Corrections for Scanner-Related Factors - Correction Techniques in Emission Tomography - page 112

ent directions for different applications. Since MRI data is usually available
for brain imaging, this information has been taken into account (see, e.g.,
[19, 20]). For tumor imaging, a recent overview is provided in [17].
6.2 Single photon emission computed tomography
SPECT is based on the detection of photons emitted by radiotracers that
were injected into a patient. The basic detection block is a gamma detector
head, consisting of a scintillator crystal, a light guide and an array of pho-
tomultiplier tubes. In front of the scintillator crystal a collimator allows for
detection of only those photons that travel in a direction (almost) perpendic-
ular to the gamma camera surface.
6.2.1 Linearity, center of rotation, and whole body imaging
Compensation methods in SPECT for the correction of scanner-related
factors include methods to ensure
1. uniformity and linearity of the response of a gamma camera head to
incoming photons (see, e.g., [6]);
2. coinciding definitions of the center of the field of view with the center
of rotation of a gamma camera head;
3. consistency between bed motion and electronic windowing of a camera
head in whole body acquisition settings.
Most of these correction methods are built into the scanner by the vendor.
Details are not always accessible, but procedures for quality assurance have
been outlined, for example, in a NEMA standard [1].
Per point 1 above, Linearity correction for a camera head ensures that the
response of the camera to an incoming photon is as independent as possible
on the location of incidence of the photon on the camera head surface (x,
y location). However, in practice image nonlinearity is present in the raw
measured data. Straight-line objects appear as curved-line images, leading to
pincushion and/or barrel distortion. Physical reasons for this observation in
properly functioning hardware are differences in sensitivity among PM tubes
and nonuniformities in optical light guides. Malfunctions of these hardware
components can also lead to image non-linearity.
Exposing a detector crystal to a uniform flux of photons produces a flood-
field image. Even a properly functioning gamma camera head will show vari-
ations in the image intensity, which may be of the order of 5% or more.
Intrinsic ood-eld images are acquired without collimator, usually with a