COMPREHENSIVE QUALITY CONTROL OF NMR TOMOGRAPHY USING 3D PRINTED PHANTOM Mažena MACIUSOVIČ *, Marius BURKANAS *, Jonas VENIUS *, ** * Medical Physics Department, National Cancer Institute, Vilnius, Lithuania ** Laboratory of Biomedical Physics, National Cancer Institute, Vilnius, Lithuania 12TH INTERNATIONAL CONFERENCE MEDICAL PHYSICS IN THE BALTIC STATES 2015 5-7 th of November Kaunas
What is MRI? A non-invasive imaging technique for studying soft tissue structures and characteristics. The MRI technique has been used for imaging the brain, heart, muscles and joints, for early diagnosing and staging of the diseases, monitoring the effects of therapies. 2
In short The patient is placed into a magnetic field. A radio frequency wave is sent in. The radio frequency wave is turned off. The patient emits a signal. The signal is received and used for reconstruction of the picture. 3 Bayer, 2010.
Purpose To achieve the full potential of MRI and optimal images in a hospital there is a need for testing instrument performance and evaluating the system stability. Protocols for quality assurance (QA) and quality control (QC) of MRI system are required. A simple 3D printer compatible all in one phantom design was presented to fulfil the basic requirements for the assessment of stable MRI scanner operation. Daily/weekly/monthly measurements were performed and the suitability of the proposes system for the stability evaluation was confirmed. 4
Methods and materials Spherical phantom (20 cm Ø) has been made using a SOLIDWORKS software program. The phantom has been printed using a Dimension 1200es 3D Printer. Image and the view of the modelled phantom design rendered on SOLIDWORKS software. 5
Methods and materials The phantom has been printed in four different parts two hemispheres, the grid plate and the rest inner structures printed in one object (low contrast plate, spatial resolution plate, four side structures with crossing grooves). The structures were fixed together and filled with aqueous nickel chloride and sodium chloride solution. 6
Methods and materials Experiments were performed on a clinical 1.5 T MR-system Achieva XR at National Cancer Institute using Philips SENSE Head coil. All measurements were performed in one procedure no removing or repositioning of the phantom has been done. Measurements for every parameter have been performed five times, to acquire reliable data. 7
Results: Image uniformity The variation of percentage image uniformity over measurement period. PIU obtained for the properly functioning scanner should be more than 90%. Calculated PIU is 94.93% ± 1.54. 8
Results: SNR The variation of SNR values in the MRI images over measurement period. The observed SNR stability is 73.99 ± 6.05 (the allowed limits over measurement time ± 10 %). 9
Results: Geometrical distortion The variation of calculated maximum percent geometric distortion (PGD) over measurement period. The allowed value of maximum PGDs should not exceed 2%. The average maximum PGDs over the whole measurement period was 1.04 % ± 0.20. 10
Results: High contrast spatial resolution The plate consists of a line shape holes that form a high resolution test pattern of 1 16 line pair/cm. Spatial resolution is evaluated by visually estimating the amount of line pairs in centimetre that are still possible to distinguish as different lines. During the whole measurement period 6 lp/cm has been clearly visible. 11
Results: Low contrast resolution The variation of different contrast levels over measurement period. The average values of different contrast levels (from 1 to 6) are: 0.5 ± 0.01, 0.39 ± 0.01, 0.26 ± 0.01, 0.17 ± 0.01, 0.14 ± 0.01, 0.07 ± 0.004. The observed different contrast levels vary less than 10 %. 12
Results: Slice thickness Slice thickness is evaluated by measuring the width of the groove G seen in the images and multiplied by the factor of 0.5. The width of the groove is calculated as FWHM of pixel intensity profile (Line P) of the groove. 13
Results: Slice thickness The calculated slice thicknesses over measurement period. The calculated average slice thickness is 5.3 mm ± 0.15. The values are randomly distributed and no tendency can be identified. 14
Results: Slice position Verification is performed by measuring distances between crossing grooves seen in Slice 1 (distance d1) and Slice 6 (distance d2). The distance between slices is 25 mm. 15
Results: Slice position Deviations of the measured slice position from the set position over measurement period. The average difference from the set position is 0.72 mm ± 0.39 when the positioning distance is 25 mm. The deviations are distributed randomly and there is no tendency to lower or increase. 16
Conclusion The 3D printed phantom allowed to measure and calculate all required parameters to evaluate image uniformity, geometric accuracy, SNR, high contrast and low contrast spatial resolution, slice thickness and slice position. The deviation of the measured/calculated parameters has been insignificant therefore the observed constancy of these parameters is sufficient to declare the stable operation of the MRI scanner. To expand the abilities of the printable system further developments and investigations are planned to be performed. 17
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