PT  - JOURNAL ARTICLE
AU  - Chang, Yan
AU  - Chen, Qiaoyan
AU  - Rittenbach, Andrew
AU  - Xu, Jingyan
AU  - Sun, Mingshan
AU  - Tsui, Benjamin
AU  - Yang, Xiaodong
TI  - Evaluation of MR compatibility of a collimator material for simultaneous SPECT/MR imaging
DP  - 2016 May 01
TA  - Journal of Nuclear Medicine
PG  - 1937--1937
VI  - 57
IP  - supplement 2
4099  - http://jnm.snmjournals.org/content/57/supplement_2/1937.short
4100  - http://jnm.snmjournals.org/content/57/supplement_2/1937.full
SO  - J Nucl Med2016 May 01; 57
AB  - 1937Objectives In this study, we evaluate the MR compatibility of a collimator material for use in a SPECT insert for simultaneous small animal SPECT/MR imaging applications.Methods A sample cylindrically-shaped multi-pinhole (MPH) collimator with 40 mm diameter was made by a 3D printing method using a tungsten alloy from a commercial vendor (densigy-15; 3D Systems Layerwise, Leuven, Belgium). The MPH collimator was tested for its effect on the uniformity and distortion on MR images of home-built MRI scanner with a 0.5T permanent magnet. A test phantom was placed inside a single-channel Tx/Rx RF coil that in turn was inserted into the MPH collimator. The assembly was placed in the center of the MRI scanner for image acquisition. A uniformity phantom, consisting of a hollow acrylic cylinder with an inner diameter of 15 mm and height of 50 mm, was filled with a uniform CuSO4 aqueous solution was used in the uniformity measurement. It was centered inside the RF coil with its long axis parallel to that of the RF coil. The RF coil was first tuned to its resonance frequency without the MPH collimator and MR images of the uniform phantom were acquired. It was retuned to its resonance frequency with the MPH collimator and before repeating the uniformity measurement. The MRI experiments were performed with and without the MPH collimator using a spin-echo pulse sequence with TE of 12ms, TR of 1000ms. The data acquisition and image analysis of the MR images including the signal-to-noise ratio (SNR) calculation followed the NEMA study protocol. Then the uniformity of the MR images obtained without and with the MPH collimator over a 3cmx3cm field-of-view (FOV) were compared and evaluated. Similarly, a hot-rod phantom, with an inner diameter of 28 mm and outer diameter of 35 mm and consisting of 6 groups of drilled holes with depth of 10 mm and different diameters ranging from 0.75 to 2.4 mm, was used to evaluate the effect of the MPH collimator on distortion of the MR images. A 5cmx5cm FOV over the MR images was used in data analysis.Results The resonance frequency of the Tx/Rx coils was increased by ~2.71/2.47 MHz from 21.1 MHz in the presence of the MPH collimator and a returning was performed to achieve maximum SNR in the MR images. The MR images of the uniform phantom show the MPH collimator has small effect on the uniformity but reduces the SNR by a factor of ~60%. The signal drop across the phantom can be attributed to the RF power absorption and the change in the static magnetic B0 field due to the presence of the collimator materials. Negligible distortion is found in the MR images of the hot-rod phantom in the presence of the MPH collimator.Conclusions The MPH collimator with the tungsten alloy material causes a shift in the resonance frequency shift by an average of 2.59 MHz and a SNR loss of ~60%. It has negligible effect on the linearity of the MR images. The preliminary results indicate the material used in the MPH collimator fabrication is suitable for use in a SPECT system for simultaneous SPECT/MR imaging. Future work includes improving the re-shimming method and optimized pulse sequence to reduce the SNR loss.