Hybrid Utrasound and MRI Acquisitions for High-Speed Imaging of Respiratory Organ Motion
Magnetic Resonance (MR) imaging provides excellent image quality at a high cost and low frame rate. Ultrasound (US) provides poor image quality at a low cost and high frame rate. We propose an instance-based learning system to obtain the best of both worl
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Abstract. Magnetic Resonance (MR) imaging provides excellent image quality at a high cost and low frame rate. Ultrasound (US) provides poor image quality at a low cost and high frame rate. We propose an instance-based learning system to obtain the best of both worlds: high quality MR images at high frame rates from a low cost single-element US sensor. Concurrent US and MRI pairs are acquired during a relatively brief offline learning phase involving the US transducer and MR scanner. High frame rate, high quality MR imaging of respiratory organ motion is then predicted from US measurements, even after stopping MRI acquisition, using a probabilistic kernel regression framework. Experimental results show predicted MR images to be highly representative of actual MR images.
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Introduction
Magnetic Resonance (MR) imaging has gained considerable traction in the last two decades as a modality of choice for image-guided therapies [1,2], primarily due to its excellent soft-tissue contrast and its non-invasive nature. However, major challenges include relatively slow frame rates and limited physical patient access within the MR bore. Perhaps the most notable effort made toward scanner design and providing patient access has been the (now discontinued) double-doughnut 0.5 T SIGNA SP/i design [3], whereby the interventionist could step in-between two physically-separate magnets and gain direct access to the patient. Other interventional MR systems have also been developed and commercialized but patient access, MR-compatibility of instruments and overall costs have remained considerable hurdles. In contrast to MR imaging, ultrasound (US) imaging provides fast frame rates and nearly-unhindered physical access to the patient. US imaging systems are cheaper and faster than MR, yet produce images that are often found lacking in terms of contrast and overall quality. As a consequence, several noteworthy efforts have been made to combine the two complementary imaging modalities and a body of work has emerged on developing hybrid methods [4,5,6,7,8,9,10]. In [4,5], hybrid 2D US/MR systems were proposed where orientation information extracted from US data was used to update the image slice position of an SSFP sequence in real time, for prospective c Springer International Publishing Switzerland 2015 N. Navab et al. (Eds.): MICCAI 2015, Part I, LNCS 9349, pp. 315–322, 2015. DOI: 10.1007/978-3-319-24553-9_39
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motion compensation in a motion phantom. A similar hybrid system was presented in [7], where a clinical US imaging system was integrated with 1.5 T and 3 T clinical MR scanners for simultaneous 4D MRI and US imaging. The present work involves a small 8 mm-diameter single-element MR-compatible ultrasonic transducer applied to the skin of the abdomen and held in place using a simple adhesive bandage (Figure 1). A regular flexible MR coil array can readily be wrapped over this small US probe, at no detectable penalty in MR image quality. The emitted US field is not focused, it is expected to penetrate and reflect
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