Biomedical Engineering Reference
In-Depth Information
the biodistribution of a radioactively labeled pharmaceutical, which may have
been introduced into the body by injection, inhalation, or ingestion. Emission
tomography with radionuclide tracers is divided into two categories depending
on the physics of the nuclear decay and detection process. In the first and
most common form, SPECT (single photon emission computed tomography),
photons emitted by the nucleus as a result of an energy transition or nuclear
decay are detected by a gamma camera in a single event counting mode. The
other form of emission tomography, PET (positron emission tomography),
counts pairs of photons that arise from the annihilation of a positron with an
electron. The feature that distinguishes PET from SPECT is that the radioactive
nuclei decay by emitting a positron (a positively charged electron) rather than
directly emitting a photon; the positron annihilates with an electron in the
imaged body and produces two photons in exactly opposite directions.
SPECT systems have a lead collimator that defines lines of response, stored
as projections on the crystal face for a given rotational angle. Gamma cameras
commonly have one, two, or three detector heads. The heads rotate in a cir-
cular motion to acquire projection data at sufficient angles for reconstruction.
In PET detection, coincident events are recorded when two photons are
detected on opposing sides of a circular array of detectors. A line of response is
ascribed to the chord joining the locations where both photons were detected.
The detector configurations common in PET vary.
50-55
In recent years many
dual-head gamma cameras have become available with the ability to work in a
coincidence mode, thereby permitting them to perform both PET and SPECT.
56
An interesting recent development in “integrated imaging” has been to
combine emission tomography and CT scanners into a single device.
57-59
SPECT
CT devices should be commercially available within the
next few years. This approach obviously minimizes some of the problems of
registration between modalities by using the same scanning couch and
patient positioning, as well as making further use of anatomical data in atten-
uation, scatter, and partial volume correction of emission tomography data.
Similar efforts have been made to combine PET and MRI or MRS.
CT and PET
60,61
5.4.1
Sources of Spatial Inaccuracies and Measures to Prevent
or Minimize Them
The data recorded by ET systems often require processing to compensate for a
variety of effects before quantitatively accurate image reconstruction using
methods such as conventional filtered backprojection can be employed. These
include: photon attenuation
; distortions intrin-
sic to the acquisition geometry; differences in performance within as well as
between individual detector elements (uniformity); “gaps” in the projections,
e.g., between the edges of flat detectors where the detectors meet. In addition,
spatial distortions can arise at high counting rates (“pulse pileup”).
Particularly problematic for image registration techniques are effects which
lead to spatial and, to a lesser extent, intensity distortions. The main features
which potentially limit the ability to coregister data from ET with anatomical
in vivo
; photon scattering
in vivo
