Page 32 - Bio Engineering Approaches to Cancer Diagnosis and Treatment

2.3 Ultrasound 29

The efficiency of MRI CAs is related to their impact on the relaxation time. The
−1
−1
efficiency of CAs defines as r and r (m Mol s ) related to the longitudinal and
2
1
transversal relaxation, respectively:
r 1 = 1 and r 2 = 1 (2.2)
T 1 T 2 r =1T and r =1T 2
1
2
1
The r /r ratio is evaluated to determine the efficiency of CA; as the ratio of r /r
2
1
1
2
increase, the contrast of T -weighted imaging is increased. Experimentally, r and r
2
1
2
are calculated using the plot of 1/T and 1/T versus iron concentration by applying
2
1
the following equation:
1 = 1 + rC i 1,2 (2.3)
−
T i T i 0 i 1Ti=1Ti0+riC i−1,2
Where T and T are the longitudinal and transversal relaxation time in pure T 20
0
0
T10
1
2
water, respectively and C is the concentration of iron oxide [16,19].
2.2.3 Advantages and disadvantages
The advantages of MRI are as follow:
• No need for iodinated contrast
• Greater ability to image the brain and spinal cord than other modalities such as CT
• Nonradiation involved
• Excellent contrast for soft tissues
• Allow lesion detection
• Staging of cancer is possible
The possible disadvantages of MRI are listed as follow:
• Expensive
• Not portable
• Difficult for real time imaging
• Limited availability
• Lengthy acquisition time
• Limited information on lung
• No electron density mapping for the tissues
• Contraindicated in some ferromagnetic implant such as pacemaker, fusion, and
screws

2.3 Ultrasound
Ultrasound is longitudinal acoustic energy having a frequency above 20 kHz (i.e.,
above human hearing range). Ultrasound irradiation makes particles displacement
along with the wave propagation. Currently, ultrasound imaging is the most clini-
cally used technic after CT scan [20].