Biomedical Engineering Reference
In-Depth Information
do not hold for sections that pass through sinuses and regions where the ad-
jacent bone is much thicker. Alternatively, the transmission scan may be per-
formed after tracer administration, referred to as postinjection transmission
(PIT) scanning [48], which utilizes strong rotating rod (or point) sources for the
transmission source. A small fraction of “transmission” coincidences contains
in the sinogram data can be distinguished from emission coincidences that orig-
inate from the administered radiopharmaceuticals by knowing the positions of
the orbiting sources. Another approach is to integrate measured and calculated
attenuation that makes use of the advantages of each approach. A transmis-
sion scan is still required and the attenuation coefficient images derived from
the transmission and blank scans are reconstructed and then segmented into a
small number of tissue types, which are assigned with
a priori
known atten-
uation coefficients [49-51]. These processes greatly reduce noise propagation
from the transmission data into the reconstructed emission images.
2.12 Calibration
Once the acquired data has been corrected for various sources of bias introduced
by different physical artifacts as mentioned in the previous section, images can
be reconstructed without artifacts, provided that there are sufficient axial and
angular sampling of projection data. To reconstruct images in absolute units of
radioactivity concentration (kiloBecquerel per milliliter, kBq/mL, or nanoCurie
per milliliter, nCi/mL), calibration of the scanner against a source of known
activity is required. This can be accomplished by scanning a source of uniform
radioactivity concentration (e.g. a uniform cylinder) and then counting an aliquot
taken from the source in a calibrated well-counter to obtain the absolute activity
concentration, which is then compared to the voxel values in the reconstructed
images for the source (after corrections for physical artifacts have been applied)
to determine a calibration factor.
2.13 Resolution Limitations of PET
Although there has been significant improvement in PET instrumentation over
the last two decades, there is a finite limit to the spatial resolution of PET scanner.
