Abstract
Magnetic Resonance Imaging (MRI) simulation differs from diagnostic MRI in purpose, technical requirements, and implementation. We propose a semiautomatic method for image acceptance and commissioning for the scanner, the radiofrequency (RF) coils, and pulse sequences for an MRI simulator. The ACR MRI accreditation large phantom was used for image quality analysis with seven parameters. Standard ACR sequences with a split head coil were adopted to examine the scanner's basic performance. The performance of simulation RF coils were measured and compared using the standard sequence with different clinical diagnostic coils. We used simulation sequences with simulation coils to test the quality of image and advanced performance of the scanner. Codes and procedures were developed for semiautomatic image quality analysis. When using standard ACR sequences with a split head coil, image quality passed all ACR recommended criteria. The image intensity uniformity with a simulation RF coil decreased about 34% compared with the eight‐channel diagnostic head coil, while the other six image quality parameters were acceptable. Those two image quality parameters could be improved to more than 85% by built‐in intensity calibration methods. In the simulation sequences test, the contrast resolution was sensitive to the FOV and matrix settings. The geometric distortion of simulation sequences such as T1‐weighted and T2‐weighted images was well‐controlled in the isocenter and 10 cm off‐center within a range of ±1% (2 mm). We developed a semiautomatic image quality analysis method for quantitative evaluation of images and commissioning of an MRI simulator. The baseline performances of simulation RF coils and pulse sequences have been established for routine QA.
Highlights
Compared with CT, Magnetic Resonance Imaging (MRI) has the advantages of nonionizing radiation, superior soft-tissue contrast, and allowing quantitative or semiquantitative analysis of functional images.[1,2,3] Developments in radiotherapy require precise MRI images for target and normal tissue delineation, characterizing tumor features, and monitoring treatment response during and after radiotherapy.[4,5] MRI simulation is a relatively new technique for radiotherapy.[6,7] Because MRI simulation serves a different purpose from MRI diagnosis, the technical requirements are different.[8]
For ACR T1-weighted image (T1WI) and T2-weighted image (T2WI) sequences, compared with the eightchannel diagnostic head coil, the Percent image uniformity (PIU) of the simulation head coil decreased by 34.37% and 34.04%, respectively (Table 4)
For the six-channel simulation head coil, image uniformity of ACR T1WI increased to 86% and 89.49%, and of ACR T2WI increased to 85.44% and 89.83%, respectively (Table 5 and Fig. 5)
Summary
Compared with CT, MRI has the advantages of nonionizing radiation, superior soft-tissue contrast, and allowing quantitative or semiquantitative analysis of functional images.[1,2,3] Developments in radiotherapy require precise MRI images for target and normal tissue delineation, characterizing tumor features, and monitoring treatment response during and after radiotherapy.[4,5] MRI simulation is a relatively new technique for radiotherapy.[6,7] Because MRI simulation serves a different purpose from MRI diagnosis, the technical requirements are different.[8]. According to AAPM Report 100,10 the main procedures for acceptance and commissioning of a diagnostic MRI should include general system checks and MRI scanner system tests. Image quality tests play an important role in checking and monitoring the performances of an MRI scanner system. The gradient subsystem is assessed by geometric accuracy tests. Slice thickness accuracy is evaluated for combined gradient/radiofrequency (RF) subsystem. Percent image uniformity (PIU), high-contrast spatial resolution (HCSR), low contrast detectability (LCD), and percent signal ghosting are evaluated for the performances of global system
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