Chemistry Reference
In-Depth Information
1.3.2
Advantages and limitations
one of the major advantages of SPEcT over other imaging modalities is that it provides improved contrast between regions
of different functions, thus allowing improved detection of abnormal physiological functions. SPEcT also gives better
spatial localisation as well as improved quantification. It offers greater accessibility because it uses radioisotopes with longer
half-lives and, unlike PET, does not require any cyclotrons to generate these radioisotopes. Due to the selection of radioiso-
topes available, it allows simultaneous imaging because different radioisotopes produce different
γ
-rays of different energies
(Table 1.1). This is a unique advantage of SPEcT because multiple energy windows can be used for concurrent imaging of
different functions and metabolic processes.
However, this technique also has disadvantages: It is often necessary to use long scanning times, which can cause discom-
fort to patients. Artefacts can also be easily generated by numerous uncontrollable factors such as patient movement and
uneven distribution of the radiotracer.
1.4
WhAt Is coMPuted toMogrAPhy (ct) or coMPuted AxIAl toMogrAPhy (cAt)?
computed tomography (cT) or computed axial tomography (cAT) is a diagnostic technique that uses special X-ray equipment
to obtain cross-sectional pictures of the body [18]. This technique allows detailed imaging of organs, bones, and tissues and is
often used in conjunction with other diagnostic methods such as MrI and PET (Figure 1.10) [19].
1.4.1
Basic Principles
X-rays are a form of electromagnetic radiation with a wavelength in the range of 0.01 to 10 nm that can transverse along
the cross-section of an object in straight lines. The wavelength is attenuated by the object through which it passes but is
still detectable outside the object [20]. In cT imaging, the cross-section is probed with X-rays from various directions with
the use of rotating X-ray equipment where both the low energy X-ray source and the detector are rotated at 360 degrees
around the patient. The acquired attenuated signals are then recorded and converted into projections of the linear attenua-
tion coefficient distribution of the cross-section to produce volumetric data. charged coupled Device (ccD) detectors are
used to carry out photo-transduction of incoming X-rays to produce images. The contrast in cT images relies on the
tABle 1.1
types of radioisotopes used for different studies by sPect Imaging.
Study
radioisotope
Emission energy (keV)
Half-life
radiopharmaceutical
Activity (MBq)
Bone scan
technetium-99 m
140
6 hours
Phosphonates /
Bisphosphonates
800
Myocardial perfusion scan
technetium-99 m
140
6 hours
tetrofosmin; Sestamibi
700
Brain scan
technetium-99 m
140
6 hours
HMPAo; EcD
555-1110
Tumour scan
iodine-123
159
13 hours
MIBG
400
White cell scan
indium-111
& technetium-99 m
171 & 245
67 hours
in vitro
labelled leucocytes
18
Diatrizoic acid
O
OH
I
I
O
O
N
H
N
H
I
FIgure 1.10
A typical contrast agent used in cT-Gastrografin.
