Page 41 - Bio Engineering Approaches to Cancer Diagnosis and Treatment

38 CHAPTER 2 Diagnostic imaging in cancer

2.5.2 Advantages and disadvantages
The advantages of using radiopharmaceutical imaging system list as follow:
• Identify skeletal problem
• Possibility of whole-body scanning
• Monitoring the treatment response
The disadvantages of radiopharmaceutical imaging are:

• Not widely available
• Relatively higher cost that X-ray and CT
• Low spatial resolution (5–10 mm)
• Radiation risk
• Usually required I.V. injection
• Disposal of radio-activity waste
• Slow image acquisition

2.6 PET and PET/CT
Positron emission tomography (PET) is a nuclear medicine imaging system applied
to observe metabolic processes to diagnosis and follow up various diseases. PET and
PET/CT have been used to study various tumors such as lymphoma, melanoma, lung
carcinoma, pulmonary nodules, colorectal cancer, and breast cancer [44].
In general, MRI and CT are based on morphological changes for diagnosis and
follow-up malignances while PET detects abnormal metabolic activity in target as
yet doesn’t undergo anatomical changes. Also, PET is able to monitor patients sub-
jected to chemotherapy or tumor resection whom having difficulty to appearance
using MRI or CT imaging systems.
Although PET has considerable advantages as an imaging system, it is hard to
accurately find out the area of abnormal activity due to the lack of anatomical struc-
ture data. Therefore, great attempts have been done to couple functional information
of PET with anatomical information. Beyer et al. introduced PET/CT scanner to
coregister structural and functional images without moving the patient [45].
PET imaging is based on the detection of γ photons released from radionuclides
during the annihilation of positron (β) with electron (Fig. 2.5). The photons possess
energy around 0.511 MeV and are detected by scintillation crystals. Scintillation
crystals are mainly made up of gadolinium silicate, lutetium oxyorthosilicate, and
bismuth germinate. The value of 512 keV is the energy of the mass of an electron
based on the law of energy conservation.
11
64
124
15
18
The main sources of positron-emitting radioisotopes are C, Cu, F, O, I,
86 13 68 18
Y, N, and Ga. F is one of the most widely studied radioisotopes due to the
11
long half-life (around 109.8 min). The specific feature of C is that mainly the
11
tracers based on C are endogenous substances such as fatty acid acetate, hydy-
roxyephedrine and amino acids methionine while other radioisotopes are analogous