# Development and Application of MAGIC-f Gel in Cancer Research and Medical Imaging
## 5 Evaluation of MAGIC-f Gel Dosimeter  
The MAGIC-f gel is analyzed by using the same diagnostic imaging techniques as other polymer gels, such as OCT, X-ray CT [12, 44, 45, 46], magnetic resonance imaging (MRI) [3, 27, 47, 48], and ultrasound (sonography) [49, 50, 51]. Out of these techniques OCT and MRI are most popular based on the physical change occurring in gel due to irradiation [52]. Ultrasound is the least-preferred method in recent practice of dosimetry evaluation, as after 2002, there was no significant study with ultrasound for imaging gel dosimeter, except for one in 2016, by [PERSON] et al. [53]. [PERSON] et al. introduced a new approach to three-dimensional dosimetry, using the MRI techniques, based on the principle that mapping dose distribution is given by any MRI contrast parameter that alters after polymerization in the gel. They found that the spin-lattice relaxation rate (\(R1\)) changes only slightly upon irradiation-induced polymerization. The most widely used MRI property is the spin-spin relaxation rate (\(R2\)) due to its wide dynamic range and sensitivity to polymerization caused by the absorbed dose from radiation exposure. After the discovery of MAGIC-f gel in 2008, the MRI has been extensively used in the experiment by [PERSON] et al., [PERSON] et al., [PERSON] et al., and [PERSON] et al. [21, 24, 25]. \(R2\) values can be calculated by using different MR imaging sequences. The image intensity of each MRI sequence should be calculated. The intensity ratio between MRI sequences is used to derive the \(R2\) value for the gel composition. The simplest MRI sequencing method uses only two echoes to determine \(R2\) value. The following formula is used to calculate \(R2\) related to two consecutive echoes:  
\[R2=\frac{\log(E1)-\log(E2)}{\Delta T_{E}}=\frac{log(E1/E2)}{\Delta T_{E}}= \frac{\log(Intensity\ Ratio)}{\Delta T_{E}}, \tag{1}\]  
where \(E1\) is the intensity of the gel dosimeter for the first echo, \(E2\) is the intensity for the second echo, and \(\Delta T_{E}\) is the echo time between the first and second echo. The \(R2\) value can be calculated for the whole MR image, which is called the \(R2\) map. Moreover, more than two echoes of MRI sequences can be used to calculate the \(R2\) value. Increasing the number of sequences in MRI sequences will result in better accuracy in \(R2\) value. The calculated \(R2\) value can be used to determine the dose distribution in gel dosimeter. In this step, a determined dose is given to the gel dosimeter, and then this determined dose is related to the calculated \(R2\) value; the calibration curve which relates the calculated \(R2\) value to the given dose is acquired. The acquired calibration curve can be used to determine the delivered dose to the gel dosimeter for unknown dose values [54]. As an alternative to MRI technique, other types of techniques (OCTs) were used, based on the alternation of the absorption coefficient due to polymerization induced from the irradiation of polymer gel, and the ultrasonic which is based on the change of the sound speed due to the change in density and viscosity of the irradiated gel [53]. When the gel is not irradiated, it is transparent to light, but the irradiated MAGIC-f gel becomes darker and the amount of visible light which can pass through the gel decreases; thus, this feature can be used for gel dosimetry purposes. To calculate the dose delivered to a gel dosimeter in OCT method, a laser beam is projected to the gel and some detectors, such as charge-coupled device (CCD), or a complementary metal oxide semiconductor (CMOS) is used to get the required OCT image. The intensity ratio of the OCT images for irradiated and unirradiated gels is presented as follows [3]:  
\[I=I_{0}exp\left[-\int_{beam-path}\mu(s)ds\right] \tag{2}\]  
where \(I\) is the measured intensity for the irradiated gel dosimeter, \(I_{0}\) is the intensity for the not irradiated gel, \(\mu\) is the optical attenuation coefficient, and \(s\) is the distance that the laser beam travels through the sample.