T2∗ Relaxation of Agar Gel with and Without the Presence of Tumor-Like Structure as Obtained from Resting State Fmri Sequence Protocol

Abstract

The objective of this study was to determine the change in signal-to-noise ratio (SNR) as a function of TE of agar gel phantom with and without the presence of tumor-like structure, using a resting state fMRI sequence protocol. The phantoms were prepared using a conventional method. Citrus hystrix DC (kaffir lime) was used to mimick tumor and was submerged in the agar gel. The phantoms were scanned using a 3T-MRI system. An rsfMRI sequence protocol (TR = 2000 ms; TE = 51 ms - 115 ms) was used in obtaining the SNR at several TEs. In addition, a standard MRI phantom as control was also scanned using the same sequence protocol. For the prepared phantoms, the SNR was obtained from several 7-mm diameter ROIs at a distance 2, 4 and 6 cm from the centre of the image and was averaged. For the standard MRI phantom, similar ROI diameter was used for SNR determination but for three different slice thickneses; 3, 5 and 10 mm. The MRI, agar gel and agar gel with kaffir lime phantoms show T2∗ relaxation that fits the exponential equation S ∝ e-TE/T2∗. The average T2∗ for the standard MRI phantom is 86.7 ± 0.1 ms. The average T2∗ for agar gel phantom is 70.1 ± 0.1 ms while T2∗ is shortened to an average of 54.8 ± 0.1 ms for agar gel phantom with kaffir lime. This indicate an increase in local magnetic field inhomogeneity in the vicinity of the tumor. The relaxation is thus faster causing a drop in the magnetic resonance signal intensity. The agar gel and agar gel with tumor mimicking phantom are suitable for a long TE rsfMRI scans.