Chemistry Reference
In-Depth Information
nuclear isomer,
99m
Tc. This isomer has a short half-life of ~6 hours before it goes to
99
Tc by isomeric transition, a radioactive
decay process from an excited metastable state that results in
γ
-ray emission [13, 14].
In SPEcT, these
γ
-ray emissions are detected by a 360
0
rotating photon detector array around the body known as the gamma
camera, which can acquire multiple 3D projections at multiple angles. Sodium iodide or solid-state cadmium-zinc-telluride
detectors, which provide spatial resolution of 1-2 mm, are usually used. Images are formed with the information given on the
position and concentration of the radionuclide biodistribution in two dimensions [14]. However, due to the attenuation effects
of
γ
-ray emission as it is transmitted from the injected tracers inside the body, mathematical reconstruction algorithms have
been developed to improve resolution. Some other common radionuclides used in SPEcT in addition to
99m
Tc are
111
In (half-life
2.8 days),
123
I (13.2 h), and
125
I (59.5 days). Due to the different half-lives, dual tracers can be used to give simultaneous imaging
because the
γ
-ray emissions have different energies [14, 15].
A gamma detector is made up of a few cameras that are placed opposite to one another to form a cylindrical
detector that allows rotation around an axis centre. Due to its multiple camera heads, it only needs to rotate 120-180
degrees to collect data around the entire body. The gamma camera is made up of three basic layers, the first of which
contains a collimator, a special lens that only allows entry of γ-rays that are perpendicular to the plane of the camera.
The other two layers consist of a crystal and detectors. The crystal is usually a thallium-activated sodium iodide
[naI(Tl)] detector crystal, which, when absorbing γ-rays, would scintillate to cause a light signal to be detected
(Figure 1.9) [15].
The data are collected as a planar matrix of values that correspond to the number of gamma counts and can be pro-
cessed to give planar scintigrams for constructing 2D images. Typically, each row across the matrix represents an inten-
sity displayed across a single projection, whereas the successive rows represent successive projection angles. There are
different techniques to reconstruct tomographic images that are different for 2D cross-sectional images and for 3D images.
A common reconstruction method is the simple back-projection method, which generates 2D cross-sectional images of
activity from a slice within the detected object, using the projection profiles obtained for that slice. However, there is a
flaw in this method of data reconstruction: The final SPEcT images have poorer spatial resolution than the raw 2D
images used to produce them. For better spatial resolution, other processing techniques such as direct Fourier transform
reconstruction as well as data filtering can be used. Filtered back-projection is a favourable method for data reconstruc-
tion. For tomographic 3D images, 3D reconstruction algorithms can also be used to visualise the 3D biodistribution of the
radiotracers.
The resolution and sensitivity of SPEcT is dependent on the pinhole of the collimator multiple-pinhole and multiple-
solid state detector systems are often used to allow for lower radiation dosages and shorter scan times, hence improving the
sensitivity and resolution of this imaging modality [16, 17].
Rotating Nal(TI) detector module
with Pb/W collimator in front
Signals to electronics
Direction of rotation
Scintillation site for γ-ray
parallel to collimator holes
Emitted γ-rays
Absorbed in collimator
Tracer
Tracer
+ γ
99m
Tc
99
Tc
FIgure 1.9
Schematic diagram showing the basic principles of SPEcT.
