Background

One of the most commonly used materials for cranioplasty is a titanium mesh owing to its excellent biocompatibility and mechanical performance [1]. However, a titanium mesh has been reported to cause radiofrequency (RF) shielding [2]. After surgery for brain tumors, T1-weighted imaging (T1WI) with contrast medium is performed to assess the presence of remnant tumors and recurrence [3]. However, the influence of the RF shielding effect with the use of a titanium mesh is a concern. If the RF shielding effect is reduced using suitable routine sequences, high-quality images will be obtained.

Purpose

The present study aimed to determine which sequence for routine contrast-enhanced brain MRI (2-dimentional spin echo, 2D SE; and 3-dimentional gradient echo, 3D GRE) exhibits less RF shielding effect of the titanium mesh after cranioplasty, using a phantom.

Methods

A 1.5-T MRI scanners (Philips, Achieva; Siemens, MAGNETOM Avanto; and GE Healthcare, Signa Explorer) were used for imaging. The sequence parameters were as uniform as possible. A cubic bottle [152 × 152 × 195 mm (width × height × depth)] filled with 33 mg/L of manganese chloride tetrahydrate and 3.6 g/L of sodium chloride was used as a phantom to mimic the human brain. The phantom was placed at the center of the coil, and a titanium mesh board (100 × 100 × 0.8 mm; hole diameter, 1.6 mm; hole interval, 3 mm; thickness, 0.8 mm), which is identical to the material used during actual cranioplasty, was placed on the phantom. The signal attenuation ratio was calculated as the signal intensity of the image obtained in the presence of the titanium mesh divided by the signal intensity obtained in the absence of the mesh. If the signal intensity did not change, the signal attenuation ratio was 1 [2]. To evaluate uniformity as RF shielding dependency, normalized absolute average deviation (NAAD) was used. NAAD was calculated using the following equation:
$$NAAD=100\times[1-\frac{1}{N\bar{Y}}\sum_{i=1}^N(\mid Yi-\bar{Y}\mid)]$$
where$$$Yi$$$is the individual pixel value within the measurement region of interest (MROI), $$$\bar{Y}$$$is the mean of all pixels within the MROI,$$$\mid Yi-\bar{Y}\mid$$$is the absolute deviation for pixel $$$i$$$, and $$$N$$$ is the total number of pixels within the MROI.
Conventional 2D SE and 3D GRE were compared in the proximal section of the titanium mesh (most influenced by RF shielding). The Wilcoxon paired-rank test were used for statistical analyses (EZR v. 3.4.1). A P-value <0.05 was considered statistically significant.

Results

3D GRE showed less radiofrequency shielding effect than 2D SE, and it was considered as the optimal sequence for MRI in the presence of a titanium mesh. Every vendor had a similar signal attenuation ratio (Fig. 1) and NAAD in the proximal section (Fig. 2). With regard to NAAD, there was a significant difference between 2D SE and 3D GRE (P < 0.05) (Fig. 2). Fig. 3 presents sample images with contrast medium.

Conclusions

3D GRE shows less RF shielding effect of a titanium mesh after cranioplasty among routine sequences for contrast-enhanced brain MRI, and it might be the most appropriate sequence for MRI when a titanium mesh is present.

Acknowledgements

Nothing in particular.