measured by human performance in visual
tasks that are relevant to clinical diagnosis. The standard method of evaluating di-
agnostic methods is a receiver operating characteristic (ROC) study, which is time
consuming and costly because it requires a large number of human observations.
Although a detailed discussion of ROC is beyond the scope of this topic, image
quality is assessed by three basic factors.
8.4.4.1 SpatialResolution
Spatial resolution of an imaging system is defined as the smallest spacing between
two objects that can be imaged clearly. In computer monitors, resolution is the
number of pixels contained on a display monitor, expressed in terms of the number
of pixels on the horizontal axis and the number on the vertical axis. The sharpness
of the image on a display depends on the resolution and the size of the monitor.
The same pixel resolution will be sharper on a smaller monitor and lose sharpness
on larger monitors if the same number of pixels is spread out over a larger number
of inches. A computer display system will have a maximum resolution depending
on its physical ability to focus light (in which case the physical dot size matches
the pixel size). Spatial resolution is determined by characteristics of each imaging
modality and operating factors for that modality. In each case, the distance be-
tween separate objects that a device can record is a measure of its spatial resolution.
Hence, the intrinsic resolution of the detectors is the critical factor in determining
the ultimate resolution of the image. The spatial resolution of a conventional X-ray
system with direct film exposure is approximately 0.01 mm while that of a CT scan-
ner is approximately 1 mm.
8.4.4.2 ImageContrast
Image contrast is the difference in film density or image brightness between areas
within an image. The medical imaging process sees physical contrast in the body
and transfers it into image contrast. The contrast sensitivity of the imaging process
determines the lowest physical contrast that is visible. When the contrast sensitiv-
ity of an imaging process is low, many things in that image are not distinguishable.
Contrast sensitivity is determined by characteristics of each imaging modality and
operating factors for the modality. For example, the physical contrast that can be
visualized with X-ray imaging is differences in physical density. Although spatial
resolution is low in CT, it has low contrast resolution, enabling small changes in
the tissue type. The physical contrast that is visible in radionuclide imaging is usu-
ally different concentrations of radioactivity. An MRI can visualize many forms of
physical contrast. Some are characteristics of tissue and others are characteristics
of fluid movement.
In digital images, the differences in gray shades are used to distinguish different
tissue types, analyze anatomical relationships, and quantify physiological function.
The larger the difference in gray shades between two adjacent tissue types, the eas-
ier it is to make these important distinctions. The objective of an imaging system is
to maximize the contrast in the image for any particular object of interest, although
there may be design compromises accounting for noise and spatial resolution.