MRI phantom is one of the important assets in MRI scanner

MRI phantom is one of the important assets in MRI scanner. It is essential for the calibration and checking of MRI equipment including testing of equipment performance, correction of image quality and contrast. It is also useful in the development of new pulse sequences, adjustment of operational conditions, technical training of operators, simulation of specific organ visualisation, for use in various studies and evaluating safety (Ohno et al. 2008) (Hattori et al. 2013). The characteristics of the MRI phantom should have relaxation time equivalent to human tissue, dielectric properties equivalent to human tissue, homogenous relaxation times and dielectric properties throughout the phantom, sufficient strength to fabricate a torso without the use of physical reinforcements, allowing fabrication in the shape of human organs, the ease of handling and chemical and physical stability over an extended time (Kato et al. 2005). Besides, the materials used for the MRI phantom should be toxic free as the phantom might be a problematic during shipping and disposal and broken phantoms could lead to contamination of MRI instrument, staff and researchers (Hellerbach et al. 2013).
To date, there have been many different types of MRI phantom materials proposed, the phantom materials that are commonly used is water, agar and agarose. Polyvinyl alcohol (PVA), gelatin and polysaccharide can also be used. Although water is easy and safe to handle, it requires 10mins of settling time and it is easily influenced by vibrational effects. For the use of agar and agarose gel as the phantom material, its disadvantage is they are natural product. Hence, the relaxation properties differ somehow between different production and batches. On the other hand, synthetic polymers are standardised thus do not differ in relaxation properties (Hellerbach et al. 2013). In the study by (Mazzara et al. 1996), they used polysaccharide TX-151 to develop a realistic breast phantom as TX-151 is inexpensive, easy to mould, temporary stable and is tissue equivalent. At the same time, Gd-DTPA and Al powder is used as T1 and T2 relaxation time modifier which allow the development of phantoms with a wide range of relaxation times to mimic different human tissues and organs.
PVA is a non-toxic, hydrophilic, synthetic polymer recently introduced in tissue engineering for biocompatibility (Uchino et al. 2007). It is generally safe to use as MRI phatom materials. In the study by (Mano et al. 1986), PVA gel can be used to construct MRI phantom because of its suitable characteristics and properties including compatibility with other substances, homogenous, shape retaining capability and having values of H density, T1 and T2 similar to human soft tissues. Another study by (Chu & Rutt 1997) using PVA cryogel as MRI phantom material state that it is a near ideal material for fabrication of distensible phantoms. It is formed by numerous freeze thaw cycle of the PVA solution causing chemical cross-link through hydrogen bonding in order to appear to have strength similar to common gelatin. Its homogenous signal intensity, relaxation times close to human tissue and it is inherently flexible and tough made it a near-ideal phantom materials. Although studies have shown that PVA gel can act as an alternative MRI phantom material, its long-term stability still remains unknown.
In the meantime, the physical strength of the phantom can also be adjusted by chemically crosslinking the gel. Carrageenan, a polysaccharide that is extracted from seaweed (red algae) is often use as gelling agent as it has similar properties to agar. Carrageenan is commonly used as food additive because it is safe and inexpensive. Its advantages over agar includes greater elasticity and strength, able to form into large and stable phantom, easily mouldable, easy to produce and have little or no effect on T2 relaxation time allowing the phantom to have long T2 value (Ikemoto et al. 2011). Besides carrageenan, borax can also be used as a gelling agent. Borax, also known as sodium tetraborate with a chemical formula of Na2B4O7·10H2O is a salt with strong base and weak acid which can hydrolyse in water solution to produce boric acid-borate buffer having approximately a pH of 9 (Casassa, Sarquis & Van Dyke 1986). It works together with PVA glue to form slime which will be use as the phantom materials for this study. However, boric acid alone cannot produce polyol gel, it must be partially neutralised into borate ions to serve as a cross-link agent to interact with the -OH group of the PVA glue, forming a three-dimensional gel network (Casassa, Sarquis & Van Dyke 1986).
In many cases, the MRI properties of the phantom can be adjusted by using various additives such as graphite and paramagnetic ions. (Kato et al. 2005). The T1 and T2 relaxation times and conductivity of the phantom can be independently changed by modifying the concentration of the paramagnetic ions and the concentration of the gelling agent. The T1 relaxation time of human tissue are ten times longer than T2 relaxation time (Kato et al. 2005). Paramagnetic ions such as CuSO4, NiCl2, MnCl2 or GdCl3 have been commonly used as T1 modifiers. A study by (Kraft et al. 1987) stated that the relaxation time of manganese and copper are strongly temperature and magnetic field strength dependent whereas the relaxation time of nickel and gadolinium hardly dependent on these factors. Although the T2¬ relaxation time can be modified by changing the concentration of the gelling agent, the physical strength of the phantom should also take into account (Kato et al. 2005). A study by (Ikemoto et al. 2011) using carrageenan as gelling agent, GdCl3 as T1 modifier and agar as T2 modifier, it state that the T1 value decrease with the increase concentration of GdCl3 regardless of the concentration of agarose. While the T2 value decreases with the increase concentration of agarose and further decrease with the increase concentration of GdCl3. Hence, it is concluded that the T1¬ values, T2 values and the conductivity aspects of the phantom can be independently modifiable. Another similar study by (Yoshimura et al. 2003) also uses carrageenan as gelling agent, GdCl3 as T1 modifier and agarose as T2 modifier, it produces similar results where the T1 relaxation time decreases as the concentration of GdCl3 increases whereas the T2 relaxation time decrease with the increase of the concentration of agarose.