Thus if an image of 250 ×250 mm FOV is obtained in a matrix of 128 ×
128 mm, the pixel size would be 250/128 ≈2 mm. If the matrix size is changed
to 64 ×64, then the pixel size would be ~4 mm. Often, a zoom factoris
applied during data acquisition to improve spatial resolution because it
reduces the pixel size. Overall, the pixel size dcan be calculated as
d=FOV/(z ×N) (11.1)where zis the zoom factor (1.2, 2.0, etc.,) and Nis the number of pixels
across the matrix. The use of a zoom factor of, say, 2, reduces the pixel size
by half, improving the spatial resolution, but counts per pixel are reduced
thus increasing the noise on the image (see later).
The choice of pixel size and zoom factor is limited by the spatial resolu-
tion of the imaging device, particularly in tomographic systems. Ideally, the
pixel size should be less than 1/3 of the expected spatial resolution of the
SPECT system, measured at the center of rotation. That is,
d ≤FWHM/ 3 (11.2)where FWHMis the full width at half maximum of the line spread function
of the imaging system. If the expected system resolution is 18 mm, then the
pixel size in the matrix should be less than 6 mm. Pixel size larger than this
limit would degrade the image.
For a typical SPECT gamma camera, the FOV size is 400 mm across and
the spatial resolution is of the order of 18 to 25 mm. Thus, the pixel size in
a 64 ×64 matrix is 400/64 =6.25 mm, which is nearly equal to or less than
the 1/3 of the spatial resolution of the SPECT system. Thus, a 64 ×64 matrix
should be good enough in most SPECT imaging. Using a 128 ×128 matrix
(pixel size is 3.13 mm, which is much less than 1/3 of the system resolution),
would improve the spatial resolution significantly. However, as mentioned
before, the counts in each pixel would be reduced by 1/4, as the total counts
are distributed over four times the pixels, compared to a 64 ×64 matrix.
Thus the noise increases in the image and so the signal-to-noise ratio
decreases causing degradation in image contrast.
Application of Computers in Nuclear Medicine
Digital Data Acquisition
The X- and Y-signals obtained in scintigraphic studies in nuclear medicine
are digitized by ADCs in the computer and stored in one of two ways: (a)
frame modeand (b) list mode. In both modes, a technique of magnification
or zooming can be applied, whereby the pixel size is decreased by a zoom
factor. Zoom factors typically vary from 1 to 4 in increments of 0.25.
Data acquisition in the frame mode is the most common practice in
nuclear medicine and widely used in static, gated, dynamic, and single
144 11. Digital Computers in Nuclear Medicine