Biomedical Engineering Reference
In-Depth Information
same radionuclide source and from a radionuclide distribution of
130
I macroaggre-
gated albumin, to obtain a transverse section image through the thorax of a patient [
1
].
This early SPECT imaging technique can be identified as the first effective evidence
for feasibility of hybrid metabolic imaging.
Later, in the 1980s, a significant progress in the development of SPECT devices,
image reconstruction and correction techniques was reached. In particular, an iter-
ative algorithm for image reconstruction called maximum likelihood expectation
maximization (MLEM) was implemented in 1989 by Tsui and co-workers [
2
,
3
].
However, early tomographic data suffered from low photon flux and poor contrast,
facts that coupled with a low convergence speed of the MLEM algorithm finally
produced limited anatomical information available from the reconstructed images.
In 1994, a new technique was introduced (the ordered subset expectation maximiza-
tion (OSEM) technique), that led to an efficient image reconstruction when applied
to SPECT and CT data [
4
].
In the same period, the technical feasibility of simultaneous SPECT/CT imaging
was demonstrated by Hagesawa et al [
5
-
7
], who developed a new imaging system
using an array of high-purity germanium (HPGE) detectors to simultaneously acquire
emission and transmission data from a low x-ray source. From that moment on, the
development of manufactured SPECT/CT systems for clinical use started, giving rise
to a series of multimodal SPECT/CT devices. Further attempts were also made to
obtainmultimodal image visualization by developing automated algorithms to super-
pose images acquired using different techniques, specifically related to functional
and morphological imaging e.g. [
8
,
9
].
3.2.2 PET/CT
In 1992, in an independent manner from the pioneering work of Hasegawa and
co-workers on multimodality imaging, and in particular on SPECT/CT, Townsend
and co-workers proposed to combine PET and CT. They suggested the use of CT
to produce attenuation correction factors for PET, similarly to what was done by
Hasegawa et al., who used CT images to correct SPECT images for attenuation.
The possibility to acquire bimodal PET/CT images in a single scan session within
an integrated multimodal device became available as a prototype in 1998 [
10
], and
clinically only in 2001 [
11
,
12
]. Such a device enabled the acquisition of aligned
bimodal images accurately combining, in a single exam session, functional PET
information with high-resolution three-dimensional CT information. The integration
in a single device of PET and CT scanners showed a greater convenience with respect
to using two separate stand-alone machines. The integrated device provided faster
and more efficient data acquisition and accurate registration, as this process did
not suffer of patient repositioning between subsequent examinations. Technological
improvement resulted in a replacement of simple PET by combined PET/CT as a
