puted tomography (CT) or magnetic resonance imaging (MRI) anatomical
images of an organ are superimposed onto the corresponding functional
images obtained by positron emission tomography or other nuclear medi-
cine studies to match the functional abnormalities with the anatomical
defects. Computers are well utilized to perform these superimpositions of
images.
Another important utility of the computer is the subtraction of back-
ground activity from an image or one set of images from another set. An
example of the latter is to subtract the interictal images obtained in epilepsy
patients using 99mTc-ethyl cysteinate dimer (ECD) from those obtained in
ictal period using the same radiotracer. Resultant difference images provide
better delineation of epileptogenic foci in these patients.
Display
Digital images are displayed on video monitors which are either cathode
ray tubes (CRT) or flat-panel type liquid crystal display (LCD) monitors.
These monitors are characterized by parameters such as spatial resolution,
contrast, aspect ratio, luminance, persistence, refresh rate, and dynamic
range. The spatial resolution and luminance of LCD monitors are far supe-
rior to those of CRTs. These monitors are placed in what is called the work-
station where nuclear physicians view, manipulate, and interpret the images
using the computer.
Display can be in either grayscale (black and white) or colorscale. In
either case, grading of scale is achieved by variations in counts in the pixels
in the digital image. In grayscale, the number of counts in the pixel defines
the brightness level of a pixel. Thus, the black and white contrast in a digital
image is obtained by applying the grayscale.
Color hues are assigned to different pixels corresponding to counts stored
in the individual pixels in order to provide contrast between areas on the
image. In a gradient colorscale, blue, green, yellow, and red are assigned in
order to pixels with increasing counts: blue to the lowest count and red to
the highest count. Edges of color bands are blended to produce a gradual
change over the full range of the color scale.
Often a grayscale or colorscale bar is shown on the side of the image in
order to help the interpreter differentiate the image contrast. Images can
be displayed in transaxial (transverse), coronal (horizontal long axis), or
sagittal (vertical long axis) views individually or simultaneously on the
video monitor. On the simultaneous display of SPECT images, a point on
the image is chosen using the cursor and three images that pass through the
point are displayed. New sets of images are obtained by choosing a differ-
ent point on the image. Such sequential screening of images is helpful in
delineating the abnormal areas on images of the patient.
Angular projections around an object computed from the 3-D tomo-
graphic data can be displayed in continuous rotation. This presents the
148 11. Digital Computers in Nuclear Medicine